Chapter 6

Question 1

Plankton / benthos

Answer 1

• Plankton cannot swim against currents, they passively float in the environment
  > Phytoplankton = primary producers
  > Zooplankton = main primary consumers, feed on phytoplankton, bacteria, algae
  > Bacterioplankton = decomposers, feed on matter produced by phytoplankton
• Benthos = individuals that live at bottom of water body, on the sediments

Question 2

Pelagic / littoral

Answer 2

• Pelagic zone - light does not penetrate to bottom, has no vegetation but has phytoplankton
• Littoral zone is near the shore, sunlight penetrates all the way to sediment = has emergent + submerged aquatic vegetation

Question 3

Periphyton

Answer 3

• Algae growing on macrophytes (aquatic plants) and other hard substrates
• Form a slimy layer around underwater stems of aquatic plants - use macrophytes as substrate or as nutrient source

Question 4

Macrophytes

Answer 4

Large vegetation that can be found in lakes, photosynthetic activity releases oxygen in lake

Question 5

Submerged / emergent / floating-leaved macrphytes

Answer 5

• Submerged: rooted in sediment but does not reach the surface, important for nutrient cycle e.g. pondweed
• Emergent: located in littoral zone, rooted in shallow depths with vegetative parts coming up above the surface
• Floating-leaved: do not root in sediment but float on surface - affect the quantity of light penetrating water, affecting photosynthesis of plants in the lake. Potential problem in tropical aquatic systems - high density covering e.g. water lily

Question 6

Microbial loop

Answer 6

Dissolved organic matter/carbon is returned to higher trophic levels via incorporation in bacteria biomass. Aquatic macrophytes, algae and cyanobacteria excrete nitrogen rich compounds -> heterotrophic bacteria grow on dissolved organic molecules and are fed upon by protists

Question 7

Compensation depth

Answer 7

Photosynthesis is limited to surface layers where there is light. Compensation depth is where net photosynthetic rate = 0 so gross photosynthesis = losses due to respiration + other causes. Influenced by occurrence of high density of algae, differs among taxa e.g. cyanobacteria can grow at greater depth than green algae because they have additional pigments

Question 8

Explain how thermal stratification in a lake builds up

Answer 8

Wind action combined with temp-density characteristics of water generates thermal stratification, can change with season

Question 9

Epilimnion, hypolimnion, metalimnion, thermocline

Answer 9

• Aphotic = stratum below compensation point
• Epilimnion (mixing zone)= water temp is relatively high, + uniform mixing due to wind-induced mixing
• Hypolimnion = cold and relatively undisturbed, not influenced by wind-induced currents. Temp difference prevents mixing with epilimnion
• Thermocline = plane with strongest thermal change with depth
• Metalimnion = stratum in which thermocline occurs

Question 10

Why is thermal stratification important to understand the functioning of lakes?

Answer 10

• Because mixing of water affects temperature, conc of nutrients, dissolved oxygen conc
  > Dissolved oxygen is essential to metabolism of all aerobic organisms (almost all expect bacteria)
  > When turnover happens, mixing of water column brings hypolimnetic water into photic zone - this water is much richer in nutrients because they have not been absorbed by phytoplankton. Flow also resuspends nutrients from the sediments

Question 11

Discuss the seasonal dynamics of thermal stratification in a typical lake and its direct an indirect consequences for living organisms

