In 2012/2013 surveys are held among users of the Wadden area. The following monitoring questions related to climate and nature emerged.
As for other coastal seas in the world, the Wadden Sea’s carrying capacity for individual species or communities is highly dependent on food availability and quality (Figure 1, Herman et al. 1999; Kemp et al. 2005). These are determined by primary production, the process in which microscopic algae use the energy of 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, thus laying the basis for the carrying capacity of fish, birds and marine mammals.
|Estuaries in Figure 1 (Herman et al. 1999, Kemp et al. 2005)|
|1||Ythan Estuary||6||Ems estuary (Wadden)||11||Bay of Fundy|
|3||Oosterschelde||8||Long Island Sound||13||Ems Estuary (inner)|
|4||Balgzand (1980’s)||9||San Fransisco Bay||14||Columbia Estuary
|5||Veerse Meer||10||Balgzand (1970’s)||15||Upper (a), Mid (b)
& Lower (c)
In addition to the level of primary productivity by algae, carrying capacity is also determined by the efficiency of food transfer to successive trophic levels. The efficiency of the first step, the transfer of algal biomass to zooplankton and benthos, is mainly determined by the timing of primary production and the species composition of algal populations, both in the water column (phytoplankton) and on the sediment-water interface (microphytobenthos).
Changes in primary production and its effects on the food web are partly determined by the supply of nutrients (nitrogen, phosphate and silicate) from rivers to coastal areas. Through the use of fertilisers in agriculture worldwide, many coastal areas experienced an increase in the supply of nitrogen (Levin et al. 2015). But the supply of other nutrients for algae growth, such as phosphate and silicate, has also changed as a result of human activities (Conley et al. 1993).
The consequences of this so-called eutrophication were region-specific. Typical effects included an increase in catches of (mobile) benthic animals and fish and reduced oxygen levels in the water (Figure 2). As a result of governmental measures, loads and concentrations of nutrients were lowered in several coastal areas, including the Wadden Sea. The effects of these reductions were highly variable (Philippart et al, 2007), in part due to a delayed release of phosphate stored in the seabed (Leote & Epping, 2015) and concomitant with other changes in these areas, including climate change, intensive fishing and introductions of exotic species (Levin et al. 2015).Changes in primary production and its effects on the food web are partly determined by the supply of nutrients (nitrogen, phosphate and silicate) from rivers to coastal areas. Through the use of fertilisers in agriculture worldwide, many coastal areas experienced an increase in the supply of nitrogen (Levin et al. 2015). But the supply of other nutrients for algae growth, such as phosphate and silicate, has also changed as a result of human activities (Conley et al. 1993).
Evaluating the dynamics within and between different trophic levels in the Wadden Sea requires monitoring of primary production and algal biomass. This monitoring will be complemented with information on the composition of algal species, and monitoring of the factors that affect growth and loss (Figure 3). To date there are few observations that enable us to make accurate estimates, reflecting the strong and varying gradients in factors affecting productivity and species compositions of phytoplankton and microphytobenthos.
Climate change will affect all drivers of biomass and growth rate of microalgae in the water and on the mud flats (e.g. Van Beusekom et al. 2008; De Jonge et al. 2012). Changes in temperature affect the rate of algal growth, but also the rate at which the algae are grazed by zooplankton and macrozoobenthos and the speed at which the nutrients get remineralised, and thereby become available again for the growth of algae. Changes in light conditions are determined by changes in hours of sunshine, but also by changes in wind, with consequences for water turbidity influenced by resuspended sediment material.
The KNMI scenarios (KNMI 2014) include wetter winters, heavier rainfall and a higher probability of drier summers leading to changes in river discharges, and thus in supplies of nutrients to coastal regions such as the Wadden Sea. If changes in large-scale weather patterns lead to changes in wind speed and direction, this also has implications for the exchange of water and materials between the Wadden Sea and the North Sea, and thus for the import and export of nutrients, silt, microalgae and zooplankton in the water column. It may also have an impact on the estuarine circulation (Burchardt et al. 2008) as one driver of sustained sediment, and hence nutrient supply from the German Bight, as changes in precipitation and wind will most likely affect the horizontal density gradients, the driver of estuarine circulation.
In addition, there may also be some indirect effects of climate change expected on the primary production, as a result of modifications of plants and animals to the new conditions. Thus, relatively warm winters may lead to better local survival of some predators (zooplankton such as jellyfish and crustaceans such as shrimp) on grazers (microzooplankton and young shellfish). This would allow for the grazing pressure on microalgae in the water column and on the seabed in the spring and summer to decrease, the species compositions to change and / or the time of the annual peak in production to shift (Wiltshire & Manly, 2004; Philippart et al. 2003). Shifts in the timing of reproduction of shellfish would also lead to a change in the dynamics of grazing season of microalgae in the water and on the seabed (Philippart et al. 2014). Such effects on predators and grazers can have implications for the entire food web and habitats, for example on the stability of the mud flats, the so-called “cascade effect” (Carpenter et al. 1985). In addition, zooplankton and temperature influence the phytoplankton diversity (Lewandowska et al. 2014).