Adjacent Seagrass Meadows Research Articles

The impact of a Reno mattress installation on an adjacent seagrass meadow and its macrofauna in a South African estuary

Armouring of coastlines and the banks of estuaries to protect property against erosion can have a negative impact on natural habitats and their communities. Within estuaries, one of the most threatened habitats is seagrass meadows. The impact of newly installed Reno mattresses on an adjacent Zostera capensis meadow and associated fauna was investigated in the Keurbooms Estuary, South Africa. The impact on Z. capensis was studied by determining percentage cover and the length of blades before and after placement of Reno mattresses. At the impact site, percentage cover and blade length decreased during the installation phase, however, recovered shortly after installation was completed. At the control site no changes in eelgrass percentage cover were noted. Invertebrate and fish species richness, diversity and abundance were compared in the eelgrass meadows before and after the installation. Whilst invertebrate species richness increased significantly after the installation at the Impacted site, there was no significant difference at the Control site. There was also no significant difference in invertebrate diversity and fish species richness and diversity at the Control and Impacted sites before and after installation. This suggests that the installation had no net negative impact at the Impacted site. Within a month of installation, the Reno mattresses were colonised by two invertebrate species (Amphibalanus amphitrite and Littoraria sp.). Although the introduction of Reno mattresses at the study site created a hybrid habitat that continued to provide a habitat for fish and invertebrates, we suggest that the use of these artificial structures should be in combination with natural vegetation (e.g. as a hybrid habitat) and not one that replaces natural habitat. There is limited information regarding the ecological impacts of using Reno mattresses in estuaries and this study provides new information on their ecological efficacy thus providing valuable insights in future coastal erosion control practices.

Prioritizing Conservation of Coastal Ponds for Wintering Redheads

ABSTRACTThe Laguna Madres of Texas, USA, and Tamaulipas, Mexico, are the most important wintering areas for redheads (Aythya americana) as most of the continental population winters in these lagoons. Redheads forage in the saline waters of the Laguna Madre and make daily flights to coastal freshwater ponds on the adjacent mainland to drink. The abundance and spatial distribution of coastal ponds varies depending on precipitation and can influence the foraging pressure on adjacent seagrass meadows. We conducted weekly aerial surveys to monitor coastal pond use by wintering redheads from mid‐October through mid‐March along the entire length of the Laguna Madre of Texas, during 2000–2003 and in 2012–2014. We developed 3 parameters to provide a measure of biological value of each coastal pond to redheads: amount of foraging habitat within 10 km of each pond, water permanence of the pond, and the potential to distribute redheads if inundated. During 101 aerial surveys across 5 years of study, we identified 140 coastal ponds that were used by redheads. We developed a prioritization scheme to identify wetlands that remain inundated in all years and targeted them for conservation protection. We identified those coastal ponds that, if enhanced through increasing their water permanence, would provide additional drinking sites during dry years and help distribute redheads on more foraging habitat, thereby reducing potential overgrazing on seagrass meadows. We identified 3,624 ha of foraging habitat (21.5% of all foraging habitat) in the lower Laguna Madre that had no coastal ponds within a 10‐km radius and, thus, was proximal to potential areas for coastal pond creation. Our results provide guidance for resource managers to protect, enhance, or create coastal ponds to reduce foraging pressure on seagrass meadows in the Laguna Madre and help sustain future populations of wintering redheads. © 2021 The Wildlife Society.

How Does Mangrove Expansion Affect Structure and Function of Adjacent Seagrass Meadows?

Temperatures are increasing globally and causing species-specific geographic range expansions. In the Gulf of Mexico, mangroves are encroaching regions historically dominated by temperate salt marshes, changing animal communities and nutrient cycling in the intertidal zone. Marine systems are highly connected; therefore, we expect that changes in the intertidal will alter functions of adjacent subtidal seagrass meadows. We surveyed seagrass meadows adjacent to mangroves, salt marshes, and a mixture of the two and asked, do changes in intertidal plant composition influence (1) environmental conditions (subtidal water and sediment characteristics); (2) biogeochemical cycling (net oxygen and nitrogen gas fluxes); (3) seagrass meadow cover, biomass, and productivity; and (4) invertebrate community assemblage? There are clear differences in sediment organic matter and net nitrogen gas (N2) fluxes between adjacent intertidal habitats, but the magnitude or direction of change differs seasonally. We hypothesize that this seasonal pattern is due to outwelling from the intertidal, as mangroves senesce in fall, and marshes senesce later in winter. Therefore, changes in adjacent intertidal habitat can impact the timing of organic matter delivery. This also has implications for seagrass biomass. Thalassia testudinum belowground biomass adjacent to mangroves substantially decreased over the winter, suggesting vulnerability to stressors as the intertidal plant community shifts from marsh to mangrove dominance. Epifauna density and diversity did not vary among seagrass meadows based on adjacent intertidal habitats, but subtle differences in community assemblages associated with shifts in intertidal plant community were detected. This work demonstrates that impacts of species range expansions are far-reaching due to connectivity in marine systems.

