Nitrogen-alkalinity interactions in the highly polluted Scheldt basin (Belgium)

We present results of one year observations in highly heterotrophic and oxygen-depleted rivers of the polluted Scheldt basin. Monthly measurements revealed a high variability for dissolved inorganic carbon and nitrogen, with the following strong parallelism: highest alkalinity and NH4+ were associated with lowest NO3− and oxygen and vice-versa. In river water incubations, nitrification lowered the alkalinity whereas denitrification raised it; in an anoxic, NO3−-free incubation an increase of alkalinity was observed, partially due to ammonification. A stoichiometric analysis, taking into account the amount of protons produced or consumed by each process involving nitrogen, revealed that monthly variations of NO3− and NH4+ with ammonification, nitrification and denitrification could explain the 28 and 62% alkalinity variations at all stations, except one. The remaining part of the alkalinity variations was attributed to other anaerobic processes (Mn-, Fe- and SO4-reductions). This trend seems to be the result of the whole catchment metabolism (riverine waters and sediments, sewage networks and agricultural soils). The observed HCO3− concentrations in the Scheldt basin were 2–10 times higher than the representative concentrations reported in pristine basins and used in chemical weathering models. This suggests the existence of an anthropogenic source, originating from organic matter decomposition. We conclude that in highly polluted basins, nitrogen transformations strongly influence the acid–base properties of water

Dissolved inorganic carbon dynamics and CO₂ atmospheric exchanges in the inner and outer Scheldt estuary

Since 1992, the Chemical Oceanography Unit of the University of Liège has carried out on a regular basis field cruises in the Scheldt inner estuary and the river plume (outer estuary), during which were measured: pH, total alkalinity, dissolved inorganic carbon, partial pressure of CO2 (pCO2), dissolved oxygen and atmospheric flux of CO2. In the inner Scheldt estuary, pCO2 values in the upper estuary can be as high as 9000 ppm that is about 25 times the value of atmospheric equilibrium (presently around 370 ppm). These high pCO2 values induce a high CO2 efflux and the entire Scheldt can emit up to 790 tons of carbon per day (tC day-1) to the atmosphere. The annually integrated CO2 emission is estimated to 456 tC day-1 (Frankignoulle et al 1998 Science 282: 434-436). Along the salinity gradient, dissolved inorganic carbon dynamics are dominated on one hand by nitrification at salinities around 5 and on the other hand by dilution. Total alkalinity is not conservative in the upper estuary (salinity 0 to 5) due to intense nitrification which produces H3O+ and leads to a decrease of total alkalinity and a minimum of both pH and oxygen saturation level. For salinities higher than 5, total alkalinity has a conservative behaviour (Frankignoulle et al 1996 Limnol. Oceanogr. 41: 365-369). A simple carbon budget shows that aerobic heterotrophic activity and nitrification produce similar amounts of CO2 and can explain most of the CO2 emission from the inner estuary to the atmosphere. The input of CO2 from fresh water inputs represents only 10% of aerobic heterotrophic activity and nitrification and 10% of the estuarine emission to the atmosphere. The advective flux of CO2, from the river to the estuary and from the estuarine mouth to the North Sea are one order of magnitude lower than atmospheric exchange in the estuarine zone (Abril et al 2000 Comptes Rendus de l'Académie des Sciences Paris 330: 761-768). In the outer Scheldt estuary, pCO2 shows a distinct seasonal evolution related to the cycle of biological activity. Throughout the year, the river plume is over-saturated (average pCO2 value of about 450 ppm) except during the Phaeocystis bloom when values of pCO2 as low as 50 ppm are observed (Borges and Frankignoulle 1999 J. Mar. Syst. 19: 251-266). The outer Scheldt estuary emits CO2 on an annual basis at a rate of about 110 tC day-1 that corresponds to about 25% of the emission of CO2 by the inner estuary. A simple carbon budget shows the input of CO2 from the inner estuary contributes to about 30% of the emission of CO2 from the outer estuary. The remaining emission of CO2 is from the net heterotrophic activity fuelled by organic carbon inputs from the inner Scheldt estuary and the Belgian coast (Borges and Frankignoulle 2002 Biogeochemistry 59: 41-67)

