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CLIMATE CHANGE,
FRESHWATER
ECOSYSTEMS
1. Introduction
Long-term data from surface waters already show changes associated with
climate warming. Rising air temperatures are reflected in increasing surface
temperatures in lakes and streams, in higher thermal lake stability and in longer
ice-free season in lakes, with a later freezing in autumn or winter and an earlier
melt in spring or summer. Increasing hypolimnetic temperatures in lakes may
lead to a higher risk of deep-water anoxia. Changing wind patterns may alter
the input of mixing energy to lakes, and hence affect their overall heat balance
and internal heat distribution. Changes in wind and air temperature will be
reflected in changes of physical behavior of lakes, which may go hand in hand
with a modification of the chemical and biological characteristics of surface
water. Changing precipitation patterns, like changes in the total amount, sea-
sonality or intensity, may alter hydrological cycles including river runoff
regimes. Wetland in particular, may be affected by changes in flooding. A change
in the amplitude frequency, duration, or timing of floods may affect biogeochem-
ical processes, plant nutrient dynamics, and plant communities.
2. Direct Impact
2.1. Physical Impact. Regional climate variability is often related to
recurrent patterns of atmospheric circulation such as North Atlantic Oscillation
(NAO), the Northern Annular Mode or the El Ni~
no-Southern Oscillation. NAO is
the most prominent pattern of atmospheric variability. It corresponds to
changes in the westerly winds. Potentially, the NAO can have an impact on
temperature and precipitation over large areas of Western and Northern
Europe, and freshwater ecosystems have been shown to be sensitive to changes
in the NAO.
Thermal Regimes. The thermal regime of water bodies is mainly deter-
mined by the local weather. The net heat exchange across the air-water interface
is given by the sum of energy fluxes relate to radiation, latent, and sensible heat
(1). A shift in climate variables such as air temperature, radiation, cloud cover,
wind or humidity will influence these heat fluxes and thus alter the heat balance
of lakes and rivers. Model studies predict that lake temperatures, especially in
the profiles, thermal stability, and mixing patterns are expected to change as a
result of climate change (2). Analysis of long-term data series demonstrate that
such a change has already occurred in recent decades. One of the first studies on
increasing water temperatures was on boreal soft water lakes in the Experimen-
tal Lakes Area of north-western Ontario and showed that the lakes experienced
an increase in water temperature of ca 2C per decade between 1969–1988 and
that water renewal rates decreased as a result of higher than normal evaporation
and lower than average precipitation. At Lake Baikal, the world’s largest lake,
with a maximum depth of 1600 m surface waters have warmed at a rate of
0.2C per over the past 60 years. Lake Baikal was expected to be rather resistant
1
Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
to climate change due to its enormous volume, but even here increasing water
temperatures and a longer ice-free season are having major implications for
nutrient cycling and food-web structure.
Fluctuations in lake surface water temperatures are transported down-
wards by vertical mixing and can reach the deep waters when the thermal stra-
tification is weak. In particular, the hypolimnetic temperatures of deep lakes,
which are determined by winter meteorological conditions and the amount of
heat reaching deep-water layers before the onset of thermal stratification may
act as a climate memory. Increasing air temperature may thus lead to a progres-
sive rise in deep-water temperature (3). Lake Zurich provides a good example of
the response of deep temperate lakes to long term changes in air temperature.
The lake has undergone long-term warming at all depths. However the tempera-
ture increase has been more rapid in the surface layers than in the deep-water
layers, resulting in increased thermal stability in all seasons and in an extended
period of summer stratification by about 2–3 weeks from the 1950s to the 1990s.
The summer of 2003 was exceptionally hot with air temperatures similar to those
predicted for the late 21st century. The highest epilimnetic temperatures ever
recorded exceeded the long term mean by almost three standard deviations. By
contrast hypolimnetic temperatures were slightly lower than average. The
potential ecological consequences of this, such as the release of phosphorus
from the sediments, possibly ultimately resulting in an increase in the intensity
of algal blooms, may thus counteract management and restoration efforts under-
taken in the past to mitigate anthropogenic eutrophication.
Rivers and streams have also warmed during the past few decades, and
stream and water temperatures are projected to increase further in future
warmer climates.
The effects of warming on populations of cold-water fish, such as brown
trout, are expected to be deleterious at the warmer boundaries of their habitat
and positive at the cooler boundaries.
Ice cover plays an important role in freshwater systems and changes in the
thickness and duration of the ice layer are of ecological importance and have con-
sequences for human activities. According to IPCC, freeze-up is defined concep-
tually as the time at which a continuous and immobile ice-cover forms, while
break-up is generally the time when open water becomes extensive in a lake or
when ice-cover starts to move downstream in a river. Dates of freeze-up and ice
break-up have proved to be good indicators of climate variability at local to regio-
nal scales.
2.2. Chemical Impact. Climate not only has an impact on physical char-
acteristics on surface waters, but also is a master variable for ecologically impor-
tant chemical processes. There are two examples of how climate may directly or
indirectly affect surface water chemistry. First with respect to changes in con-
centration of dissolved organic carbon (DOC) and secondly related to increases
in the release of major ions and heavy metals from active rock glaciers in high
mountain regions.
Dissolved Organic Carbon. DOC is an important constituent of many
natural waters. It is generated by the partial decomposition of organic matter
and may be stored in soils for varying lengths of time before transport to surface
waters. The humic substances generated by organic matter decomposition
2 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
impart a characteristic brown color to the water due to the absorption of visible
light by these compounds. DOC thus influences light penetration into surface
waters, as well as their acidity, nutrient availability, metal transport and toxi-
city. During the past decades rising DOC concentrations have been observed
across much of the British Isles, parts of Central Europe, and northeastern
North America. When first observed, these increases were widely interpreted
as evidence of climate change impacts on terrestrial carbon stores due to rising
temperatures and the increasing frequency and severity of summer droughts
(4–6). Increasing precipitation could also lead to increasing DOC concentrations,
first by increasing the proportion of DOC-rich water derived from upper organic
horizons of mineral soils and secondly by reducing water residence time, and
hence DOC removal, in lakes (7). Rising levels of atmospheric carbon dioxide,
influencing plant growth and litter quality were also proposed to explain
increased rates of DOC production.
Studies over a 35-year period (1970–2004), most of the inter-annual varia-
bility of concentrations of dissolved organic matter (OM) in 28 Scandinavian
river catchments were explained by discharge and concentrations of sulfate,
with discharge being the more important driver. Despite the heterogenicity of
the catchments with regards to climate, size, and land use, there was a high
degree of synchromeity in chemical oxygen demand (COD), a common proxy
for O, across the entire region. Multiple regression models with discharge and
sulfate concentration explained up to 78% of annual variability in COD, while
other candidate drivers, air temperature, and chloride concentration in river
water added little explanatory value (8).
During the period 1990–2004, the observed increases in DOC concentra-
tions in Europe and eastern North America have clearly been driven mainly
by reductions in atmospheric sulfur deposition, resulting from international leg-
islation to reduce the pollutant emissions since the 1980s (9) as sulfur deposition
has fallen, reductions in acidity and ionic strength have allowed more DOC to
remain in solution in soil water, and therefore to be leached to surface waters.
European sulfur emissions have fallen dramatically and while some further
reductions are likely, they clearly will not be on the scale of those observed in the
past. On the other hand, climatic changes are expected to continue or even accel-
erate into the future. Experimental data suggest that rates of soil DOC produc-
tion are increased under higher temperatures and in response to a shift from
anaerobic to aerobic conditions in saturated soils (10).
In coastal areas, climate change may also impact DOC through decadal-
scale variations in the deposition of sea-salt. High levels of sea-salt deposition,
which are linked to high wind speeds and hence to positive phases of the
North Atlantic Oscillation, may affect DOC solubility through a mechanism ana-
logous to the effect of sulfur deposition, by causing transient periods of acidifica-
tion and increased ionic strength, supporting DOC release.
Findings suggest that as sulfate deposition returns towards background (ie,
pre-industrial) levels, climatic factors such as discharge and temperature may
become major drivers of variability in dissolved organic matter.
Solutes in High Mountain Lakes. Remote high mountain lakes are excel-
lent sensors of environmental and climate change. Over the past two decades, a
substantial rise in solute concentration at two remote high mountain lakes in
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 3
catchments of metamorphic bedrock (gneiss and mica schists) in the European
Alps has been observed (11). At Rasass See in Italy, a high altitude lake south
of the main alpine divide, electrical conductivity has increased by a factor of
18 during the last two decades and concentrations of the most abundant ions,
magnesium, sulfate, and calcium by 68-fold, 26- and 18-fold respectively. At
Schwarzsee ob Solden in Austria, a high mountain lake north of the main alpine
divide, the solute increase was less pronounced. Electrical conductivity has
increased by a factor of 3 during the same time and the concentrations of magne-
sium, calcium, and sulfate have increased six-fold compared with values in 1985.
These high solute values cannot be explained by weathering of the meta-
morphic bedrock as has been postulated earlier for corresponding high mountain
lake water sin the Alps (12). Neither do current levels of atmospheric deposition
nor any recent direct anthropogenic impact account for solute increase. This is
particularly relevant for the nickel concentrations of 243 ug/L at Rasass See,
which are more than 20 times above the drinking water limit. The high concen-
trations can be only be explained by an increase in the mobilization and release
of solutes from active rock glaciers in the lake catchments entering the lakes and
melt water, related to the observed increase in average air temperature in the
region over recent decades (13).
Findings have been supported by other studies (14–16).
