Elevated Particulate Organic Carbon Research Articles

Changes in soil organic carbon stocks and its physical fractions along an elevation in a subtropical mountain forest

Soil microorganisms are the drivers of soil organic carbon (SOC) mineralization, and the activities of these microorganisms are considered to play a key role in SOC dynamics. However, studies of the relationship between soil microbial carbon metabolism and SOC stocks are rare, especially in different physical fractions (e.g., particulate organic carbon (POC) fraction and mineral-associated organic carbon (MAOC) fraction). In this study, we investigated the changing patterns of SOC stocks, POC stocks, MAOC stocks and microbial carbon metabolism (e.g., microbial growth, carbon use efficiency and biomass turnover time) at 0–20 cm along an elevational gradient in a subtropical mountain forest ecosystem. Our results showed that SOC and POC stocks increased but MAOC stocks remained stable along the elevational gradient. Soil microbial growth increased while microbial turnover time decreased with elevation. Using structural equation modeling, we found that heightened microbial growth is associated with elevated POC stocks. Moreover, MAOC stocks positively correlate with microbial growth but show negative associations with both POC stocks and soil pH. Overall, the increase in SOC stocks along the elevational gradient is primarily driven by changes in POC stocks rather than MAOC stocks. These findings underscore the importance of considering diverse soil carbon fractions and microbial activities in predicting SOC responses to elevation, offering insights into potential climate change feedbacks.

REE + Y uptake in bioapatite revisited: Facies-controlled variability in coeval conodonts

REE + Y concentrations in bioapatite have been repeatedly shown to be ambiguous carriers of information for paleo-seawater reconstructions. The uptake of REE + Y begins rapidly after deposition, and bioapatite can, therefore, potentially record the pore water conditions during early diagenesis. In this study, we examined the REE + Y composition of conodonts belonging to a single species from three coeval facies across the Ordovician Baltic Paleobasin carbonate shelf. We show that the REE + Y concentrations, as well as Ce/Ce*, bell-shape index (BSI), Y/Ho and LaN/SmN ratios, vary systematically in conodonts from facies with different host rock compositions. Dissimilarly, REE + Y patterns and ratios of conodont dental elements from the same facies are uniform, indicating that the local depositional environment imposes principal control on the REE + Y uptake by bioapatite. Furthermore, our results show that even though REE + Y concentrations in different histological tissues of the same conodont vary significantly, no REE-specific fractionation occurs, resulting in similar REE + Y ratios in all tissue types. Our results illustrate that conodonts can record modified seawater-like characteristics of REE compositions if the input of FeMn oxyhydroxides and particulate organic carbon (POC) is low. In contrast, under elevated POC and FeMn oxyhydroxide loading, the conodonts show increasingly positive Ce anomalies and MREE enrichment. Collectively, these results suggest that, although primary seawater signatures are not captured in bioapatite, the conodont REE + Y patterns are controlled by local depositional environments and can record early diagenetic pore water conditions.

Lagrangian Observation of 234Th and Its Application in Constraining the Sinking of Particulate Organic Carbon on the Slope of the Northeastern South China Sea

The 234Th–238U disequilibrium has been widely used to quantify the sinking flux of particulate organic carbon (POC) out of the upper ocean. However, the influence of the advection on the quantification is poorly understood due to the lack of in situ measured physical parameters. Here, a Lagrangian observation was deployed for 39 h to track the variability of 234Th along with the current on the slope of the northeastern South China Sea (SCS). Contrasting to the general ocean interior, 234Th showed deficits relative to 238U in the mesopelagic waters, indicating an enhanced removal of 234Th. Concurrently, elevated total particulate matter (TPM) and POC contents were observed in the mesopelagic waters, supporting the driving force of the cross-shelf dispersion of re-suspended shelf/slope sediments for the 234Th removal. The widely used 234Th-model (ignoring physical processes) produced a much lower sinking flux of POC than the sediment trap-derived POC flux during the same observation, indicating an unneglectable influence of advection and diffusion. By considering the main horizontal advection and vertical diffusion, the 234Th–238U method gave rise to comparable results to sediment trap. 234Th-derived POC fluxes showed an increased pattern from 300 to 1,000 m, consistent with the more abundant POC where. These results indicated that advection represents an unneglectable process during the quantification of the sinking flux of 234Th over the slope of the SCS.

