Nitrogen runoff, constructed wetlands and the effect on catch of fish in the Öresund strait

Abstract: Eutrophication and low dissolved oxygen levels are spreading all over the world, causing changes in fish distribution and decline in fish abundance and catch of fish. This study assesses the current nitrogen (N) surface runoff to the Öresund strait, the seafloor oxygen demand it cause, the subsequent effect on fish catch, and the constructed wetland area needed to retain current N loads. This is assessed using a nitrogen; phytoplankton; zooplankton; detritus (NPZD) model and historical N load and fish catch data. N loads to the Öresund strait have steadily decreased since 1995 as the total annual fish catch declined with 77% between 1997 and 2017. The results show no negative correlation between annual N load and annual fish catch, instead a positive correlation can be seen for catch of benthopelagic species and total catch. The annual average oxygen demand produced by surface runoff N loading is equivalent to 0.37 mℓ ℓ-1. No conclusion can be drawn that current N loads cause hypoxia or have a negative impact on fish catch in the Öresund strait. The diminishing fish catch can instead be explained by lowered fishing quotas due to environmental status and declining fish populations in adjoining basins.