Tidal Passes Research Articles

Estuarine-coastal connectivity and partial migration of southern flounder in the Gulf of Mexico

Southern flounder (Paralichthys lethostigma) from an estuarine complex in the Gulf of Mexico were tagged with acoustic transmitters (n = 60) from 2016 to 2018 to assess estuarine-coastal connectivity during fall/winter spawning runs. Both egress and residency behaviors were observed for adult southern flounder caught in the Galveston Bay Complex (GBC), and individuals were classified as migrators or residents based on their maximum distance detected (MDD). Migrators (MDD >5 km; mean 13.8 km) displayed directed egress out of the GBC, with individuals moving through the tidal pass in November and December, peaking in mid to late December. In contrast, residents (MDD <5 km; mean 0.6 km) showed limited movements and were often detected in the same general area throughout the fall, winter, and spring, indicating overwintering in the GBC. Conventional tagging was also performed on over 1300 southern flounder and mean MDD for all recaptured fish was 5.7 km. Mean monthly MDD of conventionally tagged fish was also highest in December and linked to egress from the GBC. Although directed migrations into the Gulf were observed for southern flounder with both tagging approaches, a meaningful fraction of the population displayed sedentary tendencies with MDD less than 1–2 km and retention within the GBC. The coexistence of two migratory contingents with contrasting estuarine-coastal migration behaviors is symbolic of partial migration by southern flounder, which may influence the resilience and stability of the population. Given that the timing and magnitude of peak egress out of this estuarine complex did not align with management regulations intended to protect spawning adults (i.e., peak movement after reopening of fishery), our findings also indicate the need to extend future closures to protect migrators during the primary egress period.

Nonstationary streamflow effects on backwater flood management of the Atchafalaya Basin, USA

A non-stationary increasing streamflow trend has been observed on the Mississippi River and other major river basins around the world. The current study analyzed the non-stationary streamflow effects (NSFEs) on flood management in backwater areas adjacent to the Atchafalaya Basin floodway in Louisiana, USA. A continuous simulation hydrology model coupled with a quasi-two-dimensional hydrodynamic model of the basin floodway and surrounding regions was used to develop over 180 simulation scenarios by superimposing local flood events (early summer 2014 and late summer 2016) against 90-years’ worth of daily Atchafalaya River streamflow hydrographs. The NSFE on the Atchafalaya River induced substantial reductions in the performance of major flood regulating structures with seasonal effects based on the annual flood cycle. Capacity reductions at the structures were demonstrated to trigger a cascade of effects in ostensibly protected backwater areas including amplification of erosion potential near the levee and within tidal passes during early summer floods. Increases in mean and peak flood levels on the order of 15–20 cm during local storm events were shown to extend as far as 20 km away from the floodway protection levee during both early and late summer local flooding scenarios. Low-lying areas closest to the levee were adversely affected during both the high (early summer) and low flow (late summer) periods of the annual discharge cycle. The approach and findings of this study are relevant for risk management in river basins around the world affected by NSFEs.

Breakdown of Hydrostatic Assumption in Tidal Channel with Scour Holes

Hydrostatic condition is a common assumption in tidal and subtidal motions in oceans and estuaries.. Theories with this assumption have been largely successful. However, there is no definite criteria separating the hydrostatic from the non-hydrostatic regimes in real applications because real problems often times have multiple scales. With increased refinement of high resolution numerical models encompassing smaller and smaller spatial scales, the need for non-hydrostatic models is increasing. To evaluate the vertical motion over bathymetric changes in tidal channels and assess the validity of the hydrostatic approximation, we conducted observations using a vessel-based acoustic Doppler current profiler (ADCP). Observations were made along a straight channel 18 times over two scour holes of 25 m deep, separated by 330 m, in and out of an otherwise flat 8 m deep tidal pass leading to the Lake Pontchartrain over a time period of 8 hours covering part of the diurnal tidal cycle. Out of the 18 passages over the scour holes, 11 of them showed strong upwelling and downwelling which resulted in the breakdown of hydrostatic condition. The maximum observed vertical velocity was ~ 0.35 m/s, a high value in a tidal channel, and the estimated vertical acceleration reached a high value of 1.76×10-2 m/s2. Analysis demonstrated that the barotropic non-hydrostatic acceleration was dominant. The cause of the non-hydrostatic flow was the that over steep slopes. This demonstrates that in such a system, the bathymetric variation can lead to the breakdown of hydrostatic conditions. Models with hydrostatic restrictions will not be able to correctly capture the dynamics in such a system with significant bathymetric variations particularly during strong tidal currents.

