Freshwater ecology unit 2

Water/aquatic chemistry

  • how different types of elements (C, Ca, S,) how they move throughout a water body.

  • Chem of aquatic ecosystems

    • aquatic ecosystems are variable: how fast the water is moving, sunlight, etc. what drives this variablity?

      • chemical reactions…algae is critical

    • dissolved vs. particulate material. how do we distinguish? simply by size. cutoff is 0.45 microns… anything over 0.45 is the particulate load. the rest is the dissolved. Cpalm 1mm>

    • major ions in inland waters:

      • cations: Ca2+> Mg2+> Na+>K+

      • anions: HCO3^- > SO4²-

    • what determines the chem compositions of inland waters?

      • 1. rates of chemical weathering: reactions that take place in soils and rocks of water sheds which release anions…H2O+ CO2 → H2CO3 makes weak acid. breaking down of chemical weathering which causes chemical change

      • 2. rock type influences weathering: ease of weathering decreases and ion concentrations decrease… limestone < sandstone < granite < quartzite. Geology plays a large role in determining ion concentrations in pristine waters. underlying rock type determines the chemistry within the water and can affect wildlife

      • 3. ionic concentrations matter…low alkalinity in northern Minnesota where its hard to weather rocks/low production. high alkalinity in southern Minnesota where its easier to weather rocks/high production.

      • 4. other important determinants of water chem: climate, age/history of geological substrata, distance from ocean, human lnad use, terrestrial and aquatic biota

    • Red alder: fixes nitrogen turning it into ammonia. bacteria then convert to nitrite and nitrate which leach into streams nearby

    • some lakes have very high ion concentrations: endorheic (closed) lake basins. small inputs and exports of water and high evaporation.

    • Report ion concentrations: conductivity (microS/cm): relative amount of electricity can be conducted by water.

      • total dissolved solids (mg/L): similar but also non-ionic compounds (silica, dissolved organics)

      • hardness: Ca2+, Mg2+

      • turbidity: light absorption or light scattering of water…high turbidity (muky water) low turbidity (clear water)

      • hydrogen ions are extremley important in freshwater ecosystems despite their low concentrations: pH=-log10[H+]

  • Ecosystem thinking

    • chemical transformations

      • energy is needed for chem reactions..change state of an element. diff forms of materials have diff potential energy (stored)

      • ex: photosynthesis

      • loose electrons: oxidized…gain electrons: reduced

    • redox reactions

      • redox potential: measure of the relative availability of transferable electrons in solution for chem reactions to occur EH(mV) …charge

      • high postive numbers: oxidizing environment…low availability of electrons

      • low numbers: reducing environment..high availability of transferable electrons

      • Redox potential: 1. will occur without an input of energy 2. require energy 3. favored in environment

  • oxygen and photosynthesis

    • importance : comprises 21% of atomosphereric gas

    • aerobic/oxic

    • anaerobic/anoxic—hypoxic (low oxygen)

    • amount of oxygen dissolved in water influenced by: diffusion, temperature, atmospheric pressure, metabolic activity of organisms

    • saturation concentration of oxygen: equilibrium concentration of oxygen when pure water is in contact w atomosphere

    • photosynthesis dominates in the epilimnion region of a lake

    • hypolimnion will dominate in respiration…little to no light

      • oxygen is high at the surface of a lake, but declines when the lake goes deeper

2/12/26

  • CPOM:FPOM…

    • course particulate organic matter: fine particulate organic matter

    • ex: leaf particle (1mm) is CPOM

    • ratio number 1…numerator > or < than 1 difference of CPOM vs FPOM

  • P:R ratio…

    • production to respiration (photosynthesis to respiration)

    • relative to 1

    • P/R is low in the headwater streams…below 1 means respiration is dominating…when it’s above 1 photosynthesis is dominating

  • pattern and biodiversity hypothesis…

  • change in temp throughout the day is largest in middle stream orders.

    • swings in temp should be maximized in mid-ordered streams…biology should follow pattern

    • richness of biodiversity during this swing of temp should be highest because when the variation in temp is high, species that are cold adapted are exposed to warmer environments and vise versa. adapted to both warm and cold environments.

  1. Point sources of pollution from two different watershedds at the HUC-8 level

2/17/25

Lake mixing, nutrient pollution, and trophic state

  • Stratification and mixing

    • thermal stratification and lake turnover

    • thermal stratification: formation of layers of diff temp in a lake/reservoir resulting from large differences in density between warm and cold waters.

      • ex: warm: lighter density than cold water. top water is warmer, with the bottom layer being cold

    • the diff in density as you change temp varies depending on where you are on the curve (x-density diff, y-temp. density diff depending on water temp

    • 4°C is small value density, with desnity diff at 35°C is large density diff.

