Freshwater Ecology

Light attenuation

• Light being lost
  • Greater light attenuation at shallower waters through all levels of light transmission
    • Eutrophic lakes have more (more algae, not as clear)
  • Less light attenuation in deeper waters at low light transmission
    • Oligotrophic lakes have less (less algae, clear)

In water

• Blue light doesn't get removed by water as much as red & green
• Red light gets removed fast, first color to be lost in a lake
• Green light doesn't get removed as fast as red, but faster than blue

Within a highly Chlorophyll populated lake

• Red will not go deep at all, high attenuation
• Blue won't go as deep either, since chlorophyll absorbs blue light (that's why it can survive at deeper regions)
• Green will be the least changed since chlorophyll does not absorb green light
  • This is why lakes with a lot of algae appear to be green, since water, algae, and other plants reflect and don't absorb it

Lentic systems - wetlands and lakes

Services of wetlands

What ecosystem services are provided by freshwater wetlands?

• Natural water filtering, flood control, carbon storage/sequestration, pollutant processing, biodiversity, long-term preservation of environmental change

Defining and delineating wetlands

Wetlands are an ecotone, or transitional habitat, between the terrestrial and aquatic realms

Three characteristics to define or delineate wetlands:

• 1. Hydrology
  • a wetland must be wetted during some or the full portion of the plant growing season
• 2. Plants
  • Specific adapted plant species must be present, called hydrophytes (emergent or submerged)
  • Herbaceous = not woody, absence of woody tissues
  • Emergent = at the interface between the air and the water
  • Submerged = under water. Some can only live in submerged wetlands
  • Woody = plants that have special adaptations to survive in swamps
• 3. Soils
  • Must have characteristics of frequent flooding, called hydric soils (often very dark, organic rich)
    • Dark coloration
    • Lots of organic matter e.g. peat because decomposition is very slow
      • NRCS - Indicators of Hydric Soils

Wetlands come in a wide variety of types (Table 5.2 highlights major types & their characteristics)

• Coastal wetlands
  • often in salty or brackish waters and include mangroves, tidal marshes, and salt marshes
    • Marshes are dominated by herbaceous plants
• Freshwater marshes
  • Well-lit wetlands dominated by floating and emergent vegetation
    • No trees
    • Adapted to drying out
    • e.g. The Everglades
• Freshwater swamps
  • Shadier wetlands dominated by trees
    • Okefenokee swamp
    • Great Dismal Swamp

Wetland losses

Wetlands are among the most threatened habitats globally

What are some of the causes of wetland losses?

• Agriculture
  • wetlands are nutrient rich
• Harvesting energy (peat) & peat moss from peatlands
• Undesirable b/c associated with diseases like malaria and yellow fever
  • Often converted to desirable ponds, lakes, or reservoirs

Wetland mitigation

Mitigating wetland losses is a charged topic:

• Government role
  • No net loss of wetlands
    • Lots of $$$ spent on defining and delineating wetlands for regulation
    • Established by Jimmy Carter as an executive order in 1977; has been upheld since
  • When wetlands are removed, wetlands must be restored or constructed elsewhere
  • Not all wetlands are created equal
    • Criticism: "no net loss" does not go far enough to give priority to pristine wetlands

Formation of Lakes

• Lake is a very slowly flowing or static (lentic) open body of water in a depression, not in contact with the ocean
• Lakes are often categorized by how they form: (Table 7.3 shows definitions) (Table 7.1 shows notable characteristics)
  • Tectonic (formed by movement of tectonic plates)
  • Glacial (pothole/kettle/moraine), or
  • Fluvial (Earthslide, Oxbow, Reservoir = but not "geological")
• Dendritic shapes are caused by damming
• Glacial lake formation
  • Lakes in basins left behind as glacier retreats
    • E.g. prairie potholes, Great Lakes
  • A string of glacial lakes
• Tectonic lake formation
  • Caused by faults slipping, causing a block to forma depression for a lake basin
    • E.g. Lake Baikal (Russia), Lake Victoria (Africa)
  • Produces biggest and oldest lakes on the planet
• Notable characteristics
  • Mixis
    • How often lake mixes
  • Area
    • Caspian Sea has highest area
  • Depth
    • Lake Baikal is deepest
  • Volume
  • Length
  • D_L
    • a measure of shoreline development index
    • 1.0 for a "perfect circle"
    • Higher for long, narrow lakes
  • Retention time
    • How long a water molecule stays in the lake, on average
    • Same as residence time
    • Sink
      • No drainage

Distinct physical attributes of lakes

How do total area and number of lakes change with lake surface area classes? Any compare/contrast to streams?

• More small lakes than large lakes
  • Can be compared to stream order
    • More streams are low order than higher order
  • Can be contrasted as well
    • Most stream length comes from small streams
• Most surface area of lakes on Earth is composed of large lakes
• Bathymetry
  • Describes the water depth contour of lakes
• Most lakes go through seasonal stratification
  • Layering of physical/chemical features with depth
  • During stratification, a lake forms into 3 layers
    • Epilimnion
      • top layer, warmest and least dense
    • Metalimnion
      • Thermocline
        • layer of rapid change
    • Hypolimnion
      • bottom layer, coldest and waters most dense
  • Lake mixing can weaken or even prevent stratification
  • Fetch
    • Length of continuous water over which wind can act to form waves and mix a lake
  • Wind causes deep but weak mixing below the surface
    • Less water is moved further down
    • Needs big waves to mix a big/deep lake

Think about contrasting features of reservoirs versus natural lakes (e.g. Great Lakes). How do fetch and wind mixing differ between these, and why?

• Bigger, natural lakes will have a bigger fetch since there is more surface area for wind to generate waves
• The deeper the lake, the more difficult it is to get that deeper water to the surface

Lake stratification and mixing can help us categorize lakes as

• Monomictic
  • Mixes 1x per year
• Dimictic
  • Mixes 2x per year
• Meromictic
  • Never mixes
  • Usually lakes that are very salty (e.g. Dead Sea)
• Polymictic
  • Mixes often

Global mean warming of +0.34 degrees Celsius per decade

• High regional variation in degree of warming
  • Due to lake variation
• Lakes generally are warming with air temperature, but lots of variation

Freshwater Biodiversity: Microbes, Plants (Ch.9), and Multicellular Animals (Ch.10)

Classifying biodiversity

• Phylogenetic tree
  • Shows relationships among taxonomic groups
• Metabolism
  • How organisms acquire their energy through the environment
  • Photosynthesis
    • Light energy stored in chemical energy (glucose)
  • Heterotrophic
    • Rely on other organisms (reduced organic carbon, e.g. glucose) for energy
  • Chemosynthetic
    • Draw energy from reactions among inorganic chemicals
      • Methanogenesis & Denitrification
• What diseases do you know that are associated with freshwater microbes?
  • Multiple diseases caused by multiple groups of organisms (Bacteria, Virus, Protozoa, Helminths)
    • Table 9.1 in textbook

