Water quality and survivability of Didymosphenia geminata

Abstract

Didymosphenia geminata or Didymo has become a world-wide invasive aquatic species. During blooms, the algae can form thick mats covering entire reaches of stream bottom, which in turn creates negative aesthetic, ecologic, and economic impacts. Although Didymo is historically present in the United States, it is spreading quickly into areas that were previously free of it, and is even growing in waters that were thought not ideal habitat for Didymo. Previous research on how water quality affects Didymo growth and spreading appear to be influenced by streamflow rates and water pH levels. Other water quality parameters have not been fully tested on Didymo, which would contribute to a better understanding of what controls Didymo growth. The first goal of this study was to colonize Didymo in an artificial stream within a laboratory setting. The second goal was to evaluate the survivability of Didymo by exposing it to different water quality parameters. Artificial stream configurations with various light intensity and duration, water temperature and velocity, source water chemistry, and different growth media were used. In all attempts colonization of Didymo was unsuccessful as Didymo slowly deteriorated and became covered by other algae that were more successful in the artificial conditions. Didymo survivability as affected by a 60 minute exposure to different water quality parameters followed previously determined results in that known algaecides did affect cell viability, while other non-toxic parameters showed no effect on Didymo. Nitrate, nitrite, phosphate, chloride, calcium, and magnesium did not affect Didymo survivability. Ammonia also did not affect Didymo but signs of cells lysis were observed and possible mortality may occur with longer exposure times. Copper, zinc, chlorine, and pH affected Didymo survivability. Copper showed the greatest affect on Didymo survivability with the median lethal concentrations (LC50) for copper at 9.3°C and 13.0°C being 3.3 mg/L and 5.4 mg/L respectively at pH 7.7. For copper toxicity in waters with a lower pH (6.7) the resulting LC50 was 33 mg/L. Generally, both colder water temperature and higher pH increased copper toxicity on Didymo. The affect of temperature on copper toxicity was shown to be statistically significant (p-value 0.02). However, there was no statistically significant affect of pH on copper toxicity (p-value 0.07). The LC50 could also not be determined for all three zinc tests but the highest zinc concentration of 40 mg/L had on average 56% of Didymo cells surviving. No apparent trend on the affect of temperature to zinc toxicity on Didymo could be determined; however, the interaction of temperature on zinc toxicity was statistically significant (p-value 0.02). Chlorine at temperatures of 11.5°C and 17.3°C had LC50s of 5.67 and 8.46 mg/L respectively. The affect of temperature on chlorine toxicity was statistically significant (p-value <0.001). Didymo survivability was affected in water with pH 4.3 but not in water with pH 5.9 and 6.9. Cell lysis was occurring in water with pH 10.7 but no sign of any affect on Didymo survivability was found in water with pH 9.9.