A Study on Biological Threats to Texas Freshwater Resources

This thesis consists of two separate experiments on unique biological threats to Texas freshwater ecosystems. The first experiment sought to understand the interaction between the harmful alga Prymnesium parvum and the cyanobacteria, Anabaena sp. The second experiment sought to determine the effectiveness of triploid grass carp, Ctenopharyngodon idella, as a biological control for two species of invasive macrophytes, which were giant salvinia, Salvinia molesta, and hygrophila, Hygrophila polysperma.

Prymnesium parvum blooms have become more frequent in the south-central United States, leading to significant ecological and economic impacts. Allelopathic effects from cyanobacteria were suggested as a mechanism that might limit the development of P. parvum blooms. This research focused on the effects of cultured cyanobacteria, Anabaena sp., on P. parvum. Over a 6-d period, daily additions of filtrate from the senescent Anabaena culture were made to P. parvum cultures growing in log phase. All treatments, including several types of controls, showed reductions in P. parvum biomass over the course of the experiment, but the treatments receiving Anabaena filtrate were reduced to a lesser degree, suggesting that filtrate from the senescent cyanobacteria culture was beneficial to P. parvum in some way.

Aquatic vegetation is an important component of most freshwater systems and provides numerous valuable ecosystem services, providing food, habitat and refuge for a variety of organisms. A significant threat to beneficial aquatic vegetation abundant in many United States waterways is the introduction and spread of invasive macrophytes. Two of the newest invasive species, giant salvinia (Salvinia molesta) and hygrophila (Hygrophila polysperma), have quickly established in Texas waters. This research evaluated the potential use of triploid grass carp (Ctenopharyngodon idella) as a biological control agent for these two novel invasive species. Using a controlled mesocosm experiment, consumption rates and feeding preferences were measured. Giant salvinia and hygrophila were compared to six native and introduced species common in Texas and the Southern US. Grass carp were found to be potentially useful in controlling giant salvinia in the preliminary stages of an infestation but an overall poor control option for hygrophila.