Pathways to Spatial Subsidies by Kelp in Seagrass Meadows

Spatial subsidies often occur from highly productive donor ecosystems to recipient systems with low productivity, but little is known about subsidies between highly productive ecosystems. Here, we investigated whether the kelp Ecklonia radiata, which is frequently dislodged from reefs, provides spatial subsidies in adjacent seagrass meadow where in situ productivity is already high. We clarified the pathways of the allochthonous kelp as a spatial subsidy by adding 15N-enriched kelp to Posidonia sinuosa seagrass meadows, tracing the uptake of kelp-derived nitrogen by seagrass epiphytes and epifauna. We observed a 32.5–52.2% loss of kelp biomass (wet weight), representing almost 13,000 μg of 15N from the labelled kelp. This corresponded with an increase in δ15N and 15N biomass of detritivores and epiphytes, showing either a direct or indirect (via epiphytes) assimilation of kelp-derived nutrients. The densities and biomass of the gastropod Strigosella lepidus and shrimps were higher in seagrass plots with input of E. radiata, but not for amphipods or the biomass of epiphytes. We estimate that the detritivores and epiphytes in the seagrass plots assimilated about 3% of the 15N released from the labelled kelp, and based on a data from previous study, the seagrass itself could have assimilated up to a further 6% of the released kelp 15N. Our findings show that in a highly productive seagrass ecosystem, where autochthonous food sources are available, the input of allochthonous resources such as kelp can supplement those resources and appears to play an important a role as a vector for a spatial subsidy.

Microbial and nutrient dynamics in mangrove, reef, and seagrass waters over tidal and diurnal time scales

In coral reefs and adjacent seagrass meadow and mangrove environments, short temporal scales (i.e. tidal, diurnal) may have important influences on ecosystem processes and community structure, but these scales are rarely investigated. This study examines how tidal and diurnal forcings influence pelagic microorganisms and nutrient dynamics in 3 important and adjacent coastal biomes: mangroves, coral reefs, and seagrass meadows. We sampled for microbial (Bacteria and Archaea) community composition, cell abundances and environmental parameters at 9 coastal sites on St. John, US Virgin Islands that spanned 4 km in distance (4 coral reefs, 2 seagrass meadows and 3 mangrove locations within 2 larger bays). Eight samplings occurred over a 48 h period, capturing day and night microbial dynamics over 2 tidal cycles. The seagrass and reef biomes exhibited relatively consistent environmental conditions and microbial community structure but were dominated by shifts in picocyanobacterial abundances that were most likely attributed to diel dynamics. In contrast, mangrove ecosystems exhibited substantial daily shifts in environmental parameters, heterotrophic cell abundances and microbial community structure that were consistent with the tidal cycle. Differential abundance analysis of mangrove-associated microorganisms revealed enrichment of pelagic oligotrophic taxa during high tide and enrichment of putative sediment-associated microbes during low tide. Our study underpins the importance of tidal and diurnal time scales in structuring coastal microbial and nutrient dynamics, with diel and tidal cycles contributing to a highly dynamic microbial environment in mangroves, and time of day likely contributing to microbial dynamics in seagrass and reef biomes.

Stronger Together: Do Coral Reefs Enhance Seagrass Meadows “Blue Carbon” Potential?

