Monitoring the Responses of Freshwater Ecosystems to Climate Change - Climate Change Impacts on Freshwater Ecosystems - page 86

difficult to classify. Simple indicators are required to judge how advanced these
processes already are, and which of them could be integrated into presently
applied assessment systems. Given the different types of impact in cold,
temperate and warm ecoregions, different sets of indicators may be required.
The theoretical background for indicator selection is described below and in
earlier chapters, while a potential set of indicators for lakes in cold, temperate
and warm ecoregions, respectively, is given in Table 5.1. Most indicators could
be easily incorporated into routine monitoring programmes, e.g. by adding
indices reflecting the impact of temperature increase on phytoplankton already
monitored for the purpose of the Water Framework Directive. In many cases,
simple physicochemical measurements are most appropriate, as they are easy
to make, are often included in routine monitoring programmes and are located
at the starting points of cause-effect chains, thus giving context to subsequent
changes.
Hydrology and physicochemistry
The timing of ice cover is directly dependent on winter and spring temperatures,
and therefore 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. They
have been discussed in Chapters 3 and 4. Likewise, there are many consequent
chemical features of these that have already been discussed and provide strong
initial indicators.
Primary production
Climate-sensitive physicochemical and hydrologic conditions can be major
determinants for primary production in lakes. Phytoplankton community
composition may be altered by changes in winter and spring temperatures,
depending on lake type and location (Anneville et al . 2002; Christoffersen et al .
2006; Elliott et al . 2006). The phytoplankton assemblages of shallow cold-water
ecosystems seem to be especially sensitive to temperature changes (Schindler
et al . 1990; Findlay et al . 2001).
A shift towards dominance of cyanophytes in warmer water, with possible
implications for water quality, is widely predicted and may lead to a progressive
loss of phytoplankton biodiversity (Chapter 6). Models suggest that cyanobacterial
dominance will be greatest if high water temperatures are combined with high
nutrient loads. At low nutrient levels, the effect of water temperature change is
reduced considerably (Anneville et al . 2004; Elliott et al . 2005, 2006).
Generally, increased phytoplankton productivity and biomass are correlated
with higher spring water temperatures as well as changes in hydrochemistry such
as increased nutrient availability (Schindler et al . 1996; Straile & Geller 1998;
Findlay et al . 2001). Earlier stratification and deeper thermoclines may have an
opposite effect, however. Furthermore, 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 a levels (Pettersson et al . 2003).