An important question is why have Rasass See and Schwarzsee reacted so
differently in respect to the solute increase, although their catchments are situ-
ated only 45 km apart and are characterized by the same geology. The probable
explanation is that the catchments different in the size of the active rock glaciers
occupy ca 20% of the catchment equivalent to 200% of the lake surface, whereas
at Schwarzsee ob Soden, rock glaciers occupy only ca 5% of the catchment, which
is equivalent to ca 30% of the lake surface, and the volume of Rasass See is four
to five times smaller than Schwarzsee. In addition, Rassas See is situated at an
altitude 100 m lower than Schwarzsee. Although these factors probably explain
the differences between lakes, the specific source and pathways of solutes and
heavy metals released from the melting rock glaciers into adjacent surface
waters is unknown.
3. Hydrology and Morphology
Climate affects freshwater ecosystems directly through social and economic sys-
tems such as land management as well as directly by precipitation and tempera-
ture. Freshwater diversity is at present affected by over-exploitation of natural
resources, water pollution, flow modification, habitat degradation and by inva-
sive alien species. In the future effects of climate change are expected to become
more prominent especially if the change is of the larger magnitude. For streams
and rivers changes in precipitation and discharge regimes are as important as
changes in temperature. Climate change will alter the dominant pattern of pre-
cipitation which in turn changes run-off and discharge, including spates and
droughts in streams and rivers. During spates, habitats may be destroyed; dur-
ing low flow, they will be silted and during base flow conditions, habitats will be
generated again (17). The fluctuations in habitat conditions will become greater
4 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
in the future and stream communities will fluctuate more. The predictability of
resources will decrease and species will have to adapt or become locally extinct.
3.1. Impacts on Lake Hydrology. Impacts will be mainly in residence
time and water level as well as through receptors and sources of stream flow.
Short residence times mean that pollutants such as nutrients from point sources
are flushed out of lake ecosystems, whereas with longer residence times they will
accumulate with likely changes in phtyoplantkton communities (18). Phyto-
plankton may increase with higher temperatures due to higher nutrient avail-
ability and eutrophication problems may be more severe. A decline in water
level due to decreased precipitation may cause changes in nutrient status and
acidity of lakes with low buffering capacities (19). Water level change also affects
phytoplankton development in lakes. Longer ice free periods potentially lengthen
the growing season for algae and aquatic macrophytes. Higher temperatures
may raise the mineralization rate of organic matter. Sediment accumulation
rate is one of the most important physical variables affecting the functioning
of lake ecosystems. It affects lake morphology, physical and chemical stratifica-
tion, the characteristics of the lake’s inhabitants, and hence the distribution of
aquatic flora particularly in littoral areas. More rainfall and increased frequency
of extreme events could increase catchment soil erosion resulting in more
allochthonous materials reaching lake sediments. Susceptibility to erosion will
depend on soil type, vegetation, and land use, but upland peat soils are already
known to be eroding quickly and could be made worse by climate change.
3.2. Hydraulics and Morphology at the Reach Scale. The pattern
and variation of current velocity within a stream reach has a major influence
on longitudinal and transverse channel morphology, species, diversity food-web
structure and ecological processes. It is often assumed that monthly or daily
means, which are often available, are sufficient to characterize flow regime. How-
ever even single events can cause substantial changes in physical habitat and
can affect ecological functioning. Gore and co-workers (20) list five major hydrau-
lic conditions that most affect the distribution and ecological success of stream
biota: suspended load; bed-load movement, water-column effects, such as turbu-
lence and velocity profile, and substratum interactions (near-bed hydraulics).
However, stream organisms are generally adapted to a wide variability in stream
discharge and can accommodate large changes (21).
3.3. Habitat Scale. At the habitat scale, subtle variations in current,
near-bed velocities, bottom roughness, grain sizes, and distribution together
influence the distribution and abundance of particular species of plants and ani-
mals. The primary need of stream macroinvertebrates is related to current, as it
provides oxygen and food particles, shapes habitats, and transports wastes. A
cross-section profile of current velocity in a straight stream has a more or less
parabolic shape. The velocity is highest in the middle of the profile close to the
surface and decreases towards the banks and bottom due to friction. In a straight
gutter, this pattern continues over long distances giving low habitat variability.
Natural streams are not straight gutters; they are irregularly shaped channels
that induce complex current velocity profiles. Turbulent flow patterns character-
ize natural streams, and continuously changing discharge causes turbulence pat-
terns to change continuously. In a review of seven case studies concerned with
links among land use, hydromorphology and river biota at different spatial scales
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 5
found that the strongest relationship was between in-stream variables or
hydraulic variables and biota rather than catchment characteristics (22).
Stream and river species are adapted to unidirectional flow, modified by
turbulence. Morphological and behavioral adaptations can be classified into the
those involving position of the organisms, such as locomotion and body shapes,
those associated with feeding or nutrient intake and physiology (eg, respiration
and temperature) and those pertaining to reproduction.
With fewer, but more extreme rainfalls events together with land use,
siltation will be an even more serious problem in stream and river ecosystems.
4. Monitoring Responses
Since 1970, freshwater biodiversity has decreased more drastically than marine
or terrestrial (23). This is a result of a complex mix of stressors and impacts (24).
The major drivers can be summarized as multiple use (such as fisheries, naviga-
tion, and water abstraction), nutrient enrichment, organic and toxic pollution,
acidification, and habitat degradation. Climate change is adding further stresses
(temperature increase, hydrological changes) and interacts in complex ways with
existing ones (25).
As with other stressors, climate change will result in complex-cause effect
chains, the link between them provided by many interacting environmental
parameters, which are directly or indirectly influenced by temperature and pre-
cipitation. The response of biota is, therefore less predictable than the response
chemical or hydrological variables. On the other hand, biotic parameters such as
species richness, community composition or functional diversity integrate the
complex effects of many stressors on freshwater ecosystems, including those
directly or indirectly associated with climate change. This is the reason for
using biotics such as phytoplankton, invertebrates or fish, for monitoring
ecological integrity of European surface waters.
Indicator as used here is a simple detectable sign of a complex process can
be used as an early warning sign of ecological change.
4.1. Biological Indicators on Lakes. A variety of organisms groups
(such as phytoplankton, macrophytes, benthic invertebrates, and fish) have
now been monitored, supplemented by hydomorphological and physiochemical
measurements.
Most biological assessment systems aim to reflect the deviation of the
observed assemblage from an undisturbed reference state, thus providing an
integrated appraisal of a water’s ecological quality. Climate change impacts
will be among the most important stressors on freshwaters in the future and
will initiate chains of processes that are complex and difficult to classify.
Given the different types of impact in cold temperature and warm ecoregions, dif-
ferent sets of indicators for lakes in cold, temperate, and warm ecoregion. Table 1
gives a list of potential indicators covering these regions.
Hydrology and Physiochemistry. The timing of ice cover is directly
dependent on winter and spring temperatures and therefor is one of the early
indicators for climate change in lakes in cold and temperate regions as are the
nature and duration of summer stratification.
6 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
Table 1. Direct and Indirect Impacts of Climate Change on Lakes. c, t, w: Variable Relevant in Cold (c), Temperature (t) or Warm (w)
Ecoregions
Category Response Indicator Justification of indicator c t w
Hydrology
Ice cover Higher air, and thus higher
water temperature,
leads to a shorter
ice cover period.
The relationship
between air temperature
and timing of lake ice
break-up shows an arc
cosine function. This nonlinearity
results in marked differences
in the response of ice break-up
timing to changes in air temperature
between colder and warmer regions.
Ice-cover duration,
timing of ice
break-up, ice
thickness
Ice cover duration
is simple to monitor,
eg, by remote sensing.
x(x)
Stratification Higher temperatures result in earlier
onset and prolongation of summer
stratification. As a result, changing
mixing processes occur and systems
may change from dimictic to warm
monomictic. A lack of full turnover
in winter might lead to a permanent
thermocline in deeper regions.
Duration of summer
stratification as
reflected by water
temperature
Water temperature
reflects the status
of lake stratification.
x x (x)
Water level Increased temperature and decreased
precipitation in conjunction with
intensive water use will decrease
water volumes. This will lead to
water level imbalances and, in many
cases, to the complete loss
of water bodies.
Lake surface Easy to monitor
by remote
sensing.
(x) (x) x
Oxygen depletion High temperatures will stimulate
phytoplankton growth, which will
lead to oxygen depletion of profundal
habitats.
Oxygen concentration
of the bottom water
in summer
The parameter is
easy to record
and often
incorporated
into routine
water chemistry
monitoring.
(x) (x) x
7
Table 1. (Continued )
Category Response Indicator Justification of indicator c t w
Physicochemistry
Sulfate
concentration
With less precipitation in El Ni~
no years
and resulting droughts, stored
reduced S in anoxic zones (wetlands)
is oxidized during drought, with
subsequently high sulfate export rates.
Elevated sulfate concentrations in
lakes will be the result.
Sulfate
concentration
Directly reflecting
the responding
parameter;
often incorporated
into routine water
quality monitoring
x(x)
DOC Rising temperatures in combination with
declining acid deposition cause increasing
DOC concentrations.
DOC Incorporated into
routine water
quality monitoring
x(x)
Acidification
effects on
phytoplankton
Acidification pulses occur due to drought
(El Ni~
no). Acidification pulses will cause
changes in phytoplankton richness
and biomass.
pH; biotic acid
indices
pH is easy to record
and often incorporated
in water chemistry
monitoring. As pH
varies seasonally
and daily, biotic
indices are often
more stable.
x
Salinity Warmer winters cause extreme rainstorms
and heavy sea-salt deposition, which
might affect water chemistry.
Acidifying
substances
These parameters
are easy to
record and often
incorporated into
routine water
chemistry
monitoring.
xx
Total Organic Carbon
(TOC) runoff
patterns
Warmer winters produce higher levels
of runoff TOC release with
subsequently increasing
TOC water concentrations.