An evaluation of the use of shore-based surveys for estimating spatial overlap between deep-diving seabirds and tidal stream turbines

As we approach the first array deployments of tidal stream turbines, the risk of collisions between moving components of devices and deep-diving seabirds need assessing at population levels. Part of assessing risks is quantifying the extent to which the foraging distributions of deep-diving seabirds overlap with the potential locations of devices in the large tidal pass habitats favoured for array installations (spatial overlap). Shore-based surveys using single vantage points and extensive grid systems to record the locations of foraging birds are often used in large tidal passes. Here, an application of these methods in the Fall of Warness (FOW), Orkney, UK are used to evaluate their effectiveness for quantifying and statistically analysing spatial overlap in large tidal passes. Whilst it was possible to quantify and statistically analyse spatial overlap using the methods evaluated here, viable statistical analysis required data sacrifices through the use of presence rather than abundance data. Moreover, comparisons between simultaneous shore-based and boat-based surveys, which are assumed to represent an accurate method, showed that sightings in the former were biased towards non-turbine microhabitats. This shore-based survey bias reflected low detection rates of foraging birds in fast unidirectional currents away from coastlines. Therefore, the methods evaluated here are unsuitable for quantifying and statistically analysing spatial overlap in large tidal passes. It is suggested that future shore-based surveys should overcome aforementioned issues by using several vantage points spread throughout large tidal passes, and comparing species use of neighbouring turbine and non-turbine microhabitats at reasonable distances to the vantage points (<2km) during favourable sea states (Beaufort Scale<3). It is hoped that these recommendations lead to the development of standardised shore-based survey methods, providing comparable measurements of spatial overlap across tidal passes earmarked for array deployments: a key step towards estimations of collision risks at population levels.

Movement patterns of immature and adult female Kemp’s ridley sea turtles in the northwestern Gulf of Mexico

The Kemp's ridley sea turtle Lepidochelys kempii is recovering from declines that reduced nesting from a single-day estimate of 10 000 to 40 000 females in 1947 to <300 during the entire 1985 nesting season. Although beach monitoring is crucial to estimating nesting population size and activity, in-water data are essential for understanding population dynamics, evaluating management strategies, and ensuring the species' continued recovery. Fifteen immature and 7 adult female ridleys were fitted with platform terminal transmitters and released off the upper Texas coast during 2004 through 2007. Immature individuals were tracked primarily during warmer months and exhibited preferences for tidal passes, bays, coastal lakes, and nearshore waters, although movement patterns varied among years. Females tracked during their inter- nesting intervals remained in the vicinity of the upper Texas coast and, upon entering the post- nesting stage, moved eastward along the 20 m isobath to foraging areas offshore of central Louisiana. Satellite telemetry indicated that inshore and continental shelf waters of the northwest- ern Gulf of Mexico serve as developmental, migratory, inter-nesting, and post-nesting habitat for the Kemp's ridley. Projected population growth will likely lead to increased use of the northwest- ern Gulf by the species and more frequent encounters with human activities. The extent of such anthropogenic interactions and need for mitigation measures should be examined and considered by natural resource managers to facilitate continued recovery of this and other sea turtle species in the Gulf of Mexico. Likewise, research efforts should be continued to better understand sea- sonal in-water distributions, abundances, population dynamics, and mortality risks to all life his- tory stages.

