Where are the mussels on Cook Strait (New Zealand) shores? Low seston quality as a possible factor limiting multi-species distributions

Abstract

Examination of the physiological ecology of 3 sympatrically occurring mytilids (the ribbed, blue and greenshell mussels, Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus, respectively) was carried out during summer and winter at 2 sites which differ markedly in their environmental characteristics (Mahanga Bay inside Wellington Harbour, where mussels are abundant, and Island Bay on the exposed Cook Strait shore, where mussels are absent). Seston components (particle counts per ml in the size range 2.5 to 63 μm, total particulate matter [TPM], particulate organic matter [POM], and percent organic matter [PCOM]) all varied significantly between sites and between seasons. Mahanga Bay seston exhibited characteristics typical of productive temperate coastal waters, whereas Island Bay seston was characterised by low POM and low PCOM values. Weight-standardised clearance rate (CR(s)), net absorption efficiency (AE) and Scope for Growth (SFG) values were determined for all 3 mussel species at both sites during both seasons. At Island Bay there was no evidence of seasonal differences in CR(s) whereas at Mahanga Bay species-specific CR(s) values were all highest in the winter. Regardless of season, CR(s) estimates for all 3 species were higher at Mahanga Bay than at Island Bay. At both sites, AE estimates were highest in the winter, and AE estimates were higher at Mahanga Bay than at Island Bay during each season. SFG estimates mirrored AE estimates, with SFG at both sites being highest in winter, and higher at Mahanga Bay than at Island Bay during each season. The SFG data support the hypothesis that mussels are absent from sites along Cook Strait shores because they do not enjoy a positive energy balance for long periods of time, consistent with the low seston organic matter levels at such locations. The SFG data also indicate that mussels inside Wellington Harbour can maintain a large, positive net energy balance throughout the year, consistent with the considerable biomass and very large size of all 3 species in this environment.