    • lake turnover: complete mixing of water column from top to bottom due to wind and changes in temp

    • lake mixing: amictic, monomictic, dimictic (column mixes/turns over twice a year in spring and fall), polymictic, meromitic

      • winter stratification: ice is at -4°C

      • spring turnover: ice melts and becomes a little more dense and starts to sink and mixes with the upper layers. add wind and the entire water column mixes

      • summer stratification: surface water beginst o heat and become less dense. layers become more and difficult to mix.

      • epilimnion and metalimnion no mixing together. epilimnion and metalimnion and hypolimnion do not mix together. their densities are too different.

      • fall turnover: air and surface water temp cools. metalimnion erodes away, and starts to get more dense and sinks. mixing over water column afterwards

      • Meromitic, partially mixed.

  • nutrients limitation and stoichiometry

    • nutrients limitation: control of growth or production by an element instead of light, temp, predation, or other limiting factors.

      • ex: ammonium lower levels at upper water, but grows more in lower depth of water.

    • C,N,P needed for biomolecules for organism growth

    • Liebig’s Law of the Minimum. growth is controlled by the single nutrient that is shortest in supply.

    • what determines whether an organisms is chem limited? by their chemical composition. organisms stoichiometry will determine which element is limited in an organism

    • stoichiometry: relationship of reactions and products in a balanced chem reaction

    • along w lake mixing dynamics, food web structure may affect nutrient cycling and nutrient limitation

      • phosphorus content of zooplankton can determine their role in lakes

  • trophic state

    • classifying lakes based on their level of productivity

      • olgotrophic (very few nutrients/food),mesotrophic,eutophic(plenty of food),hypereutrophic (most nutrients/food)

    • classification of ‘trophic state’ in lakes is based on: water clarity, phytoplankton biomass, nutrient concentration

    • lots of nutrients/food=lots of algae??? more nutrients=higher phytoplankton biomass

    • lakes may not fall into any particular classification scheme…same with rivers

    • streams are much harder to classify by ‘trophic state’ especially if you try

    • nonbiological factors (floods) in rivers can drastically reduce algal biomass (aka Chl a)

  • excess nutrients

    • eutrophication: change in structure and function of aquatic ecosystems in response to excess nutrients from humans

      • eutrophic: change in structure and function of ecosystem in response to excess nutrition like waste water, septic systems, ag systems

    • BOD (biological oxygen demand) inc. as there is more nutrients which leads to too much algae growth which can kill fish

    • types of sources of excess nutrients:

      • point sources (we know what and where is contribution to excess nutrients into a water system)

      • non point source: ag lands, rural lands, city streets. don’t know where pollution is coming from. majority of pollution, but harder to manage

  • Fick’s Law

    • oxygen in sediment and groundwater

      • depleted due to septic systems, feedlots (high cont. of animals), and subsurface sewage effluent

      • elevated due to: animal activity and burrowing (crayfish inc. oxygen delivery) (mayflies dig into sediment and inc. o2 sediment)

    • Diffusion and movement of molecules

      • nutrients or oxygen from a feedlot could move to alter conditions in a nearby waterway…diffusion o fink in water

      • concentration gradient, distance, temp, water current, molecule size, sediment/sticky polymer, direct movement of organisms

      • modeled by Fick’s Law- rate of movement (J)

        • positively related to concentration difference. inversely related to distance

        • J= D [c1-c2/ x1-x2]

        • temp can influence the constant D

    • Redox in a lake

      • Redox potential is the transfer of electrons to and from different atoms; creating a negative/positive charge. An atom with a high potential of reduction carry the most electrons, and vise versa. Redox potential is measured in mV

      • The charts indicate that oxygen, redox, and iron all seem to have a coorelated relationship. Spring at medium depth (4m) redox and o2 is high. but negative relatinoship w iron. total iron is low when o2 is high. oxidizing/high redox environment the form of iron is Fe3+ (precipitate/sink into lake) concentration of iron is 0 since its all at the bottom of the lake.

      • late summer—> fall. stratified lake (warm at surface and cool in deeper section) O2 gets low, which means low redox (reducing environment) iron will be soluble (Fe2+) and will be leaching up from sediment

    • Ice-Off

      • lakes loosing ice///climate change

      • earlier dates of ice off…warmer and earlier springs

      • historically, match of predators and prey. ex: algae (prey) inc with also inc. in zooplankton (predator). predators closley following by the prey. However, these two groups algae responds to change in temp; zooplankton respond to photoperiod. ice off (influenced by temp) potentially change how predators and prey interact.

      • Now, algae respond to temp (happens earlier) and zooplankton respond to photoperiod, but prey aren’t available for them. predator/prey mixmatch. lead to low population predator numbers

Interpret Redox in a Lake