Oyster reefs as processors of estuarine materials

Oyster reefs are dense concentrations of filter-feeding animals in estuarine ecosystems. A flow-through plastic tunnel is a feasible method of determining significant changes in material concentrations in tidal waters passing over an oyster reef. The oyster reef reduces the amplitude of the particulate organic carbon and chlorophyll a signals while increasing the amplitude, of the ammonia signal. The observations suggest that oyster reefs have one of the highest reported release rates of ammonia (1680–7250 μg at.·m^?2·h^?1), and thus are probably important in material cycles in marsh-estuarine ecosystems. The magnitude of particulate organic carbon removal by the oyster reef is many times greater than that expected from biofiltration alone, suggesting that removal due to physical factors may be important.     

The impact of Hurricane Ivan on the primary productivity and metabolism of marsh tidal creeks in the NorthCentral Gulf of Mexico

Past research has examined hurricane impacts on marine communities such as seagrass beds, coral reefs, and mangroves, but studies on how hurricanes affect marsh tidal creeks are lacking despite the important ecological roles that marsh tidal creeks have in coastal ecosystems. Here we report on the impact of Hurricane Ivan, which made landfall on September 16, 2004, on the primary productivity and metabolism of six marsh tidal creeks in the NorthCentral Gulf of Mexico. The hurricane did not seem to have any large, lasting impact on nutrient concentrations, primary productivity, metabolism, and chlorophyll a concentration in the water-column of the marsh tidal creeks. In contrast, the hurricane seemed to largely decrease gross primary productivity, net productivity, and chlorophyll a concentration in the sediment of the marsh tidal creeks. The results observed for Hurricane Ivan were coincident with those observed for four other major storms that made landfall close to the study area during 2005, Tropical Storm Arlene and Hurricanes Cindy, Dennis, and Katrina. However, the apparent negative impact of major storms on the sediment of the marsh tidal creeks did not seem to be long-lived and appeared to be dissipated within a few weeks or months after landfall. This suggests that marsh tidal creeks mostly covered with bare sediment are less disturbed by hurricanes than other types of marine communities populated with bottom-attached and/or more rigid organisms, such as seagrass meadows, coral reefs, and mangroves, where hurricane impacts can be larger and last longer.     

Tidal and diel fluctuations in the temporal concentrations of chlorophyll a and pheophytin at a station monitoring a high-marsh creek

The temporal distribution of chlorophyll a and pheophytin at a transect monitoring the flow at a high-marsh creek was investigated. The observed fluctuations in chlorophyll a concentration consisted of complex, superimposed, tidal and diel rhythms; pheophytin variability, on the other hand, was controlled by the tides. Transport measurements and correlation analyses supported the hypothesis that tidal forces have a major influence on the temporal fluctuation of chlorophyll a and phaeophytin concentrations in high-marsh creeks. The data indicate that it is important to consider tidal flux when designing programs to study seasonal effects, primary productivity, and phytoplankton species composition.     

Nutrient limitation and algal blooms in urbanizing tidal creeks

Tidal creeks are commonly found in low energy systems on the East and Gulf Coasts of the United States, and are often subject to intense watershed human development. Many of these creeks are receiving urban and suburban runoff containing nutrients, among other pollutants. During the period 1993-2001, we studied three tidal creeks located in southeastern North Carolina, a rapidly urbanizing area. All three creeks received anthropogenic nutrient loading. Oligohaline to mesohaline stations in upper tidal creek regions had much higher nutrient (especially nitrate-N) concentrations than lower creek areas, and hosted spring and summer phytoplankton blooms that at times exceeded 200 μg chlorophyll a l⁻¹. Phytoplankton biomass during winter was low at all stations in all three creeks. Spring and summer nutrient addition bioassay experiments were conducted to characterize the nutrients limiting phytoplankton growth. Water from high salinity stations in all three creeks always showed significant positive responses to nitrate-N inputs, even at concentrations as low as 50 μg N l⁻¹. Low salinity stations in upper creek areas often showed significant responses to nitrate-N inputs, but on occasion showed sensitivity to phosphorus inputs as well, indicating the influence of anthropogenic nitrate loading. During several experiments, one of the upper stations showed no positive response to nutrient inputs, indicating that these stretches were nutrient replete, and further phytoplankton growth appeared to be light-limited either by phytoplankton self-shading or turbidity. Water from upper creek areas yielded much higher chlorophyll a concentrations in bioassay experiments than did lower creek water. In general, these urbanizing tidal creeks were shown to be very sensitive to nitrogen loading, and provide a physical environment conducive to phytoplankton bloom formation in nutrient-enriched areas. Tidal creeks are important ecological resources in that they are considered to be nursery areas for many species of fish and shellfish. To protect the ecological function of these small, but very abundant estuarine systems, management efforts should recognize their susceptibility to algal blooms and focus on control of nonpoint source nutrient inputs, especially nitrogen.     

