Short-term sedimentation dynamics in the Rhône River Delta. France: The importance of riverine pulsing

Short-term sedimentation patterns were evaluated from August 1992 to May 1993 in different wetland habitats characteristic of the Rhône Delta, including impounded and seasonally-dry saline Arthrocnemum marshes, brackish Juncus, Phragmites, and Scirpus riverine wetlands directly connected to the Rhône River, and Arthrocnemum-dominated marine marshes influenced by the Mediterranean. Short-term sedimentation was measured as sediment accumulation on paper filters which had been placed on the soil surface for several weeks. Total sedimentation and material lost on ignition was significantly related to individual sampling periods, reflecting the importance of short-term processes. High sedimentation at riverine sites (up to 22 g m^?2 d^?1) was related to a combination of river stage and wind events. Marine and impounded wetlands of the Rhône Delta experienced low sedimentation throughout the period of study. Sedimentation rates averaged over the study period were 0.8 g m^?2 d^?1, 1.8 g m^?2 d^?1, and 5.4 g m^?2 d^?1 for marine, impounded, and riverine sites, respectively. Percent material lost on ignition was low in all habitats (average 15%) and followed a seasonal trend with a minimum in late fall and winter (1%). Soil percent organic matter was also low in the top several centimeters (13%), suggesting that inorganic sedimentation is very important for accretion on these wetland surfaces. Coastal flooding was not a significant mechanism for sedimentation in the marine sites during the period of study. Sedimentation is an important factor in elevation change, and this study shows that impounded habitats, the most common “natural environment“ left in the delta, may become vulnerable to sea-level rise in the future if management practices continue to isolate these wetlands from riverine sources of sediment.     

A method for measuring ingestion rate of deposit feeders and its use with the polychaeteNereis succinea

In situ ingestion rates of some infaunal deposit feeders can be determined without collecting feces by labeling the sediment with fluorescent particles and using these to trace ingested material through the gut of the animals. This technique was applied to the polychaeteNereis succinea and showed that ingestion rate, expressed as material ingested per body weight, increased with temperature and decreased with body size. Total annual ingestion of sediment and detritus for aN. succinea population in a salt marsh near Beaufort, N.C., was estimated to be 5 kg of dry material m^?2, more than 4 times that reported for salt marsh epifaunal deposit feeders. *** DIRECT SUPPORT *** A01BY009 00005     

Amino acid composition of suspended particles,sediment-trap material,and benthic sediment in the Potomac Estuary

Sediment trap deployments in estuaries provide a method for estimating the amount of organic material transported to the sediments from the euphotic zone. The amino acid composition of suspended particles, benthic sediment, and sediment-trap material collected at 2.4 m, 5.8 m, and 7.9 m depths in the Potomac Estuary was determined in stratified summer waters, and in well-mixed oxygenated waters (DO) in late fall. The total vertical flow, or flux, of material into the top traps ranged from 3 g m^?2 d^?1 in August to 4.9 g m^?2 d^?1 in October. The carbon and nitrogen fluxes increased in the deepest traps relative to the surface traps during both sampling periods, along with that of the total material flux (up to 47.3 g m^?2 d^?1 in the deepest trap), although the actual weight percent of organic carbon and organic nitrogen decreased with depth. Amino acid concentrations ranged from 129 mg g^?1 in surface water particulate material to 22 mg g^?1 in particulate material in 9-m-deep waters and in the benthic sediment. Amino acid concentrations from 2.4-mg-depth sediment traps averaged 104±29 mg g^?1 in stratified waters and 164±81 mg g^?1 in well-mixed waters. The deep trap samples averaed, 77.3±4.8 mg g^?1 amino acids in summer waters and 37±16 mg g^?1 in oxygenated fall waters. Amino acids comprised 13% to 39% of the organic carbon and 12% to 89% of the orgnaic nitrogen in these samples. Analysis of the flux results suggest that resuspension combined with lateral advection from adjacent slopes can account for up to 27% of the material in the deep traps when the estuary was well-mixed and unstratified. When the estuary was stratified in late summer, the amino acid carbon produced by primary productivity in the euphotic zone decreased by 85% (86% for total organic carbon) at the pycnocline at 6 m depth, leaving up to 15% of the vertical organic flux available for benthic sediment deposition.     

