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Turbidity

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.
Regarding permanent effects, the turbidity is defined constant over time, until the tidal flat disappears. In this case, the turbidity declines since the sand has completely washed to lower parts. Regarding temporal effects, it is defined that when applying suppletion, an increase of turbidity occurs within the year of suppletion to a maximum of 800 mg/l.

Substrate type

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.
It is assumed that the layer of sand increases due to the sand hunger effect, This increase is greatest in the tidal flat ecotype, and least in the deep water ecotype. When suppletion takes place, the suppleted area instantly shifts to the lowest substrate type class.

Water depth

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).
Within the depth range of an ecotype however, there is an effect of depth on ecotype quality, since the functionality of for instance a tidal flat ecotype is less when the average depth is far below the water table. Therefore, for each ecotype a quality index is created that reflects the functionality of the ecotype at different average depths. This effect is greatest for the tidal flat ecotype, where the water depth range is smallest.
Regarding temporal and permanent changes, the change over time and for different policy alternatives is linked to the area change (see step 1c).

Light penetration

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

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.
Periods of drought are positive for benthic feeding birds, since they depend on these periods to forage. For macropytes, periods of drought are negative because they need submergence for strength, respiration and foraging.
Without suppletion the periods of drought decline since the tidal flats slowly decay. Suppletion causes the period of drought to increase again, when the water depth falls into a new ecotype.