Regulation of phytoplankton biomass in estuarine enclosures

During April, June/July and September 1986, phytoplankton biomass was followed in marine enclosures in the Roskilde Fjord, Denmark, manipulated by the addition of benthic suspension feeders, planktivorous fish, nutrients and contact to the sediment. During periods with high insolation and temperatures from 11 to 22°C, the manipulations caused marked changes in the development of phytoplankton biomass.

Additions of Mytilus edulis reduced phytoplankton biomass to between 10 and 59 % of controls, whereas addition of nutrients raised phytoplankton biomass to an average of 256 % of controls. Generally, low growth rates of mussels were found in enclosures containing mussels alone. Addition of planktivorous fish and nutrients increased growth rates of mussels.

During June/July, when inorganic nitrogen limited phytoplankton growth, autotrophic picoplankton (1 to 2 μm cell diameter) constituted 70 to 93 % of phytoplankton biovolume in enclosures containing mussels compared to 4 to 20 % in controls. The mussels reduced phytoplankton biomass by only about 50 % during this period, presumably due to low retention efficiency of the small cells.

In April and September, however, when nitrogen did not control phytoplankton growth, picoplankton comprised < 0.001 % of phytoplankton biovolume. During September (temperatures 11 to 13°C), M. edulis reduced chlorophyll levels to 10 % of controls and the effects of nutrient additions were significantly reduced in enclosures containing mussels.

The effect of fish additions revealed that zooplankton grazing removed (average for June and September) about 20 % of phytoplankton biomass. The sediment acted primarily as a nutrient source during summer. However, in April at temperatures > 6 °C and in September, benthic suspension feeders maintained chlorophyll levels below those in enclosures with no sediment contact.

During April, only minor changes were recorded in the chlorophyll levels in the various enclosures, presumably due to the low temperature (0 to 4.5°C) and a phytoplankton population of decaying diatoms. Thus, several factors or combinations of factors controlled the phytoplankton biomass in the enclosures.

The most important of these factors was the balance between nutrient input, phytoplankton size structure and the physiological state of the mussels.