Effects of nutrients (phosphorous, nitrogen, and carbon) and zooplankton on bacterioplankton and phytoplankton - a seasonal study

The effects of inorganic nutrients (P and N), organic C, and metazoan zooplankton on bacterioplankton production and abundance and on phytoplankton biomass were studied in five experiments (from May to September) in Lake Erken. In addition, the seasonal dynamics of bacterioplankton and phytoplankton were followed in the lake from April to November. Bacterioplankton production was P limited from May to August. N alone never stimulated bacterioplankton production, but bacterioplankton growth was close to colimited by P and N in July and August. Organic C stimulated bacterial production in June and September. Zooplankton enhanced bacterioplankton production in June, when bacterioplankton production was limited by P and C and the phytoplankton biomass in the lake was low. N alone stimulated phytoplankton growth in all experiments. In addition, P alone stimulated phytoplankton growth in May and July and the combination of P and N stimulated phytoplankton growth in July and August. Zooplankton additions resulted in a decrease in phytoplankton biomass in May and September, mainly owing to grazing on Cryptophyceae. The experimental results indicate that bacterioplankton and phytoplankton growth were uncoupled during most of the open-water period because P primarily limited bacterioplankton growth and N limited phytoplankton growth. The response of the bacterioplankton community was most likely a direct effect of nutrient additions. Primary production and bacterioplankton production were correlated during the season, but partial correlations analysis indicates that this relationship can be attributed to the fact that both primary production and bacterioplankton production showed strong positive correlations with temperature. We suggest that uncoupling of bacterioplankton production and phytoplankton production may be a common phenomenon in lakes.