Food web studies in a Norwegian kelp forest based on stable isotope (δ13C and δ15N) analysis

Abstract

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 13C depleted in spring and there was a significant difference between the basal (δ 13C: -16.65‰) and distal (δ13C: -18.67‰) parts of the lamina. Young kelp plants were more 13C depleted (-23.59‰). The red algae fell into 2 groups: one in the same range as kelp plants (δ13C: -18.93 to -22.27‰) and the other highly depleted (δ13C: -32.85 to -34.38‰). Phytoplankton showed an average δ13C value of -24.44‰, δ15N values from L. hyperborea differed between the basal (δ15N: 5.54‰) and distal (δ15N: 3.54‰) part of lamina. δ15N values for all primary producers values were in the range of 3.52 to 5.78‰. 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 13C-depleted red algae. δ15N values for 2 other gastropods, Gibbula sp. and Calliostoma zizyphinum (8.64 and 10.30‰, 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 δ13C signals in the same range as phytoplankton were amphipods. Based on δ15N 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 δ15N 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.