An in Situ Study of Seasonal Dissolved Organic Carbon and Nutrient Fluxes from a Spartina Alternifora Salt Marsh in North Carolina, USA

Derek Detweiler

An in situ study of seasonal dissolved organic carbon and nutrient fluxes from a Spartina alternifora salt marsh in North Carolina, USA

Derek Detweiler University of North Carolina Wilmington Faculty Mentor: Ai Ning Loh University of North Carolina Wilmington

ABSTRACT Salt marshes are among the most productive and biogeochemically active ecosystems on Earth. While they are known sources of dissolved organic carbon (DOC), and organic and inorganic nutrients (including nitrogen, N, and phosphorus, P) to the coastal ocean, it has not been well quantifed experimentally. The purpose of this study was to quantify seasonal DOC and organic and inorganic N and P fuxes from a fringing temperate salt marsh in North Carolina, USA. This experiment was conducted using in situ benthic microcosm chambers in which seawater samples were collected during ebbing tides over 4.5 hours. Water samples were analyzed for DOC and organic and inorganic nutrient concentrations over time, and fuxes from vegetated and non-vegetated marsh sediments were calculated. Results showed that there were no signifcant differences in fuxes between vegetated and non-vegetated sediments within the same season. However, sediments were a minor source of DOC, N, and P in July compared to a signifcant sink in December. These data suggest that the remineralization of organic matter occurs more strongly in the winter with a more active microbial loop. Results also provide insight as to how environmental variability may affect coastal biogeochemical cycles.

alt marshes are an essential transition is of major focus in current climate change Sfrom the terrestrial environment to the research (Osburn et al., 2015). coastal ocean (Bianchi, 2007). Not only do Carbon can be present in the environment they provide a plethora of ecosystem services as dissolved organic carbon (DOC) which, in for humans and wetland organisms, but they addition to organic forms of nutrients such as are an important part of the global carbon dissolved organic nitrogen (DON) and phos- cycle that affects both terrestrial and aquatic phorus (DOP), can be formed by salt marsh environments. Though not fully understood, plants and associated organisms via primary it is believed that wetland ecosystems such as production and respiration. These dissolved salt marshes contribute to the fate and stor- compounds can then be released into the age of terrestrial and atmospheric carbon in coastal ocean with daily changes in tide (Cai the environment (Bauer et al., 2013). The et al., 2000; Hedges, 1992; Winter et al., ability of salt marshes and other shallow, 1996). The dominant plant in North Carolina coastal, vegetated ecosystems such as man- salt marshes is the smooth cordgrass Spartina groves and seagrass beds to sequester carbon alternifora which has the ability to store and has been termed “blue carbon storage” and release large amounts of DOC, DON, and

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DOP (collectively referred to as dissolved or- half of all carbon burial occurs in shal- ganic matter, DOM), and inorganic nutrients low water ecosystems such as salt marshes. - - (as nitrate, NO3 , nitrite, NO2 , ammonium, While increasing atmospheric CO2 levels + 3- NH4 , and orthophosphate, PO4 ). This stor- are expected to increase rates of salt marsh age and release occurs in vegetative aboveg- carbon sequestration and assimilation, rapid round shoots as well as in surrounding sedi- environmental changes due to climate change ments (Turner, 1993; Bianchi, 2007). may lessen the effectiveness of these pro- The release of DOC, N, and P from plants cesses. Under normal rates of relative sea such as S. alternifora and surrounding sedi- level rise (RSLR) and CO2 abundance, salt ments is thought to be an important compo- marshes can respond by accreting more sedi- nent of carbon and nutrient sources to the ment and storing higher concentrations of or- coastal ocean. This DOM can fuel secondary ganic matter within the sediment (Kathilankal production, but there are complex processes et al., 2008). However, at current RSLR that create uncertainty in how this can be ex- rates, marsh accretion cannot occur quickly perimentally quantifed (Bauer et al., 2013; enough, resulting in a fooded marsh with Childerset al., 1993; Dame et al., 1986). A low sequestration capabilities (Kathilankal theoretical exchange diagram is represented et al., 2008; Osburn et al., 2015). Thus, as in Figure 1 which shows this complexity. shown in Kirwan and Mudd’s (2012) climate Aside from biological or physical processes model, the positive feedback that is associ-

