Effectiveness of phosphorus control under extreme heatwaves: implications for sediment nutrient releases and greenhouse gas emissions

Eutrophication has been identified as the primary cause of water quality deterioration in inland waters worldwide, often associated with algal blooms or fish kills. Eutrophication can be controlled through watershed management and in-lake measures. An extreme heatwave event, through its impact on mineralization rates and internal nutrient loading (phosphorus—P, and nitrogen—N), could counteract eutrophication control measures. We investigated how the effectiveness of a nutrient abatement technique is impacted by an extreme heatwave, and to what extent biogeochemical processes are modulated by exposure to heatwaves. To this end, we carried out a sediment-incubation experiment, testing the effectiveness of lanthanum-modified bentonite (LMB) in reducing nutrients and greenhouse gas emissions from eutrophic sediments, with and without exposure to an extreme heatwave. Our results indicate that the effectiveness of LMB may be compromised upon exposure to an extreme heatwave event. This was evidenced by an increase in concentration of 0.08 ± 0.03 mg P/L with an overlying water volume of 863 ± 21 mL, equalling an 11% increase, with effects lasting to the end of the experiment. LMB application generally showed no effect on nitrogen species, while the heatwave stimulated nitrification, resulting in ammonium loss and accumulation of dissolved oxidized nitrogen species as well as increased dissolved nitrous oxide concentrations. In addition, carbon dioxide (CO2)-equivalent was more than doubled during the heatwave relative to the reference temperature, and LMB application had no effect on mitigating them. Our sediment incubation experiment indicates that the rates of biogeochemical processes can be significantly accelerated upon heatwave exposure, resulting in a change in fluxes of nutrient and greenhouse gas between sediment and water. The current efforts in eutrophication control will face more challenges under future climate scenarios with more frequent and intense extreme events as predicted by the IPCC.

Stakeholder Perspectives on Blue Mussel Farming to Mitigate Baltic Sea Eutrophication

Here, we present an application of systems thinking to controlling Baltic Sea eutrophication—a wicked environmental problem characterized by multiple stakeholder perspectives and no single, agreed upon solution. The Baltic Sea is one of the most polluted waterbodies in the world. More than 40 years of land-based (linear) measures have failed to adequately control eutrophication, yet internal (circular) measures are rarely used. Farming native blue mussels for nutrient capture has been proposed as one measure for eutrophication control, but the relevant stakeholders disagree as to its environmental, social and economic benefits. Here, we present the views of four Swedish stakeholder groups—academics, entrepreneurs, municipal government employees and representatives of non-governmental organizations (NGOs)—on the sustainability of native blue mussel farming, a circular measure for eutrophication control. Using semi-structured interviews, we elicited stakeholder perspectives on the environmental, economic and social dimensions of blue mussel farming. The interviewees generally agreed that blue mussel farming is not currently economically sustainable, but that it can contribute to the social sustainability of coastal regions. Academics were skeptical of the environmental benefits, claiming that farms could reinforce eutrophication, whereas the remaining stakeholder groups argued for its potential to mitigate eutrophication. In a roundtable discussion conducted one year after the original interviews, all stakeholder groups agreed that blue mussel farming alone will not fix Baltic Sea eutrophication, but can be part of the solution together with land-based measures. All groups also agreed on the need for cautious upscaling, continuous environmental monitoring and constant improvement if blue mussel farms are to be part of a “toolkit” for eutrophication control. Our results highlight the fact that wicked environmental problems can be addressed when multiple stakeholder groups with differing perspectives have the opportunity to achieve consensus through dialog.

What Colin Reynolds could tell us about nutrient limitation, N:P ratios and eutrophication control

Colin Reynolds exquisitely consolidated our understanding of driving forces shaping phytoplankton communities and those setting the upper limit to biomass yield, with limitation typically shifting from light in winter to phosphorus in spring. Nonetheless, co-limitation is frequently postulated from enhanced growth responses to enrichments with both N and P or from N:P ranging around the Redfield ratio, concluding a need to reduce both N and P in order to mitigate eutrophication. Here, we review the current understanding of limitation through N and P and of co-limitation. We conclude that Reynolds is still correct: (i) Liebig’s law of the minimum holds and reducing P is sufficient, provided concentrations achieved are low enough; (ii) analyses of nutrient limitation need to exclude evidently non-limiting situations, i.e. where soluble P exceeds 3–10 µg/l, dissolved N exceeds 100–130 µg/l and total P and N support high biomass levels with self-shading causing light limitation; (iii) additionally decreasing N to limiting concentrations may be useful in specific situations (e.g. shallow waterbodies with high internal P and pronounced denitrification); (iv) management decisions require local, situation-specific assessments. The value of research on stoichiometry and co-limitation lies in promoting our understanding of phytoplankton ecophysiology and community ecology.