Settlement and Survival of the Oyster Crassostrea virginica on Created Oyster Reef Habitats in Chesapeake Bay

Efforts to restore the Eastern oyster (Crassostrea virginica) reef habitats in Chesapeake Bay typically begin with the placement of hard substrata to form three‐dimensional mounds on the seabed to serve as a base for oyster recruitment and growth. A shortage of oyster shell for creating large‐scale reefs has led to widespread use of other materials such as Surf clamshell (Spisula solidissima), as a substitute for oyster shell. Oyster recruitment, survival, and growth were monitored on intertidal reefs constructed from oyster and Surf clamshell near Fisherman’s Island, Virginia, U.S.A. and on a subtidal Surf clamshell reef in York River, Virginia, U.S.A. At the intertidal reefs, oyster larvae settlement occurred at similar levels on both substrate types throughout the monitoring period but higher levels of post‐settlement mortality occurred on clamshell reefs. The oyster shell reef supported greater oyster growth and survival and offered the highest degree of structural complexity. On the subtidal clamshell reef, the quality of the substrate varied with reef elevation. Large shell fragments and intact valves were scattered around the reef base, whereas small, tightly packed shell fragments paved the crest and flank of the reef mound. Oysters were more abundant and larger at the base of this reef and less abundant and smaller on the reef crest. The availability of interstitial space and appropriate settlement surfaces is hypothesized to account for the observed differences in oyster abundance across the reef systems. Patterns observed emphasize the importance of appropriate substrate selection for restoration activities to enhance natural recovery where an underlying habitat structure is destroyed.     

A comparison of the zooplankton of Lake Kainji and of the Rivers Niger and Swashi

Seventy-nine taxa of periphyton were collected on Plexiglas plates at seven stations in the three streams between March, 1975, and February, 1976. More taxa and lower densities were generally observed at the upstream stations in the creeks than at the downstream stations resulting in greater species diversity values at the upstream stations. Species diversity was generally lower in the river than in the creeks, reflecting the lower numbers of taxa and the presence of several abundant species. Species diversity did not reflect extremes in physiochemical conditions. Although several different species were abundant in the streams during the present study, the most commonly occurring dominants were Gomphonema olivaceum and species belonging to the genera Navicula and Nitzschia. No consistent trend was observed in ash-free weight; however, the concentration of chlorophyll a was generally greater at the downstream stations in the creeks than at the upstream stations.     

Water column oxygen demand and sediment oxygen flux: patterns of oxygen depletion in tidal creeks

