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In situ enclosure experiments on the influence of cultured mussels (Perna canaliculus) on phytoplankton at times of high and low ambient nitrogen |
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| Authors: | Shaun C Ogilvie Alex H Ross David R Schiel |
| Institution: | a Ecology and Entomology Group, Lincoln University, PO Box 84, Canterbury, New Zealand b National Institute of Water and Atmospheric Research, PO Box 8602, Riccarton, Christchurch, New Zealand c National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand d University of Canterbury Zoology Department, Private Bag 4800, Christchurch, New Zealand |
| Abstract: | The influence of the cultured mussel Perna canaliculus (Gmelin 1791) on the abundance of phytoplankton was investigated in Pelorus Sound, New Zealand. Four in situ enclosure experiments were undertaken, two in summer when ambient nitrogen was low, and two in winter when it was high. Each experiment had four manipulation types: added mussels; added nitrogen; both mussels and nitrogen added; and control (no additions). In summer, there was a significant increase of chlorophyll a in response to added nitrogen, indicating that the phytoplankton were nitrogen-limited. At this time, mussels caused an increase (11-17%) in phytoplankton biomass, possibly by converting particulate nitrogen to ammonium, making the nitrogen available for phytoplankton utilisation. The highest ambient chlorophyll a concentrations coincided with high ambient nitrogen in the winter. At this time, mussel grazing caused a significant decrease (5-14%) in phytoplankton concentration, indicating that within-farm depletion of phytoplankton is most likely to occur in winter. On an annual time scale, the mussels had a stabilising influence on phytoplankton biomass, reducing high ambient levels in winter and slightly increasing low levels in summer. |
| Keywords: | Enclosure experiments Perna canaliculus Phytoplankton Nitrogen |
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