The Impact of Climate Change on European Lakes by Glen George

Author:Glen George
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

14.2.3 Summer and Autumn

In many of the CLIME lakes, there has been a significant change in the phytoplankton growth patterns observed in early summer. At that time of year, many lakes experience a clear water phase, i.e. a period when the biomass of the phytoplankton declines sharply. In most cases, this decline has been related to zooplankton (Daphnia) grazing. Detailed accounts of this phenomenon have been given for lakes in Northern Europe (Gerten and Adrian, 2000; Weyhenmeyer, 2001; Adrian et al., 2006), Western Europe (Talling, 2003) and Central Europe (Straile, 2000; Straile and Adrian, 2000; Anneville et al., 2002a, b). In warmer years, a biomass of Daphnia, large enough to limit the growth of phytoplankton, is reached earlier in the year and results in an earlier and longer lasting clear-water phase. These effects appear to be lake specific: in shallow Müggelsee, spring water temperatures and Daphnia abundance both increased more rapidly than in large, deep Lake Constance. Consequently, the clear water phase started about three weeks earlier in Müggelsee than in Lake Constance (Straile and Adrian, 2000). The climatic responses observed in autotrophic species may not, however, be mirrored by heterotrophic species (Blenckner, 2005) since the processes responsible for the decay and recycling of the autotrophs are often lake specific. For example, in Lake Stechlin, spring diatoms simply sink as soon as the lake starts to stratify and the clear water phase is not connected with any grazing effects (Padisák, et al., 2003b). In some cases, the processes responsible for the breakdown and decomposition of the cells are markedly non-linear whilst in others they have critical thresholds. Cell death through parasitism may also account for significant proportion of phytoplankton loss in many lakes (Jassby and Goldman, 1974).

At some sites, causal links have been established between the meteorological conditions experienced in winter or early spring and events in the plankton the following summer. In large and shallow Lake Võrtsjärv in Estonia (270 km2, mean depth 2.8 m), large year-to-year differences in the water level have a very pronounced effect on the development of the phytoplankton (Nõges, 2004; Nõges et al., 2003). The magnitude of the spring floods, determined largely by the winter air temperature and precipitation, explains most of the variability in annual mean water levels (R = 0.85, p < 0.0001). When the level is low, the water is enriched with phosphorus by sediment resuspension and there is an associated reduction in the nitrate concentration due to denitrification. Since 1964, the phytoplankton biomass has been significantly lower in years of high water level, a pattern that was not related to any change in external loading of nutrients. These fluctuating water levels have also had an effect on the qualitative composition of the phytoplankton. During high-water periods, Limnothrix redekei and L. planktonica have typically accounted for more than 90% of the total wet weight of phytoplankton, which has remained under 30 g m–3, even when the external nutrient loading was high. Limnothrix species can, by virtue of their shape and photoadaptive

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.