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Benthos is a species group with many different species, sessile and mobile, filter feeding and predating. This great variety is also found in the Eastern Scheldt estuary, which makes this species group difficult to define. Regarding the a-biotic parameters and their changes over time, benthos is here represented by benthic, sessile species (i.e. mussels) only. Also, mussels have been widely studied as a keystone species in many aquatic systems, so cause-effect relations are more underpinned. Most relations have been derived from the HABITAT-wiki of DELTARES. Turbidity, water depth, type of substrate and drought duration are input parameters for this species group.
In the Eastern Scheldt estuary case study example, vegetation only consists of submerged vegetation (macrophytes). Within this species group, there is a great variety of species that vary in resilience against drought and salty soil. Based on expert judgement and cause-effect relations from the HABITAT-wiki from DELTARES, vegetation is influenced by light penetration, water depth, substrate type and drought duration.
The species group fish is divided into the subgroups plant eating fish and fish eating fish. This was done because these subgroups respond differently to the a-biotic parameters and are both important as food source for birds.
Plant eating fish
The subgroup plant eating fish roughly consists of the smaller fish species that mainly live in the shallow water zone (but also in the deep water ecotype, as long as there is vegetation, where they feed and hide in the vegetation. The input parameters for plant eating fish are turbidity, water depth and vegetation. Turbidity works in favour of the plant eating species because most of the occurring fish eating fish are visual hunters, meaning that they need good vision (and thus less turbidity) to forage. Plant eating fish serve as a food source for fish eating fish, birds.
Fish eating fish
In comparison to the subgroup plant eating fish, fish eating fish is negatively influenced by turbidity in the model. As described above, they feed on the species group plant eating fish and are influenced by the input parameters turbidity, water depth, vegetation and plant eating fish.
The species group birds is divided into three subgroups, namely benthos eating birds, plant eating birds, fish eating birds and omnivorous birds. This was done because these subgroups respond differently to a-biotic parameters and because of their food sources have different place in the food web (see figure 4.1).
Benthos eating birds
This subgroup forages on the species group benthos only. Because the biotic factors such as drought duration are already reflected in benthos, this is the only input parameter for benthos eating birds.
Plant eating birds
This subgroup forages on the species group vegetation only. Because the biotic factors such as water depth are already reflected in benthos, this is the only input parameter for plant eating birds.
Fish eating birds
This subgroup forages on the species group fish only. Although adults of most of the fish eating fish species are too large to serve as food for fish eating birds, the juveniles are eaten. It was decided not to distinguish between age groups, so the sub group completely contributes to fish eating birds. Because the biotic factors such as water depth and turbidity are already reflected in fish, this is the only input parameter for plant eating birds.
A lot of the bird species in the Eastern Scheldt estuary are omnivorous, meaning that they can switch between diets when scarceness occurs. This subgroup is defined by all three input parameters, under the condition that the maximum value is taken.