Succession and Vertical Distribution of Phytoplankton in Response to Thermal Stratification in a Lowland Mere, with Special Reference to Nutrient Availability

Abstract

Changes in the populations of the more common algae occurring in the plankton of Crose Mere [England] during the 1973 period of thermal stratification are described. Green algae (chiefly Volvox and Eudorina) were dominant during June; blue-gree algae (Anabaena, Aphanizomenon and Microcystis) were abundant during July; the dinoflagellate Ceratium was the dominant species during August; diatoms (especially Melosira granulata) increased during July. The vertical distributions of populations were also investigated. Whereas the green algae were confined largely to the superficial layers of the mere, Peridinium, Cryptomonas and the blue-green algae each showed a tendency to preferentially inhabit deeper layers. The distribution of Ceratium was variable, but there was a tendency for the population to descend to the metalimnion during times of nutrient stress. The diatoms tended to occupy only the deepest epilimnetic layers, except when conditions of turbulent mixing prevailed. The vertical distribution of major nutrients is described. Depletion of nutrients (especially NO3) in the upper layers roughly coincided with the demise of the green algae, and the termination of growth in the blue-green species followed the exhaustion of epilimnetic N. Growth of Ceratium appears to have depended partly upon its ability to migrate to still greater depths in the water column. The seasonal population maximum appears not to have been ultimately limited by lack of nutrients. A hypothesis is proposed to explain the successional changes in terms of the cycle of thermal stratification and the extension of nutrient depletion into progressively deeper water in relation to the competitive advantages provided by behavioral adaptations of the motile species, and hence, their opportunities to exploit the nutrients available.