Effects Of The Exotic Invader, Tamarix spp., On Aspects Of The Aquatic Ecosystem At McComb Creek, Presidio County, Texas
Moore, Jane Blaisdell Nellan
MetadataShow full item record
The effects of saltcedar, Tamarix spp., invasion in riparian zones of the western United States have been well documented. These include, but are not limited to soil salinization, and shifts in hydrology and water use. Comparatively unknown are the effects on the associated aquatic systems and if a significant increase in water salinity accompanies these invasions. Shifts in stream metabolism have been reported, but the impacts of saltcedar invasion on the algal species, which are the dominant primary producers in desert aquatic ecosystems, are unknown. A two year study was conducted in an oligotrophic desert stream, currently experiencing an invasion, to determine the influence of saltcedar on specific environmental factors and their effect on benthic algal assemblages and stream metabolism. Saltcedar presence in this system was expected to increase salinity. A potential increase in phosphorus availability, resulting from this salinity increase, may cause a shift from species-poor communities of oligotrophic and halosensitive algae to species-rich communities of eutrophic and halotolerant species. The invasion may also influence other aquatic environmental parameters that are fundamental determinants of algal community composition, such as temperature, pH, conductivity and nitrogen. Canopy cover of the stream would also be increased, thereby lowering daily photosynthetically active radiation (PAR) and increasing allochthonous input into the stream in the form of leaf litter. A greater allochthonous input is expected to drive an increase in respiration, effectively counteracting any increase in photosynthesis that may result from increased nutrients in the saltcedar invaded reach. Therefore, no significant difference in stream metabolism between the upstream native and downstream saltcedar reaches was anticipated. Species composition and diversity were assessed across reaches by deploying artificial substrates and performing a series of experiments utilizing nutrient-diffusing substrates (NDS) and transplantation of artificial substrates. Environmental measurements of temperature, pH, conductivity, daily PAR, dissolved oxygen, soluble reactive phosphorus (SRP), nitrate-nitrogen and ammonium-nitrogen were measured to determine if saltcedar presence exerted any effects on these parameters. Leaf litter and chlorophyll a measurements were used to determine allochthonous and autochthonous inputs into the study reaches, respectively. Stream metabolism was assessed as the ratio of photosynthesis to respiration (P:R) using dissolved oxygen data over a 24 hour period. Species diversity of soft algae and diatoms was significantly higher in the saltcedar reach than in the native reach (p=0.015 and p=0.001, respectively). This is contrary to previous research indicating that increases in salinity decreases algal diversity. The native communities were dominated by Achnanthidium minutissimum, which is the most widespread diatom in the world, characteristically found in oligotrophic systems. Saltcedar communities were dominated by N2-fixing species of the genera Rhopalodia and Anabaena. Significantly different environmental variables that were higher in the saltcedar reach were specific conductance and salinity, while pH was higher in the native reach. Leaf litter mass in the saltcedar reach was significantly higher than that in the native reach, but there was no significant difference in chlorophyll a between the sites. Increased allochthonous input was not enough to shift stream metabolism, i.e. the P:R ratio. SRP in the saltcedar reach was significantly increased, indicating a possible relationship between SRP and salinity leading to increased diversity in benthic algal communities. Increased phosphorus availability in the saltcedar reach stimulated proliferation of Rhopalodia species with endosymbionts capable of N2-fixation, thus overcoming the nitrogen limiting conditions. It is shown here that saltcedar invasion along the riparian corridor has shifted a characteristically oligotrophic native system to an ecosystem dominated by a more eutrophic algal community utilizing increased phosphorus. Nutrient enrichment with nitrogen and phosphorus resulted in higher diatom species diversity in experimental treatment samples from the native reach, with community assemblages more closely resembling those communities found in the saltcedar reach. The present results on species diversity of both diatoms and soft algae contradict previous research concluding saltcedar invasion negatively impacts all aspects of both aquatic and terrestrial ecosystems.