Mission-Aransas Estuary Research Articles

Paired passive acoustic and gillnet sampling reveal the utility of bioacoustics for monitoring fish populations in a turbid estuary

Abstract Estuaries worldwide provide critical habitat to many ecologically and socioeconomically important fish species. However, the high turbidity and strong environmental gradients inherent to many estuaries make the monitoring of fish populations a challenging task. To assess the health and diversity of local fish populations, resource managers often employ fisheries-independent sampling methods, which result in valuable population assessments, but suffer from low-temporal resolution and high rates of mortality. Passive acoustic monitoring (PAM) provides a near-continuous, non-invasive means for monitoring soniferous fish activity over extended periods, but its utility for quantifying entire fish communities is largely untested, especially in turbid estuaries. Here, we use synchronized acoustic and gillnet sampling at 51 locations in the Mission-Aransas Estuary (TX), across two sampling seasons to test correlations between acoustic indices and gillnet catch. Positive correlations were observed between gillnet catch and sound pressure levels in both broadband (50–22050 Hz) and low-frequency (50–2500 Hz) bands. Additionally, sciaenid calling activity was positively correlated with sciaenid catch during the spring, when most spawning activity occurs. Our results indicate that PAM can be used to monitor the relative abundances of fish in Gulf of Mexico estuaries and beyond, although this is subject to species identity and seasonal dynamics.

A Biogeochemical Alkalinity Sink in a Shallow, Semiarid Estuary of the Northwestern Gulf of Mexico

Estuarine total alkalinity (TA), which buffers against acidification, is temporally and spatially variable and regulated by complex, interacting hydrologic and biogeochemical processes. During periods of net evaporation (drought), the Mission-Aransas Estuary (MAE) of the northwestern Gulf of Mexico experienced TA losses beyond what can be attributed to calcification. The contribution of sedimentary oxidation of reduced sulfur to the TA loss was examined in this study. Water column samples were collected from five stations within MAE and analyzed for salinity, TA, and calcium ion concentrations. Sediment samples from four of these monitoring stations and one additional station within MAE were collected and incubated between 2018 and 2021. TA, calcium, magnesium, and sulfate ion concentrations were analyzed for these incubations. Production of sulfate along with TA consumption (or production) beyond what can be attributed to calcification (or carbonate dissolution) was observed. These results suggest that oxidation of reduced sulfur consumed TA in MAE during droughts. We estimate that the upper limit of TA consumption due to reduced sulfur oxidation can be as much as 4.60 × 108 mol day−1 in MAE. This biogeochemical TA sink may be present in other similar subtropical, freshwater-starved estuaries around the world.

Radioactive and stable isotopes reveal variations in nearshore submarine groundwater discharge composition and magnitude across low inflow northwestern Gulf of Mexico estuaries

To determine how submarine groundwater discharge (SGD) magnitudes and composition (fresh or saline/recirculated) vary in nearshore low inflow estuaries across ⁓125 km of a semiarid coastline, this study assessed three south Texas estuaries, using radon [222Rn], radium [226Ra and 224Ra], and water isotopes [δ18O and δD]. Mass balance models of time-series 222Rn, found to be representative of total SGD in this study, revealed much higher SGD inputs to the Nueces Estuary (average [x̅] Nueces, Corpus Christi and Oso Bays: 120, 83, and 44 cm·d−1, respectively), attributed to anthropogenically-disturbed substrates and potentially surfacing growth-faults. The lowest 222Rn-derived SGD occurred in the Upper Laguna Madre Estuary (x̅: Upper Laguna Madre and Baffin Bay: 21 and 18 cm·d−1, respectively), explained by the drier climate, lower anthropogenic disturbance, and neighboring groundwater cone of depression. Aransas Bay in the Mission Aransas Estuary received greater average annual precipitation but exhibited low total SGD rates (x̅: 23 cm·d−1). Seasonally, average 222Rn-derived SGD rates increased following Hurricane Harvey (43 cm·d−1 in spring to 64 cm·d−1 in summer). In the Nueces Estuary, the overall 222Rn-derived SGDs were substantially higher than SGDs from 224Ra and 226Ra. The closer agreement between 224Ra and 222Rn-derived SGD and larger 224Ra rates in the Upper Laguna Madre Estuary, Aransas Bay and Oso Bay indicate that saline/recirculated SGD contributions were significant. Values of δ18O and δD confirm these types of inputs, with effects of evaporation/salinization more pronounced where recirculation was predominant and the opposite where terrestrial/222Rn-derived SGD inputs dominate. 226Ra-derived SGDs were lower than the 224Ra due to different behavior of the two isotopes while released into water following transport through saline and fine-grained estuarine sediments or due to wind-driven disturbances.

