Use of annular flumes to determine the influence of current velocity and bivalves on material flux at the sediment-water interface

A benthic annular flume for both laboratory and in situ deployment on intertidal mudflats is described. The flume provides a means of quantifying material flux (i.e., biodeposition of suspended particulates, sediment resuspension, nutrients, oxygen, and contaminants) across the sediment-water interface in relation to changes in current velocity and benthic community structure and/or population density of key macrofauna species. Flume experiments have investigated the impact of the infaunal bivalveMacoma balthica and the epifaunal bivalveMytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbatorMacoma was found to increase the sediment resuspension and/or erodability by 4-fold, at densities similar to those recorded at the Skeffling mudflat (Humber estuary) (i.e., >1000 individuals m^?2). There was a significant correlation between sediment resuspension andMacoma density (r=0.99; p<0.001), which supported previous in situ field observations indicating bioturbation byMacoma enhanced sediment erodability. Biodeposition rates (g m^?2 h¹) ofMytilus edulis andCerastoderma edule were quantified and related to changes in population density in a mussel bed (Cleethorpes, Humber estuary). Biodeposition rates were up to 40-times the natural sedimentation rates. At the highest mussel bed densities (i.e., 50–100% cover or >1400 mussels m^?2) the physical presence of this epifaunal bivalve on the sediment surface reduced erosion by 10-fold. The shift from net biodeposition to net erosion occurred at current velocities of 20–25 cm s^?1. These results demonstrate that infaunal and epifaunal bivalves can have a significant impact on seston flux or sediment deposition and on sediment resuspension or erodability in estuaries where there are extensive mudflats.     

Composition and vertical flux of organic matter in a large Alaskan estuary

Observations of the composition and rate of input of organic matter to the sea floor were made at three locations in lower Cook Inlet, Alaska, during five cruises taken in the spring and summer of 1978. Total particulate, plant pigment, carbon, nitrogen, fecal pellet, and phytoplankton cell fluxes, inferred from sediment trap samples, were related to algal biomass and production in overlying waters. A daily average of 7.5% of the phytoplankton biomass was lost to the bottom. Of this loss, 83% was attributable to zooplankton grazing and fecal pellet production. At the three sampling sites, an average of 39 g C m^?2 (range of 17–60 g C m^?2, was sedimented to the bottom between May and August. This carbon flux represented an average of 12% of the total primary production measured for that time period. Kachemak Bay eastern arm of the inlet, is identified as an extremely productive embayment in which large amounts of organic matter were transferred to the sea floor.     

Bivalve contribution to benthic metabolism in a California Lagoon

Populations of suspension feeding bivalves constitute a metabolically important component of the benthos in Colorado Lagoon, Long Beach, California. Oxygen and nutrient flux were measuredin situ at monthly intervals over a two-year period. Estimates of bivalve metabolism were based on regressions of oxygen and ammonium flux on a measure of clam biomass adjusted allometrically. The introduced bivalve,Mercenaria mercenaria, occurs at maximum densities exceeding 400 per m². Based on mean densities (143 per m²), bivalves contributed more than 50% of the 77 mg O₂ per m² per hr mean annual oxygen uptake and the 191 μM per m² per hr mean ammonium release. Although bivalve biomass was not correlated with other inorganic nutrient flux, on an annual basis the sediments were a source of phosphate (26 μM per m² per hr annual average, range 5 to 50) and a small sink for nitrate and nitrite. Net primary production, ammonium flux, and phosphate flux showed great annual variability while respiration was relatively constant. Total community oxygen uptake was not correlated with temperature. Long term incubations revealed no obvious rhythms having a period between four hours and four days. The benthic flux of ammonium and phosphate was calculated to supply more than the annual requirement of Lagoon phytoplankton.     

