Abstract
Sources and transformation of C were quantified using mass balance and ecosystem metabolism data for the upper segments of the James, Pamunkey and Mattaponi Estuaries. The goal was to assess the role of external (river inputs & tidal exchange) vs. internal (metabolism) drivers in influencing the forms and fluxes of C. C forms and their response to river discharge differed among the estuaries based on their physiographic setting. The James, which receives the bulk of inputs from upland areas (Piedmont and Mountain), exhibited a higher ratio of inorganic to organic C, and larger inputs of POC. The Pamunkey and Mattaponi receive a greater proportion of inputs from lowland (Coastal Plain) areas, which were characterized by low DIC and POC, and elevated DOC. We anticipated that transport processes would dominate during colder months when discharge is elevated and metabolism is low, and that biological processes would predominate in summer, leading to attenuation of C through-puts via de-gassing of CO2. Contrary to expectations, highest retention of OC occurred during periods of high through-put, as elevated discharge resulted in greater loading and retention of POC. In summer, internal cycling of C via production and respiration was large in comparison to external forcing despite the large riverine influence in these upper estuarine segments. The estuaries were found to be net heterotrophic based on retention of OC, export of DIC, low GPP relative to ER, and a net flux of CO2 to the atmosphere. In the James, greater contributions from phytoplankton production resulted in a closer balance between GPP and ER, with autochthonous production exceeding allochthonous inputs. Combining the mass balance and metabolism data with bioenergetics provided a basis for estimating the proportion of C inputs utilized by the dominant metazoan. The findings suggest that invasive catfish utilize 15 % of total OM inputs and up to 40 % of allochthonous inputs to the James.
Highlights
The estuaries were found to be net heterotrophic based on retention of OC, export of delivering C products of mineral weathering (DIC), low GPP relative to ER, and a net flux of
The effective tidal exchange derived from the Cl mass balance was equivalent to 7.4% and 14 ± 1% of the tidal prism
For DIC, this resulted in a dilution response in both the river and estuary, whereas high discharge resulted in a flushing response of POC in the river and estuary
Summary
Inland waters occupy a small proportion of surface area but play a disproportionately large role in global C cycling (Cole et al 2007; Butman et al 2016; Tranvik et al 2018; Holgerson and Raymond 2016). Decomposition of aquatic and terrestrial organic matter returns C to the atmosphere, which, along with C sequestration via sediment burial, results in the attenuation of C fluxes to the coastal zone (Richey et al 2002; Vorosmarty et al 2003; Middelburg and Herman 2007; Tranvik et al 2009). Acting against these processes are fluvial forces that hasten through-puts of C and favor transport over processing. Current efforts focus on understanding the net effect of inland waters on landscape scale fluxes of C In this context, comparatively little attention has been focused on processes occurring at the river-estuarine transition
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