Integrating High-Resolution Coastal Acidification Monitoring Data Across Seven United States Estuaries.

Beginning in 2015, the United States Environmental Protection Agency’s (EPA’s) National Estuary Program (NEP) started a collaboration with partners in seven estuaries along the East Coast (Barnegat Bay; Casco Bay), West Coast (Santa Monica Bay; San Francisco Bay; Tillamook Bay), and the Gulf of Mexico (GOM) Coast (Tampa Bay; Mission-Aransas Estuary) of the United States to expand the use of autonomous monitoring of partial pressure of carbon dioxide (pCO2) and pH. Analysis of high-frequency (hourly to sub-hourly) coastal acidification data including pCO2, pH, temperature, salinity, and dissolved oxygen (DO) indicate that the sensors effectively captured key parameter measurements under challenging environmental conditions, allowing for an initial characterization of daily to seasonal trends in carbonate chemistry across a range of estuarine settings. Multi-year monitoring showed that across all water bodies temperature and pCO2 covaried, suggesting that pCO2 variability was governed, in part, by seasonal temperature changes with average pCO2 being lower in cooler, winter months and higher in warmer, summer months. Furthermore, the timing of seasonal shifts towards increasing (or decreasing) pCO2 varied by location and appears to be related to regional climate conditions. Specifically, pCO2 increases began earlier in the year in warmer water, lower latitude water bodies in the GOM (Tampa Bay; Mission-Aransas Estuary) as compared with cooler water, higher latitude water bodies in the northeast (Barnegat Bay; Casco Bay), and upwelling-influenced West Coast water bodies (Tillamook Bay; Santa Monica Bay; San Francisco Bay). Results suggest that both thermal and non-thermal influences are important drivers of pCO2 in Tampa Bay oxygen, National Estuary Program and Mission-Aransas Estuary. Conversely, non-thermal processes, most notably the biogeochemical structure of coastal upwelling, appear to be largely responsible for the observed pCO2 values in West Coast water bodies. The co-occurrence of high salinity, high pCO2, low DO, and low temperature water in Santa Monica Bay and San Francisco Bay characterize the coastal upwelling paradigm that is also evident in Tillamook Bay when upwelling dominates freshwater runoff and local processes. These data demonstrate that high-quality carbonate chemistry observations can be recorded from estuarine environments using autonomous sensors originally designed for open-ocean settings.

Oyster growth across a salinity gradient in a shallow, subtropical Gulf of Mexico estuary

Abstract An increase in oyster aquaculture as a sustainable method of shellfish production is one response to overharvest and degradation of natural oyster reefs over the past century. Successful aquaculture production requires determining the environmental conditions optimal for oyster growth. In this study, the salinity, temperature, chlorophyll a concentration and the growth of Crassostrea virginica were monitored at four locations within the Mission-Aransas Estuary, Texas (USA), a shallow subtropical estuary influenced by relatively low freshwater inflow. Mean growth of the oyster shell (0.205 mm d–1 and 0.203 g d–1) and soft tissues (3.447 mg d–1) was highest when salinity was low (mean = 15.5) and chlorophyll a concentration was high (8.4 μg l–1). Oyster growth also varied temporally with periods of spawning. In low-inflow estuaries such as the Mission-Aransas Estuary, oyster farms should be sited close to river mouths so that oysters can benefit from freshwater inflows and lower salinities.

