Hydrologic Shifts Create Complex Transient Distributions of Particulate Organic Carbon and Biogeochemical Responses in Beach Aquifers

Abstract

Biogeochemical reactions within intertidal zones of coastal aquifers have been shown to alter the concentrations of terrestrial solutes prior to their discharge to surface waters. In organic‐poor sandy aquifers, the input of marine organic matter from infiltrating seawater supports active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing have been documented, there is limited understanding of the transience of various organic carbon pools (pore water particulate, dissolved, sedimentary) within the aquifer and how these relate to the location and magnitudes of biogeochemical reactions over time. To understand the relationship between changes in groundwater flow and the seasonal migration of geochemical patterns, beach pore water and sediment samples were collected and analyzed from six field sampling events spanning 2 years. While the seasonally dynamic patterns of aerobic respiration closely followed those of salinity, redox conditions and nutrient characteristics (distributions of N and P, denitrification rates) were unrelated to contemporaneous salinity patterns. This divergence was attributed to the spatial variations of reactive particulate organic carbon distributions, unrelated to salinity patterns, likely due to filtration, retardation, and immobilization dynamics during transport within the sediments. Results support a “carbon memory” effect within the beach, with the evolution and migration of reaction patterns relating to the distribution of these scattered carbon pools as more mobile solutes move over less mobile pools during changes in hydrologic conditions. This holds important implications for the prediction and quantification of biogeochemical reactions within beach systems.

Seasonality of Organic Matter Degradation Regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach

“…Due to its reduced mobility, particulate organic matter is further trapped in intertidal sediments. This “carbon memory” has been observed to enhance microbial rates for up to 2 months (Kim et al, ). It is thus likely that both August and October rates have been affected by the high pelagic activity in prior months.…”

“…Advective flow of seawater through sediments supplies benthic microbial communities with oxygen (O 2 ) and nitrate (NO 3 − ), as well as particulate and dissolved organic matter, thus leading to high rates of organic matter degradation and nutrient turnover (Anschutz et al, 2009;Huettel & Rusch, 2000b;Kim et al, 2019). As such, coastal permeable sediments can contribute substantially to marine organic carbon remineralization, thereby affecting coastal nutrient and metal budgets (Huettel et al, 1998;Huettel et al, 2014;Van Raaphorst et al, 1990).…”

Modeling Hydrologic Controls on Particulate Organic Carbon Contributions to Beach Aquifer Biogeochemical Reactivity

“…In beach aquifers, such particulate transport dynamics and frequent shifts in hydrologic conditions result in a spatially variable distribution of POC across the beach aquifer ( Figure 1a). Recent field investigations at Cape Shores, Lewes, DE, USA, showed that these pools of reactive POC, potentially from algal fragments, beach wrack, and seagrass, retarded in their movement relative to groundwater flow, are important sources of nutrients and biogeochemical reactivity to the adjacent porewater (Kim et al, 2019). Numerical models also suggest that dissolution of POC from buried wrack can serve as an important organic carbon source to fuel reactivity in the beach (Heiss, 2020).…”

“…During wave and tide activity, seawater travels up the beachface and infiltrates into the aquifer, overtopping seaward flowing fresh groundwater (Abarca et al, 2013; Michael et al, 2005; Robinson et al, 2006). The mixing between fresh groundwater and saline seawater creates a saltwater circulation cell in the intertidal zone (Heiss & Michael, 2014; Michael et al, 2005; Robinson et al, 2006; Vandenbohede & Lebbe, 2006), which hosts dynamic biogeochemical reactions (Hays & Ullman, 2007; Kim et al, 2017, 2019; Slomp & Van Cappellen, 2004; Spiteri, Slomp, Tuncay, & Meile, 2008; Ullman et al, 2003). Fresh groundwater delivers terrestrial nutrients such as nitrate to the beach aquifer, which can be utilized as an alternative electron acceptor after oxygen depletion.…”

“…Field studies often measure surface water or porewater DOC to quantify and characterize organic carbon supply into the system (Beck et al, 2007; Chaillou et al, 2016; Charbonnier et al, 2013; Kim et al, 2012, 2013; O'Connor et al, 2018; Oh et al, 2017; Reckhardt et al, 2015; Seidel et al, 2014, 2015), and numerical modeling studies use DOC associated with seawater infiltration to support reactions (Anwar et al, 2014; Bardini et al, 2012; Heiss et al, 2017; Spiteri, Slomp, Charette, et al, 2008; Spiteri, Slomp, Tuncay, & Meile, 2008). While particulate organic carbon (POC) has not been completely disregarded in field settings (Beck et al, 2017; Charbonnier et al, 2013; Kim et al, 2017, 2019; Reckhardt et al, 2015), especially in its association with metal oxides (Charette et al, 2005; Roy et al, 2010, 2013), to date, most of the focus has been on the distribution, supply, and molecular quality of DOC and how DOC relates to biogeochemical reactions within beach sediments (e.g., Seidel et al, 2015), with few studies considering in situ POC (e.g., Heiss, 2020).…”