Lakes

Three sources of water

1. precip

2. overland flow

3. groundwater

Lake classification

1. water color reflects two factors: abundance of photosynthesis, inputs or org. matter from watershed

2. physical structure (density stratification): temp. change with depth, effects of solute on density, seasonality of density

Light and zonation in lakes

1. light decays exponentially with depth. Surface waters are exposed to light way more than deeper waters. Makes it warmer than the cold deeper parts of water.

stratification and chemistry

1. surface water affected by atmosphere and living organisms.

What prevents the deep water from mixing with surface waters

density gradient

What does decomposition do to deep water chemistry

1. reactants get depleted (O2) and products build up (CO2, NH4, PO4)

Primary Producers

1. macrophytes: submerged aquatic veg., in freshwater the plants are usually higher, much less structural tissue than on land

2. phytoplankton: free floating, microscopic single celled plants, can form colonies

What is the most important source of NPP in shallow clear lakes

Floating vegetation

What does floating veg do for lakes

1. phytoplankton fight with rooted plants for light, plants reduce nutrient loss from sediments to water column, also provide key habitat/refuge for fish and invertebrates

Phytoplankton groups

More common: diatoms, cyanobacteria, chlorophytes

Less common: dinoflagellates, chrysophytes

At some depth when is the compensation light reached

where NPP = 0 (compensation depth)

What is the critical mixing depth

mixing depth where NPP over a day = 0

Short term limitation of primary prod. in lakes

CO2, N, Si, P

Long term limitations for primary prod. in lakes

accumulation of biomass in whole ecosystem experiments, comparisons between lakes

CO2 usually limiting to NPP ONLY in short term

Co2 removed by photosynthesis replenished by

1. carbonate buffer system- increases pH

2. carbon concentrating mechanisms

3. diffusion from atmosphere- decreases pH

Phosphorus in freshwater systems

1. lakes with higher P loading have more chlorophyll

Sources of nutrients to lakes

1. dust-mineral particulate P

2. riverine inputs (mostly particulate P, dissolved P, agricultural P, sewage P)

3. groundwater

4. animal vectors

Internal loading

release of P from sediments

Short term N is often limiting

denitrification can remove excess nitrate, N-fixation can be limited by Mo, light and other factors

Losses to herbivory

more than 70% of aquatic algae are eaten by herbivores. 30% of rooted macrophytes are eaten.

ecosystem heterotrophy

ecosystem respiration exceeds ecosystem NPP in most lakes

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