Historical Comparison of Fish Community Structure in Lower Chesapeake Bay Seagrass Habitats

Seagrass beds provide important habitat for fishes and invertebrates in many regions around the world. Accordingly, changes in seagrass coverage may affect fish communities and/or populations, given that many species utilize these habitats during vulnerable early life history stages. In lower Chesapeake Bay, seagrass distribution has contracted appreciably over recent decades due to decreased water clarity and increased water temperature; however, effects of changing vegetated habitat on fish community structure have not been well documented. We compared fish community composition data collected at similar seagrass sites from 1976–1977 and 2009–2011 to investigate potential changes in species richness, community composition, and relative abundance within these habitats. While seagrass coverage at the specific study sites did not vary considerably between time periods, contemporary species richness was lower and multivariate analysis showed that assemblages differed between the two datasets. The majority of sampled species were common to both datasets but several species were exclusive to only one dataset. For some species, relative abundances were similar between the two datasets, while for others, there were notable differences without directional uniformity. Spot (Leiostomus xanthurus) and northern pipefish (Syngnathus fuscus) were considerably less abundant in the contemporary dataset, while dusky pipefish (Syngnathus floridae) was more abundant. Observed changes in community structure may be more attributable to higher overall bay water temperature in recent years and other anthropogenic influences than to changes in seagrass coverage at our study sites.     

Changes in the abundance of the seagrassesZostera marina L. (eelgrass) andRuppia maritima L. (widgeongrass) in San Diego, California, following and El Niño Event

Changes in environmental conditions can be accompanied by shifts in the distribution and abundances of organisms. When physical factors become unsuitable for growth ofZostera marina (eelgrass), which is a dominant seagrass species in North America, other more ruderal seagrass species, includingRuppia maritima (widgeongrass), often increase in abundance or replace the dominant species. We report the proliferation of widgeongrass into eelgrass beds in Mission Bay and San Diego Bay in San Diego, California, during the 1997 to 1998 El Niño Southern Oscillation (ENSO). Widgeongrass persisted in these eelgrass beds at least one year after a return to non-ENSO conditions and an increase in eelgrass density. We suggest that a warming of the water in two bays in San Diego by 1.5–2.5°C could result, in a permanent shift in the local seagrass vegetation from eelgrass to widgeongrass. This shift, could, have substantial ecosystem-level ramifications.     

Growth rates of juvenine pinfish (Lagodon rhomboides): Effects of habitat and predation risk

Predation is often the largest source of mortality for juvenile fish and the risk of predation can influence growth rates by either forcing young fish into suboptimal foraging habitats or reducing the amount of time spent foraging. We used field experiments to test effects of predation risk by gulf flounder (Paralichthys albigutta) on juvenile pinfish (Lagodon rhomboides) growth rates by measuring changes in length and weight in three habitats (sand, low density, and high density shoalgrass,Halodule wrightii) in Perdido Key, Florida. Benthic cores, seagrass samples, and stomach contents were also analyzed to examine differences in pinfish prey densities, grass densities and epiphyte coverage, and diet, respectively, among habitat and predator treatments. Both length and weight growth rates were determined and showed similar results. We found that pinfish inhabiting seagrass habitats, particularly low densityHalodule displayed the fastest growth rates in the beginning of the growing season (June) and those in sand had the fastest growth rates later in the season (October). These differences in growth rates did not appear to be influenced by densities of pinfish prey items since the treatment having the highest density of prey was not that in which growth rates were the greatest. This seasonal shift may be attributed to increasing pinfish size. Larger pinfish in October may have been inhibited by high density grass, reducing foraging efficiency. These results demonstrate how occupying a suboptimal foraging habitat can affect juvenile pinfish growth rates. Predation risk significantly reduced length and weight growth rates of pinfish in June, but not October. This suggests that smaller pinfish early in the season traded time spent foraging for predator avoidance, while larger pinfish were likely to have reached a size refuge from predation. This study demonstrates that nonlethal effects from predation are also important influences on juvenile pinfish.     

