Gulf Of Mexico Estuaries Research Articles

Relative Value of Deep Subtidal Oyster Reefs to Other Estuarine Habitat Types using a Novel Sampling Method

Subtidal eastern oyster Crassostrea virginica (Gmelin) cover large expanses of many Gulf of Mexico estuaries; however, few researchers have attempted to quantify the value of deep, open-water, subtidal reefs as habitat for fishes and crustaceans as a result of gear limitations. We developed quantitative sampling gear for live oyster reefs by slightly modifying an epibenthic sled. Gear comparison trials showed similar effectiveness among marsh edge, submerged aquatic vegetation, and nonvegetated bottom for both epibenthic sled types. We then quantified the density and community assemblage of nekton and benthic crustaceans on deep subtidal oyster reefs in Lavaca Bay, TX, and compared it with densities found in nearby marsh edge, submerged aquatic vegetation, and nonvegetated bottom habitats. We found significantly fewer nektonic and benthic crustaceans on nonvegetated bottom and oyster reefs than in marsh edge and submerged aquatic vegetation over all seasons and regions, and community analysis revealed similar differences among habitat assemblages. Using gill nets, the greatest catch of transient fishes and crustaceans were collected on oyster reefs and nonvegetated bottom. Although relatively low densities of small juvenile fishes were observed over deep oyster reefs, our community analyses and the high catch-per-unit-effort of large, transient species provide evidence that subtidal reefs are a critical habitat for numerous estuarine fishes and crustaceans.

Success of constructed oyster reefs in no-harvest sanctuaries: implications for restoration

Dramatic declines in populations of the eastern oyster Crassostrea virginica are a symp- tom of degradation in many US Atlantic and Gulf of Mexico estuaries. We sampled 94 oyster reefs (88 constructed, 6 natural) within 11 no-harvest sanctuaries in estuaries of central and northern North Carolina, USA, to evaluate the success of oyster sanctuaries as a conservation tool. The sanctuaries have been in existence from 3 to 30 yr; 10 sanctuaries protect constructed ('restored') oyster reefs and 1 sanctuary protects natural reefs. Measurements of vertical relief, live oyster density, recruitment, abundance of market-sized oysters, and biomass as well as disease prevalence and severity indicated that 7 of the 11 sanctuaries met criteria for minimal success by having vertical relief >20 cm in height, living oysters (>10 oysters m -2 ), and evidence of recent recruitment in 1 of 2 yr of the survey. Most reefs within the 7 sanctuaries far surpassed these relatively low benchmarks. For reefs that failed, burial by sedimentation appeared to be the primary cause in 2 sanctuaries, poor water quality (low dissolved oxygen) in 1, and poor oyster recruitment in another. All intertidal reefs were successful and had significantly higher densities of all size categories of live oysters (spat, adult, marketable size) than subtidal oyster reefs. Disease prevalence and severity were low in sanctuary reefs despite high oyster densities and increased longevity of oysters on these reefs. Pronouncements that restora- tion of the native eastern oyster is a failure prove incorrect when a decade-long history of oyster reef sanctuaries is evaluated. The proposed introduction of a non-native oyster into the US Atlantic coast estuaries cannot be justified by claiming failure of native oyster restoration in light of promising suc- cesses within sanctuaries.

Benthic Herbivores are not Deterred by Brevetoxins Produced by the Red Tide Dinoflagellate Karenia Brevis

Gulf of Mexico blooms of the dinoflagellate Karenia brevis produce neurotoxic cyclic polyethers called brevetoxins. During and after a red tide bloom in southwestern Florida, K. brevis cells lyse and release brevetoxins, which then sink to the benthos and coat the surfaces of seagrasses and their epiphytes. We tested the possibility that these brevetoxin-laden foods alter the feeding behavior and fitness of a common benthic herbivore within Floridean seagrass beds, the amphipod Ampithoe longimana. We demonstrated that coating foods with K. brevis extracts that contain brevetoxins at post-bloom concentrations (1 microg g(-1) drymass) does not alter the feeding rates of Florida nor North Carolina populations of A. longimana, although a slight deterrent effect was found at eight and ten-fold greater concentrations. During a series of feeding choice assays, A. longimana tended not to be deterred by foods coated with K. brevis extracts nor with the purified brevetoxins PbTx-2 and PbTx-3. Florida juveniles isolated with either extract-coated or control foods for 10 days did not differ in survivorship nor growth. A similar lack of feeding response to brevetoxin-laden foods also was exhibited by two other generalist herbivores of the southeastern United States, the amphipod A. valida and the urchin Arbacia punctulata. Given that benthic mesograzers constitute a significant portion of the diet for the juvenile stage of many nearshore fishes, we hypothesize that the ability of some mesograzers to feed on and retain brevetoxins in their bodies indicates that mesograzers may represent an important route of vertical transmission of brevetoxins through higher trophic levels within Gulf of Mexico estuaries.

