Flux of dissolved organic carbon and pore water through the substrate of a Spartina alterniflora marsh in North Carolina

Central aspects of carbon and sulfur biogeochemistry were studied along a transect extending from an unvegetated mudflat into a Spartina anglica salt marsh. Conditions along the transect differed with respect to tidal elevation, sediment characteristics, vegetation coverage, and benthic macrofauna abundance. Dark sediment O 2 uptake and CO 2 emission at the highly bioturbated mudflat were low and relatively unaffected by tidal coverage. Sulfate reduction accounted for 30-60% of the daily CO 2 emission from the open mudflat sediment. Sediment O 2 uptake within the nonbioturbated and vegetated marsh was up to seven times higher during air exposure than during inundation, whereas the difference in CO 2 emissions always was less than a factor of 2. The contribution of sulfate reduction to CO 2 production was low (Ͻ21%) and decreased progressively with tidal elevation as a result of the oxidizing capacity of S. anglica roots in the vegetated marsh. The boundary between the mudflat and the retreating marsh is a unique environment. High near-surface pore-water concentrations of dissolved organic carbon (DOC) above the marsh cliff and highly elevated total carbon dioxide (TCO 2 ) pore-water concentrations at both sides of the cliff during air exposure coincided with extremely high TCO 2 emissions and apparent respiratory quotients (up to 14) only below the marsh cliff during inundation. We propose that substantial seepage of DOC-poor and HCO -rich pore water may have occurred from the elevated marsh to the unvegetated sediment below during low tide followed by massive release of HCO during high tide. Accordingly, sulfate reduction accounted for more than Ϫ 3 the TCO 2 release above the marsh cliff, but only for about 40% below the cliff. Mineralization rates and pathways in salt-marsh sediments vary considerably on small spatial and temporal scales and are dependent on inundation frequency as well as the composition and distribution of flora and fauna.

Section: The Impact Of Plants and Animals On Sulfate Reduction-mentioning

confidence: 99%

Benthic metabolism and sulfur cycling along an inundation gradient in a tidal Spartina anglica salt marsh

Gribsholt¹,

Kristensen²

2003

“…During the decomposition of Spartina substantial quantities of dissolved organic matter are produced, but studies of water movement and dissolved organic C fluxes in pore waters indicate that there is very little export of dissolved organic matter from salt-marsh sediments (Howes et al 1985;Yelverton and Hackney 1986). Rates of C mineralization (to C02) in salt-marsh sediments are high enough to account for the decomposition of 80-100% of Spartina belowground production (Howes et al 1985).…”

mentioning

confidence: 99%

Diagenesis of belowground biomass of Spartina alterniflora in salt‐marsh sediments

Benner¹,

Fogel

Sprague

1991

218

103

Belowground biomass of Spartina alterniflora lost 55% of its organic matter during 18 months of decomposition in salt-marsh sediments. Significant losses of lignin were observed under highly reducing conditions confirming previous reports of lignin degradation in the absence of molecular oxygen. The submolecular components of lignin varied in susceptibility to degradation and were ranked in the following descending order of resistance to decomposition: V-P> S-C. The preferential degradation of 1 he cinnamyl and syringyl components constrains the quantitative usefulness of these compounds for estimating the contributions ofherbaceous and angiosperm tissues to pools of dissolved and particulate organic matter. The relative concentration of lignin increased about 1.6-fold during decomposition owing to the more rapid loss of polysaccharides.Two phases in nitrogen dynamics were evident during the decomposition study. During the first 4 months of decomposition there was a net loss of N from Spartina tissues at a rate of about 10 gg N g-l tissue d-I. After this initial phase of net N loss, a phase of N immobilization was evident and was tightly coupled with rates of microbial degradation of the tissues. Net rates of N immobilization were -19 pg N g-' tissue d -I; by the end of the 18-month study more N had accumulated in the tissues than was initially present. Stable N isotope composilions of decaying Spartina were also very dynamic and reflected the sources of N that were immobilized during decomposition and the N transformations that occurred on the detrital particles.The stable C isotope composition of the polysaccharide components of Spartina was -6%~ heavier than that of the lignin component. The isotopic compositions of these components retained the 6o/oo difference throughout the 18-month study. The stable C isotope composition of the bulk tissues, however, decreased gradually during decomposition as the isotopically heavier polysaccharides were preferentially lost. Residual material was relatively enriched in the isotopically lighter, lignin-deriveci C.'

“…Expanding on the idea that tidal marshes act as a ''leaky dam'' (Yelverton and Hackney 1986), we propose that tidal marshes act as a Si sponge, storing Si for a period of time and often transforming Si from organic to inorganic before release to estuarine systems. In turn, we hypothesize that salt marshes increase the residence time of Si in northeastern estuaries.…”

Section: Discussionmentioning

confidence: 99%

Salt marsh tidal exchange increases residence time of silica in estuaries

Carey

Fulweiler

2014

We present flux measurements of dissolved silicon (DSi), biogenic Si (BSi), and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) on a temperate salt marsh in Narragansett Bay, Rhode Island, over neap, mid, and spring tide cycles during the spring and the summer seasons to determine the stoichiometry of marsh tidal export and the potential effect on Si availability in the receiving estuary. During the spring, when incoming waters were depleted in DSi (average 1.7 mmol L 21 ), the marsh was a net sink of BSi (132 mol h 21 ) and a source of DSi (31 mol h 21 ) to the estuary. During this period, the DIN : DSi ratio of ebbing water was more than five times lower than that of flood water. Together, these data indicate that marsh nutrient fluxes ''outwell'' nutrients in ratios that support spring diatom growth in Narragansett Bay. However, during the entire sampling period the marsh served as a net sink for both DSi and BSi, importing, on average, 14 mol DSi h 21 and 86 mol BSi h 21 . We hypothesize that the net import and retention of Si by salt marshes provides a previously overlooked ecosystem service of increasing the residence time of Si in estuarine systems. In the absence of salt marshes, we calculate that 5.1 3 10 4 kmol of Si would be exported from the estuary during the growing season, decreasing the availability of Si in the system and having direct repercussions for phytoplankton species composition in nearby estuarine waters.