Foraminifera and nematodes - Responses to seasonal hypoxia

Lake Grevelingen is an artificial, oligotrophic saline lake in the Netherlands that was created in 1971 after construction of the Brouwers- and Grevelingendam, which closed off the former estuary from both its connection to the sea and large rivers of the Delta. It is characterized by strong summer stratification and stagnant bottom-water with seasonally occurring hypoxia (= 2 ml O2 l -1) and anoxia (no oxygen). While eutrophication-associated oxygen depletion is spreading worldwide in shallow coastal seas and estuaries, the main cause for the formation of hypoxia in Marine Lake Grevelingen is the heavily affected hydrology due to the closure of the estuary. This makes the study site ideal for testing the effects of low dissolved oxygen on benthic fauna in the natural environment not affected by eutrophication. The central objective of this research project is to identify at monthly resolution the temporal (1 year) and spatial (3 water depths) dynamics of two key representatives of the meiofauna, namely benthic foraminifera and nematodes, in response to hydrographic and sediment geochemical parameters, seasonal oxygen depletion (including hydrogen sulphide development in the deeper site) and primary production. These two meiofauna taxa are prominent model organisms, used to assess the quality of the environment and how it changes over time. This study will include the identification and quantification of nematodes and Foraminifera in terms of abundance, species richness, diversity and assemblage composition. More specifically, on a fine scale we aim to elucidate the short- and longer- term responses and adaptations of species to (recurrent) seasonal hypoxia, i.e. changes in vertical distribution in the sediment as well as variations in pore density and in pore size in foraminifera. The study is part of a larger multidisciplinary project examining the spatial and temporal dynamics of hypoxia in Marine Lake Grevelingen from various perspectives (i.e. water column chemistry, sediment biogeochemistry, primary production, benthic microbial, meiofauna, and macrofauna community). The individual data sets, linked and analysed in an integrated metadata approach will be used to obtain a comprehensive overview on feedback responses between benthic fauna and sediment geochemistry in relation to the duration and intensity of hypoxia/anoxia. Our study will therefore yield a holistic overview on the current state of Marine Lake Grevelingen and provide important insights into the status of marine shallow-water ecosystems threatened with oxygen depletion worldwide.

The occurrence of low oxygen waters in shallow, coastal and estuarine areas are increasing worldwide, most likely accelerated by human activities. The enrichment of waters by nutrients is a main cause for the formation of hypoxia (? 2 ml O2 l -1) and anoxia (0 ml O2 l -1). In the study site, Marine Lake Grevelingen, the Netherlands, however, seasonal hypoxia/anoxia occurs due to the heavily affected hydrology after the closure of the estuary in 1971, which makes this site ideal for testing the effects of low oxygen on benthic fauna in the natural environment.The central objective of this study on disturbance and subsequent recovery was to identify at a bimonthly resolution the temporal (1 year) and spatial (3 water depths, along a bottom water oxygen gradient) dynamics of two meiofauna taxa benthic Foraminifera (results described below) and nematodes which have become promising model organisms to assess the quality of the environment and how it changes over time.Overall, the identification and quantification of the benthic Foraminifera assemblage revealed a markedly higher abundance and diversity in Lake Grevelingen than originally expected. Based on these results, and unlike our hypothesis, the population in the deepest station, was clearly characterised as autochthonous. While the population dynamics during the course of the year in the middle station (affected by hypoxia) rather imply that bottom water oxygen concentration did not vary enough to cause significant changes in the overall benthic foraminiferan assemblage, strong response patterns were observed in the deepest site (seasonal anoxia). We clearly determined more tolerant (e.g. Trochammina globigeriniformis) and more sensitive species (e.g. Quinqueloculina laevigata). Moreover, our results support previous findings of a reversed ratio in abundance of Ammonia cf. A. batava/Elphidium ex. gr. E. excavatum, suggesting a significantly higher tolerance level of the latter to prolonged hypoxia than is generally assumed. Also, we determined a high number of malformed individuals from different species as well as Foraminifera with low test strength, most likely related to seasonal low oxygen concentrations and/or changes in the pH value. Another unexpected observation was the abnormal and not yet specified encrustation of the Foraminifera species Elphidium magellanicum. Finally, the analysis of morphological features, i.e. pore density and pore size in the species Ammonia cf. A. batava, identified different morphotypes.The results represent a highly valuable contribution to Foraminifera research and add concrete input into the discussion both in the current and historical context on the issue- complex of climate change, acidification and biodiversity loss.