Assessing Water Clarity as a Component of Water Quality in Gulf of Mexico Estuaries

The U.S. Environmental Protection Agency Environmental Monitoring and Assessment Program (EMAP) uses water clarity as a water quality indicator for integrated assessments of coastal waters. After the publication of the first National Coastal Condition Report (USEPA, 2001c), the national water clarity reference value of 10% of ambient surface light at 1 m depth was reevaluated and modified to reflect expected differences in regional reference light conditions. These regional differences range from naturally turbid estuaries like those found in Mississippi and Louisiana to clear water estuaries expected to support extensive beds of submerged aquatic vegetation in, e.g., Florida and Tampa Bays. For the second National Coastal Condition Report, water clarity was assessed based on regional reference values (USEPA, 2004). Different regional water clarity reference values and data collection methods necessitated the development of a water clarity index based on light attenuation coefficients (k). This index incorporates regional reference conditions and is interchangeable with secchi depth and percent light transmission calculated from light meter measurements. Evaluation of the water clarity index shows that k values based on transmissivity at 1 m can be estimated from secchi depth measurements and successfully used as a surrogate for transmissivity calculated from light meter data. An approach for assessing water clarity in Gulf of Mexico estuaries using light meter data and secchi depth is presented.

Growth and reproduction of gelatinous zooplankton exposed to low dissolved oxygen

The lobate ctenophore Mnemiopsis leidyi and the scyphomedusan jellyfish Chrysaora quinquecirrha are seasonally important consumers in the food web of Western Atlantic and Gulf of Mexico estuaries, including Chesapeake Bay. The abundance and importance of these gelatinous species may be increasing as a result of anthropogenic alteration of these systems, particularly the increasing severity and extent of low dissolved oxygen. Ctenophores and jellyfish are more tolerant of hypoxia than co-occurring finfish, and can sustain high feeding rates in hypoxic waters. We exam- ined the effects of hypoxia exposure on M. leidyi and C. quinquecirrha growth rates and M. leidyi reproduction over 4 d periods in 1 m 3 mesocosms at a range of natural prey densities. Both small (0.2 to 2.0 ml biovolume) and larger (8.0 to 17.6 ml biovolume) ctenophores had significantly reduced growth at oxygen levels of 1.5 and 2.5 mg l -1 as compared to air-saturated water, especially at high prey densities. Egg production by large ctenophores was also significantly reduced by exposure to low dissolved oxygen concentrations. In contrast, C. quinquecirrha growth rates were unaffected by low dissolved oxygen concentrations tested. These results are counter-intuitive as M. leidyi preferen- tially utilizes moderately hypoxic bottom waters in the field, while C. quinquecirrha avoids such waters. Our findings suggest that hypoxia may differentially affect population growth of these domi- nant gelatinous species.

Molecular Phylogeography and Evolutionary History of the Estuarine Copepod, Acartia Tonsa, on the Northwest Atlantic Coast