Benthic nutrient remineralization in a coastal lagoon ecosystem

In situ measurements of the exchange of ammonia, nitrate plus nitrite, phosphate, and dissolved organic phosphorus between sediments and the overlying water column were made in a shallow coastal lagoon on the ocean coast of Rhode Island, U.S.A. The release of ammonia from mud sediments in the dark (20–440 μmol per m² per h) averaged ten times higher than from a sandy tidal flat (0–60 μmol per m² per h), and while mud sediments also released nitrate and phosphate, sandy sediments took up these nutrients. Fluxes of nutrients from mud sediments, but not from sandy areas, markedly increased with temperature. Ammonia release rates for mud sediments in the light (0–350 μmol per m² per h) were lower than those in the dark and it is estimated that some 25% of the ammonia released to the water column on an annual basis may be intercepted by the benthic microfloral community. Estimates of the annual net exchange of nutrients across the sediment-water interface, weighted by sediment type for the lagoon as a whole, showed a release of 450 mmol per m² of ammonia, 5 mmol per m² of phosphate, 5 mmol per m² of dissolved organic phosphorus, and an uptake of 80 mmol per m² of nitrate. Although rates of ammonia and nitrate exchange were comparable to those described for the deeper heterotrophic bottom communities of nearby Narragansett Bay, rates of benthic phosphate release were significantly lower. On an annual basis the Bay benthos released approximately 20 times more inorganic phosphate per unit area than did the lagoon benthos. As a result., the N/P ratio for the flux from the sediments was 74∶1 in the lagoon, compared with 16∶1 in “average” marine plankton and 8∶1 for the benthic flux from Narragansett Bay. The lack of remineralized phosphate in the lagoon, is reflected in water, column phosphate concentrations (always <1 μm) and water column N/P ratios (annual N/P=27) and suggests that the lagoon may show phosphate limitation rather than the nitrogen limitation commonly associated with marine systems.     

Variability of dissolved reactive phosphate flux rates in nearshore estuarine sediments: Effects of groundwater flow

Theoretical diffusive flux rates for dissolved reactive phosphate (DRP) were determined for sediments in a small area of the Indian River, Florida for the period March–May 1982. Flux rates from the sediment varied from 29 to 70 × 10^?6g per m² per day in seagrass associated sediments to 3–25 × 10^?6g per m² per day for an area devoid of seagrass. Simultaneous measurements of groundwater seepage velocities indicated greater velocities in seagrass associated sediments (1.03 × 10^?6m per sec) than an area devoid of grass (0.77 × 10^?6m per sec). Measured seepage flux accounted for more than 99% of the combined estimated diffusive and seepage flux of DRP for nearshore seagrass sediments. Also noted was an apparent direct relationship between tidal height, DRP and seepage velocity in nearshore sediments (25 m from shore) which further demonstrates the importance of hydrogeologic variables to these areas.     

Decadal Change in Sediment Community Oxygen Consumption in the Abyssal Northeast Pacific

Long time-series studies are critical to assessing impacts of climate change on the marine carbon cycle. A 27-year time-series study in the abyssal northeast Pacific (Sta. M, 4000 m depth) has provided the first concurrent measurements of sinking particulate organic carbon supply (POC flux) and remineralization by the benthic community. Sediment community oxygen consumption (SCOC), an estimate of organic carbon remineralization, was measured in situ over daily to interannual periods with four different instruments. Daily averages of SCOC ranged from a low of 5.0 mg C m^?2 day^?1 in February 1991 to a high of 31.0 mg C m^?2 day^?1 in June 2012. POC flux estimated from sediment trap collections at 600 and 50 m above bottom ranged from 0.3 mg C m^?2 day^?1 in October 2013 to 32.0 mg C m^?2 day^?1 in June 2011. Monthly averages of SCOC and POC flux correlated significantly with no time lag. Over the long time series, yearly average POC flux accounted for 63 % of the estimated carbon demand of the benthic community. Long time-series studies of sediment community processes, particularly SCOC, have shown similar fluctuations with the flux of POC reaching the abyssal seafloor. SCOC quickly responds to changes in food supply and tracks POC flux. Yet, SCOC consistently exceeds POC flux as measured by sediment traps alone. The shortfall of ~37 % could be explained by sediment trap sampling artifacts over decadal scales including undersampling of large sinking particles. High-resolution measurements of SCOC are critical to developing a realistic carbon cycle model for the open ocean. Such input is essential to evaluate the impact of climate change on the oceanic carbon cycle, and the long-term influences on the sedimentation record.     

