Phytoplankton biomass associated with mussel farms in Beatrix Bay, New Zealand

Abstract

The spatial and temporal variability in phytoplankton abundance is a major factor determining the productivity of mussel ( Perna canaliculus) farms. During periods of low phytoplankton abundance, food depletion may be a significant factor in the productivity of mussel farms. Measurements of phytoplankton abundance (as chlorophyll a) were made over the entire depth of the water column both inside and outside four mussel farm sites in Beatrix Bay, over a 13-month period. Ambient (outside) concentrations of dissolved inorganic nitrogen (NO 3–N and NH 4–N) and dissolved reactive phosphorus (DRP) were also measured. The highest ambient chlorophyll a concentrations in the surface waters occurred during autumn–winter, with peak levels of around 5 μg l −1. At this time, all farms had a significant reducing impact on phytoplankton biomass ( P<0.05 in all cases). The lowest ambient chlorophyll a concentrations of <0.5 μg l −1 were recorded in the summer, coinciding with low nitrogen concentrations. In November, there was significantly more phytoplankton inside all the farms ( P<0.05 in all cases). It was hypothesised that this increase occurred because mussels are net producers of dissolved inorganic nitrogen. In five out of the 7 months when farms were surveyed, the highest chlorophyll concentrations were in the deeper water, associated with a pycnocline. Two management options to increase mussel productivity are presented: (1) Deployment of mussel dropper ropes to deeper waters to take advantage of chlorophyll maxima in summer; (2) Artificially increasing nitrogen inside farms during spring and summer to increase phytoplankton supply.