Answer 11

• Summer: as surface waters are warmed during spring, they become less dense and remain near surface. Difference in temp and therefore density impedes mixing by wind-induced currents = further tendency for stratification with surface layers getting warmer by solar radiation and resistance to mixing getting higher. There is little heat exchange between hypolimnion and epilimnion. Hypo is very homogeneous in temp + temp changes little throughout summer. If warming during spring occurs slowly and weather conditions are calm = thermocline will be shallow. Depth of thermocline tends to lower gradually during course of the summer due to small gains of heat
• Fall: decline in air temps result in surface layer becoming cooler and denser - as it sinks, it is mixed by convection currents and wind-induced water circulation = leads to gradual erosion of metalimnion from above and increase in thickness of isothermal epilimnion. Finally, entire volume of lakewater is included in circulation = fall turnover -> entire water column has same temp and whole water column is prone to circulation by wind-induced currents. Gradual cooling continues until a temp of 4 C is reached in entire water column
  > Turnover periods important for structure of lake ecosystem + productivity. Mixing of water column brings hypolimnetic water (rich in nutrients because they have not been depleted by phytoplankton and because gradual enrichment from sedimentation and decomposition of organic material) into photic zone.
• Winter: as temp of water reaches maximum density (4 C) , inverse stratification begins to build up with cooler water overlying warmer water. However difference in density of water of 0 and 4 C is weak so in the beginning stratification is quite easily disrupted by wind. If ice is formed, ice seals lake off from effects of wind. Under ice = thermal gradient from 0 to 3-4 C.
• Spring: when ice melts, water is still thermally stratified but density differences are low. Heating of surface water by solar radiation results in density currents + wind leads to spring turnover phase - water column tends to be isothermal before summer stratification

Question 12

Discuss the vertical distribution of oxygen in a oligotrophic / eutrophic lake

Answer 12

• Oligotrophic lakes (low nutrient load, low organic productivity): summer - oxygen conc in epilimnion decreases as temp increases. So oxygen concs in meta and hypolimnion are higher than epilimnion with bottom of lake having almost 100% saturation. Fall turnover = mixing until all at saturation. Winter: ice cover - exchange of oxygen is almost completely lost - oxygen profile reflects saturation in relation to temp at all depths
• Productive (eutrophic) lakes: summer - oxygen conc in hypolimnion becomes progressively reduced due to respiration + breakdown of organic material - this loss is not compensated by photosynthesis as light intensity in this region is low = strong reduction in oxygen concentration in hypolimnion, can eventually become anaerobic until fall turnover phase. After fall = oxygen conc is saturated throughout water column. Winter - similar to summer but oxygen consumption is lower during winter due to lower temperatures = lower metabolic rate

FIGURE

Question 13

Meromictic lakes

Answer 13

• Only part of water column undergoes circulation, layers of water that do not mix - most of the time arise because lower layer of lake is highly saline + denser. Due to continuous decomposition of organic matter in absence of any circulation, deeper water layer becomes anoxic. Nearly all very deep lakes of the equatorial tropics are meromictic. Also small lakes with low area to depth ratio and are sheltered from wind action
• Measure saline concentrations and oxygen concentrations in deep water, temp over water column + over time
• Extreme clinograde oxygen profile - oxygen is quickly depleted + water remains anaerobic

Question 14

Which factors determine the concentration of dissolved oxygen in a lake or a given site in the lake

Answer 14

• Supply of oxygen: atmosphere and photosynthetic activity. Losses: metabolic activity (respiration, decomposition of organic material)
• Temperature: solubility of oxygen increases with decreasing temp
• Pressure: at higher pressures (deeper depths), more gas can be dissolved in water
• Salinity reduces solubility of oxygen, 20% less oxygen in seawater than freshwater

Question 15

What are the major factors determining oxygen lack in a lake or a given water layer in a lake?

Answer 15

Losses of oxygen are due to metabolic activity of organisms (respiration + decomposition of organic material by bacteria).

Question 16

Is the amount of oxygen that can be dissolved in water dependent on the temperature? On salinity?

Answer 16

• Temperature: solubility of oxygen increases with decreasing temp
• Pressure: at higher pressures (deeper depths), more gas can be dissolved in water
• Salinity reduces solubility of oxygen

Question 17

Why are the sediments of standing waters often anaerobic (without oxygen), certainly at deeper depths (e.g. more than a few centimetres deep)?

