Phosphorus recycling and availability in the western Wadden Sea

Abstract

Phosphorus is a main and often limiting nutrient for phytoplankton growth, as suggested for the western Wadden Sea. In this area, freshwater discharge was a major nutrient source in the past. However, pollution reduction measures dramatically reduced its contribution, particularly for phosphorus. In shallow coastal areas, the sediments can also represent significant nutrient sources to the pelagic environment, given their major role in nutrient cycling. This thesis intended to re-assess the seasonal cycling of P in the western Wadden Sea, three decades after the introduction of policies to reduce freshwater phosphorus loading, and to investigate its present availability to primary producers. Phosphorus can be stored in various forms in the sediments, ranging from short-term retention to the long-term incorporation in new sedimentary rock. Sorption is one of the main processes involved in temporary retention, controlling the sediment release of phosphorus. Several incubations of sediment in a seawater phosphorus solution were performed to obtain isotherms, i.e., curves that describe the distribution of a compound between the dissolved and adsorbed phase and usually assumed to be independent of the amount of adsorbent used. However, significant bias was found in the sorption isotherms when different particle concentrations were used in incubations of marine sediment/iron-coated sand and a proper isotherm description that takes into account this particle concentration effect was used. Even though large amounts of phosphorus can be sorbed in the sediment and be easily exchanged with the water column, not all sedimentary phosphorus is in an exchangeable form. A proper quantification of the sedimentary phosphorus includes a discrimination between the different functional pools. Sequential extractions were used to identify the easily exchangeable, redox sensitive, non-easily reducible Fe/Mn bound, Ca bound/apatite, and refractory organic phosphorus pools from subtidal and intertidal sediments. The potentially bioavailable pool corresponded to 12–42% of the total sedimentary phosphorus and a budget for the western Wadden Sea, including the first 9 cm of sediment, varied between 0.6 and 1.0 x 108 moles . The sediment-water exchange was measured by whole-core incubations and estimated using porewater profiles of nutrient concentration.Two distinct reactive sedimentary zones were found: one corresponding to the depositional areas, with high silt and organic C contents, no macrofauna, and a diffusion-controlled release; and another, with coarser grain size and low organic C content, and the presence of macrofauna whose activity clearly enhanced the sediment-water exchange. The phosphorus sediment-water exchange increased from February until September with a conservative estimation pointing to an annual release of 1.0 x 107 moles. Considering the present relatively high primary production and the significant decrease in the freshwater nutrient loading, it is interesting to compare the seasonal contributions of freshwater discharge and sediment release to the pelagic primary production. A direct link between phytoplankton growth and nutrient availability was found, with freshwater discharge showing a higher contribution in winter and sediment release being the main phosphorus source in spring–autumn. The sediment release had the potential to sustain most of modern primary production levels.