Macroinvertebrate Production in the Submerged Aquatic Vegetation of the Mobile–Tensaw Delta: Effects of an Exotic Species at the Base of an Estuarine Food Web

This study, conducted in 1997, reports the first estimates of the impacts of the proliferation of an exotic submerged aquatic vegetation (SAV) species (Myriophyllum spicatum) on macroinvertebrate production via comparisons with two co-occurring native SAV species (Heteranthera dubia and Vallisneria americana) in the tide-influenced Mobile–Tensaw Delta (located in the north-central Gulf of Mexico, 30°40′ N, 87°55′ W). Production of macroinvertebrates was greatest on M. spicatum and H. dubia and least on V. americana. The key determinant of these differences was a greater abundance of amphipods (Gammarus mucronatus) found on the leaves of M. spicatum and H. dubia. Macroinvertebrate production on M. spicatum was three times greater (>1 kg m^?2 year^?1) than on either of the native SAV species. No-choice palatability tests showed that these differences could not be attributed to differences in invertebrate grazing on these plants. Instead, it is probable that the high production within the structurally complex M. spicatum and H. dubia was the result of reduced predator foraging efficiency. If true, then the presence of this exotic species probably renders this elevated production inaccessible to most high-order predators.     

Seasonal dynamics and production of the hydromedusan Clytia hemisphaerica (Hydromedusa: Leptomedusa) in Southampton water

In 1990 and 1991, a zooplankton sampling program revealed the presence of the hydromedusan Clytia hemisphaerica (Hydromedusa: Leptomedusa) from early May through late September throughout Southampton Water. The most notable aspects of the population dynamics of C. hemisphaerica in Southampton Water were the considerable interannual variation in peak abundance—10.65 m^?3 in 1990 and 2.44 m^?3 in 1991—and temporal patterns of mean bell diameter. The suggestion of almost continuous reproduction in 1990, as indicated by high abundance and large size range on each sampling date, is in contrast to 1991, where at the Cracknore site, five cohorts were produced at 21 d to 80 d intervals. The maximum bell diameter attained in each generation was typically 10–11 mm. At the Cracknore site in 1991, in situ bell diameter growth was linear within each cohort. A trend of increased growth rate with each subsequent cohort, from 26 μg DW d^?1 to 106 μg DW d^?1 was coincident with increasing water temperature. Annual production ranged from 0.98 mg C m^?3 yr^?1 to 1.08 mg C m^?3 yr^?1 in 1990 and 0.35 mg C m^?3 yr ^?1 to 0.48 mg C m^?3 yr^?1 in 1991. Daily P: B ratios varied in each cohort from 0.05 to 0.35.     

The relationship between primary production and the vertical export of particulate organic matter in a river-impacted coastal ecosystem

As part of the National Oceanic and Atmospheric Administration’s (NOAA) Nutrient Enhanced Coastal Ocean Productivity program, we have conducted four research cruises, July–August 1990, March 1991, September 1991, and May 1992, in the Mississippi River plume and adjacent shelf regions. Over this time period, photic-zone-integrated primary production varied significantly in both the river plume and shelf study regions, with greatest variability observed in the river plume region. In the river plume and the adjacent shelf, highest production occurred during July–August 1990 (8.17 g C m^?2 d^?1 for the plume and 1.89–3.02 g C m^?2 d^?1 for the shelf) and the lowest during March 1991 (0.40–0.69 g C m^?2 d^?1 for the plume and 0.12–0.45 g C m^?2 d^?1 for the shelf). The vertical export of POC from the euphotic zone, determined with free-floating MULTITRAP sediment trap systems, also varied temporally in both study regions, with highest values occurring in May 1992 (1.80±0.04 g C m^?2 d^?1 for the plume and 0.40±0.02 g C m^?2 d^?1 for the shelf) and the lowest values occurring during July–August 1990 (0.29±0.02 g C m^?2 d^?1 for the plume and 0.18±0.01 g C m^?2 d^?1 for the shelf). The fraction of production exported out of the photic zone was highly variable and was dependent, in part, on phytoplankton species composition and on the grazing activities of microzooplankton and mesozooplankton. The lowest ratio of export to production coincided with the time when production was greated and the highest ratios occurred when production was the lowest.     

