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

Over the last decades, hypoxia in marine coastal environments has become more and more widespread, prolonged and intense. Hypoxic events have large consequences for the functioning of benthic ecosystems. In severe cases, they may lead to complete anoxia and the presence of toxic sulfides in the sediment and bottom-water, thereby strongly 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 sulfide (H2S) in the uppermost part of the sediment. Our results indicate that foraminiferal communities are impacted by the presence of H2S in their habitat, with a stronger response in the case of longer exposure times. At the deepest site (34 m), in summer 2012, 1 to 2 months of anoxia and free H2S 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 H2S was shorter (1 month or less), a dense foraminiferal community was found throughout the year except for a short period after the stressful event. Interestingly, at both sites, the foraminiferal community showed a delayed response to the onset of anoxia and free H2S, suggesting that the combination of anoxia and free H2S does not lead to increased mortality, but rather to strongly decreased reproduction rates. At the deepest site, where highly stressful conditions prevailed for 1 to 2 months, the recovery time of the community takes about half a year. In Lake Grevelingen, Elphidium selseyense and Elphidium magellanicum are much less affected by anoxia and free H2S than Ammonia sp. T6. We hypothesise that this is not due to a higher tolerance for H2S, but rather related to the seasonal availability of food sources, which could have been less suitable for Ammonia sp. T6 than for the elphidiids.