Answer 17

Lower levels of the lake: no mixing with the atmosphere (oxygen from surface doesn’t reach deeper layers), no photosynthetic activity (light doesn’t reach), bacterial decomposition deposits in these layers (they don’t need oxygen to live) and they consume excretions + dead organisms = consuming oxygen

Question 18

Summer and winter fish kills

Answer 18

• Oxygen regime in a lake is determined by duration of stratification periods + amount of organic matter. Sometimes oxygen depletion during stratification can be so pronounced that whole water body turns anoxic and major fish kills can be observed
• Summer: oxygen content of littoral zone is severely reduced e.g. when large populations of macrophytes die at the end of the growing season and get decomposed. In shallow + productive lakes, macrophytes can grow over entire lake basin - macrophyte decomposition can be so intense that oxygen content is severely reduced to near anoxia = fish kills
• Winter: under heavy snow cover, light intensity under ice is reduced to low levels, disabling photosynthetic production of oxygen. If sustained for several weeks, heavy respiratory demands + decomposition of dead organic matter lead to extremely low levels of oxygen in highly eutrophic systems. At low temps, most fish cannot survive low oxygen concs = fish kills

Question 19

Are productive lakes richer or poorer in oxygen than unproductive lakes?

Answer 19

Productive lakes have less oxygen - eutrophic lakes have lost so much oxygen that aquatic life begins to die

Question 20

What is the main distinction between productive and unproductive lakes with respect to spatial / temporal variability in oxygen concentration ?

Answer 20

• In more productive (meso- to eutrophic) lakes, the oxygen concentration of the hypolimnion becomes progressively reduced due to respiration + breakdown of organic material by bacteria. This loss of oxygen is not compensated by oxygen production through photosynthesis, since light intensities in the hypolimnion in most lakes is low
• See earlier answers

Question 21

Give the definition of salinity

Answer 21

Total amount of salts present as ions in 1 kg of water, most salts are present as ions in water. Ca2+, Mg2+, Na+, K, HCO3-, CO32-, SO42-, Cl- usually constitutes total ionic salinity. Expressed in mg/L or meq/L.

Question 22

Soft / hard waters

Answer 22

• Soft = low salinity/ion content, usually derived from drainage of acidic rocks e.g granite
• Hard wanters = large concentrations of alkaline compounds, usually derived from drainage of calcareous deposits e.g. lake in limestone area

Question 23

Open / closed lakes

Answer 23

• Open lakes (with outlet) - chemical composition largely determined by composition of influents + precipitation
• Closed basins: chemical composition (salinity) determined by evaporation + precipitation of salts - can give rise to saline lakes because outflow of water is restricted

Question 24

Saline lakes: origin, characteristics, some characteristic organisms and their adaptations

Answer 24

• Saline lakes form when: outflow of water is restricted (closed basin), evaporation exceeds inflow and inflow is sufficient to sustain a standing body of water
• Most originate through evaporation of freshwater (rather than marine origin like Caspian sea).
• Na+ is most often dominant cation, Cl- dominant anion
• Salt lakes stratify more easily, freezing point is lowered, high osmotic stress = high ionic content of water results in permanent tendency of organisms to dehydrate - low biodiversity but often high productivity for species that are present and often occur in high densities e.g brine shrimp Artemia, cyanobacteria Spirulina. Fish are normally completely absent. Most roseate flamingos depend on saline lakes in east Africa (e.g. Lake Turkana) - feed by filtering on cyanobacteria and zooplankton

Question 25

Where do we expect saline lakes ? Give a few examples of areas with many saline lakes

Answer 25

Most common in semi-arid regions e.g. central asia, west australia, east and southern africa.

Question 26

Conservative / dynamic ions

Answer 26

• Major cations and anions of salinity in freshwater can be separated:
  > Concentrations of conservative ions undergo relatively minor changes from biotic utilization or biotically mediated changes in the environment e.g. Mg, Na, K - biological requirement for these cations are small
  > Concentrations of dynamic ions can be influenced strongly by metabolism e.g. Ca, inorganic carbon can show marked spatial and temporal dynamics

Question 27

Osmoregulation problems in freshwater organisms / saline water organisms

Answer 27

Because of osmotic pressure, organisms in saline water must be adapted to have different osmoregulation than freshwater. Hypertonic organisms (cells let water out) live in freshwater while hypotonic organisms (cells only let water inside) live in saline ecosystems.

Question 28

What is so special about the life cycle of eels?