Low dissolved oxygen (DO) levels often occur during summer in tidal creeks along the southeastern coast of the USA. We analyzed rates of oxygen loss as water-column biochemical oxygen demand (BOD₅) and sediment oxygen flux (SOF) at selected tidal creek sites monthly over a 1-year period. Ancillary physical, chemical and biological data were collected to identify factors related to oxygen loss. BOD₅ rates ranged from 0.0 mg l^?1 to 7.6 mg l^?1 and were correlated positively with organic suspended solids, total suspended solids, chlorophyll a concentrations, temperature, and dissolved oxygen, and negatively with pH and nitrate + nitrite. SOF rates ranged from 0.0 to 9.3 g O₂ m^?2 d^?1, and were positively correlated with temperature, chlorophyll a, and total suspended solids, but negatively with dissolved oxygen. Both forms of oxygen uptake were seasonally dependent, with BOD₅ elevated in spring and summer and SOF elevated in summer and fall. Average oxygen loss to sediments was greater and more variable than oxygen loss in the water column. Oxygen deficits at three of five locations were significantly related to BOD₅ and SOF, but not at two sites where ground water discharges were observed. Correlation and principal component analyses suggested that BOD₅ and SOF responded to somewhat different suites of environmental variables. BOD₅ was driven by a set of parameters linked to warm season storm water inputs that stimulated organic seston loads, especially chlorophyll a, while SOF behaved less strongly so. Runoff processes that increase loads of organic material and nutrients and ground water discharges low in dissolved oxygen contribute to occurrences of low dissolved oxygen in tidal creeks.     

Guidelines for evaluating performance of oyster habitat restoration should include tidal emersion: reply to Baggett et al.

Baggett et al. (2015) identified a set of three universal environmental variables to be monitored for evaluating all oyster habitat restoration projects: salinity, temperature, and dissolved oxygen. Perhaps evidencing a bias toward subtidal reefs, this set of parameters omits another first‐order environmental factor, tidal emersion. Intertidal oyster reefs can be the dominant reef habitat in estuaries, with clear zonation in oyster performance across the intertidal exposure gradient. Therefore, we propose to include tidal emersion as a fourth universal environmental parameter when designing and evaluating oyster restoration projects to better encompass the whole environmental spectrum along which reefs occur.     

Ectoenzymatic activity and glucose heterotrophic metabolism in a shallow estuary (Ria de Aveiro,Portugal): influence of bed sediments and salt marshes

Bacterioplankton abundance and activity were studied in the estuarine system of Ria de Aveiro (Portugal) to test if tidal resuspension of sediments and transport of particles from the salt marshes may act as factors of variability of bacterial communities. The total and attached cell abundance, ectoenzymatic activity and the heterotrophic metabolism of glucose, as well as seston, chlorophyll a and particulate organic carbon (POC) were monitored during four 10-h periods along the tidal cycle at four sampling sites across a transect. The variation of particulate materials (seston, POC and chlorophyll a ) along the transect was not significantly correlated with either distance to the margin or distance to the sediment surface. Nevertheless, proximity to the salt marsh or to the bottom sediment surface favoured glucose incorporation and aminopeptidase activity. A multiple stepwise linear regression analysis using temperature, salinity, seston, POC, chlorophyll a , distance to sediment surface and distance to the margin as independent variables explained 66.5% of the variability of the fraction of particle-attached bacteria and only a very small proportion (12–43%) of the observed variability of total bacterial abundance, ectoenzymatic activity and glucose utilization. The spatial patterns of variation of the concentration of particulate material (seston, POC and chlorophyll a) do not clearly indicate the occurrence of sediment resuspension and runoff from the margins. This, together with the poor contribution of these parameters to the transversal and tidal variability of bacterial activity, dismisses the importance of inputs of suspended material across the sediment/water interface and from neighbouring salt marshes in the control of bacterial density and activity.     

Biophysical Constraints on Optimal Patch Lengths for Settlement of a Reef-Building Bivalve

