THE SEASONAL DISTRIBUTION, ABUNDANCE AND DIVERSITY OF DESMIDS (CHLOROPHYTA) IN A SOFTWATER, NORTH TEMPERATE STREAM1

Abstract

ABSTRACTSuspended and benthic algal communities from a mildly acidic, third‐order Rhode Island stream were examined to determine the seasonal distribution, abundance and diversity of the lotic desmids. Within a one‐year sampling period, 148 species and 202 subspecific taxa of desmids were identified, representing 23 genera. Species ofCosmariumandClosteriumaccounted for approximately 70% of the desmids present, and were the most diverse and abundant taxa during all seasons except spring, whenHyalotheca dissilienswas the dominant desmid species. Average abundance and species richness generally were greatest during summer for both suspended and benthic desmids. Most desmids occurred in benthic habitats, and were randomly distributed among substrata. Average seasonal abundance was 7.4 × 104cells·g−1dry wt substratum, among 13 types of substrata. Highest desmid abundance was measured among substrata with intricate morphologies, such asFontinalisspp., which was associated with 1.2 × 106desmid cells·g−1dry wt substratum, or 1.7 × 103cells·cm−2substratum. Cell division was observed for 70 desmid taxa, and average seasonal reproduction (based on cell numbers) among all substrata ranged from 4% in winter to 20% during summer. In addition, sexually produced zygospores were found occasionally forH. dissiliens.Desmids were distributed among most substrata examined in this stream, with abundance comparable to reported estimates from softwater lakes and acid bogs. In contrast to established dogma, lotic desmids are not incidental drift organisms, but rather comprise a viable and persistent component of the stream periphyton.