(e.g. tidal currents and waves) which may ated with CO2 assimilation in salt marshes alter the composition of DOM in the envi- will eventually diminish. Furthermore, ways ronment, anthropogenic infuences such as to accurately quantify carbon and nutrient wetland destruction, wetland modifcation, dynamics are being researched to enhance nutrient inputs, and climate change are con- the understanding of the capacity at which stantly altering the dynamics of the coastal salt marshes infuence DOM cycling. carbon and nutrient cycles (Childers and Day, One of the most well-known ideas regard- 1990; Koch and Gobler, 2009; Loomis and ing carbon and nutrient export in estuarine Craft, 2010). For instance, Koch and Gobler systems is the outwelling hypothesis. The (2009) showed that in salt marshes that have hypothesis states that estuarine systems and - been ditched for drainage purposes, NO3 associated aquatic infuences such as river- export was greater than that of intact salt ine and tidal exchanges occur too quickly for - marshes which were a sink for NO3 and other signifcant utilization of organic matter by or- nutrients. ganisms to occur. Thus, estuaries simply act It has also been shown that increased as exporters of these compounds, and there is levels of carbon dioxide (CO2) in the atmo- virtually no biogeochemical activity (Odum, sphere, in addition to enhanced nutrient load- 1980 as cited in Hazeldon and Boorman, ing of coastal waters, may ultimately result 1999). This has, however, been supported as in the production of more DOC by salt marsh well as challenged many times since its in- organisms (Bauer et al., 2013; Marsh et al., ception as technological advances and new 2005; Osburn et al., 2015). This refects the techniques have given rise to a more accurate importance of DOC and nutrient fuxes from characterization of salt marsh DOC and nu- wetlands like the S. alternifora salt marshes trient fuxes. found so ubiquitously along the eastern coast For instance, Taylor and Allanson (1995) of the United States. The latest report by the stated that the outwelling hypothesis is Intergovernmental Panel on Climate Change not universal and the heterogeneity of salt (IPCC) also supports the role of wetlands as marshes is so extreme that areas such as a crucial reservoir for CO2 and as a possible high marsh habitats are not accurately con- source or sink for DOC (Ciais et al., 2013). sidered. Different conclusions have also in- According to Kirwan and Mudd (2012), volved study sites that vary geologically,

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Figure 1: Diagram showing the fate of DOM in estuarine environments and the relationship between sediments, the water column, and associated organisms (Adapted from Hansell and Carlson, 2002 as cited in Bianchi, 2007).

95 Explorations |Natural Sciences and Engineering geographically, biologically, chemically, supporting the transfer of DOM from sedi- and physically. Murray and Spencer (1997) ments to the above water column (Burdige, identifed the need to incorporate tidal pro- 2002; Childers and Day, 1990; Tyler et al., cesses into fux calculations and overall 2003). Some have suggested that abundant budgets for tidal wetlands, further under- inorganic nitrogen imported to estuaries is scoring the complexity of quantifying and quickly removed via denitrifcation processes characterizing fuxes of compounds in salt before ebbing tides are able to carry it back marshes. Furthermore, it has been found that to the coastal ocean, and that sedimentary the source-sink dynamics of salt marshes de- processes cause salt marshes to be a sink for pend on a variety of factors including marsh nitrogen (Cai et al., 2000; Dame et al., 1986; maturity, available tide energy, salinity, and Osburn et al., 2015). balance between microbial loop processes Maher and Eyre (2010) provide further (Figure 2; Childers et al., 1993; Dame et al., evidence of sedimentary microbial remin- 1986; Hopkinson et al., 1999; Negrin et al., eralization and have suggested that DOC 2011; Tyler et al., 2003). production is directly correlated with meta- It has been suggested that sediments may bolic bacterial production, while others have be the primary source of DOM (as carbon cited remineralization processes as a driver and nitrogen) where microbial remineral- of fxed nitrogen export (Anderson et al., ization of highly refractory organic matter 1997; Caffrey et al., 2007). The exact mecha- occurs (Burdige, 2002; Koch and Gobler, nism remains unknown, however, as there 2009). Observations have shown that DOM are seasonal variations and uncertainty as in pore waters is more highly concentrated to how microbial communities are affected than water column concentrations, further by the aforementioned complexities of salt