Tropicalization of the Northern Gulf of Mexico: Impacts of Salt Marsh Transition to Black Mangrove Dominance on Faunal Communities

The tropically associated black mangrove (Avicennia germinans) is expanding into salt marshes of the northern Gulf of Mexico (nGOM). This species has colonized temperate systems dominated by smooth cordgrass (Spartina alterniflora) in Texas, Louisiana, Florida and, most recently, Mississippi. To date, little is known about the habitat value of black mangroves for juvenile fish and invertebrates. Here we compare benthic epifauna, infauna, and nekton use of Spartina-dominated, Avicennia-dominated, and mixed Spartina and black mangrove habitats in two areas with varying densities and ages of black mangroves. Faunal samples and sediment cores were collected monthly from April to October in 2012 and 2013 from Horn Island, MS, and twice yearly in the Chandeleur Islands, LA. Multivariate analysis suggested benthic epifauna communities differed significantly between study location and among habitat types, with a significant interaction between the two fixed factors. Differences in mangrove and marsh community composition were greater at the Chandeleurs than at Horn Island, perhaps because of the distinct mangrove/marsh ecotone and the high density and age of mangroves there. Infaunal abundances were significantly higher at Horn Island, with tanaids acting as the main driver of differences between study locations. We predict that if black mangroves continue to increase in abundance in the northern GOM, estuarine faunal community composition could shift substantially because black mangroves typically colonize shorelines at higher elevations than smooth cordgrass, resulting in habitats of differing complexity and flooding duration.     

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Fishes

The structure of the fish community associated with eelgrass beds in the lower Chesapeake Bay was studied over a 14 month period. A total of 24,182 individuals in 48 species was collected by otter trawl with Leiostomus xanthurus (spot) comprising 63% of the collection, Syngnathus fuscus (northern pipefish) 14%, Anchoa mitchilli (bay anchovy) 9%, and Bairdiella chrysoura (silver perch) 5%. The density and diversity of fishes were higher in vegetated areas compared to unvegetated areas; fishes were more abundant in night collections Fish abundance and species number increased in the spring and early summer as both water temperature and eelgrass biomass increased and decreased in the fall and winter as temperature and eelgrass biomass decreased. Gill netting revealed some of the top predators in the system, especially the sandbar shark, Carcharhinus milberti. The fish community in the Chesapeake Bay was quite different from North Carolina eelgrass fish communities. Most notable was the rarity of the pinfish, Lagodon rhomboides, which may be a very important predator in the structuring of the epifaunal communities.     

Abundance and Distribution of Reef-Associated Fishes Around Small Oil and Gas Platforms in the Northern Gulf of Mexico’s Hypoxic Zone

Oil and gas platforms (platforms) provide high-relief habitat in the northern Gulf of Mexico’s hypoxic zone that are important to associated fishes. Hypoxia develops near the bottom and reef-associated fishes utilize vertical structure in the well-oxygenated waters overlaying hypoxia. A video array was used to profile the water column and to estimate abundances and depth distributions of fishes before, during, and after summer hypoxia at platforms experiencing intense (seaward) and mild hypoxia (shoal). Gray snapper abundance increased at shoal platforms (10× greater after vs. before the hypoxia season), while abundance remained stable at seaward platforms. However, there was no significant relationship between gray snapper abundance and oxygen concentrations. Sheepshead, Atlantic spadefish, blue runner, and Atlantic bumper abundances varied throughout the summer, but there was no significant effect of hypoxia. Occupation of bottom waters by fishes was consistent throughout the study period at shoal platforms, but fishes were rarely observed in the bottom 3 m and congregated in the water immediately above the hypoxic layer when hypoxia was present at seaward platforms. Nevertheless, patterns of fish abundances were not driven by the presence or absence of hypoxia. The vertical dimension of platforms is a unique and key aspect of their ecological value, especially in the hypoxic zone, and should be considered for artificial reef management.     

Regional Correspondence in Habitat Occupancy by Gray Snapper (Lutjanus griseus) in Estuaries of the Southeastern United States