Biological control of air-sea CO₂ fluxes: effect of photosynthetic and calcifying marine organisms and ecosystems

A simple expression enables prediction of the effect of photosynthetic and calcifying systems on air-sea CO2 exchange at all spatial scales (from organism to ecosystem). Input data are: gross primary production (Pg), respiration (R), net calcification (G) and the ratio of CO2 released to CaCO3 precipitated ( psi ); the output is the amount of dissolved inorganic carbon (FCO2 which needs to be exchanged with the atmosphere to balance biologically mediated changes in the concentration of dissolved inorganic carbon in an open sea water system: FCO2 = -Pg + R + psi G. Coral reef data were used in the model to illustrate the relative influence of organic and inorganic carbon metabolism on ocean-atmosphere CO2 cycling. A coral reef comprised of calcareous and non-calcareous organisms can be shown to act as a sink for atmospheric CO2 when excess (= net) production is high and CaCO3 precipitation is low. These characteristics are not typical of actively developing reef systems which typically exhibit a nearly balanced organic carbon metabolism (Pg/R similar to 1) and relatively high rates of calcification. In these circumstances, reef communities can be expected to cause CO2 evasion to the atmosphere. This prediction is confirmed by the only existing measurement of air-sea CO2 flux in a coral reef system

Atmospheric CO₂ fluxes in a highly polluted estuary (the Scheldt)

Dissolved CO2 concentration and exchange with the atmosphere were investigated simultaneously in the Scheldt estuary. CO2 partial pressures as high as 5,700 µatm, corresponding to oversaturation with respect to the atmosphere of 1,600%, were observed in the upper estuary. The corresponding atmospheric CO2 fluxes reached values of up to 1.2 mol m-2 d-1. The estimated flux for the entire estuary amounts to 600 t of C d-1 for a river discharge of 6 m3 d-1

Dissolved inorganic carbon cycle in the maximum turbidity zone of the upper Scheldt estuary

The Scheldt Estuary is one of the most polluted macro-tidal European estuaries due to a high anthropogenic pressure around its catchment area. High load of suspended organic matter (with at least two third directly related to human activities) associated to a long residence time within the estuary (three months) contribute to an intense bacterial degradation (Wollast, 1988). The most striking feature of this work, compared to the previous studies carried on the Scheldt (Frankignoulle et al.,1996, 1998; Abril et al., 2000) is the continuous measurement of the CO2 partial pressure of the surface brackish water in the maximum turbidity zone of the inner Scheldt Estuary since November 2002 to nowadays. Our results show that pCO2 in the surface brackish water is outstandingly high, ranging from 2000 to 10000 ppm, which represents up to 2700% of the CO2 atmospheric pressure. CO2 also shows strong meso- and macroscale variabilities and on an annual scale it appears that pCO2 is mainly controlled by temperature and heterotrophy

Carbon fluxes in coral reefs. II. Eulerian study of inorganic carbon dynamics and measurement of air-sea CO₂ exchanges

Air-sea CO2 exchanges and the partial pressure of CO2 were measured in surface water overlying 2 coral reefs: Moorea (French Polynesia, austral winter, August 1992), where coral diversity and surface cover are low, and Yonge Reef (Great Barrier Reef, austral summer, December 1993), where coral diversity and cover are comparatively higher. A procedure is proposed to estimate the potential CO2 exchange with the atmosphere by taking into account both the saturation level of oceanic seawater and the equilibration process occurring after water leaves the reef. It is shown that both sites were net sources of CO2 to the atmosphere as a result of the effect of calcification on the dynamics of the inorganic carbon system. The potential global CO2 evasion from the ocean to the atmosphere is about 4 times higher at Yonge Reef than at Moorea. It is also demonstrated that, at both sites, the major exchange of CO2 from sea to air occurs as seawater returns to chemical equilibrium after it has crossed and left the reef. The dynamics of inorganic carbon were studied using the so-called homogeneous buffer factor [beta = dln(pCO(2))/dln(DIC)] (where pCO(2) is the CO2 partial pressure in surface water and DIC is dissolved inorganic carbon), which gave estimates that approximately 80% of the change in inorganic carbon was related to photosynthesis and respiration. This approach showed that the calcification rate was proportional to the net organic production during the day and to the respiration rate at night

