Foraminiferal community response to seasonal anoxia in Lake Grevelingen (the Netherlands)

Abstract

<p><strong>Abstract.</strong> Over the last decades, hypoxia in marine coastal environments have become more and more widespread, prolonged and intense. These hypoxic events have large consequences for the functioning of benthic ecosystems. They profoundly modify early diagenetic processes involved in organic matter recycling, and in severe cases, they may lead to complete anoxia and presence of toxic sulphides in the sediment and bottom water, thereby severely affecting biological compartments of benthic marine ecosystems. Within these ecosystems, benthic foraminifera show a high diversity of ecological responses, with a wide range of adaptive life strategies. Some species are particularly resistant to hypoxia/anoxia and consequently, it is interesting to study the whole foraminiferal community as well as species specific responses to such events. Here we investigated the temporal dynamics of living benthic foraminiferal communities (recognised by CellTracker™ Green) at two sites in the saltwater Lake Grevelingen in the Netherlands. These sites are subject to seasonal anoxia with different durations and are characterised by the presence of free sulphide (H<sub>2</sub>S) in the uppermost part of the sediment. Our results indicate that foraminiferal communities are impacted by the presence of H<sub>2</sub>S in their habitat, with a stronger response in case of longer exposure times. At the deepest site (34 m), one to two months of anoxia and free H<sub>2</sub>S in the surface sediment resulted in an almost complete disappearance of the foraminiferal community. Conversely, at the shallower site (23 m), where the duration of anoxia and free H<sub>2</sub>S was shorter (one month or less), a dense foraminiferal community was found throughout the year. Interestingly, at both sites, the foraminiferal community showed a delayed response to the onset of anoxia and free H<sub>2</sub>S, suggesting that the combination of anoxia and free H<sub>2</sub>S does not lead to increased mortality, but rather to strongly decreased reproduction rates. At the deepest site, where highly stressful conditions prevailed for one to two months, the recovery time of the community takes about half a year. In Lake Grevelingen, <i>Elphidium selseyense</i> and <i>Elphidium magellanicum</i> are much less affected by anoxia and free H<sub>2</sub>S than <i>Ammonia</i> sp. T6. We hypothesise that this is not due to a higher tolerance of H<sub>2</sub>S, but rather related to the seasonal availability of food sources, which could have been less suitable for <i>Ammonia</i> sp. T6 than for the elphidiids.</p>