Reef fishes, such as gray snapper, support important recreational and commercial fisheries and use a variety of habitats throughout ontogeny. Gray snapper juveniles may be found in estuarine nursery areas, such as seagrass beds, or mangrove shorelines, while adults are most often found in deep channels and farther offshore, associated with hard-bottom habitats. Juvenile and subadult gray snapper were collected from 1996 through 2009 during long-term fishery-independent monitoring of several estuarine systems along the Gulf and Atlantic coasts of Florida. Indices of abundance and habitat suitability were constructed for gray snapper to determine size-specific relationships between abundance, habitat, and environmental conditions. Juvenile and subadult gray snapper were collected year-round only in the southernmost estuaries but were most common from July through December in all estuaries sampled. In addition to timing of estuarine occupancy, abundance varied with latitude; gray snapper were more frequently collected in warmer, southern estuaries. In general, gray snapper were most abundant in euhaline areas with a high percentage of submerged aquatic vegetation (SAV) and, in most cases, where overhanging shoreline vegetation was also present. Annual abundance varied over the sampling period, with some juvenile peaks in abundance translating to subadult peaks in subsequent years. Although strong correspondence between juvenile and subadult populations was not observed in all systems, long-term, broad-scale habitat selection patterns as described in this study are critical to more effectively assess populations of estuarine-dependent species.     

SAV Communities of Western Biscayne Bay,Miami, Florida,USA: Human and Natural Drivers of Seagrass and Macroalgae Abundance and Distribution Along a Continuous Shoreline

Nearshore benthic habitats of Biscayne Bay fit the prediction of communities at risk due to their location adjacent to a large metropolitan center (Miami) and being influenced by changes in hydrology through the activities of the Comprehensive Everglades Restoration Plan (CERP). We examine whether the proposed programmatic expansion of mesohaline salinities through the introduction of additional fresh water would result in: (1) increases in seagrass cover; (2) expansion in the distribution and cover of Halodule; and (3) a reduction in the dominance of Thalassia, as hypothesized by CERP. Seagrasses were present at 98 % of sites where they covered 23 % of the bottom. Salinity was the only physical variable with a significant relationship to the occurrence of all SAV taxa. Occurrence of Thalassia, Halimeda, and Penicillus increased significantly with increasing salinity, but Halodule, Syringodium, Laurencia, Udotea, Batophora, Caulerpa, and Acetabularia showed a significant negative relationship with salinity. Mesohaline habitats had higher cover of seagrass and Halodule, and reduced dominance by Thalassia. Thus, we expect increases in the extent of mesohaline habitats to achieve the established CERP goals. We also examined the nutrient content of seagrass blades to evaluate whether: (1) nutrient availability is higher in areas close to canal discharges; and (2) tissue nutrient levels are related to seagrass abundance. The low abundance of Thalassia along the shoreline is not only due to its exclusion from low-salinity environments but also by higher nutrient availability that favors Halodule. Percent N and P, and N:P ratios in seagrass tissue suggest that Biscayne Bay receives high N inputs and is P-limited. Thus, increased P availability may facilitate an expansion of Halodule. The data presented suggest that increased mesohaline salinities will increase seagrass abundance and support co-dominance by Halodule and Thalassia as hypothesized, but raise concerns that current high N availability and increases in P may prompt a shift away from seagrass-dominated to algal-dominated communities under scenarios of enhanced fresh water inputs.     

Immigration of larvae of fall/winter spawning marine fishes into a North Carolina estuary

Larval fishes were sampled weekly from late fall to early spring in the Newport River estuary just inside Beaufort Inlet, North Carolina. Quantitative samples were taken during darkness at mid-flood tide with paired 60-cm bongo nets (505-μm mesh). Larvae of 22 species from 15 families were collected. Seventy-seven percent of the species and 97% of the individuals were fishes that had been spawned on the continental shelf and had immigrated to the estuary. In descending order, the five most abundant species, accounting for 90% of the individuals, were spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus), Atlantic menhaden (Brevoortia tyrannus), pinfish (Lagodon rhomboides), and speckled worm ell (Myrophis punctatus). Most species immigrated through-out the winter and into spring, but there were distinct patterns in their temporal abundances.     

Hurricane effects on seagrasses along Alabama's Gulf Coast

In November 2004, we evaluated the effect of Hurricane Ivan on seagrass meadows in Alabama by surveying all coastal locations known to support seagrass prior to Hurricane Ivan's Iandfall in September 2004. We found that 82% of the sites containing seagrass in 2002 still supported seagrass, and that, as in 2002, the most abundant species wasHalodule wrightii (shoalgrass). We also found more sites containingRuppia maritima (widgeongrass) than previously recorded. We confirmed the existence ofThalassia testudinum (turtlegrass) in Little Lagoon, Alabama, whose first record in the state had been noted in 2002. A resurvey of the western half of Alabama's coastal waters in October 2005 after Hurricane Katrina found no loss of seagrass, with all sites that supported seagrass in 2004 still containing seagrass in 2005. There was no major loss of Alabama's seagrass resources due to Hurricanes Ivan or Katrina, even though both category 3 hurricanes severely affected the northern Gulf Coast.     

