Field Experiences in South Texas
The University of Texas Marine Science Institute


Dissolved Organic Carbon Cycling In Coastal Marsh Waters

Presenter:    Alexis Boggs, University of Texas at Austin, Marine & Freshwater Biology
Mentor:    Dr. Chris Shank

photo of Alexis BoggsMy study investigated the dynamics of dissolved organic carbon (DOC) in shallow waters of a Port Aransas salt marsh.  This research included an examination of daily fluctuations in DOC levels and the influence of solar radiation on DOC concentrations.  For my study, water samples were taken daily from four locations within the marsh at approximately 9:00 and 14:00 for a period of several weeks.  DOC concentrations, measured on 0.2μm-filtered water using a Shimadzu Total Organic Carbon Analyzer, ranged from 4.83 mg C/L to 25.27 mg C/L.  The magnitude of colored dissolved organic matter (CDOM), the light absorbing portion of the DOM pool, was also measured using a Lambda 35 UV/VIS spectrophotometer.  CDOM (a305) values ranged from 61 m-1 to 119 m-1.  Results show that as the water levels dropped, DOC and CDOM levels increased.  Controlled experiments involving simulated sunlight (Atlas Suntes XLS+ solar simulator) were conducted to assess the role of sediment resuspension on DOC levels using water and sediments from two sites in the marsh.  Photoproduction of DOC from sediment resuspension has recently been recognized as a significant source of dissolved organic carbon.  This carbon can then be incorporated by bacteria and reintroduced into the food web.  In my study, DOC concentrations of irradiated samples containing 1g of sediment increased from 10 mg C/L to 12 mg C/L after 24 hours of exposure.  The concentration of DOC in a dark control sample containing 0.5g of resuspended sediment decreased from 10 mg C/L to 9 mg C/L.  This confirms our predictions that solar radiation plays a role in DOC cycling in shallow salt marsh systems.

Effects Of Temperature On Sensory And Locomotive Systems Of Red Drum Larvae During Escape Responses

Presenter:    Mary Clark, Sewanee-The University of the South, Environmental Studies
Mentors:    Dr. Lee A. Fuiman, Dr. Alfredo F. Ojanguren

photo of Mary ClarkThe activity of all ectotherms is partially regulated by environmental temperatures. Some species have broad thermal tolerance ranges that allow them to endure major thermal fluctuations, but most species are restricted to narrower ranges of temperatures. The red drum (Sciaenops ocellatus) is an interesting species because it spawns during a period of the year (September-November) when water temperatures are highly unpredictable. Little is known about how these thermal fluctuations affect the red drum’s reproductive strategy, but previous studies have indicated that larval survival decreases throughout the spawning season as temperatures decrease. The objective of this study was to determine whether temperature directly affects escape performance of red drum larvae and to estimate the range of temperatures at which performance is optimum. Escape performance of 102 larvae was tested during a digitally simulated predator attack at 5 different temperatures that are within the range of variability for a typical spawning season (20, 24, 27, 29, 31 ºC). High-speed video analysis was used to measure 9 variables of locomotive and sensory performance based on routine swimming and escape performance trials. Larval responsiveness to the stimuli showed a highly significant positive relationship with temperature, with an increase from approximately 45% to 90% between 20 and 31 ºC. Average and maximum swimming speeds during the escape responses fit within a typical dome-shaped thermal performance curve and showed significant differences among the five temperatures—with an optimum performance temperature of 28 ºC for both variables. The contrast between responsiveness and speed in these trials suggests that temperature affects sensory and locomotive systems differently and that the temperature at which larvae are most responsive may be significantly higher than the temperature at which overall escape performance is highest.

Experimental Studies In Green Water Aquaculture In Larval Red Drum

Presenter:    Rynnie Henderson, Spring Hill College, Marine Biology
Mentors:    Dr. G. Joan Holt, Cindy Faulk

lab photoThe presence of live algae in larval fish rearing tanks has been shown to improve growth rates, survival rates, and feeding, ultimately increasing fingerling production. Red drum (Sciaenops ocellatus) is no exception. However, the use of live algae is time and cost inefficient. This study examines two possible alternatives to live algae, a green dye (Chroma®) and a manufactured algal substitute (Sanolife ALG®) by comparing the larval growth rate, survival rate, and trypsin activity of these treatments with a clear water control and in the presence of a microalgae (Isochrysis galbana). Standard length of the larvae increased from an average of 2.6 mm at 1 dph to 7.0 mm at 18 dph. There were no significant differences (P>0.05) in standard length noted among treatments.  Survival means ranged from 21.3% to 30.5% with no significant differences among treatments. Trypsin activity increased from 0.25 U/larva/mm at 10 dph to 0.40 U/larva/mm at 18 dph, again with no significant differences among treatments. While both Chroma® and Sanolife ALG® still appear to be potential replacements, an alternative to live algae was not definitively determined by this study. Low overall survival in comparison with previous studies suggests incomplete weaning on dry feed. Future studies will need to first determine a better weaning regime to ensure results are not compromised by poor feeding or weaning before retesting these and other potential algae alternatives.

