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

Abstract

The use of riverine allochthonous organic matter by plankton in the northern Baltic Sea was studied using stable isotopes of carbon and nitrogen. Dissolved and particulate material was sampled in the main Swedish rivers entering the Bothnian Bay and the Bothnian Sea. At 4 sea stations, dissolved matter, plankton and nekton were sampled in 11 size-classes: below 0.7 μm filtrate, 0.7-5, 5-20, 20-50, 50-100, 100-200, 200-500, 500-2500 μm, mysids (~10 and ~20 mm) and herring. A riverine influence could be detected in δ13C, but not in δ15N. Except for the 5-20 μm fraction, the carbon became enriched in 13C with increasing salinity in all plankton size-fractions. This mainly reflected differences in the δ13C of phytoplankton, due to the different δ13C of marine and riverine dissolved inorganic carbon. Plankton use of riverine dissolved organic carbon (DOC) was tested with a 2-component model. The δ13C of riverine DOC and the basin phytoplankton were taken to represent the allochthonous and autochthonous signals, respectively. Use of riverine DOC was indicated for larger zooplankton (200-2500 μm) in the Bothnian Bay, confirming earlier suggestions of extensive use of riverine DOC in the zooplankton food webs in this basin. A new method to adjust δ13C in zooplankton for variations in lipid content by analysis of covariance was successfully demonstrated. The isotope data reflect the presence of 2 trophic structures, coinciding with the microheterotrophic food web (0.7-5 μm) and the classic grazing food web (> 50 μm). From phytoplankton to large zooplankton, δ15N increased linearly with the logarithm of organism size in all basins. This increase was steeper in the Bothnian Sea and the Baltic Proper, where zooplankton diversity is higher, than in the Bothnian Bay, possibly reflecting more complex food-web interactions.