EEMB 142A Final - Lakes

lentic system

“still waters” - slowly flowing open body of water in a depression not in contact with the ocean

what are the three general characteristics of rivers?

1. distinct edges

2. homogenous bottoms

3. well-mixed water

tectonic lakes

• formed by faulting

  • rift lakes or grabbers

  • commonly steep sided and very deep

• formed by uplifting of sea floor

  • Horst lakes (block tilting)

  • commonly shallow

volcanic lakes

• formed within calderas of extinct volcanos

  • commonly steep and very deep

  • formed with lava cuts off and dams rivers

glacial lakes

• formed by actions of glaciers

  • most common type of lake

  • cirque (tarn lakes), valley, and kettle lakes

other types of lakes

• artificial (reservoirs)

• dammed lakes (natural and artificial)

• sinkholes (karst regions)

• floodplain lakes (oxbow, levee)

• playa, dune, meteorite lakes and ponds

morphometry

size, shape, depth

• attributes impact lake function (productivity)

zonation of lakes

1. littoral (can drive photosynthesis)

2. pelagic (limnetic - middle of the lake)

3. profundal (benthic, too deep to drive photosynthetic work)

photic zone

deep enough for light, P/R > 1

aphotic zone

no light, P/R < 1

thermocline

steep transition of temperature, based on the density of the water

compensation depth

P = R

light

• reflected, absorbed, scattered

• declines with depth

• photic, aphotic zones

• affected by turbidity, color, algae, POM

temperature

varies with season, depth, latitude, and altitude - thermal stratification

epilimnion

high oxygen

metalimnion

thermocline

hypolimnion

low oxygen

water motion

driven by wind

substratum

basin dependent

dissolved oxygen

• may be low in hypolimnion of some lakes (clinograde profile - stratified lake)

• may be high in hypolimnion of some lakes (orthograde profile - well-mixed lake)

salinity

• species richness declines with salinity

• may cause stratification

acidity pH = -log(H+)

• few species tolerate very high or very low pH

• a bell curve

nutrients (nitrogen and phosphorus)

P (often the limiting nutrient in lakes)

oligotrophic (unproductive)

• low nutrients

• not much oxygen saturation

• low biota and productivity

• high light

• deep lake with steep sides

eutrophic (productive)

• high nutrients

• great variation in oxygen saturation

• high biota and productivity

• low light

• shallow lakes with gently sloping sides

biological taxa

macrozooplankton (>200 um)

• malacostracans, copepods, cladocerans, daphnia

microzooplankton (<200 um)

• rotifers, protozoa, copepods

phytoplankton taxa: cyanophyta

gas vesicles, nutrient fixing, grazing resistant

phytoplankton taxa: chlorophyta

increased SA, stay near the surface, rapid reproducers

phytoplankton taxa: bacillariophyta

diatoms, grazing resistant, asexual and sexual reproduction

phytoplankton taxa: chrysophyta

golden/brown algae, two flagella, can transfer between autotrophic and heterotrophic

phytoplankton taxa: cryptophyta

extremely small, grazing resistant

phytoplankton taxa: pyrrophyta

grazing resistant, has toxins, spinning mechanism

basic food chain: pelagic zone

large fish (piscivorous), smaller fish (zooplanktivorous), zooplankton, phytoplankton

basic food chain: profundal (benthic) zone

fish, invertebrates (collectors), detritus

complex food web: pelagic zone

• piscivorous fish also eat nektonic invertebrates

• planktivorous fish also eat nektonic invertebrates and carnivorous zooplankton

• nektonic invertebrates and carnivorous zooplankton eat herbivorous zooplankton

• herbivorous zooplankton eat in the microbial loop

• protozoan eat bacteria (DOM/POM), very small algae, cyanobacteria, flagellates

complex food web: profundal (benthic) zone

• fish can directly eat detritus

• in shallow lakes, periphyton may be important

• in some lakes, there may be fish that eat detritus and/or algae

littoral zone

• rocky shores (much like stony streams)

• plants (macrophytes) provide complex structure producing complex food webs

submergent plants

all photosynthetic parts are underwater

floating plants

rooted & unattached

all photosynthetic parts are on the surface

emergent

rooted in sediment - photosynthetic parts can be either above or below

four energy sources in the littoral zone

detritus, macrophytes, periphyton, phytoplankton

diversity

littoral > pelagic > profundal

what are the 6 reasons that cause complex interactions in food webs?

