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.
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
1 Introduction
Fresh groundwater in coastal aquifers mixes with saline groundwater prior to its discharge to the ocean as submarine groundwater discharge (SGD; Li et al., 2008; Michael et al., 2005; Robinson et al., 1998, 2007). SGD includes the discharge of fresh groundwater (blue, Figure 1), convectively recirculated seawater at the base of the beach (lower interface, Figure 1), and circulating brackish water due to tide and wave activity on the beach (intertidal circulation cell, Figure 1). The influx of oxygen and reactive marine organic carbon from saline seawater into hypoxic to anoxic fresh groundwater creates strong geochemical gradients along freshwater-seawater (FW-SW) mixing zones, promoting biogeochemical reactions within the intertidal circulation cell. The chemical reactivity within the cell and of coastal aquifers has been the subject of a number of field (Beck et al., 2017; Charbonnier et al., 2013; Reckhardt et al., 2015; Seidel et al., 2015) and numerical modeling studies (Abarca et al., 2013; Anwar et al., 2014; Heiss et al., 2017; Robinson et al., 2007), due to its ability to transform and mitigate inputs of nutrients (Hays & Ullman, 2007a& Hays & Ullman, 2007b; Kim et al., 2017; Slomp & van Cappellen, 2004; Ullman et al., 2003), metals and trace elements (Bone et al., 2006; Charette et al., 2005; Charette & Sholkovitz, 2002, 2006; Jung et al., 2009; McAllister et al., 2015), and contaminants (Geng et al., 2015; Robinson et al., 2009) to coastal systems.
Seasonal sea level and groundwater table fluctuations, climate-driven sea level anomalies, wave conditions, storms, and precipitation patterns create variability in SGD flux (Gonneea et al., 2013a; Heiss & Michael, 2014; Michael et al., 2005; Moore & Wilson, 2005; Wilson et al., 2011; Xin et al., 2014; Yu et al., 2017), and consequently, the fluxes of nutrients, trace elements, and radioactive tracers to the adjacent ocean (Beck et al., 2007; Gonneea et al. (2013b); Jeong et al., 2012; Kelly & Moran, 2002; Roy et al., 2013; Seidel et al., 2015; Santos et al., 2009; Waska & Kim, 2011). The circulation cell is spatiotemporally dynamic, responding primarily to seasonal freshwater gradient fluctuations, and also to spring-neap cycles, and tidal stage to a lesser extent (Heiss & Michael, 2014). These physical shifts alter groundwater flow paths within the beach aquifer and consequently change the delivery of gases, solutes, and particles necessary to support biogeochemical reactions at a given location. It has been shown that the locations and rates of solute-dependent reactions, such as aerobic respiration and denitrification, are related to groundwater flow paths defined by the geometry of the circulation cell (Kim et al., 2017). Therefore, transient distribution of various reactants that result from seasonal hydrologic changes imply an equally dynamic movement of reaction centers and characteristics over the seasons.
Sandy sediments are not often associated with large reservoirs of organic matter (Anschutz et al., 2009; Boudreau et al., 2001). Therefore, the delivery and transport of reactive carbon into the beach aquifer is a principal factor controlling the rates and distribution of biogeochemical reactions within the circulation cell. Continental shelf and intertidal sandflat sediments have been shown to entrap organic matter, bacteria, and particles which then undergo bioturbation, resuspension, and/or degradation (Bacon et al., 1994; Huettel & Rusch, 2000; Pilditch & Miller, 2006; Rusch et al., 2000; Rusch & Huettel, 2000). Analogously, beach sediments also act like a filter, entrapping fine particles (Anschutz et al., 2009; Charbonnier et al., 2013; McLachlan et al., 1985) and bacteria (Gast et al., 2015), resulting in spatial variations in the respiration rate of sediments (Beck et al., 2016). However, while the seasonality of hydrology, surface water primary production, pore water dissolved organic carbon (DOC) concentrations, and subsequent changes to reaction rates are relatively well-documented (Beck et al., 2008; Charbonnier et al., 2013; O'Connor et al., 2018; Seidel et al., 2014; Seidel et al., 2015), there has been limited focus on the transport and immobilization of particulate organic matter distributions and related reaction characteristics. As the extent and geometry of the intertidal circulation cell evolve over seasons, changing flow and transport patterns would variably distribute particulate organic carbon (POC) within the aquifer.
