Spatial distribution of seawater carbonate chemistry and hydrodynamic controls in a low-inflow estuary
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Coastal and estuarine systems play an important role in the global carbon cycle and often have complex carbonate chemistry dynamics due to a multitude of biogeochemical and physical drivers. Compared to classic estuaries, mechanisms driving the distribution of carbonate parameters in low-inflow estuaries are understudied. The spatial distribution of carbonate chemistry and hydrodynamic parameters were characterized in Morro Bay, a short and seasonally hypersaline estuary on the Central California Coast, during the dry, low-inflow season to better understand in situ modifications. Sampling transects were completed in the main channel in June, August, and September of 2018, bracketing both a high and low tide on each date. Temperature, salinity, total alkalinity, and dissolved inorganic carbon all increased from the mouth to the back of the estuary, with larger values observed during the low tide. pH values decreased towards the back of the bay, and had little variation between high and low tide for June and August transects. Flushing times (estimated using a salt-budget model approach) also increased toward the back of the bay which led to hypersaline conditions. Salinity alone only explained 20–33% of observed changes in total alkalinity and 13–22% of observed changes in dissolved inorganic carbon throughout the bay. The remaining changes in total alkalinity and dissolved inorganic carbon were likely driven by biogeochemical modifications enhanced by extended flushing times, particularly in the back bay. Prior to this project, Morro Bay experienced a recent, rapid collapse of eelgrass, the major biogenic habitat. In the last four years eelgrass in Morro Bay appears to be on a recovery trajectory; therefore, this study provides a baseline whereby future studies can evaluate carbonate chemistry changes associated with potential eelgrass recovery and expansion. This study highlights the unique hydrodynamic exchange in seasonally low-inflow estuaries and its potentially large role in influencing local carbonate chemistry and ocean acidification.Keywords:
Biogeochemical Cycle
Alkalinity
Abstract. The marine CaCO3 cycle is an important component of the oceanic carbon system and directly affects the cycling of natural and the uptake of anthropogenic carbon. In numerical models of the marine carbon cycle, the CaCO3 cycle component is often evaluated against the observed distribution of alkalinity. Alkalinity varies in response to the formation and remineralization of CaCO3 and organic matter. However, it also has a large conservative component, which may strongly be affected by a deficient representation of ocean physics (circulation, evaporation, and precipitation) in models. Here we apply a global ocean biogeochemical model run into preindustrial steady state featuring a number of idealized tracers, explicitly capturing the model's CaCO3 dissolution, organic matter remineralization, and various preformed properties (alkalinity, oxygen, phosphate). We compare the suitability of a variety of measures related to the CaCO3 cycle, including alkalinity (TA), potential alkalinity and TA*, the latter being a measure of the time-integrated imprint of CaCO3 dissolution in the ocean. TA* can be diagnosed from any data set of TA, temperature, salinity, oxygen and phosphate. We demonstrate the sensitivity of total and potential alkalinity to the differences in model and ocean physics, which disqualifies them as accurate measures of biogeochemical processes. We show that an explicit treatment of preformed alkalinity (TA0) is necessary and possible. In our model simulations we implement explicit model tracers of TA0 and TA*. We find that the difference between modelled true TA* and diagnosed TA* was below 10% (25%) in 73% (81%) of the ocean's volume. In the Pacific (and Indian) Oceans the RMSE of A* is below 3 (4) mmol TA m−3, even when using a global rather than regional algorithms to estimate preformed alkalinity. Errors in the Atlantic Ocean are significantly larger and potential improvements of TA0 estimation are discussed. Applying the TA* approach to the output of three state-of-the-art ocean carbon cycle models, we demonstrate the advantage of explicitly taking preformed alkalinity into account for separating the effects of biogeochemical processes and circulation on the distribution of alkalinity. In particular, we suggest to use the TA* approach for CaCO3 cycle model evaluation.
Alkalinity
Biogeochemical Cycle
Total inorganic carbon
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Biogeochemistry
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Vaz, N., Mateus, M. and Dias, J.M., 2011. Semidiurnal and spring-neap variations in the Tagus Estuary: Application of a process-oriented hydro-biogeochemical model. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), 1619 – 1623. Szczecin, Poland, ISSN 0749-0208 The main objectives of this work are the presentation of a hydro-biogeochemical model for the Tagus estuary and the study of the interaction between tides and river discharge. Special emphasis will be given to hydrographic, fine sediment dynamics and biogeochemical variations at two different time scales: the tidal cycle and the fortnight cycle. The numerical model validation was performed comparing harmonic analysis results of measured and model predicted sea surface height for 12 stations covering the whole estuary. For the M2 constituent the difference is less than 5% of the local amplitude for almost all the stations. Differences in phase are small, with an average value of 5o. The results show strong salinity and suspended sediments gradients which are advected up and downstream according to the tidal cycle. Moreover, the estuary is divided in three main regions: a marine, a mixing and a freshwater influence area. The suspended sediments and chlorophyll concentrations reveal strong fortnight time dependence near the estuary mouth. Higher concentrations of fine sediments are found during spring tides that on neaps, which induces a decrease (increase) in the chlorophyll concentration during spring (neap) tides. This study presents the first results of the Tagus estuary predictive model, which are consistent with observations and with previously knowledge about the estuary. Preliminary outcomes obtained with this high resolution numerical modeling system are very encouraging to pursuit the development of a numerical tool suitable to deliver products that can be used both in scientific endeavors and in estuarine management, in order to assess the ecological estuarine quality.
Biogeochemical Cycle
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Alkalinity
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