Freshwater Ecology

Question 1

Describe the different types of wetlands, their hydrology,
dominant vegetation and important ecosystem properties or
functions.

Answer 1

Wetland Hydrology:
Gains:
* precipitation, surface and
groundwater
* tidal/coastal – bidirectional flow

Losses:
* Evapotranspiration
relatively more important
* Directional flow – losses
when rivers flow through

Hydrologic regime is the most important
abiotic factor determining wetland ecology

Wetlands degrading overtime due to agriculture.

High biological diversity of both aquatic and
terrestrial organisms
Ephemeral (drying up) or extreme abiotic
conditions
Often with locally adapted biodiversity as a result

Question 2

Discuss how variation in discharge and velocity of a river
system can lead to habitat and biological diversity.

Answer 2

In-channel features, such as woody debris and submerged vegetation, can also give rise to significant variability in water velocity and depth within a reach of river. This spatial variability can be important for maintaining habitat diversity and biodiversity, including different life stages within individual species.

Question 3

Discuss how self-organisation can lead to the characteristics
of a flowing water feature and affect the landscape it is
flowing through.

Answer 3

A system is self-organizing if it acquires a spatial, temporal, or functional structure without specific interference from the outside. By “specific” we mean that the structure or functioning is not impressed on the system but that the system is acted upon from the outside in a non-specific fashion.

Question 4

Describe the transport of materials by rivers and discuss
their role in the global carbon cycle.

Answer 4

Atmospheric carbon dioxide dissolves in rainwater, making it slightly acidic. This dissolves rocks, allowing the carbon to flow down rivers and into the ocean, deposited in new rocks — usually calcum carbonate. This moves carbon from the atmosphere into the lithosphere.

river ecosystem metabolism consumes organic carbon derived from terrestrial ecosystems, which produces CO2 emitted into the atmosphere

Question 5

Discuss the processes that lead to boreal kettle lakes being
one of the most common lake types globally – and most
important in a climate change context

Answer 5

Kettles form when a block of stagnant ice (a serac) detaches from the glacier. Eventually, it becomes wholly or partially buried in sediment and slowly melts, leaving behind a pit. In many cases, water begins fills the depression and forms a pond or lake—a kettle.

The massive Lakes act like heat sinks that moderate the temperatures of the surrounding land, cooling the summers and warming the winters. The lakes also act like giant humidifiers, increasing the moisture content of the air. In the winter, this moisture contributes to heavy snowfall known as “lake effect” snow.

Question 6

What are the main ways in which lakes are formed? (Tectonic and glacial,)

Answer 6

Tectonic activity

Lake origins – glacial activity

The Great Lakes (US / Canada)

Lake Superior, Michigan, Huron, Ontario & Erie

Michigan is the world’s largest freshwater lake by area

Formed during glacial retreat at end of last Ice Age

Retreating ice sheets exposed the basins they had carved into the land

Filled with meltwater

Question 7

how are volcanic lakes formed?

Answer 7

Crater lakes usually form through the accumulation of rain, snow and ice melt, and groundwater in volcanic craters. Crater lakes can contain fresh water or be warm and highly acidic from hydrothermal fluids.18 Apr 2023

Crater lakes form as the depression left behind following a volcanic eruption fills with
water. This may come from precipitation, groundwater, or melted ice. Its level rises
until an equilibrium is reached between the rates of incoming and outgoing water.

Question 8

Lake origins – damming

Answer 8

Beavers, landslides and reservoirs.

Question 9

Lake origins – ephemeral

Answer 9

Ephemeral means temporary

Many ponds are an example (dry up in hot weather)

Many other natural cavities that host freshwater life (treeholes, pitcher plants, tank bromeliads)

Question 10

What is an ecosystem?

Answer 10

All of the living things (organisms) and non-living things (abiotic factors) in a specific area

A single type of organism is a species

Every member of a species in one area is called a population

All of the populations in an area make up a community

The species found in an ecosystem are determined by both abiotic factors and other organisms

Question 11

What are the major abiotic factors influencing lake ecosystems? (Including details on solar radiation)

Answer 11

Solar radiation

Light penetration

Mixing and stratification

Almost all solar radiation completely absorbed in first metre of water column

Varies daily due to angle of sun

95% of incident radiation is transmitted when sun is high

Reflection increases when sun is lower in the sky
Reflected light cannot be used for photosynthesis

Algal production will vary both seasonally and daily due to solar radiation

Question 12

Light penetration details

Answer 12

Water clarity determines how deep light can reach

Dissolved organic matter can lead to brown colour in lakes, e.g. decomposed plants and animals, tannic & humic acids

Turbid water can result from suspended sediments

Surface run-off from soil also increases turbidity

Transparency often reflects abundance of phytoplankton

Clear lakes in winter, phytoplankton blooms in summer

Nutrient enrichment can also promote phytoplankton blooms

Question 13

Mixing & stratification

Answer 13

Temperature of water determines its density

Peak density is at 4 °C

Water is lighter at both higher & lower temps

Ice floats on liquid water

Bottom of lake will be last part to freeze in winter

This also leads to thermal stratification

Epilimnion is the warm top layer of stratified lake

Metalimnion or thermocline contains a temperature gradient

Hypolimnion is the cold bottom layer of a stratified lake

Stratification broken down by wind or convective overturn

Mixing affected by depth, size, shape, climate, topography, vegetation & inflow from streams

Question 14

Different types of mictic lakes.

