A statistical method to estimate the biochemical composition of phytoplankton in the Southern Bight of the North Sea

Abstract

The knowledge of changes in the biochemical composition of phytoplankton and its degraded products is regarded as important for the understanding of an eco-system because it influences the heterotrophic activities of the seawater column and/or sediments. The linear regressions method of some specific metabolite (protein, carbohydrate, lipid) on chlorophyll a is considered as the better method to estimate the biochemical composition of natural phytoplankton in the coastal area of the Southern Bight of the North Sea. The validity of this statistical method depends on a correct assemblage of the results in time and space, and requires a good knowledge of the growth conditions of the phytoplankton cells. This essential condition is however difficult to realize for post-bloom periods where the growth conditions are more heterogeneous. The linear regressions lead on the one hand to the quantification of two components of the total particulate organic matter (phytoplankton and bacteria/detritus) and on the other hand to their biochemical characterization. The comparison of the biochemical composition of the two components shows that bacteria/detritus component has an effective nutritive value when phytoplankton is less abundant. The study of the influence of changes in the environmental conditions (light intensity and nitrogen availability), on the biochemical composition of the phytoplankton shows a similar protein content of about 50% when nitrogen is not limiting. On the other hand, a change in the light intensity does not change the biochemical composition of natural phytoplankton but increases the cellular chlorophyll content when nitrogen is sufficient to ensure a good phytoplanktonic growth.