TOC levels and/or
absorbance
(water color)
Water TOC
concentrations
reflect changes
in runoff and
input of
allochthonous
material.
x
8
Primary
production
Water temperature
effects on
phytoplankton
Increasing water temperatures
lead to shifts from a dominance
of diatoms and cryptophytes to
cyanobacteria. This effect
is especially pronounced at
temperatures >20C, since
cyanobacteria (especially large,
filamentous) and green algae are
favored at higher temperatures.
Phytoplankton biomass
and composition,
cyanobacterial algal
blooms
The shift in
community
composition
gives information
about the response
of biota to changed
lake characteristics
as water temperatures.
Phytoplankton
community composition
is routinely monitored
for the Water
Framework Directive.
xx(x)
Water temperature
effects on
macrophytes
Inter-annual variation in water
temperature results in deeper
macrophyte colonization, greater
wet weight biomass, and an increase
in whole lake biomass.
Water temperature The parameter is
easy to record and
often incorporated
into routine
monitoring
programs.
x
Secondary
production
Water temperature
effects on
zooplankton
Higher water temperature leads to
shifts in zooplankton community
composition. Higher, earlier population
growth rates of Daphnia and earlier
summer decline occur due to higher
spring temperatures. As a result, higher
Daphnia biomass leads to earlier
phytoplankton suppression and a
shift from a dominance of large-bodied
to smaller species.
Zooplankton biomass
and composition,
size classes
The response of
zooplankton
(although not
monitored for the
Water Framework
Directive) might
be a good indicator
for changes in food
web dynamics due
to temperature
increase.
(x) x (x)
Water temperature
effects on cold
water fish
Higher water temperatures (especially in
the epilimnion) lead to the progressive
reduction of thermal habitats for,
eg, Salvelinus namaycush. As a result,
cold-water species will disappear from
littoral areas in spring and summer.
Furthermore, higher water temperatures
will reduce reproduction success of
cold-water species and increase parasitic
and predator pressure on the egg and
young life stages.
Summer water
temperature or
air temperature
Water temperature is
easy to measure, but
even air temperature
reflects warming up
of mixed layer
temperature.
xx
9
Table 1. (Continued )
Category Response Indicator Justification of indicator ct w
Spread of alien
species
Higher temperatures often
favor alien fish,
macrophyte or
macroinvertebrate species.
Share of alien
species in the
community
This parameter can often
be inferred from routine
monitoring for the Water
Framework Directive.
(x) x x
Food webs
Water temperature
effects on food
webs
Increased water temperature
generates principal shifts
in food webs. As cyprinid
planktivorous fish species
are supported, large
zooplankton species
are suppressed and
grazing intensity is
reduced.
Proportion of
planktivorous
and piscivorous
fish species;
proportion of large
and small
zooplankton
species
Food web structure is well
reflected by these two
parameters. The share
of large zooplankton
species determines the
effects on phytoplankton,
the share of planktivorous
species determines the
effects on zooplankton.
xxx
Category: ecosystem component being affected by direct or indirect climate change effects. Response: describes how the variables change under the stressor considered. Indicator: a
judgemental selection of the variables that most clearly reflect climate change.
10
Primary Production. Climate sensitive physiochemical and hydrologic
conditions can be major determinants for primary production in lakes. Phyto-
plankton community composition may be altered by changes in winter and
spring temperatures depending on lake type and location (Annesville) (26).
The phytoplankton assemblages of shallow cold water ecosystems seem to be
especially sensitive to temperature changes. A shift towards dominance of cyano-
phytes in warmer water, with the possible implications for water quality, is
widely predicted and may lead to the loss of phytoplankton biodiversity. Models
suggest that cyanobacterial dominance will be greater if high water tempera-
tures are combined with high nutrient loads. At low nutrient levels, the effect
of water temperature is reduced considerably.
Generally increased phytoplankton productivity and biomass are correlated
with higher spring water temperatures as well as changes in hydrochemistry
such as increased nutrient availability (27). Earlier stratification and deeper
thermoclines may have an opposite effect, however. Improved light conditions
affect phytoplankton biomass. The better light conditions during warmer winters
with shorter ice cover and less snow promote phytoplankton growth in winter,
even doubling chlorophyll alpha levels (28). Increasing spring temperatures
and light availability can cause phytoplankton species to grow earlier in sum-
mer, nutrient limitation may occur earlier. Larger growths can be expected
in autumn and winter benefitting from higher temperatures and delayed light
limitations before ice forms.
Secondary Production. Trends in average temperature sometimes corre-
late significantly with changes in zooplankton community compositions, even over
comparatively short periods of 10–15 years (29). Water temperature may be asso-
ciated with a shift of zooplankton assemblages form larger to smaller bodied
forms, a shift in food availability to inedible cyanobacteria and possibility heigh-
tened sensitivity to algal toxins. Individual taxa have very different threshold or
maximum temperatures for growth that are species specific rather than func-
tional group specific. A shift from Daphnia galeata to smaller D. cucullata has
been observed with higher spring temperatures (30). Changes in the vertical tem-
perature gradient of a lake may affect zooplankton vertical migration. In warmer
months zooplankton occurred closer to the surface. There are, however, powerful
effects of predation and zooplankton communities that may mask climate effects.
Community composition and species richness in lake fish communities is
strongly related to air temperature. For many species, the relationships with
temperature have long been investigated, eg, Coregonus albula, an autumn
spawning fish species, is vulnerable to spring temperature increases because of
a timing mismatch of hatching and the spring development of zooplankton, in
combination with higher rates of predation by warm-water predatory fish species
(31). Another threat for this species is a general reduction in occurrence of the
cold oxygen-rich hypolimnetic conditions that it requires in summer (32). Gener-
ally, higher water temperatures increase growth and production for warm water
fish and inhibit growth and production for fish at or above their thermal
optimum (33).
Changes in precipitation and temperatures may have opposing effects on
fish populations. Exceeding water temperature thresholds may limit the survival
of fish in lakes. More fish die from oxygen deficiency or physiological stress in
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 11
warmer water (33). On the other hand in shallow eutrophic lakes, winter fish-kill
caused by low dissolved oxygen under ice will be reduced or eliminated.
Increased hypolimnetic temperatures may lead to a loss of juvenile fish requiring
cool water as a summer refuge, thus climate change can eliminate fish popula-
tions at the margins of their range (34). Hydrologic changes together with
changes in temperature will probably favor invasive species over rare and
threatened native species (35).
Food Webs. The principal alteration in lake food webs caused by global
warming may be a reduction in zooplankton grazing intensity, leading to eutro-
phication effects. This is caused by a variety of factors; the density of planktivor-
ous cyprind fish species is enhanced (36) and the density of piscivorous species
reduced, which may lead to a strong top-down control of large zooplankton spe-
cies (37). Warmer spring temperatures may disrupt food web linkages between
phytoplankton and zooplankton because of different sensitivities to warming.
The timing of thermal stratification and spring diatom growth may advance sig-
nificantly with increasing spring temperatures. Thus, a long-term decline in
Daphnia populations, frequently a keystone herbivore, may also be associated
with an expanding temporal mismatch with the spring diatom bloom (38). A
timing mismatch between phytoplankton maxima and the peak abundance of
Daphnia may lead to the absence of a clear water phase. That is the feature of
many lakes in late spring. Even modest warming (>2C) during a short but
critical seasonal period may induce changes in whole lake food webs and thus
alter entire ecosystems.
4.2. Biological Indicators of Rivers. Table 2 gives a selection of
potential indicators for climate change impacts for small rivers in cold,
temperate and warm ecoregions of Europe. As for lakes, many of the biotic indi-
cators could be easily incorporated into routine monitoring programs, eg, by
adding indicators reflecting the impact of temperature change on benthic inver-
tebrates. Physiochemical variables are most appropriate as early warning
indicators.
Primary Production. Changed runoff and water temperature are expected
to cause changes in riparian vegetation (39) and may enhance macrophyte and
algal growth. Lower water levels and increased nutrient availability will lead to
greater proportion of terrestrial plant species in floodplains (40). In Fennoscan-
dia, macrophtye species richness decreases with latitude and altitude, mainly
due to decreased July temperature. The macrophyte biodiversity is expected to
respond strongly to climate change (41).
Secondary Production. Water temperature, flow regime, channel mor-
phology and sedimentation, which are all subject to impacts from climate change
are decisive for invertebrates and fish. Key variables are summer and maximum
temperatures.
Invertebrates. The vulnerability of freshwater organisms to the direct
and indirect effects of climate change can be estimated by the ecological prefer-
ences of species. Species may be classified as follows:
Species with limited distribution (endemic species) are characterized by a
restricted ecological niche and limited dispersal capacity, and are thus
more affected by climate change then widely distributed species (42).
12 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
Table 2. Direct and Indirect Impacts of Climate Change on Rivers. c, t, w: Variable Relevant in Cold (c), Temperate (t) or Warm (w)
Ecoregions
Category Response Indicator Justification of indicator c t w
Hydrology
Decrease in ice
cover duration
Higher temperatures will
reduce ice cover duration.
Ice cover duration Ice cover is a key factor
for the productivity
of boreal aquatic
ecosystems and
easy to monitor.
x(x)
Increase in drought
frequency and
duration
Decreased summer
precipitation and
increasing air temperature
in some parts of Central,
Eastern and Southern
Europe change the
character of several
small streams from
permanent to temporary.
Drought periods As gauging stations
are not installed in
most small headwater
streams, drought
periods can be easily
recorded by visiting
the respective streams.
xx
Change of permanent
to intermittent
regime
Due to less precipitation
and increased demand
for freshwater, higher
temperatures, and higher
transpirations, many small
rivers will become
intermittent with long
dry phases in summer.
Drought periods As gauging stations
are not installed in
most small headwater
streams, drought
periods can be easily
recorded by visiting
the respective streams.
(x) x
Morphology
Increased fine
sediment entry
Extreme precipitation
events increase surface
runoff and lead to large
amounts of fine sediments
entering the streams;
sediments accumulate
and clog the bottom
interstitial.
