Does differential particulate food supply explain the presence of mussels in Wellington Harbour (New Zealand) and their absence on neighbouring Cook Strait shores?

Rocky intertidal reef communities in Wellington Harbour, New Zealand, are dominated by mussels (the ribbed mussel Aulacomya maoriana, the blue mussel Mytilus galloprovincialis, and the greenshell mussel Perna canaliculus). Only a few kilometres away, outside the Harbour on exposed Cook Strait shores, these mussels are absent. We tested the hypothesis of bottom-up food limitation as the explanation for this distributional difference. The water column at three Harbour sites and five Cook Strait coastal sites was sampled over an 18-month period for temperature, salinity, dissolved oxygen, total particulate matter (TPM), particulate organic matter (POM), percent organic matter (PCOM), chlorophyll a (Chl a), particle counts (number of particles mL−1 in the range 2.5–63 μm), percent carbon, percent nitrogen, and C:N ratio. Mean values of PCOM and Chl a were significantly higher (p < 0.05) in the Harbour than in Cook Strait. On two separate occasions mussels were transferred from Wellington Harbour to the Island Bay Marine Laboratory (IBML on Cook Strait) and sampled at regular intervals to permit the determination of body condition index (CI) and mortality rate to measure their response to the coastal seston regime. On both occasions monthly CI values of all three species held at IBML decreased significantly when compared with monthly CI values of mussels collected from Harbour sites. Mortality rates at IBML exhibited consistent taxon-specific responses (P. canaliculus > M. galloprovincialis > A. maoriana). We interpret these field-based and laboratory-based findings as providing support for the hypothesis that multi-species mussel distributions and hence intertidal community structure at Cook Strait sites are regulated at least in part by particulate food supply.