The provisioning of kelp-derived organic matter to higher trophic species is an ecological process integral to the health and function of temperate rocky reef communities. Suspension-feeding bivalves often constitute a large biomass within these systems, filtering large quantities of organic matter that provide an important trophic link between benthic and pelagic communities. In this regard, they are viewed as a sentinel organism used to evaluate ecosystem function. Kelp-derived organic matter has long been posited as a significant food source for bivalve suspension feeders. However, the extent to which they rely on kelp detrital production is poorly understood. In the present study, a combination of soft tissue and shell material stable isotope analyses was used to resolve the relative contribution of kelp-derived organic matter to the New Zealand green-lipped mussel (Perna canaliculus).

Surveys of kelp community structure revealed notable disparities in the composition of the six dominant canopy-forming species among sites along the Otago coast, although this did not directly translate to variability in total kelp biomass. The relative kelp-derived organic matter consumed by P. canaliculus were therefore attributed to differences in macroalgal species composition, and the environmental factors influencing kelp and phytoplankton production. A greater assimilation of kelp-derived organic matter by P. canaliculus was observed in winter compared to summer when phytoplankton contribution was greater. These findings coincided with the phytoplankton bloom cycle, but also kelp detrital production patterns which typically peak during late fall or winter. Additional analysis of P. canaliculus gut content and secondary production validated these seasonal patterns and illustrated phytoplankton as an abundant organic matter resource during the summer period.

To isolate the organic material within P. canaliculus shell layers, a sequential hydrochloric acid addition was performed. The findings demonstrate that extracted organics from P. canaliculus shells are sufficient for bulk stable isotope analysis. δ15N and δ13C site and whole tissue level comparisons showed meaningful ecological patterns that corresponded with environmental attributes. The observed variations in δ15N values across different tissue types were the result of distinct nitrogen assimilation rates specific to each tissue, and spatial differences in nitrogen loading within the Otago Harbour. Furthermore, P. canaliculus shell layers were shown to be an effective proxy of dietary seasonal shifts and will be a valuable approach over longer time scales where traditional soft tissue applications may be unsuitable.

Kelp forests are currently experiencing a global decline due to a combination of human induced stressors. Resolving the ecosystem level effects of the observed decline is therefore needed to guide effective ecosystem management strategies. Here it is suggested that stable isotope analysis of bivalve soft and shell tissues is an approach ecologists can utilize to detect shifts in ecosystem function and anthropogenic influence within kelp forest habitats. Additionally, the importance of kelp as a trophic resource for the New Zealand green-lipped mussel is demonstrated. As P. canaliculus represents an ecological, cultural, and commercially important species, the results presented here carry substantial implications for the management of both P. canaliculus and kelp forest communities.