Spatial and seasonal phosphorus dynamics in a eutrophic estuary of the southern Baltic Sea

Phosphorus (P) is one of the major drivers of eutrophication in aquatic systems. Thus, knowledge of riverine P inputs into the Baltic Sea is essential to evaluate the eutrophication level. In eutrophic estuaries such as the Warnow Estuary, the traditionally monitored parameters of total P (TP) and dissolved molybdate-reactive P (DRP) are not adequate for such an evaluation. Over two years (September 2016 to August 2018), an extended monitoring programme was applied in the Warnow Estuary, determining dissolved non-molybdate-reactive P (DNP), particulate molybdate-reactive P (PRP), and particulate non-molybdate-reactive P (PNP) in addition to TP and DRP. Within the estuary, the concentrations of the P fractions varied on both spatial and seasonal scales. While a gradient in the surface water's PNP was found towards the Baltic Sea, in the bottom water the DRP was distinctive. Particulate P fractions dominated the productive phases (>50% of TP), whereas during regenerative periods dissolved P forms were dominant (>50% of TP). Even though diffusive pore water flux estimates showed a release of dissolved P from the sediment, the calculated TP loads (44 t TP a(-1) in 2017) revealed a P retention of 12% within the Warnow Estuary. Although the reduction targets of the Baltic Sea Action Plan are presumably able to reach, further reductions are necessary to achieve the aims (good status according to eutrophication) of the Marine Strategy Framework Directive and the Water Framework Directive in the highly anthropogenically impacted Warnow Estuary.