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.     

Responses of coastal lagoon plant communities to levels of nutrient enrichment: A mesocosm study

An experiment was conducted to quantify the effects of different levels of nutrient enrichment on the plant communities of temperate coastal lagoons, specifically the lagoons of the northeast U.S. Ten mesocosms, each containing coastal water, lagoon sediments, and plants and animals found in natural lagoons, were subjected to five levels of enrichment. Two mesocosms served as controls, and received no experimental nutrient additions. The remaining 8 mesocosms were enriched in duplicate with ammonium plus phosphate at 1.0 and 0.11 mmol N or P m^?2 d^?1, 2.0 and 0.19 mmol N or P m^?2 d^?1, 4.0 and 0.35 mmol N or P m^?2 d^?1, and 8.0 and 0.67 N or P mmol m^?2 d^?1. At all levels of enrichment, and through much of the experiment, water column concentrations of dissolved inorganic nitrogen (DIN) were drawn down to background levels. Despite the efficient drawdown of added DIN even at the highest loadings, differences in plant biomass among the 5 treatments were difficult to detect. Enrichment at the highest loadings increased standing stocks of phytoplankton for one month mid-experiment. No significant effect of loading could be detected for dry biomass of eelgrass (Zostera marina), epiphytic material, drift macroalgae, or for all plant components combined. The experiment has demonstrated that the enrichment responses of coastal lagoons can be diverse, especially at intermediate loadings.     

A method for the measurement of bedload sediment transport and passive faunal transport on intertidal sandflats

A simple and inexpensive sampler to measure bedload sediment transport in shallow subtidal or intertidal areas is described. The cylindrical sub-sediment trap with an aspect ratio of 20 (height: diameter) is an improvement over conventional bedload samplers which are difficult to use in shallow areas or fail to collect the biological material associated with bedload. Traps deployed on a low-energy intertidal sandflat for six months provided daily estimates of bedload transport (quartz grains: 0.001–40 kg m^?1 d^?1), passive infaunal transport (e.g., the bivalveMya arenaria, max: 800 ind m^?1 d^?1), and organic detrital flux (e.g., macrophyte fragments, max: 400 g dry wt m^?1 d^?1). Bedload rates estimated with traps were compared to predictions from a numerical bedload model to evaluate the trap’s collection and retention efficiency. A significant linear regression between observed (trap) and predicted (model) rates (r²=0.65, p<0.001, n=97) indicated that the traps were useful for the measurement of high- and low-frequency variability in bedload transport. Potential applications of the traps in benthic oceanography include recruitment and recolonization studies.     

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

The release of remineralized N and P from the organic-rich anoxic sediments of Cape Lookout Bight is controlled by processes occurring within the sediment column and at the sediment-water interface. The relatively rapid rates of temperature dependent microbial degradation of organic matter support seasonally varying nutrient fluxes ranging from 20 to 1200 μmol·m^?2·hr^?1 for dissolved ammonium and from ? 20 to 120 μmol·m^?2·hr^?1 for total dissolved phosphate (measured in situ over the period October, 1976 to October, 1978). Molecular diffusion along steep vertical pore water concentration gradients measured simultaneously cannot explain the high fluxes observed during warmer months. Gradients for ammonium and phosphate ranged from 0.33 to 1.10 and from 0 to 0.29 μmol·cm^?3_pw·cm^?1_s respectively. These high summertime fluxes appear to result from increased sediment-water transport associated with bubble tubes created and maintained by low-tide methane gas bubble ebullition. When these tubes are present, apparent bulk sediment diffusivities calculated from concurrent studies of methane and radon-222 sediment-water exchange are 1.0–3.1 times greater than molecular diffusivities. Nutrient fluxes calculated via Fick's first law taking into account this enhanced transport and the differential diffusive mobilities of dissolved ammonium, phosphate and phosphate ion pairs indicate that removal by aerobic adsorption and/or biological uptake at the sediment-water interface plays an important role in controlling nutrient exchange in these sediments.     

