Ecosystem recovery after hypoxia: what can foraminifera indicate?

Abstract

The many resources and services provided by coastal ecosystems (e.g. food, fertile soils), make these areas valuable habitats for marine life and human occupation. Expanding human population sizes and the associated increase of human exploitation of coastal zones has made these areas prone to perturbations. In recent decades (human-induced) hypoxia became one of the prominent phenomena to cause destabilization of marine communities. Positive feedback mechanisms often amplify the impact of hypoxia and induce a nonlinear response of ecosystem functioning during and after disturbance. Restoring the oxygen concentrations towards a more natural situation does not necessarily culminate in the return of lost species. Recovery of harmed ecosystems to the status preceding the disturbance may never be reached, and the patterns of recovery are generally different from the reversed patterns of deterioration. The predictability of the effect of management and restoration effort is hindered by this nonlinearity in response and recovery pathways, thus posing an extra hurdle to convince policy makers to take appropriate action. In order to prevent the occurrence of hypoxic disturbance and to stimulate recovery of affected ecosystems, insight needs to be gained on the recovery pathways and the coupling of abiotic and biotic mechanisms within the ecosystems. Living foraminifera are increasingly acknowledged for their value as indicator for ecosystem health. The unicellular eukaryotic phylum Foraminifera comprises an ecologically and biologically very diverse group of organisms, abundantly present in nearly all marine environments. Although several species live a planktonic life style in the water column, most foraminifera find their habitat either within, or on top of marine sediments. These benthic species, the subject of this thesis, are distributed along the vertical sediment profile. Environmental circumstances determine the taxonomic and spatial structure of the foraminiferal assemblage - the foraminiferal species found in association with one another. Especially the availability of oxygen and food has been considered to shape foraminiferal communities and determine the vertical and geographical distribution of the individual members of the communities. Studying foraminiferal dynamics in relation to environmental parameters is interesting and valuable in its own right, but additionally it holds the potential to shed light on the recovery potential of disturbed ecosystems as a whole. Living foraminifera may play an eminent role also in environmental research on extant coastal ecosystems. With field and laboratory experiments we aimed to improve our understanding of the contribution of direct and indirect effects of hypoxia – and the combination of both – on foraminiferal communities in coastal ecosystems. We investigated hypoxia-induced foraminiferal dynamics during ecosystem recovery to gain better insight in the use of foraminifera as indicator species for ecosystem status and to improve our understanding of ecosystem development. These insights will help to improve their role as indicator species for ecosystem health in hypoxia-affected marine coastal areas.