Effects of embanking an intertidal-flat area in the south-western part of the Netherlands, the Markiezaat, on abundance and condition of benthic animals were followed at 3 stations during spring, 1983. Part of the former intertidal flats became permanently submerged with stagnant salt water, whereas another part became permanently emerged. At the submerged sampling station, no clear changes were found during the first 3 months of submersion. However, within 6 months, most species died off. At the drained stations, all individuals of most species died off within 2 months. During some preliminary experiments with drained sediment cores, a high mortality of the animals was reached within much shorter periods. During these experiments the animals showed a retarded mortality after they had been returned to a normal tidal cycle. Species living in the lower part of the intertidal zone were found to be less tolerant to drainage than those living in the higher part. The observed mortality rates in the Markiezaat probably represent minimal values, because of the heavy rainfall and the absence of extreme temperatures during drainage. At high (summer) and low (winter) temperatures and low precipitation, benthic animals may be less tolerant to drainage. Neither permanent submersion nor emersion affected the condition determined as dry weight per unit of volume. Therefore, the suitability of this condition-index as an indicator of stress for intertidal benthic animals may be questioned. |
