Simon Pliscovaz

Several decades of remedial work have reduced the supply of nutrients from land to sea. Despite this, large-scale eutrophication remains a significant environmental problem along the Swedish coast and release of phosphorus from the sediments exacerbate the problem and reduces the effect of land load reductions. Thus, there is a need to identify where targeted remedial actions would have the most significance, and the highest likelihood to succeed. Additionally, there is a need to map the long-term relation between oxygen concentration and phosphate release from marine sediments in the Swedish coastal zone, which will support the aim to provide a scientific basis for prioritizing remedial targeted actions.

The results presented in this report indicate that there are more than 100 waterbodies along the Swedish coast with significant correlation between low oxygen concentrations and increased release of phosphorus from bottom sediments. However, oxygen concentrations below 1 ml L-1 is not the main driver for phosphorus release from Swedish coastal sediments and only three water bodies were found to actually have a net long-term internal source of phosphorus. The results also indicate that the number of years and autumns with an internal net release of phosphorus in the Swedish coastal zone seems to be decreasing, generally, and especially in late summer and autumn. An exception is the coastal zone bordering the northern Baltic Sea where, on an annual basis, the number of years and autumns with a net phosphorus outflux from the sediments seem to increase instead. Finally, it should be noted that locations with a significant correlation between low oxygen concentrations and increased release of phosphorus from bottom sediments do not necessarily overlap with areas with a generally high sediment phosphorus outflux. Thus, both types of areas need to be considered for targeted actions.

Colored Dissolved Organic Matter (CDOM), is the light absorbing fraction of the Dissolved Organic Matter (DOM) and is mainly of terrestrial origin. These substances are highly variable in the coastal zone and absorb and scatter light at different wavelengths and increase the light attenuation in the water column. As a consultation response from the latest Action Program for the Marine Environment an initial literature search of CDOM in Sweden's coastal waters and summary of the available literature and data was completed. It showed that the Swedish coastal observations of CDOM in the SHARKweb database are sparse and only locally occuring. However,  the availiable litterature indicates that CDOM likely has a significant effect on the light attenuation in Swedish coastal waters. Future changes in light attenuation in the coastal zone are assumed to be driven largely by the changes in the terrestrial inputs. River discharge is expected to increase in a future climate, and thus also the supply of CDOM and nutrients to the coastal zone.