Tidal nitrogen exchanges across a freshwater wetland succession gradient in the upper Cooper River,South Carolina

Tidal freshwater sections of the Cooper River Estuary (South Carolina) include extensive wetlands, which were formerly impounded for rice culture during the 1,700s and 1,800s. Most of these former rice fields are now open to tidal exchange and have developed into productive wetlands that vary in bottom topography, tidal hydrography and vegetation dominants. The purpose of this project was to quantify nitrogen (N) transport via tidal exchange between the main estuarine channel and representative wetland types and to relate exchange patterns to the succession of vegetation dominants. We examined N concentration and mass exchange at the main tidal inlets for the three representative wetland types (submerged aquatic vegetation [SAV], floating leaf vegetation, and intertidal emergent marsh) over 18-21 tidal cycles (July 1998–August 2000). Nitrate + nitrite concentrations were significantly lower during ebb flow at all study sites, suggesting potential patterns of uptake by all wetland types. The magnitude of nitrate decline during ebb flow was negatively correlated with oxygen concentration, reflecting the potential importance of denitrification and nitrate reduction within hypoxic wetland waters and sediments. The net tidal exchange of nitrate + nitrite was particularly consistent for the intertidal emergent marsh, where flow-weighted ebb concentrations were usually 18–40% lower than during flood tides. Seasonal patterns for the emergent marsh indicated higher rates of nitrate + nitrite uptake during the spring and summer (> 400 μmol N m^-2 tide^-1) with an annual mean uptake of 248 ± 162 μmol m^–2 tide^–1. The emergent marsh also removed ammonium through most of the year (207 ± 109 μmol m^–2 tide^–1), and exported dissolved organic nitrogen (DON) in the fall (1,690 ± 793 μmol m^–2 tide^–1), suggesting an approximate annual balance between the dissolved inorganic N uptake and DON export. The other wetland types (SAV and floating leaf vegetation) were less consistent in magnitude and direction of N exchange. Since the emergent marsh site had the highest bottom elevation and the highest relative cover of intertidal habitat, these results suggest that the nature of N exchange between the estuarine waters and bordering wetlands is affected by wetland morphometry, tidal hydrography, and corresponding vegetation dominants. With the recent diversion of river discharge, water levels in the upper Cooper estuary have dropped more than 10 cm, leading to a succession of wetland communities from subtidal habitats toward more intertidal habitats. Results of this study suggest that current trends of wetland succession in the upper Cooper River may result in higher rates of system-wide inorganic N removal and DON inputs by the growing distributions of intertidal emergent marshes.     

Seasonal and spatial distribution of the Carolina marsh clamPolymesoda caroliniana (Bosc) in a Mississippi tidal marsh

The abundance of the marsh clamPolymesoda caroliniana at four stations on a Mississippi tidal marsh was followed from February 1977 through April 1978. The stations varied with respect to inundation time and vegetative cover. The largest population was found in an area flooded 12% of the year (136 individuals per m²) and in an area flooded 3.2% of the year (126 individuals per m²). However, the latter area contained no large clams (>25.5 mm). Smaller populations were found in the upper reaches of a tidal creek draining the marsh (68 individuals per m²) and in an area flooded 5.8% of the year (73 individuals per m²). The average abundance of the clams on the marsh was 100 per m². At least three recruitment periods were apparent. In laboratory studies, clams larger than 15 mm survived longer than 30 days without water. Clams smaller than 12 mm survived less than 6 days without water. Several decapods (Callinectes sapidus, Uca longisignalis, andSesarma reticulatum) consumed clams smaller than 10 mm but were unable to eat larger clams under laboratory conditions.Polymesoda caroliniana may be important to the functioning of the marsh by removing large quantities of particulate matter from flooding water.     

Plant and infaunal communities associated with a created marsh

Destruction of tidal wetlands has led to a growing interest in the restoration and creation of new wetland habitat. However, while natural stands of vegetation have been successfully duplicated, less is understood about the establishment of faunal communities in created or restored tidal marshes. Infauna, which may form an important link between detrital production and commercially important finfish and decapods, have received limited attention in vegetated marsh habitats. We examined the infauna, changes in vegetation composition, and selected physical parameters in created marshes of different ages. Infauna were sampled using standard core sampling techniques. Vegetation composition and changes in relative abundance were observed using plot-point techniques. Vegetation plots indicated ongoing replacement ofSpartina alterniflora bySchoenoplectus robustus, a pattern supported by comparisons of vegetation at one of the sites to that reported in a previous study. Infauna exhibited significant differences between sites of different ages, with the intermediate-age site having intermediate densities for several taxa. These results suggest that both infauna and vegetation in created marshes undergo long-term change (ongoing after 10–20 yr), with both the plant and infaunal communities having qualitatively similar overall species composition to natural marsh areas.     