Environmental influences on the occurrence of coastal sharks in estuarine waters

Long-term fisheries independent gill net surveys conducted in Texas estuaries from 1975 to 2006 were used to develop spatially explicit estuarine habitat use models for 3 coastal shark species: bull shark Carcharhinus leucas, blacktip shark C. limbatus, and bonnethead shark Sphyrna tiburo. Relationships between environmental predictors and shark distribution were investigated using boosted regression trees (BRT). Bull shark was the most abundant species (n = 5800), followed by blacktip (n = 2094), and bonnethead sharks (n = 1793). Environmental conditions influenced dis- tribution patterns of all species and relationships were nonlinear, multivariate, and interactive. Results showed very good model performance and suggested shark distribution is most closely linked to salinity, temperature, and proximity to tidal inlets. By interpolating the BRT models, maps of the probability of capture were produced using ordinary kriging. Results showed that the central region along the Texas coast contains the most important estuarine shark habitat. This area was character- ized by warm temperatures, moderate salinities, and abundant tidal inlets. Bull sharks also extended into low salinity estuaries, while blacktip and bonnethead sharks were restricted to areas near tidal passes with moderate salinities. Juvenile sharks were frequently captured, suggesting the Texas coast may constitute important nursery areas for all 3 species. The development of these spatially explicit models allows for prioritization and conservation of areas in a region that has great potential for human disturbance and climate change impacts. These results provide new insight into the habi- tat requirements of coastal sharks in the northwestern Gulf of Mexico and practical information for managing this resource.

Storm surge induced flux through multiple tidal passes of Lake Pontchartrain estuary during Hurricanes Gustav and Ike

In September 2008, Hurricanes Gustav and Ike generated major storm surges which impacted the Lake Pontchartrain estuary in Louisiana. This paper presents analyses of in situ measurements acquired during these storm events. The main data used in the analyses were from three bottom mounted moorings equipped with conductivity, temperature, and depth sensors, acoustic Doppler current profilers (ADCPs), and a semi-permanent laterally mounted horizontal acoustic Doppler profiler (ADP). These moorings were deployed in the three major tidal channels that connect Lake Pontchartrain with the coastal ocean. A process similar to tidal straining was observed: the vertical shear of the horizontal velocity was negligible during the inundation stage, but a shear of 0.8 m/s over a less than 5 m water column was recorded during the receding stage, 2–3 times the normal tidal oscillations. The surge reached its peak in the Industrial Canal 1.4–2.1 h before those in the other two channels. The inward flux of water lasted for a shorter time period than that of the outward flux. The inward flux was also observed to have much smaller magnitude than the outward flux (∼960–1200 vs. 2100–3100 million m 3). The imbalance was believed to have been caused by the additional water into Lake Pontchartrain through some small rivers and inundation over the land plus rainfall from the hurricanes. The flux through the Industrial Canal was 8–12%, while the flux through the other two tidal passes ranged between 17% and 70% of the total, but mostly split roughly half-half of the remaining (∼88–92% of the total).

Impact of Multiple Freshwater Diversions on the Salinity Distribution in the Pontchartrain Estuary under Tidal Forcing

Numerical experiments of multiple freshwater diversions into the Pontchartrain Estuary under tidal forcing were conducted to evaluate the impact on salinity and tidal flow distribution. A validated numerical hydrodynamic and transport model was used to assess the impacts on tidal flows, circulation, and salinity as a function of additional freshwater input in the estuary from hypothetical diversions combined with channel modifications in the Mississippi River Gulf Outlet. The cumulative and specific impacts were compared with existing conditions. It was concluded that upper and middle estuarine salinity regimes are coupled, and diversion flows need to be managed in accordance with historic inputs. This study also showed that if total freshwater input is not of an order similar to the existing natural tributary flow, the average salinity in the upper estuary could be reduced by 1.5 ppt (±0.5 ppt), which is approximately 40% of the existing long-term salinity of the upper estuary. The additional flow into the upper estuary will produce changes in the flow through the tidal passes on the order of 5%–6%, will decrease hydraulic detention times in the estuary, and will cause an additional increase in the ebb-dominance of the estuary.