Figure 2: Simple schematic of the microbial loop and associated microorganisms responsible for remineralization of DOM (Adapted from Foreman and Covert, 2003 as cited in Bianchi, 2007).

96 Derek Detweiler marsh heterogeneity and associated physical, dissolved inorganic carbon fux in salt chemical, and geological effects. Seasonal marshes, the use of chamber microcosms fo- comparative studies have shown that salt cusing directly on the in situ release of DOM marshes uptake DOM in the winter while it and nutrient fuxes from S. alternifora salt is exported in the highest concentrations in marshes has not been attempted. the summer (Bouchard, 2007; Hopkinson et In order to constrain the current coastal al., 1999; Osburn et al,. 2015; Yelverton and carbon budget, it is important to quantify the Hackney, 1986). Variations in these fndings amounts of DOM and inorganic nutrients exist, however, as Childers and Day (1990) exported to coastal waters and to determine and Dame et al. (1986) observed an uptake how salt marshes are acting as sources or of DOC in summer with a release of DOC sinks of organic carbon (Bauer et al., 2013). and DON in winter and spring. Besides these In a broader sense, as CO2 levels in the atmo- generalizations, many studies cited in this pa- sphere increase contributing to global climate per exhibit nuances that become easily appar- change, it is essential to know where this car- ent when compared. These nuances include bon is going. differences in DOC and nutrient concentrations, fuxes, study sites, seasons, tides, dura- METHODS tion of study, etc. Thus, the objective of this particular Study Site. project was to characterize in situ DOM (as This study was conducted at the DOC, DON and DOP) and inorganic nutri- University of North Carolina Wilmington’s ent fuxes from a temperate North Carolina Center for Marine Science and the surround- S. alternifora salt marsh during the summer ing Spartina alternifora salt marsh ecosys- and winter seasons. Average daily fuxes tem that borders the Intracoastal Waterway from vegetated and non-vegetated sediments (Figure 3). This particular marsh area is ap- were derived from in situ benthic chambers proximately 1.32 ha and experiences semi- and compared between treatments and sea- diurnal tides in which the marsh is inundated sons. Ratios of C:N:P for fuxes were also with seawater for roughly half of the day. The calculated. We hypothesized that there would fringing experimental marsh is infuenced by be no differences in sediment fuxes between fooding and ebbing tidal processes. There vegetated and non-vegetated salt marsh sedi- were no freshwater infuences for the dura- ments. However, vegetated sediments would tion of the experiment other than surface run- exhibit greater fux magnitudes than non- off. The study was conducted twice over the vegetated sediments. In addition, we also course of a year to account for the variable hypothesized that summer fuxes would be environmental conditions and ecosystem greater than winter fuxes. responses to temperature changes. The frst In relation to the methods that were used was conducted in July 2015 to mimic sum- in this study, similar experimental procedures mer sediment fuxes of DOM and nutrients, attempting to quantify DOC and nutrient while the second experiment was conducted fuxes in the region have included the labo- in December 2015 to mimic winter fuxes. ratory incubation of S. alternifora (Wang et Both feld experiments followed the same al., 2014) and the leeching of S. alternifora procedures. leaves in situ (Turner, 1993). In addition, Howes and Goehringer (1994) and Ketover Field Experiments. (2011) used an in situ chamber microcosm Field methods closely followed those of method in order to characterize DOM and Neikirk (1996) and Ketover (2011). In situ nutrient fuxes from salt marsh sediments and microcosms were placed in the marsh dur- mangrove swamps, respectively. Also, while ing low tide. These microcosms were acrylic Neubauer and Anderson (2003) characterized benthic chambers with a height of 61 cm

