Samenvatting

Anthropogenic changes to estuaries, usually highly productive systems, may result in increased turbidity levels, suppress pelagic primary production and negatively affect ecosystem functions.
The Ems-Dollard estuary, enclosed between the Netherlands and Germany, is such an estuary with a long history of human induced changes. From the mid-19th century until 1992, discharges of potato starch and strawboard industry waste water, with high contents of organic matter, negatively affected oxygen concentrations and increased nutrient levels of the system. The growing awareness of the deteriorating effects of these discharges on the ecosystem invoked a large research program, BOEDE, between 1976 and 1980.
The aim of the current paper was to establish the recent gross pelagic primary production, to compare the results with the late seventies data and to relate possible changes to environmental variables such as suspended matter and nutrients.
To this end, six stations from the North Sea into the inner area of the Dollard were visited 39 times in 2012–2013. During each sampling cruise, a so-called PocketBox, containing a number of sensors, provided a high spatial data resolution between stations. At each station, samples were collected for carbon fixation incubations using 14C.
Average chlorophyll-a concentrations as reported in this study were lower compared to 1978–1980 for the seaward stations, and higher for the most inward station in 2012 and almost similar in 2013. Light attenuation coefficients in the outer areas were lower in 2012–2013 (1.22 m− 1) compared to the values reported in the period 1976–1980 (1.59 m− 1), indicating somewhat less turbid conditions. For the two other stations, light attenuation coefficients in the present study were similar to the late seventies’ values, for the station near the entrance of the Ems River in the Ems-Dollard area, the coefficient was substantially higher.
Pelagic primary production in the 2012–2013 period was about 120 g C m− 2 y− 1, mainly determined by the most seaward areas. The largest differences in gross primary production in the current study compared to the 1976–1980 period were found at the seaward stations where production in the late seventies was about 60% higher. In the Dollard-area, production hardly changed since the late seventies, but also stations near the Ems-river entrance into the Ems-Dollard area showed a clear decrease. Changes at the inward stations are to be contributed to an increased turbidity; the decrease at the outer areas to a decreased nutrient level, especially dissolved phosphorus and silicate, but possibly also nitrogen.