Conserving Freshwater and Coastal Resources in a Changing Climate

Photography by Avi Hesterman
lake is remixed in late fall. If a lake bottom contains a
greater amount of rotting biomass, oxygen depletion occurs
more rapidly (Kevern et al., 1996).
There is a balance between spring mixing, fall mixing
and summer stratification. If the summer stratification
period lasts too long, summerkill can occur, where
higher mortality and lower production rates can lead to
fish deletion from a lake (Fang et al., 2004). As winters
get shorter with a changing climate, spring remixing
occurs earlier which leads to a longer period of summer
stratification and increased risk of oxygen depletion and
deep-water dead zones (Kling et al., 2005).
Shifting Thermal Regions
As water temperatures change, lake habitats will change
in their ability to support aquatic life. Fish depend on specific
ranges of temperature within which physiological
functions, such as growth, activity, and swimming performance,
are maximized. In some instances, a specific
life stage will have an optimum temperature (Coutant,
1990), especially the reproductive stages. Cold-water
fish have a niche centered around 15° C, cool-water
fish around 24° C, and warm-water fish around 28° C
(Shuter & Meisner, 1992). When optimum temperatures
are not available, fish will move to the best alternative
conditions, a change that may result in a decrease in
metabolic efficiency (Coutant, 1990).
Changes in thermal habitats are not uniform across
lake types. Very large lakes like the Great Lakes that
are deep and large are anticipated to experience an increase
in suitable thermal habitats for all species with
an increase of temperature. This is because there will
be increased areas of warmer waters while the fish and
other species in the deeper layers of water will benefit
from the slight warming. In smaller,
shallower lakes the warmer water
will reduce the habitat available for
cold water fish (Poff, et al., 2002).
In a comprehensive series of
studies that looked at 27 lake types
in 209 locations in the continental
United States, Fang et al. evaluated the
differing effects of changing thermal
niches on warm, cool, and cold-water
fish (2004a, b & c). The number of
locations that can support cold-water
fish (five types of salmon, four types
of trout and one variety of whitefish)
habitat is projected to decrease by
38%, resulting in fewer lakes able to
support cold water fish. Additionally,
the area of lakes that cannot support
cold-water fish is expected to extend significantly further
north (Fang et al., 2004b).
The study found that for lakes of medium depth and
size, warm-water fish (four species of bass, shad, carp
and catfish among others) could survive in all 209 locations
despite potential increases in temperature. Some
lakes in the south central and southeastern United States
are expected to experience a loss of cool-water fish due
to increased summerkill, the highest percentage of
which will occur in shallow lakes (Fang et al., 2004a).
Lower Lake Levels
Earlier runoff, increased evaporation, and changes in
stream in flow are predicted to lower lake levels in the
mid-Atlantic and northeastern United States (Kling et
al., 2005). Even large lake systems, like the Great Lakes,
may experience decreased lake levels that are greater
in magnitude than the anticipated rate of sea level rise.
Issues related to permanent lowering of lake levels
include isolation of lake-fringing wetlands, possibly
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Conserving Freshwater and Coastal Resources in a Changing Climate