Number and discharge
or flood events in
unusual seasons
(recorded by gauging
stations)
Extreme precipitation
events will wash out
fine sediments from
adjacent cropland and
other land-use types.
They are well reflected
by the discharge
of a river.
(x) x (x)
13
Table 2. (Continued )
Category Response Indicator Justification of indicator c t w
Physicochemistry
Increase of eutrophicating
substances
N flux in the runoff and
decomposition of soil
organic matter increases
with temperature, which
increases nutrient
concentrations.
Eutrophication is
further promoted by
high water retention time
through low discharge, while
denitrification counteracts
this effect.
Nitrate, total N,
phosphate
Nutrients are routinely
monitored in most
European countries.
xxx
Reduced water quality Increasing water temperatures
enhance production and
decomposition intensity,
thus leading to oxygen
depletion, particularly at
night.
Saprobic indices Saprobic indices reflect
the organic load in
streams and eventually
the oxygen content.
Species with a high
oxygen demand
(typical for low saprobic
indices) will disappear
while species with a
low oxygen demand
(typical for high
saprobic indices)
will benefit.
(x) x x
‘Potamalization’– effects
on nutrients
Higher water temperatures lead
to a more rapid mineralization
of organic matter (leaves, wood)
and thus to eutrophication effects.
As a result, small streams
(‘rhithral’) will change character
and resemble larger rivers
(‘potamal’).
Water temperature
(maximum monthly
values)
The response of
communities is mainly
determined by extremes;
secondary effects
(eg, oxygen depletion
at night times) are
most extreme in
summer.
(x) x (x)
14
Acidification Increased precipitation increases
acid runoff from borealic coniferous
forests leading to cascading
acidification effects on aquatic
biota.
pH, invertebrate-based
acid-indices
pH-values decrease
with increasing acid
deposition. Since these
events are of short
duration, community
based indices are often
better at reflecting
acidification.
x
Primary
production
Increased
macrophyte/algae
growth
Higher water temperatures and
lower discharge enhance
macrophyte and algae growth.
Furthermore, higher temperatures
increase mineralization processes
and deliver more nutrients for
macrophyte and algae growth.
Water temperature
(mean monthly values),
macrophyte coverage
Mean monthly
temperatures indicate
overall temperature
increase. Macrophyte
coverage is simple to
record and well
correlated with
biomass.
xxx
Secondary
production
and food webs
Increase of respiration
rate
The metabolic rates of bacteria
and fungi and the metabolic
rates of detritivorous species
will rise with increasing
temperatures. The proportion
between primary production
and respiration will decrease.
Percentage of collectors
in the invertebrate
community
Collectors gather organic
material. If this food
source increases, the
percentage of collectors
rises to about 40%.
x x (x)
Reduced availability
of leaves
Processing rates of leaves and
wood increase with temperature.
Floods in winter cause more
than 50% of leaf inputs to be
exported, leaving little detrital
material available for
invertebrate consumption.
Share of the feeding type
‘shredder’in the
invertebrate
community
Benthic invertebrates
are routinely monitored
in most European countries.
If the availability of leaves
decreases, the share
of shredders will decrease,
too; in small headwater
streams, shredders should
typically account for 30%–40%
of the invertebrate community.
x x (x)
Replacement of cold
water species
(fish, macroinvertebrates)
Many fish and invertebrate
species in cold regions are
highly adapted to cold water
temperatures (cold stenotherms)
and vanish with higher
temperatures.
Water temperature
(maximum monthly
values)
Physiologic barriers are
mainly determined by
extremes. For cold
water species, these
are too warm temperatures
in crucial phases of their
life cycle.
x x (x)
15
Table 2. (Continued )
Category Response Indicator Justification of indicator c t w
Increase or decrease
of species number
Low temperatures are a
migration and physiological
barrier for many aquatic species.
With temperature increase,
several species can invade
rivers in cold ecoregions.
In contrast, in temperate
and warm regions, increasing
water temperatures lead to the
extinction of cold stenothermic
taxa.
Number of species
(eg, fish, selected
invertebrate groups)
The increase of species
numbers is best evaluated
by a simple richness index,
eg, the number of species
that can be easily inferred
from routine monitoring
results.
xxx
Increase of
r-strategists
Invertebrate r-strategists
benefit from unpredictable
flood events, eg, in summer,
which remove most invertebrates
and thus favour species rapidly
colonizing the competition-free
space.
Number and discharge
of flood events in unusual
seasons (recorded by
gauging stations)
Unusual hydrological events,
eg, floods in summer,
cause catastrophic drifts
of invertebrate species
and favor r-strategists.
(x) x x
Changes in life
strategies
If small rivers become intermittent,
species with a bivoltine or semivoltine
life cycle cannot survive and the
community will change to univoltine
species with an early emergence
period.
Drought periods As gauging stations
are not installed in
most small headwater
streams, drought
periods can be easily
recorded by visiting the
respective streams.
(x) x
‘Potamalization’—
effects on
invertebrates
Higher water temperature leads to the
disappearance of species adapted
to cold water temperature and
the associated high oxygen content,
eg, several stonefly (Plecoptera) species.
They are replaced by species typical
for warmer water previously colonizing
more downstream reaches. Thus,
invertebrate species typical for
small streams (‘rhithral’) will be
replaced by species from
larger rivers (‘potamal’).
Share of invertebrate taxa
preferring the metarhithral
(trout zone)
Benthic invertebrates
are routinely monitored
in most European countries.
The response of the
invertebrate community
to temperature increase
is reflected by their
longitudinal zonation
preference. The share
of ‘metarhithral taxa’
in an unimpacted small
stream differs between
ecoregions, but should
typically be around 50%.
(x) x (x)
16
Replacement of salmonid
by cyprinid fish species
Higher water temperatures will reduce
reproductive success of salmonid
species and increase parasitic and
predator pressure on the egg and
young larval stages. Warm water
cyprinid species will invade in cold
water regions.
Water temperature
(maximum and minimum
monthly values); fish
species composition
The eggs of salmonid species
need high oxygen
concentrations,
which will be reduced
by higher water
temperatures. Parasites
and fungi benefit from
high temperatures.
xx(x)
Standing stock of
cold water fish
Brook trout populations could either
benefit from increased growth rates
in spring and fall or suffer from
shrinking habitat and reduced
growth rates in summer, depending
on the magnitude of temperature
change and on food availability.
Abundance and biomass
of brook trout
Brook trout is a keystone
species in most Northern
European countries.
xx(x)
Spread of alien
species
Higher temperatures often favour
alien species that increasingly
colonize small streams. These could
be alien fish, macrophyte or
macroinvertebrate species.
Water temperature
(maximum and minimum
monthly values); share of
alien species in the
community
The survival and reproduction
of several alien species in
temperate ecoregions is
controlled by minimum
temperatures. The second
parameter can often
be inferred from routine
monitoring for the
Water Framework
Directive.
(x) x x
Category: ecosystem component being affected by direct or indirect climate change effects. Response: describes how the variables change under the stressor considered. Indicator: a
judgemental selection of the variables that most clearly reflect climate change.
17
Species inhabiting large rivers characterized by relatively high water
temperatures are generally physiologically adaptive and may react to glob-
ally rising temperatures by colonizing upstream river reaches; species
inhabiting springs cannot move further upstream and are thus more threa-
tened (43).
Species adapted to low water temperatures (cold stenothermic species) are
threatened by climate change more than eurythermic species) (44).
Insect species potentially endangered by climate change are unevenly dis-
tributed in Europe, but there are also differences between individual orders.
Three orders were used in studies in Europe on the distribution and ecological
preferences. The three are freshwater mayflies (Ephemeroptera), stoneflies (Ple-
coptera), and caddisflies (Trichoptera). For all three orders, a high proportion of
species in the Mediterranean ecoregions and in high mountain areas is highly
vulnerable to climate change. Patterns of endemism are similar for all three
insect orders.
In general, a south-north gradient in species richness of aquatic insects can
be observed. Similar patterns are found for endemic species and to a lesser
degree, for crenal specialists and cold-stenothermic species. The patterns are
mainly a result of the fluctuations in continental ice cover during the Pleistocene,
which in turn, caused several range extensions and regressions of species (45).
While glaciers covered most of Northern Europe, species retreated to Southern
Europe or to ice-free parts of the mountains. The isolation of populations resulted
in many new species and increased diversity in these areas. Most aquatic insect
species occurring in Northern Europe live in central or Southern Europe also.
Mainly generalists and species with a high dispersal capacity recolonized North-
ern Europe after the last ice age, while specialist species and those with limited
dispersal capacities extended their range only slightly. In consequence, most of
the species in Northern Europe are likely to be capable of resisting climate
change since they are generalists or able to rapidly colonize other areas.
Fish. Temperature and hydrological factors are major environmental
determinants for fish communities, thus alterations induced by climate change
are expected to modify fish assemblage structure (46). Changing river discharges
causes a reduction on community richness and life cycle changes (44). Water tem-
perature may be a threat to cold stenothermic fish species because habitats for
cold stenotherms decline, isolating them in increasingly confined headwaters
(44). As a result fish assemblages are expected to shift to fewer cold-water taxa
and more warm water taxa, as well as fewer northern taxa and more southern
taxa (47). As for invertebrates, fish responses to water-temperature increase
are highly species-specific and depend on individual thresholds. For example
the size of Atlantic salmon is negatively correlate with spring air and water tem-
peratures and with discharge and precipitation. Increases in winter temperature
and ice break-up may affect winter survival significantly, particularly in northern
populations. Because energetic deficiencies are assumed to be an important cause
of winter mortality, strong negative effects on the energy budget can be expected.