Charbonnier et al. (2013) explored temporal variations of solute concentrations in shallow pore water at Truc Vert, France, noting that in the summer, oxygen depletion and associated nitrate enrichment in the lower beach areas were decoupled from surface water chlorophyll concentrations, indicating an alternative organic carbon source. Beck et al. (2016) observed increases of ammonium within the sediments that may be related to solid-phase organic matter degradation. Similarly, O'Connor et al. (2018) observed the seasonality of redox conditions in the intertidal aquifer at Gloucester Point, VA, and documented increases in DOC above the conservative mixing curve in the summer that may be linked to the breakdown of particulate carbon. Detailed molecular characterization of dissolved organic matter on samples from the Northern Germany Wadden Sea suggested the release of labile marine DOC and other nutrients from buried algal and microbial biomass, which were preferentially consumed relative to DOC of terrestrial origin (Seidel et al., 2014). Dissolved organic matter composition of intertidal sands also showed significant correlation to the supplied marine and terrestrial organic matter, while organic matter composition in tidal flats were decoupled from seasonality and more related to early diagenetic processes (Beck et al., 2017; Seidel et al., 2015). These studies collectively indicate that organic matter dynamics within sediments are influenced by both hydrologic transport and diagenesis and call for further work that comprehensively integrate the roles of particulate and dissolved carbon dynamics in beach systems.
In this paper, we present a 2-year pore water and sediment study to illustrate the transient behavior of reaction zones and particulate organic matter distributions within sandy beach aquifers. Our results show that while salinity and aerobic respiration rates respond and equilibrate to hydrologic changes, reaction and nutrient distributions deviate from changes in salinity patterns. We attribute this asynchronous behavior to the variable distribution of reactive organic carbon and the transport dynamics of reactants. This study demonstrates the spatiotemporal complexity of reaction characteristics with beach aquifers, caused by, but spatially and temporally decoupled from, positional shifts of the intertidal circulation cell.
2 Field Site and Methods
2.1 Field Site
The research was conducted at Cape Shores, Lewes, Delaware, a public beach adjacent to Cape Henlopen State Park. The beach is tide-dominated (semidiurnal, 1.42-m range) with limited wave activity due to offshore breakwaters. The beach aquifer is mostly coarse sand (540–668 μm), with two pebbly layers interfingering the aquifer and finer sediments (128–360 μm) in deeper locations (>2 m).
The intertidal circulation cell at Cape Shores has been extensively characterized, with previous work confirming the remineralization of organic matter and cycling of nitrogen, phosphorus, silica, iron, and sulfur (Hays & Ullman, 2007a; Hays & Ullman, 2007b; Kim et al., 2017; McAllister et al., 2015; Ullman et al., 2003). Heiss and Michael (2014) measured and modeled the dynamics of the cell over tidal, spring-neap, and seasonal cycles and showed that the most prominent changes to the cell geometry occurred over seasonal cycles. The spatial distributions of reactions within the intertidal circulation cell and their relationships to its flow field have also been documented by Kim et al. (2017), with higher oxic respiration rates near the point of seawater infiltration at the beach surface and along the landward edge of the FW-SW mixing zone at depth. Because the intertidal circulation cell at Cape Shores displays migrations seasonally, and reaction rates related to the flow field have been well characterized, this field site is well suited for further research on how reaction rates in the aquifer are related to the flow and sedimentary characteristics in the aquifer over seasonal time scales.
2.2 Pore Water Sampling and Incubation Experiments
Multilevel pore water samplers were constructed from polyethylene tubes mounted along a PVC support pipe. Pore water was collected from the samplers using a peristaltic pump (GeoTech, Denver, Colorado) along a shore-perpendicular transect over six time periods in 2014 (July, September, and November) and 2015 (May, July, and September) (Figure 2). Lost or damaged samplers were replaced as close to their original locations as possible to capture the full extent of the intertidal circulation cell, and best efforts were made to sample from as close to the same locations and depths as possible. As a consequence of the beach dynamics and damage to samplers, some ports could not be used for sample collection on all sampling dates. Inland water table was measured every 15 min at a well installed at the dune landward of the intertidal circulation cell using a TD-Diver with barometric corrections (Van Essen Instruments, Delft, Netherlands).
