Phaeocystis Globosa Bloom Research Articles

Mesozooplankton grazing during the Phaeocystis globosa bloom in the southern bight of the North Sea

During spring blooms 1998 and 1999, three complementary methods were used to evaluate the in situ feeding activities of the dominant copepod species of the Belgian coastal zone: gut pigment content analysis using HPLC, the 14C tracer method, and cell count experiments. The results obtained by all three methods consistently showed that Phaeocystis globosa is not an adequate food source for the spring copepods in the Belgian coastal zone. Our results demonstrated that, among the potential prey, copepods strongly selected diatoms and microzooplankton, and that these types of prey accounted for the major part of the ingested carbon. However, diatoms and microzooplankton ingestion did not always seem sufficient in terms of carbon to avoid food limitation. Comparison of clearance rates exerted on different potential prey types during the P. globosa peak with those before and after the P. globosa peak showed that the copepods' feeding pressure on diatoms was reduced during the P. globosa peak while that on microzooplankton was not. The low grazing pressure on P. globosa, together with the preferential grazing on diatoms, which reduces the competition for nutrients, and the predation on microzooplankton organisms, which reduces the microzooplankton grazing pressure on P. globosa cells, are likely to favour the P. globosa bloom in the Southern Bight of the North Sea.

Accumulation and sedimentation of Phaeocystis globosa in the Dutch Wadden Sea

In the Marsdiep tidal inlet, Phaeocystis globosa blooms have occurred every year since phytoplankton monitoring started in the early 1970s. A high spring peak is usually followed by lower peaks in summer. Detailed cell counts are presented for the years 1993, 1994, and 1995, of which 1994 was exceptional with a low spring peak but a much higher peak in June/July. Over tidal flats in the western Dutch Wadden Sea, I found Phaeocystis colony accumulation and cell densities one to two orders of magnitude higher than in the Marsdiep tidal inlet; colonies settled on the tidal flats, but did not become part of larger aggregates. The accumulation process resembles similar processes earlier described for fine sediments in the Wadden Sea and may be caused by similar processes. The small benthic gastropod Hydrobia ulvae at low tide grazed on colonies settled on the sediment. This loss of colonies from the water column by settling and grazing may help in rapidly decreasing P. globosa cell numbers after the spring peak in the Wadden Sea. Massive sedimentation and subsequent mineralization of Phaeocystis colonies might induce low oxygen events and mass mortality of macrobenthos, but up to now no direct relation has been reported between Phaeocystis sedimentation and mass mortality in zoobenthos.