Spatiotemporal Changes in Zostera marina Extent in Skälderviken and variations in Riverine Nutrient Inputs and Water-Quality Indicators, 2006-2023

Sammanfattning

This study investigates spatiotemporal changes in eelgrass (Zostera marina) extent in Skälderviken between 2006 and 2023, and compares these changes with riverine nutrient loads, modelled water-quality indicators and depth distribution. Eelgrass extent was mapped from orthophotos and high-resolution satellite imagery for 2006, 2010, 2016 and 2023. Annual nitrogen and phosphorus loads from Rönne å and Vege å were used as indicators of riverine nutrient input, while modelled chlorophyll a concentrations and Secchi depth from Copernicus Marine data were analysed for the May-October growing season. Estimated benthic light availability was calculated from Secchi depth to assess approximate light conditions across depth zones. Mapped eelgrass extent remained almost unchanged between 2006 and 2010, decreased by 30.2 % between 2010 and 2016, and increased by 54.8% between 2016 and 2023. Across the full study period, mapped extent was slightly larger in 2023 than in 2006. The largest mapped decline occurred during 2010–2016, but this period did not coincide with the highest riverine nutrient loads, the highest chlorophyll a concentrations or the lowest Secchi depth. Instead, the highest mean nutrient loads occurred during 2006–2010, when mapped eelgrass extent remained almost unchanged, while the highest chlorophyll a concentrations and lowest Secchi depth occurred during 2016–2023, when mapped eelgrass extent increased. This indicates that the period-based comparison did not show a clear one-directional correspondence between total eelgrass extent and the selected environmental indicators. The depth-based analysis showed that the main decline period was proportionally strongest in the 3–4 m depth zone, which was the deepest zone where eelgrass occurred in substantial amounts. Estimated benthic light availability suggested that mean growing-season light at 3.5 m depth remained above the commonly cited minimum requirement for eelgrass growth. This indicates that broad-scale seasonal mean Secchi depth alone was unlikely to explain the mapped decline in the deeper part of the meadow. However, short-term or local light limitation cannot be excluded, since seasonal mean values may hide temporary turbidity events and sediment resuspension. The results highlight the complexity of eelgrass dynamics in shallow coastal systems. Rather than showing a clear one-directional correspondence between mapped eelgrass change and the selected indicators, the study suggests that the observed patterns need to be interpreted in relation to several interacting local and methodological factors. These include depth distribution, wave exposure, sediment movement, local turbidity, seasonal variation in eelgrass biomass and image timing. In particular, the early-season 2016 orthophoto may have influenced eelgrass detectability and affected the apparent magnitude of the mapped decline.