Viruses

• Common but understudied in freshwaters
• Small size (25-350 nm)
  • Implications for suspension
• Inactivated by UV, contact w/ non-host, or predation
• Major roles:
  • Parasitic / infect hosts and reduce host fitness
• Debated whether they are "alive" - why?
  • Can't replicate without a host
• Virus structure is variable
  • Capsid protein, RNA vs. DNA genetic information

Archaea

• Prokaryotes
  • Single-celled, lack a nucleus, and no cellular organelles
• More closely related to Eukaryotes than to Bacteria
• Extreme environments:
  • Hot springs, hypersaline environments (salt lakes)
    • Thermopile Archaea are common in hot springs such as in Yellowstone National Park
• Major functions:
  • Many chemosynthetic. Includes the methanogens which produce methane in anoxia

Bacteria

• Prokaryotes
  • Single-celled, lack a nucleus, and no cellular organelles (similar to Archaea)
• Extreme metabolic diversity:
  • Photosynthetic, chemosynthetic, and heterotrophic
• Very small: 1-2 μm diameter
• Greater living biomass than any other group on Earth?
• Classification based on shape, cell walls (gram-positive vs. gram-negative), and metabolism
  • Shapes:
    • Coccus (sphere)
    • Bacillus (rod)
    • Spirilla (spiral shaped)
    • Filamentous
    • Streptobacillus
    • Streptococcus
    • Vibrio (curved rod)
    • Staphylococci
    • Sarcina
    • Spirochete
• All have a low Reynolds number
• Why do we rely on cell staining, morphology, and culturing to determine bacterial species?
  • Modern species concept is difficult to link to bacteria
  • Culturing allows description of metabolism, but... note only about 1% of bacteria can be cultured
  • Metagenomics and DNA sequencing increasingly used to identify bacterial taxa

Cyanobacteria

• Formerly called "Bluegreen algae", but they are just photosynthetic bacteria - not algae
• Can form blooms or dense aggregations of cells
• Billions of years old - earliest photosynthetic organisms?
  • Stromatolites
• Heterocysts are specialized cells used for Nitrogen fixation
• Harmful Cyanobacteria blooms are becoming more common due to climate change and nutrient pollution
• Some species produce toxins like Microcystin

Protoctista

• also called "Protists", includes a wide variety of organisms from single to multi-cellular
• Both photosynthesis (algae) and heterotopic protozoa
• Algae are nonvascular eukaryotic organisms capable of oxygenic photosynthesis that contain chlorophyll-a
• What does "nonvascular" mean? Oxygenic photosynthesis?
  • Oxygenic photosynthesis means that oxygen is produced
  • Nonvascular means that there is an absence of tissues that carry water and sugars
• Focus on Cyanobacteria, Bacillariophyceae (diatoms), Chlorophyceae (green algae), and Charophyceae

Algae: Diatoms

• Bacillariophyceae (Diatoms) are common in freshwaters
• Silica glass cell wall called a frustule
• Sediments w/ diatoms can be used to track environmental change
• As silica levels decline within a system, diatoms will tend to be less present
• Diatoms are more effective at capturing low light

Green Algae, Stoneworts

• Chlorophyceae (Green) and Charophyceae (Stoneworts)
• Range from single to multi-cellular algae
• Most diverse freshwater algae, widespread
• Filamentous green algae mats (e.g. Cladophora) in streams, often associated with nutrient pollution
• Unicellular in lakes
• Stoneworts (e.g. Chara) are evolutionary pre-cursors to land plants
• Calcareous deposits in cell walls = rigid structure

Protozoa

• Includes both photosynthetic autotrophs and heterotrophs - "zoa" associated with motility
• Flagellated - 1 or more flagella; open water (pelagic)
• ameboid - move by protoplasmic flow; benthic and sediments
• Ciliated - 4+ cilia for locomotion and free-swimming

Aquatic Fungi

• Less diversity in aquatic systems, but at least 600 aquatic species
• Saprophytes - heterotrophs which live on dead organic matter (detritus)
• Key decomposers of organic matter, help channel energy and nutrients into detritivores (animals that eat detritus)
• Major groups includes the hyphomycetes which lives in flowing, well-oxygenated waters and yeasts which are abundant in polluted waters
• Fungi can form a "mycoloop" by affecting planktonic systems
• Chytrids are fungi ranging from saprotrophic to parasitic

Practice question

Compare the different taxonomic groups we have discussed. Which groups could survive under the following conditions? Why?

• Complete darkness
  • Fungi
  • Viruses
  • Bacteria
  • Protozoa
• Absence of oxygen (anoxia)
  
  • Algae
  • Cyanobacteria
  • Fungi CANNOT and become inactive because they use oxygen for their energy

Multicellular Animals

• Wide range of diversity
• Multicellular and heterotrophic - but many have chemosynthetic or photosynthetic symbionts (examples?)
• Invertebrates are all non-Chordates including Porifera (sponges), Cnidaria, Mollusca, Arthropoda
• Vertebrates are Chordates including fishes, Tetrapods such as amphibians and reptiles/birds/mammals

Where in the phylogenetic tree are invertebrates? Vertebrates?

Both at the top by Eukarya. They split after looking at true tissues, bilateral symmetry, and protostome development

Porifera

• Sponges (Porifera) occur in marine & fresh waters
• Many are filter-feeders and/or house symbiotic algae
• Composed of collagen or silica spicules which give structure
• Specialized epithelial/reproductive/digestive cells, but no true tissues

Cnidaria

• Jellyfish and polyps are mostly marine, but include some freshwater taxa
• Nematocysts sting and capture prey (e.g. zooplankton)
• Many have symbiotic algae
• Includes corals

Mollusca

• Major groups are Gastropods (snails and limpets) and Bivalves (clams and mussels)
• Soft-bodied and unsegmented. The mantle secretes a calcareous shell, often exterior to the body
• Snails are most diverse class within Mollusca:
  • Feed on diverse food sources including detritus and periphyton (mixture of algae, bacteria, detritus)
• Bivalves include freshwater mussels - shells have two halves; have ciliated filaments for filter feeding
• Unionid mussels = freshwater mussels that produce glochidia, larval parasites of fish hosts for dispersal
• Asian clams, zebra mussels, and quagga mussels are widespread invasive bivalves in North America
  • Great Lakes - zebra mussels profoundly altered phosphorus cycle
  • Huge shifts in lake and river ecology, and costs to the economy

Arthropoda (Table 10.2)