Biogeochemical processes affecting the distribution and vertical transport of hydrocarbon residues in the coastal Mediterranean

Sediments, sediment trap material, dominant surface plankton and collected fecal material were sampled concurrently with surface seawater in a coastal Mediterranean ecosystem and analyzed for non-volatile hydrocarbons and chlorinated hydrocarbons. Results showed consistent partitioning of hydrocarbon classes between dissolved and particulate phases of surface waters which appeared to be related to component solubility and particle availability. Analysis of biological materials showed the biota were important not only in packaging residues into large, fast sinking particles, but also in modifying the composition of components through metabolism and selective incorporation into body tissues and feces. Apparent sedimentation rate was calculated by analysis of ²¹⁰Pb in sediment core samples and used to estimate average deposition rates of organics to the sea/sediment interface. The flux of particles through 100 m, as measured in the trap material in this sampling interval, was sufficient to balance most of the petroleum input to the sediments but accounted for only 17% of the average flux of PCBs to the sediments, and virtually none of the more soluble chlorinated hydrocarbon flux. Vertical transport via large fecal material compared to average background particles was seasonally low corresponding to a seasonal minimum in plankton biomass in late summer. Results show that hydrocarbon residues transported long distances away from input sources are highly modified, pointing to the geochemical significance of physical-chemical partitioning between seawater phases, incorporation into organisms and fecal material and biological/chemical degradation.     

Biogeochemical cycling in an organic-rich coastal marine basin—I. Methane sediment-water exchange processes

Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m ^?2 · hr ^?1 during November–May and June–October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June–October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m^?2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2–3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10^?5 cm² · sec^?1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m^?2 · hr^?1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5–15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.     

Vertical Carbon Flux of Biogenic Matter in a Coastal Area of the Aegean Sea: The Importance of Appendicularians

This work focuses on the direct measurement of the vertical flux of appendicularian houses in order to assess their importance as a component of vertical carbon flux in coastal areas. For this purpose, arrays of cylindrical sediment traps were deployed for 5 to 8 days at two depths in a coastal area of the northern Aegean Sea (inner Thermaikos Gulf) during spring. The data support the contention that resuspension was minimal. Fecal pellet (FP) production and grazing experiments with the dominant copepods (Acartia clausi) were conducted to provide additional information on the potential FP contribution to the total carbon flux. The magnitude of the vertical flux of particulate organic carbon (POC) ranged between 310 and 724 mg C m^?2 day^?1. The proportion of phytoplankton carbon in the POC vertical flux was up to 45 %. The contribution of zooplankton FPs to the total carbon never exceeded 5 %. On the contrary, appendicularian houses were an essential component of the biogenic carbon flux contributing up to 55.3 % of the total vertical carbon flux. Consequently, both phytoplankton and appendicularian houses contributed equally to the biogenic carbon flux exceeding 80 % of the total sinking POC. Taking into account the sinking speed of the particles and the environment in the area, all this carbon probably reaches the seafloor, thus indicating a strong pelagic–benthic coupling.     

Vertical sediment fluxes and wave-induced sediment resuspension in a shallow-water coastal lagoon

The present study describes variations in the vertical fluxes measured concurrently with sediment traps at both a shallow water (4 m) and a deeper water (7.5 m) position in a coastal lagoon in April 1995. A tripod equipped with five sediment traps (trap openings at 0.35 m, 0.75 m, 1.05 m, 1.40 m, and 1.80 m above the seabed) was placed at the shallow water position. This tripod was deployed three times during the study period and deployment periods varied between 2 d and 5 d. The second sediment trap, placed at the deep water position in the central part of the lagoon, measured vertical flux for intervals of 12 h at 1.4 m above the seabed. The horizontal distance between the sediment traps was 8 km. The average maximum vertical flux at the shallow water position reached 27.9 g m⁻² d⁻¹ during a period of high, westerly wind speeds, and a maximum vertical flux of 16.9 g m⁻² d⁻¹ was reached at the deep water position during a period of high, easterly wind speeds. Both strong resuspension events were closely related to increased wave shear stress derived from surface waves. Maximum wave-induced resuspension rate was 10 times higher at the shallow water position and 3.8 times higher at the deep water position compared with the net sedimentation rate in the lagoon. Small resuspension events occurred at the shallow water position during periods of increased current shear stress, Estimations of conditions for transport of sediment between shallow water and deep water showed that particles must be resuspended to a height between 3 m and 4 m and that current speeds must be higher than about 0.1 m s⁻¹. An average sedimentation rate of 3.8 g m⁻² d⁻¹ was obtained at the shallow water position during a period without wave shear stress and low current shear stress. This rate measured by sediment traps is similar to a net sedimentation rate in the lagoon of 4.4 g m⁻² d⁻¹, which was determined by radiocarbon dating of a sediment core (Kristensen et al. 1995).     

