Guadalupe Estuary Research Articles

Aragonite saturation states in estuaries along a climate gradient in the northwestern Gulf of Mexico

In the northwestern Gulf of Mexico (nwGOM), the coastal climate shifts abruptly from the humid northeast to the semiarid southwest within a narrow latitudinal range. The climate effect plays an important role in controlling freshwater discharge into the shallow estuaries in this region. In addition to diminishing freshwater runoff down the coast, evaporation also increases substantially. Hence, these estuaries show increasing salinity along the coastline due to the large difference in freshwater inflow balance (river runoff and precipitation minus evaporation and diversion). However, this spatial gradient can be disrupted by intense storm events as a copious amount of precipitation leads to river flooding, which can cause temporary freshening of these systems in extreme cases, in addition to freshwater-induced ephemeral stratification. We examined estuarine water aragonite saturation state (Ωarag) data collected between 2014 and 2018, covering a period of contrasting hydrological conditions, from the initial drought to multiple flooding events, including a brief period that was influenced by a category 4 hurricane. Based on freshwater availability, these estuaries exhibited a diminishing Ωarag fluctuation from the most freshwater enriched Guadalupe Estuary to the most freshwater-starved Nueces Estuary. While Ωarag values were usually much higher than the threshold level (Ωarag = 1), brief freshwater discharge events and subsequent low oxygen levels in the lower water column led to episodic corrosive conditions. Based on previously obtained Ωarag temporal trends and Ωarag values obtained in this study, we estimated the time of emergence (ToE) for Ωarag. Not only did estuaries show decreasing ToE with diminishing freshwater availability but the sub-embayments of individual estuaries that had a less freshwater influence also had shorter ToE. This spatial pattern suggests that planning coastal restoration efforts, especially for shellfish organisms, should emphasize areas with longer ToE.

Timescales and Magnitude of Water Quality Change in Three Texas Estuaries Induced by Passage of Hurricane Harvey

Tropical cyclones represent a substantial disturbance to water quality in coastal ecosystems via storm surge, winds, and flooding. However, evidence to date suggests that the impacts of tropical cyclones on water quality are generally short-lived (days-months) and that the magnitude of the disturbance is related to proximity to storm track. Discrete and continuous water samples were collected in three Texas estuaries before and after Hurricane Harvey made landfall in 2017. Of the three estuaries, the Guadalupe Estuary and its watershed received the highest rainfall totals and wind speeds. An ephemeral increase in salinity was observed (mean of 9.8 on 24 August 2017 to a peak of 32.1 on 26 August 2017) due to storm surge and was followed by a rapid decrease to < 1 as floodwaters reached the estuary. Salinity returned to pre-storm levels within 1 month. During the low salinity period, bottom water hypoxia developed and lasted for 9 days. In all three estuaries, there was an increase in inorganic nutrients post-Harvey, but the nutrients largely returned to pre-storm baseline levels by winter. The lack of long-term water quality impacts from Harvey despite its severity corroborates previous findings that estuarine water quality tends to return to baseline conditions within days to a few months after storm passage.

A Tool for Rapid Assessment of Hydrological Connectivity Patterns in Texas Coastal Wetlands: Linkages between Tidal Creeks and Coastal Ponds