4.3. Biological Indicators of Wetlands. Several variables may reflect
the effects of climate change on wet lands ecosystems particularly on processes and
functioning. Table 3 lists suitable indicators for climate change impacts on wetlands.
18 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
Table 3. Direct and Indirect Impacts of Climate Change on Wetlands
Category Response Indicator Justification of indicator
Hydrology
Ice cover duration Increased air temperatures
lead to a later thaw.
Date of ice break-up Indicates direct temperature
effects. Influences length
of season.
Retention of flood water Increased temperature may
lead to increased rates
of evaporation.
Water table height Retention of flood water will be
enhanced if the water table is
lowered but reduced if the water
table is higher.
Recharge of groundwaters Ability to recharge aquifers
is affected by desiccation.
Water table height If the water table is high, the rate
of recharge of groundwater (if any)
will be increased.
Retention of sediment Scouring of sediments by extreme
weather events.
Frequency and severity
of storms
Storms and associated flash floods and
spates may wash away sediments
and detritus, reducing their retention.
Physicochemistry
Acquisition of carbon I Warm and wet conditions together
increase carbon acquisition.
Temperature and
precipitation
Indicates gross C dynamics.
Acquisition of carbon II Early snow melt followed by wet
and warm conditions lead to high
carbon acquisition through
photosynthesis.
Date of snow melt,
temperature and
precipitation
A combination of early spring plus wet
and warm weather promotes
vegetation growth. Photosynthesis
sequesters carbon, while rates
of respiration remain comparatively
stable year to year. Thus, carbon is
accumulated in these conditions.
Export of organic carbon Organic carbon is provided for
downstream ecosystems
in runoff water.
Water table height Organic carbon can be released as
dissolved organic carbon (DOC)
in runoff water.
Release of CH
4
Lowered water table reduces
CH
4
emission.
Water table height Water table height and greenhouse gas
emission are directly correlated.
Retention of carbon Retention of carbonaceous material
will be enhanced if warmer temperatures
increase primary production while water
availability is sufficient, but will be
reduced if runoff events increase in
frequency.
Rate of primary
production
Retention of carbon in vegetation and
detritus will be enhanced by increased
production.
19
Table 3. (Continued )
Category Response Indicator Justification of indicator
Mineralization Lowered water table stimulates enzyme
activity leading to increased
mineralization.
Rates of enzyme activity Indicates carbon loss from peat.
Export of nutrients Increased production leads to greater
provision of organic detritus, which
is then available for export downstream.
Production of litter Export of nutrients will increase if
detritus production is high.
Retention of nutrients Longer season and warmer temperatures
can lead to increased primary production.
Rate of primary production Retention of nutrients is potentially
greater where production is high.
Primary production
Tree survival Increase in flooding can lead to progressive
replacement of forest with bogs.
Water table height Water table height and tree survival
are directly correlated.
Vegetation
assemblages
Elevated water table leads to increase in
bryophytes and reduction of shrubs in
the bog, but increase in graminoids and
forbs in the fen. Lowered water table leads
to increased proportion of dicotyledonous
plant species.
Water table height vegetation
assemblages
Water table height and vegetation
assemblages are directly
correlated.
Vegetation type Lowered water table leads to succession
to forest-type vegetation from the
graminoids and mosses occurring in
pristine conditions.
Water table height, occurrence
of higher plant species
Water table height and vegetation
assemblages are directly
correlated.
Secondary production
Insect species Milder winters and hot summers are
important factors in the survival
of temperature-sensitive species.
This will probably alter the tolerable
ranges of some species, including pest
species, and may lead to increased
invasions into new areas by exotic
species.
Taxonomic composition and
abundance of insect
species, especially
butterflies and aquatic
insects
Indicates impacts on habitat
integrity.
Bird migration Spring migrations start earlier with
warming. This is more pronounced
early in the season and with terrestrial
and wetland birds than with waterfowl.
Beginning of spring
migration
period
20
Food webs
Ecosystem support Ecosystems potentially will suffer if detritus
and species are lost due to severe flooding
and runoff events, and if drought levels
exceed the tolerance limits of species.
Frequency and severity
of storms
Reduction of biomass and species
due to wash out.
Support of food webs Drought and flooding both contribute
to mineralization and release of nutrients
from organic matter. This can increase the
build up of plant-available nutrients in
the sediments, which are readily washed
into water courses and wetlands in runoff.
Frequency and severity
of storms
Increase in eutrophication may result
from large runoff events.
Category: ecosystem component being affected by direct or indirect climate change effects. Response: describes how the variables change under the stressor considered. Indicator: a
judgemental selection of the variables that most clearly reflect climate change. As less data than for lakes and rivers is available we do not distinguish between wetlands in
different climatic regions.
21
Hydrology. Wetland hydrologic regimes face impacts from both land use
and climate. A rise in ambient temperature will result in increased water
abstraction, and, with more frequent droughts predicted, irrigation demands
will increase. Changes in land use may require the construction of dams, with
barrier effects for hydrological conditions. This has implications for the size
and spatial distribution of wetlands. Additionally, nutrient enrichment and pol-
lution are possible consequences of prolonged growing seasons and urbanization.
Changes in precipitation, evaporation and temperature determine the ground-
water level, which influences the wetland cover cycle, the transition between per-
manent and temporary wetlands and hydrochemical variables (48). Water table
height reflects the influence of climate change in many wetland types.
Physicochemistry. Mineralization and release of nutrients are deter-
mined by hydrology and temperature. Moisture and temperature influence
microbial enzyme activity and decomposition rates. Drier, warmer conditions
could stimulate nutrient mineralization and enhanced release from sediments
to runoff waters. Mineralization of C, N, and P may differ significantly among
wetland types. In bog peats, nitrogen mineralization and carbon dioxide produc-
tion may decrease with increasing ambient temperatures and lower water tables,
whereas in fen peats, nitrogen mineralization may decrease and methane pro-
duction may increase with higher water tables, but the generality of these find-
ings is questionable. Stored reduced sulfur in anoxic zones of wetlands oxidizes
during drought periods. Owing to the subsequent efflux into streams and lakes,
sulfate concentrations and acidity can increase after droughts. Carbon acquisi-
tion may increase under warmer and wetter conditions, whereas in warmer
and drier years, wetlands experience significant carbon losses (49). Climate
change is predicted to cause a doubling of net total C loss rates in wetlands. Max-
imum soil temperatures are correlated with maximum methane emission values,
whereas reduced water table levels suppress methane emissions. Thus, long-
term climatic changes with less precipitation and decreased water tables may
reduce the incidence of methane from wetland. Nonetheless, lowered ground-
water changes due to climate change may lead to increased nitrous oxide fluxes
in natural peatland soils. Under extreme drought, emissions may increase expo-
nentially with linear decrease in the water table.
Primary Production. A key driver of changes in the community composi-
tion of wetland vegetation is the altered hydrological regime. Drought results in
a proportional loss of native species, an influx of invasives, and a community
change towards dicotyledonous species in northern wetlands. In bogs increased
temperature, together with an experimentally lowered water table, caused a 50%
increase in the cover of shrubs, and a 50% decrease in the cover of graminoids
(50). More frequent droughts cause wetland vegetation to become both woodier
and drier. Pond-meadow wetlands may acquire more species, particularly non-
wetland species (46).
Soil temperature rise cause a shift in the productivity of plant communities.
A higher water table caused bryophyte productivity to increase in bog samples,
while shrub productivity was lower (51). Altered riparian vegetation and an
altered biomass and productivity, affects detritivores and results in lower decom-
position rates. Decomposition in river marginal wetlands is highly dependent on
precipitation, whereas climate change or river flow management could disrupt
22 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
floodplain nutrient dynamics. Species sensitivity to climate change is dependent
on plant traits and niche properties. Besides water table height, which reflects
many of these changes to some degree, the occurrences of certain species and
vegetations assemblages may be used as indicators for climate change in plant
communities (Table 3).
Secondary Production. Lower water tables, increased temperatures and
more frequent drought lead to a loss of habitat for obligate wetland species.
Invertebrates are affected directly by changing water table or temperature but
also indirectly by shifts in nutrient availability. Invasive species, exotic plants
and changed nutritional quality of litter affect detritivores. Other taxa affected
by increased drought, weaker spring flows, and reduced inundation are fish,
amphibians, waterfowl, and muskrat (44).
Climate change advances in the spring arrival of migrating birds, in both
short and long distance migrants (52). It also changes the winter distribution
of shorebirds. Further climate change alters the ranges and population state of
differing breeding bird species and populations. The impact of global warming on
terrestrial and wetland birds is more evident than on waterfowl. Variability of
precipitation in wetlands affects population and community dynamics of wetland
birds owing to egg and nesting predation, which negatively correlated with water
levels in wetlands (53). Suitable indicators might include the beginning of the
bird’s migration period and metrics related to taxonomic composition of indicator
taxa groups (see Table 3).
5. Acid Deposition
Chronic emissions of sulfur and nitrogen compounds to the atmosphere, long
range transport and the resultant deposition of S and N pollutants have cause
acidification of freshwaters over large regions of Europe and eastern North
America (54). Ecological damage includes loss of salmon and trout populations
from thousands of water bodies and changes in the species composition of inver-
tebrate, aquatic macrophyte and agal communities. Two factors are need for
acidification: the water must be acid-sensitive and the area must receive suffi-
cient amounts of acid deposition (55). Acid-sensitive lakes and streams are
found throughout the world in catchments with weathering-resistant bedrock
such as granite and quartzite and young, often poorly developed podsolic and
organic rich soils. In these waters, the dominant inorganic anion is usually the
weak-acid bicarbonate HCO
3
, whose source is from plant roots and dissolution in
the soil water. Calcium and magnesium ions are also found. Acid-sensitive
waters have low concentrations of these ions. The second factor is the amount
of acid deposition. The most sensitive waters are affected when the pH of rain
is about 4.7. In acidic waters sulfate is usually dominant and replaces HCO
3
.