• The Arthropods have chitinous exoskeletons and stiff-jointed, segmented appendages
• Chelicerata (Arachnids)
  • Includes water mites (mostly parasites, especially on other Arthropoda)
• Uniramia (Insects)
  • Huge diversity in freshwater, including flies, caddisflies, stoneflies, mayflies. Mostly aquatic larvae and terrestrial adults
• Crustacea
  • Crayfish, shrimp, isopods, cladocerans, and copepods
  • Most are strictly aquatic
• Classification by Functional Feeding Groups:
  • Shredders feed on coarse detritus (coarse particulate organic matter, CPOM)
  • Filtering or gathering collectors feed on fine particles (fine particular organic matter, FPOM)
  • Grazers/scrapers feed on algae (or periphyton - a mixture of algae, bacteria, detritus, fungi)
  • Predators feed on other animals
• Ephemeroptera (mayflies)
  • External abdominal gills and extended filaments on cerci behind abdomen
  • Includes collector gatherers and grazers, mostly (FPOM or periphyton)
  • Sensitive to fine sediments because of exposed gills
  • Mass mayfly emergence (hatching) can be tracked by Doppler radar
  • Adult life stage only mates & lays eggs. Adult survives for days
• Plecoptera (stoneflies)
  • Reduced gills and highly sensitive to water pollution, temperature
  • Includes both predators, collectors, and shredders
  • Emerge as adults. Poor flyers (b/c primitive wings)
• Trichoptera (caddisflies)
  • Case-making or free-living species with aquatic larval stages; terrestrial adults
  • Use silk glands to create cases or weave silken "nets" to filter particles. Shredders, grazers, filter-feeders
  • Emerge as adults - resemble moths w/ hairy wings
• Diptera (flies and midges)
  • 40% of all aquatic insects
• Chironomidae (midges)
  • Extremely high densities and diversity, small-bodied. Also called "gnats"
  • Includes nuisance flies such as mosquitoes, black flies, biting midges (some major disease vectors)
• Odonata (dragonflies and damselflies)
  • Predaceous as both larvae and adults
  • Extend long, hinged labium to rapidly seize moving prey. Generally sit-and-wait predators
  • Many have extended lifespans >1 year
• Crustacea includes Decapods like crayfish
  • Largest and longest-lived Crustacea. Chelae (pincers) are used to grasp food items and for defense
  • Crayfish are omnivorous - "everything eats crayfish, and crayfish eat everything"
  • Global hotspots for freshwater crayfish diversity include the Ozarks/Ouachitas, Appalachians, and Australia
• Crustacea - zooplankton Cladocera (water fleas)
  • Cladocera - abundant in lentic systems. Larger-bodied filter feeders on suspended particles (esp. algae)
  • Cladocera respond to predator chemical cues and change their morphology
  • Capable of both parthenogenesis (cloning) and sexual reproduction
• Crustacea - zooplankton Copepods
  • Smaller zooplankton, more associated with benthic zones. Range from herbivores, carnivores, to parasitic
  • Sexual reproduction usually. Better-adapted to drying and eggs hatch into larval stages called nauplii
  • Diverse coloration –blend in or protect against UV damage
  • Females have egg sacs attached.

Vertebrates: Fish

• Ichthyology is the study of fishes. Fishes are the most diverse group of freshwater vertebrates
• Jawed, bony fishes (Osteichthyes) are the most diverse group of fishes
• Morphology matches ecology (feeding, habitat):
  • Streamlined for active predators (salmon)
  • Flattened bodies of bottom-dwellers (sturgeon)
  • Sit-and-wait predators have pointed snouts (gar)
• Fish exhibit wide feeding strategies including herbivores (grazers), predators, and detritivores
• Some fish move between salty and fresh environments:
  • Diadromous - spend part of lifetime in marine
  • Catadromous - move from freshwater to saltwater
  • Anadromous - move from saltwater to freshwater
• Useful as indicator species

Stuff to Know

What do I expect you to know?

• Differences across groups, and classes within groups
• Distinguishing/special features of groups
• Relevant, big picture ecology of the group or species

Where do we go now?

• Try to apply this knowledge elsewhere in the semester

Redox and Carbon Cycling - Chapters 12-13

Some principles of water chemistry

• What's in water? Materials are
  • Dissolved
    • Always stays suspended in water; small
  • Colloidal
    • Often larger in size and are able to see by the naked eye. Can change viscosity of a solution (i.e. milk)
  • Gravitoidal
    • (Mostly) Particles affected by gravity
• The concentration of dissolved materials depends on both abundance and solubility in water
• What compounds here are more common in freshwater? Rare?
  • Common
    • HCO3-, Total dissolved solids, Na+
  • Rare
    • Al3+, Mn2+, Fe2+

How is this relevant to redox?

• Dissolved ions provide the chemical "bth" in which organisms live and undergo energetic transformations
• Redox potential
  • Relative availability of electrons under a given condition in the environment
    • Oxidation Involves Losing electrons (OIL)
    • Reduction Involves Gaining electrons (RIG)
    • Exchange of electrons results in a change of energy state
  • If Oxygen is available - Redox potential is high, and there are few electrons
  • In oxygen-depleted environments (anoxia), Redox potential is low and electrons are more available

Redox and chemical reactions

• It will help to review how to determine oxidation state
  • What is the oxidation state of Nitrogen in the following compounds?
    • NO3-
      • (+1 for negative charge, -6 for Oxygens) -> +5 for N
    • NH4+
      • (-1 for positive charge, +4 for Hydrogens) -> for N
    • N2 gas
      • (no charge, no distinct pull of electrons) -> 0 for N
• Potential Energy is the potential to do work during a chemical transformation
  • What is activation energy?
    • The energy needed to transform a compound
• Some reactions are thermodynamically favorable, and some are not
• Thermodynamics of redox reactions can change. In a reducing environment, Nitrate can be more readily reduced to Ammonium
• Different transformations result in different energy yields
  • Eh(mV)
    • Redox potential. Positive, higher values are more oxidizing

Oxygen in aquatic systems

• Dissolved oxygen can be influenced by the following metabolic processes:
  • (CH2O)X + O2 → CO2 + H2O (cellular respiration)
  • CO2 + H2O→(CH2O)X + O2 (photosynthesis)
  • Which of these processes will increase dissolved O2 availability?
    • Photosynthesis, since Oxygen is a product
  • Which will decrease dissolved O2 availability?
    • Cellular respiration, since Oxygen is an ingredient
• We can calculate the net Oxygen balance using relations between Gross Primary Production (GPP), Respiration (R), and Net Primary Production (NPP)
  • NPP = GPP - R
    • NPP
      • Photosynthesis that exceeds respiration
    • GPP
      • Total rate of photosynthesis
    • R
      • Total rate of respiration
    • This equation can be used for an individual organism (e.g., an algal cell) up to an entire ecosystem (e.g., lake)
• Remember that photosynthetic organisms undergo both photosynthesis and cellular respiration
• How do you expect NPP, GPP, and R to compare between the epilimnion vs. hypolimnion in a stratified lake?
  • NPP is more positive during the day in the epilimnion because GPP>R
  • NPP is negative in the hypolimnion because GPP<R
• Temporal O2 dynamics are related to organism metabolism through photosynthesis and respiration
• Explain how changes in O2 are related to GPP & R in the graph below
  • When the light is off the periphyton is consuming O2, performing cellular respiration and decreasing O2. R>GPP
  • When the light is on, the periphyton performs photosynthesis which increases O2. GPP>R

