Does Seston Quality Limit the Presence of Mussels on Wellington’s South Coast? FlowCAM investigation of seston particle type

Temperate rocky shores around the world are characterised by so-called ‘universal’ zonation. An interesting exception to this rule can be seen along the shores of Wellington’s South Coast where there is a virtual absence of mussels and poorly developed rocky shore intertidal community. Yet just kilometres away in Wellington Harbour there is a fully developed intertidal community, including extensive multi-species mussel beds. This thesis aims to determine if the quality of seston is limiting the presence of mussels on Wellington’s South Coast. We now have the technology to see what types of particles the mussels are selecting in low and high quality seston conditions using a FlowCAM that allows identification of particle types and their physical properties.  This study compared environmental data for Wellington Harbour (seston-rich) and the South Coast (seston-poor). These data included chlorophyll a, turbidity and suspended sediment concentrations and were collected by CTD and satellite remote sensing. Mussel feeding experiments took place under a high and low quality diet during summer, autumn and winter using Perna canaliculus and Mytilus galloprovincialis. The physiological responses recorded were clearance rate, absorption efficiency and net energy balance for individual mussels. Environmental variables recorded were total particulate matter, particulate organic matter and percent organic matter. Water samples were collected during the feeding experiments and processed using the FlowCAM. By comparing the control chambers to chambers that had mussels feeding in them it was possible to see what particles the mussels were selecting.  The environmental variables revealed that Wellington Harbour had a much higher quality seston whereas in Cook Strait the seston quality was too low for mussels to be able to inhabit, as the chlorophyll a concentrations did not reach the required levels for mussel growth. Perna canaliculus and Mytilus galloprovincialis both showed physiological responses that would allow them to grow in Cook Strait waters, both species had positive absorption efficiencies and net energy balances. These responses were greater in the high quality diet in the enriched pond water in Nelson during summer and winter. The FlowCAM analysis revealed an inter-specific difference in preferential particle selection, which varied as a function of site and season. With more particles being preferentially selected in the high quality diet compared to when the mussels were feeding on Cook Strait seawater. This new information of particle selection helps to determine why mussels are absent from Wellington’s South Coast and contributes to the extensive information on mussel feeding.