Salinity, temperature, oxidation-reduction potential (ORP), and dissolved oxygen (DO) were measured in the field while other solutes and constituents (chlorophyll, DOC, particulate nitrogen, NO3−, NH4+, particulate phosphorus, PO43−, dissolved Si) were measured in the laboratory. Pore water POC was quantified by filtering 1 L of pore water through a 0.7-μm glass fiber filter, which were then combusted using the Costech CHN Elemental Analyzer to measure carbon. For May 2015, accurate ORP measurements were unavailable due to a probe malfunction. Five unfiltered replicate pore water samples were incubated in the laboratory in the dark to determine the successive decrease of oxygen and increase of nitrogen to obtain apparent oxygen consumption and denitrification rates. Additional details about the design and construction of samplers, solute, and particulate sampling and analyses, and incubation experiments can be found in Kim et al. (2017).
2.3 Sediment Vibracores
Two vibracores were collected in July 2015 adjacent to Sampler C (Core C, 1.1-m depth) and Samper F (Core F, 1.9-m depth; Figures 3 and 8) to characterize the sediments and potential respiration rates at the field site. Pebbly layers and finer sand at depth, as well as color variations along the core (light brown to grayish black) were visible to the naked eye. Each core was divided into four sections according to grainsize and color. Then, each section was homogenized, and five replicate sediment samples were taken from the homogenized batch. The sediment samples were then incubated in borosilicate biological oxygen demand bottles with filtered, oxygenated seawater to obtain rates of oxygen consumption and nitrogen gas production at the sampling sites. Oxygen consumption rates from sediment-free control samples (filtered, oxygenated seawater with no sediment input) were subtracted from measured rates to obtain reaction rates that reflect the contributions of sedimentary organic matter to bulk reaction rates. Sediments were analyzed for carbon content (here referred to as entrapped sedimentary carbon) using the loss on ignition method. Elemental concentrations (Al, B, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Zn, As, Cd, Cr, Ni, and Pb) were also analyzed using the Environmental Protection Agency method 3051 for digestion and Inductively Coupled Plasma Mass Spectrometry.
2.4 Sand Column Deployments
Sand column deployments were conducted to distinguish respiration associated with pore water-delivered organic matter that had been entrapped by the sand from in situ organic matter deposited with sediments. Five 3.05 m PVC pipes (1.27 cm OD) were slotted over their full length and filled with homogenized beach sand that had been precombusted overnight to remove all organics. Top and bottom of the columns were glue capped, and a rope handle was attached at the top for easy retrieval. In 31 July 2015, the columns were installed by hand augering along a transect adjacent to the pore water sampling transect, 1.83 m east of each pore water sampler (Figure 8, Samplers B, C, E, F, and H). All columns remained 3.05 m in length, save for one column next to Sampler C that had to be trimmed in half due to difficulties during its deployment. On 13 October 2015, after they had time to equilibrate with the beach aquifer system, the columns were pulled out of the beach. The retrieved columns were then divided into lower, mid, and upper sections by cutting them at their 1.22- and 2.44-m mark from the bottom (except for the sand column next to Sampler C, which were only sectioned into two sections due to its shorter length). Lower 25.4 cm of sediment from each section was then divided among five borosilicate incubation bottles and were incubated following the same sediment incubation procedures outlined above. An incubation with filtered, oxygenated seawater with combusted sediment was used as a control.