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Figure 3: Map of Wilmington, NC and surrounding features where feld experiments were conducted. The black star indicates the location of the study site along the Atlantic Intracoastal Waterway. Image courtesy of Melissa Smith. and an inner diameter of 30 cm (Figure 4). on the bottom of all the sediment chambers Each chamber contained two holes drilled were plugged. In addition, one sealed con- 15 cm from the bottom which allowed them trol chamber flled with ambient seawater to be flled during a fooding tide. Chambers was placed in the marsh upon the start of the were driven into 10-15 cm of sediment a experiment. It should be noted that the data few days prior to the start of the experiment collected from the control chamber was sub- so disturbed sediment could settle within tracted from the data collected in the other the chamber. Three of these chambers were six chambers to account for any microbial placed over a patch of S. alternifora and activity that may have occurred in the water three chambers were placed over bare sedi- column for the duration of the experiment. ment without the infuence of S. alternifora. Water samples from each chamber were Hereafter, the chambers placed over a patch collected after each was gently stirred for ho- of S. alternifora and over bare sediment will mogenous mixing to avoid anoxia. Samples be referred to as vegetated and non-vegetated were collected at the beginning of the experi- treatments, respectively. ment and every 45 minutes thereafter for a

The experiments were conducted in the total of 270 minutes (T0, T45, T90…T270). The morning at the beginning of the ebbing tide. frst four samples were taken in natural light Prior to the start of the experiments, the holes and the second three samples were taken in

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Figure 4: A model of the in situ chambers used in this experiment. The chambers were placed over vegetated and non-vegetated sediments. The control chamber, flled with ambient seawater, has a sealed bottom and does not have holes. Chamber design is modifed from Neikirk (1996). the dark by placing black trash bags over the they were stable throughout the experiment chambers. This allowed for the quantifca- and the chambers did not undergo anoxia. tion of daily changes (presence or absence Surface sediment samples corresponding to of light) in DOM and inorganic nutrient each sediment chamber were also collected fuxes. Each sample was immediately fltered and stored in baked (500 °C for 4 h.) glass through baked (500 °C for 4 h.), 0.7 μm glass- jars and frozen until analyses. To account for fber flters (Whatman GF/F) into 50-mL any leakage from the chambers during the ex- acid-washed (10% hydrochloric acid, HCl), periment, the volume of each chamber was centrifuge tubes and frozen until analyses. noted and used as a correction factor when Temperature and dissolved oxygen con- calculating fux values. centrations were monitored using a YSI Water Quality Field Meter to ensure that