The Carbon Cycle

• How do organisms break down organic compounds in the environment?
  • Polymeric compounds (proteins, carbohydrates) are first hydrolyzed into monomers by enzymes
    • This takes energy
  • Organic compounds are oxidized, which yields energy. Microbes must reduce a second compound (add electrons) when they metabolize organic compounds
    • This yields energy
• Example
  • Cellulase enzymes break down (hydrolyze) cellulose (polymer) into its glucose monomers
  • During cellular respiration, C6H12O6 (glucose) is oxidized (electron donor) and O2 is reduced (electron acceptor)
    • Results in a net yield of energy
• Oxidation of organic compounds --> different electron acceptors can be used under different redox states
• Important processes in the carbon cycle?
  • Methanogenesis - Production of methane, from either acetate or CO2 and H2. Occurs in anoxic, reducing environments
  • Methanotrophy - Consumption of methane. Methane is an electron donor and oxygen is an electron acceptor
  • Fermentation - Oxidation of reduced organic matter (e.g. glucose) under anaerobic conditions. Results in slightly more oxidized organic products, such as alcohols (ethanol)

Nutrient Cycling (Chapter 14)

• What is the value of understanding cycling of nutrients like nitrogen, phosphorus, etc. in freshwaters?
  • Critical to life: link between abiotic, biotic environment
  • Humans have profoundly altered nutrients globally
  • Nutrient pollution associated with global water quality problems (for example, marine dead zones)
  • Critical to understand for human use of water
    • Aesthetics, recreation, fishing, drinking water

Revisiting a General Nutrient Cycle

• Pools
  • also called compartments
• Processes
  • also called fluxes or transformations
• Organic
  • Bound into organic form, e.g. with carbon
• Inorganic
  • Occurring in mineral or ion form, typically not bound to carbon
• Mineralization
  • Conversion from organic to inorganic forms (AKA release, or excretion if done by animals)
• Immobilization
  • Conversion from inorganic to organic forms (AKA uptake)

The Nitrogen Cycle

• Major pools of Nitrogen?
  • Organic
    • Living organisms (R-NH2) Amino groups
    • Detritus (R-NH2) Amino groups
    • Forms can be dissolved (DON) or particulate (PON)
  • Inorganic
    • Ammonium/ammonia
    • Nitrate
    • Nitrite
    • Nitrous oxide
    • Nitrogen gas
• Practice redox: Arrange these pools in order from most reduced to least reduced form of N
  • The more oxygens that are on the Nitrogen, the more oxidized it is
  • NH2, NH3/NH4 (-3/-2)
  • N2 (0)
  • N2O (+2)
  • NO2 (+3)
  • NO3 (+5)

Major processes in the N cycle

• 1. Atmospheric N2 -> Organic -NH2
  • Nitrogen fixation (Bacteria, Cyanobacteria)
    • Reduction of N
• 2. Organic -NH2 -> NH3/NH4
  • Ammonification (primarily heterotrophs)
    • This can be both Reduction and/or Oxidation (don't sweat the details)
• 3. NH3 -> NO2 or NO3
  • Nitrification (Bacteria & Fungi)
    • Oxidation of N
• 4. NO2/NO3 -> NH3 -> Organic (NH2)
  • Nitrate & Ammonium Assimilation (Bacteria)
    • Reduction of N
• 5. NO3 -> N2O -> N2
  • Denitrification (Bacteria and Fungi)
    • Reduction of N

The Nitrogen Cycle and Lake Dynamics

• How does [NO3-] change seasonally in Wintergreen Lake, Michigan? (Refer to figures in slides)
  • Nitrate is highest in March, but decreases in the summer. This is due to algal growth during summer
• How does [NH4+} change seasonally?
  • Ammonium accumulate in the bottom of the lake during the summer. This is due to anoxic conditions in the lower part of the lake
• What processes occur in the epilimnion versus the hypolimnion during lake stratification?
  • Epilimnion
    • Oxygen allows nitrification to convert NH4 to NO3
      • Thus: [NH4] stays low
    • NO3 and NH4 assimilation are high because of high light, algae
      • Thus: [NH4] and [NO3] stay low
    • This is why Nitrogen fixation is a key supply of N during summer
  • Hypolimnion
    • Anoxia allows denitrification to convert NO3 to N2
      • Thus: [NO3] stays low, N2 goes up
    • Mineralization of accumulating organic N converts -NH2 to NH4
      • Thus: [NH4] goes up
    • Nitrification cannot occur (no O2)
      • Thus: [NH4] stays around

The Phosphorus Cycle

• Major pools of P?
  • Organic
    • Living Organisms (Organic-PO4)
    • Detritus (Organic-PO4)
    • Dissolved organic P (DOP)
  • Inorganic
    • PO4 (dissolved or bound to minerals)
  • There is no major gaseous phase of the P cycle

Major Processes of the P Cycle

• 1. PO4 -> Particulate organic P
  • Assimilation
• 2. Particulate organic P -> Dissolved organic P
  • Leakage, lysis, or exudation
• 3. PO4 -> FePO4 (and other precipitates)
  • Precipitation (NOT RAIN)
• 4. FePO4 (and other precipitates) -> PO4
  • Dissolution and weathering
• 5. Particulate organic P -> PO4
  • Excretion or Mineralization

Important attributes of the P cycle

• 1. Phosphorus is extremely low in concentration in most pristine freshwaters (below 1 microgram P L^-1)
• 2. Organic phosphorus can be found in several key molecules
  • Fatty acids, nucleic acids, polyphosphate
• 3. P is strongly limiting for growth of most organisms, including both photoautotrophs and heterotrophs
• 4. No major organismal redox transformations

• Precipitation and dissolution are key processes
  • 1. When O2 is present: PO4^3- binds to Fe3+ (oxidized iron) and forms FePO4 as a precipitate. This precipitate then sinks
  • 2. Anoxic conditions: FePO4 re-dissolved or "dissociates" back into the water as Fe3+ and PO4^3-
• Think about which processes in the P cycle might differ between epilimnion and hypolimnion in a stratified lake
  • Epilimnion
    • Oxygenated waters allow PO4 to precipitate as FePO4, if Fe3+ is available
    • High assimilation by growing algae in epilimnion can remove PO4 as well
      • Thus: draw-down of PO4
  • Hypolimnion
    • Anoxic deep waters receive precipitated FePO4, which dissociates as PO4 into water/sediments
    • Accumulating particulate and dissolved organic P re mineralized to PO4 in sediments
      • Thus: Accumulation of PO4

Other Nutrient Cycles?

