LIGHT CLIMATE AS THE KEY FACTOR CONTROLLING THE SPRING DYNAMICS OF PHYTOPLANKTON IN LAKE-ZURICH

During the spring seasons of 1983, 1986 and 1987 the development of ph ytoplankton in Lake Zurich was investigated (from February to May) usi ng samples taken at short term intervals. The aim was to describe the effects of the short term dynamics of environmental factors on the alg al growth. The results could then be used to discuss the existing theo ries to assess the start of phytoplankton growth pulses in spring. Onl y 7 to 10 days without wind driven vertical mixing were required in sp ring to start the first growth pulse, despite of a still very unstable water column (sometimes inverse thermal stratification). Mainly flage llates and Stephanodiscus hantzschii increased their biomass and achie ved net growth rates of 0.1 and up to 0.65 d(-1) respectively During s uch a phase the mixing depth was always smaller than the euphotic dept h. Later on, at the start of the spring bloom (= last growth pulse in spring before the clear water stage), the intensity of vertical mixing as well as the mixing depth were markedly reduced due to an increase in heat input and low wind. Then flagellates dominated (contribution u p to 75.5 % of the areal biomass reaching 60 g fresh weight m(-2)) and the growth rate rose to a maximum of 0.65 d(-1). Standard models of c ritical depth considers that there is only a biomass increase if the m ixing depth is smaller than the depth of a water layer positive balanc ed between production and respiration. This model for determining the beginning of a phytoplankton growth pulse in spring takes no account o f the favorable light conditions for phytoplankton cells at calm and s unny days in February and March. The newly developed threshold value m odel takes these situations into account: It assumes that the phytopla nkton biomass increases when the calculated effective light climate is equal or greater than a previously fixed threshold. The calculations are based on the mean light intensity within the mixed layer at windy days or within the euphotic depth (z(eu)) at calm days. In Lake Zurich a minimum of 0.2 10(6) J m(-2) d(-1) (= 0.9 mol quanta m(-2) d(-1)) h as to be reached or surpassed in at least 3 days before an exponential increase of algal biomass can occur. The value does not depend on sho rt term fluctuations in neither radiation nor mixing depth. It seems t hat this value is rather low comparing with those of investigations in other water bodies (up to 0.8 10(6) J m(-2) d(-1)) but high related t o values from algal cultures (0.02 10(6) J m(-2) d(-1)). As the weathe r can only be forecasted a few days ahead with any certainty the perio d for a more or less accurate prediction of an algal bloom is restrict ed to about 1 to 5 days.