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Sediment Oxygen Uptake in the seasonally hypoxic Lake Grevelingen

Royal Netherlands institute of Sea Research (NIOZ), Ecosystem Studies (YES), The Netherlands

Marine Lake Grevelingen (surface area 11 km2) is a former estuary in the south-west Delta area of The Netherlands. In response to severe flooding in 1953, the Grevelingen was closed off by a dam from the North Sea in 1971, turning the estuary into a marine lake. Due the exclusion of tidal motions, summer stratification strongly increased, leading to seasonal hypoxia in the bottom waters of the deeper basins. To investigate the role of sediments in the development and duration of seasonal hypoxia, the NIOZ (PI Filip Meysman) conducted an integrated ecosystem study, targeting water column chemistry, sediment geochemistry, microbiology, meiofauna and macro fauna, and which involved a multi-disciplinary team of partners from the universities of Utrecht, Angers, and Aarhus. One particular aspect of this study was to look in detail at benthic-pelagic coupling, and to this end, we determined the fluxes across the sediment-water interface of oxygen, nutrients, dissolved inorganic carbon and alkalinity during monthly sampling campaigns in 2012 (PHD project Dorina Seitaj).

To determine they Total Oxygen Uptake of the sediment, intact sediment cores were collected with a UWITEC gravity corer (Fig.1) from the NIOZ research vessel LUCTOR. Benthic fluxes were determined ship-board via closed core incubations in a temperature controlled incubator (Fig.2). Three replicate sediment cores were incubated at in situ conditions, and as control, one core filled with bottom water was incubated, to be able correct for respiration activity withinn the overlying water of the sediment cores. Oxygen Spot Sensors (OXSP5) were attached to the inner wall of each core liner before sampling (Fig.2), and calibrated This procedure enabled continuous measurement of the O2 concentration in the overlying water (without physical insertion of a sensor into the core). Oxygen optodes (OXROB10) and temperature sensors (TSUB21 via TEX4) were connected via a Firesting Oxygen Meter (4 channnels) to a laptop, which was located outside the incubator. Figure 3 shows the temporal evolution of the O2 concentration in the four core incubations. The initial slope of this O2 profile was used to calculate the Total Oxygen Uptake, accounting for enclosed sediment area and overlying water volume.

Scientific team: Dorina Seitaj (PhD student), Pieter van Rijswijk (Technical Engineer), Filip Meysman (Senior Researcher) at NIOZ

Fig. 1: Intact sediment collection with UWITEC gravity corer.

Fig. 2: On board incubation set up. Right side: cooling system. Left side: laptop and close up of oxygen spot sensor on core walls.

Fig. 3: Temporal evolution of the O2 concentration in the three closed sediment core incubations (blue, green, red) and one control incubation only filled with bottom water (green). The initial slope of the O2 evolution in the sediment incubations was used to calculate the Total Oxygen Uptake of the sediment, accounting for the (small) background respiration in the overlying water.