Seagrass meadows are important for carbon storage, this carbon is known as “blue carbon” and represents a vital ecosystem service. Recently there has been growing interest in connectivity between ecosystems and the potential for connected ecosystems to facilitative ecosystem services. Tropical seagrass meadows are connected to coral reefs, as the reef barrier dissipates waves, which facilitates sediment accumulation and avoid erosion and export. Therefore, coral reefs might enhance the seagrass meadows capacity as a blue carbon sink. We tested this hypothesis through an assessment of blue carbon across a gradient of connected seagrass meadow and coral reef sites. We assessed attributes of seagrass meadows along a transect in addition to classifying the sites as exposed and sheltered. Classification of sites was completed through analyzing wave crest density in photographs and using granulometric evenness index. Organic carbon and organic matter were measured in sediment core samples and within seagrass living biomass (both above and below ground). Lastly, we measured changes in above and below ground traits of seagrass plants across the same sites. Gaps in the reef barrier were linked to high wave disturbance and exposed conditions, whilst barrier continuity to low wave disturbance and sheltered conditions. Organic carbon in sediments was 144 Mg ha-1 in the most sheltered (with reef barrier) and 91 Mg ha-1 in the most exposed (without reef barrier) meadows. Sheltered conditions also showed a redistribution of seagrass biomass to a greater quantity of roots compared to rhizomes. Whilst in exposed conditions the opposite occurred, which could be due to increased rhizome biomass have to enhanced anchorage or greater nutrient availability. This study found that coral reefs facilitate blue carbon potential in seagrass meadows indicating that coral reefs support this important ecosystem service. Also, results suggest that loss of coral reef structure due to bleaching and other stressors will likely result in a reduction of the blue carbon storage capacity of adjacent seagrass meadow. Further research should investigate how combined global and regional stresses may impact on the potential for coral reefs to buffer seagrass meadows, and how these stresses affect the functional traits of seagrass plants.

Decadal trends in mangrove and pond aquaculture cover on Hainan (China) since 1966: mangrove loss, fragmentation and associated biogeochemical changes

Mangrove forests suffer from large-scale conversion into pond aquaculture worldwide. However, rarely can the detailed development of these changes and the consequences for coastal biogeochemistry be traced back to baseline conditions. We analyzed decadal changes in mangrove forest and aquaculture pond cover of five estuaries along the east coast of Hainan, northern South China Sea, using aerial photos and satellite images from 1966 to 2009. In addition, we reconstructed historical changes in the biogeochemistry by analyzing three sediment cores from the largest remaining mangrove area in east Hainan (Wenchang/Wenjiao Estuary, WWE). Overall mangrove loss was 72% (from 3697 ha in 1966 to 1041 ha in 2009), ranging from 63% in WWE to virtually 100% loss in Qingge. Land cover of aquaculture ponds in the five estuaries increased from 550 ha in 1966 to 3944 ha in 2009. 55% of the former mangrove area was directly replaced by aquaculture ponds accounting for 76% of the mangrove loss. An increase in the number of individual mangrove area patches from 230 larger to 2134 smaller patches indicates severe fragmentation of the remaining mangrove areas, likely with adverse consequences for ecosystem functioning. The sediment cores from the WWE show that the primary organic matter source changed from mangrove- to aquaculture-derived suspended matter since the 1980s. Moreover, the land cover change likely increased the nutrient export from land due to loss of the mangrove filter and creation of a significant nutrient source by pond effluents with negative impact for adjacent seagrass meadows and coral reefs. This is one of the longest time series documenting massive mangrove decline in recent decades. It highlights the strong and persistent ecological and biogeochemical changes associated with mangrove conversion in tropical estuaries, negatively affecting ecosystem services provided by undisturbed mangrove forests. The speed and magnitude of land conversion and the biogeochemical consequences for adjacent coastal waters observed in Hainan serve as an extreme example of similar activities in Southeast Asia.

Sediment Resuspension and Deposition on Seagrass Leaves Impedes Internal Plant Aeration and Promotes Phytotoxic H2S Intrusion.

HIGHLIGHTS: Sedimentation of fine sediment particles onto seagrass leaves severely hampers the plants' performance in both light and darkness, due to inadequate internal plant aeration and intrusion of phytotoxic H2S.Anthropogenic activities leading to sediment re-suspension can have adverse effects on adjacent seagrass meadows, owing to reduced light availability and the settling of suspended particles onto seagrass leaves potentially impeding gas exchange with the surrounding water. We used microsensors to determine O2 fluxes and diffusive boundary layer (DBL) thickness on leaves of the seagrass Zostera muelleri with and without fine sediment particles, and combined these laboratory measurements with in situ microsensor measurements of tissue O2 and H2S concentrations. Net photosynthesis rates in leaves with fine sediment particles were down to ~20% of controls without particles, and the compensation photon irradiance increased from a span of 20–53 to 109–145 μmol photons m−2 s−1. An ~2.5-fold thicker DBL around leaves with fine sediment particles impeded O2 influx into the leaves during darkness. In situ leaf meristematic O2 concentrations of plants exposed to fine sediment particles were lower than in control plants and exhibited long time periods of complete meristematic anoxia during night-time. Insufficient internal aeration resulted in H2S intrusion into the leaf meristematic tissues when exposed to sediment resuspension even at relatively high night-time water-column O2 concentrations. Fine sediment particles that settle on seagrass leaves thus negatively affect internal tissue aeration and thereby the plants' resilience against H2S intrusion.

Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes, and invertebrates.

Plants are important in urban environments for removing pathogens and improving water quality. Seagrass meadows are the most widespread coastal ecosystem on the planet. Although these plants are known to be associated with natural biocide production, they have not been evaluated for their ability to remove microbiological contamination. Using amplicon sequencing of the 16S ribosomal RNA gene, we found that when seagrass meadows are present, there was a 50% reduction in the relative abundance of potential bacterial pathogens capable of causing disease in humans and marine organisms. Moreover, field surveys of more than 8000 reef-building corals located adjacent to seagrass meadows showed twofold reductions in disease levels compared to corals at paired sites without adjacent seagrass meadows. These results highlight the importance of seagrass ecosystems to the health of humans and other organisms.

Mangroves as a major source of soil carbon storage in adjacent seagrass meadows

Mangrove forests have the potential to export carbon to adjacent ecosystems but whether mangrove-derived organic carbon (OC) would enhance the soil OC storage in seagrass meadows adjacent to mangroves is unclear. In this study we examine the potential for the contribution of mangrove OC to seagrass soils on the coast of North Sulawesi, Indonesia. We found that seagrass meadows adjacent to mangroves had significantly higher soil OC concentrations, soil OC with lower δ 13C, and lower bulk density than those at the non-mangrove adjacent meadows. Soil OC storage to 30 cm depth ranged from 3.21 to 6.82 kg C m−2, and was also significantly higher at the mangrove adjacent meadows than those non-adjacent meadows. δ13C analyses revealed that mangrove OC contributed 34 to 83% to soil OC at the mangrove adjacent meadows. The δ13C value of seagrass plants was also different between the seagrasses adjacent to mangroves and those which were not, with lower values measured at the seagrasses adjacent to mangroves. Moreover, we found significant spatial variation in both soil OC concentration and storage, with values decreasing toward sea, and the contribution of mangrove-derived carbon also reduced with distance from the forest.

Typhoon-induced precipitation impact on nutrient and suspended matter dynamics of a tropical estuary affected by human activities in Hainan, China

Typhoons regularly hit the coasts along the northern South China Sea during summer monsoon. However, little is known on the effects of typhoon-related heavy precipitation on estuarine dynamics and coastal ecosystems. We analyzed physico-chemical characteristics, and concentrations and composition of dissolved and suspended matter in the Wenchang/Wenjiao Estuary (WWE) on the tropical island of Hainan, China, prior to and after typhoon Kammuri in August 2008. Before the typhoon, the estuary displayed vertical and horizontal gradients. High nutrient inputs from agriculture and widespread aquaculture were to a large extent converted into biomass inside the estuarine lagoon resulting in low export of nutrients to coastal waters and a mainly autochthonous origin of total suspended matter (TSM). Heavy typhoon-associated precipitation increased river runoff, which moved the location of the estuarine salinity gradient seaward. It resulted in an export of dissolved and particulate matter to coastal waters one day after the typhoon. Dissolved nutrients increased by up to an order of magnitude and TSM increased approximately twofold compared to pre-typhoon values. Lower δ 13C org and δ 15N and elevated C/N ratios of TSM together with lower chlorophyll a (chl a) concentrations indicated an increased contribution of terrestrial material originating from typhoon-induced soil erosion. Local uptake of excess nutrients inside the lagoon was inhibited because of reduced water transparency and the lack of phytoplankton, which had been washed out by the initial freshwater pulse. Two weeks after the typhoon, TSM concentration and composition had almost returned to pre-typhoon conditions. However, physico-chemical properties and nutrients were still different from pre-typhoon conditions indicating that the estuarine system had not fully recovered. Unusually high chl a concentrations in the coastal zone indicated a phytoplankton bloom resulting from the typhoon-induced nutrient export. The typhoon-induced flushing of the WWE resulted in hyposalinity, reduced water transparency, siltation, as well as temporary eutrophication of coastal waters. These are physiological stressors, which are known to impair the performance of adjacent seagrass meadows and coral reefs. The predicted increase in typhoon frequency and intensity will lead to a frequently recurring exposure of coastal ecosystems to these threats, particularly in the South China Sea region where aquaculture is widespread and tropical cyclone frequency is at a maximum.