Cost-effective use of aerial imagery to quantify faunal-habitat associations across multiple spatial scales

We investigated the utility of publicly available map products to identify faunal-habitat associations to inform fisheries management. The influence of structural and abiotic variables on the abundance of 4 focal species (red drum Sciaenops ocellatus, spotted seatrout Cynoscion nebulosus, blue crab Callinectes sapidus, and brown shrimp Farfantepenaeus aztecus) was examined at multiple spatial scales in the Mission-Aransas estuary, Texas, USA. Structural habitat data were derived from the 2004 Benthic Atlas of Texas and paired with species abundance and hydrographic data collected by the Texas Parks and Wildlife Department’s Fisheries Independent Monitoring program. Blue crab and brown shrimp distributions at the ecosystem scale were driven primarily by abiotic factors such as salinity and distance to inlet, but structural habitat became relatively more important at smaller spatial scales. Variation in spotted seatrout abundance was primarily driven by seagrass and saltmarsh extent at the ecosystem scale but was driven by abiotic factors at the sub-region scale. Red drum distribution was driven primarily by abiotic factors, and association with structural habitats was bay-specific; there was no clear pattern in the influence of spatial scale on the relative importance of structural versus abiotic habitat variables for red drum. Results demonstrate that leveraging existing mapping efforts can allow fisheries managers to incorporate habitat data into fisheries management when funding or logistical challenges may otherwise prevent habitat monitoring at management-relevant scales. More research is warranted to investigate the temporal limitations to this approach, such as identifying the ‘shelf life’ of historical mapping products for utility in fisheries management.

Spatial and temporal settlement patterns of blue crab (Callinectes sapidus and Callinectes similis) megalopae in a drought-prone Texas estuary

The Mission-Aransas Estuary is the wintering ground for the only sustained wild population of the endangered whooping crane (Grus americana), and blue crabs (Callinectes sapidus) are an important component of their diet as well as being a major food source for important sport fishes such as the red and black drum. Blue crabs also support a commercial crabbing industry, and fisheries data indicate that blue crab populations have been declining since the 1980s. Possible factors leading to decline in blue crab populations include overfishing, increased populations and predation by regulated sport fishes, reduced freshwater inflows into estuaries, and reduced larval recruitment. Little is known about blue crab recruitment dynamics in this region, but restricted passes between coastal estuaries and the Gulf of Mexico along with extended periods of drought that often lead to hypersaline conditions in coastal bays may limit larval recruitment from the Gulf into the bays. To investigate blue crab larval recruitment patterns, citizen scientist volunteers used hogshair settlement collectors to sample five monitoring sites over a four year period. Results show that large numbers of blue crab megalopae are common in nearshore waters of the Gulf of Mexico, but only a small fraction (∼1%) recruit into the estuary. Peak periods of ingress into the estuary occur during fall and winter months, with C. sapidus primarily contributing to the fall peak and C. similis dominating the winter peak. Increased salinity in the estuary during droughts may reduce the ability of blue crab larvae to detect and enter passes into the estuary.

Influence of Major Storm Events on the Quantity and Composition of Particulate Organic Matter and the Phytoplankton Community in a Subtropical Estuary, Texas

Variations in the freshwater inflow regimes of estuaries due to perturbations, such as storm events, alter the source and composition of particulate organic matter (POM) and the phytoplankton community, which are key links in estuarine carbon and nitrogen cycling. To evaluate the impact of varying freshwater discharge on POM quantity and composition and the phytoplankton community, monthly samples of surface water were collected at four long-term monitoring stations from 2012-2015 in a subtropical estuarine system, the Mission-Aransas Estuary, Texas (USA). This system is characterized by a semiarid climate, drought, and sporadic precipitation events. We analyzed organic carbon and nitrogen content, carbon and nitrogen stable isotopes, and chlorophyll a and accessory pigments of the POM samples. Following a prolonged dry spell (2012-2015), consecutive major storm events in spring 2015 led to a dramatic freshening of the entire estuary. Large increases in particulate organic carbon (POC) concentrations and decreases in δ13C values of POC over several weeks following the storms suggest an increase in in-situ production at lower salinities as a result of increased freshwater inflow. These changes in bulk POM coincided with an increase in chlorophyll a concentration, further indicating a significant contribution by phytoplankton to the elevated POM. Concurrently, pigment biomarkers revealed a significant (10-fold) increase in the cyanobacterial pigment, zeaxanthin, and further 16S rRNA analysis showed that Cyanobium spp. was responsible for the observed bloom. The combination of environment conditions, including freshening, high temperature, and high nutrients, likely contributed to the cyanobacteria bloom. These results show that episodic rain events can substantially affect estuarine phytoplankton community composition, which impacts the available energy resources for secondary production and thus may have ecosystem-wide implications on productivity.

