Estuarine phosphorus transformation, retention and bioavailability: an example from the southern Baltic Sea

Knowledge of phosphorus (P) inputs into aquatic systems is important to evaluate the level of eutrophication therein as P is one of the major driving forces of eutrophication. In eutrophic estuaries as the Warnow Estuary in the southern Baltic Sea, the traditionally monitored parameters total P (TP) and dissolved molybdate-reactive P (DRP) are not adequate to evaluate. Over two years (September 2016 to August 2018), an extended monitoring program was applied at the study site and a bioavailability experiment (four runs) was performed. In addition to TP and DRP, dissolved non-molybdate-reactive P (DNP), particulate molybdate-reactive P (PRP) and particulate non-molybdate-reactive P (PNP) were determined and their bioavailability was estimated. Additionally, sediment samples were analyzed according to P. The concentration of the different P fractions in the water column varied at the spatial scale within the estuary. In the surface waters the TP concentrations ranged between 1.0 and 2.8 µM with a decreasing trend towards the Baltic Sea. The remaining P fractions also decreased towards the Baltic Sea, except DNP which stayed nearly on the same level. The observed seasonal changes involved dominance of particulate P fractions in productive phases while dissolved P forms were increased in vegetative periods. As bioavailable P fractions DRP (62 - 100 %), DNP (9 - 50 %) and PRP (97 - 100 %) were identified. Although the estimated diffusive pore water fluxes showed a release of dissolved P from the sediment, the calculated TP loads revealed P retention (ca. 27 %) within the Warnow Estuary. Consequently, the international (European Water Framework Directive, Marine Strategy Framework Directive) and national (Surface Water Ordinance) regularities should be reconsidered, and it should be estimated and decided whether the highly anthropogenically impacted Warnow Estuary can achieve a good status according to eutrophication in the given timeframe.