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Sediment Analysis. 1983): Collected sediment samples were analyzed for sedimentary organic C and N [DON] = [TDN] – [DIN] (Eq. 1) (SOC and SON, respectively) content using [DOP] = [TDP] – [DIP] (Eq. 2) a ThermoQuest NC 2100 sediment analyzer (UNESCO, 1994). To prepare samples for Flux Calculations. SOC and SON analyses, thawed samples Based on DOC and nutrient concentra- were dried to remove water, ground to a very tions obtained over time, DOC and nutrient fne particle size, and removed of large pieces (organic and inorganic) fuxes were obtained of debris. Two subsamples of each were by creating a linear regression of the con- acidifed with 10% TraceMetal Grade HCl to centrations measured over time. The slope remove any carbonates. After again drying, of each regression was used to calculate fux each sample was weighed and packed into tin by taking into consideration the volume and capsules for analysis. areas of the chambers. If the chambers expe- Samples were analyzed in duplicates in rienced any sort of leakage during the experi- which C and N amounts in each sample were ment and therefore a change in volume, it was given as a function of grams of dry sediment factored into the equation when determining weight (gdw). Molar ratios of C:N were also fnal mean daily fuxes to ensure all data was calculated for each sample. consistent and normalized. The equation be- low represents the fux calculation: Nutrient Analysis. (Eq. 3) Each water sample collected from the feld was analyzed for DOC, dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), dissolved inorganic phospho- where J = fux, dC/dt = slope of linear regres- rus (DIP), and dissolved organic phosphorus sion, V = volume of chamber, and A = area (DOP). of chamber. Concentrations of DOC were analyzed All concentration measurements were using a Shimadzu TOC-V high temperature obtained in μmol L-1 while daily fux calcu- combustion instrument following a modifca- lations were expressed as μmol m-2 d-1 after tion of the Benner and Strom (1993) method factoring in the duration of the experiment (Loh and Bauer, 2000). Total dissolved nitro- compared to a full 24-h. day. Fluxes from the gen (TDN) and total dissolved phosphorus control chamber were then subtracted from (TDP) concentrations were determined us- the sediment chamber fuxes to correct for ing a modifcation of the persulfate diges- any microbial activity in the water column tion method from Koroleff (1983). Samples that was not being considered for this experi- - were digested to nitrate (NO3 ) or phosphate ment. Lastly, C:N:P fux ratios for DOM and 3- (PO4 ), respectively, and analyzed using a inorganic nutrients were calculated. Bran+Luebbe AutoAnalyzer 3 (Loh, 2005). + Concentrations of DIN as NH4 and Statistical Analysis. NOx (the combined concentrations of ni- Multivariate Analysis of Variance - - 3- trite, NO2 and nitrate, NO3 ) and DIP (PO4 ) (MANOVA) assuming unequal variance and were also determined using a Bran+Luebbe non-normality according to Levene’s Test of AutoAnalyzer 3 without the persulfate oxida- Equality of Error Variances among the data- tion step. Subsequent concentrations of DON sets was conducted using the SPSS statisti- and DOP were calculated by subtracting the cal software to simultaneously observe any respective DIN and DIP concentrations from differences between vegetated and non-veg- the measured TDN and TDP concentrations etated treatments and between summer and using the following equations (Koroleff, winter fuxes. Differences were determined

100 Derek Detweiler to be signifcant if p < 0.05 after conducting Sediment Data. Dunnett T3 post-hoc tests for non-equal vari- Sediment samples were analyzed for ances, and standard errors were calculated sedimentary organic carbon (SOC) and sedi- from the slope of each regression. Results mentary organic nitrogen (SON) content and from these analyses can be categorized into ratios of C:N calculated (Table 1). Vegetated four components: July vegetated sediments, sediments had SOC content ranging from July non-vegetated sediments, December 15.77 - 20.23 mg C/gdw, while SON con- vegetated sediments, and December non- tent ranged from 0.5906 - 0.9457 mg N/gdw. vegetated sediments. Ratios of C:N of vegetated sedimentary organic matter ranged from 21.82:1 to 27.03:1. RESULTS Non-vegetated sediments had SOC content ranging from 13.86 - 16.12 mg C/gdw, while Hydrographic Data. SON content ranged from 0.5393 - 0.6420 The summer experiment was conducted mg N/gdw. There were no signifcant differ- on July 17, 2015. In the feld, air temperature ences in SOC and SON content between sea- was 31.1 °C, water temperature 28.7 °C, and sons or between treatments within the same salinity 35.97. The winter experiment was season. Ratios of C:N of vegetated sedimen- conducted on December 9, 2015. Air tem- tary organic matter ranged from 26.72:1 to perature was 15.6 °C, water temperature 15.0 29.00:1. There were signifcant differences °C, and salinity 33.0. Air and water tempera- (p < 0.01) in C:N ratios between July and ture were typical for each season in which December vegetated sediments but not be- the experiment was conducted, and salinity tween non-vegetated sediments or between remained stable at typical values for coastal different treatments within the same season. waters. Additionally, dissolved oxygen (DO) content of the water in the chambers was DOC and Nutrient Fluxes. monitored to ensure that the sampling col- Mean daily fuxes expressed in μmol umn did not undergo anoxia. In both seasons, m-2 d-1 for DOC and nutrients are shown in DO (mg/L) and DO (%) were normal and Figures 5-7. Positive fux values indicate that remained oxic throughout the experiment. sediments are a source of DOM or nutrients The July experiment exhibited mean val- while negative values indicate that sediments ues of 4.82 ± 0.105 mg/L and 77.5 ± 1.91% are a sink for DOM or nutrients. Data from while the December experiment exhibited one vegetated chamber in July was not fac- mean values of 6.33 ± 0.102 mg/L and 72.8 tored into fux results due to major chamber ± 1.00%. leakage and inadequate sample retrieval during the feld experiment.