Coastal salt marshes are heterogeneous, spatially complex ecosystems. The degree of hydrological connectivity in these systems can be a significant driver in the flux of energy, organisms, and nutrients across the marsh landscape. In tidally driven systems, the frequency and magnitude of hydrological connection events results in the creation of a matrix of intermittently connected coastal wetland habitats, some of which may be hydrologically isolated or partially drained at any given time. Previous approaches to understanding landscape-level hydrologic connectivity patterns have required either intensive long-term monitoring or spatially explicit modeling. In this paper, we first describe a 13-month field study in the Guadalupe Estuary of the Texas Gulf Coast that linked hydrological connectivity patterns between a saltwater pond to water levels in an adjacent tidal creek and nearby San Antonio Bay. We next describe the integration of these field data with high-resolution digital elevation models and environmental parameters to develop a spatially explicit model that is a Simulation of Landscape-level Oscillations in Salt Marsh Hydroperiod (SLOSH). We evaluated the ability of SLOSH to simulate trends in landscape-level patterns of hydrological connectivity between a tidal creek and an inland marsh pond. Magnitude and periodicity of simulated and observed water-level fluctuations in the pond were similar. Highest creek water levels, resulting in high frequency and duration of hydrological connectivity with the pond, corresponded with the highest bay water levels, which occurred during September and October. Lowest creek water levels, resulting in low frequency and duration of hydrological connectivity, corresponded with the lowest bay water levels, which occurred during December through February. By simulating the pulsing structure of salt marsh hydrology, SLOSH creates the foundation on which to assess how additional drivers (precipitation, wind, freshwater inflows, etc.) can influence coastal marsh hydrology and overall ecology. Citation: Swannack TM, Wozniak JR, Grant WE, Davis SE III. 2019. A tool for rapid assessment of hydrological connectivity patterns in Texas coastal wetlands: linkages between tidal creeks and coastal ponds. Texas Water Journal. 10(1):46-59. Available from: https://doi.org/10.21423/twj.v10i1.7073.

The relationship between suspended solids and nutrients with variable hydrologic flow regimes

The hypothesis that “freshwater inflow variability over space and time can drive suspended solids and nutrient concentrations” was tested by comparing three micro-tidal estuaries (Guadalupe, Lavaca-Colorado, and Nueces) in Texas with different hydrologic flow regimes over three years with wet and dry conditions. In all three estuaries, Total suspended solids (TSS) was less than 50 mg/L most of the time. In the Nueces Estuary, TSS of higher than 100 mg/L occurred during frontal events. Dissolved inorganic nitrogen (ammonia+nitrite+nitrate) concentrations were most of the time lowest in the Nueces Estuary (i.e. ≤0.5μmol/L), with low inflow rates and high average salinity of 37.6. Salinity was highest in the river-estuary mouth (average salinity 38.3) of the Nueces Estuary relative to the other oceanic-side stations (average salinity 37), indicating that the system was a “reverse estuary” where evaporation exceeds freshwater inflow, resulting in net inflow of marine water into the estuary. The inverse correlation between ammonium and salinity in all three estuaries and the corresponding negative correlation between nitrite+nitrate concentrations and salinity in the Guadalupe Estuary indicate that the quantity of inflow controls nitrogen concentrations and transformations in the three estuaries. Drought conditions limited riverine transport of nitrogen and sediment to the three estuaries, demonstrating the importance of freshwater inflow to maintaining these constituents. Average silica and orthophosphate concentrations correlated positively with chlorophyll-a in combined data from all three estuaries. Silica and orthophosphate concentrations remained constant over the study period, but correlated with chlorophyll-a when suspended solid was low. Therefore, inflow dynamics drive changes in the salinity regime, suspended solids, and act to maintain nutrient concentrations.

Inorganic nitrogen release from sediment slurry of riverine and estuarine ecosystems located at different river regimes

The purpose of the present study was to compare the nature of dissolved inorganic nitrogen (DIN=ammonium (NH4+) and nitrite+nitrate (NO2+3=(NO2–+NO3–)) release from aerobic sediment slurry at two different hydrologic flow regimes. The watershed of the Guadalupe River–Estuary system receives more freshwater inflow than does the watershed of the Nueces River–Estuary system; thus, the Nueces Estuary is more saline than is the Guadalupe Estuary. Sediment samples were collected using cores, analysed for organic matter and grain size, and used to perform laboratory experiments to measure DIN release. During the experiments, DIN concentrations in overlying water were measured for 48h in five different salinity treatments. Ammonium concentrations were higher in the Nueces River and Estuary treatments than in similarly treated samples from the Guadalupe River and Estuary. An increase in NO2+3 concentrations along salinity gradients of the Nueces Estuary treatments indicated favourable condition for nitrification. The Guadalupe River sediments that were not exposed to salinity had an increase in NH4+ concentration at 7.5ppt. The different DIN release among salinity treatments indicated that hydrologic forcing on organic matter deposition and salinity have an important role on the retention and release of inorganic nitrogen at the sediment–water aerobic layers in rivers and estuaries.

