The influence of nutrient addition on growth rates of phytoplankton groups, and microzooplankton grazing rates in a mesocosm experiment

The influence of nutrient addition on the growth and microzooplankton grazing rates of individual phytoplankton groups was investigated in a mesocosm experiment including nutrient manipulation and using the dilution technique. The nutrient addition caused phytoplankton biomass to increase; chlorophyll a was less than 3.5 μg l −l in the control enclosure, while chlorophyll a increased to 19 and 35 μg l −l in enclosures added nitrogen and phosphate (NP) and nitrogen, phosphate and silicate (NPSi), respectively. The growth and grazing rates were highly variable during the experimental period and the grazing rates were significantly correlated to the growth rates. Diatoms, prymnesiophytes, and dinoflagellates occasionally reached very high growth rates, up to 2.8 d −l. The phytoplankton in the NPSi enclosure consisted mostly of diatoms, which proved unsuitable for incubation, since they formed aggregates in the incubation bottles, and the dilution experiments carried out in this enclosure were consequently not successful. The addition of nutrients (N, P, Si, Mn, and Fe) to the dilution experiment caused an increase in the growth rates of the phytoplankton both in the control and the NP enclosures. In both the control and NP enclosures, the growth rates of the diatoms increased significantly owing to nutrient addition.