Coastal estuaries are useful model systems to study the ecological and evolutionary responses of organisms to highly variable, discontinuous habitats. For this study, the molecular population genetic diversity of the planktonic calanoid copepod Acartia tonsa (Dana, 1849) was described based on DNA sequence variation for a 183 base-pair region of the mitochondrial 16S rRNA gene. Samples of A. tonsa were collected from four estuaries on the Atlantic coast of the USA during 1993 and 1994, one estuary on the Gulf of Mexico coast in 1994, and one site on the Pacific coast of the USA in 1994. Dispersal of A. tonsa was shown to be restricted, with significant population genetic structuring between different estuaries. For all but the closely-adjacent MA and RI samples, frequencies of haplotypes and/or length polymorphisms within one haplotype (caused by insertion/deletion mutations) revealed highly significant genetic differentiation and geographic isolation. Mt16S haplotypes of A. tonsa from Atlantic and Gulf of Mexico estuaries were assorted among four deeply-diverged clades. Haplotypes within each clade differed by <2%, while differences among clades of 10% to 14% approached those between described Acartia species (e.g., 19% to 28% among A. clausi, A. hudsonica, and A. longiremis). Atlantic and Pacific coast samples identified as A. tonsa had no haplotypes in common and genetic differences between haplotypes ranged from 18% to 29%; phylogenetic analysis supported the separation of Pacific coast A. tonsa as a distinct species. We hypothesize that the observed patterns of molecular genetic diversity and structure of A. tonsa resulted from responses to historical climatic variation, including episodic range compression and displacement, and alteration of NW Atlantic coastal and estuarine environments.

Of Manatees, Mangroves, and the Mississippi River: Is There an Estuarine Signature for the Gulf of Mexico?

Important parameters of estuarine variability include morphology, flushing times, nutrient loading rates, and wetland: water ratios. This variability both reflects and disguises underlying relationships between the physics and biology of estuaries, which this comparative analysis seeks to reveal, using the Gulf of Mexico (GOM) estuaries as a starting point. A question used to focus this analysis is: are the GOM estuaries unique? The GOM receives the Mississippi River, a uniquely large, world-class river, which dominates the freshwater and nutrient inflows to the GOM continental shelf, whose margins include 35 major estuarine systems. These GOM estuaries have 28% and 41% of the U.S. estuarine wetlands and open water, respectively. Within the GOM, estuarine nitrogen, phosphorus, and suspended matter loading varies over 2 orders of magnitude. Anoxic estuarine events tend to occur in estuaries with relatively slow freshwater turnover and high nitrogen loading. Compared to estuaries from other regions in the U.S., the average GOM estuary is distinguished by shallower depths, faster freshwater flushing time, a higher wetland area:open water area ratio, greater fisheries yield per area wetland, lower tidal range, and higher sediment accumulation rates. The average GOM estuary often, but not always, has a flora and fauna not usually found in most other U.S. estuaries (e.g., manatees and mangroves). Coastal wetland loss in the GOM is extraordinarily high compared to other regions and is causally linked to cultural influences. Variations in nutrient loading and population density are very large among and within estuarine regions. This variation is large enough to demonstrate that there are insufficient systematic differences among these estuarine regions that precludes cross-system analyses. There are no abrupt discontinuities among regions in the fisheries yields per wetland area, tidal amplitude and vegetation range, salt marsh vertical accretion rates and organic accumulations, nitrogen retention, or wetland restoration rates. These results suggest that a comparative analysis emphasizing forcing functions, rather than geographic uniqueness, will lead to significant progress in understanding how all estuaries function, are perturbed, and even how they can be restored.

Effects of Sediment Contaminants and Environmental Gradients on Macrobenthic Community Trophic Structure in Gulf of Mexico Estuaries

Macrobenthic communities from estuaries throughout the northern Gulf of Mexico were studied to assess the influence of sediment contaminants and natural environmental factors on macrobenthic community trophic structure. Community trophic data were also used to evaluate whether results from laboratory sediment toxicity tests were effective indicators of site-specific differences in benthic trophic structure. A multiple regression model consisting of five composite factors (principal components) was used to distinguish the effects of sediment contaminants and environmental variables on benthic community trophic structure. This model explained 33.5% of the variation in macrobenthic trophic diversity (p<0.001), a variable derived from the distribution of taxas among nine original trophic categories. A significant negative relatinship was found between principal components reflecting concentrations of sediment contaminants and macrobenthic trophic diversity.