In situ measurements of saltwater flux through tidal passes of Lake Pontchartrain estuary by Hurricanes Gustav and Ike in September 2008

Storm surge induced saltwater flux through multiple inlets has never been documented. This article reports in situ measurements of saltwater flux through major tidal passes by two consecutive storm surges impacting Lake Pontchartrain estuary in Louisiana during the first two weeks in September 2008. The main peak of inward saltwater flux lasted for 1.9 days during the flood, followed by more than 8 days of outward salt flux. The salt flux rate reached close to 200 tons/sec through the northern channel and 16 tons/sec through the southern channel. Outward flux of salt during Gustav amounted to more than 16 million tons during the receding stage, more than twice of that measured for the flux into the lake. Apparently, overland inundation, which was not measured, caused this biased estimate of influx of salt. Hurricane Ike, however, did not cause such a dramatic difference in influx and outflux of salt.

Geomorphic and Sedimentologic Heterogeneity Along a Holocene Shelf Margin: Caicos Platform

Abstract Reef-rimmed margins on isolated carbonate platforms are heterogeneous, and studying Holocene platforms provides a valuable means for characterizing the spectrum of facies variability and understanding the processes behind facies changes. In this study, insights from petrographic data, granulometric measurements, bottom observations, and remote sensing and subbottom profiler data from the northwestern part of Caicos Platform make this one of the more intensely studied shallow-marine carbonate systems in the Caribbean. Collectively, analyses of the thicknesses and heterogeneity of shallow-water Holocene carbonates and the platform margin reveal details of the considerable geomorphic variability present on this part of the margin, on which the orientation changes markedly. In this area, the shelf margin arcs from northwest- to north- to northeast-facing, accompanied by changes in morphology as well as the distribution, thickness and type of sediments. The NW-facing margin has discontinuous reefs near the margin, flanked platformward by coarse sands in poorly developed aprons that transition to a deeper rocky bottom (interpreted as Pleistocene) with patches of sand to fine gravel sized sediment. Locally, lobate-to-arcuate shoals with medium sands ( 500 μm mean size) form a reef apron that is continuous along strike and over 1000 m wide. Because the interpreted top-Pleistocene surface is gently dipping seaward, Holocene sediment thicknesses vary with changes in bathymetry and generally thin away from the reefs. Sediments in most areas are between 1.5 and 3 m thick but can reach over 6 m locally. Although the interpretation of the base of the Holocene can be ambiguous under shelf-margin reefs, subbottom profiles illustrate that backreef shelf patch reefs do not necessarily nucleate on Pleistocene bedrock highs. Facies trends are interpreted to reflect a combination of physical oceanographic, sedimentologic, and biologic processes acting within the framework of Pleistocene bedrock configuration. The NE-facing margin, with the continuous reef and expansive sand apron, is interpreted to be more wave-dominated, influenced by swells from the open Atlantic, and by locally derived wind waves. Comparison with Bahamian platforms suggests that this trend is not unique; margins facing open Atlantic swells from the north–northeast have the best-developed reefs, not east-facing (windward) margins. In contrast, the beaches and tidal deltas on the NW-facing margin suggest tidal currents and longshore transport are more pronounced here, consistent with the nature of geomorphic changes through time. The enhanced tidal influence reflects the inlets open to the open bank interior, a factor which also may discourage growth of reefs on this part of the margin. Nonetheless, other Bahamian platforms with fewer tidal passes have comparable discontinuous reefs, suggesting that island continuity is not the most influential parameter on leeward reef development. Collectively, these results illustrate the nature, scale, and causes of along-strike heterogeneity and provide a conceptual model for ancient carbonate platform margins, some of which may be equally complex in their facies architecture.