Answer 14

Monomictic lakes only mix from top to bottom once a year, e.g. polar lakes mix during ice-free summer, ice-free temperate lakes may mix throughout winter

Dimictic lakes have two seasons of mixing (spring and autumn) and two seasons of stratification (summer and winter)

Polymictic lakes stratify and mix several times a year, e.g. tropical lakes

Meromictic lakes rarely mix from top to bottom

Amictic lakes are permanently covered in ice and never mix, e.g. Arctic, Antarctic, alpine regions

Question 15

Littoral zone – periphyton

Answer 15

Phytoplankton uncommon in littoral zone

Instead, microscopic algae live in biofilms attached to submerged surfaces

Most common on rocks and attached to macrophytes

Includes diatoms, green algae & cyanobacteria, but also detritus, polysaccharides & heterotrophic bacteria

Question 16

Limnetic zone

Answer 16

Open water zone where sunlight can penetrate

Low diversity, but high density of organisms

Concentration of phytoplankton is strongly related to concentration of phosphorus (Krebs 2009)

Nutrient status determines amount of primary production

Question 17

Littoral zone

Answer 17

Typically dominated by macrophytes and diverse animal life

Macrophytes can be emergent, floating, or submerged

Question 18

Limnetic zone – phytoplankton

Answer 18

Oligotrophic = nutrient poor, few phytoplankton, clear water

Eutrophic = nutrient rich, many phytoplankton, murky water

Question 19

Limnetic zone – zooplankton

Answer 19

Zooplankton grazers feed on phytoplankton in water column

Most common groups are:
Cladocera (Daphnia spp.)
Copepoda
Rotifera (wheel animals)

Question 20

Limnetic zone – rotifers

Answer 20

Small generalist filter feeders (<1 mm)

Consume bacteria, algae & protozoa in 0.5 – 18 μm range

Distinctive morphological features

Corona of cilia used in locomotion and food collection

Pharynx used to grind food

Most species are parthenogenic – huge reproductive potential

Question 21

Limnetic zone – defensive strategies in phytoplankton

Answer 21

Phytoplankton:

Thick cell walls to decrease digestibility

Spines to increase handling time

Large size to avoid some grazers

Colonial forms to increase overall size

Mucilaginous sheaths for movement

Toxins

Question 22

Profundal zone

Answer 22

No light, no algae and often low oxygen concentration

Dominated by bacteria and fungi

Animal life relies on organic matter sinking from the surface or on migration upwards to obtain food

Bloodworms (Chironomidae) are deposit-feeders in the muddy benthos, adapted to anoxic conditions

(2) Clams filter organic matter from water column & sediment

(3) Phantom midges (Chaoborus spp.) live in profundal zone during day and migrate to limnetic zone at night

Question 23

Seasonal changes in eutrophic lakes

Answer 23

Shifts in light:
low in winter
high in summer

Shifts in nutrient concentrations:
high in winter & spring
low in summer

Shifts in grazing pressure:
low in winter & spring
high in “clear-water phase”
low in early summer
high later in summer

Question 24

Eutrophic versus oligotrophic lakes

Answer 24

Oligotrophic lakes follow a different trajectory

Nutrients are too limited for a summer bloom

Zooplankton abundance is much greater (and later) as a result

Question 25

Temperate versus tropical lakes

Answer 25

Tropical lakes have a lower magnitude of variability of incident light, water temperature, and nutrient availability

Also less seasonal variability, but more short-term (day-to-day and week-to-week) variability

Question 26

What makes a river a river?

Answer 26

No official definition, but related to its size

Rivers can be categorised according to their order (Strahler number)

1st order streams have no other water flowing into them (source or spring-fed streams)

If two 1st order streams join, they become a 2nd order stream

If a smaller order stream joins a higher one, the new branch retains the higher number

1st – 3rd order are called headwater streams – they make up about 80% of the world’s waterways

4th – 6th order are medium streams or mid-reaches

7th – 12th order are generally considered rivers

Question 27

The major biotic groups in lotic systems are:

Answer 27

Basal resources:
(a) algae (periphyton / biofilms / bryophytes / macrophytes) = green pathway
(b) dead organic matter (carrion / leaves / fine particles / dissolved material) = brown pathway

Microbes (e.g. bacteria, fungi)

Meiofauna (e.g. ciliates,
flagellates)

Macroinvertebrates

Fish

Question 28

The 5 functional feeding groups in a river ecosystem.

Answer 28

Shredders = Break up leaves and other coarse particulate organic matter (CPOM)

Collector gatherers = Collect periphyton & fine particulate organic matter (FPOM) from the benthos

Filter feeders = Filter FPOM and phytoplankton from the water column

Sometimes called collector filterers and pooled with collector gatherers as “collectors”

Scrapers / grazers = Scrape encrusting periphyton off the surface of rocks

Predators = Feed on other invertebrates and are typically the biggest organisms

Question 29

Biofilms

Answer 29

Phytoplankton are only found in the largest and deepest lower reaches of big rivers

Most algal resources occur in the form of periphyton on stones, i.e. biofilms

Diatoms, green algae, and cyanobacteria dominate

Crustose, prostrate, gelatinous forms can only be accessed by scrapers

Stalked or filamentous forms are mainly accessed by collector gatherers

Question 30

Abiotic factors change along rivers

Answer 30

Moving from headwaters to larger rivers:

Slope and substrate size decrease

Channel width, depth & discharge increase

General transition from erosion through transport to deposition

This has consequences for resources and their consumers

Question 31

The River Continuum Concept (Headwaters)

Answer 31

Headwaters:

