Climate Change and the Hydrology and Morphology of Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 69

This fluctuation in habitat conditions will become greater in future, and stream
communities will fluctuate more (Milne 1991). The predictability of resources
will decrease, and species will have to adapt or become locally extinct.
Several studies have focused on the biological implications of lower resource
predictability (Palmer et al . 1995; Palmer & Poff 1997; Townsend et al . 1997),
and many show how ecosystems are directly influenced by flow (Death &
Winterbourn 1995; Gasith & Resh 1999; Poff 2002; Bond & Downes 2003; Fritz
& Dodds 2004). High flows and spates scour accumulated sediment and debris,
redistribute streambed material and organic matter in the channel, change channel
morphology and form new erosion (runs and riffles) and deposition (point and
mid-channel bars, pools, sand accumulations) zones. High flows and spates may
also disturb in-channel and encroaching riparian vegetation, homogenize water
chemistry among the stream channel and adjacent water bodies and increase shear
stress on organisms. In contrast, low flows and droughts bring siltation of fine
mineral and organic material, decrease oxygen concentrations and increase those
of some nutrients and minerals. They promote mineralization of organic material
in the stream bottom and drying of the banks; they reduce bank stability, expose
large parts, or all, of the stream bed and stress many organisms. Change in flow
variability and timing can also be important, resulting in unpredictable erosion-
deposition processes with frequent shifts in channel morphology and habitat
availability and loss of synchronization of flow stages with stages in organisms'
life cycles, such as egg deposition, growth and pupation.
Direct climate change impacts on lake hydrology
Climate change will impact lake hydrology mainly through effects on residence
time and water level as well as through receptors and sources of stream flow.
Short residence times mean that pollutants such as excess nutrients from point
sources are flushed out of the lake ecosystem, whereas with decreasing
precipitation and longer residence times, they will accumulate, with likely changes
in phytoplankton communities (Schindler et al . 1990, 1996; Hillbricht-Ilkowska
2002) and in food-web composition and structure. In lakes with long residence
times, internal processes may become more important. For example, phytoplankton
production may increase with higher temperatures due to increased nutrient
availability, and eutrophication problems may thereby become more severe
(Mooij et al . 2005). A decline in water level due to decreased precipitation may
cause changes in the nutrient status and acidity of lakes with low buffering
capacities (Carvalho & Moss 1999).
Water-level change can also directly affect phytoplankton development in a
lake. For example, a strong influence of the North Atlantic Oscillation on the lake
water level was observed in Lake Vortsjarv (Estonia) (Nõges et al . 2007) where
water-level changes influenced phytoplankton composition and biomass
independently of the nutrient loading. In addition, less severe winters cause a
reduction in winter ice cover, which can lead to lower lake water levels and lake
system changes through the following summer months (Croley 1990). Longer ice-
free periods potentially lengthen the growing season for algae and aquatic macro-
phytes. Higher temperatures may raise the rate of mineralization of organic matter