Alkalinity in Tidal Tributaries of the Chesapeake Bay

Abstract

Despite the important role of alkalinity in estuarine carbon cycling, the seasonal and decadal variability of alkalinity, particularly within multiple tidal tributaries of the same estuary, is poorly understood. Here we analyze more than 25,000 alkalinity measurements, mostly from the 1980s and 1990s, in the major tidal tributaries of the Chesapeake Bay, a large, coastal-plain estuary of eastern North America. The long-term means of alkalinity in tidal-fresh waters vary by a factor of 6 among seven tidal tributaries, reflecting the alkalinity of nontidal rivers draining to these estuaries. At 25 stations, mostly in the Potomac River Estuary, we find significant long-term increasing trends that exceed the trends in the nontidal rivers upstream of those stations. Box model calculations in the Potomac River Estuary indicate that the main cause of the estuarine trends is a declining alkalinity sink. The magnitude of this sink is consistent with a simple model of calcification by the invasive bivalve Corbicula fluminea. More generally, in tidal tributaries fed by high-alkalinity nontidal rivers, alkalinity is consumed, with sinks ranging from 8% to 27% of the upstream input. In contrast, tidal tributaries that are fed by low-alkalinity nontidal rivers have sources of alkalinity amounting to 34% to 171% of the upstream input. For a single estuarine system, the Chesapeake Bay has diverse alkalinity dynamics and can thus serve as a laboratory for studying the numerous processes influencing alkalinity among the world's estuaries. Plain Language SummaryAlkalinity, which is the capacity of a water body to neutralize acid, is a useful quantity when studying the cycling of carbon in water bodies, including estuaries. Here we analyze alkalinity measurements in tidal tributaries of the Chesapeake Bay. Average alkalinity levels in the freshest parts of the estuaries varied by sixfold among seven tidal tributaries. Alkalinity was also found to increase over several decades at several locations, partially due to alkalinity increases in the rivers draining to Chesapeake Bay and also probably due to a reduction in the processes that remove alkalinity from estuarine waters. Evidence also supports the role of an invasive species, the Asiatic Clam, in the alkalinity removal in the Potomac River Estuary. More generally, we found evidence that tidal tributaries fed by high-alkalinity rivers consumed alkalinity while tidal tributaries that are fed by low-alkalinity rivers produce alkalinity. For a single estuarine system, the Chesapeake Bay has a wide range of alkalinity levels and a wide variety of processes that influence its alkalinity. Therefore, the Chesapeake Bay can serve as a laboratory for studying the alkalinity of many of the world's estuaries.

Challenges in Quantifying Air‐Water Carbon Dioxide Flux Using Estuarine Water Quality Data: Case Study for Chesapeake Bay

“…The relationship with time recognizes the substantial seasonal and interannual variability of TA of the Susquehanna River. TA in the mainstem bay has increased over the past several decades at a rate that is approximately consistent with expectations from the increase in the riverine source and conservative mixing with a fixed marine endmember (see also Najjar et al, 2020). On the seasonal time scale, however, it is not clear to what extent river TA is felt in the mainstem bay.…”

“…Studies of the carbonate system of the mainstem Bay are relatively recent. Observations indicate relatively poor acid buffering capacity (Cai et al, 2017); biological sources and sinks of DIC that vary with time, depth, and distance along the axis of the estuary (Brodeur et al, 2019; Friedman et al, 2020; Shadwick, Friedrichs, et al, 2019); and nonconservative TA behavior (Brodeur et al, 2019; Najjar et al, 2020). The only estimates of air‐water CO 2 fluxes in the mainstem bay were based on an 8‐month data set in mesohaline and polyhaline waters (Friedman et al, 2020), a time series estimate at a single location in the polyhaline bay (Shadwick, Friedrichs, et al, 2019), and numerical models (Shen et al, 2019; St‐Laurent et al, 2020).…”

“…Support for this premise is found in the multidecadal p CO 2 studies of Carstensen et al (2018) and Prasad et al (2013), who computed p CO 2 using pH and total alkalinity (TA) data from monitoring programs, which increased in number in the 1970s and 1980s in response to a growing public awareness of water pollution. Only recently have monitoring program data sets begun to be used for long‐term studies of the estuarine carbonate system, with a focus on acidification and alkalinization (Baumann & Smith, 2017; Carstensen & Duarte, 2019; Duarte et al, 2013, Hu et al, 2015; Najjar et al, 2020; Waldbusser et al, 2013). However, there are at least three challenges associated with using water quality data for studying p CO 2 dynamics.…”

Mechanisms Driving Decadal Changes in the Carbonate System of a Coastal Plain Estuary

“…Najjar et al. (2020) identified TA increases over many decades in tidal tributaries of the Chesapeake Bay, including the upper main stem Bay, which reflected a combination of increasing riverine TA and, in the Potomac River Estuary, a declining TA sink.…”

Relative impacts of global changes and regional watershed changes on the inorganic carbon balance of the Chesapeake Bay

“…These processes include the diffusion of CO 2 across the air-sea interface, the dissolution of CO 2 and its dissociation into bicarbonate and carbonate ions, and the transport of anthropogenic dissolved inorganic carbon into the ocean interior by vertical mixing and subduction. Thus, early estimates of the uptake of anthropogenic CO 2 by the ocean did not explicitly include marine biological processes (Oeschger et al, 1975). However, if biological processes change during the uptake of anthropogenic CO 2 into the ocean, then they can alter that uptake.…”