Seasonal and interannual variability of the Queen Maud Gulf ecosystem derived from sediment trap measurements

AbstractSeveral years of monitoring in the northern Canadian Arctic Archipelago contrast with a lack of repeated measurements in Queen Maud Gulf. As sea ice cover declines and maritime traffic increases along the Northwest Passage, Queen Maud Gulf, including the protected national historic sites of the HMS Erebus and HMS Terror wrecks, may experience drastic environmental changes. As part of the Kitikmeot Marine Ecosystems Study, moored oceanographic devices including a sediment trap were deployed in Queen Maud Gulf over three consecutive annual cycles from October 2015 to August 2018 to study the functioning of this geographic bottleneck along the Northwest Passage. Seasonal and interannual variability in total particulate matter (TPM), particulate organic carbon (POC), and microalgal fluxes were investigated in relation to wind speed, snow depth, sea ice cover, water temperature, and current velocity. Zooplankton and meroplankton present in the sediment trap were enumerated to monitor their composition and seasonal development. Landfast ice breakup consistently occurred in early June, followed by sea ice breakup in July. The release of sea ice algae was initiated in March 2016, May 2017, and June 2018, whereas peaks in diatom fluxes occurred in August or September. Sustained abundance of pelagic copepods and meroplankton suggested an ecosystem sufficiently productive to support the year‐round development of the pelagic and benthic communities. Elevated autumn TPM and POC fluxes reflected wind‐induced mixing and resuspension in the absence of ice cover. Our results provide a baseline for evaluating the impact of environmental changes on the Queen Maud Gulf marine ecosystem.

Variability of particulate bioavailable phosphorus, particulate organic carbon and nitrogen in agricultural and urban rivers

Suspended sediments (SS) play a key role as carriers of particulate nutrients which support primary productivity in aquatic systems. In this study, we assessed the relationship between particulate bioavailable phosphorus (PBAP), particulate organic carbon (POC) and particulate organic nitrogen (PON) in rivers flowing through contrasting land use at high and low flow. PBAP as a percentage of particulate phosphorus varied from 5 to 54%, and at low flow, there was a significant increase in PBAP fraction in the urban river as compared to the agricultural river. In addition, elevated POC and PON concentrations in both rivers were associated with low flow. POC and PON concentrations in both rivers declined at high flow presumably due to the mixing effects of water from different sources in the watershed which could have altered the particulate organic matter composition. A significant correlation existed between POC and SS in both rivers, and POC to PON ratios of SS from the rivers point to the change in the nature of particulate organic matter at varying flow conditions. PBAP exhibited a positive relationship with POC and PON concentration in both rivers possibly an indication that particulate organic matter contributes to bioavailable phosphorus in the rivers.

Elevated carbon flux in deep waters of the South China Sea

We measured particulate organic carbon (POC) fluxes from the euphotic zone into the twilight zone and deep waters (>1000 m) that occurred between the shelf and the basin in the South China Sea (SCS) and at the SouthEast Asia Time Series Station (SEATS) using floating sediment trap arrays. Additionally, selected sinking particles were imaged by scanning electron microscope (SEM) to reveal particle morphology and composition. Results showed large variations in POC fluxes with elevated values (32–104 mg-C m−2 d−1) below the euphotic zone and a trend towards lower values in the deep SCS. Vertical POC fluxes measured in deep waters between the shelf and the SCS basin were much higher than those estimated by Martin’s attenuation equation. These elevated POC fluxes in deep waters were attributed to lateral particle transport as opposed to enhanced settling out of the euphotic zone. SEM images of sinking particles at 150 m show abundant marine biogenic detritus, while those in deep waters contained a higher proportion of lithogenic material. A great deal of the spatial variability in POC fluxes across the twilight zone and deep waters of the SCS cannot be represented by current biogeochemical models.