Sulfur Cycle

• Includes many redox states. Very important in reduced conditions where SO4 is a terminal electron acceptor
• Redox range: H2S to SO4^2-
• Not a cycle critical for primary producers (algae) because S is not a limiting nutrient for most algae

Silicon cycle

• Silicon is very abundant in Earth's crust, but solubility of silicate (SiO4^2-) in water is low
• Diatoms use silica (SiO2) to build frustules (cell walls), thus their abundance is limited by Silicate
• Some emergent macrophytes use silica as a defensive and structural compound

Practice Question

• Lake Mendota in Madison, Wisconsin is among the best-studied lakeside the world. NH4 and NO3/NO2 levels in surface water over the >6 years are below. What seasonal patterns do you see? What processes likely drive this?
• The lake is stratified in the summer and mixes towards the fall. Ammonia spikes toward the end of summer because the lake mixes, causing ammonium to rise from the hypolimnion to the epilimnion. Nitrate/nitrite peaks right after because of the conversion of ammonium to nitrate/nitrite by nitrification by the nitrifiers in oxic conditions.

Nutrient Limitation and Remineralization (Chapter 17)

What is a nutrient?

• Nutrient

  • Any element required by organisms for growth - but we mostly refer to elements that are commonly limiting
    • Ex. Nitrogen (N), Phosphorus (P), Iron (Fe), Silicon (Si)
    • These elements can be obtained from the environment (typically in dissolved inorganic forms)

• How do organisms acquire nutrients?

  • Uptake
    • Nutrients taken up from water within the organisms' environment
  • Assimilation
    • Nutrients incorporated into organic molecules and used for growth
  • Consumption
    • Typically used for animals (heterotrophs) that ingest nutrients bound in organic material

Modeling organism nutrient acquisition

• Nutrient Acquisition Strategies for Organisms

  • Photoautotrophs
    • Can only acquire dissolved inorganic nutrients within the water column
    • Nutrients can be assimilated independently
  • Heterotrophic microbes
    • Many can acquire both dissolved inorganic and organic forms of nutrients
    • Nutrients come bound in organic forms (macromolecules)
  • Animals
    • Can only acquire organic nutrients
  • Macrophytes
    • Can acquire inorganic nutrients both dissolved (water column) and in sediments

• What factors can affect organism nutrient uptake rates?

  • Light availability (autotrophs need more for photosynthesis)
  • Oxygen presence/availability (heterotrophs need for cellular respiration)
  • Temperatures (higher temperatures result in higher uptake rates)
  • Nutrient availability
  • Body or cell size (and/or total biomass)
• Michaelis-Menten relationships help us model nutrient uptake in response to nutrient availability
  • V = Vmax([S]/(Ks+[S]))
    • V = nutrient uptake rate
    • Vmax = Maximum nutrient uptake rate
    • [S] = Substrate (nutrient) concentration
    • Ks = Half-saturation concentration for Vmax
  • How will increased [S] affect rates of nutrient uptake?
    • Higher [S] results in higher levels of nutrient uptake
  • What about higher Vmax and higher Ks?
    • Higher Vmax will increase nutrient uptake rate because it "raises the ceiling" (raises the maximum)
    • Higher Ks will decrease nutrient uptake rate
• The Droop equation predicts organism growth rates based on their cell nutrient contents
  • u = umax(1-(Q0/Q))
    • u = organism growth rate
    • umax = Maximum growth rate
    • Q = Cell quota (concentration of nutrient in cell)
    • Q0 = Organism minimum cell quota of a nutrient
  • How does this model differ from the Michaelis-Menten model?
    • This models growth rate, while the other models nutrient uptake
    • Measures dry mass instead of content
  • How will growth rates change as Q increases?
    • As Q increases, growth rate increases
  • Some organisms (especially those with large cells - algae) can make up and store excess nutrients in a process called luxury consumption
    • Organisms can use these stored nutrients later
• Another factor limiting nutrient availability is light availability
  • Why do NO3 and NH4 uptake increase with greater light?
    • Adding NH4 decreases phytoplankton NO3 uptake
    • NH4 is energetically favored for assimilation - organisms take it up more easily
      • This is due to redox

Nutrient Limitation

• Nutrient limitation is the control of growth or production of an organism by a nutrient or multiple nutrients
• Liebig's Law of the Minimum
  • The nutrient in shortest relative supply for demands will most limit growth
• Which nutrient is most limiting in freshwater systems?
  • Phosphorus
• Bioassays can be used as a method to quantify nutrient limitation of microbial growth
  • Carboys amended with nothing (Ctrl), +N, +P, or +NP
• Nutrient-diffusing substrata can be used to test for nutrient limitation in situ(within the natural environment
  • Agar is amend with various nutrients like N, P, K, or carbon (e.g., glucose)
  • Nutrients diffuse out and stimulate microbial growth
  • Measure microbial biomass as indicator of growth
  • What nutrient is most limiting of algae in streams?
    • Nitrogen and Phosphorus together
• N and P co-limitation (both nutrients are limiting) is most common in many ecosystems
• What does co-limitation mean for Liebig's Law of the Minimum?
• What about nutrient uptake models?

• The Paradox of Plankton

• The Paradox of the plankton is that lakes have very diverse phytoplankton species, yet lakes are also very homogenous in nutrient levels
• Homogeneity should lead to competitive exclusion among phytoplankton -> lower diversity. But it doesn't.
• What is the solution?
  • There's potentially a zooplankton that eats phytoplankton, keeping them diverse
• Some solutions include:
  • Herbivores suppress dominant competitors and facilitate species coexistence ("keystone effect")
  • Nutrients are not totally homogenous
  • There is more than one resource - many nutrients
  • Lakes are not at equilibrium over long time scales

Nutrient remineralization

• Nutrient remineralization (AKA nutrient regeneration) is an important process that alleviates nutrient limitation
  • Any process that converts an organic compound into an inorganic compound (ex. ammonification)
• How are nutrients remineralized?
  • Death and decomposition of organisms
  • Excretion (e.g., by animals) or lysis/leakage
  • Breakdown of dissolved organic nutrients
  • Non-organismal nutrient regeneration - P dissociation
• We can think of ecosystems as having a nutrient balance between net uptake and remineralization
  • Vnet = Vgross - M
    • Vnet = net nutrient uptake
    • Vgross = gross nutrient uptake
    • M = Mineralization
  • Does this resemble something we have seen before?
    • Yes, the Net Primary Production formula
  • Nutrient levels stable -> uptake equals remineralization
• Animal excreta are important for nutrient remineralization
  • Freshwater mussels eat and filter organic forms of nutrients, and then excrete them back out as inorganic forms of nutrients
    • At non-mussel sites there is more nitrogen limitation
    • Mussels shift the algal community assemblage toward Diatoms
      • Reducing the amount of Blue-green algae