These waters are also rich in aluminum. The acid and aluminum are toxic to
fish and other aquatic species.
5.1. Climate Effects. Climate change can affect the chemical and biolo-
gical recovery of freshwaters from acidification. Long-term, seasonal and episodic
changes in climate all potentially affect a variety of processes in catchments and
surface water bodies. For example, warming can be expected to increase rates of
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 23
mineralization of soil organic matter, which in term might release nutrients such
as nitrogen in the catchments in runoff. The most biologically damaging effects of
acidification often occur during short acid episodes in lakes and streams. These
episodes typically coincide with climatic extremes such as droughts, storms,
snowmelts and periods of winter freezing. Acid pulses following storms have
been documented from Ontario, Canada (56). The prognosis for future climate
in Europe is generally warmer and wetter in the north but drier in the south
and stormier in all areas with more frequent extremes. Climate change may
reverse some of the ongoing recovery as S and N emissions are declining.
There are three approaches for the study of the effect of climate change on
acid deposition of surface waters. The first is the analysis of data collected reg-
ularly at one site over time, the second is experimental either in the laboratory,
in mesocosms, or in large-scale whole ecosystems. The third is by modeling.
5.2. Modeling. Climate change and acid deposition have many connec-
tions, from the economic activities that give rise to emissions of gases and parti-
cles into the atmosphere to long range transport of S and N, to their combined
effects on terrestrial ecosystems and finally on their effect of aquatic ecosystems.
For all these links, models have been used to evaluate future scenarios.
A switch from fossil fuels to renewable energy sources will entail both a
reduction in greenhouse gases and a reduction in emissions of S and N com-
pounds. A study by Mayerhoff and co-workers looked at linkages between climate
change and regional air pollution in Europe (57) using the Integrated Model to
Assess the Global Environment for climate and the Regional Air Pollution Infor-
mation and Simulation model (RAINS) for air pollution. One of the tradeoffs is
that decreased emissions of S will result in fewer particles in the atmosphere,
which in turn, will increase solar radiation inputs through reduced cloud
formations.
There are many processes in terrestrial ecosystems that will affect the
amount and chemical composition of runoff. The combined effects of these pro-
cesses can be quantified by the use of process-oriented biogeochemical models
MAGIC is such a model. It has been used to study the potential effect of climate
change on soil and water chemistry. Wright and co-workers used MAGIC to
examine the relative sensitivity of eight major climate-sensitive processes on
the recovery of soil and water from acidification (58) (see Table 4). They found
several factors are of minor importance, several are important in only specific
areas and several are of widespread importance. The effect of climate change
on production and loss of organic acids is one of the most important processes,
and mineralization and loss of N from soil is another.
Dissolved organic carbon (DOC) has been the focus of other modeling inves-
tigations. The INCA-C model (Integrated Catchments Model for Carbon) pre-
dicted increases in stream water DOC as a result of warming but does not
fully account for the full effects of increased temperature on DOC and surface
water acidity. A further study using exploratory simple empirical DOC models
indicated changes in S deposition or temperature could have a confounding influ-
ence on the recovery of surface waters and the corresponding increase in DOC
may offset the recovery of pH due to reductions in S and N depositions (59).
For more information and other studies refer to Climate Change Impacts on
Freshwater Ecosystems (see Acknowledgment).
24 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
6. Distribution of Persistent Organic Pollutants and Mercury
in Freshwater
Persistent organic pollutants and some trace metals (mercury, cadmium,
and lead are toxic substances released into the atmosphere by a range of
urban, industrial, and agricultural processes. Once emitted into the atmosphere,
these pollutants are widely dispersed and are deposited onto waterbodies. They
can enter the food web where they bioaccumulate and become toxic to aquatic
and terrestrial organisms. Many of the diverse aspects of climate change (ie,
temperature increase, variations in rainfall, wind patterns, and dust deposition)
affect the distribution and mobility of the toxic substances in freshwater systems.
Volatile heavy metals and polychlorobiphenyls (POPs) present a much more
cryptic problem than those of habitat change, acidification, or eutrophication.
Their effects on communities are largely unknown and are difficult to trace,
because they are generally sublethal. Health implications for fish and their
human consumers are nonetheless clear and the volatility and condensation
effects make it very likely that a warmer climate in particular will increase
their transfer to polar and mountain regions. One of the most poignant
observation of recent years has been the widespread contamination even in the
Antarctic (60).
However, in the areas closer to the emission sites, warmer temperatures
may result in higher concentrations of these compounds in air than at present.
This change will increase the dispersion capacity of hydrophobic organic pollu-
tants and mercury, leading to higher rates of toxification of humans and higher
organisms through respiration. This effect will also be important for PAHs
although in this case the ultimate environmental impact will depends on the
balance between improved combustion sources and future energy demands,
which may increase following population and wealth growth.
The implications of increased temperatures will be similar for mercury.
Transformation into methylmercury may be enhanced at higher temperatures
due to increased microbial activity. Higher temperatures in Arctic and northern
Table 4. Summary of the Relative Importance of Factors Drivers by Climate Change
that Affect Soil and Water Acidification
a
Factor
Potential effect
on ANC Importance Relationship to climate
Sea salts Decrease Coastal sites only Empirical links to NAO
Dust Increase Southern sites only Empirical links to NAO
Runoff Increase Medium Precipitation, temperature
from GCMs
Weathering Increase Low Temperature, from GCMs
Organic acids No change High Poorly understood
Soil pCO
2
No change Low From GCMs
Forest growth Decrease Low Temperature, moisture,
pCO
2
, complicated
Organic matter
decomposition
Decrease High Temperature, moisture,
complicated
a
Modified from Ref. 58.
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 25
environments, where cold conditions currently limit methylation and mercury
mobility, are of concern. In Scandinavia, thousands of lakes have fish with
mercury levels above the health line. The projected decreases in atmospheric
precipitation may further increase these levels due to a higher proportion of
groundwater percolating through mercury-rich soils.
These possible consequences are detailed below.
6.1. Effects of Temperature Increase. Temperature influences the
majority of physicochemical properties and processes that determine the
environmental behavior of chemical compounds, affecting thermodynamic
aspects (eg, equilibrium constants, partition constants, absorption isotherms,
vapor pressure, and solubility) as well as kinetic aspects (transport and reaction
rates). Hence variations in temperature can affect the dynamics, transport, and
fate of contaminants in the environment especially in aquatic environments.
Temperature also determines the relative proportion of water phases in
each ecosystem. Warmer climate leads to higher air humidity and less ice and
snow in cold areas. Longer ice-free periods can increase catchment soil erosion
elevating the release of previously deposited contaminants bound to the soils,
but modeling and quantitative evaluation of these processes is difficult due to
the complexity of the interactions and to the uncertainties in many of the para-
meters implied.
6.2. Organohalogen Compounds. Many POPs have a vapor pressure
that enables a certain level of volatilization. Hence, they can enter the atmo-
sphere in warm or temperate zones of the planet and then condense or accumu-
late as solids or liquids in cold areas. This phenomenon, known as the global
distillation effect, depends primarily on temperature (61).
Variations in temperature due to climatic change can have significant effect
on the dynamics of these pollutants, affecting their long range bioaccumulation,
bioavailability, biodegradation, and above all, environmental persistence and
incorporation into trophic zones. For example, the total quantity of PCBS pro-
duced on earth is about 1.3 million tons, 97% of which have been produced in
the Northern Hemisphere. The majority of these compounds remain trapped in
the soil of the zones in which they have been produced (62). Nonetheless, a frac-
tion entered the atmosphere and subsequently accumulated in the cold zones. It
has recently been observed that the process of accumulation via condensation
affects not only distant zones, but high mountain regions (63). In other words
countries have not only exported part of their pollution abroad, but they have
also transferred part of it to what were previously the best-preserved areas of
the industrialized world.
Fig. 1 shows the distribution of various organochlorine compounds (OCs) in
fish from European lakes. It reveals that the most contaminated lakes are those
which are the furthest from the sources of pollution. This may seem like a para-
dox; traditionally dumping of waste to distant zones was considered dilution,
which diminishes effect on environment. However the net transfer of contami-
nants to distant areas via dilution leads to subsequent condensation. It is
shown here that the effect of POPS on ecosystems are not diluted. They simply
move from one location (warm) to another (cold). The same phenomenon is
observed for other compounds with different applications. The determining
factor is their volatility. Compounds with vapor pressures lower than 10
2.5
Pa
26 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
accumulate in high mountain zones. Compounds with vapor pressures above this
value are not retained in the range of the annual average temperatures repre-
sented by the mountain lake series but rather in the high latitude areas. Tem-
perature is also a key factor in the environmental distribution of recently
manufactured or used organobromine compounds.
