Phytoplankton can bypass nutrient reductions in eutrophic coastal water bodies

Abstract

The EU-water framework directive aims at nutrient reductions, since anthropogenically induced eutrophication is a major threat for coastal waters. However, phytoplankton biomass in southern Baltic Sea coastal water bodies (CWB) remains high and the underlying mechanisms are not well understood. Therefore, a CWB data set was analysed regarding changes in phytoplankton biomass and nutrient concentration of nitrogen (N) and phosphorus (P) from 2000 to 2014. It was expected to find imbalances between produced phytoplankton biomass and total nutrient concentrations. Inner CWB were cyanobacteria-dominated and showed up to five times higher chlorophyll a-concentrations compared to outer CWB with similar total phosphorus-concentrations. Phytoplankton tended to be P-limited during spring and N-limited during summer. Phytoplankton biomass and nutrient concentrations were even higher during very humid years, which indicated a close coupling of the CWB with their catchment areas. This study suggests that re-mesotrophication efforts need to consider the importance of changed phytoplankton composition and nutrient availabilities.

Managing Eutrophication in the Szczecin (Oder) Lagoon-Development, Present State and Future Perspectives

“…This is further enhanced by the hysteresis effect (Scheffer, 2009), as macrophytes are missing to stabilize the sediments (Karstens et al, 2015) and to reduce the resuspension, resulting in a further lowered water transparency. Berthold et al (2018) combined nutrient concentrations and phytoplankton biomass in several coastal waters of the southern Baltic Sea and emphasized the importance of hysteresis and resilience factors when reversing eutrophication, concluding that the pure reduction of nutrient loads will not be sufficient to reach the GES thresholds. Duarte (2009) showed for coastal waters, which received high external loads in the 1970ties and 1980ties that returning to oligotrophic conditions is extremely difficult, long time taking, and might in practice be impossible.…”

“…Possible internal measures cannot replace nutrient input reductions, but they are adequate supplements if they affect the central problems, which are the missing submerged vegetation and the frequent resuspension of sediments. Berthold et al (2018) discussed therefore bio-manipulation approaches, like the planting of submerged macrophytes or the transition of the food web from planktivorous to piscivorous fish. More promising is to increase the biomass of filter feeders by supplying them appropriate material to grow on (Stybel et al, 2009) as suitable hard substrate is missing in Small Lagoon.…”

“…For Curonian Lagoon the potential of mussel farms is already discussed (Bagdanavičiūtė et al, 2018). Other coastal waters like Darss-Zingst-Bodden-Chain (Schubert et al, 2010) or Rügensche Binnenbodden (both German Baltic Sea; Selig et al, 2007) experienced a comparable strong reduction of submerged vegetation and water transparency, and are not reacting to decreased nutrients loads (Berthold et al, 2018).…”

Handling the phosphorus paradox in agriculture and natural ecosystems: Scarcity, necessity, and burden of P

“…P input from the drainage basin was 36 200 t in 2010 (HELCOM 2014 , 2015 ). The slow recovery of the environmental status of the Baltic Sea is also a consequence of the long residence times and the large quantities of P stored and recycled in sediments (e.g., Leipe et al 2017 ), although mobilization from marine sediments in a shallow oxic lagoon was found to be relatively small (Berthold et al 2018 , this issue).…”

Magnitude and influence of atmospheric phosphorus deposition on the southern Baltic Sea coast over 23 years: implications for coastal waters

“…Hence, more reliable data are necessary to justify such assumptions, especially in the Baltic Sea where such long-term monitoring programmes for P deposition are rare [19]. Most coastal waterbodies of the southern Baltic Sea are dominated by cyanobacteria [20]. Those cyanobacteria can be highly adapted to low P concentrations [21].…”