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 zoopl ankton on bacterioplankton production and abundance and on phytoplankton bi omass were studied in five experiments (from May to September) in Lake Erke n. 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 cl ose to colimited by P and N in July and August. Organic C stimulated bacter ial 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 phy toplankton growth in all experiments. In addition, P alone stimulated phyto plankton growth in May and July and the combination of P and N stimulated p hytoplankton growth in July and August. Zooplankton additions resulted in a decrease in phytoplankton biomass in May and September, mainly owing to gr azing on Cryptophyceae. The experimental results indicate that bacterioplankton and phytoplankton g rowth were uncoupled during most of the open-water period because P primari ly 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 w ere correlated during the season, but partial correlations analysis indicat es that this relationship can be attributed to the fact that both primary p roduction and bacterioplankton production showed strong positive correlatio ns with temperature. We suggest that uncoupling of bacterioplankton product ion and phytoplankton production may be a common phenomenon in lakes.