Mexico Estuaries Research Articles

The relative influence of hypoxia, anoxia, andassociated environmental factors as determinants ofmacrobenthic community structure in aNorthern Gulf of Mexico estuary

Seasonal patterns (i.e., December 1986, and April and October 1987) in benthic macroinfaunal abundance,distribution, and taxa composition at 19 sites in Perdido Bay, AL/FL, are evaluated to assess the relative importance of environmental factors as determinants of community structure. A total of 46 taxa from five phyla were collected with diver-held bottom corers. Polychaetes were numerically dominant followed by crustaceans. Seventeen taxa co-occurred in samples during all three study periods. Maximum animal densities and taxa richness showed no statistically significant bay-wide seasonal pattern,however, a bay-wide trend was detected where these response parameters tended to be greater in April than December or October. Deeper upper bay stations were depauperate during December and October. Low dissolved oxygen (DO) largely explained the depauperate pattern. Mean taxa richness per core(10 cm dia.) ranged from 0.0 to 5.0, 1.2 to 4.6, and0.0 to 4.4 in December, April and October,respectively. Mean densities ranged from zero to 368,0 to 960, and 0 to 430 individuals per 0.1 m2 in December, April, and October, respectively. Results of a three-season statistical regression model indicated that DO deficiency was a primary determinant of taxa richness (partial R 2: 0.27) but was less important in explaining animal densities (partialR 2: 0.16). For December, when additional environmental variables were measured, DO was supplanted by weight loss on ignition (R 2: 0.24)and the sediment C:N ratio (R 2: 0.44) as highest explanatory factors for taxa richness and density,respectively. Application of a benthic index of environmental condition indicated wide-spread ecological stress on the benthic macroinfaunal assemblages.

Macrobenthic Responses to Natural and Contaminant-Related Gradients in Northern Gulf of Mexico Estuaries

Macrobenthic Responses to Natural and Contaminant-Related Gradients in Northern Gulf of Mexico Estuaries

FRESHWATER INPUT TO A GULF ESTUARY: LONG-TERM CONTROL OF TROPHIC ORGANIZATION

A long-term (9.5 yr) study addressed the relationship of the trophic orga- nization of a river-dominated Gulf of Mexico estuary with interannual trends of freshwater input and biological controlling features. Alluvial river flow characteristics were evaluated with respect to seasonal and interannual changes in physical, chemical, and biological trends in the receiving estuary. Infaunal and epifaunal macroinvertebrates and fishes taken over the period of sampling in the Apalachicola Bay system were transformed into their trophic equivalents. The long-term trophic organization of the bay was then related to observed changes in the physical and chemical conditions in the receiving estuary with particular attention to long-term response to a 2-yr drought. Within limited natural bounds of fresh- water flow from the Apalachicola River, there was little change in the trophic organization of the receiving estuary over prolonged periods. The physical instability of the estuary was actually a major component in the continuation of a biologically stable estuarine system. However, when a specific threshold of freshwater reduction was reached during a prolonged natural drought, we suggest that the clarification of the normally turbid and highly colored river-estuarine system led to rapid changes in the pattern of primary production, which, in turn, were associated with major changes in the trophic structure of the system. Increased light penetration due to the cessation of river flow was an important factor in the temporal response of bay productivity and herbivore/omnivore abundance. There was a dichotomous response of the estuarine trophic organization, with herbivores and omnivores responsive to river-dominated physicochemical factors whereas the carnivores responded to biological factors. Trophic response time could be measured in months to years from the point of the initiation of low-flow conditions. The reduction of nutrient loading during the drought period was postulated as a major cause of the loss of productivity of the river-dominated estuary during and after the drought period. Recovery of such productivity with resumption of increased river flows was likewise a long-term event. Based on the observed trends in the bay, postulated permanent reductions of freshwater flows due to anthropogenous ac- tivities could lead to major reductions of biological productivity in the Apalachicola Bay system. The long-term data indicated that, with reduction of freshwater flow below a level specific for the receiving system, the physically controlled, highly productive river-estu- arine system would become a species-rich, biologically controlled bay with substantially reduced productivity.

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.

An Index to Assess the Sensitivity of Gulf of Mexico Species to Changes in Estuarine Salinity Regimes

An index of biological sensitivity to changes in freshwater inflow was developed for 44 species in 22 Gulf of Mexico estuaries for adult and juvenile life stages of fishes and macroinvertebrates. The BioSalinity Index (BSI) provides an innovative approach to quantify estuary-specific sensitivity of organisms to changes in estuarine salinity regimes based upon our knowledge of species salinity habitat preferences, the availability of this preferred habitat, and the relative abundance and distribution of species in time and space. We found that a significant difference exists between adult and juvenile life stage sensitivity, with juveniles exhibiting a lower sensitivity to salinity changes than adults, and that a considerable disparity exists in species-specific sensitivities among Gulf estuaries. Likewise, when the full complement of 44 species-level BSIs are averaged, marked differences in assemblage-wide sensitivity are evident across estuaries. The availability of preferred salinity habitat had a greater influence on the BSI for estuarine species than did their relative abundance and temporal distribution. The BSI was applied by members of a 1995 Gulf of Mexico freshwater inflow workshop to identify a subset of estuaries which appear more sensitive to freshwater inflow changes and are candidates for further study.