Channel is narrow, steep
Water is fast flowing

Riparian vegetation is typically dominant
Canopy cover limits light

Major energy input is allochthonous ((of a deposit or formation) having originated at a distance from its present position.)
Dominated by leaves & CPOM

Major invertebrate groups are shredders & collectors

Question 32

The River Continuum Concept mid-reaches

Answer 32

Channel widens, allowing more light through

Temperature increases

Algal production now greatest (autochthonous)

Increasing importance of FPOM transported from CPOM broken down upstream

Dominant inverts are scrapers & collectors (including filter feeders)

Question 33

The River Continuum Concept (Large rivers)

Answer 33

Widening channel & decreasing flow

Macrophytes become more abundant

Phytoplankton can even be present

FPOM is the main source of energy

Dominant inverts are collectors, particularly filter feeders

Question 34

Serial Discontinuity Concept

Answer 34

Most rivers affected by human activities

Dams create a discontinuity in the river

Gradual downstream transition of RCC is disrupted

Movement of animals is impeded

Organic matter transported by river is deposited behind dam

Availability of food downstream is reduced

Question 35

Hyporheic Corridor Concept 1/3

Answer 35

Water does not just flow downstream

It flows vertically & laterally in the hyporheic zone

Hypo = below; rheos = flow

Hyporheic zone is the space beneath & beside the stream bed

There is mixing of oxygen-rich surface water with nutrient-rich groundwater in this zone

Question 36

Hyporheic Corridor Concept 2/3

Answer 36

Important component of floodplain dynamics

Links secondary channels to main river

Provides exchange of nutrients & organisms with wetlands

Water flows more slowly in hyporheic zone, so it is a valuable source of carbon and dissolved nutrients from groundwater

Also provides an important shelter / habitat for organisms

Question 37

Hyporheic Corridor Concept 3/3

Answer 37

Salmon lay their eggs under gravel – cooled by groundwater

Alevins use the hyporheic zone for shelter during development

Hyporheic zone is also a refuge during drought or other stress

Groundwater flow persists through gravel

Crayfish and inverts can burrow down to reach this.

Question 38

Lotic vs lentic dispersal

Answer 38

Freshwater habitats are either lentic (standing) or lotic (running). On average, lotic habitats are more stable and predictable over space and time than lentic habitats. Therefore, lentic habitats should favour the evolution of higher dispersal propensity which ensures population survival of lentic species.

Question 39

Summary for lentic and lotic dispersal

Answer 39

Dispersal is more than movement
* Determines genetic structure of populations
* Contributes to Landscape scale diversity
* Can be part of metapopulation
* “Fish” populations are heterogeneous movers
* Connectance is key at larger spatiotemporal
scales
* Species interactions play a large role in successful
dispersal and colonisation

Question 40

What is alpha, beta and gamma diversity?

Answer 40

Alpha diversity is the species diversity present within each ecosystem on a landscape. Beta diversity is represented by the species diversity between any two communities. Gamma diversity of is the species diversity across the entire landscape, taken as one unit. Scale is absolutely critical in ecological studies.

Question 41

Summary of
Dispersal in Communities

Answer 41

Dispersal a key process in colonisation and
community assembly
* Invasion sequence could lead to multiple
stable states
* Results relatively consistent between field and
lab studies
* Dispersal of predator and prey both important
* Connectance , and time, governs dispersal

Question 42

sympatric and allopatric speciation

Answer 42

In allopatric speciation, groups from an ancestral population evolve into separate species due to a period of geographical separation. In sympatric speciation, groups from the same ancestral population evolve into separate species without any geographical separation.

Question 43

Patterns of interaction strengths in food webs

Answer 43

Stability of coupled strong & weak interactions demonstrated by McCann et al. (1998)

Consumer feeds its preferred resource close to extinction (strong interaction)

It can switch to its less preferred resource(s), so it still has food (weak interaction)

This allows the population size of its preferred resource to recover

This type of module is extremely common in food webs

Facilitates stable consumer-resource dynamics through time

Question 44

Food webs like games of Jenga!

Answer 44

Changes in species identity and strength and composition of trophic links

Rewiring of ecological networks compared to a game of Jenga (DeRuiter 2005)

A Jenga structure is constantly changing with addition & deletion of stones

Stability depends on contribution of ingoing & outgoing stones to underlying structure

Question 45

Sampling of streams for food webs

Answer 45

Electrofishing to quantify fish (e.g. 3 passes of a 50 m reach)

Surber sampling to quantify macroinvertebrate community plus leaf litter, FPOM, etc.

Rock scrapes to quantify biofilm community

Question 46

Sampling of lakes for food webs

Answer 46

Electrofishing / seine nets / gill nets / fyke nets for sampling fish

Tow net (or hand net) for sampling zooplankton

Van Dorn sampler for sampling phytoplankton

Question 47

Quantifying food web structure

Answer 47

Stomach flushing (or dissection) of fish to quantify:

Which prey species they have been eating

How many of each prey they have been eating

Identifying stomach contents under the microscope
Prey items often heavily digested, difficult to identify
Potential to use DNA analysis to determine which prey have been consumed

Question 48

What is a trophic cascade?
and different types

Answer 48

Trophic cascades result in inverse patterns in abundance or biomass across more than one trophic link in a food web (Pace et al. 1999)

Species-level cascade: occurs within a subset of the community – changes in predator numbers affect a subset of plant species

Community-level cascade: substantial alteration to distribution of biomass throughout an entire ecosystem (Polis et al. 2000)

First conceptualised as Green World Hypothesis (Hairston, Smith & Slobodkin 1960)
Carnivores suppress herbivores, thus indirectly allowing plants to grow
Introduced the concept of top-down forces and indirect effects shaping ecological communities.

