Effects of wastewater treatment plant effluent inputs on planktonic metabolic rates and microbial community composition in the Baltic Sea
- 1Interdisciplinary Ecology group, Department of Biology, University of the Balearic Islands, 07122, Palma, Spain
- 2National Institute of Aquatic Resources, Section for Oceanography and Marine Ecology, Technical University of Denmark, Charlottenlund, Denmark
- 3Centre for Ecology and Evolution in Microbial model Systems, EEMiS, Linnaeus University, 39182 Kalmar, Sweden
- 4Department of Geology, Lund University, 223 62, Lund, Sweden
- 5Department of Biology, Lund University, 223 62, Lund, Sweden
- apresent address: Department of Oceanography, Center for Microbial Oceanography Research and Education (C-MORE), University of Hawaii at Manoa, 96822, Honolulu, USA
Abstract. The Baltic Sea is the world's largest area suffering from eutrophication-driven hypoxia. Low oxygen levels are threatening its biodiversity and ecosystem functioning. The main causes for eutrophication-driven hypoxia are high nutrient loadings and global warming. Wastewater treatment plants (WWTP) contribute to eutrophication as they are important sources of nitrogen to coastal areas. Here, we evaluated the effects of wastewater treatment plant effluent inputs on Baltic Sea planktonic communities in four experiments. We tested for effects of effluent inputs on chlorophyll a content, bacterial community composition, and metabolic rates: gross primary production (GPP), net community production (NCP), community respiration (CR) and bacterial production (BP). Nitrogen-rich dissolved organic matter (DOM) inputs from effluents increased bacterial production and decreased primary production and community respiration. Nutrient amendments and seasonally variable environmental conditions lead to lower alpha-diversity and shifts in bacterial community composition (e.g. increased abundance of a few cyanobacterial populations in the summer experiment), concomitant with changes in metabolic rates. An increase in BP and decrease in CR could be caused by high lability of the DOM that can support secondary bacterial production, without an increase in respiration. Increases in bacterial production and simultaneous decreases of primary production lead to more carbon being consumed in the microbial loop, and may shift the ecosystem towards heterotrophy.