Research on the food webs of these mountain lakes provides information on
the accumulation of these organic compounds in organisms. The OC composition
in water, chironomids, terrestrial insects, cladocerans, molluscs, cyanobacteria,
and brown trout has been investigated in Lake Redon in the Pyrenees. It was
concluded that distribution of these compounds does not reach equilibrium
within the life span of the food web organisms. Examinations of fish gill and
gut showed a net gill loss and net gut uptake for all compounds. The OC concen-
trations in aquatic insects change depending on the life-history stage. Increasing
concentrations of OCs and PBDEs from larvae to pupae independent of physic-
chemical properties have been observed. These concentration increases may
result from the weight loss of pupae during metamorphosis as a consequence
of mainly protein carbon respiration and lack of feeding. The intake of these
1.000
10.0
1.0
0.1
10.0
1.0
0.1
0.100
0.010
0.001
1.000
0.100
0.010
1000.0
100.0
10.0
1.0
0.1
1000.0
100.0
10.0
1.0
0.1
10
1
0.1
0.01
10
1
0.1
0.01
100.0
10.0
1.0
0.1
100.00
10.00
1.00
0.10
0.01
100
10
1
0.1
0.01
100
10
0.1
0.01
1
10.00
1.00
0.10
0.01
10
α-HCH
PCB 101 351 BCP811 BCP
Lake altitude
Annual average temperature
PCB 138 PCB 180
PCB 101
Temperature (K) Temperature (K) Temperature (K) Temperature (K) Temperature (K)
PCB 118 PCB 153 PCB 138 PCB 180
Altitude (m) Altitude (m) Altitude (m) Altitude (m) Altitude (m)
α-HCH γBCHHCH- 4,4 25 BCPEDD-
PCB 52
γBCHHCH- 4,4 -DDE
1
0.1
0.01
10
1
0.1
0.01
10
1
0.1
0.01
10
1
0.1
0.01
10
1
0.1
0.01
10
1
0.1
0.01
0 1000 2000 3000
0 1000 2000 3000
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0.0035 0.0036 0.0037 0.0035 0.0036 0.0037
0.0035 0.0036 0.0037
0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000 0 1000 2000 3000
0 1000 2000 3000
10
1
0.1
0.01
0 1000 2000 3000 01000 2000 3000 0 1000 2000 3000
0.001
R
R
R
R
RR RR
RR
R
R
RR RR
RRRR
Fig. 1. The concentrations of various OCs in fish (ng g
1
) from high-mountain European
lakes depend on altitude and temperature (these two variables are roughly related in the
series). Each point is the mean for the fish analysed in each lake. As observed, there is a
correlation between the concentrations of high molecular weight compounds (4,40-DDE,
and PCBs 118, 153, 138 and 180) and altitude or temperature. This dependence, which
implies greater contamination in the highest and most remote zones, is not observed for
the most volatile compounds. (Based on Ref. 63).
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 27
OCs in trout (or any other predator) is between two- and fivefold higher per
calorie gained when feeding on pupae rather than larvae (65).
Temperature dependence is also observed for lacustrine sediments, soils,
snow and mosses.
Once these compounds are introduced into the environment temperature
becomes a determining factor in their distribution. Climatic changes imply a
change in temperature and increases are particularly high in currently cold
and mountainous areas. Hence temperature variations should result in a redis-
tribution of contaminants accumulated in these areas. High mountain areas are
extremely important since they represent the most remote ecosystems in Europe.
These zones are of primary importance for the production of water resources for
human use, constituting the headwaters of river systems, Maintaining these
zones is required for ensuring low levels of water contamination. Studies of
the atmospheric deposition of these compounds are important for gaining knowl-
edge of the processes associate with temperature dependence. Investigations
were carried out in Tenerife (66), Redon Lake (64), and in Norway (67). For all
of these locations, greater deposition of organic compounds was observed during
warm periods, which reflected a greater rate of volatilization from the different
environmental compartments in which they were stored. However, the retention
capacity of the compounds is greater in colder lakes and the least volatile com-
pounds are the most retained.
Another major factor is the general concentration of these compounds in the
atmosphere where temperature changes associated with climate change first
appeared. In general, the same POP distributions are found in all sites, which
confirms the capacity of these compounds for volatilization and their global
impact. The atmosphere acts a store of these pollutants that are transferred to
continental freshwater ecosystems by precipitation. This trend is clearest among
the least volatile compounds and at the site of the lowest temperature. The alti-
tudinal accumulations of these compounds have toxic effects on the organisms
living in these remote areas.
6.3. Polycyclic Aromatic Hydrocarbons. A study of polycyclic aro-
matic hydrocarbons (PAHS) in the livers of high-mountain lake fish did not
reveal any dependence on temperature (64). This can be explained by the lack
of bioaccumulation of PAHS in fish and because the global transport mechanisms
differ from those of organochlorine pollutants. The distribution of PAHs in
remote and urban zones occurs in association with particles especially soot.
Thus, the concentrations of these compounds are more reflective of local sources
than OCs.
Most organisms exhibit PAH distributions largely dominated by phenan-
threne. Total PAH levels are higher in organisms from littoral habitats than
from deep sediments or the pelagic water column. However, organisms from
deep sediments exhibit higher proportions of higher molecular weight PAH
than those in other lake areas. Brown trout show an increase capacity for meta-
bolic degradation since they have lower PAH concentrations than in their food
and they are strongly enriched in lower molecular weight compounds. The
PAH levels in trout depend greatly on prey living in the littoral areas.
The atmospheric concentrations of PAHS tend to be higher in the winter
when more fuel is consumed (68). The concentrations of PAHS in high mountain
28 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
lakes decreased 30% (69) due to improvements in combustion processes in power
stations. Additional improvements can be made through greater use of solar and
wind power.
6.4. Mercury. The problems with mercury depend on the different forms
found. In air, elemental mercury vapor is found. In water, soils, and sediments,
oxidized divalent compounds are found. In these media, methylmercury gener-
ally accounts for a small percentage (<1%), but this is the most toxic form and
one capable of bioaccumulation. Increased temperatures will change the cycling
of mercury because the rates of transformation and the rates of transport
between compartments (ie, exchange between air and surfaces or water and sedi-
ments) depend on temperature. Little is known about the cycling but it is
expected that increases in methylation of inorganic mercury could occur, but
again the rate of demethylation may also be enhanced. Increased temperatures
in Arctic and Northern environments where cold climates currently limit methy-
lation and mercury mobility are of concern.
7. Eutrophication
The conversion of much of the earth’s surface to agriculture or urban land has
had major effects on nutrient flows from the local to the global scales. Natural
ecosystems generally conserve nutrients and store organic matter effectively.
In all agricultural systems, such conservation mechanisms are weakened and
management increases nutrients inputs over natural levels. These altered
features of ecosystems together result in massive leakage of nutrients into
waterways. The consequent eutrophication of the world’s waters has become
widespread and an expensive problem. Eutrophication profoundly affects
aquatic systems by altering biodiversity, trophic structure, and biogeochemical
cycling.
Biological processes are temperature sensitive. Increasing temperature,
expected under future climate change might (1) change growth and respiration
of organisms potentially leading to lower net primary production; (2) enhance
oxygen consumption and increase nutrient release from sediments; (3) affect
life history in the direction of shorter life spans and earlier reproduction;
(4) change phenology and trophic dynamics. Such temperature-induced changes
are expected to interact strongly with existing increased nutrient flows and
create new problems for freshwater biota or exacerbate existing ones (70).
There is a substantial body of literature on inland eutrophication which has
been summarized in various reviews (71,72). It is perhaps a more widespread
problem in freshwater ecosystems now that aerial deposition of nitrogen ferti-
lizes ecosystems at the global scale. Similarly the anticipated effects of global
warming on freshwater ecosystems have been repeatedly reviewed (70).
This section describes new results on the interaction between eutrophica-
tion and climate change based on studies carried out as part of the Euro-limpacs
project during 2004–2008. Key questions for this research were presented and
some answers to the questions are listed below. For more details on the indivi-
dual studies, please refer to Climate Change Impacts on Freshwater Ecosystems
(see Acknowledgment).
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 29
7.1. Do Nutrients Structure Ecosystems in Different Ways Under
Current and Anticipated Future Climatic Conditions? There is a clear
change in trophic structure in lakes along the climate gradient from simple,
often elongated, food webs in cold climates to truncated webs in warm ones.
Preliminary results indicate this conclusion, well supported by data, also holds
for streams.
The effect of increasing nutrient supply also differs among climate regions.
In temperate lakes, a shift often occurs from high proportions of potential
piscivorous fish, few and large plankti-benthivorous fish, high abundance of
zooplankton and clear water more often with macrophytes to a turbid state
with dominance of plankti-benthivorous fish and phytoplankton. The shift is
associated with increased nutrient supply, although there may be other trigger
factors. In the subtropics, lakes subject to both low and high nutrient loading, are
typically dominated by omnivorous fish that exert pressure on zooplankton. Such
systems can have clear water when nutrient loading is low. They are very vul-
nerable to increases in nutrient levels because the top-down effect of zooplankton
is weaker in temperate lakes due to high predation on zooplankton by fish.
Although less well studied than in lakes, the same probably holds true for
streams and rivers. There remains a big gap in knowledge on how wetlands
will change in response to climate change and eutrophication. As many have a
strong terrestrial character, analyses have focused on vegetation rather than
whole food webs, and although animal communities, particularly birds, have
been described in wetlands, overall trophic structures and their potential
changes are not well understood.
7.2. Will Changing Climate Interact with Increased Nutrient Supply
to Alter Ecosystem Processes? Whether changing climate interacts with
increased nutrient supply to alter ecosystems is more uncertain at the moment
than change in trophic structure. There is evidence that processes like deoxy-
genation, decomposition, and denitirfication are influenced both by nutrients
and warming, but the interaction among factors seems to be complex and vari-
able. In lakes, one expects (1) higher internal loading of phosphorus in response
to higher temperatures, more prolonged stratification in deep lakes, and higher
sedimentation rates as phytoplankton becomes more abundant and grazing by
zooplankton declines. Possibly (2) a higher likelihood of losing submerged macro-
phytes and thereby shifting shallow lakes from bentic- to pelagic-dominated sys-
tems and reducing biodiversity, and (3) higher nutrient and carbon turnover and
higher productivity.
There is evidence that nitrogen in freshwater systems may have counterin-
tuitive effects, eg, by inhibiting decomposition, but also interacting with tem-
perature to change the structure of aquatic plant communities. There is also
evidence that increased temperature will lead to rapid expansion of more exotic
species and more thermo-sensitive fish will fail to survive.