Elevated particulate organic carbon export flux induced by internal waves in the oligotrophic northern South China Sea

To understand the biogeochemical response to internal waves in the deep basin of the northern South China Sea (NSCS), particulate organic carbon (POC) export fluxes were quantified for the first time during the passage of large internal waves using drifting sediment traps attached with hydrographic sensors. Results revealed large variations in temperature, nitrate and chlorophyll a (Chl a) concentrations during and after internal waves, suggesting that cold nutrient-replete waters may be brought to the euphotic zone in the dissipation zone during and after the passage of internal wave packets, resulted in phytoplankton flourished. Most importantly, POC export fluxes (110.9 ± 10.7 mg C m−2 d−1) were significantly enhanced after internal waves compared to non-internal wave area (32.6–73.0 mg C m−2 d−1) in the NSCS. Such elevated POC fluxes may be induced by downward flourished biogenic particles, particle aggregation or converged particles from mixed layer triggered by internal waves.

Enhanced POC export in the oligotrophic northwest Pacific Ocean after extreme weather events

AbstractTo study effects of extreme weather events (EWEs, e.g., dust storm and typhoon) on the export of particulate organic carbon (POC) measured by a floating sediment trap in the oligotrophic ocean, eight sea‐going expeditions were conducted in the oligotrophic northwest Pacific (NWP) in 2007 and 2008, covering all four seasons and the passage of several EWEs. Results of year‐round field observations demonstrate that the POC export fluxes in the oligotrophic NWP did not exhibit apparent seasonal variations yielding an average flux of 36.9 ± 5.8 mg‐C m−2 d−1 without EWE effects. With EWE effects, however, the POC export flux (51.7 ± 13.2 mg‐C m−2 d−1) showed an approximately 40% increase compared to the average flux measured without EWE effects. These results suggest that EWEs can trigger elevated POC export from the euphotic zone in the oligotrophic ocean.

Fluxes of particulate organic carbon in the East China Sea in summer

Abstract. To understand carbon cycling in marginal seas better, particulate organic carbon (POC) concentrations, POC fluxes and primary production (PP) were measured in the East China Sea (ECS) in summer 2007. Higher concentrations of POC were observed in the inner shelf, and lower POC values were found in the outer shelf. Similar to POC concentrations, elevated uncorrected POC fluxes (720–7300 mg C m−2 d−1) were found in the inner shelf, and lower POC fluxes (80–150 mg C m−2 d−1) were in the outer shelf, respectively. PP values (~ 340–3380 mg C m−2 d−1) had analogous distribution patterns to POC fluxes, while some of PP values were significantly lower than POC fluxes, suggesting that contributions of resuspended particles to POC fluxes need to be appropriately corrected. A vertical mixing model was used to correct effects of bottom sediment resuspension, and the lowest and highest corrected POC fluxes were in the outer shelf (58 ± 33 mg C m−2 d−1) and the inner shelf (785 ± 438 mg C m−2 d−1), respectively. The corrected POC fluxes (486 to 785 mg C m−2 d−1) in the inner shelf could be the minimum value because we could not exactly distinguish the effect of POC flux from Changjiang influence with turbid waters. The results suggest that 27–93% of the POC flux in the ECS might be from the contribution of resuspension of bottom sediments rather than from the actual biogenic carbon sinking flux. While the vertical mixing model is not a perfect model to solve sediment resuspension because it ignores biological degradation of sinking particles, Changjiang plume (or terrestrial) inputs and lateral transport, it makes significant progress in both correcting the resuspension problem and in assessing a reasonable quantitative estimate of POC flux in a marginal sea.

