Fate of increased production in late-summer plankton communities due to nutrient enrichment of the Baltic Proper

Abstract

The fate of increased production due to nutrient enrichment was studied experimentally in late-summer plankton communities in the central Baltic Sea in 1997 and 1998. In the first year, inorganic nitrogen and phosphorus, as well as labile organic carbon, were added to natural and 0.8 μm pre-filtered seawater in a 23 factorial design, and the response of the heterotrophic bacterial community was followed. In the second year, 2 experiments were carried out, where the response of the whole microbial community to nutrient and carbon enrichments was studied. In the first of those experiments, the seawater was pre-filtered through a 40 μm filter to exclude filamentous cyanobacteria. In the second experiment, a 40-fold concentrate of >90 μm plankton was added, resulting in a 9-fold concentration of phytoplankton biomass compared to natural levels. All experiments were run for 3 d in a deck incubator equipped with continuous seawater flow. Phytoplankton primary production (PP) was limited by both nitrogen and phosphorus, while bacterial production (BP) was co-limited by inorganic nutrients and labile carbon source. The increased PP resulted in a higher biomass of picocyanobacteria and phototrophic flagellates, while filamentous cyanobacteria showed no positive response. The increased BP did not result in a higher bacterial biomass (BB), indicating grazing control by protozoa. In accordance, higher flagellate and/or ciliate biomasses were observed in the enriched treatments. Picocyanobacteria therefore seemed to be less edible to protozoa than the heterotrophic bacteria. The increase in BP by carbon addition increased biomasses at the higher trophic levels more than the increase in PP, indicating a tighter link in the heterotrophic part of the microbial food web and an important pathway for organic material in regenerated conditions.