7.3. Will Effects of Climate Change be Distinguished from those
of Eutrophication? It is patently difficult to distinguish climate change
effects from eutrophication in long-term records and palaeolimnological studies
since symptoms accompanying both changes have occurred simultaneously, at
least over the past 150 years. However, the shift in trophic structure, phenology,
and life histories of organisms can be used as indicators of warming effects. As
30 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
the change typically will be larger than expected from those of eutrophication
alone. Results of controlled experiments can separate distinct effects in relatively
simple though not necessarily unrealistic systems, but more complex ecosystems
involve many more pressures so that the demonstration of individuals effects of
possibly reduced nutrient loading and anthropogenic CO
2
emissions in the future
will continue to be challenging.
7.4. Are there Lessons to be Learned from the Past? The answer
to this question is yes, particularly if several approaches are employed in concert
to analyze the past. If records are extended back sufficiently and longtime
changes considered, a situation is reached where climate change was an impor-
tant driver of ecosystem change while anthropogenic effects such as eutrophica-
tion were less pronounced. Information in the past may therefore provide a
clearer picture of ecosystem change driven by climate.
In several lakes, there is evidence of ecological responses to change in both
nutrient and climate, although in most cases, the eutrophication signal tends to
eclipse the climate signal. Furthermore, several data sets indicate that climate
change is likely to have a confounding effect on recovery from eutrophication
(70) suggesting that lake responses to nutrient reduction may be somewhat
slower and wetter conditions than original envisaged based on current climate.
7.5. Can We Mitigate Negative Effects of Climate Change on
Ecosystems in Terms of Enhanced Eutrophication? There is much
scope for combating eutrophication symptoms that will be aggravated by global
warming by taking measures to reduce external nutrient loading to freshwaters
than those already planned. These include: (1) less intensive land use in catch-
ments with sensitive freshwaters to reduce nutrients; (2) re-establish riparian
vegetation to buffer nutrient transfers, and improve in-channel structures to
increase retention of organic matter and nutrients; (3) improve land manage-
ment to reduce sediment and nutrient export for catchments; (4) improve sewage
works to help cope with flooding and low flows of receiving waters; and (5), more
effective reduction of nutrient loading from point sources and for N from the
atmosphere.
8. Restoration of Freshwater Ecosystems
There are many ways of physically restoring streams such as reforestation of the
floodplain, re-meandering and the removal of dams and bank structures. Newer
approaches include the addition of coarse woody debris (73) and the removal of
sediment deposits in floodplains and various methods to combat the deep cutting
of streams.
For effective stream restoration, the complex link among physical para-
meters, habitat diversity and biodiversity need to be understood. When a stream
has been physically restored, success, measured by an increase in biodiversity
depends on the extent of re-colonization by the original (indicator) species.
This is the ‘‘field of dreams’’ hypothesis, which states, ‘‘if we build, they will
come.’’ However this does not guarantee a positive outcome. Whether the original
species are able to re-colonize the restored stream depend not on the quality of
the restored habitat but also on a number of actors such as dispersal capacities
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 31
of the species and the presence or absence of migration barriers between the
source populations and the restored areas. Establishment of an invasive or
non-native species could also hinder re-colonization.
RestorationofstreamsandriversispromotedbytheEUWaterFramework
Directive and other legislations. Restoration success has been lower than
expected. Failure is mainly due to a too small scale of attempts and a lack
of an ecosystem approach and understanding of the biological processes
involved. Future climate change is likely to reduce the chances of restoration
success.
The term ‘‘stream (or river) restoration’’ is used for a wide variety of project
objectives ranging from conventional bio-engineering to the restoration of nat-
ural processes aiming to generate natural in-stream structures and a natural
channel pattern. In the United States, stream restoration is defined as ‘‘the
return of an ecosystem to a close approximation of its condition prior to dis-
turbance’’ (74). However, in many parts of the world irreversible changes of
the natural setting have occurred and hence the re-creation of a previous histor-
ical state is impossible (75). This is especially true for a densely populated cul-
tural landscape in Europe and other parts of the old World, where streams
have been altered by human activity since the Mesolithic period. The present
natural setting may differ from the historical natural setting as a result of irre-
versible changes in the natural setting.
The restoration of hydromorpholically degraded rivers has become a widely
social objective and scientific interest. In densely populated areas such as Cen-
tral Europe, a large proportion of rivers are heavily degraded. Thus there is a
strong demand for simple and cost-effective restoration measures. Large wood
(defined as logs with a diameter >0.1 m and a length >1 m) is an important
component of stream ecosystems in temperate forested ecoregions. Considering
these beneficial effects, even in densely populated regions, up to one third of
the streams could potentially be improved by restoration with wood (76). After
evaluating the use of large wood in 50 streams restoration projects Kail and
co-workers (77) concluded that (1) potential effects of wood placement must be
evaluated within a catchment- and reach-scale context; (2) wood measures are
most successful if they mimic natural wood; (3) effects of wood structures on
stream morphology are strongly dependent on conditions such as stream size
and hydrology; (4) wood placement has positive effects on several fish species;
and (5) most projects revealed a rapid improvement of their hydromorphological
status.
The high natural variability of large wood standing stock and ongoing cli-
mate change make it difficult to define reference and target conditions for
streams. Therefore, the use of ‘‘passive restoration’’ methods (restoring the pro-
cess of wood structures on a reach scale) is desirable for a number of reasons.
First local active restoration measures are at risk of neglecting the more general
processes at the catchment scale causing degradation, and often treat symptoms
rather than causes of stream degradation, and thus prone to failure. It has been
proposed that restoration projects are more likely to be successful if they are
undertaken in the context of entire catchments. The findings of Kail and Kering
(76) further give support to the conjecture that restoration activities using large
wood must consider other factors such as catchment land use. Also active
32 CLIMATE CHANGE, FRESHWATER ECOSYSTEMS
restoration measures may create conditions that do not correspond to the poten-
tial natural state.
A conservative estimate of Kail and Hering showed that about 7% of the
streams in Central Europe can potentially be restored by large wood recruitment
from native or non-native riparian forests.
8.1. Connectivity and Species Dispersal. For the restoration of
freshwater ecosystems to be considered successful, the reference conditions
need to be attained or at least closely approximated as discussed above. Accord-
ing to the European Water Framework Directive (78), this involves more
than just restoration of abiotic reference conditions such as water nutrient sta-
tus and pH. It also involves the restoration of reference communities, ie, the
assemblages of the fauna and flora typical of unperturbed conditions. Whereas
the first step, abiotic reference conditions can be restored by applying external
measures, the restoration of reference communities is in most cases expected
to follow a natural recovery. This often requires that species that have been
absent from the restored site re-colonize from outside source populations.
However, even when abiotic conditions have been successfully restored, the
re-colonization of reference species may be disappointing if dispersal is a limit-
ing factor (79).
Dispersal of organisms to restored sites is dependent on the connection of
the site to source populations or the connectivity of the site and the dispersal
abilities of the organisms themselves. Generally organisms with source popula-
tions nearby are the first to colonize restored sites. A main categorization is
often made between organisms that can actively move substantial distances
such as macroinvertebrates with an aerial life stage and fish and those that
are predominantly passively dispersed over larger distances such as phytoplank-
ton, macrophytes, and most invertebrates lacking an aerial life stage. The
dispersal of individual species can be enhanced by the construction of corridors
or removal of barriers.
It is important to consider connections and dispersal in the context of cli-
mate change. Important mechanisms that climate change is expected to alter
are changed hydrodynamics of streams and rivers due to more extreme rain
events, which will alter the connections between sites and surface water flows
and faster drying of ponds and shallow lakes which will reduce stepping stone
connections between wetlands. General guidelines for restoration are difficult
to prepare because the effects will likely be specific per site and landscape. A
landscape-scale approach to restoration that addresses these spatial issues,
such as the OLU approach, in combination with climate change prediction at
the same working scale, will result in more successful, realistic, and climate
change-proof restorations.
8.2. Operational Landscape Units (OLU). Identification of OLUs,
which are defined as combinations of landscape patches with their biotic and
hydrogeological connections, has recently been proposed as a tool to facilitate
wetland restoration in catchments with a high degree of fragmentation and
strongly altered hydrology (80). The combined consideration of biotic (ie, disper-
sal, transports of organisms), and hydrological (flooding, events, groundwater
flow-path connections across landscapes factors is novel in this context. To
synthesize an OLU, the three step approach can be used. In the first step the
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 33
focus is on defining the restoration targets, ie, which plants and animals need to
be restored and which ecosystem function is to be restored, ie, water quality or
floodwater detention). In the second step identification of the features of hydrol-
ogy and dispersal that are critical for restoration. In most cases linkage plays an
important role. The third step is to define the extent of the OLU by identifying
landscape components which are necessary as building blocks for restoration of
hydrological functioning and water regime and the dispersal of plants and/or
animal species.
The OLU approach will often involve the identification of source areas (ie,
intact, species-rich nature reserves which need to be conserved) as well as recep-
tor areas, which would be suitable for restoration and encompass the restoration
target area and connecting pathways such as streams or other hydrological flow
paths. To identify the receptor areas and connecting pathways, historical maps
and other records of present and past hydrological functioning are useful. A
further step is the creation of a map that represents the combination of
landscape elements that are required for regional survival of a species or a
self-sustaining ecosystem function or for creating target conditions for plant
communities or ecosystem functions.
Cases studies if the OLU concept can be found in the original work (see
Acknowledgment).
Acknowledgment
This article is a condensation of the material in Martin Kernan, Richard W.
Battarbee, and Brian Moss, eds., Climate Change Impacts on Freshwater Ecosys-
tems, Wiley-Blackwell, Hoboken-UK, 2010. Chapters 3, 4, 5, 6, 7, 8, and 9 were
used for this condensation. Please refer to the original for further information.
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MARTIN KERNAN
RICHARD W. BATTARBEE
University College London
BRIAN MOSS
University of Liverpool
CLIMATE CHANGE, FRESHWATER ECOSYSTEMS 37