Answer 28

They migrate from the sea into the rivers (not very common). They live in rivers then go spawn in the sea and then return to freshwater habitat. Resulting larvae are carried passively by the stream and then they actively swim up to the rivers

Question 29

Definition of pH

Answer 29

• Measure of acidity/alkalinity - measure of H+ concentration. Less than 7 is acidic etc
• pH of most waters is between 6-9

Question 30

What are bicarbonate lakes

Answer 30

Have high enough conc of carbonate/bicarbonate to have buffer system. Hard water lakes often have high concs of carbonates, are slightly alkaline - are strongly buffered against acidification. The more productive the water, the higher pH tends to be and the more pH fluctuates associated with photosynthesis (fluctuations caused by changes in conc of carbon dioxide)

Question 31

Can lakes acidify in a natural way?

Answer 31

• Volcanic regions that receive strong mineral acids (like sulphuric acid resulting from oxidation of pyrite of rocks) have natural waters with pH less than 4.
• Natural waters that are rich in dissolved organic matter have low pH - bog lakes in which littoral zone is dominated by moss

Question 32

Human-induced acidification of lakes: causes, explain the distribution of impacted lakes

Answer 32

• Acidic rain from industrial pollution can increase acidity of poorly buffered waters e.g. low nutrient soft water in drainage basins dominated by granite or volcanic rocks
• No buffer (carbonates) to counteract addition of hydrogen ions = acidification

Question 33

How does the inorganic carbon content of water increase

Answer 33

• Inorganic carbon can be derived from the atmosphere by dissolution of carbon dioxide at the air-water interface. Carbon dioxide is very soluble in water so water can easily get enriched with CO2 upon exposure to the atmosphere - bicarbonate and carbonate dissociate but pH normally unaffected as produces both H+ and OH-
• Water in drainage basin can get loaded with carbonates as it percolates through soil and becomes enriched with CO2 from plant + microbial respiration. This forms carbonic acid which dissolves limestone of calcium-enriched rock formations and produces calcium bicarbonate which is soluble in water and increases conc of calcium and bicarbonate in the water = hard water, alkaline. Water is brought to lake by surface runoff or through groundwater

Question 34

Explain the bicarbonate – carbonate buffering system

Answer 34

As long as water contains enough bicarbonates, water resists changes in pH and just changes in proportion of different organic carbons is seen e.g. conc of (bi)carbonates increase with pH

Question 35

Why are lakes in northern Europe in general more impacted by acid rain than lakes in Western Europe?

Answer 35

• Western europe the soils are rich in calcium carbonate - gives lakes higher buffering capacity
• Air pollution in Western Europe has been declining due to implementation of cleaner technology

Question 36

Explain the carbon and carbonate dynamics in a hardwater lake

Answer 36

• Have high enough conc of carbonate/bicarbonate to have buffer system. Hard water lakes often have high concs of carbonates, are slightly alkaline - are strongly buffered against acidification.
• High conc of (bi)carbonates comes from calcareous drainage basin (see answer above)

Question 37

Definition: Detritus

Answer 37

Detritus = dead organic matter

Question 38

What is DOC / POC

Answer 38

• DOC = dissolved organic carbon
• POC = particulate organic matter (mostly from dead material)
• Ratio of DOC to POC is commonly between 6:1 and 10:1

Question 39

What is denitrification / nitrification, and what role does oxygen play in this ?

Answer 39

• Nitrification: conversion of nitrogenous compounds from reduced state to a more oxidized state - oxygen is necessary for this (aerobic conditions). NH4+ -> NO2- -> NO32-
• Denitrification: reduction of nitrate (NO32-) to nitrite (NO2-) to ammonia (NH4+) or molecular nitrogen (N2). Denitrification occurs mostly in an anaerobic conditions e.g. hypolimnion of eutrophic lakes and anoxic sediments

Question 40

Sources and losses of nitrogen in a lake

Answer 40

Sources:
- Precipitation falling directly on lake surface - precipitation used to be negligible compared to input from terrestrial runoff but due to industrial development + agricultural intensification the nitrogen inputs from atmospheric sources have increased
- Nitrogen fixation both in water and the sediments. Some cyanobacteria, bacteria and fungi are capable of transforming molecular N2 into ammonia (NH4+) which can then be incorporated into organic molecules. Nitrogen fixation occurs under aerobic conditions and is dependent on light (photosynthetic activity).
- Inputs from surface and groundwater drainage, especially in limestone areas

Losses:
- Effluent outflow from basin
- Bacterial denitrification resulting in N2 that can be lost to the atmosphere
- Permanent sedimentation loss of nitrogen- containing compounds to the sediments

Question 41

How can one determine whether a given nutrient is limiting algal productivity in a given waterbody?

