Report: ‘Black spots: East Frisian Wadden Sea (1990-1995)»
Description
For several years, anaerobic spots on the sediment surfaces have been observed in East Frisian watts with increasing frequency. Their size and manifestations are diverse. A type occurs in connection with summery green algae growths. It can, especially after the seaweed mats rot in autumn, take on particularly large dimensions, in extreme cases up to orders of square kilometers. However, these anaerobic sediment surfaces caused by algae appear to be reoxidised relatively quickly. Black spots created independently of algae are considerably more durable and sometimes also enduring during the winter. They are preferably found in sand and mixed wadding, their scale ranges from small areas (a few cm² to dm²) in hollows and ribbed valleys to areas of one or more square meters. The present report presents the results of a first study of such anaerobic spots and discusses the possible causes and effects.
The colonisation structure in the anaerobic spots changes depending on the nutrient content in the sediment and the duration and intensity of the anaerobic phenomena. Basically, soil fauna impoverishes under long-lasting anaerobic conditions. The appearance of black, anoxic spots on the usually grey, oxic surface of the tidal flats at the East Frisian coast of Germany was firstly detected in 1984 and the phenomenon has been increasing since then. The “black spots” differ in dimension and character. One type is caused by the decay of macro algae, which in recent summers covered wide areas of the tidal flats at the German coast. These patches can reach dimensions of square kilometers, but, after the decomposition of the algae, seem to reoxidise within a short time. Another type of black spots, appearing independently of macro algae, occurs, though migration and changing in size, over longer periods, even during winter. Preferably they are located on sand and muddy sand flats often in toughs and ripple-troughs. Their size reaches from small spots are patchily distributed. A general shrinking of the oxidised layer on the sediment surface could not be found on the basis of the available data.
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- makrozoobenthos-schwarze-flecken-reduzierung-nlwkn-anaerob