3 Results
3.1 Physicochemical Parameters
Salinity, a conservative tracer, outlines the geometry and location of the intertidal circulation cell. The measured salinity confirms the observations of Heiss and Michael (2014), with the intertidal circulation cell displaying seasonal variations in its size and position in response to seasonal changes in tidal amplitude and freshwater head (Figure 3, left column). In both 2014 and 2015, the circulation cell expanded horizontally, and salinity increased through summer and into the fall (Figure 3, left column, July to November in 2014; May to September in 2015), in response to the falling freshwater head after its spring (May 2014) or winter maximum (February 2015). In July 2015, intense rain prior to sampling increased the upland water table measured near the top of the beach from 37 to 50 cm over 3 days. This increased freshwater gradient constricted and pushed the saline circulation cell seaward, consistent with simulations of Yu et al. (2017). Salinity sampling for a subset of sampling points 8 days (16 July 2015) after the July 2015 sampling event (08 July 2015) displayed higher salinities in Samplers C and D and lower salinities in Sampler F, indicating a landward progression of the intertidal circulation cell as the inland freshwater head decreased (unpublished data). These results demonstrate the dynamic response of the intertidal circulation cell to both seasonal changes and episodic hydrologic events.
The cross-sectional distributions of dissolved oxygen had higher saturation levels located on the upper part of the circulation cell, where the highest rates of infiltration occurred, as indicated by salinity (Figure 3, middle column). Seawater infiltrates the beach aquifer at fully oxygenated conditions, so the decrease in dissolved oxygen saturation along the circulating flow path indicates active consumption of oxygen (see also Kim et al., 2017). Overall, more dissolved oxygen was available within the intertidal circulation cell during colder months when O2 is more soluble, biogeochemical reactions were slower, and less seawater carbon from primary production was available. November 2014 and September 2015 had 3 and 6 μg/L of surface water chlorophyll, respectively, compared to the 15.3 μg/L average for other months sampled.
ORP patterns, reflecting the net effects of oxidation and reduction within the beach aquifer, did not covary with salinity, consistent with previous findings at this site (Figure 3, right column; Kim et al., 2017). More reducing conditions were found toward the discharge zone and in some months, near the landward FW-SW mixing zone (May and September 2015). However, the seasonal patterns of ORP were largely independent of salinity shifts within the aquifer.
3.2 Dissolved Nutrients
Dissolved nutrients within the aquifer responded to different hydrological and geochemical conditions. While variable in concentrations and distribution over time, NO3− and NH4+ had a distinct spatial separation from each other. Nitrate (~150 μM) was elevated in the shallower, more landward parts of the aquifer, while ammonium (~8 μM) was elevated in deeper parts of the aquifer and near the discharge zone. Hot spots (local relative maxima) of ammonium were often spatially correlated to elevated concentrations of POC (Figure 6), indicating ammonium release during the degradation of POC (section 4.3). However, increased ammonium toward the discharge zone despite low POC in that region (i.e., Figure 4, September 2015 and Figure 6, September 2015) supports active nitrification within the aquifer (Boufadel et al., 2010; Kroeger & Charette, 2008). Silica concentrations were generally high in freshwater and low in saline groundwater, likely due to differences in water residence times during which dissolved silica is produced by dissolution in freshwater aquifers (Dove & Nix, 1997; Loucaide et al., 2008). Concentrations elevated above the mixing line in brackish zones indicate mixing-promoted silica dissolution (supporting information, Figure S2). Phosphate, on the other hand, was less linked to salinity and displayed more patchiness. Phosphate-rich zones were related to elevated POC in the pore water, supporting active leaching and release of phosphate from organic material. This behavior was also displayed by particulate N (see section 4.3).
3.3 Pore Water Reaction Rates
The results of four of the six sampling events for pore water reaction rates and organic carbon content are presented (September 2014, November 2014, July 2015, and September 2015; Figures 5 and 6). Incubation data from July 2014 and May 2015 were not dependable due to prolonged incubation time.
Oxygen gas consumption and nitrogen gas production rates determined by incubation reveal the spatial migration of biogeochemical hot spots within the beach aquifer (Figure 5). Oxygen consumption rates were the highest near the saline water infiltration point and continued along the saltwater circulation flow path, along the landward FW-SW mixing zone. This was consistent throughout the months sampled, and high oxygen consumption rates adhered to the changes in salinity distributions. Since aerobic respiration is most dependent on oxygen supplied by saline water infiltration to the system, this observation is consistent with expectations. High respiration rates, however, did not correlate with hot spots of other solutes, or organic carbon (both particulate and dissolved, Figure 6), suggesting that aerobic respiration within this beach is limited primarily by oxygen and only secondarily by carbon availability.