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.

Ichthyoplankton Assemblage Structure in Mississippi Sound as Revealed by Canonical Correspondence Analysis

The spatiotemporal dynamics of the ichthyoplankton assemblage structure were investigated in Mississippi Sound, a northern Gulf of Mexico estuary. The study was based on a comprehensive survey constituting 528 collections from 22 stations over 12 months at two tow depths. Important environmental correlates of assemblage structure were identified using canonical correspondence analysis (CCA). A wide taxonomic diversity was encountered during the ichthyoplankton survey, including 152 taxa distributed among 48 families. Strong physical gradients in water temperature, dissolved oxygen and salinity correlated predominantly with ichthyoplankton assemblage structure, and reflected substantial spatiotemporal variation. The CCA results were interpreted in light of adult life-style characteristics. Ecological groups of larvae, as defined by adult life-style characteristics for respective taxa, showed distinctive dispersion patterns in 2-dimensional canonical space. A diverse group of offshore demersal taxa clustered as larvae in a region of CCA space corresponding spatiotemporally with stations located in passes and outside of the Sound during late summer and autumn. Other ecological groups, such as the inshore nektonic and offshore pelagic fishes, were widely dispersed as larvae in canonical space, with member taxa occurring under dissimilar physical conditions. Estuarine embayments in the north-central Gulf of Mexico, like Mississippi Sound, might be viewed appropriately as landwardmost sections of the wider, highly productive nursery grounds of this region.

An assessment of water quality and primary productivity in Perdido Bay, a Northern Gulf of Mexico Estuary

Perdido Bay is a shallow estuarine system of approximately 130 km(2) with three major freshwater inputs. On a seasonal basis the productivity and chlorophyll a concentration of phytoplankton in Perdido Bay are controlled by temperature. One input, Eleven Mile Creek, is influenced by a paper mill discharge. Eleven Mile Creek exhibits high levels of light attenuation, high concentrations of dissolved nutrients, and low rates of carbon fixation that are significantly different from the other inputs or areas of Perdido Bay and productivity in Eleven Mile Creek is light limited. Upper Perdido Bay had slightly elevated concentrations of dissolved nutrients which correlate with significantly higher rates of carbon fixation and phytoplankton biomass. Nutrients and color from Eleven Mile Creck are diluted by the Perdido River inflow, restricting nutrient and light limitations to the area at the mouth of Eleven Mile Creek.

Trophic Structure of Macrobenthic Communities in Northern Gulf of Mexico Estuaries

Trophic structure of estuarine benthic communities in the northem Gulf of Mexico was characterized according to the functional roles and geographic distributions of the macrobenthos. Macrobenthic organisms collected during two years of study were assigned to trophic groups to assess the relative utilization of detritus and other resources. Three groups of detritivores (surface-deposit feeders, subsurface-deposit feeders, and filter feeders) were numerically dominant among the benthos, each of which accounted for 2530% of total abundance across regions. Carnivorous macrobenthos also comprised an appreciable portion (12%), while omnivores (<3%) and other groups (<4%) were poorly represented. Dominance by detritivores is consistent with current concepts regarding the role of macrobenthos in processing detritus of Gulf of Mexico estuaries.

A Benthic Index of Environmental Condition of Gulf of Mexico Estuaries

An index was developed for estuarine macrobenthos in the Gulf of Mexico that discriminated between areas with degraded environmental conditions and areas with undegraded or reference conditions. Test sites were identified as degraded or reference based on criteria for dissolved oxygen levels, sediment toxicity tests, and sediment contamination. Discriminant analysis was used to identify a suite of measures of benthic community composition and diversity that would most successfully distinguish degraded from undegraded sites. The resultant benthic index was composed of a linear combination of three factors: the Shannon-Wiener diversity index, the proportion of total benthic abundance as tubificid oligochaetes, and the proportion of total benthic abundance as bivalve molluscs. This index was used to evaluate the spatial patterns of degraded benthic resources in the Gulf of Mexico.

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.

Erosion and Deposition in Northern Gulf of Mexico Estuaries: ABSTRACT

Erosion and Deposition in Northern Gulf of Mexico Estuaries: ABSTRACT

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

Storm-Related Rejuvenation of a Northern Gulf of Mexico Estuary: ABSTRACT

Storm-Related Rejuvenation of a Northern Gulf of Mexico Estuary: 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