Diagenetic cycling of arsenic in the sediments of eutrophic Baldeggersee,Switzerland

Sedimentation and benthic release of As was determined in Baldeggersee, a eutrophic lake in central Switzerland. Sediment traps recorded As sedimentation during 1994, including a flood event in spring. Diagenetic processes were studied using porewater profiles at the sediment–water interface and in deeper sediment strata deposited in the mesotrophic lake (before 1885). Sediment cores were used to calculate the accumulation and to construct the balance of sedimentation and remobilisation. The results showed that the lake sediment acts as an efficient sink for As. Only 22% of the particulate As flux reaching the sediment surface was remobilised at the sediment–water interface. The As accumulation in the recent varved section of the eutrophic lake was 40 mg As m⁻² a⁻¹. Iron reduction in older sediment caused a remobilisation of 1.2 mg As m⁻² a⁻¹. This upward flux from the deeper sediment was quantitatively immobilised in the recent sulfidic sediments. The flood event in spring contributed about 34% of the yearly sediment load and led to distinct peak profiles of dissolved As in the porewater. This evidence for rapid remobilisation disappeared within months.     

Evaluating the impact of management scenarios and land use changes on annual surface runoff and sediment yield using the GeoWEPP: a case study from the Lighvanchai watershed,Iran

This study was undertaken to evaluate land use change impact and management scenarios on annual average surface runoff (SR) and sediment yield (SY) using the GeoWEPP tool in the Lighvanchai watershed (located in northwestern Iran). Following a sensitivity analysis, the WEPP model was calibrated (2005–2007) and validated (2008–2010) against monthly observed SY and SR. The coefficient of determination (R ²), Nash–Sutcliffe efficiency (NSE), mean bias error (MBE), and root-mean-square error (RMSE) were applied to quantitatively evaluate the WEPP model. The results indicate a satisfactory model performance with R ² > 0.80 and NSE > 0.60. Therefore, the model for current land use (scenario 1) was run for a 30-year time period (1982–2011). The annual average of SR and sediment load were predicted as 93,584 m³/year and 4340 ton/year, respectively. To reduce the annual average surface runoff and sediment yield at the watershed scale, the second scenario (alfalfa cultivation with suitable tillage) and the third scenario (grassland development) as two management scenarios of land use changes were defined by identifying the critical hillslopes. The rate of SR and sediment load in the second scenario were 42,096 m³/year and 429 ton/year, respectively. For the third scenario, the model predictions were 30,239 m³/year and 226 ton/year, respectively. Compared to the first scenario, the reduction rates in annual average of sediment load were about 90 and 94%, respectively. Moreover, for the second and third management scenarios, the reduction rates in annual average of SR were about 55 and 67%, respectively.     

Natural Cycles and Transfer of Mercury Through Pacific Coastal Marsh Vegetation Dominated by Spartina foliosa and Salicornia virginica

The potential for marsh plants to be vectors in the transport of mercury species was studied in the natural, mature, tidal China Camp salt marsh on San Pablo Bay. The fluxes of organic matter, mercury (THg), and monomethylmercury (MeHg) were studied in natural stands of Spartina foliosa and Salicornia virginica. Seasonal fluxes from the sediment into aboveground biomass of live plants and subsequent transfer into the dead plant community by mortality were measured. Loss of THg and MeHg from the dead plant community through fragmentation, leaching, and excretion were calculated and were similar to net uptake. Seasonal data were added up to calculate annual mass balances. In S. foliosa, annual net production was 1,757 g DW m^?2, and the annual net uptakes in the aboveground biomass were 305 μg THg m^?2 and 5.720 μg MeHg m^?2. In S. virginica, annual net production was 2,117 g DW m^?2, and the annual net uptakes in aboveground biomass were 99.120 μg THg m^?2 and 1.990 μg MeHg m^?2. Of both plant species studied, S. foliosa had a slightly lower production rate but greater mercury species uptake and loss rates than S. virginica, and, consequently, it is to be expected that S. foliosa matter may affect the local and possibly the regional food web relatively more than S. virginica. However, the actual effects of the input of mercury-species-containing plant-derived particulate matter into the food webs would depend on trophic level, food preference, seasonal cycle of the consumer, total sediment surface area vegetated, location of the vegetation in the marsh landscape, and estuary bay landscape. Since the levels of mercury species in dead plant material greatly exceed those in live plant material (on a dry weight basis), detritivores would ingest greater mercury species concentrations than herbivores, and consumers of S. foliosa would ingest more than consumers of S. virginica. The greatest THg and MeHg losses of both plant species due to mortality and to fragmentation–leaching–excretion occurred in late spring and early autumn, which corresponds to peak MeHg levels observed in sediments of coastal systems of previous studies, suggesting enhanced THg–MeHg export from the marsh to the nearshore sediment.     