The Influence of Benthic Macrofauna on the Erodibility of Intertidal Sediments with Varying mud Content in Three New Zealand Estuaries

Fine sediment inputs can alter estuarine ecosystem structure and function. However, natural variations in the processes that regulate sediment transport make it difficult to predict their fate. In this study, sediments were sampled at different times (2011–2012) from 45 points across intertidal sandflat transects in three New Zealand estuaries (Whitford, Whangamata, and Kawhia) encompassing a wide range in mud (≤63 μm) content (0–56 %) and macrofaunal community structure. Using a core-based erosion measurement device (EROMES), we calculated three distinct measures of sediment erosion potential: erosion threshold (? _c ; N m^?2), erosion rate (ER; g m^?2 s^?1), and change in erosion rate with increasing bed shear stress (m _e ; g N^?1 s^?1). Collectively, these measures characterized surface (? _c and ER) and sub-surface (m _e ) erosion. Benthic macrofauna were grouped by functional traits (size and motility) and data pooled across estuaries to determine relationships between abiotic (mud content, mean grain size) and biotic (benthic macrofauna, microbial biomass) variables and erosion measures. Results indicated that small bioturbating macrofauna (predominantly freely motile species <5 mm in size) destabilized surface sediments, explaining 23 % of the variation in ? _c (p ≤ 0.01) and 59 % of the variation in ER (p ≤ 0.01). Alternatively, mud content and mean grain size cumulatively explained 61 % of the variation in m _e (p ≤ 0.01), where increasing mud and grain size stabilized sub-surface sediments. These results highlight that the importance of biotic and abiotic predictors vary with erosion stage and that functional group classifications are a useful way to determine the impact of benthic macrofauna on sediment erodibility across communities with different species composition.     

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.     

Determination of 137Cs activities in surface sediments and derived sediment accumulation rates in Thessaloniki Gulf, Greece

Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of ¹³⁷Cs and ²¹⁰Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of ¹³⁷Cs and excess ²¹⁰Pb in the sediment cores. The spatial distribution of ¹³⁷Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for ¹³⁷Cs and gross alpha counting using alpha counter for ²¹⁰Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18?±?0.02 to 0.22?±?0.03?cm?year^?1. The ¹³⁷Cs inventories varied from 205?±?15 to 602?±?39?Bq?m^?2, while the ²¹⁰Pb inventories of the studied cores exhibited average value of 140?±?9?Bq?m^?2. Elevated ¹³⁷Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea.     

Diel dissolved oxygen dynamics and eutrophication in a shallow,well-mixed tropical lagoon (Cancun,Mexico)

