Carbon cycling and its effect on oxygen concentrations and pH in a seasonally anoxic coastal marine system
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Coastal systems are increasingly experiencing low-oxygen conditions, due to enhanced primary production from terrestrial nutrient input and increasing thermal stratification as a result of global warming. The second effect of increased atmospheric CO2 is an enhanced uptake by surface oceans, thereby lowering the pH. Organic carbon cycling is one of the key processes that links oxygen and pH in coastal systems, as oxygen is produced and protons are consumed by photosynthesis, while oxic remineralization in deeper water has the reverse effect. The question arises how extended periods of water-column hypoxia affect the pH of the system. Lake Grevelingen, in the southwestern part of the Netherlands, was used as a case study, as this lake has seasonal periods of hypoxia that vary in extent between consecutive years. On monthly cruises in 2012 water-column data were collected using CTD measurements and DOC and nutrient analyses on samples from discrete water depths. Suspended matter was filtered from water samples and analyzed for organic carbon and total nitrogen content. Field data were combined with a newly constructed 1D-reactive transport model of the carbon cycling, which used data from Rijkswaterstaat as forcing and calibration data. pH and oxygen concentration were calculated in the model as dependent variables. Measurements of pH in Lake Grevelingen show a decline over the past 30 years with ~0.005 unit per year, which coincided with ~0.07 μg L-1 yr-1 increase in chlorophyll a. The modeled gross primary production was within the range reported in the literature, but well below the preliminary estimate for 2012. Deep-water pH in the model is only dependent on oxic mineralization, which has led to lower values than those observed in the water column. In years of more extensive hypoxia, the drop in pH was larger than during the years of shorter hypoxic periods. Anaerobic processes may enhance the buffer factor of the system, thereby mitigating the respiratory decline in pH. An assessment of the buffer factor of Lake Grevelingen showed that the system is more vulnerable to changes in pH due to uptake of atmospheric CO2 than the Eastern China Sea and the Gulf of Mexico.