Modeling estuarine-shelf exchanges in a deltaic estuary: Implications for coastal carbon budgets and hypoxia

The export of wetland-derived materials to the coastal ocean (i.e., the “Outwelling” hypothesis) has received considerable attention over the past several decades. While a number of studies have shown that estuaries export appreciable amounts of nutrients and carbon, few studies have attempted to estimate the importance of estuarine sources for the coastal carbon budgets in river-dominated coastal ecosystems. A novel tidal prism model was developed to examine estuarine-shelf exchanges in the Barataria estuary, a deltaic estuary located in the north-central Gulf of Mexico. This estuary has been the site of a massive wetland loss, and it has been hypothesized that carbon export from the eroding coastal wetlands supports the development of a large hypoxic zone in the coastal Gulf of Mexico. The model results show that the Barataria estuary receives nitrogen through the tidal passes and releases carbon to the coastal ocean. The mean calculated tidal water discharge of 6930 m 3 s −1 is equivalent to about 43% of the lower Mississippi River discharge. The annual total organic carbon (TOC) export is 109 million kg, or 57 gC m 2 yr −1 when prorated to the total water area of the estuary. This carbon export is equivalent to a loss of 0.5 m of wetland soil horizon over an area of 8.4 km 2, and accounts for about 34% of the observed annual wetland loss in the estuary between 1978 and 2000. Compared to the lower Mississippi River, the Barataria estuary appears to be a very small source of TOC for the northern Gulf of Mexico (2.7% of riverine TOC), and is unlikely to have a significant influence on the development of the Gulf's hypoxia.

Habitat Use Patterns of Newly Settled Southern Flounder, Paralichthys lethostigma, in Aransas–Copano Bay, Texas

Southern flounder Paralichthys lethostigma populations have been declining in Texas during the past 25 years. Despite their economic importance, little is known about their juvenile habitat requirements. We examined habitat use patterns of newly settled southern flounder in three zones at varying distances from the Aransas Pass inlet in Aransas–Copano Bay by measuring densities using a beam trawl in replicate estuarine habitat types in each zone. Highest abundance occurred near the inlet in vegetated sandy areas and was lowest in nonvegetated muddy bottom in regions furthest from the inlet. We also examined a 25-year fisheries data set from Texas Parks and Wildlife Department to evaluate long-term spatiotemporal recruitment patterns in Aransas–Copano Bay. These data showed generally low recruitment during the past 25 years with highest abundance near the inlets. Our results support the importance of vegetated habitat types, especially those near tidal passes, and suggest a long-term decline in recruitment densities of southern flounder.

Effects of the 1997 Bonnet Carré Spillway Opening on Infaunal Macroinvertebrates in Lake Pontchartrain, Louisiana

Abstract The Bonnet Carre Spillway was constructed in 1931 to divert floodwater from the Mississippi River into Lake Pontchartrain, an oligohaline estuary near New Orleans, Louisiana. The spillway was opened from March 17 through April 18, 1997. Infaunal macroinvertebrate population, surface and bottom salinity, and dissolved oxygen data were examined to determine the effects of this diversion. These data were obtained from November 1996 through November 1998 from five sites on an east-to-west transect across Lake Pontchartrain. A community composed of oligohaline taxa persisted during the freshwater period, but changes in dominance, and to a lesser extent composition, occurred over time and among sites. There was a pronounced spatial effect related to the distance of the sites from the spillway and from tidal passes. Based on decreases in species diversity (H′), abundance, and the number of taxa, it appears that the opening had a deleterious impact on the benthos. The cause of these changes cannot be att...