Biogeochemistry of Gulf of Mexico Estuaries

Biogeochemistry of Gulf of Mexico Estuaries

Modelling estuarine eutrophication in the context of hypoxia, nitrogen loadings, stratification and nutrient ratios

Estuarine eutrophication is a serious ecological and economic problem. Hypoxic waters resulting from decomposing phytoplankton in stratified bottom-waters are often associated with estuarine eutrophication, with the degree of the phytoplankton accumulation and subsequent decomposition corresponding to the phytoplankton's density in the overlying water. In general, high phytoplankton densities occur when nutrient availability is high enough to maintain the phytoplankton population. High nutrient availabilities generally result from high nutrient loads entering estuaries from their watersheds. Therefore, the co-occurrence of stratified bottom-waters and high nutrient loadings predisposes estuaries to developing hypoxic bottom-waters. This paper presents a multinominal logistic regression model that predicts the status of estuaries as normoxic, borderline hypoxic, hypoxic and severely hypoxic based on nitrogen loadings, salinity stratifications and «phosphorus to nitrogen» load ratios. The model was applied to 10 US Gulf of Mexico estuaries and it correctly assigned eight of the 10 estuaries to their correct eutrophication status. Summaries of each estuary's probabilities for being normoxic, borderline hypoxic, hypoxic or severely hypoxic are presented along with their modelled responses to changes in nutrient loadings. Other model results include nutrient loading reductions associated with moving hypoxic and severely hypoxic estuaries into the normoxic and borderline status. This estuarine eutrophication model follows previously developed eutrophication models for lakes which were based on similar premises and have been successfully used to develop management policies. The estuarine eutrophication modelling approach provides a platform for evaluating nutrient reduction management strategies in the context of an estuary's modelled response to proposed nutrient reductions. Therefore, the modelling approach presented here is offered in support of developing management policies concerning estuarine eutrophication.

Benthic-Pelagic Coupling in Northern Gulf of Mexico Estuaries: Do Benthos Feed Directly on Phytoplankton?

Few of the dominant benthic taxa of the northern Gulf of Mexico feed directly on phytoplankton. Rather, most of them feed on near-bottom seston and detritus. This is in contrast to models for Chesapeake Bay and San Francisco Bay. We found that detritivores represented over 80% of the macrobenthic organisms and over 90% of the biomass in Gulf of Mexico estuaries. The paucity of benthos that consumed phytoplankton led us to hypothesize that macrobenthos in Gulf of Mexico estuaries had less effect on plankton communities than was documented in U.S. east coast and west coast estuaries, where benthic communities consumed much of the watercolumn production. We provided as evidence gutcontents analyses of dominant taxa, the feeding morphology of suspension feeders (especially clams), and the lack o f vertical mixing or strong turbulent flow that is necessary for benthos to remove substantial portions of the phytoplankton.

Suspended particulate organic matter (SPOM) in Gulf of Mexico estuaries: compound-specific isotope analysis and plant pigment compositions

Pigment and stable isotopic compositions of suspended particulate organic matter (SPOM) were determined in several estuaries along the Texas coast of the Gulf of Mexico. Analysis of the composition of accessory pigments suggested that the phytoplankton taxa varied among these bays. The δ 13C values of SPOM, lipids, and chlorophylls in these bays varied from − 18 to − 27, − 20 to − 31, and − 18 to − 31‰, respectively. The δ 13C values of SPOM, lipids, and chlorophylls, and the pigment composition together suggest that the 13C-depleted organic matter in Corpus Christi Bay in June 1992 resulted from the fixation of 13C-depleted CO 2 derived from riverine water or in situ remineralization of organic matter. The lipids and chlorophyll a isolated from the nearly freshwater system of Aransas Bay in 1992 were 13C-enriched and most likely derived from local phytoplankton production. The δ 13C values of SPOM, lipids and chlorophylls in the bays where salinities were closest to that of seawater were in the range expected for marine phytoplankton, i.e., − 18 to − 24‰. The δ 15N values for SPOM, lipids, and chlorophyll a ranged from + 4 to + 14‰ with most between + 6 to + 10‰. These values are similar to the SPOM of marine systems and indicate a common source of nitrogen throughout the estuaries.

