Historic (1900) seafloor composition in the Belgian-Dutch part of the North Sea: A reconstruction based on calibrated visual sediment descriptions

Concerns about human-induced degradation of marine ecosystems are presently being translated into European Directives aimed at a restoration of healthier seas. However, anthropogenic alterations are ancient and data on the pristine ecosystem are lacking, making the evaluation of their cumulative effects in the long run challenging. In the Belgian and Dutch parts of the North Sea, a historic data-set (the Gilson's archive) exists to describe various compartments of the marine ecosystem in the first decade of the 20th century (1899-1914). The bulk of samples were acquired in the nearshore (up to 10 nautical miles off the coast) and further offshore in the Hinder banks area. In this contribution, we focus on the sediment information, which consists of 2200 visual descriptions of geo-referenced seafloor samples and 690 archived sub-samples, to reconstruct the seafloor composition 100 years ago. Estimates of mud levels, sand grain size, shell debris levels as well as occurrence of gravel, pebbles and cobbles were derived from the descriptions and standardized. Mud levels and sand grain-size categories were calibrated by means of grain-size analysis of archived samples (n=383 analyses). Large patches of very high mud levels (50-100%) used to occur close to the shore, along the central and eastern coast. The visual sand grain-size categories display a significant trend in their median grain-size, enabling us to identify distinct patches of fine ( < 250 mu m) and medium (250 < X < 500 mu m) sands. High levels of shell debris were found in the offshore; in coastal waters, they fringed the high turbidity zone typical of this area. Gravel grounds were identified in gullies bordering offshore sand banks and were colonized with an abundant bushy epifauna. Long-term changes were briefly considered. The nearshore mud levels are reduced in the present-day due to specific human impacts. Thick layers of poorly consolidated mud now occur mostly in the artificially dug navigation channels, at disposal sites and around port areas. Offshore, richly colonized biogenic reefs of native flat oyster (Ostrea edulis) were destroyed by targeted oyster fishery as early as in the 1860s. Since the 1920s, seabed disturbance caused by trawling activities steadily increased. We thereby demonstrate a degradation path from richly colonized biogenic reefs toward continuously disturbed sandy gravel grounds in the present day. Our conclusion is likely valid for many such grounds in the English Channel and southern North Sea. Major long-term changes observed thus far are clearly related to specific human activities. This reconstruction work will enable to better define the amplitude of human-induced changes in this portion of the North Sea, a necessary pre-requisite to the definition of meaningful environmental health targets. (C) 2011 Elsevier Ltd. All rights reserved.