Reef-building species form discrete patches atop soft sediments, and reef restoration often involves depositing solid material as a substrate for larval settlement and growth. There have been few theoretical efforts to optimize the physical characteristics of a restored reef patch to achieve high recruitment rates. The delivery of competent larvae to a reef patch is influenced by larval behavior and by physical habitat characteristics such as substrate roughness, patch length, current speed, and water depth. We used a spatial model, the “hitting-distance” model, to identify habitat characteristics that will jointly maximize both the settlement probability and the density of recruits on an oyster reef (Crassostrea virginica). Modeled larval behaviors were based on laboratory observations and included turbulence-induced diving, turbulence-induced passive sinking, and neutral buoyancy. Profiles of currents and turbulence were based on velocity profiles measured in coastal Virginia over four different substrates: natural oyster reefs, mud, and deposited oyster and whelk shell. Settlement probabilities were higher on larger patches, whereas average settler densities were higher on smaller patches. Larvae settled most successfully and had the smallest optimal patch length when diving over rough substrates in shallow water. Water depth was the greatest source of variability, followed by larval behavior, substrate roughness, and tidal current speed. This result suggests that the best way to maximize settlement on restored reefs is to construct patches of optimal length for the water depth, whereas substrate type is less important than expected. Although physical patch characteristics are easy to measure, uncertainty about larval behavior remains an obstacle for predicting settlement patterns. The mechanistic approach presented here could be combined with a spatially explicit metapopulation model to optimize the arrangement of reef patches in an estuary or region for greater sustainability of restored habitats.     

Tidal geomorphology affects phytoplankton at the transition from forested streams to tidal rivers

1. Coastal rivers can have long tidally influenced reaches that are affected by tides but do not contain saline water. These tidal freshwater reaches have steep geomorphic gradients where the river transitions from narrow, heavily shaded streams to wide, unshaded channels. The influence of these gradients on river ecosystem production is poorly understood. 2. We characterised gradients in irradiance, geomorphology, water clarity and chlorophyll a along 9‐ to 16‐km tidal freshwater reaches of the Newport and White Oak Rivers in North Carolina, USA, and examined the effect of nutrient enrichment on phytoplankton growth in the Newport River. Underwater irradiance was modelled at 2–4 week intervals along both rivers using measurements of the above‐canopy irradiance, canopy cover, water column light attenuation (K_d) and water depth. Suspended material (TSS), dissolved organic carbon (DOC) and chlorophyll a were sampled at 2‐week interval at five sites on the Newport River and on four dates at four sites on the White Oak River over the course of one year. 3. Phytoplankton nutrient limitation was assessed at three locations along the tidal gradient. River water was collected during March, April, June and October, and incubated in 10‐L plastic outdoor containers under ambient water temperature and sunlight. Additions of inorganic nitrogen and phosphorus served as treatments; growth rate during the 4 days of incubation was calculated from the change in chlorophyll a concentration over time. 4. Canopy cover decreased from more than 90% to <10% over the length of both tidal freshwater rivers. Water column irradiance and phytoplankton biomass increased as tree canopy cover decreased and channel width increased. Channel width exceeded predictions for non‐tidal rivers by threefold because of tidal influence. TSS and DOC decreased significantly along the length of the Newport River, but no significant gradients were observed in the White Oak River. K_d did not vary along the tidal gradient of either river. 5. Mesocosm experiments indicated that inorganic nitrogen and phosphorus enhanced the growth of phytoplankton advected from the non‐tidal river into the tidal freshwater river during spring and summer. Phytoplankton in the tidal freshwater reach were generally not nutrient limited. 6. Tidal hydrology (in the absence of saltwater) directly affected the morphology of the channel and indirectly affected biological growth and production. The significant increase in river width, irradiance and phytoplankton biomass distinguished these tidal freshwater ecosystems from their upstream (non‐tidal fluvial) counterparts, while the strong influence of riparian shading distinguished them from the saline estuaries downstream. Future development of ecosystem and biogeochemical models for tidal freshwater rivers will benefit from the linkages between geomorphology and biological processes identified here.     

