Margaret G. Forbes

Response of a Subtropical Estuarine Marsh to Local Climatic Change in the Southwestern Gulf of Mexico

We examined the contrasting, effects of floods and droughts produced by large changes in local climatology on vegetation patterns in Nueces marsh, a semi-arid subtropical salt marsh in south Texas from 1995 to 2005. Climate variations during the study included an initial 4-yr period of moderate conditions, followed by a 2-yr interval of drought, and a recent 4-yr wet period that included large-scale floods. Variation in freshwater inflow, rainfall, and potential evapotranspiration were used in conjunction with field measurements of salinity, inorganic nitrogen, and vegetation structure collected at sites located at varying distances from Nueces Bay. Tidal creek salinities varied with Nueces Bay salinity, with strength of effect inversely related to distance from the bay. Mean (±standard deviation) pore water salinities ranged from 59±54‰ at two high, marsh stations farthest from the bay (10.1 km distant) to 30±21‰ in soil at a low marsh site closest to the bay (0.5 km distant). Mean pore water ammonium was also higher at stations most distant from the bay; nitrate + nitrite did not exhibit a high marsh to low marsh gradient. Nueces Bay salinity decreased substantially when the 10-d cumulative mean daily Nueces River flows exceeded 10 m3 s−1. During periods of low and moderate flood frequency (flows mostly below 10 m3 s−1), vegetation assemblages were dominated by stress-tolerant clonal plants. A catastrophic flood, which immersed vegetation for several weeks between July and September 2002, resulted in extensive plant mortality, but within months, unvegetated areas were rapidly colonized by the obligate annualSalicornia bigelovii. With the end of major flooding by late 2004, plant community structure began a return to pre-drought assemblages at high and middle marsh stations by summer 2005. At the low marsh station, new conditions favored clonal dominants (Spartina alterniflora andBorrichia frutescens), with the latter replacingSalicornia virginica as the dominant species. Our results support the theory that the importance of competition and abiotic stress in determining community composition are inversely related.

River–reservoir transition zones are nitrogen fixation hot spots regardless of ecosystem trophic state

Reservoir hydrodynamics may create heterogeneity in nitrogen (N) fixation along the riverine–transition–lacustrine gradient. In particular, N fixation may be highest in reservoir transition zones where phytoplankton biomass is also expected to be relatively high. We investigated spatial patterns of N fixation in three Texas (USA) reservoirs of varying trophic state. We sampled 6–9 stations along the longitudinal axes of the major inflows and measured N fixation using the acetylene reduction method. Total N, total phosphorus (P), and algal biomass (as chlorophyll-a) were also measured at each sample location. Measurable N fixation was observed in all reservoirs and was light-dependent. Nitrogen fixation was consistently low in the riverine zone, highest in the transition zone, and low in lacustrine zone of all reservoirs. The absolute magnitude of N fixation was similar in two relatively unproductive reservoirs and an order of magnitude higher in the eutrophic reservoir. A similar pattern was observed in mean nutrient and chlorophyll-a concentrations among reservoirs. However, chlorophyll-a concentrations were highest in the riverine zone of each reservoir and exhibited a monotonic decrease in the downstream direction. Maximum chlorophyll-a concentrations did not coincide with maximum N fixation rates. Results of our study indicate that reservoir transition zones can be biogeochemical hot spots for planktonic N fixation, regardless of trophic state. Therefore, transition zones may be the most at risk locations for water quality degradation associated with increased reservoir productivity. Water quality managers and aquatic scientists should consider the spatial heterogeneity imposed by unique hydrodynamic controls in reservoir ecosystems.

Influence of Climate Change on Reservoir Water Quality Assessment and Management

Climatic changes are commonly recognized to alter freshwater ecosystems. This chapter provides a unique perspective on the implications of climate change for reservoir inflows, water quality assessment, and management of aquatic contaminants influenced by site-specific pH. The various physical, biological, and chemical dynamics of reservoir zones are reviewed and a case study of four reservoirs in Texas demonstrates that reduced inflows and daily pH variability have direct implications for the collection, analysis and interpretation of water quality data. The chapter concludes with recommendations for reservoir water quality assessment and management, particularly given the prospect of increased frequency and duration of drought conditions in the southwestern and south-central U.S.

