Investigating the contribution of mussel N regeneration to coastal primary production using stable isotope tracers

Determining the sources, pathways and sinks of inorganic nitrogen is integral to our understanding of one of the main determinants of primary productivity in the marine environment. The current view of rocky shore productivity is that it is largely fuelled by ‘new’ inorganic nitrogen brought to surface waters by the physical process of upwelling. However, along the rocky shores of the Washington State outer coast, the high densities of mussels (Mytilus californianus) colonizing these shores produce significant quantities of ‘regenerated’ inorganic nitrogen in the form of ammonium, a preferred nitrogen source for primary production. In this study, we seek to determine to what extent regenerated nitrogen is responsible for fueling primary production in these environments. To this end, we employed stable isotope tracers (15NH4 and 15NO3) to track the pathway of inorganic nitrogen in several rocky shore tidepools over the course of half a tidal cycle. Half of all the pools contained mussels in their natural abundance, while half were mussel control pools in which most of the mussels had been physically removed. Discrete water and algal tissue samples were taken at several time points within the study period for mass spectrometric stable isotope analysis. Preliminary results show isotope dilution of tidepool ammonium in pools containing mussels over half a tidal cycle, due to continued ammonium production by mussels. Combined concentration data, regeneration rates as well as removal rates due to autotrophic uptake and/or microbially-mediated ammonium oxidation (nitrification) will be calculated. Isotopic analysis of algal tissue samples and of the other nitrogen pools will shed further light on the contribution of regenerated ammonium to tidepool biogeochemical cycling and ultimately to coastal primary production.