Impacts of droughts and low flows on estuarine water quality and benthic fauna

The effects of droughts on benthic infaunal communities and three epifaunal species were determined in three semi-arid central Texas estuaries with different inflow dynamics and consequent salinity regimes: the Nueces, Lavaca-Colorado, and Guadalupe Estuaries. Periods (months or years) were considered to be in drought if mean salinities were in the upper quartile of historic salinities. This drought classification method was verified by comparing the Palmer Drought Severity Index of the catchments with the drought classifications within each estuary. Droughts have demonstrable effects on estuary water column condition, notably decreases in turbidity, nutrient concentrations, and chlorophyll concentrations. Droughts do not appear to be important drivers of infaunal communities in estuarine regions with normally high salinities (25–32) although they coincide with increased diversity and changes in community composition in estuarine regions that have lower salinities (10–19). Droughts cause decreases in Litopenaeus setiferus (white shrimp) and Callinectes sapidis (blue crab) abundances and spatial extents. This indicates that droughts, especially when combined with water diversions, may negatively affect primary and secondary productions in other semi-arid estuaries of the world.

Application of watershed analyses and ecosystem modeling to investigate land–water nutrient coupling processes in the Guadalupe Estuary, Texas

Estuarine nutrient enrichment is thought to be controlled by land use patterns in coastal watersheds. Hence, the objective of this work was to conduct a watershed analysis in two adjacent river basins with different land use characteristics to determine their influence on estuarine ecosystem response in the Guadalupe Estuary, Texas, U.S.A. All data sources for this study were available electronically on the Internet; the data were mined, managed, analyzed and transformed to simulate the estuarine ecosystem response to watershed-derived nutrient loads. Between 1992 and 2001, developed land use/land cover increased the most while forest cover decreased the most in both basins. Two hydrologic units nearest the coast were responsible for the greatest change in land cover. Nutrient concentrations and loads were significantly higher in the San Antonio River Basin than in the Guadalupe River Basin. Both river basins exhibited the highest flows ever recorded in 1992, however the magnitude of difference in loads between the two coastal hydrologic units for a wet and dry year was much greater in the Guadalupe River Basin (GRB) than in the San Antonio River Basin (SARB); this difference supports the concept that the GRB is a nonpoint source dominated system and SARB is a point source dominated system. There was a strong correlation between developed land use and nutrient concentrations in river water; the GRB had less developed land use and lower nutrient concentrations while the SARB had more developed land use and higher nutrient concentrations. Estuarine ecosystem response differed in the timing, duration and magnitude of DIN, phytoplankton and zooplankton when nitrogen loads from the Lower Guadalupe River were used as opposed to the Lower San Antonio. The two basins studied differ in their fundamental characteristics, i.e. precipitation, flow, human population density, etc., resulting in different drivers of nitrogen loading, point sources in the San Antonio River Basin and nonpoint sources in the Guadalupe River Basin, therefore, differing estuarine ecosystem responses.

Factors influencing algal biomass in intermittently connected, subtropical coastal ponds

Microalgae may contribute more to salt marsh productivity than realized, underscoring the importance of understanding algal dynamics in such systems. Benthic and planktonic chlorophyll-a (surrogate for total algal biomass), sediment ash-free dry weight, total suspended solids, salinity, and nutrients were examined in intermittently connected marsh ponds in the subtropical Guadalupe Estuary, Texas, USA to determine the effects of hydrologic connections on algal biomass in this system. From May 2005 to May 2006, several high water events resulting in surface water connections between ponds and nearby tidal creeks occurred, followed by disconnection and, at times, pond desiccation. During periods of disconnection, algal biomass was higher in both the benthos and the water column than during connection events when flushing occurred. Since connection events did not increase NOx, the primary source of dissolved inorganic nitrogen seemed to be nutrient cycling within the ponds. Data at the aggregated level, i.e., chlorophyll-a, indicated this system followed the flood pulse concept. During high water events, algal biomass may be exported to supplement the tidal creek food web (aquatic), whereas during low water events accumulated biomass within the ponds feed the marsh food web (terrestrial), which includes blue crabs (Callinectes sapidus) and the whooping crane (Grus americana).