Stable Nitrogen Isotopes In Vegetation Along The San Antonio Bay Watershed

Presenter:    Allison James, Bard College, Biology
Mentors:    Dr. Ken Dunton, Sarah Wallace

photo of Allison JamesPrevious studies have shown the San Antonio Bay to have significantly higher proportions of 15N in its water and biota compared to other Texas bays in relation to river inflow.  Elevated 15N levels may be an indication of anthropogenic nitrogen pollution, which may contribute to eutrophication and poor bay health.  In order to investigate the source of these elevated numbers, algae, riparian plants, and aquatic invertebrates were sampled along the Guadalupe and San Antonio Rivers.  15N values (the relative abundance of 15N to 14N) were determined for samples and compared according to site by distance upstream.  Comparisons were also made between rivers since the San Antonio River drains a much larger metropolitan area than the Guadalupe.  15N values were expected to increase at certain points along both of the rivers where large sources of heavier nitrogen (i.e. wastewater) were entering the water system.  Both the Guadalupe and San Antonio River were found to have high 15N values near their confluence before entering the bay, with the Guadalupe having about twice the water discharge of the San Antonio.  Moving downstream from the headwaters the Guadalupe 15N values were found to increase linearly with the algae.  A similar, yet less precise trend was present amongst the plants.  Aquatic invertebrates did not follow the trend displayed by the vegetation, most likely because they feed on non-aquatic insects and therefore do not reflect the nutrients of the water column.  In the San Antonio River, an upward trend comparable to that in the Guadalupe was observed, but 15N values were higher.  There are several potential explanations for the observed trend amongst vegetation.  For this case, it is most likely due to an increasing contribution of heavy nitrogen from tributaries and runoff as the rivers move downstream.

Organic Matter Dynamics In Tidally-Influenced Freshwater Sections Of The Mission And Aransas Rivers

Presenter:    Nick Klein, Augustana College, Biology
Mentor:     Dr. James McClelland

The Mission and Aransas are tidally-influenced rivers emptying into Copano Bay, both with extensive agricultural and other developed land use in their watersheds. Export from both rivers is dominated by infrequent storm events. During normal, low-flow conditions, the lower freshwater sections of both rivers are pushed upstream during incoming tides, and a salinity gradient forms only relatively near the bay. Such tidally-influenced freshwater reaches are poorly accounted for in riverine export modeling, but may significantly alter exports due to processing during lengthened residence times.

photo of Nick KleinSamples from ten sites on the Mission and sixteen on the Aransas as well as endpoints farther upstream and in Copano Bay were analyzed. DIN concentrations are near limit of detection throughout the tidally-influenced freshwater reaches, and both systems are N-limited. Autochthonous production dominates (POδ13C < -30‰), with 15N-enriched PON values likely indicating chiefly terrestrial-derived N, a product of wastewater outfalls and potentially denitrification within the river. Production (as measured by [PON], [POC], [Chl A], and surface %DO) is high throughout and peaks near the beginning of the salinity gradient, possibly due to a turbidity maximum, a particle “trap” caused physically by restricted mixing at the meeting of the two distinct bodies of water. The [DON] and [DOC] maximum lies just downstream of maximum productivity and corresponds with a peak in the [DOC]:[Chl A] to values associated with decomposition, likely caused by lysis of freshwater phytoplankton encountering the increasing salinity. DON and DOC concentrations decrease downstream and into the bay, but remain elevated. The data indicate high production and assimilation of inorganic nutrient inputs within the tidally-influenced freshwater reaches, resulting in export of organic material into the bay.

Hypoxia In South Texas Bays And Nearshore Waters During Summer 2008

Presenter:    Adrianne Lee, Wellesley College, Geosciences
Mentor:    Dr. Dong-Ha Min

Hypoxia refers to low oxygen water conditions, often defined as less than 2 mg/L of dissolved oxygen. While hypoxia is a natural phenomenon, its increasing incidence worldwide is concerning. The compromised water quality in hypoxic zones inhibits benthic organisms’ survival and lowers species population and diversity. Hypoxic zones have been linked to nutrient loading, water column stratification, sediment oxygen demand, and natural oxygen minimum zones in the open ocean.

