FIELD NOTES

serious threat to the health and stability of Lake Texoma fisheries – Prymnesium is also extremely lethal to herbivorous zooplankton, even at non-bloom densities. Zooplankton constitute the primary trophic link between algae at the base of the food web and the many important planktivorous fishes that serve as forage for most of the sport and recreationally-important piscivorous fishes at the top of the food web. Thus, not only can Prymnesium result in the direct deaths of fish during blooms, its presence in the lake may produce deleterious effects within the zooplankton assemblage that may result in lower food availabilities for fish.

A major part of the PEL research program has included weekly to monthly monitoring of Prymnesium and important environmental factors, such as nutrient concentrations, zooplankton abundances, water pH, and temperature. This monitoring program has revealed two primary factors that seem to correlate with
Prymnesium abundances – salinity and the ratio between the concentrations of nitrogen and phosphorus. The salinity connection is not surprising, since Prymnesium is believed to have originated in marine systems. While salinity is relatively high in Lake Texoma, and particularly in the Red River arm of the lake, normal lake salinities do not appear to be very conducive to Prymnesium blooms. However, when water inflow rates are relatively low, such as during the seasonally low rainfall period of late fall and early winter, but also during longer-term, multi-annual drought periods, salinities can rise high enough to provide adequate growth conditions for Prymnesium. Such periodic increases in salinity are quite typical to Lebanon Pool, the most common sight for Lake Texoma’s annual Prymnesium bloom. Since Prymnesium has been in Lake Texoma, there have been five winters in which salinities rose above normal winter levels, and in each of these years, Prymnesium bloomed.

The connection between salinity and Prymnesium is quite strong, but high salinities cannot support an algal bloom. For that, there must be nutrients, and Lake Texoma is extremely nutrient rich, with particularly high phosphorus concentrations. Typically, in balanced systems, concentrations of nitrogen should be at least seven times higher, by weight, than phosphorus concentrations. In Lake Texoma, nitrogen is relatively low compared with phosphorus, generally only four to six times higher, and frequently less. Such low N:P ratios tend to favor algae that can utilize dissolved nitrogen gas, via a process called nitrogen fixation, and are thus responsible for the blooms of nitrogen fixing cyanobacteria (bluegreen algae) that typically occur during the warm summer months each year. As noted above, Prymnesium abundances during winter in Lake Texoma correlate with N:P. Through a series of laboratory studies in which we quantified toxicity of Prymnesium grown under different nutrient concentrations and ratios, we found that toxicity to fish is strongly increased with decreasing N:P, when phosphorus levels are maintained at high levels. Even though Prymnesium is incapable of nitrogen fixation, it appears that low nitrogen availabilities (relative to phosphorus) stimulate toxicity, which, in turn, presumably enhances Prymnesium’s competitive abilities and aids in bloom formation and domination of the winter algal assemblage.

Based on these and other finds, we have recommended that management of Lake Texoma include steps to reduce the incidence and severity of drought-induced increases in salinity. More importantly, our data indicate an urgent need to reduce nitrogen and phosphorus loading to the lake. Although we will continue to monitor the lake on a routine basis, PEL researchers are now focusing their efforts toward better understanding the mechanism and regulation of toxicity in Prymnesium, whether to fish or zooplankton, in hope of discovering concrete management practices that might be useful in reducing Prymnesium’s toxic effects should the invader become a permanent resident of Lake Texoma.

Dave Hambright

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