Ecological stoichiometry

• Ecological stoichiometry is the study of how elemental ratios affect ecological interactions
  • X:Y resource + X:Y consumer --> X:Y growth + X:Y waste
• Redfield ratio 
  • atomic ratio of C:N:P in algae converges toward 106:16:1
    • Can be used to indicate nutrient limitation when algae deviate
    • Also related to remineralization
• Organisms are relatively fixed in their C:N:P contents
  • Need elements from food in certain proportions
  • Vertebrates like fish have lower C:P ratios than invertebrates, because bony tissue is very P-rich
• Comparing the stoichiometric ratios in zooplankton:
  • Copepoda vs Cladocera
    • Which group is high N:P?
      • Copepoda
    • Which group is low N:P?
      • Cladocera
• "Consumer-driven nutrient dynamics"
  • Invasive armored catfish directly alter N and P remineralization in streams
    • Has much lower N:P due to bony armor --> greater P storage

Practice Question

• Consider two stream ecosystems dominated by different animal communities - one by mostly invertebrates (higher N:P) and the other by vertebrates (lower N:P)
• 1. How do you think N vs. P remineralization will compare between these two streams?
  • Invertebrate stream will remineralize P more
    • Because they need to hold onto their N, so they release P
  • Vertebrate stream will remineralize N more
    • Because they need to hold onto their P, so they release N
• 2. What might that mean for whether N or P is most limiting for algal growth in each stream?
  • N will be more limiting in the Invertebrate stream
    • Algae will store P in their cells since it is an excess, increasing the need for N
  • P will be more limiting in the Vertebrate stream
    • Algae will store N in their cells since it is an excess, increasing the need for P
• 3. Draw an expectation for a +N/+P/+NP bioassay experiment result for algal growth in each stream
  • Invert: Low Control, high N, low P, high NP
  • Vert: Low Control, low N, High P, high NP

Trophic State & Eutrophication (Chapter 18)

What is eutrophication?

• Eutrophic - eu (many) trophic (foods)
• Eutrophication is the alteration of a system by unnaturally high nutrient levels
• Cultural eutrophication
  • Eutrophication caused by humans
• Eutrophication is caused by nutrient pollution associated with human activities
• Eutrophication can also be natural, but it takes centuries

Defining Trophic State

• Aquatic ecologists measure eutrophication using the concept of trophic state, the level of ecosystem productivity
• General categories:
  • Oligotrophic - low nutrients
  • Mesotrophic - medium nutrients
  • Eutrophic - high nutrients
• Eutrophication research in the 70's established P as a driver of algal blooms in lakes
• Lake divided: half given C+N, half given C+N+P. Why did the latter lake turn green?
  • P was possibly a limiting resource, allowing algae to grow
• Chlorophyll can be used to classify eutrophic state - measure of algal biomass
  • Higher chlorophyll correlates to higher algae
  • Higher chlorophyll possibly results in higher eutrophic state
• Secchi depth is a measure of water turbidity - lower when there are more algae
  • Clearer lakes have deeper secchi depths, meaning lower turbidity
• Total phosphorus (TP) is also often used to indicate trophic state - measures both organic and inorganic P in water

Effects of eutrophication on aquatic systems

• Cyanobacteria produce metabolites that prompt taste and odor issues on surface waters (no longer potable?)
• Higher trophic state = more taste & odor issues
• Eutrophication makes water bodies more anoxic. How?
  • Algae respires as well as photosynthesizes. At night, algae performs cellular respiration which decreases oxygen
  • Algae eventually die, sink, and become resource for heterotrophs to decompose in hypolimnion (lower oxygen)
  • Living algae also cause 24-hour (diel) swings of dissolved oxygen. This can result in anoxia esp. in early mornings
  • Results in dead zones in coastal oceans, too

Management to control eutrophication

• Controlling eutrophication - need to consider nutrients via external loading and internal loading
• What is the difference between external vs. internal loading? Examples of each?
  • External loading is a process that occurs by introducing nutrients into a system
    • Ex. agriculture runoff, deposition
  • Internal loading is a process that occurs within a system
    • Ex. nutrient loading from sediments, fish transporting nutrients from bottom to top of lake
• Point sources vs. Non-point sources provide external nutrients. Which is easier to manage and reduce, and why?
  • Point sources are easier because they are easier to identify and control (ex. waste plant, resource dumping, leaky fertilizer plant)
• Artificial lake mixing to prevent stratification
  • Mix the lake during summer so NH4 and PO4 don't build up in the anoxic hypolimnion
• Release of hypolimnetic water- Discharge P-rich deeper lake waters to export P from the lake
  • Dilutes phosphorus from the water to prevent it from accumulating downstream
• Aluminum additions: Al3+ binds to PO4^3- and precipitates out as AlPO4-
• Copper Sulfate additions: Kills phytoplankton, effective in the short-term
  • Evidence from 114 lakes treated with Alum show strong declines in TP and chlorophyll
• Macrophytes may also dominate eutrophic systems - physical, chemical, and biological control strategies

Eutrophication Case Studies

• Lake Washington (west of Seattle)
  • Much of Seattle's sewage was dumped directly into Lake Washington for decades
  • 1963
    • All major sewage inputs to lake halted
    • Major changes in lake
      • Decline of TP, chlorophyll, cyanobacteria
    • Took years to see a response, why?
      • Although the inputs of new phosphorus were eradicated, the internal cycling kept it going for several years until it was flushed out
• Lake Tahoe (California/Nevada)
  • Oligotrophic lake threatened by human development
  • Switch from septic to sewage
  • Many endemic species at risk
• Consider the two following methods to control eutrophication in lakes. How does each work to manage eutrophication, and what are the strengths/weaknesses of each?
  1. Artificial lake mixing to prevent stratification
     • Prevents ammonium buildup in the hypolimnion
  2. Release of hypolimnetic water
     • Removing the nutrient completely from the system to prevent cycling
     • Nutrient ends up somewhere else

Species Interactions and Food Webs (Chapters 19 and 20)

Types of Species Interactions

• Ecologists categorize interactions between species based on how they affect one another - negative, positive, or neutral
  • Competition
    • Negative effects on both species
  • Mutualism
    • Positive effects on both species
  • Predation/Parasitism
    • Positive effect for the predator/parasite, negative effect for the predated/parasitized
  • Commensalism
    • Positive effect for Species 1, no effect for Species 2

Food Web Structure

• Food Webs describe feeding relationships between resources and consumers (flow of energy)
• Simplified versions of food webs
  • Trophic structure are used to separate out stages of energy flow
• Food webs are usually more complicated than at first look
  • What changed with the addition of "microbial loop"?
    • The addition of microbes and Organic C
    • Included because a lot of energy goes through the microbial loop
• The microbial loop describes feeding relationships among microbes within a food web
• Much of energy can flow through microbes before it reaches upper levels in the food chain
  • Note: Greater role of organic C (esp. detritus) and heterotrophs
  • Note: Not all consumers are supported by algae