Finfish utilization of man-initiated and adjacent natural creeks of South Creek estuary,North Carolina using multiple gear types

Three gear types were used to collect finfish species from several tributaries of the South Creek estuary near Aurora, North Carolina, to ascertain whether a man-initiated marsh and creek system resembled adjacent natural areas in finfish species composition and abundance. Project Area II was the man-initiated area constructed in 1983 as up-front mitigation by North Carolina Phosphate Corporation prior to its anticipated mining in the headwaters of nearby natural wetlands. Two creeks served as controls throughout the study: Drinkwater Creek and Jacks Creek. Otter trawls were used monthly from July 1984 through December 1988. Wegener rings were used in April, July, and October from 1984 through 1987. An experimental gill net was used monthly from June 1984 through 1985. A total of 48 finfish species was captured during the study; only 14 species (29.2%) were common among gear types. The species collected were mud sunfish, blueback herring, alewife American shad, striped anchovy, bay anchovy, inland silverside, American eel, silver perch, Atlantic menhaden, crevalle jack, common carp, spotted seatrout, weakfish, sheepshead minnow, gizzard shad, ladyfish, chain pickerel, banded killifish, mummichog, striped killifish, rainwater killifish, mosquitofish, naked goby, green goby, white catfish, brown bull-head, pinfish, longnose gar, green sunfish, pumpkinseed, bluegill, redear sunfish, spot, Atlantic croaker, largemouth bass, white perch, striped bass, striped mullet, white mullet, golden shiner, summer flounder, southern flounder, yellow perch, bluefish, Atlantic needlefish, hogchoker, andTilapia species. Abundance of finfish species was a function of gear type. Bay anchovy and spot represented about 85% of all fish in trawl samples. The remainder was comprised of 27 other species. In Wegener rings, five species—bay anchovy, menhaden, rainwater killifish, spot, and inland silverside—each represented over 10% of all fish collected. Croaker and striped mullet each comprised more than 5% of all fish collected in Wegener rings, but were present in substantial numbers only in 1985. Based on trawl samples, the total number of finfish collected from Project Area II during the period 1984–1988 was statistically similar to those collected from the control creeks; Wegener ring data analysis indicated significantly greater catches in Project Area II compared to the control creeks. Bay anchovy catches were not significantly different among the three creeks by either trawl or Wegener ring. The same result was true for Atlantic menhaden and southern flounder. The abundance of spot in trawl samples from Project Area II was significantly greater than for the control creeks; abundance in ring samples from Project Area II was statistically similar to Jacks Creek, but significantly higher than Drinkwater Creek. The number of Atlantic croaker in Project Area II trawl samples was significantly lower than for the cotnrol creeks. Wegener rings did not sample croaker effectively.     

Biomass and production of benthic microalgal communities in estuarine habitats

Accurate measures of intertidal benthic microalgal standing stock (biomass) and productivity are needed to quantify their potential contribution to food webs. Oxygen microelectrode techniques, used in this study, provide realistic measures of intertidal benthic microalgal production. By dividing a salt-marsh estuary into habitat types, based on sediment and sunlight characteristics, we have developed a simple way of describing benthic microalgal communities. The purpose of this study was to measure and compare benthic microalgal biomass and production in five different estuarine habitats over an 18-mo period to document the relative contributions of benthic microalgal productivity in the different habitat types. Samples were collected bimonthly from April 1990 to October 1991. Over the 18-mo period, tall Spartina zone habitats had the highest (101.5 mg chlorophyll a (Chl a) m^?2±6.9 SE) and shallow subtidal habitats the lowest (60.4±8.9 SE) microalgal biomass. There was a unimodal peak in biomass during the late winter-early spring period. The concentrations of photopigments (Chl a and total pheopigments) in the 0–5 mm of sediments were highly correlated (r²=0.73 and 0.88, respectively) with photopigment concentrations in the 5–10 mm depth interval. Biomass specific production (μmol O₂ mg Chl a ^?1 h^?1) was highest in intertidal mudflat habitats (206.3±11.2 SE) and lowest in shallow subtidal habitats (104.3±11.1 SE). Regressions of maximum production (production at saturating irradiances) vs. biomass (Chl a) in the upper 2 mm of sediment by habitat type gave some of the highest correlations ever reported for benthic microalgal communities (r² values ranged from 0.43 to 0.73). The habitat approach and oxygen microelectrode techniques provide a useful, realistic ranged from 0.43 to 0.73). The habitat approach and oxygen microelectrode techniques provide a useful, realistic method for understanding the biomass and production dynamics of estuarine benthic microalgal communities.     