Question 49

Body size: why do we care?

Answer 49

Body size determines many aspects of an organism’s function in ecological systems
E.g. home range, ingestion rate, secondary production and nutrient turnover
Body size is also an important determinant of consumer-resource interactions
Aquatic systems in particular are highly size-structured

Metabolic rate has been shown to scale predictably with body size & temperature (Brown et al. 2004)

Changes in either or both of these properties will alter metabolic demand and the flow of energy through food webs

Question 50

Impacts of warming: decreasing body size

Answer 50

Ectotherms rely on their environment to regulate their body temperature
Smaller ectotherms are better competitors for limited resources (Reuman et al. 2014)
For example, smaller diatoms have a larger surface to volume ratio and so can absorb more nutrients per unit biomass

In contrast, endotherms regulate their own body temperature
Smaller endotherms have proportionally larger surface area & thinner boundary layers
They can “dump” heat more effectively than larger organisms
Smaller size is favoured in warm environments

Bergmann’s rule: smaller species generally found in warmer regions
James’s rule: smaller-sized populations of the same species in warmer environments
Temperature-size rule: individuals will be smaller at a given age in warmer environments

Question 51

Impacts of warming: loss of top predators

Answer 51

Trophic pyramids are extremely common in nature
Rule of thumb: only 10% of energy transferred to next highest trophic level
Top predators are most likely to suffer when energy demands increase throughout

Question 52

Evidence for increased nutrient supply in algae

Answer 52

Nutrient cycling is higher in warmer streams (Demars et al. 2011)

Nitrogen fixation (by cyanobacteria) also increases with temperature (Welter et al. 2015)

There are more nutrients to fuel the greater production of algae at higher temperatures

Question 53

Thermal performance curve

Answer 53

Thermal performance curve (TPC)
Some organisms may sit close to or at their thermal optimum
Increasing temperature leads to a decline in thermal performance
Organisms close to CTmin may have scope for increased performance

Question 54

Trophic transfer efficiency 1/2

Answer 54

Sunlight is converted more efficiently into gross primary production as stream temperature increases

This may be related to increased nutrient cycling in warmer streams (Demars et al. 2011)
More nutrient resources available to fuel production
More resources at the base of the food web

These available resources are also converted more efficiently into invertebrate biomass
GPP to invertebrate transfer efficiency increases with stream temperature

Cold streams dominated by very selective grazers (Chironomidae)
Warm streams by more efficient grazers (snails) and filter feeders (Simuliidae)?

Question 55

Trophic transfer efficiency 2/2

Answer 55

Overall, energy transfer is more efficient at each trophic level in the food web
Without external subsidy, trout would need 32% trophic transfer efficiency to persist in warm streams
With external subsidy, trophic transfer efficiency of ~9% is sufficient
O’Gorman et al. (2016) GCB

Question 56

How do they sample fish?

Answer 56

Electrofishing to remove all trout from the experiment and check the treatments every two weeks
(3 passes of each 15 m reach)

Surber sampling to quantify body mass and abundance of macroinvertebrate community
(14 × 14 cm; 250 μm mesh; 5 samples per reach)

Rock scrapes to quantify body mass and abundance of diatom community
(scrape & measure upper surface area; preserve in Lugol’s)

Question 57

How do they Quantify food web structure?

Answer 57

Gut content analysis under the microscope… and lots of it!!
49,324 direct observations of resource individuals in consumer guts

Yield effort curves constructed for every species in each stream

If <95% diet described by yield-effort curves, add links:

from species in the same genus

from same species in Hengill

from the literature

Question 58

Trophic group biomass
in the trout river

Answer 58

Presence of trout led to a major decrease in invert biomass, but only in warm streams
Driven by snails and blackfly larvae
No change in invert biomass during the experiment in cold streams

Cascading effects on the diatom community
Increase in diatom biomass in the warm-fish treatment, no change in cold streams
Similar results for total chlorophyll, i.e. including cyanobacteria and green algae

Temperature-induced trophic cascade

Question 59

Food web structure
definitions

Answer 59

Connectance = proportion of possible links in the web that are realised
Mean trophic level = average position of a species in the web
Consumer-resource ratio = no. of consumer species divided by no. of resource species

All three were significantly reduced during the experiment, but only in the warm-fish treatment

Question 60

Autecological and synecological and consequences

Answer 60

Actively choose to feed on richer sources of energy (autecological)
Benefit from more efficient flow of energy through food web (synecological)
These mechanisms combine to help trout overcome the metabolic constraints of living in a warmer environment

This can lead to a simplification of the food web
But only at higher temperatures, where top-down control by trout is particularly strong
Simpler food webs are shown to be less stable
Robustness to extinctions declines with decreasing connectance (Dunne et al. 2002)

Question 61

Explain how warming can lead to simpler and less stable food webs

Answer 61

Consequently, as temperature increases, the ratio of energy supply to demand decreases successively at each higher trophic level, and thus, increases the relative extinction probability of species populations at the upper vs lower ends of a food chain.

Question 62

What is the temperature size rule?

