Primary production studies in the southern bight of the North Sea with reference to Phaeocystis sp. and its adaptation to varying photon flux densities

Abstract

The ecology of coastal areas in Northern Europe are under pressure as a result of the accumulation of nutrients of terrestrial origin discharged from the Eastern channel to the German Bight. A perceived consequence of this ongoing eutrophication has been an increase in algal activity and a shift in plankton community equilibria. In the Southern North Sea such a shift has given rise to the dominance of the colony forming Pyrmnesiophyte Phaeocystis sp. Well adapted to the turbid, nutrient rich environment, it is now the main primary producer of this region. Subsequently the need to better understand this species, and to be able to measure both more accurately, and interpret more precisely, primary production data, has become of increasing importance. To date the bulk of primary production data comes from in vitro 14-carbon tracer experiments. Increasingly the accuracy of in vitro incubations and the 14-tracer technique has been questioned. In this thesis, a comparative study of in vitro and in situ primary production estimates was undertaken, using the light and dark bottle oxygen Winkler titration technique. The results were compared with 14-carbon derived estimates from the literature. The oxygen and 14-carbon measurements were in the same order of magnitude, however a 50&#37; discrepancy was observed between in vitro and in situ oxygen measurements. The balance between gross production and respiration, expressed as a difference (net community production), or as a ratio (R/Pmax), was observed over the seasonal cycle in the Southern North Sea. It was found that photosynthesis dominated in early April and October, whereas respiration, apparently uncoupled from photosynthesis, peaked in May, when the Phaeocystis bloom had become senescent. An apparent coupling between growth and photosynthetic rates was observed in April. The relationship between growth rate and irradiance in phaeocystis was investigated in the laboratory, and its ability to maintain high growth rates at low photon flux densities was observed. Increase in cellular chlorophyll a concentration and the increase in colony formation were possible photoadaptations of phaeocystis.</p