Abundance and productivity of bacterioplankton in relation to seasonal upwelling in the northwest Indian Ocean

The role of bacterioplankton in the Somali Current, the Gulf of Aden and the Red Sea was studied during the SW- (May–August 1992) and NE-monsoon (January–February 1993). The diversity in physical and biological characteristics of the regions and seasons is reflected in a broad range of both phyto- and bacterioplankton production. During the SW-monsoon, the Somali current showed highest bacterial production (up to 849 mgC m−2 day−1) in regions with enrichment of the surface waters by upwelling of cold, nutrient-rich, deep water. In contrast, the Gulf of Aden and the Red Sea were most productive during the NE-monsoon (average 225 mgC m−2 day−1). Depth profiles of the upper 300 m in general showed a subsurface maximum in bacterial abundance and production at 20–70 m depth. Heterotrophic activity and primary production were closely correlated, indicating the dependence of bacterioplankton on local phytoplankton-derived organic carbon and their ability to adapt quickly to changes in the environment. The bacterial carbon demand in the upper 300 m of the water column was largely supplied by phytoplankton production in the euphotic zone. Bacterial production was 18 ± 7% (average ± S.D.) of primary production. Assuming an assimilation efficiency of 50% for marine bacteria, they consumed up to half of the carbon produced by the phytoplankton. Cycling of carbon within the euphotic zone appears to be achieved by intense grazing by (micro)zooplankton and subsequent remineralization

COMPARISON OF THE PIGMENTATION OF TWO STRAINS OF THE PRYMNESIOPHYTE PHAEOCYSTIS SP.

Two strains of Phaeocystis sp., one isolated from the Weddell Sea region (Antarctica) and one from the North Sea, were compared for their growth characteristics and pigmentation during growth in batch cultures. Experiments were performed starting with identical nutrient and light conditions at 2°C, 7°C and 10°C. Division rates ranged from 0.17 to 0.94 d -1 depending on strain and temperature: the Antarctic strain grew fastest at 2°C (#=0.71 d-l), the strain from the North Sea at 10°C (#=0.94 d-l). Growth phase, phase in the diurnal cycle and temperature influenced the 19'hexanoyloxyfucoxanthin to chlorophyll a ratio in both strains. Large differences in this ratio were found between flagellates and colony cells from the same strain. Despite variability within each strain, mean levels of 19'hexanoyloxyfucoxanthin were always higher in the Antarctic strain. Another fucoxanthin-related pigment, 19'butanoyloxyfucoxanthin, showed the same trends during growth as 19'hexanoyloxyfucoxanthin in the Antarctic strain but was undetectable in the strain isolated from the North Sea. A comparison was made with field data collected during the Phaeocystis blooming period in the coastal zone of the North Sea. During this period relative amounts of 19'hexanoyloxyfucoanthin as well as the absence of 19'butanoyloxyfucoxanthin matched with the results from the North Sea strain in culture. Environmental factors influence pigment content and ratio, yet the two investigated Phaeocystis strains can be distinguished on the basis of their pigment characteristics

Responses of marine phytoplankton in iron enrichment experiments in the northern North Sea and northeast Atlantic Ocean

Short-term iron enrichment experiments were carried out with samples collected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The research area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml−1. Maximum chlorophyll a concentrations varied from about 1.0 μg l−1 in a high-reflectance zone (caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi) and about 3.5 μg l−1 in a zone in which the phytoplankton were growing, to about 0.5 μg l−1 in the northeast Atlantic Ocean. From the high-reflectance zone to the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 μM to 6.0 μM. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high-reflectance zone to <1.0 nM in the northeast Atlantic Ocean. In the research area, no signs of true iron deficiency were found, but iron enrichments in the high-reflectance zone, numerically dominated by Synechococcus sp., resulted in increased nitrate uptake. Ammonium uptake was hardly affected. Strong support for the effect of Fe on cell physiology is given by the increase in the f-ratio. Net growth rates of the phytoplankton (changes in cell numbers over 24 h) were almost unchanged. Phytoplankton collected from the northeast Atlantic Ocean, did not show changes in the nitrogen metabolism upon addition of iron. Net growth rates in these incubations were low or negative, with only slightly higher values with additional iron