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Dumping effects

Fauna species

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Changes of the sea floor after dumping of dredged material

shown on the example of dumping site 552a located about 10 nautical miles northeast of Warnemünde (Mecklenburg Bight, 4-18m deep)

 

Links in the text refer to short video clips

 

Before the dumping event, the seabed surface on the dumping site was marked by natural sediments and current-induced ripples. Shallow areas also have some "residual stones", left behind after previous glacial ages.

Benthic organisms are supplied with food particles from sinking upper layers of the water column between spring and autumn. This organic matter forms a green "fluffy layer" after algal blooms.

In late summer, high temperatures and calm weather conditions (i.e. slow flow) can lead to low levels of the near-bed oxygen concentration in deeper areas. The sediment surface then displays dark oxygen-depleted spots. Anthropogenic input of nutrients and the resulting algal growth can cause an additional oxygen consuption and thus strengthen this effect.

After a dumping event, the structure of the seabed surface is altered, depending on the type of sediment that was dumped. The dumping site monitored in this study showed an increased number of rocks and marl structures. All sessile and slow-moving benthic organisms are burried during such a dumping event. In addition, suspended lithogenic material can cause a high turbidity and thus have a negative effect on filtrating species sue to a deterioration of the food quality and a possible obstruction of their filter mechanisms. These unfavourable effects are however balanced by the presence of new substratum which can be colonised by e.g. the white piddock (Barnea candida). These new structures also offer protective areas for juvenile fish and shrimps and habitats for predators like the eel.

Which benthic organisms are affected?
How do they react to increased particle loads?

Mobile animals living on the sediment surface often have the ability to escape or dig themselves out (e.g. crabs or starfish). Sessile filtration-feeding organisms may use the suspended matter as food source. The additional load of inorganic particles is rejected as faces or pseudofaeces. However, some oranisms cannot survive being completely covered with dredge material, like e.g. the blue mussel (Mytilus edulis), barnalces (Balanus sp.) and the bristleworm (Pygospio elegans).

Endobenthic species, living inside the sediment and protected by a shell or tube, generally have a better chance of urviving a dumping event. But their tolerance level depends on their ability to move across the sediment. Some examples are the lugworm (Arenicola marina), the trumpet worm (Pectinaria koreni), the baltic tellin (Macoma balthica) and the sand gaper (Mya arenaria). Endobenthic bivalves like Mya arenaria are connected to the water column with a sipho. They are filter feeders or deposit feeders. High particle loads which may occur after a dumping event, but also during storms, are tolerated and the non-commestible fraction is rejected into the water column (bioresuspension).