Term 2 Lecture 7: Adaptations To Freshwater Living In Lakes And Rivers Flashcards
Freshwater habitats
Freshwater<1 part per 1000 salt (<1gL-¹)
Brackish water ~1-30 ppt
Marine ~30-50 ppt
>3% of land surface is covered by freshwater (mostly lakes and swamps) and just 0.2% of land surface is rivers (Downing et al. 2006)
Freshwater lakes, rivers and swamps contain <0.01% of the world’s liquid water
Yet freshwater environments contain ~6% of the world’s biodiversity and are one of the world’s most threatened habitats.
Lakes (lentic - standing freshwater)
Rivers (lotic - flowing freshwater)
^ very different habitats
Common problems for lake and river dwelling organisms
Many eukaryotes that evolved in marine environments are isosmotic so have no net osmotic flux.
Freshwater eukaryotes can’t live at the external osmotic concentration of lakes and rivers so water must be eliminated against the osmotic gradient, uptake of solutes is by active transport and solute conservation is also active.
Freshwater eukaryotes must maintain a much higher internal osmotic concentration than that of their external environment - must prevent excess water coming in and prevent nutrient loss by diffusion
E.g. adaptations include contractile vacuoles to expel water and active ion transporter organs on the cell membrane
Lotic compared to lentic freshwater
Lotic (flowing) / lentic (still)
Current:
unidirectional/ variable but slow
Water body size:
Variable shallow/ often deeper & wider
Circulation:
Lotic:Well mixed isothermal/
Lentic: In deep lakes thermal stratification occurs in summer and exotic climates leading to stagnation
Suspended material (turbidity):
Lotic: if current >0.6ms-¹ then this causes erosion and high levels of suspended material
Lentic : low levels, seasonally variable. Can be high if the body of water is shallow or exposed to strong winds
Source of organic matter (base of food web):
Lotic: more allochthonous - produced outside the system (e.g. tree leaves)
Lentic: more autochthonous - produced inside the system (e.g. phytoplankton and reeds)
Pond and lake formation
Size <1ha or 1000’s km²
depth 1m to >2000m
Sources:
- Retreating glaciers - scoured out basins
- Cut off river meanders
- shifts in earth crust e.g. part of valley sinks
- craters of extinct volcanoes
- landslides
- human activity (reservoirs etc.)
Temperature zones of lentic water through the seasons
Water has a high specific heat capacity
Slow warming/cooling of surface
Seasonal temp stratification ( in deep lakes) which limits mixing of water, dissolved gases and nutrients
Summer:
Surface water gets heated and becomes lighter floating over the cooler water below creating a thermal barrier and little mixing occurs.
The surface layer is called the epilimnion and is low density warm water.
The mid layer is called the metalimnion a steep temperature decline (thermocline) is noticeable here.
The base layer in contact with the lake bed is called the hypolimnion here the water is high density and cold
Autumn
The air temperature falls, the surface water loses heat, the metalimnion sinks and mixing occurs so that the water becomes isothermal (all one temperature.)
Winter
If no ice forms on the water then it remains a uniform temperature.
Below 4°c the water becomes lighter and accumulates at the surface, when ice forms on the surface warm water below it sinks leading to a higher temperature at the lake bottom - inverse stratification.
Spring
Ice melts, when the surface of the water reaches 4°c it sinks and water becomes isothermal once the surface water is above 4°c stratification begins again (especially in calm water)
Exception: deep tropical lakes that have a high stable temperature year round have permanent thermal stratification and many nutrients become locked up in the sediment.
How oxygen enters and leaves freshwater
It can be said that cold water holds more O2 than warm (due to higher O2 solubility at lower temperatures)
O2 in:
From atmosphere
( mixing at air/water interface)
From photosynthesis
(In euphotic zone and net production only in daytime) turbid waters have shallow euphotic zones
O2 out:
Due to increased temperature
Organisms respiration
Aerobic microbial decomposition
Summer
Lakes become thermally stratified and microbial decomposition of organic matter on the lake bed progressively uses hypolimnion O2 until the water becomes hypoxic (low O2) or anoxic (no O2) in the hypolimnion.
Over 99.9% of freshwater animals are exothermic - their activity is dictated by temperature of their surrounding water. In hotter water they respire more and require more O2 although there is progressively less available as warmer water has lower O2 solubility.
Deep eutrophic and hypertrophic lakes
Eutrophic means many nutrients
-Causes high primary productivity (phytoplankton blooms)
- high biochemical O2 demand for decomposition
- this high O2 demand leads to O2 depletion in the hypolimnion resulting in hypoxia or even anoxia which stresses fauna
- additional nutrients are often introduced by humans - sewage, factory waste, chemical fertilizer runoff etc.
- high productivity →die off due to low O2 → high O2 demand from microbial decomposers in hypolimnion
Deep oligotrophic lakes
Oligotrophic means few nutrients available
- low productivity so little demand for O2 in the hypolimnion
- little O2 depletion
- O2 lasts all summer
- good cold water refuge for cold water stenotherms (limited to narrow temperature range) with high O2 demands such as Charr (glacial relict specie in UK)
Tend to be clear lakes with light below the thermocline, photosynthesis occurs in the hypolimnion so O2 can be high even in deep water
Temperate environment O2
Spring and autumn
Water recirculates, O2 is replenished in deep water
Winter
If there’s no ice there’s plentiful O2 and less bacterial decomposition (due to low temp)
If there is ice there’s no gas exchange with air leading to O2 depletion
-small O2 store if shallow
- lower temperature so lower rate of metabolism but hypoxia can still arise and kill animals - particularly fish as they are larger so have a higher O2 demand
Ecological zones and terminology of a deep lake
Littoral- high water level to euphotic depth of plants - bottom and overlying water
Pelagic - open water
Profundal- zone below euphotic, photosynthesis is not possible here. Dim and dark though may not be below the thermocline. Food originates from surface layers.
Euphotic zone - depth at which plants can photosynthesise enough to grow - deeper where there’s better water clarity as in oligotrophic lakes.
Benthic - organisms associated with lake bed at all depths
Primary productivity in lakes
Deep, open lakes: pelagic phytoplankton in the water column.
Microscopic and low density they are moved by waves with slow sinking rate e.g. Lake Windermere
Shallow convoluted lakes: vascular macrophytes grow from the lake bed due to extensive bay and wetland areas e.g. Norfolk Broads
Phytoplankton seasonality in temperate lakes
Spring: mixing, access to nutrients, bloom
Summer: stratification, nutrient depletion, drop in populations
Autumn: mixing again, may be a smaller bloom than in spring
Animal communities (lakes)
Pelagic:
-Zooplankton (few mm) herbivores e.g. water flea Daphnea
- some small carnivores (~1mm) show vertical migration to feed in the dark and avoid fish
Nekton (large, capable of independent movement) fish such as Charr, Perch, Roach etc depending on temperature
Benthos - bottom dwellers, mostly invertebrates: worms, insects, crustaceans and molluscs. This category includes plants and bacteria but in these environments it’s mostly invertebrates
Littoral- compared to profundal- benthos
Littoral/profundal
Habitat: heterogenous/ homogeneous
Temp/O2 conc.: Warm high / cold low
Intrinsic food: yes/ no - food originated from pelagic and littoral
Microhabitats: many/ few
Complexity and species richness:
High/low
Profundal- specialist invertebrates
Have high O2 affinity specialised Hb
E.g.
chironomus larvae (non - biting midge)
Tube dwelling with specialised Hb
Can survive severely hypoxic conditions for several months
