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

• Published in: Journal of geophysical research. Biogeosciences Vol. 124; no. 10; pp. 3024 - 3038
• Main Authors: Kim, Kyra H., Michael, Holly A., Field, Erin K., Ullman, William J.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-10-2019
• Subjects: Aerobic respiration; Algae; Aquifers; beach aquifer; Beaches; Biogeochemistry; Carbon; Chemical analysis; Chemical reactions; Coastal aquifers; Coastal ecosystems; coastal hydrogeology; Coastal waters; Coasts; Denitrification; Dissolved organic carbon; Divergence; Fluxes; Geochemistry; Groundwater; Groundwater flow; Hydrology; Immobilization; Inland water environment; intertidal circulation cell; Intertidal environment; Intertidal zone; Marine environment; Mineral nutrients; Nutrients; organic carbon; Organic chemistry; Organic matter; Particulate organic carbon; Phytoplankton; Pools; Pore water; Primary production; reactivity; Salinity; Salinity effects; Seasonal variations; Seawater; Sediment; Sediment samples; Sediments; Solutes; Spatial variations; submarine groundwater discharge; Surface water; Tides; Water mixing; Water quality
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2019JG005114

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.
Plain Language Summary
Sandy beaches host mixing zones between fresh and salty water. Seawater flows up the beachface due to waves and tides, and flows into the sand to meet the fresh groundwater flowing through the aquifer. These mixing zones change over time, responding to freshwater flow, waves, and tides. The mixing of the two waters supports chemical reactions that benefit coastal ecosystems by reducing the amount of land‐derived nutrients that degrade coastal water quality. Reactions are fueled by carbon in these beach settings. Previous research has focused on dissolved organic carbon in seawater as the key driver of beach chemical reactions. However, seawater often also carries fragments of algae and phytoplankton, which are particles that can also support chemical reactions within the sands. As mixing zones change in shape, responding to hydrologic condition changes, these particles may be transported within the beach. We investigate how these carbon particles are distributed as water mixing patterns change over seasonal time scales, and how they contribute to beach chemical reactions. This work highlights that patterns of hydrologic and geochemical parameters within the beach may deviate from each other, yielding valuable insight to the spatial patterns of biogeochemical reactions and coastal solute fluxes crucial for the health of marine environments.
Key Points
Distributions of salinity and solutes are dynamic over seasonal time scales, dependent on different hydrological and geochemical processes
Particulate organic matter distributions are a result of complex filtration, retardation, and immobilization dynamics during transport and deviate from solute patterns
Entrapped particulate carbon is reactive and continues to contribute to biogeochemical reactions in the aquifer, creating a “carbon memory” effect