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Seasonal cycles of δ 13 C and δ 15 N in dissolved organic carbon and size-fractionated plankton, ranging from bacteria to the jellyfish Aurelia aurita, were studied during a 1 yr cycle at a coastal station in the Baltic Sea. The observed isotopic changes were found with time lags in all size-fractions of plankton. The δ 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 δ 15 N, the annual cycle was trimodal with 3 local maxima. 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 of these patterns are discussed in relation to measured oceanographic variables. In the summer, a depleted nitrogen isotopic signal was propagated through all size-classes of plankton, indicating direct or secondary utilisation of fixed nitrogen from cyanobacteria. The strength of the signal indicated that nitrogen-fixing cyanobacteria are more ecologically important as instantaneous nitrogen sources in the Baltic than previously assumed. Enrichment of δ 15 N in size-classes of plankton was found to be a linear function of logarithmic organism size from 20 to 500 μm, reflecting size-related 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 in the zooplankton community. The size-specific approach was found to be a simpler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per trophic level.

Seasonal variation in δ13C and δ15N of size-fractionated plankton at a coastal station in the northern Baltic proper