Pelagic metabolism of the Scheldt estuary measured by the oxygen method on an annual scale

Pelagic gross primary production (GPP), community respiration (CR) and nitrification were measured in the turbid Scheldt Estuary by the oxygen Winkler method from January to December 2003 at monthly intervals (EUROTROPH EU project). Five stations along the estuary were investigated, corresponding to a salinity (S) range of 0-25. Water was sampled and incubated until sunset in 60 ml glass bottles stored in a 5 compartment incubator kept at in situ temperature by flowing water. Irradiance was controlled in each compartment by filters having a shading capacity ranging from 0 to 100%. In order to estimate the oxygen consumption due to the respiration and nitrification processes, samples were incubated, in the dark compartment, with and without addition of nitrification inhibitors. Net community production (NCP) was most of the time negative in the estuary with values ranging from -275 to +31mmol O2.m-2.d-1 and the lowest values were found near Antwerp (S = 2). Strong pelagic GPP and positive NCP rates were observed in the freshwater part during summer with a maximal value in June (+373mmol O2.m-2.d-1), corresponding to an increase of the O2 concentration and a decrease of the partial pressure of CO2 (pCO2) in the water column during this period. Nitrification contributes 5 to 60% of the oxygen consumption in the water column with highest values measured in the inner part of the estuary due to high ammonium and suspended matter concentrations. Assuming a C/O2 molar ratio of 0.07, we estimated that nitrification represents on an annual scale 35% of organic matter production at salinity 2 which is consistent with previous estimates. NCP rates measured in 2003 are among the lowest reported in the literature and confirm the strong heterotrophic status of the Scheldt Estuary

Artificial neural network analysis of factors controling ecosystem metabolism in coastal systems

Knowing the metabolic balance of an ecosystem is of utmost importance in determining whether the system is a net source or net sink of carbon dioxide to the atmosphere. However, obtaining these estimates often demands significant amounts of time and manpower. Here we present a simplified way to obtain an estimation of ecosystem metabolism. We used artificial neural networks (ANNs) to develop a mathematical model of the gross primary production to community respiration ratio (GPP:CR) based on input variables derived from three widely contrasting European coastal ecosystems (Scheldt Estuary, Randers Fjord, and Bay of Palma). Although very large gradients of nutrient concentration, light penetration, and organic-matter concentration exist across the sites, the factors that best predict the GPP:CR ratio are sampling depth, dissolved organic carbon (DOC) concentration, and temperature. We propose that, at least in coastal ecosystems, metabolic balance can be predicted relatively easily from these three predictive factors. An important conclusion of this work is that ANNs can provide a robust tool for the determination of ecosystem metabolism in coastal ecosystems

Diurnal changes in seawater carbonate chemistry speciation at increasing atmospheric carbon dioxide

Natural variability in seawater pH and associated carbonate chemistry parameters is in part driven by biological activities such as photosynthesis and respiration. The amplitude of these variations is expected to increase with increasing seawater carbon dioxide (CO2) concentrations in the future, because of simultaneously decreasing buffer capacity. Here, we address this experimentally during a diurnal cycle in a mesocosm CO2 perturbation study. We show that for about the same amount of dissolved inorganic carbon (DIC) utilized in net community production diel variability in proton (H+) and CO2 concentrations was almost three times higher at CO2 levels of about 675 ± 65 in comparison with levels of 310 ± 30 μatm. With a simple model, adequately simulating our measurements, we visualize carbonate chemistry variability expected for different oceanic regions with relatively low or high net community production. Since enhanced diurnal variability in CO2 and proton concentration may require stronger cellular regulation in phytoplankton to maintain respective gradients, the ability to adjust may differ between communities adapted to low in comparison with high natural variability