1. predatory and grazing guilds of vertebrates and invertebrates

2. multiple types of primary producers

3. omnivory

4. ontogenic niche shift

5. feedback loops

6. detritus, algae, and macrophyte based food webs

phytoplankton season #1: ice break and spring turnover

• deep circulation (isothermal)

• high wind speed - high water movement

• light is increasing but low

• water is cold

• nutrients is high (Si, N, and P)

• diatoms and small unicellular green algae are dominant (r-selected)

phytoplankton season #2: onset of stratification (late spring-summer)

• wind slows - mixing decreases

• light is high, temp increases

• Si in epilimnion decreases

• Si:P and Si:N ratios decrease

• zooplankton increases

• edible phytoplankton decreases

phytoplankton season #3: clearwater phase

• zooplankton increases

• algal populations crash due to grazing

• water transparency increases

• nutrient saturated growth can occur or a limited time again, but Si remains low (low competition)

phytoplankton season #4: late summer (early autumn)

• algae competing at low nutrient levels

• N and P supplied by zooplankton excretion

• higher nutrient patches available to motile algae and storage specialists

• cyanobacteria, gelatinous green, and dinoflagellates increase

• fall mixing may generate secondary bloom (wind speed increases)

seasonal phases

succession of R to K selected species

bottom-up effects (eutrophic lakes)

• surveys: correlations across lakes

• uncontrolled, natural experiments: changes following increases or decreases in nutrient input

• laboratory experiments: lose control

• field experiments

  • small jars - diverge from natural conditions

  • large bags (mesocosm)

  • whole system - difficult to replicate

top-down effects (cascading effects)

important in mesotrophic and oligotrophic lakes

top-down effects: fish-zooplankton in Connecticut Lakes

observations

• alewives present - small zooplankton species

• alewives absent - large zooplankton species

natural experiment

• huge shift in size of zooplankton after the introduction of alewives

why do large zooplankton species dominate when fish are absent?

hypothesis #1: large zooplankton are competitively superior over small zooplankton

observation

• large zooplankton are very vulnerable to fish predation

results

• exploitative competition

conclusions

• large zooplankton are competitively superior if they are much larger than small zooplankton

  • if Daphnia decrease - increase in small zooplankton

• if large zooplankton are not greatly larger than small zooplankton, then the outcome of competition is complex and often unpredictable

  • outcomes may depend more on abiotic factors

why do large zooplankton species dominate when fish are absent?

hypothesis #2: invertebrate predators (large zooplankton) reduce the abundance of small zooplankton

transfer experiment

results

• small Daphnia alone, survived, not abiotic factors

• small Daphnia and large Daphnia, survived, not competition

• small Daphnia and large copepod, eliminated, predation

conclusions

• invertebrate predation kept small Daphnia out of ponds, large Daphnia resistant to invertebrate predation

Peter Lake

• 4 trophic levels

• piscivorous fish, planktivorous fish, zooplankton, phytoplankton

Tuesday Lake

• 3 trophic levels

• planktivorous fish, zooplankton, phytoplankton

Peter and Tuesday Lake Experiment

add piscivores to tuesday (decrease Pl, increase zoo, decrease phyto)

• matched the expectations

remove piscivores from peter (increase Pl, decrease zoo, increase phyto)

• did not match the expectations

• cannot remove all piscivores from the lake

• slower interactions

• planktivores hid in littoral zone

• algae dominated by forms with increased with zooplankton grazing (grazing resistant)

habitat complexity: treatments with and without fish under three levels (insectivorous fish, invertebrate predator, invertebrate grazers/collectors)

fish biomass accrual dependent on habitat complexity

low: find prey - consumes all and starves, high predation

middle: less macrophytes but enough to survive, the highest fish biomass, moderate predation

high: low fish yield, macrophytes hiding, low predation

Flathead Lake, Montana

native cutthroat and bull trout lost due to overfishing

Flathead Lake Exotic species introduction

introduced lake trout and kokanee salmon as replacement

• lake trout ate large zooplankton and small kokanee salmon

• kokanee salmon ate large zooplankton

• kokanee salmon run usotread to breed, served as food for bears and eagles

increased tourism and fishing

Flathead Lake secondary introduction

opossum shimp (Mysis) introduced into drainage as additional food for kokanee (but they also eat large zooplankton)

• opossum shrimp outcompete kokanee salmon for large zooplankton via exploitative competition and reduce large zooplankton - kokanee eliminated

• bears and eagles decrease (tourism fails), lake trout increase in number but decrease in size

secondary species introduction reduces biodiversity