Restoration potential of benthic foraminiferal communities in hypoxic areas of Lake Grevelingen

Abstract

ABSTRACT- Lake Grevelingen (SW-Netherlands) is experiencing oxygen depletion of bottom waters. This experimental study investigated the restoration potential of benthic foraminifera in anoxic parts of Lake Grevelingen. With a microcosm experiment we studied the composition and development of benthic foraminiferal communities under changed geochemical conditions. Benthic foraminifera can survive extreme oxygen conditions. They are less affected by these harmful low oxygen conditions than other faunal compartments of these benthic ecosystems. With their short life cycle they are ideally suited for short term experiments. Sediments were collected in hypoxia affected and unaffected areas of Lake Grevelingen. Hypoxia affected sediments were characterized by very low foraminiferal densities and mostly occupied by opportunistic species. The unaffected sediments comprised numerous foraminifera. The collected sediments were used in a laboratory experiment. To obtain the experimental cores, the sediment cores were split along the vertical axis. By placing two half sediment cores adjacent to each other, three types of experimental cores were made. Affected sediment halves were reunited and either exposed to ventilated, oxygen-rich bottom-water conditions for 10.5 weeks or kept under prolonged anoxic field conditions. Other hypoxia affected sediments were combined with half cores form unaffected areas. In addition, also sediments form the unaffected areas were reunited. The latter two types of experimental cores were kept under ventilated conditions for 10.5 weeks. The affected (former anoxic) sediments that were combined with unaffected sediments and were ventilated, revealed more rapid recovery rates than the anoxic ventilated sediments after 10.5 weeks in terms of population growth. There are already plans for opening the Brouwersdam (which disconnects Lake Grevelingen form the NorthSea) further to reintroduce tides and ventilate the bottom waters of the lake. The foraminiferal community could benefit from this situation because the foraminifera from the unaffected sediments could migrate to the affected sediments, as seen in the experiments.