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WFD macrophytobenthos classification at the North sea island Helgoland: a spatial approach

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Citation:
Bartsch, I. , Kuhlenkamp, R. , Gehling, C. and Boos, K. (2006): WFD macrophytobenthos classification at the North sea island Helgoland: a spatial approach , Workshop: Transitional states in transitional and coastal waters, 1-2 June, Kavala, Greece. .
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Abstract:

WFD classification at Helgoland (German Bight, North Sea) identified seven variables allowing comparison with historical pristine conditions. These were:, Abundance and spatial spread of (1) dense Fucus associations, (2) Mytilus edulis associations and (3) Ulva associations, (4) Development of the sublittoral kelp belt, (5) Sublittoral algal depth limits, (6) Floristic diversity and (7) Amount of rare species. The class limits were set assuming population development according to a logistic model thereby cutting classes off at the 90, 70, 30 and 10% level relative to the pristine conditions. Although floristic diversity and rare species are good indicators for changes they are not fully monitorable during one season. The change of subtidal depth limits of various seaweed species revealed a consistent trend. Water clarity (Secci-depths) increased during recent years at Helgoland by 1-2 m and the flora reacted by species-specific increase of depth limits. Thus, only following a set of species will show the trend reliably. As perennial dense fucoid associations are considered to be of structural importance at pristine temperate coasts and dense Ulva or ephemeral algal or Mytilus associations may show degradation due to eutrophication, a spatial mapping approach was used to classify these variables. Combination of a digital elevation model with a geo-referenced map and mapped polygons of degraded versus dense Fucus serratus associations, Mytilus and Ulva fields allowed spatial quantification of the variables and comparison with a calculated historical situation. The extent of Ulva and other ephemeral green algae could only be monitored by using spatial data (polygons and grid percentage cover data), but classification is still insecure. Comparative line transect data did not satisfactorily show abundance of ephemeral green algae. Some persistent Ulva fields seem to be a product of continuous mechanical disturbance and thereby might not be indicative for a degraded situation.

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