Answer 62

The temperature-size rule denotes the plastic response (i.e. phenotypic plasticity) of organismal body size to environmental temperature variation.[1][2] Organisms exhibiting a plastic response are capable of allowing their body size to fluctuate with environmental temperature. First coined by David Atkinson in 1996,[3] it is considered to be a unique case of Bergmann’s rule[1] that has been observed in plants, animals, birds, and a wide variety of ectotherms.[2][4][5][6][7] Although exceptions to the temperature-size rule exist, recognition of this widespread “rule” has amassed efforts to understand the physiological mechanisms (via possible tradeoffs) underlying growth and body size variation in differing environmental temperatures.

Question 63

Biological insurance

Answer 63

Different species can perform similar roles – insurance against biodiversity loss?

E.g. pollination has evolved independently and repeatedly in insects, reptiles, mammals & birds

So, how many species can we afford to lose?

Question 64

What are the multiple stressors on the world?

Answer 64

Species extinction

Global warming

Overpopulation

Pollution

Loss of rainforests

fossil fuel dependency

Question 65

Threats to freshwater ecosystems

Answer 65

Acidification

Chemicals

Warming

Drought

Flooding

Question 66

Causes and solutions to freshwater acidification.

Answer 66

The accelerated burning of fossil fuels over the past two centuries has largely contributed to the acidification of freshwater ecosystems.

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas (e.g. carbon dioxide), or by the reduction of acid anions, like sulphate and nitrate within a lake.

Lakes that have been acidified by acid rain can be neutralized by liming, the addition of limestone (CaCO3).

Question 67

The impacts of acidification on freshwater ecosystems

Answer 67

Acidification of freshwater ecosystems may have significant negative effects. Changes in pH as a result of freshwater acidification imposes physiological challenges on individual organisms, may decrease native biodiversity, and can alter ecosystem structure and function entirely.

Invertivorous river birds can suffer from direct toxic effects of aluminium combined with indirect effects of reduced food supply in acid streams
E.g. declines in UK Dipper (Cinclus cinclus) populations in upland areas during 1970s and 80s (Tyler & Omerod 1992)

Question 68

Acid pulses in freshwaters

Answer 68

Coniferous vegetation catches mist & fog precipitation and the acidifying pollutants it carries

Surface run-off leads to acid pulses during storm events and spring ice-melt

pH often dips markedly over short timescales (especially in streams)

Question 69

Ecological issues: declining health of fish

Answer 69

Gills are finely-divided with a high surface area: volume ratio

High labile aluminium at low pH causes mucification of the gills

Excess mucus clogs the respiratory surface, degrading gills & suffocating the fish

Question 70

Effects of acidification with freshwater algae

Answer 70

Algal resources typically decline in acidified streams

Sensitive species like mayflies and snails disappear

Brown pathway dominants

Hardy species like worms and dragonflies thrive

Question 71

Direct & indirect effects of acidification

Answer 71

Direct toxic effects - e.g. aluminium induces clogging of fish gills

Indirect food web effects - e.g. plants don’t do well in acidified environment; shift to detrital resources alters functional composition of higher trophic levels

Question 72

The losers and winners of acidification

Answer 72

Trout & invertebrate herbivores (mayflies, snails) are lost from the “green” pathway

Large invertebrate predators and detritivores dominate the food web in the absence of fish and herbivores

Question 73

Solving issues like acidification

Answer 73

Short term chemical solutions, e.g. liming to raise pH (lime is highly basic)
But artificially raising pH in acidified catchments is extremely expensive
It doesn’t mimic natural chemical change and may create problems of its own
Not a viable long-term solution

Question 74

Salt marsh die-off
due to drought

Answer 74

Salt marsh die-off shown to alter the distribution of marsh snails, Littorina irrorata
To avoid predation by marsh crabs, snails aggregated on the border of retreating cordgrass, Spartina alterniflora
Compounded the die-off by intense grazing at the margins

Exclusion experiments showed that cordgrass recovers if snails are prevented from overgrazing
Drought causes not only direct die-off, but indirectly through altering animal behaviour (Silliman et al. 2005)

Question 75

Impacts on interaction strength

Answer 75

Interaction strength is the magnitude of the effect of one species on another
Shown from a whole range of ecosystems that distribution is skewed towards weak interactions (Wootton & Emmerson 2005)

Demonstrated that coupling of strong and weak interactions facilitates switching between resources when densities become low
This promotes stability by avoiding extinctions (McCann et al. 1998)

Reduction in the number of weak links in the drought treatments
Weak interactions are important in promoting stability (McCann et al. 1998)
Drought is likely to reduce the stability of stream ecosystems.

Question 76

Time & Space in Evolution

Answer 76

Two factors – evolution (time) and
isolation (space)
* Creates heterogeneity between
systems
* Importance of evolution in
geologically ancient habitats – can
you name some?
* Endemic species – evidence of
adaptive radiation
* Not northern temperate lakes –
why?

Question 77

First summary for evolution of freshwater

Answer 77

F’water diversity often high, specialised and
endemic due to isolation and geological stability
* Morphological and genetic differentiation
* Important evolution models – charr, cichlids
* Common to fishes – taxa bias?
* Ecological Speciation
* Adaptive Radiation -needs “Space” with distinct
habitats

Question 78

what is adaptive radiation?

Answer 78

In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic interactions or opens new environmental niches.

Question 79

What is Dispersal?

Answer 79

Movement from one discrete patch to another
(Lincoln, 1998)
* Movement from point of origin
* Highly contentious (Benton et al. 2012)
* Not the same as “migration”….but migration
can be part of dispersal
* Does not have to involve colonisation or gene
flow

Question 80

lotic vs lentic species

Answer 80

The term lentic (from the Latin lentus, meaning slow or motionless), refers to standing waters such as lakes and ponds (lacustrine), or swamps and marshes (paludal), while lotic (from the Latin lotus, meaning washing), refers to running water (fluvial or fluviatile) habitats such as rivers and streams.