Effects of pulsed riverine versus non-pulsed wastewater inputs of freshwater on plant community structure in a semi-arid salt marsh

The use of treated wastewater to restore freshwater inputs to arid and semi-arid wetlands is a relatively new concept, and the long-term effects of such practices on plant community structure are largely unknown. We compared vegetation composition, pore water salinity, and soil moisture along permanent transects at a restoration site receiving wastewater effluent since October 1998 to three nearby downstream sites subjected only to freshwater inputs via precipitation and river flooding. Local climate during this period was highly variable and included two droughts and a wet period that began with a series of large floods. Significantly lower pore water salinities were observed at the wastewater site compared to downstream sites, particularly during droughts, when salinities were 20%.–40%. lower at the wastewater site. Between July 1997 and July 2002, cover of the clonal stress-tolerator, Salicornia virginica, decreased from 87% to 33% at the wastewater site, while cover of the clonal dominant, Borrichia frutescens, increased from 5% to 55%. In contrast, S. virginica cover increased at two downstream sites during the same period, while cover of B. frutescens remained relatively stable. Following large floods in summer 2002, which marked the beginning of a three year-wet period, B. frutescens cover increased at all sites. We concluded that constant wastewater additions and climate-driven wet periods affected plant community structure similarly by promoting expansion of the clonal dominant B. frutescens and inhibiting expansion of the stress-tolerant species S. virginica. We propose distinct management strategies for using wastewater to 1) increase plant cover, 2) promote endemic plant assemblages, and 3) maximize species richness.

Hot spots and hot moments of planktonic nitrogen fixation in a eutrophic southern reservoir.

Reservoirs have been identified as hot spots of biogeochemical activity, although they are known to exhibit pronounced temporal and spatial variability in planktonic community dynamics. Results from this 19-month study at 5 locations on a southern polymictic reservoir identify pronounced temporal (seasonal and interannual) and significant spatial variability in planktonic nitrogen fixation. Planktonic nitrogen fixation was found to be high during the warmest portion of the year and undetectable for most of the rest of the year. Interannual variability between 2 consecutive summer periods was high and likely due to differences in rainfall pattern and resulting differences in ambient nutrient concentrations and flushing rates. Rates during the dry summer exceeded those during the following summer when unexpected summer rains occurred. Spatial differences in areal and volumetric rates were also significant. Volumetric rates were highest in the inflow transition zone of a river draining a watershed impacted by dairy operations, with inflows exhibiting low N:P ratios. Areal rates were highest in the deeper and clearer portions of the reservoir. Areal rates of nitrogen fixation varied among the 5 stations by a factor of 4.4 in the dry summer and by a factor of 22 in the wetter summer.

Hydrology of Coastal Prairie Freshwater Wetlands.

T Gulf Coast region of Texas and western Louisiana contains a unique wetland type that is part of the globally imperiled Coastal Prairie Ecosystem (USGS 2000). This southernmost extension of the tall-grass prairie is a mosaic of depressional wetlands, pimple mounds, and flats (Moulton and Jacob 2000). Coastal Prairie Freshwater Wetlands (CPFWs) are characterized by microtopography and complex patterns of inundation that promote diverse plant communities. Some of these freshwater wetlands originated from ancient channel scars that have been reworked by aeolian erosion, while other “gilgai” wetlands are formed by the vertical action of clay soils (Sipocz 2002). The dominant soil types are Vertisols and Alfisols that developed over Pleistocene deposits flanking the Gulf coast. These wetlands have diverse and locally variable hydrology, ranging from temporarily flooded to intermittently exposed. CPFWs tend to have small watersheds, seasonal inundation, intermittent outflows, and hydrology driven largely by precipitation and evapotranspiration.