Changes in gastropod assemblages in freshwater habitats in the vicinity of Basel (Switzerland) over 87 years

Net pen fish farms generally enrich the surrounding waters and the underlying sediments with nutrients and organic matter, and these loadings can cause a variety of environmental problems, such as algal blooms and sediment anoxia. In this study we test the potential of biofiltration by artificial reefs for reducing the negative environmental impacts surrounding fish farms in the Gulf of Aqaba, Red Sea. Two triangular-shaped artificial reefs (reef volume 8.2 m³) constructed from porous durable polyethylene were deployed at 20 m; one below a commercial fish farm and the other 500 m west of this farm in order to monitor the colonization of these reefs by the local fauna and to determine whether the reef community can remove fish farm effluents from the water. Both reefs became rapidly colonized by a wide variety of organisms with potential for the removal of compounds released from the farms. Within the first year of this study fish abundances and the number of species reached 518–1185 individuals per reef and 25–42 species per reef. Moreover, numerous benthic algae; small sessile invertebrates (bryozoa, tunicates, bivalves, polychaetes, sponges, anemones) and large motile macrofauna (crustaceans, sea urchins, gastropods) settled on the reef surfaces. Depletion of chlorophyll a was measured in the water traversing the artificial reefs in order to assess the biofiltration capacity of the associated fauna. Chlorophyll a was significantly reduced to a level 15–35% lower than ambient concentrations. This reduction was greatest at intermediate current speeds (3–10 cm s^–1), but was not influenced by current direction. The reef structures served as a successful base for colonization by natural fauna and flora, thereby boosting the local benthic biodiversity, and also served as effective biofilters of phytoplankton.     

Energy flow to two abundant consumers in a subtropical oyster reef food web

Oyster reefs are among the most threatened coastal habitat types, but still provide critical habitat and food resources for many estuarine species. The structure of oyster reef food webs is an important framework from which to examine the role of these reefs in supporting high densities of associated fishes. We identified major trophic pathways to two abundant consumers, gray snapper (Lutjanus griseus) and crested goby (Lophogobius cyprinoides), from a subtropical oyster reef using stomach content and stable isotope analysis. The diet of gray snapper was dominated by crabs, with shrimp and fishes also important. Juvenile gray snapper fed almost entirely on oyster reef-associated prey items, while subadults fed on both oyster reef- and mangrove-associated prey. Based on trophic guilds of the gray snapper prey, as well as relative δ¹³C values, microphytobenthos is the most likely basal resource pool supporting gray snapper production on oyster reefs. Crested goby had omnivorous diets dominated by bivalves, small crabs, detritus, and algae, and thus were able to take advantage of prey relying on production from sestonic, as well as microphytobenthos, source pools. In this way, crested goby represent a critical link of sestonic production to higher trophic levels. These results highlight major trophic pathways supporting secondary production in oyster reef habitat, thereby elucidating the feeding relationships that render oyster reef critical habitat for many ecologically and economically important fish species.     

Runoff from tomato cultivation in the estuarine environment: biological effects of farm management practices

The use of plastic row covers (plastic mulch) on vegetable farms increases runoff of pesticides after rainfall events and has been linked to toxic events in adjacent tidal waters. In coastal Virginia, USA, runoff from tomato fields with plastic mulch was suspected of causing mortality of commercial hard clam larvae at a hatchery located downstream of farming operations. Concern about the putative impacts of this practice on local waters resulted in a collection of studies to: (1) determine the sensitivity of early life stages of bivalves to copper, a commonly used fungicide; (2) examine acute and chronic biological effects of runoff on tidal creeks; and (3) examine the efficacy of management practices designed to reduce the delivery of pesticides to adjacent creeks. Laboratory bioassays revealed that 48-h LC₅₀ values for embryonic clams Mulinia lateralis and Mercenaria mercenaria were 38 and 20 μg/l, respectively. In situ bioassays with Palaemonetes pugio showed that pulsed toxic conditions sometimes occur downstream of some tomato farms in plastic mulch following rainfall events. Growth, mortality rates and bioaccumulation of copper and organic pesticides in oysters were not correlated with the use of plastic mulch in watersheds. Sediment bioassays indicated potential toxicity in sediment collected downstream of some tomato fields in plastic mulch, but the effects were not consistent between years. Closer examination of management practices on the farms suggests that controlling runoff can prevent toxic impacts. Elevated levels of crop protectants measured at the outflow of farm ponds suggested that they may do little to reduce loadings of some pesticides. However, forested buffer zones and ephemeral sedimentation basins appeared to be effective in reducing pesticide concentrations in runoff and pulsed toxicity in tidal creeks.     