Answer 41

Carry out experiments where you add different kinds of nutrients (and nutrient combinations) and monitor algal growth

Question 42

What is the relationship between nitrogen concentration and lake productivity?

Answer 42

Correlation does not imply causation: in fresh waters, phosphorus is major determinant of algal productivity (nitrogen is limiting factor in many coastal areas). Normally as average N concs increase so does algal productivity - correlation likely comes from the fact that high N lakes also have high P concs

Question 43

Why do cyanobacteria often dominate the phytoplankton in hypertrophic lakes?

Answer 43

Cyanobacteria are capable of nitrogen fixation - have specialized cells that capture nitrogen. As productivity of lake increases, nitrogen fixation becomes more important as nitrogen is most often not limiting factor for organic production. With increasing eutrophication due to phosphorus loading, nitrogen may become the more limiting nutrient in sustaining the increasing productivity. Then, the growth of cyanobacteria that are capable of nitrogen fixation is promoted at the expense of other organisms that are dependent upon ammonium and nitrate. If N is limiting, cyanobacteria can take it from the air

Question 44

What is DON / PON

Answer 44

• DON = dissolved organic nitrogen
• PON = particulate organic nitrogen
• Just like with organic carbon, the DON of lakes is 5-10x greater than PON contained in plankton but the ratio decreases as lake becomes more eutrophic. Ratio is often close to 1:1 in epilimnion

Question 45

What is the form of phosphorus that is available to phytoplankton and bacteria? How should we measure phosphorus to obtain an idea of the productivity of a given waterbody?

Answer 45

• Orthophosphate (PO43-) = inorganic phosphorus - form that is readily available for primary production. However, most of phosphorus (>90%) is organic phosphate
• Phosphorus is the major contributor to lake productivity + eutrophication as it is normally the least available in freshwater environments = limits biological productivity.
• Amount of total phosphorus normally shows a positive relationship with productivity of the lake with eutrophic lakes having highest phosphorus and oligotrophic having low concs
• During summer stratification, vertical distribution of P varies:
  > In oligotrophic lakes, general little variation in P conc with depth
  > Lakes exhibiting clinograde oxygen curves: increase in soluble P content in lower hypolimnion and decrease in epilimnion due to sedimentation of living and dead organic particles. Particulate phosphorus in epilimnion fluctuates widely due to oscillations in plankton populations

Question 46

Oligotrophic / mesotrophic / eutrophic / hypertrophic lakes

Answer 46

• Oligotrophic lakes: low nutrient load, low organic productivity, most often limited by P and have excess N, slight;y acidic
• Mesotrophic: intermediate
• Eutrophic: nutrient rich, highly productive, usually alkaline
• Hypertrophic: very nutrient rich

Question 47

Eutrophication: causes, consequences, remedies

Answer 47

• For most lakes, most important nutrients causing the shift from less to more productive state are phosphorus and nitrogen. Ratio in aquatic algae tissue is normally 1 P: 7 N: 40 C per 500 wet weight - so if all other elements are in excess (P is limiting) and you add a given amount of P it will theoretically generate 500 x its weight in living algae. If P is added to unproductive lakes normally leads to rapid increase in algal productivity but increased productivity is not sustained due to losses from sedimentation of particulate P. So steady P loading = sustained increased productivity.
  > Under anaerobic conditions ferric iron (Fe3+) is reduced to ferrous iron (Fe2+) and the phosphates that were adsorbed to the ferric iron are mobilized and released
• Consequences: reduction of biodiversity, loss of of species that are dependent on presence of aquatic vegetation and clear water, economic losses:
  > Phytoplankton blooms hamper production of potable water due to clogging of filters and bad odour and taste
  > Turbid waters are less attractive for tourists - scenery, swimming, water sports
  > Quality of fish in terms of species composition and taste of meat are lower e.g. highly valued pike and salmonids disappear and are replaced with carp
  > Summerkills and winterkills of fish can occur due to high oxygen consumption
• Remedies:
  Reducing nutrient load (especially P)
  Adding ferric iron (Fe2O3) reduces phosphate loads by precipitation