Investigations of the availability and survival of submersed aquatic vegetation propagules in the tidal Potomac River

The establishment of submersed aquatic vegetation (SAV) at unvegetated sites in the freshwater tidal Potomac River was limited primarily by factors other than propagule availability. For two years, traps were used to quantify the amount of plant material reaching three unvegetated sites over the growing season. The calculated flux values provided a gross estimate of the flux of propagules that could potentially survive if other site factors were suitable. The mean flux ofHydrilla verticillata and all other species (≥0.01 gdw m^?2 d^?1) appeared sufficient to favor the establishment of vegetation, particularly considering the high viability (70–100%) of whole plants and fragments under controlled conditions. However, median water clarity values (i.e., for light attenuation, Secchi depth, total suspended solids, and chlorophylla) were below SAV restoration goals at all unvegetated sites. Additionally, sediments from unvegetated sites showed a potential for nitrogen limitation of the growth ofH. verticillata. Our findings support the hypothesis that in the tidal Potomac River, water clarity and nutrient (especially nitrogen) levels in sediment are key to plant community establishment.     

Can pelagic net heterotrophy account for carbon fluxes from eastern Canadian lakes?

Lakes worldwide are commonly oversaturated with CO₂, however the source of this CO₂ oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration (R) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O₂ and C were measured in 23 Québec lakes. All of the lakes sampled were oversaturated with CO₂ over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO₂ oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and à l’Ours, where CO₂ flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R, or net primary production (NPP), was not sufficient to account for the size of the CO₂ flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m⁻² d⁻¹ while the average annual flux of CO₂ to the atmosphere was 34 mg C m⁻² d⁻¹. In Lac à l’Ours average annual NPP was −9.1 mg C m⁻² d⁻¹ while the average annual flux of CO₂ to the atmosphere was 55 mg C m⁻² d⁻¹. In all of the lakes sampled, O₂ saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O₂ (δ¹⁸O_DO) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R, nor did the isotopic signature of dissolved CO₂ in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO₂ oversaturation. The isotopic composition of dissolved inorganic C (δ¹³C_DIC) indicates that the CO₂ oversaturation cannot be attributed to in situ aerobic respiration. δ¹³C_DIC reveals a source of excess C enriched in ¹³C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.     

The effect of chronic oil pollution on salt-marsh nitrogen fixation (acetylene reduction)

Annual acetylene reduction rates associated with interidal communities in a chronically oil polluted Virginia salt marsh were compared to rates measured in an undisturbed marsh. Chronic oil treatment resulted in visible damage to the higher plants of theSpartina alterniflora zones; however, vegetation-associated acetylene, reduction was not different from the untreated control. Sediment rates generally were affected little by oil application, except during the summer when rates in the median tidal elevation zones were considerably higher than those of the control. Acetylene reduction occurred in all transects, each of which extended from upper mudflat to theSpartina patens zone. Intertidal sediment acetylene reduction was patchy, both spatially and seasonally. Estimated rates were greatest near the surface; free-living bacterial N₂ fixation activity averaged 2.23 mg N per m² per d (range=undetectable to 365 mg N per m² per d) in the untreated and 3.17 mg N per m² per d (range=undetectable to 564 mg N per m² per d) in the oil-treated marsh during the year. Vegetation-associated N₂ fixation activity yielded highest overall mean rates (156 mg N per m² per d). The seasonal pattern of sediment and vegetation-associated fixation may be controlled by temperature and availability of oxidizable substrates.     

