Trophic ecology surrounding kelp and wood falls in deep Norwegian fjords

Abstract Food availability in fjords is unusual among deep-sea environments (defined here as systems below 200 m) due to the increased availability of nutritional sources that are comparatively rare in the open sea, such as influxes of macrophytodetritus and terrestrial organic matter. In open waters, these deep-sea ‘organic falls’ have been shown to increase beta diversity in comparison to surrounding, unenriched sediments, and support a range of specialised fauna. There has been unprecedented loss of coastal kelp forests in Norway and this is likely to greatly reduce the transfer of an important carbon subsidy to deep-sea communities in Norwegian fjords. In contrast, progress in terrestrial land management and reforestation has resulted in a large increase in the area covered by boreal forests. With such an expansion in forests fringing deep Norwegian fjords, an increase in the transport of wood material and forest detritus to these deep-sea habitats seems inevitable. Benthic landers containing experimental substrates (wood blocks and kelp parcels) were deployed for 10 months at a depth of 530 m in Osterfjorden, and stable isotope analysis (13C, 15N) was used to investigate and compare trophic relationships between wood and kelp substrates and the macrofauna that colonised them. Trophic shift analyses showed a dependence on kelp either as a direct dietary source or a primary source of carbon in most of the fauna analysed, whereas there was little evidence of wood providing an importance dietary subsidy. Modelling analyses showed that kelp detritus comprised a large percentage of the diet of all of the taxa collected from the kelp samples, and several from the wood samples. Ontogenetic trophic changes (i.e., differences in δ13C or δ15N depending on animal size) were seen in several of the taxa analysed, revealing changes in dietary preference and increasing trophic level as a function of size/age. This study provides evidence of the importance of kelp in the trophic ecology of communities living at the Norwegian deep-sea floor. The continued loss of kelp from shallow, coastal systems may have profound effects for these communities as they face a reduction in this important subsidy.