Possible Relationships Between Plankton-Diatom Species Numbers and Water-Quality Estimates

Williams, Louis

Semimonthly samples from 103 scattered stations on the major rivers and Great Lakes of the United States reveal differences in kinds and numbers of dominating planktonic organisms. Diatoms dominate at these stations. A counting of 250 to 300 individuals per sample of diatoms using special hyrax mounts gives an adequate proportional representation of the four most abundant species of the population. Samples with high representation of individuals in the four most abundant species reliably indicate low species diversity, which in most situations is an indication of enrichment or eutrophication. Samples with high standing crops (productivity) usually show low species diversity. A study of the four most abundant species of diatoms and the six most abundant genera of algae demonstrated, with few exceptiosn, that the most frequent dominants have the highest standing crops. Dominating diatom species with highest frequency and composing 50% of the total abundance of diatoms for each of 2 years were calculated for each station, and the results indicate differences among the stations for studies of enrichment. An overall "trophic index" was computed by multiplying the percentage of the total abundance by the density level for each station. There is a correlation between the trophic index value and a range of stations known to be clean to heavily enriched. Eutrophic stations generally were represented by a few species composing a large portion of the diatom population, and the density level was usually high. "Clean" stations, on the other hand, had more species composing a small portion of the total live diatom population, and the overall density level and low. A parallel study of the most frequently occurring dominant algal genera showed the same trend.

Possible Relationships Between Plankton-Diatom Species Numbers and Water-Quality Estimates

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