Carbon dynamics and community production in the Mississippi River plume

Abstract

Dissolved inorganic carbon (DIC), total alkalinity (TAlk), pH, and dissolved oxygen (DO) were determined in the Mississippi River plume during five cruises conducted in the spring, summer, and fall. In contrast to many other large rivers, both DIC and TAlk were higher in river water than in seawater. Substantial losses of DIC, relative to TAlk, occurred within the plume, particularly at intermediate salinities. DIC removal was accompanied by high DO, high pH, and nutrient depletion, and was attributed to high phytoplankton production. As a result, the carbonate saturation in the plume became much higher than in ocean and river waters. A mixing model was used to determine DIC removal. We provide evidence that the use of a two-end-member (river and ocean) mixing model was valid during late summer and fall (low discharge period). However, for other periods we used salinity and TAlk to delineate a mixing model that included two river end members and an ocean end member. Net community production rates in the plume, estimated using a box model, peaked in the summer and were among the highest reported to date for large river plumes. In the summer and fall, biological production in the river plume consumed a majority of the available nutrients, whereas during the spring only a small fraction of the available nutrients were consumed in the plume. Biological production was the dominant process influencing pH and carbonate saturation state along the river-ocean gradient, whereas physicochemical dynamics of mixing played an important role in controlling the TAlk and DIC distributions of this large river plume.

Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes

“…A complete removal of NO 3 − (arbitrarily set to occur at S = 25) leads to pCO 2 as low as 192 µatm and pH as high as 8.34 in the high carbonate river plume ([NO 3 − ] = 100 µmol kg −1 , which is diluted to 30.6 µmol kg −1 at S = 25) and pCO 2 as low as 224 µatm and pH as high as 8.18 in the low carbonate river plume ([NO 3 − ] = 15 µmol kg −1 , which is diluted to 4.6 µmol kg −1 at S = 25). Observed extreme pCO 2 and pH in the Mississippi River plume are similar to the simulations (Guo et al 2012;Cai unpublished).…”

“…7.2). Therefore, the rapid pCO 2 decrease to a minimum (or pH increase to a maximum) observed in many middle-latitude river plumes Guo et al 2012) cannot be attributed simply to thermodynamic changes during riverocean mixing as was presented in Salisbury et al (2008). Large rivers in middle-latitudes are also rich in nutrients derived from use of agriculture fertilizers and sewage.…”

“…As a result, pCO 2 (or pH) in these river plumes are expected to be very low (or high) ( Fig. 7.2); such low pCO 2 and high pH values have been observed in the Mississippi (Guo et al 2012) and Changjiang River plumes (Chou et al 2009b;Zhai and Dai 2009;Tseng et al 2011). Consequently, we suggest that although ocean acidification, which has been linked with CO 2 uptake from the atmospheric (which has increased because of fossil fuel use), has become a major ocean environmental stressor, eutrophication-induced pH increases are a dominant feature in surface water pH in the nutrient-rich river plumes worldwide (Borges and Gypens 2010).…”

“…Average TAlk values of the Mississippi River and Amazon River were estimated to be 2,300 µmol kg −1 and 300 µmol kg −1 , respectively. We also assumed a DIC = TAlk + 30 µmol kg −1 (Guo et al 2012) for both rivers, which results in reasonably expected values of pCO 2 during summer. A complete removal of NO 3 − (arbitrarily set to occur at S = 25) leads to pCO 2 as low as 192 µatm and pH as high as 8.34 in the high carbonate river plume ([NO 3 − ] = 100 µmol kg −1 , which is diluted to 30.6 µmol kg −1 at S = 25) and pCO 2 as low as 224 µatm and pH as high as 8.18 in the low carbonate river plume ([NO 3 − ] = 15 µmol kg −1 , which is diluted to 4.6 µmol kg −1 at S = 25).…”

“…during lowest discharge, owing to the dilution effect of weathering products (Cai 2003;Cai et al 2008;Chen et al 2008;Bouillon et al 2012;Guo et al 2012). High HCO 3 − content in subtropical to middle-latitude large river waters provides a high acid-base buffering capacity, comparable to that of seawater.…”