Origin, composition and quality of suspended particulate organic matter in relation to freshwater inflow in a South Texas estuary

South Texas has a semi-arid climate with a large interannual variability of freshwater inflows. This study sought to define how changes in freshwater inflow affect the composition, quantity and quality of suspended particulate organic matter (SPOM) in a South Texas estuary: the Mission-Aransas estuary. The study was implemented 1.5 months after a large rain event in September 2010 and continued for 10 months of drought conditions. The composition of SPOM originating from rivers, the Gulf of Mexico and the estuary were determined using stable isotopes (δ13C, δ15N and δ34S). The quantity and quality of SPOM were assessed using organic carbon content, chlorophyll a concentrations and C/chl a ratios. Our results demonstrated that autochthonous phytoplankton was the dominant component of SPOM in the Mission-Aransas estuary during droughts. Benthic organic matter from local primary producers (i.e., seagrass, salt marsh plants, benthic microalgae) did not influence SPOM composition, either as fresh material or as detritus. A comparison with a positive estuary (i.e., Sabine-Neches estuary, TX) indicates that decreases in freshwater inflow may lead to decreases of terrestrial organic matter inputs and to increase the ratio of autochtonous phytoplanktonic material in SPOM.

Role and Value of Nitrogen Regulation Provided by Oysters (Crassostrea virginica) in the Mission-Aransas Estuary, Texas, USA

Suspension-feeding activities of oysters impart a potentially significant benefit to estuarine ecosystems via reduction of water column nutrients, plankton and seston biomass, and primary productivity which can have a significant impact on human well-being. This study considered nitrogen regulation by eastern oysters Crassostrea virginica in the Mission-Aransas Estuary, Texas, USA, as a function of denitrification, burial, and physical transport from the system via harvest. Oyster reefs were estimated to remove 502.5 kg N km−2 through denitrification of biodeposits and 251.3 kg N km−2 in burial of biodeposits to sediments. Nitrogen is also physically transported out of the estuary via harvest of oysters. Commercial harvest of oysters in the Mission-Aransas Estuary can remove approximately 21,665 kg N per year via physical transport from the system. We developed a transferable method to value the service of nitrogen regulation by oysters, where the potential cost equivalent value of nitrogen regulation is quantified via cost estimates for a constructed biological nutrient removal (BNR) supplement to a wastewater treatment plant. The potential annual engineered cost equivalent of the service of nitrogen regulation and removal provided by reefs in the Mission-Aransas Estuary is $293,993 yr−1. Monetizing ecosystem services can help increase awareness at the stakeholder level of the importance of oysters beyond commercial fishery values alone.

A Restoration Suitability Index Model for the Eastern Oyster (Crassostrea virginica) in the Mission-Aransas Estuary, TX, USA

