Limnological studies in a large,deep, oligotrophic lake (Lake Ohrid,Yugoslavia). A summary of nutritional radiobioassay responses of the pelagial phytoplankton

In situ carbon-14 bioassay techniques were used during 1972–1974 to estimate nutritional preferences and requirements of the pelagial phytoplankton in Lake Ohrid, Yugoslavia.Bioassay measurements, conducted spatially and temporally, and corrected appropriately, showed a strong stimulation to phytoplankton photosynthesis rates during most seasons in epilimnetic waters following microadditions of inorganic silica and iron. Photosynthetic stimulation was additive for individual elements and synergistic when simultaneous additions were made. Marked stimulation occurred for both elements during the spring, especially in upper hypolimnetic waters (50–75 m) and correlated strongly with dominant diatom populations.The addition of an organic chelator, nitrilotriaceticacid (NTA), commonly stimulated photosynthetic rates and, frequently, as deep as 15o m, considerably below the depth of optimal light availability for photosynthesis. With minor exception, the simultaneous addition of this chelator and inorganic iron were always capable of producing stimulatory responses.Inorganic phosphorus was preferred over nitrogen and generally found to be more stimulatory. Phosphorus stimulation was restricted primarily to periods of spring and summer production. The addition of glucose, acetate and glycine produced positive responses, while the addition of several vitamins showed little effect except for a stimulation from microamounts of B₁₂. The addition of two organic growth substances, Gibberellic acid (GA) and Indoleacetic acid (IAA), both stimulated phytoplankton photosynthesis.The results of more than 140 bioassay measurements indicate the pelagial phytoplankton are severely restricted nutritionally due to specific physical-chemical interactions occurring in Lake Ohrid. Similar mechanisms have been postulated and evaluated in other hand-water lake systems. In spite of the great depth of the lake (300 m+) and probable age, these mechanisms are undoubtedly responsible for the low phytoplankton production occuring in the lake.