Question 48

Explain why an anoxic hypolimnion may enhance eutrophication

Answer 48

• Anaerobic conditions mobilize phosphates from the sediments:
• In very productive lakes where hypolimnetic decomposition produces anoxic conditions, the formation of hydrogen sulphide (through anaerobic metabolism of bacteria) leads to precipitation of ferrous sulphide (FeS). This leads to a reduction of iron content in hypolimnion and results in a significant portion of phosphates remaining in solution during autumnal circulation, leading to increase in productivity of lake during turnover period.

Question 49

How can one can boost the productivity of a lake without adding phosphorus?

Answer 49

Adding sulphate to a lake to increase bacterial production of hydrogen sulphide (H2S) and to accelerate the loss of iron can be used to regenerate phosphate from the sediments

Question 50

How can one remove phosphorus from the water column?

Answer 50

Adding ferric iron (Fe2O3) reduces phosphate loads by precipitation, best done on canals that supply the lake with water to substantially reduce nutrient loading. Then remove precipitated material

Question 51

Why is agriculture less important as a source for phosphorus than as a source of nitrogen?

Answer 51

• Agriculture is less important than domestic sewage when it comes to phosphorus - cleaning detergents are a major source of phosphate and have contributed to much of the eutrophication of surface waters in densely populated areas throughout the world
• Nitrogen -

Question 52

Why do turnover periods often enhance algal blooms?

Answer 52

Turnover periods resuspend nutrients from the bottom lakes, making them available again

Question 53

The dynamics of phosphorus are influenced by bacteria, phytoplankton, zooplankton, fish, benthic macro-invertebrates and macrophytes – explain.

Answer 53

• Bacteria: decompose dead organic material, resulting in the release of orthophosphate
• Phytoplankton: rapidly assimilate P
• Zooplankton: zooplankton take up P and N by ingestion of food particles. During egestion, nutrients are released - some remains in fecal materials and settles out of epilimnion, other nutrients are rapidly re-assimiliated by phytoplankton. When P conc in epilimnion is low, regeneration of P and N by herbivorous zooplankton are a major source of nutrients for algae
• Fish: bottom-feeding fish (e.g. carp) disturb the sediment-water interface (and the oxidized surface layer) and can release P OR can enhance physical penetration of oxygen and impede the release of P
• Benthic macro-invertebrates: same as fish
• Macrophytes: they can absorb nutrients from water through their leaves. They can mobilize phosphorus through their root systems - contribute to active transport of P from sediments to overlying water column - this P becomes available as soon as the macrophytes die and their leaves/roots are decomposed by microbes

Question 54

Eutrophication: definition

Answer 54

• Eutrophication: change in system (by human activity) such that productivity of the system is enhanced at a faster rate than would have occurred in the absence of interference.
• Eutrophy = nutrient rich

Question 55

Internal eutrophication

Answer 55

• In shallow lakes, recovery from eutrophication is much slower than deeper lakes. In shallow lakes wind action results in resuspension of P from sediments = internal eutrophication
• In lakes with anaerobic water-sediment interface anaerobic conditions mobilize phosphates from the sediments

Question 56

Explain the difference between the response of shallow and deep lakes to a phosphorus pulse

Answer 56

If P is added to a lake, usual response is rapid increase in algal productivity but this is not sustained if P input is not continued as particulate P settles out of epilimnion and into sediment (not true for shallow lakes). In shallow lakes, recovery from eutrophication is much slower as wind action results in resuspension of P from sediments = internal eutrophication

Question 57

Silicium is an abundant element. Yet, it can sometimes be limiting for algal growth? Why?

Answer 57

Diatoms (a specific group of algae) assimilate large quantities of silicon in the synthesis of their frustules (silica wall in which these unicellular algae live).