Bojorquez Lagoon (BL), located on the Mexican Caribbean, has received sewage and dredging impacts as a result of tourism development. The lagoon supports a high diversity of primary producers compared to sheltered adjacent lagoons dominated byThalassia testudinum communities. The Diurnal Curve Method (Odum and Hoskin 1958) was used to measure community metabolism and assess eutrophication in BL by comparing it to the nonimpacted lagoons and to other systems studied with this method. Dissolved oxygen community input to the water column in BL ranged between 8.3 g O₂ m^?2 d^?1 and 41.5 g O₂ m^?2 d^?1 during 1985 and 1986, and averaged 17.1, whereas dissolved oxygen community consumption ranged from 6.4 g O₂ m^?2 d^?1 during 1985 and 1986, and averaged 17.1, whereas dissolved oxygen community consumption ranged from 6.4 g O₂ m^?2 d^?1 to 37.6 g O₂ m^?2 d^?1 and averaged 15.2. These values are higher than those found for the adjacent lagoons and similar coastal lagoons, and are similar to results from other lagoons with sewage or seafood waste discharge. Net flux of oxygen from the community to the water column averaged 1.9 g O₂ m^?2 d^?1 and ranged from ?9.8 g O₂ m^?2 d^?1 to 8.1 g O₂ m^?2 d^?1. These values are low compared to the adjacent lagoons, and close to zero, as in dystrophic environments. Primary productivity, as estimated by oxygen input, increased in BL during the period of study, indicating that eutrophication is proceeding, but the lagoon has not reached yet a level of “critical eutrophication” as defined by Mee (1988).     

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.     

The effects of in situ light reduction on the growth of two subtropical seagrasses,Thalassia testudinum and Halodule wrightii

The effects of in situ light reductions on two species of subtropical seagrasses, Thalassia testudirum (reduced to 14% and 10% of surface irradiance; SI) and Halodule wrightii (reduced to 16% and 13% SI) were examined over a 10-mo period (October 1992-September 1993) in relation to leaf elongation rates, sediment pore-water ammonium, and blade chlorophyll concentrations. No significant changes in pore-water ammonium levels were noted among treatments with time, but blade chlorophyll concentrations in both species were higher in plants exposed to the darkest treatments (10% and 13% SI) relative to controls exposed to 50% SI. In all treatments, blade chlorophyll concentrations were highest and chlorophyll a:b ratios lowest during the warner months, coincident with higher water temperatures. Leaf elongation rates in T. testudinum plants decreased relative to unshaded controls after 1 mo of treatment in autumn, but no significant differences in leaf elongation were noted among treatments for H. wrightii in late autumn or winter when very low growth rates (<0.1 cm shoot^?1 d^?1) were recorded. There were no differences between treatments during the spring growth period for T. testudinum (no data are available for H. wrightii), suggesting that growth (ca. 1 cm shoot^?1 d^?1) was probably not related to available light but was supported by belowground reserves. After 10 mo of treatment, all H. wrightii plants at 13% SI (1,600 mol m^?2 yr^?1) and 16% SI (2,000 mol m^?2 yr^?1) disappeared from experimental plots; similarly, no T. testudinum plants exposed to 10% SI (1,300 mol m^?2 yr^?1) remained, although 4% of the plants at 14% SI (1,800 mol m^?2 yr^?1) survived nearly 12 mo of reduced irradiance. In neither species were leaf elongation rates, which showed little change among treatments, a reliable indicator of the underwater light environment.     

Primary productivity inHalodule wrightii: A comparison of techniques based on daily carbon budgets

Bimonthly variations in shoot density, biomass, and blade productivity were used to estimate diel rates of primary productivity in conjunction with in situ measurements of photosynthesis and irradiance for a monospecific meadow ofHalodule wrightii Ascherson in Laguna Madre, Texas. Four separate techniques for estimating areal primary productivity were compared to estimates of primary production calculated from in situ measurements of photosynthesis and continuous recording of underwater light using the H_sat model. The clip and reharvest method, which is commonly used to measure shoot production, provided estimates ranging from 0.003 mol C m^?2 d^?1 in winter to 0.054 mol C m^?2 d^?1 in summer. In contrast, a method using aboveground biomass values, previously determined turnover rates, and belowground: aboveground biomass ratios provided estimates of primary production ranging from 0.04 mol C m^?2 d^?1 (winter) to 0.49 mol C m^?2 d^?1 (spring), similar to values determined from the H_sat model using in situ measurements of photosynthesis and ambient light regimes. Our results indicate that the clip and reharvest method dramatically underestimates primary productivity forH. wrightii, and that the inclusion of belowground biomass in carbon budget calculations is essential to obtaining realistic estimates of plant productivity. *** DIRECT SUPPORT *** A01BY069 00022     

