Temporal and spatial variations in heterotrophic nanoflagellate abundance in North Sea sediments

Summer-winter variations in marine nanoflagellate densities at 3 depths in North Sea sediment~ (0-3, 30-33 and 60-63 mm) were studied using epifluorescence microscopy. Benthic flagellate densities in summer ranged from 7 to 859 X lo3 and in winter from 9 to l100 X 103 cells ~ m ~ . T h e effect of season on flagellate densities was different among stations. At 10 out of 15 stations summer values were significantly higher than winter values. The effect of season on flagellate densities was the same at all 3 depths. Flagellate densities in the sediment surface layer (0-3 mm) were in general 2 to 4 times higher than in the 2 deeper sampled layers (30-33, 60-63 mm). The extraordinary high flagellate densities near Esbjerg (Denmark) were remarkable: 859 X 103 cells in the surface layer in summer and 1100 X 103 cells in the 2 deeper layers during winter. In both seasons, at all depths and all stations most cells (50 to 75%) occurred in the 2 to 5 pm size class. Few flagellates were larger than 10 pm or smaller than 2 pm. Pooled winter and summer data of flagellate densities in the sediment surface layer showed a positive correlation with bacterial production and bacterial specific growth rate, explaining 20 and 30% respectively of the variance. In summer a positive correlation existed between flagellate density and bacterial specific growth rate and grain size, together explaining 53% of the variance. In winter nanoflagellate densities were significantly correlated with bacterial biomass and abundance accounting for 59% and 33%, respectively, of the variance. The data suggest that bacterial biomass/abundance during winter sets llmits to flagellate densities. Increased bacterial production was probably responsible for generally higher summer flagellate densities although grain size could become a limiting factor for flagellate densities in silty sediments during summer