Potential denitrification rates were determined by production of nitrogen gas beyond its N2/Ar equilibrium gas solubility ratios with respect to air (Kim et al., 2017). Nitrogen gas production via denitrification is expected to occur in areas of low oxygen, high nitrate, and high reactive carbon concentrations, making the zones of mixing between hypoxic, nitrate-bearing freshwater, and carbon-rich seawater ideal sites for this reaction, especially near the discharge zone where oxygen availability is low. However, some hot spots of denitrification were less consistent with the expected patterns. Unlike oxygen consumption rates that were almost always spatially associated with the landward FW-SW mixing zone, hot spots of denitrification were patchier in their distributions, and were generally located further along the circulating flow path. This may be attributed to the variability in nitrate availability and the various pools of carbon within the beach system, discussed in the following sections.
3.4 Spatial Distributions of Pore Water POC, DOC, and Sedimentary Carbon
The distribution and concentration of pore water POC shifted over seasons (Figure 6), independent from the patterns of salinity. Additionally, elevated chlorophyll (not shown) did not correlate with elevated pore water POC, indicating that particulate carbon from different sources may have contrasting transport dynamics and reactivities. All months had some degree of elevated POC at the seaward discharge zone where oxygen availability was low. This was heightened during November 2014 and July 2015. While this may suggest a persistent pool of POC in the seaward parts of the aquifer, concentrations decreased during September 2015, suggesting transience of the POC pool. Although the sampler at 157 m was missing for September 2015, the two most seaward samplers at 162 and 167 m had low POC concentrations, in contrast to the elevated POC concentrations at these locations during July 2015. Notably, some hot spots of pore water POC were found more landward of the intertidal circulation cell, upland of the boundaries of the circulation cell defined by elevated salinity (July 2015).
Zones of elevated DOC coincided spatially with zones of elevated pore water POC. Both the distributions and the concentrations exceeding that of surface water indicate that DOC is produced by active leaching from POC, along with other nutrients (see section 4.3).
Sedimentary carbon content in vibracore samples determined by loss on ignition method was minimal (1.95–7 mgorganic/gsediment), and there was no apparent relationship between depth, grainsize, organic matter content, and oxygen consumption rate (supporting information, Table S1). However, despite the low organic matter content, vibracore samples displayed measurable oxygen consumption when incubated with oxic water (9.3–77.9 ΔO2 μM/d; Figure 8i). This demonstrates that the minor amounts of sedimentary carbon found in beach sands are nonetheless sufficiently labile to contribute to bulk respiration. Oxygen consumption rates did not increase linearly with absolute carbon content (supporting information, Table S1), again suggesting differences in reactivity in various carbon sources (Seidel et al., 2014). There was insufficient nitrate present in the incubated water to determine nitrogen gas production rates in the core samples. Deeper samples at location F with iron-oxide coatings around the sediment grains also had elevated concentrations of Al, Ca, K, P, Mg, Mn, Na, Zn, As, Cr, Pb, and S compared to other samples, indicating an active interception of elements on the iron-oxide coatings (e.g., Charette & Sholkovitz, 2002).
Slotted sand column deployments demonstrated that delivery of reactive organic carbon into the sediments via pore water infiltration is a key source of organic carbon to beach systems. Retrieved sand column samples (combusted and then deployed in situ for 75 days) had a fine layer of gray flocculated material not found in the control samples with only combusted sand (Figure 7). The fine particles were evaluated using microscopy and green autofluorescence (520–560 nm), common in dinoflagellates, diatoms, and microalgae (Tang & Dobbs, 2007). Based on the autofluorescence observations, bacteria, cell fragments, and algal detritus were found within the gray material layer.
