A historical record of benthic foraminifera in seasonally anoxic Lake Grevelingen, the Netherlands

Lake Grevelingen is a former branch of the Rhine-Meuse-Scheldt estuary, which was artificially transformed into a salt-water lake by a dam in 1971. This transformation induced profound changes in the biological community of the basin, which have been described as an ecodisaster, with seasonal hypoxia/anoxia occurring in the deepest parts of the lake. Here, we investigate a sediment core sampled in 2012 in the Den Osse Basin (34 m depth) of Lake Grevelingen, recording the last ~50 years and including the transition from an estuary to a salt-water lake. Sediment molybdenum (Mo) concentrations were used to refine an existing age model based on 210Pb, giving us an estimated precision of ±3 years. The benthic foraminiferal succession reflects the anthropogenic modifications in Lake Grevelingen and allows four successive stages to be distinguished: 1) before 1971, when the system was estuarine; 2) from 1971 to 1978, when the system rapidly changed into an enclosed brackish water lake; 3) from 1978 to 1999, when a seaward sluice was opened during winter and 4) the period from 1999 to 2012, with a year-round opening of the sluice and doubling of water exchanges with the North Sea. The foraminiferal record, showing communities mainly dominated by Elphidium selseyense, also highlights the appearance of the putatively exotic Ammonia confertitesta in the mid-1980s, which thereafter progressively replaced the congeneric autochthonous Ammonia aberdoveyensis. Finally, we hypothesise that the activity of cable bacteria, S-oxidising prokaryotes present in the surface sediments of Den Osse Basin in winter and spring, causes dissolution of foraminiferal tests by decreasing the pore water carbonate saturation state in the sediments directly below the oxic zone. This explains the contrast between the abundant living populations and the very low numbers of foraminiferal shells preserved in the top ~15 cm of the sediment.