Question 81

Dispersal among Lentic Systems

Answer 81

Lotic issues apply
* Connectance is key
* Metapopulations more likely
* Interactions can cause local
extinction, i.e. sinks
* Connectance + Time =
successful Dispersal, Hien et al
2011.

Question 82

Second Summary to evolution in freshwater

Answer 82

Dispersal is more than movement
* Determines genetic structure of populations
* Contributes to Landscape scale diversity
* Can be part of metapopulation
* “Fish” populations are heterogeneous movers
* Connectance is key at larger spatiotemporal
scales
* Species interactions play a large role in successful
dispersal and colonisation

Question 83

Space and Scales

Answer 83

Large scale regions with high productivity have
greater biodiversity
Determined by Beta diversity increasing with
productivity at higher spatial scales
Chase & Leibold, Nature 2002
“Greater role of stochastic assembly than
deterministic as productivity increases”

Question 84

Summary of
Dispersal in Communities

Answer 84

Dispersal a key process in colonisation and
community assembly
* Invasion sequence could lead to multiple
stable states
* Results relatively consistent between field and
lab studies
* Dispersal of predator and prey both important
* Connectance , and time, governs dispersal

Question 85

What is the role of sexual selection in sympatric speciation?

Answer 85

his prediction leads to a hypothesis that sympatric speciation by sexual selection tends to occur as predation pressure is reduced (with other conditions unchanged) by, for example, a population entering into a new enemy-free habitat such as an island or a lake.

Since sympatric speciation occurs in the same geographic area there needs to be a driving force behind the divergence of the traits in the species, leading to speciation. If we add in the factor of sexual selection, this can lead to specific individuals seeking specific traits in their mates.

Question 86

What is “biodiversity”?

Answer 86

Species’ presences and abundances

Species, traits and genes

Describes the range of ‘things’ present in a place and time

At different levels of biological organisation (genes to landscapes)

At different spatial scales (small sample to whole world).

Question 87

Photosynthesis

Answer 87

Most photosynthesis is oxygenic, e.g. phytoplankton, macrophytes.

Dissolved O2 (D.O.)
in an English Chalk Stream
* Diel (day/night) changes in
concentration
* Oxygen production strongly
dependent on solar radiation
* Imbalance of photosynthesis
and respiration by night
* Modified by water-air
exchange (diffusion)

Question 88

Anoxygenic & Chemosynthesis

Answer 88

Anoxygenic photosynthesis occurs where sulphide is present, also
on and in sediments, e.g. Chlorobium, Chloroflexus

Question 89

Cyanobacteria

Answer 89

nitrogen fixing and O2 producing
- bloom forming – exhibiting fast growth
in low O2 and N conditions
- contain vacuoles that affect their
buoyancy – surface slicks
- can manipulate these vacuoles –
competitive advantage
- linked to toxins in freshwaters…a
recreation and drinking hazard

Question 90

Periphyton

Answer 90

Photosynthetic
microorganisms that live in
biofilms attached to
submerged surfaces
* Algal components:
– cyanobacteria
– microalgae
– filamentous algae
* Biofilm also includes:
– polysaccharides
– detritus
– heterotrophic bacteria
– protozoa

Question 91

Bryophytes (mosses & liverworts)

Answer 91

lack true roots
* grow attached to firm substrates
* Fontinalis antipyretica in clear
turbulent waters
– adapted to low light in
forested upland streams

Sphagnum moss in bogs (Mer
Bleue Bog, Ottawa, Canada)
* Can hold >20 times their dry
weight in water
* Helps to retain water during dry
conditions

Question 92

Vascular plants

Answer 92

Angiosperms
* have true roots
* found in
– sluggish flowing waters
– wetlands
– shallow lake margins

Can be classified by life-forms
– emergent (9.17)
– floating emergent (9.15)
– submergent (9.16)
– free-floating

occur in zones of differing
water depth

Question 93

Emergent macrophytes

Answer 93

perennials with large rhizomes
– can expand rapidly through belowground growth
– common reed (Phragmites)
– bulrush or cattail (Typha latifolia)

habitats
– wetlands
– shallow waters (<1.5 m deep)

optimal access to both light and nutrients
– access nutrients from the sediment
– photosynthesis above water surface

Question 94

Adaptations to anoxic sediments

Answer 94

Diffusion of CO2 and O2 is
10,000 times faster in air than
in water

O2 transported to rhizome
(roots) by living and even dead
stems (culms)

cavities or spaces (lacunae)
make up to 70 % of the plant
volume (hollow stems)

-provides O2 to tissues buried in
waterlogged anoxic soil &
sediment

Question 95

Submerged macrophytes

Answer 95

For example, pondweed (Potamogeton)
– occur to depths of about 10 meters in
clear lakes
* water provides buoyancy
– less need for structural tissue (lignin)
* rarely nutrient-limited
– nutrients absorbed from water through
foliage
– nutrients absorbed from sediment
through root and rhizomes
– translocation in both directions.