Table 1: Mean C:N ratios and sedimentary organic carbon and nitrogen (SOC, SON) content are expressed as mg/gdw (with standard error) for each season and treatment. (*) indicates a signifcance level of p < 0.01.

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Figure 5: Treatments (vegetated and non-vegetated sediments) are plotted as a function of mean daily fux (μmol m-2 d-1) of DOC from three replicates of each treatment. Positive values indicate a DOC source while negative values indicate a DOC sink for a) summer and b) winter experiments. Standard error (SE) is not plotted as error bars due to variability in feld data. Asterisks represent signifcance between seasons (p < 0.05).

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Figure 6: Nutrients are plotted as a function of mean daily fux (μmol m-2 d-1) from three replicates of vegetated sediment chambers. Positive values indicate a nutrient source while negative values indicate a nutrient sink for a) summer and b) winter experiments. Bars represent one standard error (+1SE).

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Figure 7: Nutrients are plotted as a function of mean daily fux μmol m-2 d-1) from three replicates of non-vegetated sediment chambers. Positive values indicate a nutrient source while negative values indicate a nutrient sink for a) summer and b) winter experiments. Bars represent one standard error (+1SE), and letters represent signifcance between seasons (p < 0.05).

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For DOC fuxes, there were no signif- magnitude of fuxes were greater than in July. cant differences in fuxes between treat- When inorganic and organic sediment ments (vegetated vs. non-vegetated) within fux ratios were calculated for C:N, C:P, and the same season (Figure 5). However, there N:P ratios, there were no signifcant differ- was a signifcant difference (p < 0.05) in non- ences found between seasons and treatments. vegetated DOC fuxes between the July and The highest values were found in DOC:DOP December experiments. Vegetated sediments ratios, followed by DOC:DON, DON:DOP, were a minor sink of DOC and non-vegetated and DIN:DIP (Table 2). sediments were a very minor source of DOC in July while in December both vegetated DISCUSSION and non-vegetated sediments were a sink for DOC (Figure 5). Additionally, fuxes oc- The purpose of this experiment was to curred with greater magnitude in December characterize in situ DOC and nutrient fuxes compared to July (>-400 μmol m-2 d-1 vs. from vegetated and non-vegetated sediments <100 μmol m-2 d-1). in a Spartina alternifora salt marsh under two For nutrient fuxes, a similar pattern was distinct seasonal conditions and to determine revealed in that there were no signifcant any differences in sediment fuxes and mag- differences between treatments (vegetated nitude between treatments (vegetated versus vs. non-vegetated) within the same season non-vegetated sediments) and between sea- (Figures 6-7). In addition, there were also no sons (July versus December). It was initially signifcant differences in organic or inorganic hypothesized that there would be no differ- nutrient fuxes between seasons in vegetated ences in sediment fuxes between vegetated sediments (Figure 6). However, in non-veg- and non-vegetated salt marsh sediments and etated sediments, signifcant differences (p between summer and winter seasons.