Oyster reefs are one of the most threatened marine habitats on earth, with habitat loss resulting from water quality degradation, coastal development, destructive fishing practices, overfishing, and storm impacts. For successful and sustainable oyster reef restoration efforts, it is necessary to choose sites that support long-term growth and survival of oysters. Selection of suitable sites is critically important as it can greatly influence mortality factors and may largely determine the ultimate success of the restoration project. The application of Geographic Information Systems (GIS) provides an effective methodology for identifying suitable sites for oyster reef restoration and removes much of the uncertainty involved in the sometimes trial and error selection process. This approach also provides an objective and quantitative tool for planning future oyster reef restoration efforts. The aim of this study was to develop a restoration suitability index model and reef quality index model to characterize locations based on their potential for successful reef restoration within the Mission-Aransas Estuary, Texas, USA. The restoration suitability index model focuses on salinity, temperature, turbidity, dissolved oxygen, and depth, while the reef quality index model focuses on abundance of live oysters, dead shell, and spat. Size-specific Perkinsus marinus infection levels were mapped to illustrate general disease trends. This application was effective in identifying suitable sites for oyster reef restoration, is flexible in its use, and provides a mechanism for considering alternative approaches. The end product is a practical decision-support tool that can be used by coastal resource managers to improve oyster restoration efforts. As oyster reef restoration activities continue at small and large-scales, site selection criteria are critical for assisting stakeholders and managers and for maximizing long-term sustainability of oyster resources.

Role of Flood Disturbance in Natural Oyster (Crassostrea virginica) Population Maintenance in an Estuary in South Texas, USA

A 2-year period with flood versus drought conditions provided the opportunity to examine the effects of flood disturbance on subtidal eastern oyster Crassostrea virginica biology and population dynamics in a south Texas estuary. Oysters were sampled monthly in 2007 and 2008 to examine the impacts of changing environmental conditions on oyster populations. Oysters were also examined quarterly for the presence of Perkinsus marinus. Filtration rates were calculated as a function of oyster size, temperature, salinity, and total suspended solids. Flood events in 2007 caused temporary reductions in salinity and were associated with reductions in oyster abundance, spat settlement, disease levels (weighted prevalence and percent infection), and filtration rates. Oyster populations had generally recovered within 1 year’s time—the oysters were younger and smaller but were just as abundant as pre-flood levels. The rapid return of oysters to pre-flood abundance levels is attributed in part to the ability of oysters in Gulf coast estuaries to spawn multiple times in a single season and in part to their relatively high growth rates. Although flood disturbance may temporarily reduce or destroy oyster populations, the ability of the Mission–Aransas Estuary to retain freshwater pulses within the system and maintain low salinities that are unfavorable for predators and disease can facilitate oyster population recovery. Episodic flood events appear to play a critical role in promoting long-term oyster population maintenance in the Mission–Aransas Estuary. The response of oysters to changing environmental conditions over the short term provides some insights into the potential long-term effects of changing climate.

FIRST HARMFUL DINOPHYSIS (DINOPHYCEAE, DINOPHYSIALES) BLOOM IN THE U.S. IS REVEALED BY AUTOMATED IMAGING FLOW CYTOMETRY1

Imaging FlowCytobot (IFCB) combines video and flow cytometric technology to capture images of nano- and microplankton (∼10 to >100 μm) and to measure the chlorophyll fluorescence associated with each image. The images are of sufficient resolution to identify many organisms to genus or even species level. IFCB has provided >200 million images since its installation at the entrance to the Mission-Aransas estuary (Port Aransas, TX, USA) in September 2007. In early February 2008, Dinophysis cells (1–5 · mL−1) were detected by manual inspection of images; by late February, abundance estimates exceeded 200 cells · mL−1. Manual microscopy of water samples from the site confirmed that D. cf. ovum F. Schütt was the dominant species, with cell concentrations similar to those calculated from IFCB data, and toxin analyses showed that okadaic acid was present, which led to closing of shellfish harvesting. Analysis of the time series using automated image classification (extraction of image features and supervised machine learning algorithms) revealed a dynamic phytoplankton community composition. Before the Dinophysis bloom, Myrionecta rubra (a prey item of Dinophysis) was observed, and another potentially toxic dinoflagellate, Prorocentrum, was observed after the bloom. Dinophysis cell-division rates, as estimated from the frequency of dividing cells, were the highest at the beginning of the bloom. Considered on a daily basis, cell concentration increased roughly exponentially up to the bloom peak, but closer inspection revealed that the increases generally occurred when the direction of water flow was into the estuary, suggesting the source of the bloom was offshore.