Distribution of sedimentary phosphorus in Gulf of Mexico estuaries

Sediment samples were collected from all the major bays and estuaries along the US Gulf of Mexico Coastline in 1986–1988 by the NOAA NS & T Program (NOAA (1989), The National Status and Trends Program for marine environmental quality; NOAA/OMA, Rockville, MD, USA). These have been analyzed for total phosphorus as well as leachable Al-P, Ca-P and Fe-P. Total phosphorus concentration ranges from 1·8 to 45·9 μmol/g and averages 11·3 μmol/g. Higher values were found in Louisiana estuaries than in those in Texas but the highest values occur in the phosphate rock mining region of southern Florida. Ca-P is the most abundant leachable species and is higher in samples from higher salinity sites. Previous unpublished data by one of the authors (D.A.) on solid phase and pore water phosphate from 18 cores collected on the Mississippi River Delta, are reported for the first time here. Low dissolved phosphate (< 5 μM/liter) was found at the surface of several cores but values at 1 m depth were mostly much higher (up to 145 μM/liter). Pore water phosphate gave a significant positive correlation with solid phase inorganic phosphorus, but a poor correlation with solid organic phosphorus, showing the importance of inorganic reactions in controlling dissolved PO 4 in pore water.

Denitrification, nutrient regeneration and carbon mineralization in sediments of Galveston Bay, Texas, USA

Rates of benthic denitrification, oxygen consumption and nutrient regeneration were measured during winter, spring and summer in Galveston Bay (Texas, USA) sediments. Denitrification ranged from 0 to 47 pm01 N2 m-2 h' with maximal rates generally occurring in the summer and the upper estuary. Oxygen consumption rates ranged from 38 pm01 O2 m-2 h' in the winter to 353 pm01 O2 m-' h-' in the summer and were correlated with denitrification rates. Variations in bay water temperature accounted for 52 % of the variability associated with denitrification rates whereas only 28% of the variability could be attributed to organic carbon content and 15% to salinity, indicating a predominance of temporal over spatial factors in controlling estuarine rates of denitrification. In the spring and summer, denitrification was responsible for the majority (73 and 80%, respectively) of the total benthic inorganic nitrogen efflux while in the winter, nitrogen fluxes were dominated (80 %) by ammonium. At salinities less than 6 L , cation exchange interactions may have played an important role in retaining ammonium in the sediment, producing the higher rates of denitrification found in the upper estuary. Dissolved inorganic carbon flux was used as a measure of total organic carbon mineralization. The average molar C:N of the remineralized substrate (5.2) was lower than the average C:N of the sediments (12.6) indicating preferent~al remineralization of nitrogen relative to carbon. Molar C:O ratlos suggested that anaerobic carbon m~neralization and the storage of its reduced end-products is more prevalent in the lower estuary and In the winter. Denitrifiers were responsible for 37 and 13 % of the total benthic carbon mineralization in the upper and lower estuary, respectively. Denitrification appears to be a greater contributor to total carbon mineralization than previously considered. Nearly one-third of the total sediment oxygen consumption was attributed to nitrification. Galveston Bay sediment denitrification and oxygen consumption rates and nutrient fluxes were lower but comparable to those of other Gulf of Mexico estuaries. Differences among the estuaries examined are attributed mainly to sediment organic matter content.

Evaluation of sampling strategies to characterize dissolved oxygen conditions in northern Gulf of Mexico estuaries

Dissolved oxygen was continuously monitored in eight sites of northern Gulf of Mexico estuaries in August, 1990. Monte Carlo analyses on subsamples of the data were used to evaluate several commonly used monitoring strategies. Monitoring strategies which involve single point sampling of dissolved oxygen may often misclassify an estuary as having good water quality. In the case of shallow, often well-mixed estuaries that experience diurnal cycles, such monitoring often does not occur at night, during the time of lowest dissolved oxygen concentration. Our objective was to determine the minimum sampling effort required to correctly classify a site in terms of the observed frequency of hypoxia. Tests concluded that the most successful classification strategy used the minimum dissolved oxygen concentration from a continuously sampled 24-hour period.

Aging and Sediment Characteristics of Northern Gulf of Mexico Estuaries: ABSTRACT

Aging and Sediment Characteristics of Northern Gulf of Mexico Estuaries: ABSTRACT

Prospective study of infectious and noninfectious diseases in oysters and fishes in three Gulf of Mexico estuaries

Prospective study of infectious and noninfectious diseases in oysters and fishes in three Gulf of Mexico estuaries