Phytoplankton Dynamics in Marlborough Sounds

Phytoplankton are microscopic free-floating plants in the water. A signature feature of phytoplankton dynamics is a sporadic increase in their population, often referred to as "blooms." Mussels extract phytoplankton from the water by filter feeding, and it is the most important food source for mussels. Blooms provide pulses of food to mussels and higher trophic levels. Blooms of some species may be toxic, and are the cause of heath advisories against harvesting mussels during certain times of the year.

Phytoplankton growth is affected by nutrient supply, light, temperature, water column stratification, and grazing by shellfish and zoolplankton. This study investigated the interaction of light and nitrate in structuring phytoplankton communities in Pelorus Sound, the most heavily farmed region in the country.

Nitrate availability was found to be limiting to phytoplankton growth during spring and summer, and there is considerable variation in size classes in their response to nitrate and light. Long-term monitoring revealed that the species which responded most rapidly to nitrate enrichment have historically dominated the phytoplankton assemblage, suggesting that competition for nitrate is a key component in structuring the community. Taxa that maintained biovolume in shaded treatments were historically both abundant and rare, suggesting that light is secondary to nutrient supply in structuring phytoplankton communities.