< 0.05) were found for NOx and DIN fuxes The results from this study support the ex- between seasons (Figure 7). pectation that there would be no differences In vegetated sediments in July, there in DOC and nutrient fuxes between treat- + were positive fuxes of NOx, NH4 , and DIP ments (i.e., between sediments that were veg- (Figure 6) suggesting that the vegetated salt etated with S. alternifora and those which marsh sediments were a source of these nutri- were bare sediment without any vegetation) ents. Vegetated sediments were also a sink for within the same season (Figures 5-7). The DON and DOP in July while in December, insignifcance of fuxes between treatments sediments were a sink for all measured com- may indicate that most of the biogeochemi- ponents. July exhibited a greater magnitude cal cycling that occurs in salt marshes does of nutrient fuxes than in December. In non- so in the sediments which is consistent with vegetated sediments in July, sediments were previous conclusions (Anderson et al., 1997; a source of all nutrients including DON Burdige, 2002; Caffrey et al. 2007; Cai eet and DOP (Figure 7). In December, sedi- al., 2000; Tyler et al. 2003). It also appears ments were a sink for all components with that much of the remineralization could be

Table 2: Mean fux ratios for each season and treatment. There were no signifcant differences between fux ratios.

105 Explorations |Natural Sciences and Engineering occurring in pore waters which may host a common idea is that salt marshes actively community of microbes that actively use “flter” or control nutrient concentrations DOM for metabolic processes (Cai et al., especially when considering anthropogenic 2000). Additionally, this experiment did not inputs and stability of the coastal ocean. At analyze for grain size which could also im- a global scale, these results may also support pact pore water dynamics and therefore mi- the idea of CO2 sequestration in salt marshes crobial loop processes and fux rate in or out as is consistent with additional insights from of the sediment. Anderson et al. (1997), Caffrey et al. (2007), Conversely, while there were no differ- Duarte et al. (2005), Kirwan and Mudd ences present between seasons within veg- (2012), and Kathilankal et al. (2008). etated sediments, fuxes greatly differed be- Initially, the observed small P fux values tween July and December in non-vegetated may also suggest that the biogeochemical po- sediments (Figure 7). It can be generalized tential of salt marsh ecosystems are limited that in July, salt marshes are a source for by P availability (Figures 6-7). However, the DOC, N, and P in non-vegetated sediments, high negative fux of DON indicates a strong while the vegetated sediments are a sink for utilization by the microbial community even DOC, N, and P compared with December in July vegetated sediments when most other where the entire marsh is a sink of DOC, N, components exhibited positive fuxes (Figure and P. These results suggest that in December, 6). Under further investigation, ratios of N:P microbial activity in the sediment is intensi- (Table 2) are relatively low, and even though fed, and DOM is being actively remineral- N is utilized most often, it may also be the ized (Bianchi, 2007; Burdige, 2002; Cai et limiting nutrient due to the utilization of in- al., 2000; Dame et al., 1986; Hopkinson et organic N in addition to organic N. In a simi- al., 1999). These processes are dominated by lar system used in Anderson et al.’s (1997) N remineralization as seen in the signifcant study, results are consistent except in the case differences in NOx and DIN fuxes between of high N loading circumstances. July and December non-vegetated sediments. As to issues regarding the experimental In addition to the disparity in sinks and procedure, there were few instances where sources for DOM and nutrients, the magni- the benthic chambers in the feld leaked. This tude of fuxes also differed. In addition to was due to natural seepage from underneath December being a sink for DOC and nutri- the sediments. However, this volume change ents, there was a larger magnitude of fuxes was corrected when fuxes were calculated. during this experiment. It is apparent that salt Additionally, every attempt was made to en- marsh sediments are capable of taking up sure that experimental patches were removed large amounts of DOM in the winter, an ob- of all visible organisms such as snails, oys- servation that is important when considering ters, and algae. However, it is possible that blue carbon research. If salt marsh sediments unseen epifaunal or infaunal organisms were are able to store large amounts of DOM in not removed which could also have some in- such a small area, it raises the question of fuence on the cycling of DOM. In addition, how impactful large salt marsh areas may be this study was conducted at a relatively small in biogeochemical activity and therefore how site where anthropogenic disturbances such infuential it is to the coastal ocean which is as foot traffc and experimental equipment so vital to ecosystem health. It seems that are evident. This could potentially infuence nutrients can be effectively contained in salt the results in that pristine and vast areas of marshes and perhaps facilitate greater DOC salt marshes may produce different results. production as supported by studies within However, it can be argued that the disrupted Bauer et al. (2013), Cai et al., (2000), Koch and fragmented nature of the salt marsh is and Gobler (2009), Loomis and Craft (2010), representative of salt marshes currently un- Osburn et al., (2015) and Turner (1993). One dergoing anthropogenic stresses such as

