Dissolved organic phosphorus: An indicator of organic matter turnover?

Seasonal and interannual variations of dissolved organic phosphorus (DOP) are analysed for five stations in the Dutch Wadden Sea (southern Wadden Sea, 1991–2007) and one station in the northern Wadden Sea (List tidal basin near Sylt, 2000–2009). A clear seasonal cycle is observed with low winter DOP values of around 0.1–0.2 μM and a summer maximum of up to 1.6 μM. Mean summer DOP concentrations show a decreasing trend in line with a decrease in the eutrophication level of the Wadden Sea. Regional differences exist in the mean summer DOP levels with highest values in the eastern part of the Dutch Wadden Sea and lowest values in the northern Wadden Sea near Sylt. A regional comparison for the years 2000–2007 shows that average summer DOP values are correlated with average summer phytoplankton chlorophyll-a (proxy for biomass) and autumn ammonium concentrations suggesting that highest DOP values are found in those regions where highest phytoplankton production and highest autumn remineralisation occurs. We conclude that the summer DOP values may be used as a useful indicator of regional differences and is worth to further investigate as indicator of interannual trends in organic matter turnover in the Wadden Sea.