Transfer of microbial energy into the food web

• Omnivory (feeding on a mixture of different resources) is very common in aquatic food webs
  • Many organisms are ultimately supported by a mixture of both autotrophic and detrital energy
• Many animals directly feed off of the microbial loop:
  • Nutritional importance of bacteria and fungi
  • Mayfly Stenonema derives a lot of carbon from "exopolymers" secreted by bacteria within periphyton
  • Could heterotrophic carbon from bacteria and fungi be more nutritionally valuable than algal carbon?
  • The majority (~90%) of carbon incorporated by the shredder-detritivore caddisfly Pycnopsyche is fungal

Species Interactions: Competition

• Competition is an interaction between two or more individuals which share a resource, in which resource limitation causes one or both individuals to be negatively affected by each other
  • Can be interspecific or intraspecific
    • Interspecific - Competition between different species
    • Intraspecific - Competition within a single species
• Interspecific competition is why most populations grow logistically - resources become limiting
• Carrying capacity is maximum population size, given limiting resources
  • As population increases, birth rates (b) and death rates (d) change and affect net population growth rate
• Exploitation competition - Indirect species interaction through a shared exploited resource
• Interference competition - Direct species interaction over a shared resource, e.g. aggression over nesting or mating territory
• The competitive exclusion principle states that two species with overlapping resource requirements cannot coexist indefinitely
  • --> Eventually, one species will win and exclude the other species
  • --> Species evolve to minimize competitive overlap and therefore coexist
  • Certain ranges of resource availability (ratios) may allow two competing species to coexist

Species Interactions: Mutualisms

• Mutualism involves positive interactions between two species, often by mutual resource exchange
  • Nitrogen-fixing bacteria associate with plants. Plants provide photosynthate in exchange for nitrogen
  • Animal gut microbiota - Animals provide shelter in exchange for metabolic products (energy) from microbes
  • Syntrophy: complementary mutualism, i.e. exchange of metabolic wastes between microbes
    • e.g. bacteria and algae exchange CO2 & O2 in periphyton
• Facilitation is a looser term of mutualism in which one species can increase the survival/growth/reproduction of another species
  • Example: Shredders convert CPOM to FPOM, facilitating downstream collectors

Species Interactions: Predation

• Predation is a type of exploitation interaction (+/-) where one organism uses another organism as its resource
  • In aquatic systems, herbivory is a kind of predation which usually kills the prey (algae, macrophytes)
  • "True predation" involves one animal killing and consuming another animal
• Top predators can cause trophic cascades throughout a food web. What does this mean?
  • Trophic cascades cause nutrient level breakdowns within the trophic level
  • Top-down control
    • Predation by upper levels limits organisms
  • Bottom-up control
    • Resources limit organisms via competition
  • Adding planktivorous fish should increase algae by causing a trophic cascade throughout the food web
  • Adding piscivores to a lake should reduce algae by causing a trophic cascade throughout the food web

Ecosystem Ecology (Chapter 24)

The Ecosystem Concept

• The biological and physical components of the environment are a single interactive ecosystem
• Ecosystems include both biotic and abiotic components and their interactions
• An ecosystem is a spatial concept
  • It has boundaries and can be viewed in the context of its surrounding environment
  • Often difficult to separate ecosystems
  • Mostly a system we define, to permit study
• Ecosystems vary in their degree of openness to both inputs and outputs
  • How might different freshwater systems compare in their degree of openness?
    • Some can be very open (i.e. rivers) and some can be very closed (i.e. groundwater)
  • How do ecosystem boundaries relate to themes or concepts in the film RiverWebs?
    • Streams are not totally separate from the surrounding terrestrial (riparian) ecosystem
    • Subsidies of energy/material like leaves and mobile insects can connect different systems together
    • Subsidies can be reciprocal (both ways) - but are not always equal in quantity or quality
• Ecosystems are simplified into trophic levels:
  • Primary producers - Autotrophs like algae, macrophytes which conduct photosynthesis
  • Primary consumers - Heterotrophs, herbivores which consume plant material
  • Secondary consumers- Heterotrophs, carnivores or omnivores which consume animal material
    • "Green" food web based on living autotrophs
  • Detritus - Accumulated dead organic matter like carcasses, feces, leaf-litter, and woody debris
  • Decomposers - Heterotrophs like fungi and bacteria which feed and decompose dead organic matter
  • Detritivores- Heterotrophs (animals) which consume detritus and decomposer microbes
    • "Brown" food web based on detritus + heterotrophs

Energy Flow within an Ecosystem

• Each trophic level in an ecosystem includes attributes of biomass and production
  • Biomass - Amount of standing energy or material in a given compartment (e.g. g dry mass m^-2)
  • Production - Rate of adding new biomass for a compartment (e.g. g dry mass m^-2 yr^-1)
• Ecosystem shows a biomass pyramid (AKA Eltonian pyramid) where most biomass it at the base of the ecosystem, with less biomass in the upper levels
  • Why does biomass decrease among upper trophic levels? Where does the energy in producers go?
    • Nutrients decrease as the trophic level goes higher
    • Energy is lost through heat
• Production in ecosystems involves the fixation and transfer of energy derived from the sun
  • Follow the Laws of Thermodynamics
    • Law 1: Energy cannot be created or destroyed, but is converted from one form to another
    • Law 2: Every energy transformation results in a reduction of the usable (free) energy of the system
• Energy losses during energetic/chemical transformations in an ecosystem?
  • 1. Wastes - Not retained within biomass after ingestion by a trophic level (e.g. defection)
  • 2. Respiration - Unusable for next trophic level because used for energy of living organisms
  • 3. Mortality / offspring - Not all energy in one trophic level is used by the level above it
• Organism energy budges can account for major transformations: ingestion, assimilation, production
  • Assimilation efficiency = assimilation/ingestion
  • Production efficiency = production/assimilation
• There is considerable variation in assimilation and production efficiency across organisms. Why?
  • Assimilation efficiency:
    • Strongly affected by diet digestibility
    • Detritivores < Herbivores < Carnivores (Why?)
      • Carnivores obtain higher quality nutrients while detritivores get the scraps
    • Lower values -> more energy/material egested
  • Production efficiency:
    • Affected by organism activity and metabolism
    • Endotherms < Ectotherms (Why?)
      • Producing your own heat is expensive
    • Lower values -> more energy/material respired
• Trophic transfer efficiency describes the efficiency of transfer across trophic levels in an ecosystem
• Secondary production describes the rate of production of heterotrophs in an ecosystem
  • Strong link to detritus in forested streams
• Overall energy flow within an ecosystem is also measured using ecosystem metabolism
• Gross Primary Production = GPP = gross rate of photosynthesis
• Ecosystem Respiration = ER = gross rate of respiration
• Net Ecosystem Production = GPP - ER = NEP, the net rate of energy gained or lost by ecosystem
• Ecosystem metabolism generates "heartbeat" of the ecosystem - relates to energy and C cycling

