Biomass of phytoplankton (pelagic microalgae) has been measured in the long-term on the basis of chlorophyll-a concentrations by RWS (factsheet P2) and NIOZ (factsheet P3). The RWS measurement programme consists of six stations in the Wadden Sea, including 2 situated in the Ems-Dollard tidal basin. The NIOZ station is located on the northern edge of the Marsdiep tidal basin.
The measurement and analysis protocols differ; for the RWS measurements, a water sample is taken about 20 times per year, the frequency of the NIOZ sampling programme is approximately 40 times per year. Under the NIOZ programme, biomass is estimated on the basis of hyperspectral measurements of the water colour at a high-frequency (1x per 15 min). A comparison of the measurements illustrates that more or less comparable estimates of the average annual biomass can be made for both high and low frequency measurement series, but that high frequency measurements better describe the season dynamics (in particular, during the spring bloom) than the low frequency measurements (Figure 4).
The long-term growth rate of phytoplankton is only monitored at NIOZ, on the basis of the same samples from which chlorophyll-a is determined. Because it is expected that growth rate is (even) more variable than biomass, the technique for manual production measurements by NIOZ is now automated (Fast Repetition Rate Fluorometry; FRRF). Growth rate can now be directly measured in the field and at a high frequency (1x per 2 hours). The automated FRRF has been deployed for the long-term measurements at the NIOZ jetty since 2014.
The biomass and growth rates of microphytobenthos (benthic microalgae) are not monitored.
Regarding the driving factors for biomass and production of microalgae (Figure 3), RWS automatically measures high frequency salinity (factsheet H3), seawater temperature (factsheet H8) and water levels (factsheet H4) at a number of stations in the Wadden Sea and the North Sea. The KNMI measures irradiance (J m-²) and hours of sunshine per day (factsheet H10) at a number of stations on the mainland and on the islands (see locations for stations and actual values: wad-nu.nl).
Sea temperature is also published weekly by the KNMI, and mapped on the basis of satellite data when weather conditions permit (factsheet H9). Water depth (and therefore exposure duration of the tidal flats) can be calculated using a combination of water levels (factsheet H4) and bathymetry (factsheet H1) of the Wadden Sea (InterTides).
Like phytoplankton biomass (pelagic microalgae), a number of environmental factors (salinity, sea temperature, turbidity, suspended solids, nutrients) are also measured long-term from water samples by RWS (factsheet P2) and NIOZ (factsheet P3). The RWS environmental factors measurement programme consists of 6 stations in the Wadden Sea tidal basins, of which 2 are located in the Ems-Dollard tidal basin, with a sampling frequency of approximately 20x per year. The sampling frequency of the NIOZ programme at 1 station in the Marsdiep tidal inlet, is approximately 40 times per year.
The freshwater supply (m3 s-1) from the IJsselmeer is measured in the Wadden Sea at the catchment locks of Den Oever and Kornwerderzand (factsheet H5). In combination with the nutrient concentrations as measured in the IJsselmeer (live.waterbase.nl), an estimate of the supply of nutrients to the Wadden Sea via this route can be made.
Loss (mortality) of biomass due to grazing by benthic fauna in the tidal flats can be estimated on the basis of long-term data (species composition, size, emersion time) of benthic animals in a part (factsheet B8; factsheet B9) or at all tidal flats of the Wadden Sea (factsheet B10, factsheet B13, factsheet B27). To estimate the loss of biomass due to grazing by benthic animals in the sublittoral, data derived from mussel surveys can be used both from natural mussel beds (factsheet B13) and from mussels on plots (factsheet B14), and for all benthic species for a very limited number of locations (factsheet B13). No area-wide monitoring of benthic fauna in permanently submerged areas of the Wadden Sea is conducted.
Loss of biomass due to grazing by zooplankton cannot be estimated because there are no long-term measurements of zooplankton (densities, species composition). However, the presence of shellfish larvae (only absence/presence of species, no densities) is included as part of the plankton measurements in the Marsdiep tidal inlet (factsheet P3).
Import and export of microalgae (phytoplankton and resuspended microphytobenthos) resulting from exchange with the North Sea is not measured, but may be estimated on the basis of models. Imports of phytoplankton from the IJsselmeer can be estimated by proportion of freshwater species in microscopic counts of microalgae, as implemented within the RWS (factsheet P2) and NIOZ (factsheet P3) monitoring programmes. No long-term measurements of mortality of microalgae, sedimentation of phytoplankton or resuspension of microphytobenthos are performed.