Biogeochemical Responses in the Southern East China Sea After Typhoons

To understand the biogeochemical impact of typhoons in marginal seas, 16 sea-going expeditions were conducted from 2007 to 2009, covering all four seasons and including periods following the passage of several typhoons in the southern least China (SECS). Higher surface nitrate and chlorophylil a (Chl a) concentrations were measured in winter and spring but surface nitrate (<0.1 mu M) and Chl a (0.47 +/- 0.17 mg m(-2)) concentrations were loss in summer under nontyphoon conditions. In comparison, elevated surface nitrate (0.2-2.3 mu M) and Chl a concentration (1.11 +/- 0.40 mg m(-2)) were recorded in the SECS several days after the passage of each of three typhoons in 2008. The results demonstrate that nutrient-rich water are brought to the surface after the passage of typhoons, after which phytoplankton flourish. Most importantly, elevated particulate organic carbon (POC) fluxes (552 +/- 28 mg cm(2) d(-2)) were observed after Typhoon Jangal, about a threefold increase from the monthly mean value (184 +/- 37 mg m(2) d(2)). These field investigations demonstrate that typhoons can have a profound effect on nutrient supply, phytoplankton growth, and POC: fluxes to marginal seas.

Downward particulate organic carbon export at high temporal resolution in the eastern Fram Strait: influence of Atlantic Water on flux composition

A sediment trap was deployed at 340 m from April to July 2003 to monitor the down- ward export of particulate organic carbon (POC) at high temporal resolution in the marginal ice zone of the eastern Fram Strait. Although POC fluxes remained <15 mg m �2 d �1 , variations in the magnitude and composition of the exported POC were observed during deployment. A first period of elevated POC export associated with an increase in diatom fluxes and low zooplankton fecal pellet fluxes was recorded at the beginning of May, suggesting a mismatch between phytoplank- ton production and zooplankton grazing. A second period of elevated POC export composed of coccolithophores, diatom resting spores and empty diatom frustules was observed in June. This transition in the composition of the export fluxes reflected a shift in water masses caused by the onset of an ice-edge eddy bringing warm Atlantic Water into the region at the beginning of June. The cyclonic eddy also contributed to the rapid export of Phaeocystis pouchetii, a microalga that does not significantly contribute to carbon export in stratified waters. The main contributors to the zooplankton fecal pellet flux also varied according to the prevailing water mass, with copepod fecal pellets dominating throughout the deployment, except at the beginning of June, when the fecal pellet flux in Atlantic Water was dominated by appendicularian fecal pellets. These results indicate that a prevalence of Atlantic Water may have a large impact on the magnitude and com- position of POC export in the eastern Fram Strait.

Downward particulate organic carbon export and jellyfish blooms in southeastern Hudson Bay

A long-term sediment trap was deployed at 100 m (station AN03; 55°24′N; 77°55′W; water depth = 136 m) from October 2005 to August 2007 to monitor downward carbon export in southeastern Hudson Bay in the Canadian subarctic region. Elevated particulate organic carbon (POC) and total fluxes during fall 2005 and 2006 were the result of wind-induced resuspension of settled POC. The absence of a significant increase in POC fluxes in summer 2006 in contrast to summer 2007 suggests variation in the processes regulating carbon export in southeastern Hudson Bay. Large quantities of the holoplanktonic jellyfish Aglantha digitale were collected in the sediment trap during fall 2006 and summer 2007. The large collection of A. digitale during fall 2006 corresponds to the seasonal descent of the population, while the large abundance during summer 2007 remains unexplained. The increased POC fluxes in summer 2007 and the large occurrence of A. digitale in 2006 and 2007 may be early signs of environmental changes in southeastern Hudson Bay, however more sampling is needed to capture the variability of the ecosystem.

