Phytoplankton Mardiep tidal inlet (NIOZ jetty)
Analysing the phytoplankton dynamics
Local primary production is the main source of energy for the Wadden Sea’s food web. Primary production is carried out in the water column by phytoplankton, and on the shallow seabed by microphytobenthos. The NIOZ has been studying long-term phytoplankton dynamics in the western Wadden Sea since 1974, by taking about 40 water samples per year from their jetty located close to the Marsdiep tidal inlet.
Previous analyses of parts of this dataset revealed that the phytoplankton chlorophyll-a concentrations (often used as an index of phytoplankton biomass) doubled at the end of the 1970s as the result of eutrophication caused by a doubling of riverine nutrient loads. A reduction in nutrient loads was, however, not directly and not fully followed by a reduction in chlorophyll-a concentrations. Figure 1 shows the annually averaged chlorophyll-a concentrations (mg per m3) as measured during high tide at the NIOZ jetty from 1974 to 2015. The dotted line depicts the long-term mean.
Possible explanations of this decoupling between nutrient reduction and chlorophyll-a concentrations include a delayed release of P stored in the sediment during eutrophication, and a decrease in grazing pressure by zooplankton and suspension-feeding bivalves. These aspects are now further explored, a.o. by the compilation of long-term nutrient budgets of the western Wadden Sea. In terms of temperature, 2015 was a relatively warm year (see previous newsletter). For chlorophyll, 2015 still seems to be in line with an increase that started around 2009.