Shallow water benthic foraminifera as proxy for natural versus human-induced environmental change

Abstract

Ecosystem composition and functioning is not only subjected to human-induced alterations, ecosystems also subjected to natural (e.g. climate-induced) variability. To quantify human impacts on ecosystems, these natural fluctuations must be accounted for. Since long-term biological monitoring programs are rare and usually do not include the pre-human state, we must rely on traces of past ecosystems found in the geologic record. Foraminifera (Protista) are close relatives of the amoeba, that live predominantly in the sea and have a unique feature that makes them popular proxies: many build a shell (a so-called test) of calciumcarbonate during their life. Since they are abundant in most marine environments and their tests are often preserved in sediments, they are widely used in paleoceanography and paleoclimatology. Besides two practical issues that are dealth with in this thesis, most chapters are concerned with developing proxies to reconstruct human influences on near-shore ecosystems by collecting living foraminifera from the North Sea and Dutch Wadden Sea. In the southern North Sea and Wadden Sea foraminiferal distributions did not appear to be limited by total food abundance or in-sediment oxygen concentrations. Additionally, foraminiferal community composition did not seem to be influenced by macrofaunal community composition (dominated by either filter feeders or burrowing species). We hypothesize that distribution of benthic foraminifera in the North Sea is mainly controlled by the type of food available (labile or refractory) and by the level of environmental variability. Different combinations of these two variables are found across habitats beneath tidal mixing fronts and therefore, benthic foraminifera in temperate, shallow seas are particularly suited to reconstruct hydrodynamic regimes. These conclusions were used to reconstruct the history of the western Wadden Sea. In this analysis, the interplay between anthropogenic and natural influences shows that the effects of human alterations had sudden and dramatic consequences for the functioning of this ecosystem.