Particulate organic carbon budget in the open Algero-Balearic Basin (Western Mediterranean): Assessment from a one-year sediment trap experiment

With the aim of improving the knowledge of the open ocean carbon cycle, we present a budget of particulate organic carbon (POC) fluxes carried out in the deep central part of the Algero-Balearic Basin (ABB) at 2850 m water depth based on a single mooring equipped with five automated sediment traps deployed from April 2001 to May 2002 at depths of 250, 845, 1440, 2145 and 2820 m. Suspended particulate matter (SPM) and superficial sediments were also used as indicators of hydrodynamics and carbon burial, respectively. The data reveal that the fraction of primary production buried in the sediment, which finally leads to the sequestration of carbon dioxide from the atmosphere, is 0.16%, lower than the values found in the nearby continental margin regions such as the Alboran Sea (0.48–0.89%) but of the same order as recorded at other Mediterranean sites at similar depths, such as the Ionian Sea (0.11%). As they sink through the water column, the particles exhibit decreases in flux that are similar to those observed elsewhere, but also show variations that appear to correlate with hydrological features of the water masses present in the basin, as revealed by SPM concentrations and compositions. The input of the tyrrhenian deep water (TDW) into the ABB at 800–1500 m of water depth exhibits low suspended POC concentrations and low sinking POC fluxes were also observed in this depth range. Gulf of Lions water mass formation appears to also contribute to elevated suspended POC concentrations and perhaps POC accumulation in the traps and sediments by spreading of dense cold water along the whole ABB that supplied POC at depths higher than 2000 m.

The effect of Hurricane Lili on the distribution of organic matter along the inner Louisiana shelf (Gulf of Mexico, USA)

On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus. Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m 2/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m 2/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m 2 and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.

Seasonal and spatial distribution of particulate organic matter (POM) in the Chukchi and Beaufort Seas

As part of the Western Arctic Shelf–Basin Interactions (SBI) project, the production and fate of organic carbon and nitrogen from the Chukchi and Beaufort Sea shelves were investigated during spring (5 May–15 June) and summer (15 July–25 August) cruises in 2002. Seasonal observations of suspended particulate organic carbon (POC) and nitrogen (PON) and large-particle (453mm) size class suggest that there was a large accumulation of carbon (C) and nitrogen (N) between spring and summer in the surface mixed layer due to high phytoplankton productivity. Considerable organic matter appeared to be transported from the shelf into the Arctic Ocean basin in an elevated POC and PON layer at the top of the upper halocline. Seasonal changes in the molar carbon:nitrogen (C:N) ratio of the suspended particulate organic matter (POM) pool reflect a change in the quality of the organic material that was present and presumably being exported to the sediment and to Arctic Ocean waters adjacent to the Chukchi and Beaufort Sea shelves. In spring, low particulate C:N ratios (o6; i.e., N rich) were observed in nitrate-replete surface waters. By the summer, localized high particulate C:N ratios (49; i.e., N-poor) were observed in nitrate-depleted surface waters. Low POC and inorganic nutrient concentrations observed in the surface layer suggest that rates of primary, new and export production are low in the Canada Basin region of the Arctic Ocean. r 2005 Elsevier Ltd. All rights reserved.

Estimation of particulate organic carbon in the ocean from satellite remote sensing

Measurements from the Southern Ocean show that particulate organic carbon (POC) concentration is well correlated with the optical backscattering by particles suspended in seawater. This relation, in conjunction with retrieval of the backscattering coefficient from remote-sensing reflectance, provides an algorithm for estimating surface POC from satellite data of ocean color. Satellite imagery from SeaWiFS reveals the seasonal progression of POC, with a zonal band of elevated POC concentrations in December coinciding with the Antarctic Polar Front Zone. At that time, the POC pool within the top 100 meters of the entire Southern Ocean south of 40 degrees S exceeded 0.8 gigatons.

