Abstract
Aquatic ecosystems nowadays are under constant pressure, either from recent or historical events. In most systems with increased nutrient supply, submerged macrophytes got replaced by another stable state, dominated by phytoplankton as main primary producer. Yet, reducing the nutrient supply did not yield the aimed goal of restored habitats for submerged macrophytes in systems worldwide. The question arises, why submerged macrophytes do not re-colonize, and if they are actually competitive. Therefore, primary production assays were conducted in ex-situ bentho-pelagic mesocosms and compared to the actual ecosystem, a turbid brackish lagoon of the southern Baltic Sea. Mesocosm were either manipulated to be colonized by macrophytes, or stayed phytoplankton dominated. Oxygen evolution was monitored over a period of five months in 5 min (mesocosms) to 10 min (ecosystem) intervals. Surface and depth-integrated production was calculated to analyse seasonal and areal resolved production patterns. It was found that macrophyte mesocosms were more stable, when considering only surface O2 production. However, calculating depth-integrated production resulted in net-heterotrophy in both shallow mesocosms approaches and the actual ecosystem. This heterotrophy is likely mediated by sediment respiration and POC accumulation in mesocosms, and a low share of productive to respiring water column in the actual ecosystem. Therefore, it seems unlikely that macrophytes will re-settle, as constant net-heterotrophy may allow for high-nutrient turnover at sediment-water interfaces and within the water column, favouring phytoplankton. These results will assist decision makers in developing more effective restoration measures that can mitigate the negative effects of eutrophication on ecosystem function and services.
Highlights
Oxygen depletion caused by eutrophication and its consequencesTransitional water bodies such as lagoons or estuaries are prone to eutrophication due to nutrients received from their catchment area
Zooplankton and other macrozoobenthos suffer from suboxic conditions, which lead to death and overall lowered grazing rates on phytoplankton
The Darß-Zingst lagoon system (DZLS) covers an area of 197 km2, has a total volume of 397 106 m3, and a catchment area of 1600 km2 [18]
Summary
Oxygen depletion caused by eutrophication and its consequencesTransitional water bodies such as lagoons or estuaries are prone to eutrophication due to nutrients received from their catchment area. Phytoplankton dominance of aquatic systems is associated with high turbidity and light limitation [3, 4] This reduced light availability decreases the euphotic depth, that means the net-autotrophic water column, which can reduce the habitable zone for submerged macrophytes, as well as light-limiting the phytoplankton itself. This decrease lowers the overall primary production per m-2 of the water body, even in shallow waters. Increased nutrient recycling and supplies at reduced grazing control causes high phytoplankton biomass and can trap such eutrophic systems within a phytoplankton dominated state, as phytoplankton turn-over and generation times are higher compared to submerged macrophytes
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