Climate Change and the European Water Dimension - Agri ...

summer blooming species in Lake Erken (Pettersson et al., 1993; Tymowski and
Duthie, 2000), might be strongly influenced.
Temperature effects
Most planktonic species can survive and grow at temperatures well in excess of
those predicted for a warmer world. Hawkes (1969) has examined the temperature
tolerance of different groups of algae and suggested that diatoms grow best at
temperatures below 25 C and blue-green algae at temperatures above 30 0 C. There
are, however, notable exceptions, such as the diatom Acnanthes marginulata, which
can tolerate temperatures up to 41 C (Patrick, 1969) and the blue-green alga
Oscillatoria rubescens that is commonly described as a cold-water form. In
physiological terms, most groups of algae photosynthesise most efficiently at
temperatures of around 25 0 C. The rate of carbon fixation could therefore increase
with increasing temperature, but factors other than temperature usually limit net
production in most lakes.
If the climate becomes warmer, with warmer winters, the composition of
phytoplankton might be totally changed, as observed in 1989 from only one
extremely mild winter (Weyhenmeyer et al., 2002). In addition to the direct (in terms
of no time lag) response, the outbreak of blooms in the summer period can also be
influenced by the warmer winter period (Hallegraeff, 1993; Guess et al., 2000).
Density and viscosity of water directly influence the ability of phytoplankton
organisms to remain in suspension. Viscosity decreases by about 50% from 0°C to
25°C. Consequently sinking rates of phytoplankton double over this range. Any
prolonged rise of water temperature will therefore result in higher loss rates due to
sinking or selective growth of species which can more easily compensate sinking
such as cyanobacteria, increasing the risk of algal bloom formation and the
appearance of toxic species (Dokulil, 2000, 2003).
Another temperature-related phenomenon is the change in species distribution
areas. The tropical bloom-forming cyanobacterium Cylindrospermopsis raciborskii is
causing increasing concern because of its potential toxicity and invasive behavior at
mid-latitudes (Neilan et al., 2003; Briand et al., 2004). This species has recently been
identified in several temperate areas in Hungary, Germany, France, and Portugal. It
is suggested that the colonization of mid-latitudes by C. raciborskii may result from a
combination of its ability to tolerate a rather wide range of climatic conditions and
climate warming, which provides this species with better environmental conditions for
its growth.
Mixing vs. stability
In general, increasing mixing depth increases the proportion of abiotic light
attenuation within the mixed layer, leading to a decrease of phytoplankton production
averaged over the mixed layer (Huisman et al., 1999; Diehl, 2002). Windy and rainy
periods (and the NAO) also affect the mixing depth and therefore lead to a lower
phytoplankton biomass compared to calm and sunny periods (Arvola et al. 2002).
Warm water and longer periods of stratification can promote a dominance of
potentially toxic cyanobacteria (George and Harris, 1985; George et al., 1990;
Hyenstrand et al., 1998). Additionally, large cyanobacteria colonies (besides other
phytoplankton groups) are resistant against grazing by zooplankton, leading to a
dominance of cyanobacteria in the last stages of the stratification. Figure IV.B.6
shows the results of a long-term study of the factors influencing the growth of the
blue-green alga Aphanizomenon in Esthwaite Water (George et al., 1990). The solid
line shows the de-trended and smoothed summer abundance of the species and the
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