Diel variation in immigration of fishes and decapod crustaceans to artificial seagrass habitat

Fish and decapod entry into small (1.5 m²) artificial seagrass habitats positioned on an open sand area in a New Jersey estuary was examined to determine if immigration varied between day and night. To encounter the structured habitats, colonizers had to cross an expanse of bare sand, with its presumably higher predation risk. Contrasts in abundance in the artificial seagrass plots between dawn and dusk indicated higher nighttime immigration for four species, including the fishesFundulus heteroclitus andMyoxocephalus aenaeus, and the caridean shrimpsPalaemonetes vulgaris andHippolyte pleuracanthus. Size-frequency distributions of colonizers varied between day and night for two fish species,Menidia menidia andSyngnathus fuscus, with a greater proportion of smaller individuals immigrating to the artificial seagrass at night.Callinectes sapidus also displayed a diel contrast in size distribution but, for this species, proportionately more small individuals colonized the plots during the day. We suggest that diel variability in predation risk and/or diel patterns in motor activity may be responsible for these patterns in immigration.     

Fish and invertebrate assemblages in seagrass, mangrove, saltmarsh, and nonvegetated habitats

Many studies compare utilization of different marine habitats by fish and decapod crustaceans; few compare multiple vegetated habitats, especially using the same sampling equipment. Fish and invertebrates in seagrass, mangrove, saltmarsh, and nonvegetated habitats were sampled during May–August (Austral winter) and December–January (Austral summer) in the Barker Inlet-Port River estuary, South Australia. Sampling was undertaken using pop nets in all habitats and seine nets in seagrass and nonvegetated areas. A total of 7,895 fish and invertebrates spanning 3 classes, 9 orders, and at least 23 families were collected. Only one fish species,Atherinosoma microstoma, was collected in all 4 habitats, 11 species were found in 3 habitats (mangroves, seagrass, and nonvegetated), and 13 species were only caught in seagrass and nonvegetated habitats. Seagrass generally supported the highest numbers of fish and invertebrates and had the greatest species richness. Saltmarsh was at the other extreme with 29 individuals caught from two species. Mangroves and nonvegetated habitats generally had more fish, invertebrates, and species than saltmarsh, but less than seagrass. Analyses of abundances of individual species generally showed an interaction between habitat and month indicating that the same patterns were not found through time in all habitats. All habitats supported distinct assemlages although seagrass and nonvegetated assemblages were similar in some months. The generality of these patterns requires further investigation at other estuaries. Loss of vegetated habitats, particularly seagrass, could result in loss of species richness and abundance, especially for organisms that were not found in other habitats. Although low abundances were found in saltmarsh and mangroves, species may use these habitats for varying reasons, such as spawning, and such use should not be ignored.     

Postsettlement patterns of habitat use by sciaenid fishes in subtropical seagrass meadows

Spatial and temporal patterns of distribution and abundance were examined for postsettlement sciaenids collected from seagrass meadows in the Aransas Estuary, Texas. Overall, 5443 sciaenid larvae and early juveniles were identified from biweekly epibenthic sled collections taken from August 1994 to August 1995. Eight species were present in seagrass meadows, with five accounting for over 99.9% of sciaenids collected: silver perch (Bairdiella chrysoura), spotted seatrout (Cynoscion nebulosus), spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus), and red drum (Sciaenops ocellatus). Settlement to seagrass meadows was partitioned temporally with little overlap among the five species. Postsettlers from inshore spawners (B. chrysoura, C. nebulosus, S. ocellatus) inhabited seagrass meadows during the spring and summer, while individuals from offshore spawners (L. xanthurus, M. undulatus) were present in the late fall and winter. Densities ofB. chrysoura, C. nebulosus, S. ocellatus were highest for small individuals (4–8 mm SL) and these taxa remained in seagrass sites through the early juvenile stage. Conversely,L. xanthurus andM. undulatus maintained longer pelagic periods and generally entered seagrass meadows at larger sizes (10–14 mm SL). Moreover, these taxa were only temporary residents of selected seagrass meadows, apparently migrating to alternative habitats shortly after arrival. During peak settlement, mean and maximum densities among species ranged from 0.1 m^?2 to 0.8 m^?2 and 0.7 m^?2 to 23.8 m^?2, respectively. Density and mean size of possettlement sciaenids differed significantly between seagrass species (Halodule wrightii, Thalassia testudinum) and among sites within the estuary.     