Seasonal abundance and diversity of spiders in two intertidal marsh plant communities

The spiders of two Mississippi marsh communities were studied from January 1982 through March 1983. Monthly collections were made in two adjacent marsh plant zones dominated bySpartina cynosuroides (L.) Roth andJuncus roemerianus Scheele respectively. A total of 38 species of spiders (36 inSpartina, 33 inJuncus) representing 13 families were collected. The dominant species in theSpartina zone includedPirata mayaca Gertsch,Lycosa watsoni Gertsch (Lycosidae),Clubiona saltitans Emerton,Scotinella formica (Banks) (Clubionidae),Floricormus sp. (Linyphiidae),Dictyna sylvania Chamberlin & Ivie (Dictynidae),Paramaevia hobbsae (Barnes) (Salticidae), andAgelenopsis barrowsi Gertsch (Agelenidae). The dominant species in theJuncus zone includedLycosa watsoni, Pirata mayaca, Clubiona saltitans andSarinda hentzi (Banks) (Salticidae). Density, biomass, species richness and equitability peaked in May in theJuncus zone and in June in theSpartina zone. Peak levels of density and biomass corresponded to the reproductive activity of the common species, while diversity patterns were attributable to the reproductive activity of the less common species. Mean values of density and biomass over the study period were 84.8 spiders per m² and 155.6 mg per m² in theSpartina zone and 39.4 spiders per m² and 133.0 mg per m² inJuncus zone. The Juncus zone was flooded more frequently, contained less litter, and supported lower overall density and diversity of spiders.     

Direct counts of bacteria in the sediments of a North Carolina salt marsh

The number of bacteria in sediments of a North Carolina salt marsh was determined by direct counts with epifluorescent illumination and acridine orange stain. Cell number decreased from 8.36–10.90×10⁹ cells/cm³ of sediment at the surface to 2.19–2.58×10⁹ cells/cm³ of sediment at a depth of 20 cm. No significant difference was found among four stations located on a transect which crossed the marsh and spanned subtidal to intertidal sediments.     

Distribution and abundance of early life history stages of the blue crab,Callinectes sapidus,in tidal marsh creeks near Charleston,South Carolina

A 16-month study of estuarine habitats in poly-, meso-, and oligohaline salinity regimes near Charleston Harbor assessed the distribution and abundance of megalopae and early crab stages of the blue crab,Callinectes sapidus. Blue crab were sampled with a plankton net and a cylindrical drop sampler. Blue crab were most abundant in plankton collections at night, accounting for 68% of the megalopae and over 88% of the juveniles collected in day and night tows combined. At night, densities of megalopae were greatest in surface samples, whereas densities in daylight collections were greater on the bottom. Juvenile densities were greatest on the bottom in both day and night collections, although catch rates at night were more variable than those of the megalopae. This suggests that megalopae, and possibly juvenile stages, experience a diel vertical migration. Results indicate that ingress to estuarine nursery areas occurs at the megalopal stage. Megalopal densities were highest at the polyhaline site, while juvenile blue crab were most abundant in the oligohaline area. Habitat utilization by juvenile blue crab was estimated using a cylindrical drop sampler and Venturi suction pump on three bottom types in the intertidal zone. Densities were greatest over the sandy-mud substrate, although catch rates were much lower than those reported for other geographical areas. These results suggest that juvenile blue crab do not occur in abundance on the marsh surface but remain on the creek bottom, possibly because creek physiography and large tidal amplitudes may restrict accessibility to the marsh surface.     