Mercury in Baltic Sea sediments—Natural background and anthropogenic impact

In total 27 short and one long sediment core, and 278 surface sediment samples from the Baltic Sea were analyzed for mercury (Hg), and organic carbon contents. Thirteen short cores and the long core were dated by radionuclide methods (²¹⁰Pb, ¹³⁷Cs, AMS¹⁴C). The dataset allows discriminating between natural and human induced changes on the Hg levels in Baltic Sea sediments. Preindustrial Holocene background concentrations vary between 20 and 50 μg Hg per kg dry sediment and are positively correlated with organic carbon changes. Strong human induced pollution is recorded for the second half of the past century and caused high Hg concentrations of up to several hundred μg Hg per kg dry sediment even in Baltic Sea basins. Maximum concentrations are found at industrial and war waste dumping sites (local hot spots). An Hg concentration decreasing trend toward the present day is observed at most coring sites, a result of environmental measures undertaken during the last two decades. At sites where it is possible to calculate Hg fluxes, the natural accumulation rates vary between 2.1 and 5.4 μg Hg per m² per year. Anthropogenically sourced Hg accumulation rates vary in a wide range of 30 and 300 μg Hg per m² per year for the time span of maximum pollution. In areas characterized by discontinuous sedimentation only “inventories” of human sourced Hg expressed as the total amount of deposited Hg (above the natural background) per m² can be calculated. The inventories of the investigated cores vary in the range of 1 and 8 mg Hg per m². Additionally, influences of sediment dynamics on spatial distribution pattern of Hg concentrations in surface and subsurface sediments are discussed.     

Biogeochemical cycling in an organic-rich coastal marine basin 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis

In situ carbon flux measurements and calculated burial rates are utilized to construct an organic carbon budget for the upper meter of sediment at a single station in Cape Lookout Bight, a small marine basin located on the Outer Banks of North Carolina, U.S.A. (34°37′N, 76°33′W). Of 149 ± 20 mole · m^?2 · yr^?1 of total organic carbon deposited, 35.6 ± 5.2 mole · m^?2 · yr^?1 is recycled to overlying waters, 84 ± 18% as ∑CO₂ and 16 ± 8% as CH₄. Approximately 68 ± 20% of the upward carbon flux is supported by sulfate reduction while 32 ± 16% takes place as the result of underlying methanogenesis. Measured ∑CO₂ and CH₄ sediment-water fluxes range seasonally from 1900–6300 and 50–2500 μmole · m^?2 · hr^?1 respectively.The mean residence time of metabolizable organic carbon in the upper 80 cm of sediment is approximately four months with greater than 98% of the calculated total remineralization taking place within three years. In spite of large upward fluxes of methane, larger molecules derived from metabolizable sedimentary organic carbon appear to be the dominant reductants for dissolved sulfate.     

Vertical distribution and feeding types of nematodes from Chetumal Bay, Quintana Roo, Mexico

The vertical distribution and feeding type of nematodes in sediments of Chetumal Bay, Mexico, were studied in five intertidal transects along the urbanized zone in June and December 1995. Sediments were collected with a PVC corer to 6-cm sediment depth and cut immediately into three equal 2-cm depth fractions. Nematode density varied from 7.4 × 10³ to 5.3 × 10⁵ m^?2 in June and from 1.7 × 10⁴ to 7.2 × 10⁵ m^?2 in December. In June, the epistrate feederPseudochromadora sp. was the most abundant in the deepest sediment fraction (4–6 cm), whereas epistrate feeders,Neotonchoides sp.,Desmodora sp., and the deposit feederBathylaimus australis were dominant in the top most sediment (0–2 cm). In December, deposit feeders,Desmolaimus zeelandicus, Parodontophora sp., and the epistrate feederOncholaimus oxyuris were the most abundant in the deepest sediment, whereasNeotonchoides sp. andPseudochromadora sp. dominated the first 2 cm of sediments. Highest nematode density was recorded in the uppermost sediment layer (0–2 cm). Feeding types showed different abundance among transects and between months. There was a seasonal change in vertical distribution of nematodes, with the highest abundance in the deepest sediment layer in December, possibly due to the effect of wind waves on sediments of Chetumal Bay. The trophic composition of the nematode fauna in Chetumal Bay showed a dominance of deposit feeders and epistrate feeders, most likely in response to organic enrichment that is typical of eutrophic environments.     

Methane flux from mangrove sediments along the Southwestern coast of Puerto Rico

Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH₄ m^?2 d^?1, 42 mg CH₄ m^?2 d^?1, and 82 mg CH₄ m^?2 d^?1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH₄ flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.