Utilization of Saltmarsh Shorelines by Newly Settled Sciaenids in a Texas Estuary

Post-settlement patterns of habitat use along saltmarsh shorelines of Galveston Bay, Texas were examined for three sciaenids; spotted seatrout (Cynoscion nebulosus), red drum (Sciaenops ocellatus), and Atlantic croaker (Micropogonias undulatus). Collections were made summer through fall of 1997 and 1998 using a 1.5-m beam trawl hand-towed along the outside edge of salt marshes. Sciaenids were collected from tidal pass, bay, and remote tidal creek areas to assess large-scale (bay-wide) patterns of distribution and abundance. Cynoscion nebulosus were smaller and most numerous at bay stations, with densities peaking in June. Conversely, S. ocellatus were collected in higher numbers, and smaller sizes, at stations near the tidal pass, with peak densities in September. Micropogonias undulatus occurred in high numbers near both the tidal pass and remote tidal creeks, with undetectable size differences among areas and peak densities in November. Densities of C. nebulosus and S. ocellatus were greater at sites nearer the larval supply; patterns were less clear for M. undulatus. Small-scale patterns of habitat use were investigated within remote tidal creeks, with trends in density and size apparent for M. undulatus, while few C. nebulosus and S. ocellatus were collected. Interannual variability in densities occurred within and among stations, although total densities for each species were relatively similar between 1997 and 1998.

Transport, Mixing and Stirring Processes in a Louisiana Estuary: A Model Study

Transport and mixing processes in a broad and shallow estuary in Louisiana, Terrebonne/Timbalier Basin, are examined using a depth-integrated two-dimensional numerical model. Using current meter records previously obtained, the model calibration yielded correlation coefficients between simulated and observed current components of 0·89–0·95. It appears that the bottom friction in the bay is relatively large with a Manning's coefficient of 0·07 producing the best results. The large bottom friction appears to be due to a combined effect of currents and the surface wave field which is not explicitly accounted for in the numerical model. Despite the small tidal range, tidal forcing dominates circulation in the bay. During equatorial tides, tidal currents on the order of 20cms−1could develop in a broad area of the bay while in tidal passes currents could reach 50–60cms−1. During tropic tides, strengths of the currents in the bay could easily be double those during equatorial tides. Local wind forcing is also important in controlling general flow direction inside the bay, in particular during equatorial tides. Flushing time, estimated by a particle tracking technique, was 27 days, that appears to be in agreement with observations. Horizontal diffusivities computed using tracer particles are comparable to the previous estimates of horizontal diffusion coefficients compiled by Okubo (1974). The larger values appear to be due to coastal trapping. Mixing of water masses, based on particle tracking, is found to consist of continuous stretching, folding and break-up of material lines due to interaction of wind-driven and tidal currents with bottom and coastal topography. Time evolution of the boundary between the two water masses depends on the initial tidal phase. However, this dependence lasts only until coastal trapping becomes dominant in controlling the time evolution of the boundary. Coastal trapping appears to be an important process by which stirring and mixing processes are enhanced, thus making stirring and mixing ‘more efficient’ and ‘chaotic’. Those observations point to the need to resolve small-scale shear flow patterns, in both space and time, and detailed bottom and coastal topography in order to understand transport, stirring and mixing processes in the broad, shallow estuaries typical of the northern Gulf of Mexico.