Changing elevation,accretion, and tidal marsh plant assemblages in a South San Francisco Bay tidal marsh

Analyses of organic content, pollen, and the carbon-isotopic composition of a 3.5-m sediment core collected from a subsided tidal marsh located in South San Francisco Bay, California, have provided a 500-yr record of sediment accretion and vegetation change before, during, and after a rapid 1 m increase in sea level. Core chronology was established using¹⁴C dating of fossil plant material, the first appearance of pollen types produced by plants not native to California, and changes in lead concentrations coincident with anthropogenic contamination. Prior to the mid 19th century, rates of sediment accretion were between 1 and 4 mm yr⁻¹; sediment accretion accelerated to an average of 22 mm yr⁻¹ following the initiation of subsidence. Changes in tidal marsh vegetation also accompanied this depositional change. Vegetation shifted from a high to low marsh assemblage, as indicated by a larger percentage of grass pollen, rhizomes ofSpartina foliosa, and a strong C₄ signal. Between 1980 and 2001, Triangle marsh again developed high marsh vegetation, as indicated by higher percentages of the Amaranthaceane pollen type, seed deposition, includingSalicornia spp., and more negative carbon isotopic ratios.     

Flocculation and sedimentation in the ACE Basin, South Carolina

Extremely rapid deposition of fine-grained sediment has been observed near an artificially created channel between the Edisto and Ashepoo Rivers in South Carolina. Accumulation rates on the order of 0.05 m mo⁻¹ formed a region of fluid mud known as the Mud Reach that was present during a study carried out in May 1998. Subsequent surveys in March and May 1999 found no evidence of fluid mud in this area. Image analysis of photographs of the suspended sediment from 1998 showed that the water column was populated by flocs with a median diameter on the order of 0.3 mm which tended to remain in suspension over the tidal cycle, and flocs on the order of 0.8 mm which remained in the near bottom region except at maximum current speeds. In 1999, both the size and concentration of flocs at the surface remained the same, but the size and abundance of flocs near the bottom was reduced. Disaggregated inorganic grain size analysis of the suspension and samples from a core collected in the Mud Reach indicated that in 1998 the accumulation of material on the bottom was the result of floc settling and that in spite of highu _s values, little sorting of this material was occurring. Grain size analysis of samples deeper in the core showed that the rapidly floc-deposited mud was underlain by coarser, well-sorted sediment, confirming that the Mud Reach was a transient feature. The observed flocculation processes may play a significant role in the trapping of fine-grained sediment throughout this region.     

三江平原典型沼泽湿地能量平衡和蒸散发研究

基于涡度相关技术对三江平原典型沼泽湿地的水热通量进行了连续观测,研究沼泽湿地能量平衡和蒸散发的季节变化,确定观测期内沼泽湿地总蒸散量,并通过逐步回归方程估算沼泽湿地水面蒸发和植被蒸腾。结果表明,沼泽湿地的能量平衡具有明显的季节变化特征,总体看来,潜热通量是湿地的主要能量支出项,占净辐射的45.5%,感热通量和存储热通量分别占净辐射的27.9%和26.7%。2006年5~10月份沼泽湿地总蒸散量为310.6mm,月均日蒸散量最高值出现在7月。观测期内沼泽湿地水面蒸发量约为221mm,占总蒸散量的71%左右,植被蒸腾量则约占总蒸散量的29%,湿地蒸散发以水面蒸发为主。     

三江平原沼泽湿地和农田的演替过程对地下水的影响

三江平原是我国重要的商品粮基地和沼泽湿地集中分布区。60 a来,随着农田面积持续增加和种植结构调整,湿地退减和地下水水位下降备受关注,地下水是否超采争议不断。文章选取1956—2019年7期遥感影像数据,采用单一土地利用动态度进行沼泽湿地和农田的演变进程特征分析;以1980年、2019—2021年4期同期统测数据和国家地下水监测工程数据为基础,探讨了湿地农田化对地下水水位的影响。结果表明：（1）1956—2019年沼泽湿地呈现减少态势,旱田呈现先增加后减少的态势,水田呈现先增加后稳定的态势,在1956—1996年具有“沼泽湿地变旱田”的特征,2.36×104 km2的沼泽湿地变成旱田,在1996—2019年具有“旱田改水田”的特征,1.15×104 km2的旱田变成水田;（2）三江平原1980—2021年36 546 km2的区域地下水水位降幅小于5 m,3 669 km2的区域地下水水位降幅大于10 m,建三江垦区存在超采地下水现象;（3）与1980年枯水期地下水水位相比,以降深10 m计算,2021年地下水降落漏斗面积为3 669 km2,较2019年面积增大269 km2,向北东方向略有扩张;（4）2019年建三江垦区在强降水的条件下地下水仍难以实现“以丰补欠”的自然调节,地下水储量减少5.81×108 m3。此研究成果为区域水平衡研究奠定了基础,对科学认识水土资源合理开发利用具有重要意义。     