Hydroclimatic trend detection in a rapidly urbanizing semi-arid and coastal river basin

Detection of past trends, changes, and variability in the time series of hydroclimatic variables is important for understanding the potential impact of future change. The objective of this study was to analyze trends in several hydroclimatic variables in the rapidly urbanizing semi-arid San Antonio River Basin, particularly trends in freshwater inflows to the Guadalupe Estuary. Seasonal time series for 27 hydroclimatic variables from nine USGS stream gauging stations and five NCDC weather stations were analyzed for trend using the Mann–Kendall non-parametric test after removing serial and cross-correlations. The significance of detected trends was analyzed using a permutation method. Variability in total annual and seasonal precipitation was characterized and the effect of precipitation on regional streamflow trends during coinciding periods was analyzed. Results of the analysis revealed a definitive line just below the City of San Antonio above which the influence of the increase in impermeable surfaces can be clearly seen. Above this line nearly all significant trends were negative (decreasing). The percent contribution of baseflow to total streamflow in the upper watershed decreased for almost every season for average, high, and low precipitation events. Below this line all significant trends were positive (increasing). A greater number of significant trends was seen in all hydroclimatic variables during all seasons at stations in the lower watershed and the percent contribution of baseflow to streamflow is increasing across most seasons for average, high, and low precipitation. Some significant positive trends were found in precipitation, but only in the winter season and with no significant spatial pattern. Streamflow has been increasing at the outlet of the watershed, however, freshwater inflow contributions from this watershed are only being met during years that have higher than average precipitation. This has very real implications for management in this watershed.

The role of inflow magnitude and frequency on plankton communities from the Guadalupe Estuary, Texas, USA: Findings from microcosm experiments

Changes in magnitude and frequency of inflow results in subsequent alterations in the delivery of nutrients essential for phytoplankton growth and competition producing variations in community composition and nutritional value of phytoplankton. Zooplankton demographics are likely directly influenced by pulsed inflows due to flushing losses, whereas they are also indirectly affected by changes in prey quality. In this study, we report the potential effect of pulsed inflows on the plankton community of the Guadalupe Estuary. Microcosms were used that allowed control of light intensity and photoperiod, turbulence, temperature, nutrient loading, and flushing magnitude and periodicity. Our microcosm experiments were novel as they utilized natural plankton communities, thereby allowing the simultaneous interaction between hydrology, resource availability and grazing. Results show differences in microcosms according to magnitude and frequency of flushing. For example, copepod population density was greatest at the annual mean inflow magnitude. At half the annual mean inflow a decline in prey quality likely resulted in poorer grazer performance, and at double the annual mean inflow magnitude increased flushing losses prevented the incidence of higher copepod densities. Similarly, pulsed inflows resulted in greater copepod population densities, higher overall phytoplankton biomass, and dominance of centric diatoms (known to be faster growing and more edible). While reduced freshwater inflow associated with increasing anthropogenic demands often strains the needs of estuarine systems, the effects of reduced river input may be alleviated with several management options including manipulation of the timing, frequency, and magnitude of freshwater inflows. Before implications for management can be discerned from these findings, however, larger scale experiments are needed focus on the roles of inflow magnitude and frequency.