Stable Isotopes Reveal Complex Changes in Trophic Relationships Following Nutrient Addition in a Coastal Marine Ecosystem

Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.     

Impact of habitat edges on density and secondary production of seagrass-associated fauna

Species richness and abundance of seagrass-associated fauna are often positively correlated with seagrass biomass and structure complexity of the habitat. We found that while shoot density and plant biomass were greater in interior portions of turtle grass (Thalassia testudinum) beds than at edges, mean faunal density was significantly greater at edges than interior sites during 1994. This pattern was also observed in 1995, although differences were not significant. The four numerically dominant taxonomic groups showed varying degrees of elevated densitities at edges ofT. testudinum beds. Peracarids and polychaetes had significantly greater densities at edges oft. testudinum beds, while both decapods and gastropods showed dramatic temporal variability in density, with reversals in density between edge and interior occurring during the course of the study. This within-habitat variability in abundance may reflect both active accumulation of fauna at edges and settlement shadows for species with pelagic larvae. Active accumulation of highly mobile taxa seeking refuge in seagrass beds may explain the differences in density between edge and interior ofT. testudinum patches for peracarids in 1994 and in 1995. Active accumulation at edges may also explain differeces in density for some decapod taxa. Chauges in gastropod densities between habitats may reflect larval settlement patterns. Results showed a distinct settlement shadow for the gastropodCaecum nitidum whose densities (primarily second stage protoconch) increased by more than an order of magnitude in 1994. Settlement shadows and post-settlement processes may also explain density differences of polychaetes between the edge and interior ofT. testudinum patches. The differences in faunal densities between edge and interior habitat resulted in habitat specific differences in secondary production among the major taxonomic groups. On four of five dates in 1994 and in 1995, secondary production was greater at edge than interior locations. These unexpected results suggest that differences in faunal densities and secondary production between edges and interiors of seagrass patches represent a potentially vital link in seagrass trophic dynamics. If this elevated secondary production leads to increases in trophic transfer, then edges may serve as a significant trophic conduit to higher-level consumers in this system.     

Monitoring and Modeling of Syringodium filiforme (Manatee Grass) in Southern Indian River Lagoon

Decreased salinity and submarine light associated with hurricanes of 2004?C2005 impacted seagrass habitats in the Florida coastal zone. A combination of salinities ??20 and light attenuation ??1.5?m^?1 resulting from the freshwater discharge in 2005 were among the drivers for a widespread decrease in the coverage and biomass of Syringodium filiforme (manatee grass) in 2006. These observations provided an opportunity to develop and apply a modeling framework to simulate responses of S. filiforme to variable water quality. The framework connects water column variables to field monitoring of seagrass abundance and salinity growth response experiments. The base model was calibrated with macrophyte abundance observed in southern Indian River Lagoon (IRL) from 2002 to 2007 and tested against shoot data from a different time (1997?C2002) and nearby location in the IRL. Model shoot biomass (gC?m^?2) was similar to field observations (r ²?=?0.70) while responding to monthly seasonal fluctuations in salinity and light throughout the 6-year simulations. Field and model results indicated that S. filiforme growth and survival were sensitive to, and increased with, rising salinity throughout 2007. This modeling study emphasizes that discharge, salinity, and submarine light are inter-dependent variables affecting South Florida seagrass habitats on seasonal to inter-annual time scales.     