Trophic coupling between two adjacent benthic food webs within a man-made intertidal area: A stable isotopes evidence

This study aimed at establishing the effects of human-made physical modifications on the trophic structure and functioning of an intertidal benthic food web in Arcachon Bay (France). The main food sources and the most representative consumers were sampled on an artificial rocky dyke and its adjacent seagrass meadow. The food sources of consumers were inferred through the use of carbon and nitrogen stable isotopes. The contributions of the different food sources to the diets of the consumers were established using the Isosource mixing model. In order to reduce the range of feasible contributions, additional non-isotopic constraints were added when necessary to the outputs of this model. We observed a more complex food web than previously shown for artificial habitats. Moreover, it appears that several consumers inhabiting the artificial environment base most of their diet on allochtonous eelgrass-derived detritus. In turn, several consumers inhabiting the eelgrass meadow consumed significantly macroalgae-derived material originating from the adjacent artificial rocky area. These results point out that the food webs associated to adjacent habitats can influence each other through the utilisation of exported organic matter.

Reefs as contributors to diversity of epiphytic macroalgae assemblages in seagrass meadows

The links between macroalgae-covered reefs and diversity and biomass of epiphytic algae assemblages in adjacent seagrass meadows were investigated. Algae assemblages were sam- pled in 3 habitats: on-reef, seagrass meadow near-reef ( 3000 m), off the coast of Fremantle, Western Australia. We interpreted differences among these habitats as evidence that proximity to reefs influences epiphytic algae assemblages. Propagules in the water column, epiphytes recruiting onto artificial seagrass units (ASU) and assem- blages on natural Posidonia sinuosa were sampled in each habitat. For propagules, ASU and natural P. sinuosa, there were significant differences in the structure of algae assemblages near and distant from reefs, while assemblages near reefs tended to resemble those found on reefs. Multivariate analysis (ANOSIM) confirmed that propagule assemblages near reefs and on reefs were similar, but those distant from reefs differed from those on reefs. Near-reef habitat recorded the highest total number of algal taxa growing on ASU (53), followed by on-reef (44) and distant-from-reef (37) habi- tats. ANOSIM again confirmed that assemblages at on- and near-reef sites were not significantly dif- ferent, but both differed from those at sites distant from reef. We identified 59 species of algal epi- phytes growing on mature P. sinuosa leaves at near-reef and distant-from-reef sites, with only 19 species being common to both habitats, and multivariate analyses were suggestive of differences in assemblage structure. Biomass was only recorded for the ASU epiphyte assemblages and was great- est in the near-reef habitat (0.63 ± 0.11 g shoot -1 ) followed by the on-reef habitat and the distant- from-reef habitat (0.18 ± 0.04 and 0.11 ± 0.01 g shoot -1 , respectively). Our results demonstrate that diversity and biomass of macroalgae epiphytes in seagrass ecosystems differ between habitats of varying proximity to reefs.

Tidal exchange of macrolitter between a mangrove forest and adjacent seagrass beds (Gazi Bay, Kenya)

In the present study the tidal transport of macrolitter between the mangrove forest in Gazi bay (Kenya) and the adjacent seagrass meadows in the bay was investigated, by deploying large standing nets, which extended over the entire height of the water column, in the transition zone between both ecosystems. In addition, the presence of macrolitter on the floor ofRhizophora mucronata andCeriops tagal stands was studied. The macromaterial (>2 mm) that was collected with the nets consisted of mangrove material (26%, mostly leaf material), seagrass leaves (60%) and macroalgae (14%). Transport was bidirectional, indicating shuttle movements of the litter, driven by the opposite flow direction of flood and ebb tides. Litter from the mangrove species consisted mainly of leaves from species occurring at the outer zone of the forest,i.e., Rhizophora mucronata andSonneratia alba. This finding suggests that the complex spatial structure of the forest hampers outflow of macrolitter from the more inner parts. Consequently, this material remains trapped within the forest. The dominant presence of seagrass litter in the macromaterial transported with the tidal water, and the conspicuous and persistent presence of seagrass litter in the low lying, peripheralR. mucronata plots (but not in the more elevatedC. tagal plots) suggest that the mangrove forest of Gazi bay is the recipient of carbon and nutrients from the seagrass system. It is hypothesized that the element cycling of the inner parts of the mangrove forest proceeds as that of a rather closed system, whereas element cycling in the outer parts has conspicuous reciprocal connections with the adjacent seagrass meadows.