Nekton assemblage structure in natural and created marsh-edge habitats of the Guadalupe Estuary, Texas, USA

Natural and created Spartina brackish marsh habitats in the Guadalupe Estuary, adjacent to the Aransas National Wildlife Refuge, Texas, USA were surveyed during spring, summer, and fall 2004 to evaluate the equivalence of nekton assemblages in an old (>30 years) created marsh. During each season, six replicate samples were collected in each marsh type using a 1-m 2 drop sampler. Multivariate analysis revealed significant differences in nekton assemblage structure among marsh type, both within and across seasons. Species richness was significantly higher in the natural marsh in spring and summer but not in fall. Several species that were dominant in the natural marsh but rare or absent in the created marsh had strong correlations with the presence of oyster substrate that was only encountered in natural marsh samples. Although cumulative richness was greater in the natural marsh, eight species were collected only from the created marsh. Shrimp and fish biomass was significantly higher in natural marsh. Analysis of the density, biomass and size structure of three commercially important crustaceans indicated that the created marsh supported similar biomass of some species (white shrimp, blue crab); however, the size structure of some populations was variable among marshes (blue crab, brown shrimp). We conclude that lower substrate complexity (specifically oyster) and soil organic content in the created marsh reduced measures of nekton similarity and recommend that these features be addressed in future restoration efforts.

Multi‐objective risk assessment of freshwater inflow on ecosystem sustainability in San Antonio bay, Texas

: San Antonio Bay is located on the coast of Texas between Galveston Bay and Corpus Christi Bay and is the primary bay in the Guadalupe Estuary. Three rivers feed San Antonio Bay from two river basins, including the Blanco and Guadalupe Rivers in the Guadalupe River Basin and the San Antonio River in the San Antonio River Basin. The Canyon Reservoir regulates the flow of fresh water in the middle and lower reaches of the Guadalupe River. These inflows are a primary regulator of salinity and, thus, the productivity of commercially important estuarine species. Increasing demand for water has prompted plans for an increased diversion of 49.3 million m3 (40,000 acre‐feet) from the reservoir. An additional amount of 61.6 million m3 (50,000 acre‐feet) from the mouth of the river is to be pumped back to San Antonio to relieve over‐pumping of the Edwards Aquifer. Because the Guadalupe River Basin contributes 58.1 percent of the freshwater inflow to the estuary, it is not known what the impact of these actions will have on the ecological integrity of the San Antonio Bay. Water resource management in the San Antonio Basin consists of decision making under risk and uncertainty related to randomness in the critical parameters such as the salinity in the bay, biological productivity, and total flow into the bay. The aim of this study is to investigate the trade‐offs between the competing objectives of maximizing biological productivity in the bay and minimizing flow using Stochastic Compromise Programming (SCP). The SCP model solves a multi‐objective function subject to constraints that must be maintained at three different prescribed levels of probability providing a global set of solutions for the water resource management problem under input uncertainty. The SCP model provides information on the trade‐offs among the objective function value, tolerance values of the constraint at the prescribed levels of probability, which could be valuable to policy makers in risk assessment. Solutions were found using three distance functions. Model outputs may suggest the minimum amount of freshwater needed to maximize biological productivity of the bay at specified risk level for assessing the impact of upcoming diversion program. Results indicate that current flows in the Guadalupe River are of sufficient volume to satisfy harvest requirements.

The Effect of Freshwater Inflow on Meiofaunal and Macrofaunal Populations in the Guadalupe and Nueces Estuaries, Texas

Two estuaries with very different inflow characteristics were compared to test the hypothesis that benthic standing crops are enhanced by freshwater inflow. Assuming predation pressure is similar in both estuaries, this would imply that freshwater inflow enhances secondary production. The Guadalupe Estuary had 79 times more freshwater inflow than the Nueces Estuary, and a third of the salinity. The Guadalupe had higher macrofaunal densities and biomass than the Nueces, and both parameters increased with decreasing salinity within the Guadalupe Estuary. Macrofauna density increased with increasing salinity in the Nueces Estuary, due to invasion by marine species. However, meiofauna population size responds differently than macrofauna. Meiofaunal densities were higher in the low-inflow Nueces Estuary, and increased with increasing salinity in both estuaries. Macrofauna diversity increased with salinity, both within and between estuaries. The macrofauna response supports the hypothesis that increased freshwater inflow stimulates secondary production. A review of past benthic studies in these estuaries and the historical climatic patterns indicate that wet years with high inflow result in increased macrofaunal productivity. Since, macrofaunal diversity decreased with lower salinity both within and between the estuaries, the enhanced productivity is due to increases by freshwater and estuarine species that can tolerate low salinities. Increased macrofaunal densities are associated with decreasing meiofaunal densities. The latter result could be due to either increased macrofaunal competition with or predation on meiofauna, or a lack of low-salinity tolerance by meiofauna.