The retrieved sand samples were very reactive, with oxygen consumption rates up to 48 ΔO2 μM/d, comparable to rates measured in vibracore sediment samples (Figure 8). Oxygen consumption rates across the beach were virtually zero at the most landward location (Figure 8ii, Location B) and increased toward the ocean across the region where saline infiltration occurs (Figure 8ii, Location H). While the particles were identified through microscopy as algae and bacteria, their green autofluorescence and seaward increases of reactivity cannot independently serve as conclusive evidence of the particles' marine origin. However, they collectively support a marine origin of at least the reactive portion of the particles. While the absolute carbon concentrations of the sand column samples were not quantified, oxygen consumption rates presented in Figure 8ii serve as a proxy of labile carbon distribution within the beach aquifer. The respiration patterns did not correspond to the salinity, depth, or POC patterns of either the deployment or the retrieval month, indicating divergent transport dynamics within the aquifer between solutes and fine particulates.
4 Discussion
4.1 Seasonal Dynamics of Salinity, Nutrient Distributions, and Reaction Characteristics
Solute distributions over seasons show the spatial and temporal dynamics of both conservative and reactive components of the intertidal circulation cell (Figure 3) resulting from pore water advection, filtration, redox conditions, and dissolution. Oxygen concentrations and oxic respiration rates closely followed the anticipated patterns along advective flow paths prescribed by the geometry and salinity of the intertidal circulation cell. This behavior is expected, as oxygen, salinity, and reactive organic carbon share a common seawater source. However, distributions of chemically reactive parameters such as ORP, dissolved inorganic nitrogen, and nitrogen gas production displayed dynamic patterns over seasonal time scales that were not tightly coupled to changes in salinity (Figures 4-6). Nitrogen gas production rates, while often elevated farther along groundwater flow paths where oxygen availability is low, also had patches of high rates in other areas. This can be attributed to the nitrate and carbon distributions that were not correlated to salinity, suggesting a divergence between reaction characteristics and groundwater flow paths. The distributions of salinity and various solutes collectively indicate that on seasonal time scales, hydrology influences, but is not the sole driver of, shifts in the distributions of pore water constituents and reaction characteristics. The divergent migration of reaction characteristics from salinity patterns within the beach system is likely influenced by heterogeneous pools of organic matter with varying forms and degrees of reactivity resulting from hydrologic transience.
4.2 Carbon Pool Distributions and Dynamics
Three main types of carbon are found within the beach aquifer: DOC (Figure 9, seawater [9i] and leached [9iii]), entrapped sedimentary carbon (Figure 9i, 9ii, and 9v), and pore water POC (Figure 9 iv). We show that (1) the three pools are distributed heterogeneously in the beach aquifer and do not correspond to salinity-indicated saltwater circulation flow paths, (2) that carbon is mobile, reactive, and at least partially of marine origin, and (3) the distribution of pore water POC and entrapped sedimentary carbon likely represents deposition during both current and past hydrologic conditions, thus particulate carbon distributions may “lag” those of fully mobile solutes.
The heterogeneous distributions of carbon pools indicate contrasting transport mechanisms between particulates and solutes. Hot spots of pore water POC and DOC did not correspond to inferred flow paths nor did the entrapped carbon content of vibracore samples. These observations might suggest that these hot spots are associated with a geologic deposit, rather than the result of seawater infiltration and circulation through the aquifer. However, we believe that is not the case for two reasons. First, both particulate and dissolved reactive carbon pools are mobile, and second, they are of marine origin. Both conclusions are supported by the sand column deployments, as samples displayed active aerobic respiration (an indicator of labile organic matter, often associated with marine primary production) that increased seaward (Figure 8ii), and green autofluorescence common in dinoflagellates, diatoms, and microalgae (Figure 7). Because the particulate carbon sources do not always coincide with hydrologic flow despite originating with infiltrating seawater and moving through the aquifer, we infer that transport and degradation/consumption occur more slowly than hydrologic shifts (Figure 9, conditions b to a). While marine DOC travels within the beach aquifer synchronous with seawater, larger carbon fragments (>0.7μm) may experience retarded movement once delivered into the sediments. As the delivered POC travels within the aquifer along groundwater flow paths, it may become entrapped and immobilized (Figures 9i, 9ii, and 9v), depending on particle size, grainsize distributions, groundwater flow velocity, and sorption behavior. July 2015 data clearly show this lagged behavior. After the rainstorm, the intertidal circulation cell was constricted and pushed offshore (Figure 3). The pool of elevated pore water POC was found landward of the high-salinity region of active saltwater circulation, in the zone where seawater was likely infiltrating prior to the storm. This shows the divergent movement between fully mobile solutes (i.e., salt) and particulates in beaches. The mobile nature of fine particles, immobilization, and remobilization due to physical or chemical forces in other porous settings have been well described (El-Farhan et al., 2000; Gast et al., 2015; McDowell-Boyer et al., 1986; Pronk et al., 2009).