Seagrass patch characteristics alter direct and indirect interactions in a tritrophic estuarine food web

We used a mesocosm approach to examine how_ratch characteristics influenced predation and habitat selection in a tritrophic food web. Our experiments included juvenile red drum (Sciaenops ocellatus; RD), juvenile pinfish (Lagodon rhomboides; PF), and grass shrimp (Palaemonetes sp.; GS), members of a food web common in seagrass meadows of the northern Gulf of Mexico. We added an additional level of complexity to the experiment by including a predator that could feed at two different trophic levels. RD were top predators, PF were both prey items for RD and predators of GS, and GS were prey for RD and PF. We used 4 different artificial seagrass habitats that varied by size (0.049 and 0.203 m²) and shape (circular and stellate) to control for covariation between patch size and seagrass density. Predation on GS was measured in each habitat when PF, RD, and PF+RD were present, and predation on PF was measured when RD and RD+GS were present. Habitat selection by each of these 3 species was measured individually and in the presence of every other combination of the 3 species. Neither predation nor habitat selection were consistently influenced by patch characteristics (size, shape, or perimeter: area ratios) or the number of trophic levels. For GS, there was a significant negative relationship between patch size and predation rates in the GS+PF+RD treatment. Habitat selection by GS without the threat of predation suggested a preference for smaller habitats, but when in the presences of RD or RD+PF, GS preferred larger habitats. In predation experiments, PF predation by RD showed no significant relationships with patch characteristics or trophic structure. For our habitat selection experiments, PF preference was for larger habitats in the PF only and GS+PF+RD treatments. There were no significant relationships between patch size, shape, or trophic structure and RD habitat selection.     

Nonnative Species in British Columbia Eelgrass Beds Spread via Shellfish Aquaculture and Stay for the Mild Climate

Nonnative species cause economic and ecological impacts in habitats they invade, but there is little information on how they spread and become abundant. This is especially true for nonnative species in native Zostera marina eelgrass beds in coastal British Columbia, Canada, which play a vital role in estuarine ecosystems. We tested how nonnative species richness and abundance were related to both arrival vectors and environmental factors in northeast Pacific eelgrass. Using correlation tests and generalized linear models, we examined how nonnative macroinvertebrates (benthic, epifaunal, and large mobile) and some algae species were related to arrival vectors (shipping and aquaculture) and environmental factors (climate variables, human population density, and native richness and abundance). We found 12 nonnative species, 50 % with known negative impacts within eelgrass habitats. For benthic organisms, both nonnative richness and abundance were strongly correlated with shellfish aquaculture activities, and not with shipping activity. For epifaunal nonnative richness and abundance, neither vector was significantly correlated. Climate (temperature and salinity) helped explain nonnative richness but not abundance; there was no relationship of nonnative richness or abundance to native species richness and abundance or population density. Results suggest that aquaculture activities are responsible for many primary introductions of benthic nonnative species, and that temperature and salinity tolerances are responsible for post-introduction invasion success. While aquaculture and shipping vectors are becoming increasingly regulated to prevent further international spread of nonnative species, it will be important when managing nonnatives to consider secondary spread from intraregional transport through local shellfish aquaculture and shipping.     

Recruitment of Estuarine-Dependent Nekton Through a New Tidal Inlet: the Opening of Packery Channel in Corpus Christi,TX, USA

The US Army Corps of Engineers recently dredged and permanently reopened Packery Channel, historically a natural tidal inlet, to allow water exchange between the Gulf of Mexico and the Laguna Madre, TX, USA. The main objective of this study was to characterize estuarine-dependent recruitment and community structure in seagrass habitats adjacent to Packery Channel pre- and post-channel opening. We sampled fish and crustacean abundance using an epibenthic sled in Halodule wrightii seagrass meadows in both control and impact locations over 2 years, 1 year before the opening of Packery Channel (October 2004–May 2005) and 1 year after (July 2005–April 2006). Using the before–after control–impact design, we found significantly fewer nekton post-channel opening. However, we found significantly higher mean densities of newly settled estuarine-dependent species (Sciaenops ocellatus, Micropogonias undulatus, Lagodon rhomboides, Callinectes sapidus, and penaeid shrimp) post-opening. Multivariate analyses showed significant community assemblage changes post-opening with increased contribution of estuarine-dependent species post-opening. Our results show that estuarine-dependent nekton are using Packery Channel as a means of ingress into areas of the upper Laguna Madre’s seagrass meadows that were previously inaccessible, which may lead to higher fisheries productivity for some of these economically and ecologically important fishery species.     

Effect of Sediment Nutrient Enrichment and Grazing on Turtle Grass <Emphasis Type="Italic">Thalassia testudinum</Emphasis> in Jobos Bay,Puerto Rico

We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5 m plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls.