Trophic importance of <i>Laminaria hyperborea</i> to kelp forest consumers and the importance of bacterial degradation to food quality

The nutritional value of kelp Laminaria hyperborea (Gunn.) Foslie and the importance of bacterial degradation in making kelp available to consumers were tested by no-choice feeding experiments. Fresh and dead kelp material degraded in situ was sampled. In the laboratory, fresh kelp was degraded for 3, 14 and 44 d by kelp associated bacterial communities under aerobic and anaerobic conditions. The fresh and degraded kelp products were fed to the kelp associated amphipods Jassa falcata, Lembos websteri, Ampithoe rubricata and Gammarus locusta and the gastropod Rissoa parva. None of the amphipods tested could survive or grow on fresh kelp, but bacterial degradation decreased the C:N ratio and the phlorotannin content and made kelp available as food. In contrast, the gastropod survived on both fresh and degraded kelp. C-13: C-12 and N-15: N-14 isotope analysis of fauna and possible food items sampled in the field suggested that kelp derived particulate organic matter (POM) is an important food source for kelp epifauna.