LATE CENOZOIC DEPOSITIONS PALEOENVIRONMENTS OF SOUTHEAST FLORIDA INTERPRETED FROM CORE LOGS

The use of core logs from water wells and engineering-geological site investigations permitted spatial refinement of known paleodepositional environments along the southeastern coast of Florida where the narrow continental shelf was influenced by changes in sea level and shoreline position during the late Cenozoic. Lithological materials have been formalized in terms of the Tamiami Formation (early Pliocene), Fort Thompson Formation (late Pliocene), Anastasia Formation (Pliocene-Pleistocene), Miami Limestone (late Pleistocene), and Pamlico sands (Pleistocene) in southeastern Florida. The lack of good exposures, scarcity of biostratigraphically important microfossils, similarity of sedimentary deposits, and pervasive diagenesis of fossiliferous sections have mitigated against a consensus of interpretation for late Cenozoic stratigraphic sections. Inspection of more than 18,300 m of unpublished core log data combined into cross-sections that crisscrossed the Broward County study area, permitted mapping of vertical and horizontal relationships for Miocene, Miocene-Pliocene, early Pleistocene, and late Pleistocene/Holocene time frames. These lithostratigraphic interpretations of changing paleoenvironments formed the basis of a preliminary model for the coastal geological framework of Broward County. This study confirmed general late Cenozoic trends in sedimentation from marine conditions (Tamiami Formation) to nearshore (Anastasia Formation, Fort Thompson Formation) settings to very shallow marine environments (oolitic facies of the Miami Limestone) and eolian materials in the upper Pamlico sands. Late Pleistocene to Recent trends showed a shore-parallel pattern of shallow paleobathymetries that included barrier bars and tidal passes (paleoinlets). Erosional unconformities (interpreted as irregular karstified groundsurfaces with low paleotopographic relief) in the sections were frequently capped by dense subaerial weathering crusts of calcite which today act as semi-impermeable barriers to groundwater flow. Knowledge of this coastal geological framework is relevant to increased understanding of submarine groundwater discharges, modes of saltwater intrusion, boundary conditions for groundwater models, and has practical implications for optimized water management along the southeast coast of Florida.

Food Habits and Dietary Overlap of Newly Settled Red Drum (Sciaenops ocellatus) and Atlantic Croaker (Micropogonias undulatus) from Texas Seagrass Meadows

Food habits and dietary overlap of newly settled larval and juvenile red drum and Atlantic croaker were examined during the period when the two species co-occur in seagrass nurseries. A total of 274 red drum (4.00 19.99 mm SL) and 205 Atlantic croaker (8.00 17.99 mm SL) were used for this analysis. Of the red drum stomachs examined, 8.4% were empty while 28.8% of Atlantic croaker stomachs contained no food. Major prey items identified for both species were calanoid copepods, harpacticoid copepods and mysid shrimp across all size classes. Ontogenetic trophic niche shifts were detected for red drum and Atlantic croaker. Type and quantity of food ingested by red drum were similar across all stations (Aransas Bay Station: lH, 2T and 3H) examined. Atlantic croaker ingested the same types of prey at all stations, but contained varying quantities of food throughout the study area. In general, high dietary overlap was observed between red drum and Atlantic croaker with most overlap values (Schoener’s index) exceeding 70%. INTRODUCTION Red drum (Sciuenops ocellutus) spend most of their adult lives offshore and migrate to tidal passes to spawn in late August through mid-November, whereas adult Atlantic croaker (Micropogonius undufutus) occupy gulf coastal waters and congregate offshore to spawn in early October through February (Johnson 1978). Pelagic larvae of both species are transported by currents through tidal inlets and into nursery habitats in bays and estuaries (Rooker et al. 1998). Consequently, larval and juvenile red drum (4 20 mm SL) occupy seagrass beds from late September to early December, while larval and juvenile Atlantic croaker (8 18 mm SL) are found in seagrass beds from early October to February (Holt et al. 1983, Rooker et al. 1998). Both species concurrently occupy seagrass beds in November at similar sizes. Conspecifics and morphologically similar species (i.e., confamilials) occupying similar habitats can potentially compete for food particularly during times when fish densities are high and prey is scarce. Intraspecific and interspecific competition among larval fishes can reduce growth rates, which in turn, may increase early-life stage mortality due to starvation or predation (Houde 1987). Therefore, it is important to understand the trophic relationships of early life stages. Fishes change resource (food) use throughout the course of their lives, especially during larval and juvenile stages. Such ontogenetic niche shifts may divide sizestructured populations into ecologically distinct stages basedon diet (Olson 19%). Duration of stages and transition among stages has the potential to minimize intraspecific competition for food. Although several studies have addressedfood habits of these two species separately (Bass and Avault 1975, Chao and Musick 1977, Oversteet and Heard 1978, Steen and Laroche 1983, Govoni et al. 1983, Cumn et al. 1984, Govoni et al. 1986, and Peters and McMichael 1987), no dietary overlap analysis has been conducted on newly settled red drum and Atlantic croaker. The primary aim of this study was to obtain an understanding of the trophic dynamics of newly settled larval and juvenile red drum and Atlantic croaker occurring in seagrass habitat. Specific objectives were to: 1) quantitatively describe the diets of larval and juvenile red drum and Atlantic croaker; 2) determine ontogenetic changes in diets of the two species; 3) determine ifdiet varies across different sites and habitats for red drum and Atlantic croaker; 4) determine interspecific dietary overlap between red drum and Atlantic croaker; and 5) determine if red drum and Atlantic croaker feed on equal quantities of food at similar sizes during the cooccurring period. MA T E R I A L S AND METHODS Diurnal sampling (0730 1700 h) was conducted weekly from October throughDecember 1994. Fish samples were taken from three stations in Aransas Bay (lH, 2T and 3H) and two stations in Redfish Bay (4H and 5T)(Figure 1). Stations lH, 3H and 4H were in shoal grass (Hulodule wrightii) while stations 2T and 5T were in turtle grass (Thulussiu testudinum) (Figure 1). A 1 m (diameter), 505 pm mesh cone net attached to a 0.75 m (length) x 0.56 m (height)epibenthic sledwas hand-towed for 20 m across the grassbed sites. Three samples f” each site were obtained picked free of grass, and preserved in 5% formalin. Standard

