Seasonal variation in delta C-13 and delta N-15 of size-fractionated plankton at a coastal station in the northern Baltic proper

Seasonal cycles of delta(13)C and delta(15)N in dissolved organic carbon an d size-fractionated plankton, ranging from bacteria to the jellyfish Aureli a aurita, were studied during a 1 yr cycle at a coastal station in the Balt ic Sea. The observed isotopic changes were found with time lags in all size -fractions of plankton. The delta(13)C showed a bimodal cycle with 2 local maxima, the first coinciding with the spring bloom and the second with the autumn bloom. In delta(15)N, the annual cycle was trimodal with 3 local max ima. The first occurred in connection with the spring bloom, the second in mid-summer and the third was a broad autumn-to-winter maximum. The causes o f these patterns are discussed in relation to measured oceanographic variab les. In the summer, a depleted nitrogen isotopic signal was propagated thro ugh all size-classes of plankton, indicating direct or secondary utilisatio n of fixed nitrogen from cyanobacteria. The strength of the signal indicate d that nitrogen-fixing cyanobacteria are more ecologically important as ins tantaneous nitrogen sources in the Baltic than previously assumed. Enrichme nt of delta(15)N in size-classes of plankton was found to be a linear funct ion of logarithmic organism size from 20 to 500 mu m, reflecting size-relat ed consumption patterns of marine plankton food-webs. The explanatory power of the Linear regression and the enrichment per unit size were stronger in spring and autumn than in the summer, reflecting time lags and diversity i n the zooplankton community. The size-specific approach was found to be a s impler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per t rophic level.