In this study, five South Texas bays, one tidal inlet, and one nearshore Gulf of Mexico shelf water transect, were surveyed with several revisits, to determine the spatial distribution of hypoxic waters during the summer of 2008, to examine whether waters of hypoxic condition in the bays and offshore were physically connected through the tidal inlet, and to observe how hypoxic waters may form or break down. Hypoxia was observed at bottom depths in Copano Bay, Corpus Christi Bay near the mouth of Upper Laguna Madre, and Gulf of Mexico shelf waters, which compose the three mixing endmembers involved in the study area. According to T-S analysis, there is direct mixing between Copano Bay and Upper Laguna Madre waters and also between Offshore and mixed bay waters through the tidal inlet. The hypoxic bottom waters are localized in distinct water masses and do not appear to be physically connected through the tidal inlet. Any particular stations where hypoxia was observed during the first survey exhibit normoxic (higher oxygen) conditions during the subsequent survey, suggesting episodic nature of hypoxia. While offshore waters exhibit a correlation between surface-to-bottom salinity difference, an indicator of water column stratification, and bottom oxygen concentrations, the relationship was not seen in inshore water surveys. Despite its episodic nature, the severity and prevalence of hypoxic waters can be a considerable threat to the South Texas estuarine ecosystem.

Crab Counting And The Controversy Of The Canal: A Preliminary Assesment Of Larval Recruitment In The Aransas Ship Channel

Presenter:    Caitlin McKimmy, Carleton College
Mentor:     Dr. Ed Buskey

The blue crab, Callinectes sapidus, spawns and hatches in the open ocean—post-larval crabs (a.k.a. megalopae) must then recruit into estuarine environments to develop into fully fledged adults. Although blue crab larval recruitment has been studied extensively in Atlantic coastal waters, little research has been conducted around the unique barrier islands on the coast of the Gulf of Mexico. Recruitment around Mustang, San José, and Matagorda Islands is of special interest in light of the recent closure of Cedar Bayou, a natural canal between the gulf and Mesquite Bay. Cedar Bayou, when opened, is the most direct route for crab megalopae to enter the estuarine waters surrounding the Aransas National Wildlife Refuge. If the blue crab population in this area is recruitment limited, it follows that the condition of Cedar Bayou affects blue crabs and their predators, such as the endangered whooping crane. My objectives were 1) to pilot the use of artificial settlement collectors in this area to produce a base data set for future research and 2) to examine historical records in an effort to determine what might limit Callinectus populations in the Mission-Aransas Estuary. The megalopal recruitment patterns I observed agreed with published data, suggesting that my methods were indeed effective, and these data regarding settlement in the ship channel are available as a resource for future research. My examination of historical data revealed a correlation between variations in climate and adult blue crab populations, implying that recruitment is not a major limitation, but more extensive studies are required to substantiate this claim.

The Role Of GPR30 And The Effects Of Estrogens On Zebrafish And Croaker Oocyte Maturation

Presenter:    Jennifer Strykowski, University of South Florida, Marine biology
Mentor:    Dr. Peter Thomas

photo of Jennifer Strykowski Zebrafish (Danio rerio) are becoming increasingly popular among scientists across the globe for biological studies. The entire genome of zebrafish is known, and due to the large amount of evolutionary conservation among zebrafish and other vertebrates, this species of freshwater fish is an excellent model organism. Therefore, zebrafish were used for many of the biological assays that were the basis of my research. Easy access, frequent spawning, and a short oocyte maturation time allowed me to use this species to perform one assay almost every day. First, it was necessary to determine under what conditions the oocytes would mature with minimal spontaneous maturation. This was determined by harvesting zebrafish at varying times throughout the day and observing the length of time that was necessary for the oocytes to experience maximal amounts of germinal vesicle breakdown (GVBD), an indicator of maturation, when treated with a maturation inducing hormone while untreated oocytes experienced minimal amounts of GVBD. After determining the precise conditions that were necessary for the assays to work effectively, an aromatase inhibitor was used as speculation that estrogens were involved in the mechanism controlling maturation. More specifically, it was hypothesized that signaling by estrogens resulted in the arrest of meiotic maturation when the aromatase inhibitor allowed for a rate of germinal vesicle breakdown nearly equivalent to the amount resulting from the addition of a maturation inducing hormone. However, the inconsistent quality of zebrafish resulted in a switch to utilizing Atlantic croaker (Micropogonias undulates) oocytes. By adding estrogens that bind to a membrane receptor, GPR30, it was possible to observe the effects on maturation. There is currently evidence that estrogens signal for the halt of meiotic maturation but ongoing research will determine if environmental doses of estrogens and xenoestrogens result in significant decreases in maturation.

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