Food web studies in a Norwegian kelp forest based on stable isotope (delta C-13 and delta N-15) analysis

Samples of kelp Laminaria hyperborea, various species of red algae, phytoplankton, material from sediment traps, invertebrates, fish and seabirds were collected on the west coast of Norway and the abundance of stable isotopes was analyzed. Canopy plants of L. hyperborea were most C-13 depleted in spring and there was a significant difference between the basal (delta(13)C: -16.65parts per thousand) and distal (delta(13)C: -18.63parts per thousand) parts of the lamina. Young kelp plants were more C-13 depleted (-23.59parts per thousand). The red algae fell into 2 groups: one in the same range as kelp plants (delta(13)C: -18.93 to -22.23parts per thousand) and the other highly depleted (delta(13)C: -32.85 to -34.38parts per thousand). Phytoplankton showed an average delta(13)C value of -24.44parts per thousand. delta(15)N values from L. hyperborea differed between the basal (delta(15)N: 5.54parts per thousand) and distal (delta(15)N: 3.54parts per thousand) part of lamina. delta(15)N values for all primary producers values were in the range of 3.52 to 5.78parts per thousand. Among the gastropods, Helcion pellucida proved to be a kelp plant grazer, Lacuna vincta probably fed on both kelp and various species of red algae, whereas Aplysia punctata grazed exclusively on the most C-13-depleted red algae. delta(15)N values for 2 other gastropods, Gibbula sp. and Calliostoma zizyphinum (8.64 and 10.30parts per thousand, respectively), suggest that they belong to higher trophic levels. Filter feeders occupied lower trophic levels and received variable carbon inputs from kelp. The only group of animals with delta(13)C signals in the same range as phytoplankton were amphipods. Based on delta(15)N values, the fish varied from intermediate to top consumers (trophic level: 2.6 to 3.3). Two species of seabirds were included in the study and their delta(15)N values suggested 2 different trophic levels, which correspond to their known feeding preferences. A mixing model suggests that kelp-derived carbon plays an important role in this nearshore system and that kelp may serve as a carbon source for marine animals with several different types of feeding strategies.