Studying interactions between eutrophication, hypoxia and benthic biomass Benthic macrofaunal communities can benefit from increased food abundance due to nutrient loading of aquatic ecossytems. On the other hand, this also increases the risk of hypoxia, resulting in a reduction or complete loss of benthic biomass.
Using a physiological benthos model with 5 functional macrofaunal groups that were linked to a 3D-hydrodynamic-ecological Baltic Sea model (BALTSEM), the researchers studied the interaction between eutrophication, hypoxia and benthic biomass, with emphasis on the balance between gains and loss of benthic biomass due to changes in nutrient loadings.
Effects of nutrient load reductions
Model results revealed that benthic biomass increased between 0 and 700% after re-oxygenating bottom waters. Nutrient reduction scenarios indicated improved oxygen concentrations in bottom waters and decreased sedimentation of organic matter up to 40% after a nutrient load reduction following the HELCOM Baltic Sea Action Plan.
The lower food supply to benthos reduced the macrofaunal biomass up to 35%, especially in areas not currently affected by hypoxia, whereas benthic biomass increased up to 200% in areas affected by eutrophication-induced hypoxia.
Hypoxia as a key structuring factor for benthic communities
Hypoxia is a key structuring factor for benthic communities in the Baltic Sea and nutrient load reductions will lead to more benthic biomass in currently hypoxic areas.
The expected changes in benthic biomass resulting from nutrient load reductions and subsequent reduced hypoxia may not only increase the food supply for benthivorous fish, but also significantly affect the biogeochemical functioning of the ecosystem.