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Turbidity is commonly defined as a measure for the suspended material in the water (in mg/l), consisting of algae, detritus and inorganic particles. These three factors explain most of the turbidity, and therefore need to be calculated. However, in the light of the key problem of sand hunger in the Eastern Scheldt estuary and with the purpose of keeping the calculations simple and effective, it is assumed here that inorganic particles, i.e. the sandhunger, explains all of the turbidity. It is also assumed, as is commonly done, that the turbidity is homogenous on every depth.
The a-biotic parameter substrate type is taken into account because a bad type of substrate can be critical for both plants and sessile filter feeders to the soil. In this case, a bad type of substrate is defined as a type of substrate with many inorganic material (sand) coming from the sand hunger process. Four classes have been defined: completely silt, silt/soil mixture with mostly silt, silt/soil mixture 50-50 and silt/soil mixture with mostly soil.
The translation in quality of change in water depth is more complex than one initially might think. The reason for this is that the ecotypes are defined by ranges in water depth, meaning that water depth changes affect the area of an ecotype (part 1 of the analysis).
Light penetration is defined as a ratio between water depth and turbidity. In several studies (Witteveen+Bos, 2006, 2010) Witteveen + Bos have distracted a rule of thumb for light penetration, based on the Euphotic depth (Scheffer, 1998) stating the following: extinction depth/water depth =relative sight depth ratio. For every ratio greater than 0.5, enough light reaches the bottom for plants to grow. This rule of thumb is used to derive a cause-effect relation for this case study (see model). As such, the changes over time and due to the policy alternatives is determined by the changes in turbidity and water depth.
Drought duration is expressed as a percentage of time at which the ecotype is not submerged. The quality effect of flood duration is relevant for the tidal flat ecotype only, since the other ecotypes are per definition constantly submerged.