Pelagic and benthic ecology of the lower interface of the Eastern Tropical Pacific oxygen minimum zone

The distributions of pelagic and benthic fauna were examined in relation to the lower boundary of the oxygen minimum zone (OMZ) on and near Volcano 7, a seamount that penetrates this feature in the Eastern Tropical Pacific. Although the broad, pronounced OMZ in this region is an effective barrier for most zooplankton, zooplankton abundances, zooplankton feeding rates, and ambient suspended particulate organic carbon (POC) peaked sharply in the lower OMZ (about 740–800 m), in association with the minimum oxygen concentration and the increasing oxygen levels just below it. Zooplankton in the lower OMZ were also larger in size, and the pelagic community included some very abundant, possibly opportunistic, species. Elevated POC and scatter in the light transmission data suggested the existence of a thin, particle-rich, and carbon-rich pelagic layer at the base of the OMZ. Gut contents of planktonic detrifvores implied opportunistic ingestion of bacterial aggregates, possibly from this layer. Benthic megafaunal abundances on the seamount, which extended up to 730 m, peaked at about 800 m. There was a consistent vertical progression in the depth of first occurrence of different megafaunal taxa in this depth range, similar to intertidal zonation. Although the vertical gradients of temperature, salinity, and oxygen were gradual at the lower OMZ interface (in contrast to the upper OMZ interface at the thermocline), temporal variability in oxygen levels due to internal wave-induced vertical excursions of the OMZ may produce the distinct zonation in the benthic fauna. The characteristics of the lower OMZ interface result from biological interactions with the chemical and organic matter gradients of the OMZ. Most zooplankton are probably excluded physiologically from pronounced OMZs. The zooplankton abundance peak at the lower interface of the OMZ occurs where oxygen becomes sufficiently high to permit the zooplankton to utilize the high concentrations of organic particles that have descended through the OMZ relatively unaltered because of low metazoan abundance. A similar scenario applies to megabenthic distributions. Plankton layers and a potential short food chain (bacteria to zooplankton) at OMZ interfaces suggest intense utilization and modification of organic material, localized within a thin midwater depth zone. This could be a potentially significant filter for organic material sinking to the deep-sea floor.

Organic carbon flux through a Delaware Bay salt marsh: tidal exchange, particle size distribution, and storms

Organic carbon exchange via tidal transport between a 190 ha Spartina alterniflora marsh and Delaware Bay was quantified during 5 seasonal sampling periods in April, June, August, October, 1980 and January, 1981. Based on hourly samples collected over 3 consecutive tidal cycles, mean ebb tide concentrations of particulate organic carbon (POC) were hlgher than flood tide concentrations, except in January when the marsh surface and creeks were frozen. Mean ebb tide dissolved organic carbon (DOC) concentrations were greater than flood during all seasons, although the difference was only significant in August and January. Hourly concentrations were variable, ranging from 0.4 to 11.4 mg POC 1-' and 1.5 to 15.5 mg DOC I-'. The relative degree of marsh surface flooding, current velocity and phytoplankton concentration are important factors influencing seasonal differences in mean flood and mean ebb concentrations of POC. A broad size spectrum of particulate detritus is available to estuarine consumers, with 60 to 84 % of the total POC on both flood and ebb tides being 243 pm, 243-63 pm, 63-20 pm). There was a net export of POC from Canary Creek marsh during all seasonal studies, except in January when exchange processes were much reduced. DOC was exported during all studies, except in June when the water volume transport estimate was significantly biased toward the flood direction. Annually, 1.05 x 10' kg was exported as POC and 1.97 x 105 kg as DOC. The influence of storms was not included in the annual organic carbon exchange budget. However, during an October tropical storm ebb-directed transports of POC and DOC were 5 to 6 tlmes greater than during the normal ebb cycles monitored in October, suggesting that major coastal storms are significant to salt marsh organic material exchange processes. Elevated POC concentrations during 2 ebb cycles following an intense rainstorm in April suggest that these episodic events also represent important marsh-estuarine exchange mechanisms.