Abstract
In May 1992 the effects of changing nutrient conditions on the algal community structure and the nutritional state of phytoplankton populations were studied in three seawater enclosures installed at the Marine Biological Field Station near Bergen, Norway. The enclosures were fertilized with nitrate and phosphate; they differed in the amount of phosphate administered. Phytoplankton groups were identified on the basis of pigment concentrations measured by means of high performance liquid chromatography. Multivariate statistical analysis of the data revealed that the algal communities in the three enclosures developed according to a comparable pattern. In the first half of May phytoplankton composition changed from a dinoflagellate-rich community into a diatom bloom culminating around 10 May; in the second half of May haptophyte blooms developed in two enclosures. Carotenoid/chlorophyll a ratio was used as a diagnostic tool to probe the nutritional state of the phytoplankton commmunity. Nutrient-stressed or senescent cells could not be detected. The observed changes in the ratio were more likely the result of changes in species composition. It is discussed that the carotenoid/chlorophyll a ratio is an invalid indicator of nutrient stress on the level of the whole algal community, due to the complex interactions in the pelagic ecosystem.