Erosion and deposition of fine-grained sediments from the Bay of Fundy

Simulations of the erosion, transport and deposition of fine-grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine-grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy. Initial results showed that the resistance to erosion of intertidal fine-grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub-tidal counterparts. The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient. The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τ_d). The closest comparisons were obtained when τ_d was assigned a value of 0.1 N m^?2 following Creutzberg & Postma. The in situ still-water particle settling rate (V_o) was constant with respect to time (2.1 × 10^?3 m s^?1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10^?4 m s^?1). The reason for this discrepancy is not apparent from our results.     

The mass balance of salt and water in intertidal sediments: Results from North Inlet,South Carolina

Salinity can be used as a conservative tracer of porewater turnover in circumstances when evapotranspiration is great enough to concentrate porewater salts in intertidal sediments. At two intertidal sites situated at mean high tide at North Inlet, South Carolina, porewater drainage was estimated by this method to be 9.4 m^?2 d^?1 and 16.6 1 m^?2 d^?1, depending on physical soil properties and assuming that solute losses occur by simple diffusion across the sediment surface, by uptake and excretion by vegetation, and by drainage. Mass balance simulations indicated that sediment physical properties, evapotranspiration, and elevation are important determinants of seasonal salinity extremes. At sites situated mear mean high tide, small differences in elevation significantly affect salinity and drainage rate. As site elevation increases, losses of solutes by drainage and diffusion decrease, and the variability of porewater salinity increases. This is significant because interannual changes in mean sea level, which average ±2.9 cm on the South Carolina coast, can have a great impact on the structure and function of estuaries due to changes in the solute balance of intertidal zone sediments. Mass balance simulations that used reduced evapotranspiration rates typical of colder climates significantly reduced the mean and variability of porewater salinity, which suggests that at lower latitudes salinity becomes a more dominant determinant of biological processes. This should influence a number of processes including primary productivity, strategies of water conservation and osmoregulation, and community structure. This conclusion is consistent with published data that show tropical mangroves to have lower photosynthetic rates, and presumably lower gas exchange rates in general, than mid- and high-latitude salt marsh grasses.     

The role of tides in redistributing macrodetrital aggregates within the Swartvlei estuary

The Swartvlei estuary possesses a prolific growth of both intertidal and subtidal eelgrass,Zostera capensis. During 1984 less than 12% of the eelgrass beds were located in the upper half of the estuary, yet deposition ofZostera/macroalgal wrack in this region, when the estuary was linked to the sea (open phase), was similar to that in the lower half. Over a period of 20 semidiurnal tidal cycles there was a net gain of 2.5 tonnes dry mass of plant material into the upper reaches. Export of aquatic macrophytes and filamentous algae from the lower reaches toward the sea over 20 tidal cycles amounted to 1.6 tonnes dry mass. The amount of plant material transported during spring tides was 2 to 3 times greater than that carried during neap tides. Shallowing of the estuary mouth due to sand deposition resulted in a decline in the tidal prism and a decrease in macrodetrital flux. Total export ofZostera and associated algae amounted to 0.87 g ash-free dry mass m^?2d^?1 and represented a monthly export of 18% ofZostera bed biomass. Deposition of plant wrack during the 1984–1985 closed phase amounted to 63 g dry mass per meter of shore per day at the lower reaches site but only 10 g m^?1d^?1 was recorded at the upper reaches site. The relatively low latter value was attributed to the absence of tidal action which transports macrodetritus from the lower and middle reaches into the upper part of the system. During the 1984 open phase 70 g m^?1d^?1 was deposited at the lower reaches site and 68 g m^?1d^?1 at the upper reaches site. The role of tides in the redistribution of aquatic macrophyte primary production in the Swartvlei estuary was therefore clearly underscored.