Question 96

Macrophytes v phytoplankton

Answer 96

sequestration of nutrients by macrophytes may limit the growth of
phytoplankton
* abundant phytoplankton absorb light in the water column and may
limit light availability to submerged parts of macrophytes
* alternative stable states
– clear macrophyte dominated
– turbid phytoplankton dominated
*
state may also depend on planktivores & predatory fish
– trophic cascades

Question 97

The building blocks of freshwater life (C,N,P)

Answer 97

Carbon is the most crucial element
for the formation of biomass in the
food web
* But nitrogen and phosphorus are
essential nutrients that give it
vitality
* Nitrogen is a major component of
chlorophyll and amino acids (the
building blocks of protein)
* Phosphorus is a vital component
of ATP (the energy unit of plants)
* Other important elements include
sulphur, silicon, iron, etc. (see
Chapter 14)

Question 98

Elemental stoichiometry

Answer 98

Carbon to nitrogen ratio is a key
determinant of the “quality” of
resources in freshwater
* A high C:N ratio equates to a lowquality resource
* Nutrient poor, takes longer to
decompose
* A low C:N ratio equates to a highquality resource
* Nutrient rich, need to consume
less material for growth and
reproduction

Question 99

Denitrification

Answer 99

A respiratory process in which NO3
- and other inorganic N forms are
used to oxidise organic matter leading to formation of N2
– Dissimilatory nitrate reduction
* Undertaken by some bacteria, the denitrifiers
– E.g. Paracoccus denitrificans, Thiobacillus denitrificans, and
some pseudomonads

Denitrification occurs where O2 is depleted
– Some bacteria use nitrate instead of O2 as a terminal electron
acceptor in respiration
– Requires an organic-rich, O2-poor environment
– Waterlogged soils, groundwater, wetlands, poorly ventilated
hypolimnion
* Denitrification is very important to reduce pollution in areas with
heavy agricultural or industrial inputs of DIN
* E.g. denitrification is promoted as a management tool to remove
nitrate from sewage or fertilisers in wetlands & riparian zones

Question 100

Perennial vs. Intermittent vs. Ephemeral Streams

Answer 100

Streams are classified by different jurisdiction in different ways. Regulations differ, and are not applied consistently.

Perennial: A stream that has flowing water year-round during a typical year. The water table is located above the streambed for most of the year. Groundwater is the primary source of water for stream flow. Runoff from precipitation is a supplemental source of water for stream flow.

Intermittent: A stream that has flowing water during certain times of the year, when groundwater provides water for stream flow. During dry periods, intermittent streams may not have flowing water. Runoff from precipitation is a supplemental source of water for stream flow.

Ephemeral: A stream that has flowing water only during or for a short duration after precipitation events in a typical year. In many states, this term refers to streambeds that are located above the water table year round and streams where groundwater is not a source of water for the stream.

Question 101

Wetland Hydrology

Answer 101

Hydrologic regime is the most important
abiotic factor determining wetland ecology

Permanence
Predictability
Seasonality

Hydrological characteristics have
consequences for biodiversity
* Regional and landscape
links/ variability
* More variability = more
diversity (see later lectures)

Question 102

Physiography of rivers

Answer 102

Before we can understand the ecology, we must study
the morphology and geology….

1st Characterisation
* Discharge
* Catchment area
* Tributaries

Then…
* Physical habitat
Finally…
* Vegetation

Question 103

Stream order classification

Answer 103

Higher order streams have a longer individual length

• But there are many more lower order streams per catchment
• Thus, lower order streams have an overall combined longer length
• Lower order streams often dominate the interaction between land and water

Question 104

Meanders and Braids

Answer 104

Unless otherwise constrained, streams and rivers will meander

• Self organising – fractal geometry
  (Stolum, 1996)
• Erosion & deposition exaggerate
  meander over time
• Faster flow picks up sediment on
  outside bend
• Slower water deposits sediment
  on inside bend
• Point bar forms

Can cut through and cut-off
to form an oxbow lake

Over softer sediments and shallow water,
braiding may develop
This involves the main channel splitting and
combining in a braided pattern
Can be very variable over time
Channels form and disappear at fast pace

Question 105

Primary production

Answer 105

The production of organic matter
from CO2 and nutrients using either
light energy or chemical energy
* Photoautotrophs obtain energy
from light
– oxygenic photosynthesis
– anoxygenic photosynthesis

Chemoautotrophs obtain energy
from the chemical oxidation of
inorganic compounds
– methanogenesis, sulphide
oxidation, nitrification
– often extremophiles living in
hostile environments, such as hydrothermal vents

Question 106

Carbon cycling

Answer 106

Primary production also plays a key role in the carbon cycle
* Photosynthesis converts inorganic carbon (CO2) to organic carbon (i.e. the
kind that can be eaten by animals – heterotrophy)
* Heterotrophic & anoxic respiration change organic carbon to CO2
* Methanogenesis/methanotrophy converts CO2 to CH4

Question 107

Submerged v emergent macrophytes

Answer 107

emergent plants benefit from greater access to light and CO2
* cost is need for large amounts of structural material (lignin) in stems
* submerged plants benefit from lower structural investment
* cost is slow diffusion of CO2 in water
* floating leaves at water surface benefit from contact with air, but
without cost of synthesising large amounts of structural material

Question 108

N- or P-limitation

Answer 108

Seston N:P of 25 to 30 delimits transition from N-limitation to P-limitation at ecological scale

N-depleted waters are often dominated by cyanobacteria

Cyanobacteria can fix N2 from air

Brings an external source of nitrogen into freshwaters

Question 109

Key components of the nitrogen cycle

Answer 109

There is very little nitrogen in rocks
* Abundant in the atmosphere (78%)
* But not available to most organisms
(including all higher plants)
* Some bacteria can fix N2 gas into
organic N (useable)
* Most planktonic N-fixation performed
by cyanobacteria

