Use of riverine organic matter in plankton food webs of the Baltic Sea

The use of riverine allochthonous organic matter by plankton in the norther n Baltic Sea was studied using stable isotopes of carbon and nitrogen. Diss olved and particulate material was sampled in the main Swedish rivers enter ing the Bothnian Bay and the Bothnian Sea. At 4 sea stations, dissolved mat ter, plankton and nekton were sampled in 11 size-classes: below 0.7 mu m fi ltrate, 0.7-5, 5-20, 20-50, 50-100, 100-200, 200-500, 500-2500 mu m, mysids (similar to 10 and similar to 20 mm) and herring. A riverine influence cou ld be detected in delta(13)C, but not in delta(15)N. Except for the: 5-20 m u m fraction, the carbon became enriched in C-13 with increasing salinity i n all plankton size-fractions. This mainly reflected differences in the del ta(13)C Of phytoplankton, due to the different delta(13)C Of marine and riv erine dissolved inorganic carbon. Plankton use of riverine dissolved organi c carbon (DOC) was tested with a 2-component model. The delta(13)C Of river ine DOC and the basin phytoplankton were taken to represent the allochthono us and autochthonous signals, respectively. Use of riverine DOC was indicat ed for larger zooplankton (200-2500 mu m) in the Bothnian Bay, confirming e arlier suggestions of extensive use of riverine DOC in the zooplankton food webs in this basin. A new method to adjust delta(13)C in zooplankton for v ariations in lipid content by analysis of covariance was successfully demon strated. The isotope data reflect the presence of 2 trophic structures, coi nciding with the microheterotrophic food web (0.7-5 mu m) and the classic g razing food web (>50 mu m). From phytoplankton to large zooplankton, delta( 15)N increased linearly with the logarithm of organism size in an basins. T his increase was steeper in the Bothnian Sea and the Baltic Proper, where z ooplankton diversity is higher, than in the Bothnian Bay, possibly reflecti ng more complex food-web interactions.