Variability of carbonate chemistry in Chesapeake Bay: a long-term study using simple and complex models

The dynamics of carbonate chemistry in estuaries are strongly influenced by biogeochemical processes that vary significantly across seasonal and regional scales. Despite recent advances in the understanding of inorganic carbon cycling in Chesapeake Bay, key questions remain regarding the seasonal coupling of dissolved inorganic carbon (DIC) and total alkalinity (TA) dynamics, regional variations in DIC and TA cycling, and the relative contribution of biogeochemical processes to TA and DIC production and consumption. In this study, we investigated axial and seasonal variations in the net biogeochemical transformation of DIC, TA, dissolved oxygen, nitrogen, and phosphorus over a thirty-year period in the main stem of Chesapeake Bay using a box model coupled with rates predicted by a 3D biogeochemical model. Model results suggested much higher early spring DIC/TA production/consumption rates in the upper bay, but comparably lower rates in the mid and lower bay during summer time. Aerobic respiration (AR) was the dominant biogeochemical process in regulating the DIC of bottom waters, particularly in the mid bay. Sulfate reduction (SR) and calcium dissolution (CD) were the other key processes in controlling the dynamics of bottom DIC and TA. A higher ΔTA/ΔDIC ratio in late summer (compared to early summer) indicated an even stronger role of SR and CD. Calcium carbonate precipitation was another dominant process other than AR in the very upper bay in summer, which likely occurred within recently expanded SAV communities where high pH (>9.5) and omega result from high rates of primary productivity. These results highlight that net transformations of DIC and TA in this large estuary exhibit distinct seasonal patterns across space that reflect alternating contributions of aerobic, anaerobic, and dissolution processes.