TEMPORAL AND SPATIAL VARIATIONS IN HETEROTROPHIC NANOFLAGELLATE ABUNDANCE IN NORTH-SEA SEDIMENTS

Summer-winter variations in marine nanoflagellate densities at 3 depth s in North Sea sediments (0-3, 30-33 and 60-63 mm) were studied using epifluorescence microscopy. Benthic flagellate densities in summer ran ged from 7 to 859 x 10(3) and in winter from 9 to 1100 X 10(3) cells c m-3. The effect of season on flagellate densities was different among stations. At 10 out of 15 stations summer values were significantly hi gher than winter values. The effect of season on flagellate densities was the same at all 3 depths. Flagellate densities in the sediment sur face 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 flagel late densities near Esbjerg (Denmark) were remarkable: 859 x 10(3) cel ls cm-3 in the surf ace layer in summer and 1100 X 103 cells cm-3 in t he 2 deeper layers during winter. In both seasons, at all depths and a ll stations most cells (50 to 75%) occurred in the 2 to 5 mum size cla ss. Few flagellates were larger than 10 mum or smaller than 2 mum. Poo led winter and summer data of flagellate densities in the sediment sur face 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 flagel late density and bacterial specific growth rate and grain size, togeth er explaining 53% of the variance. In winter nanoflagellate densities were significantly correlated with bacterial biomass and abundance acc ounting for 59% and 33%, respectively, of the variance. The data sugge st that bacterial biomass/abundance during winter sets limits to flage llate densities. Increased bacterial production was probably responsib le for generally higher summer flagellate densities although grain siz e could become a limiting factor for flagellate densities in silty sed iments during summer.