The primary production of phytoplankton in Lake Vechten

The primary production of the phytoplankton of Lake Vechten (The Netherlands) (area, 4.7 ha; mean depth, 6 m), an unpolluted and stratified sandpit was investigated from 1969 to 1980 (except in 1971, 1975 and 1976) by the in situ ¹⁴C-technique. Other data collected include: solar radiation, transparency, oxygen and thermal structure. In winter and spring diatoms, Cryptophyceae and Chlorococcales were important algal groups, while in summer Dinophyceae and Chrysophyceae were important. The chlorophyll-a concentration was compared to the cellular biovolumes (= fresh weight) of the most abundant phytoplankton species. The primary production maxima occurred in winter, spring and during the summer stratification. The vertical profiles of photosynthesis exhibit light inhibition at surface to a maximum of 4 m. The maximum of zooplankton grazing in May–June caused a sharp decrease in the phytoplankton biomass and seston concentration accompanied by the highest transparency (clear water phase).The values for cellular C-fixation range from 10 to 1307 mg C · M^–2 · day^–1 (annual mean of 280 mg C · m^–2 · day^–1). High dark fixation (up to 100%) was encountered in the metalimnion and hypolimnion from July to October together with peaks of ¹⁴C-fixation due to a crowding of phytoplankton and phototrophic anoxic bacteria. Extracellular excretion by phytoplankton, investigated in 1977 to 1979, was 15% of the annual mean of the total C-fixation. The photosynthetic efficiency, turnover rates, and activity coefficients were low, particularly in the summer months when Ceratium hirundinella was predominant. The seasonal variations were controlled mainly by solar radiation and probably phosphate, the former being more important in the non-stratification period and the latter during the stratification period.