The carrying capacity for individual species or communities in the Wadden Sea is strongly dependent on primary production, the process in which microscopic algae use energy from sunlight for the formation of biomass from CO2, water, and nutrients. Via a chain of eat-and-be-eaten this food is diverted to the higher trophic levels. Phytoplankton (microalgae in the water column) is important food for zooplankton in the water and for the filtering worms and shellfish on the tidal flats and in the permanently submerged areas of the Wadden Sea. The microphytobenthos (microalgae on the mudflats) are grazed by different types of worms, shellfish, fish such as mullets (Helon labrosus) and even birds including shellducks (Tadorna tadorna).
As a result of a very limited number of measurements and the use of different techniques, primary production in the Wadden Sea can only be roughly estimated. The annual primary production of microalgae in the western Wadden Sea is roughly 300 grams of carbon per m2 (Van Beusekom et al. 1999, Philippart & Epping, 2010). Because the production of microphytobenthos is in the same order of magnitude as that of phytoplankton (Kromkamp et al. 2006), the contribution of microphytobenthos to total production depends mainly on the relative area of the tidal flats. In the westernmost part of the Wadden Sea, an area with relatively few tidal flats, the microphytobenthos contributes less than 20% to the total annual primary production (Philippart & Cadée, 2000). For a number of areas with a high proportion of tidal flats in the northern Wadden Sea, the contribution of microphytbenthos may add up to 60 to 70% of total production (Van Beusekom et al. 1999, Baird et al. 2004).
In addition to the magnitude of primary productivity by micro-algae, the efficiency of food transfer to successive trophic levels also determines the carrying capacity of the Wadden Sea. The amount of captured energy that is channelled through the microalgae to the higher trophic levels depends not only on the amount of primary production, but also on the species composition of the microalgae (Cloern, 2001) and the synchronisation between algal blooms and grazing pressure (Nakazawa & Doi, 2012).
Not all algae have the same nutritional value. Whether an alga is good food for a grazer or not depends on its size, morphology, toxicity, and biochemical composition. In particular, the composition of the (unsaturated) fatty acids is important (Dijkman & Kromkamp, 2006). The type of growth limitation (nutrients or light) can exert a strong influence on the species composition and thus the nutritional value of the algae, and hence the carrying capacity of the Wadden Sea. Changes in the type and density of grazers are therefore not only related to the biomass and production, but also to the species composition of microalgae.
A proper quantification of processes of primary production, import and export of organic material, relationship with light (silt) and with nutrients is essential for the study of the Wadden Sea as a dynamic system. The relationship between primary production and consumption by herbivores is also of great importance. For the objective of improving knowledge, a combination of monitoring, modelling and experimental studies are needed.
At present some necessary parameters are hardly monitored, such as the abundance of zooplankton, microphytobenthos and benthic fauna in the sublittoral parts of the Wadden Sea. Existing measurements of nutrients, chlorophyll, organic matter and silt should be linked to measurements of primary production by phytoplankton and microphytobenthos. Also, experimental research on the limiting factors for production is necessary. Because ‘microalgae’ and ‘grazers’ cannot be considered as one, attention should also be given to the species and their species-specific characteristics (Herman et al. 2009).
Many of the (pelagic) monitoring programmes in the Wadden Sea are deployed with a frequency of 1x to 2x per month. During the growing season, this results in ‘under sampling’ of phytoplankton, implying that changes in the patterns of algal dynamics might remain unnoticed, and changes in the functioning of food sources unexplained. High-frequency measurements can be performed with automated equipment, enabling data gathering on many growth conditions (light, temperature, salinity, turbidity) and biomass of microalgae (fluorometers, radiometers), and should be backed up by repeated sampling.
However, the automated measurement of primary production is much more difficult. Nowadays, there are optical techniques available that measure the photosynthetic activity of phytoplankton, including the promising FRRF (Fast Repetition Rate Fluorometer) (Robinson et al. 2014). Another development is the continuous measurement of primary production of coastal waters and mud flats on the basis of the vertical gradients in the concentrations of CO2 in air, the so-called ‘eddy correlation’ technique (Zemmelink et al. 2009). Continuous oxygen measurements can also be used to estimate primary production (Cox et al. in prep.). Zooplankton can potentially be monitored with optical methods using video plankton recorders. These techniques are presently undergoing further development for future use in long-term monitoring.
To know how representative local observations are for the entire Wadden Sea, measurements should be performed at various locations. This is possible by combining manual and continuous measurements from measurement platforms and ships with remote sensing images from various platforms, including aircrafts (Burazerovic et al. 2014), drones, and satellites (WaLTER RS factsheets: InSar, Laser-altimetri en Optische technieken). There are a number of indications that the import of suspended matter and nutrients from the German Bight plays a substantial role in controlling parameters (Van Beusekom et al. 2012) that in turn control the primary production in the Wadden Sea. Hence, the monitoring programme should extend well beyond the seaside of the Wadden Sea area.
For a number of measurements, such as species composition of microalgae, zooplankton and benthic animals, samples still have to be taken to accurately estimate the growth and grazing processes. Techniques are currently being investigated to perform automatic measurements, and for the use of satellite images to provide direct and indirect information on (functional) groups of species within algae and benthic animals (WaLTER RS factsheets: Akoestische methoden, Gammaspectroscopie). However, these techniques are not yet tested for all conditions, which may occur in the Wadden Sea and in the coastal zone of the North Sea, and therefore cannot be used at present for long-term measurements.
A number of recent research and infrastructure projects (e.g. IN PLACE, PROTOOL, COSYNA, WIMO) focused on the potential of old and new monitoring techniques to gain a good picture of the spatial and temporal variations in primary production by microalgae in the Dutch and German coastal waters, comprising both the inner German Bight and the Wadden Sea. Through an appropriately designed monitoring plan (see chapter 5), we can come to a better understanding of the role of microalgae in the food web and thus the capacity and potential for human use of natural resources of the Wadden Sea under changing conditions, such as climate change.