Nuisance algae in streams: Public perception versus environmental impact

Background/Question/Methods

A public opinion survey was carried out in Montana to determine what level of benthic river & stream algae the public viewed as interfering with recreation. The survey had two parts; an On-River survey and a By-Mail survey. The On-River survey was conducted via 44 trips randomly scheduled throughout the state during which river recreationists were interviewed in-person. Selection of stream segments and survey dates/times was based on known, statewide recreational-use patterns. The By-Mail surveys were sent to 2,000 individuals randomly selected from Montana’s Centralized Voter File (CVF) available from the Montana Secretary of State (represents 85% of the state’s eligible voting population). In both surveys, eight randomly-ordered photographs depicting varying levels of stream benthic algae were presented (levels depicted were approximately 50, 100, 150, 200, 250, 300, 400, 800 mg chlorophyll a per sq. meter).  Survey participants were asked if each algal level shown was desirable or undesirable for recreation. Survey form design, selection of photographs and pre-testing followed acceptable protocols that limited unintentional bias through survey execution. There were 389 completed responses to the By-Mail survey, while the On-River survey documented 563 interviews.
Results/Conclusions

In both surveys, as benthic algal chlorophyll a (Chl a) levels increased, desirability for recreation decreased. For the majority of the public, benthic Chl a levels ≥ 200 mg/m² (reach mean) were determined to be undesirable for recreation, whereas levels ≤ 150 mg Chl a /m² (reach mean) were found to be desirable. All error rates were within the survey's statistical design criteria (≤ 5%). The largest potential error source was non-response in the By-Mail survey; however, the population represented by non-respondents would have to exhibit profoundly different perceptions of algae in rivers & streams to alter the results significantly. These results agree with earlier reports in the literature that suggest 150 mg Chl a /m² represents a nuisance level of benthic algae. This level was earlier adopted as a standard for the Clark Fork River of Montana, to protect a variety of beneficial uses, based on scientific criteria.