Eutrophication -- 21.7 Climate, nutrients and fish-kills

nogesFig14

Figure: Quantified food web structure in Lake Võrtsjärv according to Nõges et al. [335] and in L. Peipsi where fish biomass and production was calculated by A. Järvalt from data published by Kangur et al. [261], biomass and production of zoobentos was obtained from Timm et al. [468] and that of zooplankton from Haberman [205], other data from Nõges et al. [329]. Symbols: b=bulk, gC m^-2; p=production, gC m^-2 year^-1; r=requested food ration, gC m^-2 year^-1. Green arrows denote grazing food chains, blue arrows denote detrital food chains, violet arrows denote microbial loop and black arrows denote other transformations. Ovals of the same colour have an equal distance from the base of the food chain, i.e. belong to the same trophic level. Symbols: NPfish -- non-piscivorous fish; Pfish -- piscivorous fish; b - bulk, gC m^-2; p - production, gC m^-2 year^-1; r - requested food ration, gC m^-2 year^-1. Conversion factor 1 mg WW = 0.1 mg C is used for all links.

Several winter fish-kills have been documented in Lake Võrtsjärv during the last century (in 1939, 1948, 1967, 1969, 1978, 1987, 1996). Fish-kills occurred most probably in wintertime and dead fish were subsequently recovered in spring. One reason for these fish-kills could be the depletion of oxygen in low-water years during late winter when the under-ice oxygen concentration dropped faster due to smaller absolute amount dissolved in the smaller volume of water. This kind of oxygen depletion was documented in March 1996 [333] and resulted in a massive kill of eel.

In Lake Peipsi, high water temperature and algal blooms resulted in massive fish-kills during summers of 1959, 1972, and 2002. During algal blooms, phytoplankton biomass is built up faster than can be consumed by zooplankton. Intensive photosynthesis produces much oxygen during the day that partly leaves to the atmosphere when the water becomes oversaturated. At night, when algal masses consume but do not produce oxygen, oxygen deficiency may occur. Such large-scale diurnal fluctuations of oxygen concentration harm fish and make them more susceptible to other stressors. High water temperature associated with algal blooms makes the situation even more dangerous to fish. Other stressors accompanying algal blooms are high water pH caused by intensive photosynthesis and elevated concentrations of ammonium released during the decomposition of organic matter. At high pH (>9) most ammonium is converted to toxic ammonia (NH₃), which can kill fish. Moreover, cyanobacterial toxins can also significantly influence fish populations (Figure 13).