Biogeochemical processes and buffering capacity concurrently affect acidification in a seasonally hypoxic coastal marine basin

Abstract

Abstract. Coastal areas are impacted by multiple natural and anthropogenic processes and experience stronger pH fluctuations than the open ocean. These variations can weaken or intensify the ocean acidification signal induced by increasing atmospheric pCO2. The development of eutrophication-induced hypoxia intensifies coastal acidification, since the CO2 produced during respiration decreases the buffering capacity in any hypoxic bottom water. To assess the combined ecosystem impacts of acidification and hypoxia, we quantified the seasonal variation in pH and oxygen dynamics in the water column of a seasonally stratified coastal basin (Lake Grevelingen, the Netherlands). Monthly water-column chemistry measurements were complemented with estimates of primary production and respiration using O2 light–dark incubations, in addition to sediment–water fluxes of dissolved inorganic carbon (DIC) and total alkalinity (TA). The resulting data set was used to set up a proton budget on a seasonal scale. Temperature-induced seasonal stratification combined with a high community respiration was responsible for the depletion of oxygen in the bottom water in summer. The surface water showed strong seasonal variation in process rates (primary production, CO2 air–sea exchange), but relatively small seasonal pH fluctuations (0.46 units on the total hydrogen ion scale). In contrast, the bottom water showed less seasonality in biogeochemical rates (respiration, sediment–water exchange), but stronger pH fluctuations (0.60 units). This marked difference in pH dynamics could be attributed to a substantial reduction in the acid–base buffering capacity of the hypoxic bottom water in the summer period. Our results highlight the importance of acid–base buffering in the pH dynamics of coastal systems and illustrate the increasing vulnerability of hypoxic, CO2-rich waters to any acidifying process.

Mn∕Ca intra- and inter-test variability in the benthic foraminifer <i>Ammonia tepida</i>

“…2.1, Fig. S2; Hagens et al, 2015). In theory, under oxic conditions the zone of manganese reduction should lie below the microhabitat of A. tepida, which is situated close to the sediment-water interface (SWI) (Thibault de Chanvalon et al, 2015;Cesbron et al, 2016), and chambers calcified in this condition should show low Mn/Ca ratios.…”

“…S1). The sampling site experiences seasonal hypoxia; for the year 2012, monthly recordings of temperature, salinity and water column oxygen concentrations (Hagens et al, 2015), sedimentary microbial community composition , pore water geochemistry (Sulu-Gambari et al, 2016a, b), and benthic O 2 uptake rates (Seitaj et al, 2017) are available. In 2012, BWO started to decrease in April and attained a minimum of about 20 µM (∼ 8 % saturation) in August (Fig.…”

“…Ammonia tepida is abundant in coastal areas of temperate climate zones, tolerating diverse biological and environmental stress factors, including low-oxygen conditions (Moodley and Hess, 1992;Sen Gupta et al, 1996;Geslin et al, 2014;Nardelli et al, 2014;Thibault de Chanvalon et al, 2015;Cesbron et al, 2016). The salty bottom waters of Lake Grevelingen, an artificial lake created after the closure of a branch of the Rhine-Meuse-Scheldt estuary, are characterized by seasonal hypoxia ([O 2 ] < 63 µM) and anoxia ([O 2 ] < detection limit of 1 µM) (Hagens et al, 2015;Seitaj et al, 2017). Therefore, this site provides a suitable location to study short-term environmental variability (on a timescale of weeks to months) in relation to elemental incorporation into benthic foraminiferal tests.…”

Impact of Seasonal Hypoxia on Activity and Community Structure of Chemolithoautotrophic Bacteria in a Coastal Sediment

“…Lake Grevelingen is a former estuary located within the Rhine-Meuse-Scheldt delta area of The Netherlands, which became a closed saline reservoir (salinity, ϳ30) by dam construction at both the land side and sea side in the early 1970s. Due to an absence of tides and strong currents, Lake Grevelingen experiences a seasonal stratification of the water column, which in turn leads to a gradual depletion of the oxygen in the bottom waters (52). Bottom-water oxygen at the deepest stations typically starts to decline in April, reaches hypoxic conditions by end of May (O 2 concentration, Ͻ63 M), further decreasing to anoxia in August (O concentration, Ͻ0.1 M), and reoxygenation of the bottom water takes place in September (14).…”

“…Details of the water column, pore water, and solid sediment chemistry of Lake Grevelingen over the year 2012 have been previously reported (14,26,52). Sediments were recovered at three stations along a depth gradient within the Den Osse basin, one of the deeper basins in Lake Grevelingen: station 1 (S1) was located at the deepest point (34 m) of the basin (51.747°N, 3.890°E), station 2 (S2) was located at 23 m depth (51.749°N, 3.897°E), and station 3 (S3) was located at 17 m depth (51.747°N, 3.898°E).…”