Modern marine stromatolites in the Exuma Cays, Bahamas: Uncommonly common

Modern stromatolites in open marine environments, unknown until recently, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments and intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobacterial mats, whereas high relief surfaces are commonly colonized by algal turf and other macroalgae such asBatophora, Acetabularia, andSargassum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear as micritic horizons with distinct microstructures: they consist of thin micritic crusts (20–40 μm thick) overlying layers of micritized sediment grains (200–1000 μm thick); the micritized grains are cemented at point-contacts and are trucated along a surface of intense microboring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, settings and shapes of stromatolites in the Exuma Cays present excellent opportunities for future studies of stromatolite morphogenesis.

Temporal and Vertical Distribution of Crab Larvae in a Tidal Pass

We investigated the vertical and temporal distributions of portunid zoea andCallinectes megalopae in a tidal pass of the Texas coast. Zoea were equally abundant on ebb and flood tide but were more abundant during the day than at night. Megalopae were more abundant during flood tide and at night than during ebb tide or during the day. We examined the evidence for selective tidal stream transport in both life-history stages. Depth of the centers of mass of the larval distributions and the dispersions around those centers were unrelated to temperature, salinity, current velocity, and time of day. Scaling arguments suggest that the absence of pattern in the vertical distributions was not due to turbulent mixing. There was little evidence that either larval stage used these environmental characteristics as cues for changes in behaviour. However, the presence of megalopae in the water column primarily during flood tide does support the tidal transport hypothesis. Megalopae may have difficulty sensing and reacting to environmental cues in wellmixed estuaries with semidiurnal tides.

Ichthyoplankton Abundance along a Recruitment Corridor from Offshore Spawning to Estuarine Nursery Ground

Two temporally separate larval fish assemblages were identified from ichthyoplankton samples collected between November 1987 and January 1990 along a transect from offshore to Oyster Bayou tidal pass into Fourleague Bay, Louisiana. The warm water species (e.g. bay anchovy, naked goby, Microgobius sp., spotted seatrout and skilletfish) dominated in water temperatures >23 °C and the cooler water species (e.g. gulf menhaden and Atlantic croaker) dominated in water temperatures <23 °C. These assemblages were characterized by different life-history strategies, distributions, and seasonal abundances suggesting that they utilize resources in different locations and times. Analysis of Oyster Bayou tidal pass data indicated that larvae from all species were more abundant on flood than ebb tides and larval fish densities were often statistically greater near bottom, suggesting tidal transport and/or retention. In addition, larger larval bay anchovy and gulf menhaden were taken in the middle of the tidal pass during flood tides. During ebb tides, however, larger bay anchovy larvae were taken along the pass edges, suggesting behaviourally mediated transport into and/or retention within the estuary.