Major forms of organic nitrogen include:
– protein
– nucleic acids
– amino acids
– urea
* Urea is particularly important because it
is excreted by air-breathing organisms
* Most freshwater organisms directly
excrete ammonium (NH4
+), however
* Organic nitrogen is also decomposed to
form NH4
+
* Soil leachate and sewage discharge are
also sources of (dissolved) organic
nitrogen in freshwaters

Dissolved inorganic nitrogen mainly consists of ammonium (NH4
+),
nitrite (NO2
-
) and nitrate (NO3
-
)
* Under high pH, NH4
+ is converted to ammonia gas (NH3) and can
dissipate to the atmosphere
* Highly toxic to aquatic organisms
* Nitrite is also toxic, but usually found in low concentration
* There are occasionally trace amounts of nitrous oxide (N2O)
* Aquarium lovers will know the importance of introducing NH4
+ to a new
tank (either directly or with hardy fish)
* Converted to NO2
- then NO3
- before delicate fish can be introduced

Only bacteria & archaea can fix nitrogen
– in cyanobacteria, N2 fixation often occurs in specialised cells,
called heterocysts (Figure 14.1), e.g. Anabaena
– requires large amounts of energy (ATP)
– inhibited by O2
* Some freshwater cyanobacteria form large globular structures
(e.g. Nostoc)
* Inedible, so nutrients only enter food web once decomposed

Question 110

Key component of the phosphorus cycle

Answer 110

Unlike carbon and nitrogen,
phosphorus is mainly found in only
one inorganic form
* Phosphate (PO4
3-
)
* Dominant in natural waters, but
often undetectable in pristine waters

Organic phosphorus
occurs in cells as nucleic
acids, phospholipids, and
other compounds
* Cells can also store
phosphate as a polymer
(polyphosphate)

Main sources are weathering of minerals in rocks and soils,
agricultural runoff & decomposition of organisms
* Essential nutrient required for synthesis of DNA, RNA & phospholipids
* Often limiting in freshwaters due to binding by oxic soils
* Therefore, phosphates become soluble under anoxic conditions
* This has management consequences

Question 111

What is an ecosystem?

Answer 111

All of the living things (organisms) and non-living things (abiotic factors) in a specific area

A single type of organism is a species

Every member of a species in one area is called a population

All of the populations in an area make up a community

The species found in an ecosystem are determined by both abiotic factors and other organisms

Question 112

Fluvial transportation

Answer 112

Rivers transport 3 main materials downstream – water, sediment & organic matter

Water & sediment affect shape of river channel

A change in either the flow of water or sediment load of the river can lead to increased deposition or erosion

E.g. Above a dam, water pools and slows, depositing much of its transported sediment; below the dam, there is increased erosion as the river tries to replace its sediment load#

Question 113

Exceptions to the RCC
(River Continuum concept)

Answer 113

General theory of river ecosystems, does not work perfectly

Not all headwater streams receive leaf litter from forests

Many upland streams are dominated by algae and grazers

Variable importance of green versus brown pathway

Question 114

Time & Space in Evolution

Answer 114

Two factors – evolution (time) and
isolation (space)
* Creates heterogeneity between
systems
* Importance of evolution in
geologically ancient habitats – can
you name some?
* Endemic species – evidence of
adaptive radiation
* Not northern temperate lakes –
why?

Question 115

Summary of freshwater biodiversity

Answer 115

F’water diversity often high, specialised and
endemic due to isolation and geological stability
* Morphological and genetic differentiation
* Important evolution models – charr, cichlids
* Common to fishes – taxa bias?
* Ecological Speciation
* Adaptive Radiation -needs “Space” with distinct
habitats

Question 116

Summary in dispersal

Answer 116

Dispersal is more than movement
* Determines genetic structure of populations
* Contributes to Landscape scale diversity
* Can be part of metapopulation
* “Fish” populations are heterogeneous movers
* Connectance is key at larger spatiotemporal
scales
* Species interactions play a large role in successful
dispersal and colonisation

Question 117

Space and Scales

Answer 117

Large scale regions with high productivity have
greater biodiversity
Determined by Beta diversity increasing with
productivity at higher spatial scales
Chase & Leibold, Nature 2002
“Greater role of stochastic assembly than
deterministic as productivity increases”

Dispersal a key process in colonisation and
community assembly
* Invasion sequence could lead to multiple
stable states
* Results relatively consistent between field and
lab studies
* Dispersal of predator and prey both important
* Connectance , and time, governs dispersal

Question 118

Food web stuff

Answer 118

Body size is an important determinant of consumer-resource interactions

Big things typically eat small things, which eat even smaller things

Aquatic systems in particular are highly size-structured

Consumers are often constrained to only eat what will fit in their mouth

Not all species interact with each other equally
Interaction strength is the magnitude of the effect of one species on another (Laska & Wooton 1998)

Question 119

Flooding info

Answer 119

Discharge - volume of water that passes through a given cross section per unit time (usually measured in cubic meters/feet per second)

Stage - level of water surface over a datum (often sea level); as discharge increases, stage increases

Flood stage - stage at which overbank flows are of sufficient magnitude to cause considerable inundation of land and roads

Crest (or peak) - highest stage reached during a flood

The counter-intuitive process of increased frequency of both drought & flooding:
a) average summertime precipitation is predicted to greatly decrease
b) amount of extreme precipitation per year is predicted to greatly increase

Question 120

What is “biodiversity”?

Answer 120

Species’ presences and abundances

Species, traits and genes

Describes the range of ‘things’ present in a place and time