Ecosystem Nutrient Budgets

• Ecosystems include both energy and nutrient transformations - nutrient fluxes coupled to energy flow
• Use a currency of elements (C, N, P, etc.) because elements exchange between abiotic and biotic forms
• Pools - Major standing form of a nutrient (e.g. g N m^-2. Example: Organic N in phytoplankton)
• Processes - Fluxes or transformations between pools (e.g. g N m^-2 d^-1. Example: Nitrogen fixation rate)
• In streams, the nutrient cycle becomes a nutrient spiral as nutrients are immobilized and remineralized
• Spiral length in water (Sw) and particulate form (Sp) can indicate nutrient limitation and uptake during water flow
• How does animal excretion compare to nutrient uptake?
  • Animal excretion of NH4 and PO4 can support some whole-stream uptake of N, P across field studies

Stream Ecosystems

• Some major features of stream ecosystems:
  • 1. Allochthonous (origins outside system) vs. autochthonous (origins within system) inputs
    • Examples: Terrestrial leaf litter (allochthonous) & benthic algae (autochthonous)
  • 2. Lateral connectivity to the nearby terrestrial environment (especially in large floodplains)
  • 3. Longitudinal changes down the continuum from headwaters to rivers - River Continuum Concept
• River Continuum Concept - What changes down a river?
  • Ecosystem metabolism - shifts of P/R (or GPP/ER)
  • Primary food resources - shifts in type and size (including allochthony vs. autochthony)
  • Functional feeding groups - Energy flow into different groups, especially among shredders, collectors, and grazers
  • Fish community - shift to more bottom-feeders tolerant of low oxygen levels

Lake Ecosystems

• Some major features of lake ecosystems:
  • 1. Historical emphasis on pelagic (open-water) component of ecosystem (phytoplankton-zooplankton)
  • 2. More recently, we realize benthic processes can be very influential (esp. in shallower lakes)
  • 3. Also evidence that a lot of lake energy flow is supported by allochthony rather than autochthony

Threats to freshwater ecosystems (Chapter 16)

Threats to Freshwaters

• 1. Nutrient Pollution - Excess N and P inputs can cause eutrophication
• 2. Hydrologic Alteration - disrupting natural flow of water (channelization, dams...)
• 3. Freshwater salinization and alkalinization
• Other potential stressors:
  • Climate change, Overharvesting, Novel contaminants, Acidification, Invasive species, Riparian removal

Understanding stressors - Toxicology

• A toxin is a substance with the potential to cause harm within living organisms
• More than 72,000 chemicals are in commercial use...
• Only ~10% of these chemicals have been screened for toxicity (2% as carcinogens)
• Only 0.5% are regulated as toxins by governments
• Why do you think so many anthropogenic chemicals are not regulated?
  • It's expensive to screen for toxicity & stop production
  • Other alternatives are expensive to produce & you can't just tell people "no" without government intervention
• Toxin exposure can be either acute (large pulse over short duration) or chronic (low doses over long time)
• Responses to toxins range from sublethal (does not result in mortality) to lethal (results in mortality)
• Lethal dose-50 (LD50)is the dosage of a toxin that causes death among 50% of individuals from a certain species
  • Toxicology study of the mayfly Neocloeon triangulifer exposed to brine salts (varying conductivity)
    • 20 day LD50 was 2,866 uS/cm (measure of conductivity)
• Many toxins can exhibit bioconcentration, in which the substance moves from water into an organism
  • Bioconcentrations factors are higher for compounds with lower solubility in water - why?
    • If it's not soluble in water, it's probably soluble in lipids
  • Lipid-soluble toxins can build-up and become transferred from one organism to another via feeding (bioaccumulation)
  • This can result in biomagnification, or very high levels of toxins accumulated, among higher trophic levels (Mercury for example)

Major stressors and causes of impairment

• How do we know if a water body is "impaired"?
  • Criteria for impairment - Threshold exceeded, based on some response
  • How to set thresholds?
    • LD50 or other toxicology
    • Societal/policy decisions
    • Dose-response relationships

Bioassessment to measure threats

• Bioassessment is the application of biological monitoring to assess the status of an ecosystem
• Why is biological monitoring so important to establish the health of an ecosystem? Why not just measure toxin concentrations?
  • 1. Biology provides an integrated look at stressors over time
  • 2. Biology offers a clearer link to function and human uses
  • 3. Together with other measures, provides holistic assessment
• Invertebrates, fish, plants, algae, and others can be used in bioassessment
• Number of invertebrate species - also called richness - shows strong responses to stressors
  • Structural response
• Most stressors occur along a gradient of intensity...
  • Bioassessment can help ecologists assess the shift from minor to severe alteration - what is stage of impairment?
  • Most bioassessment is based on structural metrics - but more emphasis is shifting to also include functional metrics
  • Function provides a clear link to health or uses of water? Clearer links to management?
  • Example of functional response: Leaf litter decomposition across a gradient of N and P in European streams
  • Nutrient pollution impairs both structure (invertebrate community) and function (leaf decomposition rates)

National Rivers and Streams Assessment (NRSA)

• EPA conducts national assessments of rivers and streams (NRSA) and lakes (NLA)
• EPA assessed 1,853 stream and river segments in the 2013-2014 NRSA. Sampling sites methodically chosen

National Lakes Assessment (NLA)

• Systematic national assessments can also directly compare trends over time (e.g., 2007 vs 2012 for NLA)

Novel contaminants in freshwaters

• Pharmaceuticals - antibiotics, hormones, fragrances, caffeine, and more can be found in elevated levels within waterbodies
• Pesticides -Atrazine (herbicide) is carcinogenic but widely used; Glyphosate (herbicide) toxic to amphibians and can be a source of phosphate pollution
• Transgenic by-products - Bt corn pollen and leaves can enter aquatic food webs, disrupting the insect community
• Plastics - Plastics break down to micro plastics that enter the food web and can negatively affect animals (e.g. clog guts)
• Most of these pollutants are not regulated. Why do we let them get into our water?

Freshwater acidification

• Acidification was more of a concern in 1980's and 90's before regulations began - Acid Rain Program (ARP) in 1990
• Burning fossil fuels releases sulfur and nitrous oxides that can precipitate as sulfuric and nitric acid
• What are the biological effects of acidification?
  • 1. Lower rates of organic matter decomposition - lower enzyme activity, lower bacterial activity
  • 2. Lower species diversity with lower pH
  • 3. Aluminum toxicity increases at lower pH, due to higher solubility of Al3+ as pH declines

Freshwater salinization

• Freshwater salinization syndrome
  • Increasing concentrations of ions such as chloride in freshwater systems across the planet. New "chemical cocktails" in water

Other threats to freshwaters?

• Climate change- many potential changes. Examples:
  • 1. Lake stratification - stratification for longer times during summer, and at higher peak temperatures
  • 2. Lake freezing and ice cover - shorter periods of ice over and reduced ice thickness
  • 3. Streams and rivers - higher frequency and intensity of droughts as well as severe floods
• Many of the most at-risk groups are from freshwaters...why?
  • The integrity of freshwaters is already compromised and freshwater species are sensitive to these big changes