Aquatic Ecosystems

Reduced Freshwater Inflows and Productivity in the Guadalupe Estuary

Use of High-Resolution Spatial Mapping.

Daniel Roelke and Stephen E. Davis, III

Reduced freshwater inflow to estuaries is a growing problem along the Texas coast. Population growth concentrated at urban centers, such as San Antonio, result in a greater demand for freshwater. In San Antonio, to accommodate this increasing demand for freshwater a diversion project will be implemented that will pump water lower in the watershed back up into the headwaters of the San Antonio River, which will exacerbate freshwater inflow-stress to the Guadalupe Estuary. In this research we will study the relative influence of factors known to impact estuarine productivity and respiration, which include the magnitude and periodicity of freshwater inflow and the associated loading of nutrients and sediment, and the flux of nutrients and other materials from the benthos. For this research we will use a relatively new ship-board technology, i.e., Dataflow, which will allow us to generate high-resolution spatial maps of salinity, chlorophyll a, turbidity, dissolved organic matter, temperature, pH, and dissolved oxygen for the entire Guadalupe Estuary at sampling intervals ranging between weeks and months (click here for data information). Findings from our multivariate statistical analyses will increase our understanding of the relative influence, and possible synergistic interactions, of factors known to impact estuarine health.

Bridging the Gap Between Plankton Dynamics and Spatial Variability in Water Quality in the Guadalupe Estuary (Texas): The Importance of Freshwater Pulses.

Stephen E. Davis, III and Daniel Roelke

The Lower Guadalupe Water Supply Project (LGWSP) seeks to divert approximately 100,000 acre-ft yr-1 from just below the confluence of the Guadalupe and San Antonio Rivers (near the head of the Guadalupe Estuary) to meet projected needs in the greater San Antonio (TX) area. Water management activities such as these are often made without an understanding of the impact of reduced freshwater inflows on estuarine function. However, recent studies supported by agencies involved with the LGWSP will be studying the effects of the proposed diversion on marsh ecosystems and commercially important species in the estuary. We propose a 3-year study coordinated with these efforts to understand the effects of temporal variability in inflows and corresponding spatial variability in water quality on phytoplankton dynamics and water column production in the open water bays of the Guadalupe Estuary. Continuous sampling of water quality and hydrology at the mouth of the river in addition to USGS flow data will be used to understand inflows and loading of materials into the estuary. We will quantify and map the effects of these inflows on estuary-wide water quality with Dataflow, a flow-through measurement apparatus for mapping physico-chemical parameters in shallow aquatic systems such as the Guadalupe Estuary. Estuarine plankton response to a range of inflow events characterized will be analyzed from discrete field samples and supported by laboratory experimentation on natural plankton assemblages. Reduced freshwater inflow effects on the bay will include reduced loading of new nutrients and sediment to the estuary, increased areal extent of high salinity zones, and reduced magnitude of pulsed inflows, and increased period between pulses to the estuary. The latter two are expected to increase the time over which competitive exclusion processes occur, leading to a decline in phytoplankton species diversity and productivity. The synthesis of information collected from this study and the mentioned ongoing studies supported by the LGWSP will help provide a better understanding of the relationships between the nature (i.e. frequency, magnitude, and mode) of these pulsed inflow events and estuarine ecosystem health, thus allowing water managers to optimize their diversion of freshwater while minimizing impact to estuarine integrity.

Use of High-Resolution Spatial Mapping to Estimate Plankton Response to Freshwater Inflows Entering Galveston Bay: Importance to Watershed Development and Ecosystem Health

Stephen E. Davis, III, Daniel Roelke and James L. Pinckney

Freshwater inflow and degree of nutrient loading to an estuary have long been understood as factors contributing to water quality and ecosystem health. In Texas estuaries, freshwater inputs have been implicated as a primary factor controlling productivity of juvenile brown shrimp, macrophyte productivity, root:shoot ratios, and species diversity, and benthic invertebrate density and diversity. Factors equally important, but not as often addressed in estuarine studies, include the magnitude of freshwater inflows and nutrient loading, the mode of nutrient loading (e.g., continuous vs. pulsed flows, and frequency of pulsed flows), and the ratios of potentially limiting nutrients within the load. The primary objective of this study is to understand the importance of pulsed inflows into the Galveston Bay Estuary in shaping spatial variability in water quality across the estuary and as “bottom-up” controlling mechanisms on phytoplankton community structure and productivity. The proposed study will provide a continuous record of materials loading into the bay from the Trinity River. In addition, our sampling will characterize and map water quality and quantify water column productivity across the entire estuary during different inflow conditions a minimum of 12 times per year (i.e. monthly). We will also be collecting samples for HPLC pigment analyses to infer plankton community structure. The results of these analyses and subsequent pigment ratios will be periodically checked against microscopic analysis. Sampling on this project will commence in Fall 2004.

Importance of Episodic Storm Events in Controlling Ecosystem Structure and Function in the Southern Everglades Mangrove Transition Zone

Stephen E. Davis, III, Daniel L. Childers, Jaye Cable, John Day, Dave Rudnick, and Fred Sklar

Wind and precipitation strongly influence the hydrodynamics of micro-tidal estuaries and can have profound effects on the pulsing of materials, leading to enhanced primary and secondary production. Since 1996, we have been monitoring surface water quality along Taylor River, a southern Everglades mangrove creek. During this period, there were a number of meteorological events that affected south Florida. In this presentation, we highlight the effects of three major events as well as typical variability in concentrations and fluxes of materials. In 1996, an extended winter storm reduced salinity as freshwater was pushed out of the Everglades. When the winds subsided, there was a 6-fold increase in creek salinity as flow reversed. [TN] increased throughout this event while [TP] and inorganic N and P remained constant. In 1999, T.S. Harvey dropped nearly 26 cm of precipitation in south Florida with negligible winds. Harvey caused [TP] to more than triple (from 1µM to 3.8µM) and discharge to increase by over an order of magnitude. Hurricane Irene passed just west of Taylor River in 1999 producing strong southerly winds and heavy precipitation, and led to increased [TN] and no observable change in [TP]. Irene also produced the highest discharge measured in this system (730,000 m3 d-1). These 3 events exemplify the kinds of events common to this region. The effects of these events combined with a synthesis of long-term water quality and quarterly flux data indicate that the patterns of nutrient dynamics in this system are dependent upon the nature of each event.

Carbon sources supporting aquatic food webs in connected and isolated marsh habitats in Aransas National Wildlife Refuge

Stephen E. Davis and David J. Hoeinghaus

We are examining food web structure in aquatic habitats at two locations within the heterogeneous estuarine marsh ecosystem of Aransas National Wildlife Refuge.  At each location we collected samples of plants, algae, flocculated detritus, invertebrates and fishes in habitats of differing degrees of hydrologic connectivity (channel, connected lagoon, isolated lagoon) for carbon and nitrogen stable isotope analysis.  Analyses will compare the relative importance of various carbon sources supporting crabs and shrimp and large piscivorous species such as red drum.  Crabs and shrimp are important food resources of the endangered whooping crane, which winter in the refuge, and red drum are important game species as well as consumers of shrimp and crabs.

Development of Flow Recommendations for the stretch of Big Cypress Creek below Lake O’ the Pines Dam.

Kirk O. Winemiller, Anne Chin, Stephen E. Davis,
Daniel L. Roelke, Luz Maria Romero, and Bradford P. Wilcox

This project involved a synthesys of available data and literature in order to obtain sufficient information to develop ecologically based flow recommendations for the Big Cypress Creek below Lake O’ the Pines Dam. Click here to view the summary report.