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

Aulacoseira spp. favour the changed conditions during spring overturn (Lund 1966,
Round et al. 1990). These diatom blooms can collapse for a variety of reasons, but
thermal stratification and sedimentation usually accelerates their rate of decline.
Once the diatoms have departed, a variety of small flagellates then tend to dominate
the plankton. These small forms have high rates of growth but can only survive if the
water column is periodically mixed by the wind.
In lakes which do not freeze in winter, the timing of the spring bloom is mainly
dependent on light and turbulence, two factors influenced by climatic variation and
change. Lund (1950) and Talling (1971) detailed the influence of stratification and
spring rains on the timing and magnitude of spring diatom blooms. A critical factor is
the time the growing cells spend in the illuminated and dark portion of a mixed water
column. Phytoplankton suspended in ‘dark’ water cannot grow because their
respiratory losses are greater than their gross rate of carbon fixation.
Catchement processes
(Vegetation, Soil etc.)
Nutrients
Temperature
The magnitude of the bloom may be differently related to climatic factors. The loss
of ice cover or less snow on the ice might change and/or increase the algae
population during winter (Pettersson, 1990; Adrian et al., 1995). Thus, the nutrient
availability might be lower for the actual spring bloom, leading to a reduced algae
peak (Pettersson 1990). Cloud conditions in spring, the mixing regime, and the timing
of the onset of stratification strongly affect the magnitude of the phytoplankton spring
peak (Gaedke et al., 1998a,b) and the annual primary production alpine lakes
(Goldman et al., 1989). In extreme mild winters in Europe (i.e. 1989 and 1990) an
coherent increase in cyanobacterial biomass was found in a number of European
lakes (Weyhenmeyer et al., 2002).
Overwintering
Relatively small differences in the overwintering stocks of some algae can have a
major effect on their growth rate later in the year. For many slow-growing species of
phytoplankton, their growth rate in early summer is strongly influence by the number
of cells that overwinter in the open water (Heaney and Canter, 1989). Wet winters
invariably reduce the size of this spring inoculum, which may then influence the
seasonal succession of phytoplankton later in the year. Also the overwintering
success of resting stages, for example of Gloeotrichia echinulata, a dominant
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Mixing
Processes
Figure IV.B.5. Conceptual diagram of climate impacts on catchment and lake
processes, and their relations (Modified by Dokulil from Anderson, 2000)
Lake Biota