Monitoring the Responses of Freshwater Ecosystems to Climate Change - Climate Change Impacts on Freshwater Ecosystems - page 93

Changes in precipitation and temperature may have opposing effects on fish
populations. In Norway, higher winter precipitation, leading to more accumulated
snow in April, can be detrimental to the recruitment of brown trout ( Salmo trutta ),
whereas warmer summers increase recruitment to levels that may lead to
overpopulation and to the establishment of brown trout populations at higher
elevations (Borgstrom & Museth 2005). Exceeding water temperature thresholds
may limit the survival of fish in lakes. More fish may die from oxygen deficiency
or physiological stress in warmer water (DeStasio et al . 1996). On the other hand,
in shallow, eutrophic lakes, winter fish-kills caused by low dissolved oxygen under
ice will be reduced or eliminated (Fang & Stefan 2000). Increased hypolimnetic
temperatures may lead to a loss of juvenile fish requiring cool water as a summer
refuge; thus, climate change can eliminate fish populations at the margins of their
range (Gunn 2002). The effect of climate change at different latitudes has been
modelled based on temperature and the minimum oxygen requirements of cold-,
cool- and warm-water fish (Stefan et al . 1996). Zoogeographical boundaries could
move significantly north (Carpenter et al . 1992; Petchey et al . 1999), a problem
for several Salmonidae (DeStasio et al . 1996; Jansen & Hesslein 2004), whereas
Percidae and Cyprinidae may benefit from an increased thermal habitat in the
case of moderate warming (Jansen & Hesslein 2004). Hydrologic changes,
together with changes in temperature, will probably favour invasive species over
rare and threatened native species (Rogers & McCarty 2000).
As lake fish communities are well correlated with temperature patterns through
the variety of pathways described above, water (or air) temperature is an indicator
for changes in fish communities, supported by composition measures, such as the
proportion of alien species in the fish community.
Food webs
The principal alteration in lake food webs caused by global warming may be a
reduction in zooplankton grazing intensity, leading to eutrophication effects (see
Chapter 6 for detailed discussion). This is caused by a variety of factors: the
density of planktivorous cyprinid fish species is enhanced (Jansen & Hesslein
2004) and the density of piscivorous species reduced, which may lead to a strong
top-down control of large zooplankton species (Jeppesen et al . 2005). 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 significantly with increasing
spring temperatures. Thus, a long-term decline in Daphnia populations, frequently
a keystone herbivore, may be associated with an expanding temporal mismatch
with the spring diatom bloom (Winder & Schindler 2004). A timing mismatch
between phytoplankton maxima and the peak abundance of Daphnia may lead to
the absence of a clear water phase (De Senerpont Domis et al . 2007) that is a
feature of many lakes in late spring. Even modest warming (less than 2°C) during
a short but critical seasonal period may induce changes in whole lake food webs
and thus alter entire ecosystems (Strecker et al . 2004; Hampton et al . 2006;
Wagner & Benndorf 2007).