Denitrification and oxygen consumption in sediments of two south Texas estuaries

Spatial and temporal variations in rates of denitrification and oxygen consumption were measured in sediments of the Nueces and Guadalupe Estuaries in southern Texas, USA. Denitrification rates varied from 4.0 to 71 . l pm01 N2 m'2 in the Nueces Estuary and from 4.6 to 34.7 pm01 N, m-' h-' in the Guadalupe Estuary. Denitrification accounted for 29 to 80 'X of total benthic N flux in the study areas. Oxygen consun~ption rates ranged from 176 to 818 pm01 0, m-' h-' in Nueces Estuary and from 208 to 550 pm01 0, m-, h ' in Guadalupe Estuary. In both estuaries denitrification and oxygen consumption rates were generally higher in the upper estuaries where the porosity and organic matter contents of sediments were typ~cally highest. Among 3 measurements in different seasons, the lowest denitrification and oxygen consumption rates were usually observed during winter. Chemical oxygen consumption, as measured using formalin-killed controls, was about 50 % of the total oxygen consumption. A considerable portion (ca 50 %,) of biological oxygen consumption was attributed to nitrification in sediments. We estimated that carbon mineralization by denitrifiers was a s much as 40 to 179 ' % of that by aerobic heterotrophs. In the Guadalupe Estuary, denitrification removed about 38 % of the measured inputs of organic and inorganic nitrogen. In the Nueces Estuary, the amount of nitrogen removed by denitrification was ca 2 times greater than the measured nitrogen inputs from the Nueces River and precipitation, suggesting that unmeasured anthropogenic inputs along the perimeter of the estuary were very important for maintaining nitrogen balance.

Production ecology of Ruppia maritima L. s.l. and Halodule wrightii Aschers, in two subtropical estuaries

Seasonal growth and production dynamics of Ruppia maritima L. s.l. were compared over a 3-year period in two south Texas estuaries characterized by different salinity and N regimes as a result of freshwater inputs. Measurements of shoot production in the Guadalupe Estuary (0–25‰ salinity) and the Nueces Estuary (32–38‰ salinity) revealed no major differences in the magnitude of growth, but the plant populations differed in the seasonality of growth, the time of flowering, and the persistence of an over-wintering population. During the period of rapid shoot development, from March to August, leaf elongation rates usually ranged from 2 to 4 mm·day −1 (0.04–0.08 mg dry wt·mg shoot −1·day −1), although peak growth rates of up to 8 mm·day −1 were also recorded. In both estuaries, R. maritima was a strict opportunist, yearly colonizing bare areas and completing its entire growth cycle in 4 months. Overwintering populations of R. maritima existed at the lower nutrient and higher salinity site in Nueces Estuary but not at the high nutrient, low salinity site in Guadalupe Estuary. Halodule wrightii Aschers, was absent from Guadalupe Estuary, but in the Nueces Estuary, H. wrightii maintained large and persistent overwintering populations characterized by sustained year-round growth and larger below-ground root and rhizome fractions of total biomass (50–85%) compared to R. maritima (20–70%). The high nutrient regimes associated with large inputs of freshwater in the Guadalupe Estuary appear to have little beneficial effect on the growth dynamics of R. maritima. Instead, significant reductions in underwater light may be most important for growth. Fouling by algal epiphytes, higher riverine inflow, and local physiographic differences in wave exposure appear to be the primary factors regulating light levels and thus the proliferation and year-round persistence of R. maritima.