Variability in Chlorophyll and Phytoplankton Composition in an Estuarine System

The spatial and temporal variability of phytoplankton abundance (in terms of chlorophyll a and cell number), inorganic nitrogen, suspended particulate matter (SPM), and light availability was determined throughout one year in the Tagus estuary, Portugal. Chlorophyll a concentrations showed a strong seasonal variation with values ranging from 1 to 32 μg l^?1(average 5.4 μg l^?1). Chlorophyll patterns were unimodal for sites 1, 2, and 3 and bimodal for site 4. Diatoms and cryptophytes were, throughout the year, the dominant groups in this shallow and unstratified estuarine system. Nitrate concentrations were seasonally related to river flow and ammonium concentrations spatially related to sources of sewage input. Lower river inputs and long water residence times during summer initially promoted the accumulation of phytoplankton, but the resulting low dissolved inorganic nitrogen (DIN) concentrations lead to limitation of phytoplankton growth. Chlorophyll a and DIN values obtained in the present study were comparable to those reported 20 years ago for the Tagus estuary.     

牡蛎礁生境：海岸带可持续发展的潜在碳汇

牡蛎礁生境是指由聚集的牡蛎和其他生物及环境堆积形成的复合生态系统,其固碳和储碳潜力巨大,在海岸带生态系统中发挥着重要的作用。然而,目前对牡蛎礁生境碳源与汇的认识仍存在不足,主要在于牡蛎钙化和呼吸作用都释放CO₂,而碳源与汇的评估忽视了钙化、同化和沉积过程带来的整体碳汇价值及牡蛎礁生态系统功能带来的碳汇效应。因此,有必要重新认识牡蛎礁生境的碳汇价值。一方面,牡蛎礁生境的碳源和碳汇需要从牡蛎礁自身的整体碳循环中进行评估,包括牡蛎礁系统中的沉积、钙化、呼吸作用及侵蚀、再悬浮和再矿化作用; 另一方面,牡蛎礁生态系统服务引起的碳汇效应需从牡蛎礁的生态系统服务价值角度进行评估,将生态系统服务价值及碳价值进行关联,从而纳入碳汇核算体系。从实现海岸带可持续增汇角度出发,综述了牡蛎礁生境中碳的源与汇;阐述了容易被人们忽视的微生物在牡蛎礁生境碳汇中的作用;以保护和生态修复为目的,进一步提出可实现牡蛎礁生境最大潜在碳汇的策略,以期为实现海洋负排放及践行"国家双碳战略"提供理论和技术支撑。     

Oyster Reef Restoration in the Northern Gulf of Mexico: Effect of Artificial Substrate and Age on Nekton and Benthic Macroinvertebrate Assemblage Use

In the northern Gulf of Mexico (GOM), reefs built by eastern oysters, Crassostrea virginica, provide critical habitat within shallow estuaries, and recent efforts have focused on restoring reefs to benefit nekton and benthic macroinvertebrates. We compared nekton and benthic macroinvertebrate assemblages at historic, newly created (<5 years) and old (>6 years) shell and rock substrate reefs. Using crab traps, gill‐nets, otter trawls, cast nets, and benthic macroinvertebrate collectors, 20 shallow reefs (<5 m) in the northern GOM were sampled throughout the summer of 2011. We compared nekton and benthic assemblage abundance, diversity and composition across reef types. Except for benthic macroinvertebrate abundance, which was significantly higher on old rock reefs as compared to historic reefs, all reefs were similar to historic reefs, suggesting created reefs provide similar support of nekton and benthic assemblages as historic reefs. To determine refuge value of oyster structure for benthic macroinvertebrates compared to bare bottom, we tested preferences of juvenile crabs across depth and refuge complexity in the presence and absence of adult blue crabs (Callinectes sapidus). Juveniles were more likely to use deep water with predators present only when provided oyster structure. Provision of structural material to support and sustain development of benthic and mobile reef communities may be the most important factor in determining reef value to these assemblages, with biophysical characteristics related to reef location influencing assemblage patterns in areas with structure; if so, appropriately locating created reefs is critical.     