Exchange of microbial biomass between aSpartina alterniflora marsh and the adjacent tidal creek

Salt marsh systems of the southeastern United States are characterized by extensive stands ofSpartina alterniflora. These marshes may influence the concentrations of material suspended in flooding and receding tidal waters. The ability of aSpartina alterniflora-dominated marsh to influence the concentration of suspended microbial biomass was investigated through the use of a 142-m long flume. The flume extended through stands of tall-, medium-, and short-heightSpartina. Water passing through the tallSpartina lost a considerable portion of microbial biomass. Initial samples from medium-heightSpartina were collected from water that had already passed through the tall grass. These samples contained 20 to 70% less microbial biomass than did water entering the tallSpartina. Calculations of mass transport suggest that the tallSpartina zone of marsh acts as a sink for microbial biomass while the short-heightSpartina tends to export biomass (to the tallSpartina zone). The marsh as a whole acts as a sink for microbial biomass. Transport estimates from 32 individual tide cycles were modeled to obtain an annual estimate of transport. As a consequence of high variability among individual transport estimates, no annual transport estimate could be distingushed from a net-zero transport.     

Weichselian ice movement in South Norway and adjacent areas

Three main phases of ice-movement pattern in South Norway during the Weichselian are reconstructed. During Phase II (possibly of Middle Weichselian age) the ice divide was located far to the west. Phase III (assumed Middle/Late Weichselian age) had an easterly situated ice divide. During Phase IV (Preboreal age) the ice divide had moved back to the west and had partly fragmented into separate domes. The migration of the ice divide from west to east may be due to glacier surges, whilst the ice-divide migration from Phase III to IV was probably a result of a general reduction in size of the ice sheet.     

Olivine-normative dolerite dikes from western South Carolina: Mineralogy,chemical composition and petrogenesis

This investigation describes five Mesozoic dolerite dikes which intrude Paleozoic metamorphic and igneous rocks of the Inner Piedmont of western South Carolina. The dikes are vertical or nearly so and strike approximately N40° W. Three major northeast-trending faults also occur in the study area. Left lateral displacement of one dolerite is documented at a locality near Cleveland, South Carolina. Elsewhere, several of the dolerite dikes appear to terminate at or near the faults. — The dolerite dikes have subophitic to microporphyritic textures and consist principally of plagioclase (generally An_70–80), olivine (dominantly Fo_80–90) and augite with subordinate pigeonite, titanomagnetite, chromite and brown, partly glassy mesostasis. In one dike pyroxene compositions trend from augite to ferroaugite in contrast to an augitesubcalcic augitepigeonite trend characteristic of the other dolerites. The contrasting trends primarily result from differences in SiO₂ abundance in the dolerite magmas. — Major and trace element analyses indicate the presence of two different olivine-normative dolerite magma types. The two magma types are not related by near surface crystal fractionation. Models for genesis of the olivine-normative dolerite magmas by partial melting of a plagioclase peridotite upper mantle source region are presented. The models require that the source region be enriched in LREE and incompatible elements such as Rb, Ba, Hf and Th relative to Cl chondritic abundances. One magma type appears to represent a primary dolerite magma that ascended from the source region with little subsequent compositional change. The second magma type most likely experienced assimilation of clinopyroxene-garnet (eclogite) during ascent, thereby acquiring a REE pattern with a less steep negative slope for the LREE and a slight positive slope in the HREE.     