Comparison between oxygen consumption rates of pore water samples and vibracore sediment samples suggest that these carbon pools, while oxygen limited under current hydrologic conditions, have the potential to react at a later time when conditions become favorable (Figures 5 and 8). At Location F, both pore water and vibracore sediment samples were incubated, but under different oxygen concentrations. Pore water incubations were conducted using in situ oxygen concentrations (<15 μM except for surficial sample), with no additional introduction to oxygen, while sediment incubations were conducted with oxic water. Oxygen consumption rates for pore water samples were highest at the surficial sampling points for Locations D and E, and samples at Location F had respiration rates that were far below the highest rate observed across the aquifer (mean of four depths at F: 3.5 ΔO2 μM/d; highest rate across aquifer: 23 ΔO2 μM/d). However, when sediments from the same location were incubated with oxic water, the oxygen consumption rate reached 77.90 ΔO2 μM/d, displaying the full respiration potential of sedimentary carbon at this location (Figure 8i and Table S1 in the supporting information). While Location F is currently limited in oxygen, given changes in the geometry of the intertidal circulation cell and its subsequent flow path changes, oxygen or other dissolved reactants could become available in time, allowing the sedimentary carbon to respire and contribute to beach reactivity by acting as an electron donor for redox reactions. The presence of these carbon pools and potential for previously deposited carbon to react suggests that intertidal aquifers may be more reactive system than if labile DOC is the sole carbon source, as previously assumed (e.g., Heiss et al., 2017). Shifts in hydrologic conditions, such as rainstorms, tidal variations, and changes to freshwater flux, therefore have the potential to tap into unutilized reservoirs of carbon, promoting shifts in reactions in zones unpredicted by salinity patterns, and creating a “carbon memory” effect within the aquifer.
The time scale of this “carbon memory” effect within the circulation cell will depend on the amount of carbon delivered, its rate of movement, and its degradation/reaction rate. Hot spots of pore water POC did not appear to endure beyond the two months between successive sampling events (Figure 6 and unpublished data). This suggests that the memory effect in this aquifer has a lifespan shorter than 2 months, due to degradation and eventual transport, and is more relevant for spring-neap and shorter time scales rather than seasonal cycles. While changes to the intertidal circulation cell over spring-neap cycles are less pronounced than changes over seasonal time scales (Heiss & Michael, 2014), gradual movements over spring-neap tides and abrupt hydrologic changes (i.e., storms) have the potential to utilize these reactive carbon pools. On longer time scales and/or at shallow depths, the erosion and accretion of sand on beaches may be an important mechanism for POC burial. At Cape Shores, monthly surveys of topographic profiles have shown elevation differences of up to 0.5 m between seasons, and 1 m between consecutive years due to winter storms (e.g., Quartel et al., 2008). During excavation of pore water samplers, especially from the upland dune to the high tide mark, it was common to see decomposing layers of beach wrack that had been buried within the first 1 m of the surface. While these debris layers were within the dry, unsaturated zone and thus did not affect pore water sampling, they indicate that for such large, hardy forms of POC, the “carbon memory” effect may be much longer than mobile POC fragments primarily discussed in this work, even over seasonal morphological changes. However, further work is needed at different time scales to resolve the variable temporal relationship between groundwater flow and reactive carbon transport within the beach.