Quantifying Climatological Ranges and Anomalies for Pacific Coral Reef Ecosystems

Coral reef ecosystems are exposed to a range of environmental forcings that vary on daily to decadal time scales and across spatial scales spanning from reefs to archipelagos. Environmental variability is a major determinant of reef ecosystem structure and function, including coral reef extent and growth rates, and the abundance, diversity, and morphology of reef organisms. Proper characterization of environmental forcings on coral reef ecosystems is critical if we are to understand the dynamics and implications of abiotic–biotic interactions on reef ecosystems. This study combines high-resolution bathymetric information with remotely sensed sea surface temperature, chlorophyll-a and irradiance data, and modeled wave data to quantify environmental forcings on coral reefs. We present a methodological approach to develop spatially constrained, island- and atoll-scale metrics that quantify climatological range limits and anomalous environmental forcings across U.S. Pacific coral reef ecosystems. Our results indicate considerable spatial heterogeneity in climatological ranges and anomalies across 41 islands and atolls, with emergent spatial patterns specific to each environmental forcing. For example, wave energy was greatest at northern latitudes and generally decreased with latitude. In contrast, chlorophyll-a was greatest at reef ecosystems proximate to the equator and northern-most locations, showing little synchrony with latitude. In addition, we find that the reef ecosystems with the highest chlorophyll-a concentrations; Jarvis, Howland, Baker, Palmyra and Kingman are each uninhabited and are characterized by high hard coral cover and large numbers of predatory fishes. Finally, we find that scaling environmental data to the spatial footprint of individual islands and atolls is more likely to capture local environmental forcings, as chlorophyll-a concentrations decreased at relatively short distances (>7 km) from 85% of our study locations. These metrics will help identify reef ecosystems most exposed to environmental stress as well as systems that may be more resistant or resilient to future climate change.     

Linkages between tidal creek ecosystems and the landscape and demographic attributes of their watersheds

Twenty-three headwater tidal creeks draining watersheds representative of forested, suburban, urban, and industrial land cover were sampled along the South Carolina coast from 1994 to 2002 to: (1) evaluate the degree to which impervious land cover is an integrative watershed-scale indicator of stress; (2) synthesize and integrate the available data on linkages between land cover and tidal creek environmental quality into a conceptual model of the responses of tidal creeks to human development; and (3) use the model to develop recommendations for conserving and restoring tidal creek ecosystems. The following parameters were evaluated: human population density, land use, impervious cover, creek physical characteristics, water quality, sediment chemical contamination and grain size characteristics, benthic chlorophyll a levels, porewater ammonia concentration, fecal coliform concentration, and macrobenthic and nekton population and community characteristics.The conceptual model was developed and used to identify the linkages among watershed-scale stressors, physical and chemical exposures, and biological responses of tidal creeks to human development at the watershed scale. This model provides a visual representation of the manner in which human population growth is linked to changes in the physiochemical environment and ultimately the nursery habitat function of tidal creeks and the safety of seafood harvested from headwater tidal creeks. The ultimate stressor on the tidal creek ecosystem is the human population density in the watershed and associated increases in the amount of impervious land cover. Measurable adverse changes in the physical and chemical environment were observed when the impervious cover exceeded 10-20% including altered hydrography, changes in salinity variance, altered sediment characteristics, increased chemical contaminants, and increased fecal coliform loadings. Living resources responded when impervious cover exceeded 20-30%. The impacts on the living resources included reduced abundance of stress-sensitive macrobenthic taxa, reduced abundance of commercially and recreationally important shrimp, and altered food webs. Headwater tidal creeks appear to provide early warning of ensuing harm to larger tidal creeks, tidal rivers and estuaries, and the amount of impervious cover in a watershed appears to be an integrative measure of the adverse human alterations of the landscape. Through education and community involvement, a conservation ethic may be fostered that encourages the permanent protection of lands for the services they provide.