Carolina Bay geoarchaeology and Holocene landscape evolution on the Upper Coastal Plain of South Carolina

Surface water on the mainly dry, upland interfluves of the Upper Coastal Plain of South Carolina occurs currently as a sporadic distribution of shallow ponds held within Carolina bays and other small, isolated basins. At seven bays on the U.S. Department of Energy's Savannah River Site on the Upper Coastal Plain of South Carolina, we investigated Holocene changes in bay morphology, ecology, and prehistoric human activity. At Flamingo Bay, we employed archaeological survey and testing, shovel and auger testing, sediment analysis, and ground-penetrating radar to document stratigraphy and chronology of the sand rim on the eastern side of the bay. Artifact assemblage indicate changes in intensity of human use of the bay. Radiocarbon dates from a sediment core establish time scales for depositional processes at the center of the basin. Ground-penetrating radar data from the other bays indicate that the stratigraphy of all seven bays is broadly similar. We conclude that: (1) Significant modification of the bays, including rim development and basin infilling, occurred during the Holocene; (2) ponds on the early Holocene landscape were larger and more permanent than at present; (3) early Holocene climate, as indicated by both depositional processes and human activity, was not characterized by prolonged periods of extremely dry conditions; and (4) fluvial-centric models of terminal Pleistocene—early Holocene human adaptations require revision to include intensive use of isolated upland ponds. © 1996 John Wiley & Sons, Inc.     

Influence of land use on macrobenthic communities in nearshore estuarine habitats

Macrobenthic community indices were examined for their ability to characterize the influence of shoreline alteration and watershed land use in nearshore estuarine environments of the Chesapeake Bay, U.S.A. Twenty-three watersheds were surveyed in 2002 and 2003 for nearshore macrobenthic assemblages, environmental parameters (i.e., dissolved oxygen, pH, total suspended solids, salinity, and sediment composition), shoreline condition, and land use. Two indices of macrobenthic biological integrity, benthic index of biological integrity in the nearshore (B-IBI_N) and abundance biomass comparison (W-value), were evaluated for associations with environmental and shoreline condition, and riparian and watershed land use. Comparisons between nearshore measures of the B-IBI with offshore values (>2 m; Chesapeake Bay benthic index of biological integrity [B-IBI_CB]) were conducted to assess the ability of the index to reflect land use patterns at near and far proximities to shore. Nearshore macrobenthic communities were represented by a total of 94 species (mean number of species =9.2 ± 0.4 sample⁻¹), and were dominated by the phyla Arthropoda, Annelida, and Mollusca. Temporal variability in environmental conditions and macrobenthic abundance and biomass may be attributable to the notable increase in precipitation in 2003 that led to nutrient influxes and algal blooms. For the biotic indices applied in the nearshore, the highest scores were associated with forested watersheds (W-value, B-IBI_N). Ecological thresholds were identified with nonparametric change-point analysis, which indicated a significant reduction in B-IBI_N and W-value scores when the amount of developed shoreline exceeded 10% and developed watershed exceeded 12%, respectively.     

Nekton of new seagrass habitats colonizing a subsided salt marsh in Galveston Bay, Texas

Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core⁻¹ thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change.     

Sediment trapping and transport in the ACE Basin,South Carolina

A study took place during May 1998 and May 1999 to examine the processes controlling localized accumulation of fine-grained sediments in the lower Ashepoo River. This region, referred to as the Mud Reach, is an area of muddy bottom sediments bounded by fine sands. The Mud Reach is located downstream of the landward extent of the salt intrusion where an estuarine turbidity maximum commonly occurs. Tidal time-series measurements made in the Mud Reach during May 1998, when river discharge was at a 10-yr high, showed high concentrations of suspended sediment (0.05–1 g I⁻¹) during maximum tidal current velocity with concentrations in the bottom 30 cm exceeding 70 g I⁻¹ (fluid mud). A correlation between salinity stratification and increased suspended sediment concentration suggests that inhibited vertical mixing enhances the settling of flocculated sediments to the bed. Measurements made during May 1999 show a two-order-of-magnitude decrease in the concentration of near-bed sediments. A decrease in river discharge during the 1999 observation period of more than 100 m³ s⁻¹ suggests that changes in the hydrography and in the supply of sediments to the system both may be important factors in the trapping of fine-grained sediments in the region. The source of sediments is likely from muddy deposits in the Fenwick Cut, a man-made section of the Atlantic Intracoastal Waterway about 2 km north of the Mud Reach that connects the Ashepoo and Edisto Rivers. The Fenwick Cut appears to be an effective area for trapping sediments where shoaling has increased by 130% in the last decade. Current measurements show that flow velocities decrease through the Cut, likely allowing for the settling of suspended particles that form the thick deposits of unconsolidated mud observed during both years.