4.3 Nutrient Release and Leaching Dynamics
Our data provide evidence that the particulate carbon in intertidal aquifers, both immobile and mobile, is an important source of DOC and nutrients, enhancing reactivity and altering land-sea fluxes. Laboratory incubations with sand column samples show that as aerobic respiration progresses, oxygen decreases (Figure 10). The slope of nitrate decrease steepens after oxygen falls below a certain threshold, and ammonium increases. This indicates that as reactive carbon degrades, nutrients are consumed or released, which alters the local chemistry and redox conditions. These solutes and their reaction products may be subsequently transported with the groundwater.
Further, pore water POC and entrapped sedimentary carbon act as local sources of potential reactants (dissolved and particulate) such as DOC, particulate nitrogen, particulate phosphorus, and phosphate via leaching (Figure 11). DOC was higher than surface water concentrations in all sampled months, even up to 300% in July 2015, indicating a source other than seawater (Figures 6 and 11). This suggests that leaching, in addition to the seasonal changes in seawater carbon concentration and input, is another mechanism for seasonal changes in carbon concentrations. Leaf matter has the potential to increase labile DOC concentrations nearly tenfold in nearby waters (Wetzel & Manny, 1972), suggesting that the observed pore water POC leaching in beach pore waters is analogous to processes observed in other environments. The differences in leaching efficiency among seasons, presumably contingent on the quality of infiltrated carbon, could alter the DOC concentrations within the sediments independent of surface water DOC concentrations.
While the molecular quality and reactivity of pore water POC and DOC was not considered in this study, it can be expected based on previous studies that the DOC that leaches from pore water POC of surface water origin is reactive and labile (Seidel et al., 2014; Wetzel & Manny, 1972). In beach aquifer systems, where organic carbon and nutrients are generally scarce, it can be expected that these reactive organic matter pools become important reservoirs of electron donors that fuel biogeochemical reactions outside the existent intertidal circulation cell. Biogeochemical reactions within the aquifer, therefore, are not just a function of reactant transport but rather a complex system governed by hydrologic, geochemical, and biological characteristics that are spatially and temporally transient on different time scales.
5 Conclusion
Intertidal circulation cells of beach aquifers are complex and dynamic in their location, extent, and geometry due to seasonal hydrologic and oceanic conditions. However, the distributions of chemical constituents and reaction characteristics do not always synchronously correspond to hydrologic shifts. While salinity, dissolved oxygen, and oxic respiration rates closely followed shifts in groundwater flow paths, the distributions of other parameters did not. The divergence of nitrogen gas production and ORP conditions from salinity was attributed to the spatially variable distributions of solute reactants and reactive organic carbon. Hydrologic transience and the filtration effect of sediments created scattered pools of organic carbon within the aquifer that did not correspond to salinity patterns. These reactive carbon pools were at least in part delivered by seawater and displayed differential mobility within the sediments, creating reaction dynamics within the aquifer that deviated from salinity-defined hydrologic patterns. POC was shown to actively leach DOC and other nutrients into the beach aquifer system, acting as a local source of reactants and causing further deviation of nutrient distributions from salinity patterns. These carbon pools, though oxygen limited, were readily reactive, indicating their potential to support reactions once conditions become favorable as hydrologic conditions change. These results together emphasize that the evolution and migration of reaction characteristics in coastal aquifers, often highly dependent on carbon, can be caused by, but decoupled from, positional changes of the circulation cell and may not be completely predictable from current hydrologic conditions due to a “carbon memory” effect.
Acknowledgments
Data produced in this study are publicly available at Hydroshare (Kim, 2019). We thank Tobias Ackerman, Kaileigh Calhoun Scott, Beverly Chiu, Samuel Dever, Carlos Duque, Zac Duval, Haley Glos, James Heiss, Kara Hoppes, Katie Li, Eric Lunn, Trevor Metz, Veronique Oldham, Christopher Russoniello, Fang Tan, and Dan Torre and for their assistance in the field. We also thank Matthew Cottrell for his assistance in using the MIMS and Christopher Sommerfield for his assistance in vibracoring. This research was funded by NSF EAR-1246554 (to H. M. and W. J. U.).
