Bivalve nutrient cycling : nutrient turnover by suspended mussel communities in oligotrophic fjords

This study examined a range of eco-physiological processes (i.e filtration, growth, excretion, faeces production) and feedback mechanisms with the aim to investigate the contribution of suspended mussel Mytilus edulis communities to nutrient cycling in oligotrophic fjords. Previous work has shown that bivalves have the potential to play an important role in coastal nutrient cycling. Understanding bivalve nutrient dynamics is particularly essential in oligotrophic environments, where bivalve communities potentially have a higher influence as a consequence of low background nutrient levels. The eco-physiological response of individual mussels to oligotrophic conditions indicated that clearance and biodeposition rates were related to food/nutrient availability and were therefore respectively higher and lower compared to rates determined for eutrophic conditions. No specific responses to oligotrophic conditions were observed for excretion of inorganic metabolites or nutrient storage in tissue. However, in situ methods that determined nutrient dynamics along suspended communities (ropes) demonstrated that rates under field conditions may differ from what can be expected from extrapolation of rates measured in the laboratory for individual mussels. Clearance rates were lower for communities while nutrient regeneration was higher, specifically during periods with high fouling activity of ascidians. This study thereby highlights the need to consider community specific processes while evaluating bivalve-ecosystem interactions. Biodeposition is an important pathway in bivalve nutrient cycling and represented up to 47% of ingested nutrients under oligotrophic conditions. Nutrient releases from decomposing biodeposits were high for all nutrients (C-N-P-Si), and approximately 24% of carbon and 17% of nitrogen in the biodeposits were mineralized with enhanced temperatures resulting in faster decomposition (Q10=2-3). Combining mussel physiology with physical conditions of the systems showed that the fraction of ingested nutrients allocated to either nutrient regeneration (source) or nutrient removal (sink) was similar between oligotrophic fjords and eutrophic bays. Nutrient regeneration was imbalanced for each of the elements and differed from ratios observed in the ambient water. Mussel cultures thereby have the potential to influence phytoplankton community composition. However, positive and negative feedback estimates indicated that present mussel aquaculture in Norwegian fjord systems has low influence on nutrient cycling due to the low bivalve densities and physical characteristics of the fjords. This thesis provided insights in the pathways in which mussels interact with nutrient cycling, with special reference to oligotrophic conditions. The empirical data collected in this study can be applied to optimize models that simulate bivalve-ecosystem interactions, and thereby help to understand and predict the exploitation and management of coastal zones.