106 Derek Detweiler encroachment. Additionally, Hopkinson and source of DOC and nutrients to the coastal Vallino (1995) and van Heemst et al. (2000) ocean has many implications. First, it adds to suggest that human-derived particulate and the understanding of how salt marshes oper- dissolved carbon can impact the lability of ate in addition to the complex microbial pro- organic matter in estuaries. That is, reminer- cesses that occur within sediments. alization processes may involve the metabo- Future research in this area could focus lism of nutritionally different carbon sources much more on the microbial community and the ability of microbial communities to that exists in the salt marsh sediments, par- utilize those compounds. ticularly in pore waters, and the community Ultimately, the results of this experiment composition that is responsible for reminer- are a good indicator that salt marsh biogeo- alizing nutrients in the system. In relation, chemistry is much more complex than what the heterogeneity of salt marsh ecosystems was frst described by Odum in 1980 and may result in vegetation types that differ further supports studies citing microbial in- with changes in elevation or distance from fuences on salt marsh cycling (Anderson et the shoreline. Of course, this experiment was al., 1997; Burdige, 2002; Caffrey et al. 2007; limited by time and by space. Higher marsh Cai et al., 2000; Maher and Eyre, 2010; Tyler habitats may exhibit different fux proper- et al., 2003). The readily produced and avail- ties as well as those marshes with freshwa- able DOC in salt marshes may fuel the mi- ter infuences. So, while the general theory crobial activity and consequent release of of biogeochemical reactivity and source-sink remineralized DOM in salt marsh sediments dynamics is supported, it would be diffcult and provide a big picture approach to salt to use these fuxes to generate any sort of marsh biogeochemistry and how fuxes of budget for salt marsh export. DOC and nutrients can vary seasonally. The In the future, a more comprehensive study biogeochemical activity of these systems was should consider more of the microbial pro- shown to be complex and driven by micro- cesses at work in the sediment to better un- bial activity in the sediment further support- derstand the remineralization and microbial ing the claim that salt marshes are among the loop processes that operate to essentially most biogeochemically active ecosystems on suck up nutrients in the winter. Additional Earth. efforts should also be made to understand the effects of these processes on the coastal CONCLUSION ocean. How might the biogeochemical activity of salt marshes affect water quality or This research endeavor has resulted in other parameters in tidally infuenced estu- many unanswered questions about sediment aries and coasts? This information could be fuxes in Spartina alternifora salt marshes. vital to understanding the importance of salt The information that was gained from this marshes to coastal systems as valuable bio- experiment in support of salt marshes act- geochemical reactors. ing as biogeochemical reactors as well as a

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REFERENCES

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