Temporal variability in the demography of the palaemonid prawn Macrobrachium intermedium in two seagrasses

Spatial and temporal variations in the demography of the palaemonid prawn Macrobrachium intermedium were investigated in Port Hacking. New South Wales, Australia. Populations in 2 adjacent seagrass meadows. Zostera capricorni and Posidonia australis, were sampled monthly over 3 yr (1976 to 1978) using a small beam trawl. For most population variables there were no consistent differences between meadows and both meadows showed similarly variable temporal changes. Neither meadow consistently supported larger populations nor recruited more juveniles. In 1976 abundance was greater in Zostera than Poddonia, the opposite occurred in 1977, whilst there was no difference in 1978 Seasonal and annual changes in abundances were basically similar in both populations and repeated in all 3 yr. Abundances peaked between spring and autumn and were smallest in wnte r . In both meadows, abundances were greater in 1976 and 1978 than in 1977. Breeding and recruitment of juveniles occurred throughout the year, although ovigerous females were most abundant in summer/ autumn and juveniles recruited predominantly in late summer/autumn and in spring. At least 2 cohorts of juveniles were produced annually, the spring cohort being the largest. In 1976 and 1978 more juveniles recruited to the Zostera meadow whilst in 1977 more recruited to the Posldonia meadow. There were no consistent differences in the size structures of the 2 populations. During spring, juveniles predominated until summer/autumn when populations consisted of adults and only a few juveniles, after which the numbers of adults declined to virtually disappear by winter. Females grew faster and attained a larger size than males. Most individuals survived at least 15 mo, but longevity was estimated to be up to 2 yr. For most population variables, year-to-year variations within each meadow were as great as differences between the 2 meadows within each year. These variations were most likely due to factors extrinsic to the seagrasses themselves. This study stresses the need for long-term studies to verify the consistency of spatial differences between sites.

Temporal Patterns of Feeding by Blue Crabs (Callinectes sapidus) in a Tidal-Marsh Creek and Adjacent Seagrass Meadow in the Lower Chesapeake Bay

A 24-h study of blue crab feeding periodicity was conducted concurrently in a tidal marsh creek and adjacent seagrass meadow in the lower Chesapeake Bay. Crabs from the grassbed tended to have fuller guts than crabs from the marsh creek. In the grassbed, a weak trend toward nocturnal feeding was observed, with an apparent peak at dusk. During the day, crabs were not easily observed and were assumed to be feeding beneath the eelgrass canopy; at night crabs fed in the canopy. In the marsh creek, feeding was related to the tidal cycle, with guts being fullest at high tide and decreasing to lows just prior to the next high tide. This study suggests the potential importance of habitat on blue crab feeding patterns.

Comparative ecology of nekton residing in a tidal creek and adjacent seagrass meadow, community composition and structure

A structural analysis of the nekton communities occupying a tldal creek and adjacent seagrass meadow at Vaucluse Shores, Virginia (Delmawa Peninsula, USA) is presented along with a comparison of the relative value of each habit to the early life stages of marine and estuarine species. Seagrass meadows were characterized by significantly greater richness and diversity of constituent taxa; both areas, nevertheless, contained mixtures of habitat specialists and wide-ranging (ubiquitous) species that displayed no area1 preferences. Except for a few resident forms, much of the nekton community in the grassbed was comprised of less abundant 'southern' species that entered the Chesapeake Bay in late summer and fall. Reciprocal averaging and numerical classification procedures applied to pooled station collections further indicated the clinal nature of species distributions among habitats, but also clearly demonstrated several microhabitat associations for either the Zostera marina or Ruppia maritima portions of the grassbed. The sciaenid. Leiostomus xanthurus dominated the nekton in both habitats, with > 80 % of all individuals but were nearly 4 times as abundant in the tidal creek. Abundance distribution and length frequency analyses for this species indicated that the marsh is the preferred habitat but also that larger individuals in the population continuously 'bled off' into downstream areas. Two other species of regional importance. Callinectes sapidus and Paralichthys dentatus, also utilized both habitats extensively. Late in summer and early fall, juveniles of both species were more abundant in the grassbed, whereas earlier in the year, they were randomly dispersed. The apparently limited dependence on both the grassbed and tidal creek by the young of local taxa is strikingly different when compared to similar habitats at lower latitudes. In light of the differences established between these habitats and their utilization by different species, an attempt is made to identify potentially important determinants of community structure and relate these to the success of individual populations in both areas. Because few expenmental data are available, our presentation, by necessity, is hypothetical.