Chesapeake Bay Acidification
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6
The importance of fluvial processes are apparent when making comparisons across hydrogeomorphic
regions of the Chesapeake Bay watershed, and these must be considered in order to understand the
chemical nature of the Bay. Upward trends in total alkalinity in many east coast rivers, including the
Susquehanna, Potomac, and Patuxent Rivers, are apparent in recent decades, perhaps due to human
activities that accelerated carbonate rock weathering through acid deposition, mining, and other land use
changes activities (Raymond & Cole, 2009; Kaushal et al., 2013). Given the importance of these
tributaries as sources of freshwater to Chesapeake Bay, changes to total alkalinity likely have important
effects on local and regional carbonate buffering capacity.
It is clear that carbonate chemistry varies spatially and temporally in Chesapeake Bay, but the full extent
of that variation is not known because the carbonate system has not been widely measured in this context.
Charge Question B – How do biological and biogeochemical processes affect acidification in the Bay?
There is strong evidence that Chesapeake Bay is influenced significantly by both biological and
biogeochemical processes. The importance of photosynthetic CO
2
drawdown is well recognized in coastal
ecosystems, and is a phenomenon frequently witnessed during phytoplankton blooms that result in CO
2
reduction and pH elevation. Widespread hypoxia and anoxia are often telltale signs of extreme benthic
respiration but benthic respiration also plays an important role in diurnal and seasonal fluctuations in
water column pCO
2
/pH. Because of the Bay’s relative shallow water depth, organic carbon that rains out
of the water column is typically decomposed via aerobic and anaerobic processes, generating extensive
release of CO
2
back into the water column. In heterotrophic estuary reaches, high pCO
2
is associated with
escape of CO
2
to the atmosphere however, such CO
2
flux between the air and water remains to be
quantified over much of Chesapeake Bay and many other coastal ecosystems.
Unlike oceanic systems, where sediments are typically aerobic and low in organic materials (Schlesinger
& Bernhardt, 2013; Cai & Reimers, 1995), tidal marsh soils and Bay sediments are hot spots of sulfate
reduction, a biogeochemical process that generates both CO
2
and net alkalinity (Giblin, 1988; Cai &
Wang, 1998; Cai, 2011). Sulfate reduction may play an important role in carbonate chemistry dynamics
across much of Chesapeake Bay, but this process has yet to be quantitatively characterized.
Tidal saltmarshes
are important transition zones between upland terrestrial ecosystems and many
temperate coastal bays and estuaries. Depending on a marsh’s particular soil type, the cation exchange
capacity may make them important sites of cation exchange (e.g., H
+
displacing K
+
), generating net
alkalinity (Megonigal unpublished data). Furthermore, saltmarsh plants transfer significant amounts of
carbon to soils through root respiration and productivity. Root respiration releases CO
2
directly to the
rhizosphere where plant roots, soil solids, and porewater meet. Root productivity supports soil microbes
that break down organic material to CO
2
, with sulfate reduction being one of the dominant microbial
respiration pathways as discussed above. A portion of the CO
2
produced in soils is exported to adjacent
tidal creeks. Thus, tidal salt marshes are believed to be an important source of CO
2
in coastal waters. The
extent of CO
2
input, and concomitant changes to pH and other aspects of carbonate chemistry has yet to
be quantified in Chesapeake Bay or other Mid-Atlantic coastal ecosystems where tidal saltmarshes are
frequently dominant transitional habitats.
Nutrient runoff and eutrophication
are important drivers of water quality in Chesapeake Bay (Jordan &
Weller 1996), and are also expected to influence the Chesapeake’s carbonate chemistry. For example,
eutrophication-driven phytoplankton blooms draw down CO
2
from the water column, much of which is
released in benthic sediments once the bloom ends and dead biomass is subjected to microbial
decomposition. Though eutrophication by itself is unlikely to explain spatial and temporal patterns in the
Bay carbonate system, understanding the relationships between nutrient runoff and carbonate chemistry
will be important.
