Quality of suspended fine particulate matter in the Little Tennessee River

Abstract

Fine particulate organic matter is a major food resource in southeastern river food webs, but natural variability in the quality of this resource has not been assessed. We measured the quality of suspended fine particulate matter (SFPM) along the Little Tennessee River at four sites ranging from 5th to 7th order. SFPM quality was measured using traditional measures: nitrogen to carbon ratio (N/C), calories (cal g−1 DM), % lipids, % inorganic matter, bacteria (# cells g−1 DM) and % diatoms. Instantaneous growth rates of chironomids fed SFPM were used as an integrated measure of food quality. Traditional measures of SFPM quality varied among sites, with higher N/C and % inorganic downstream, higher % lipids and bacteria upstream, and no pattern in the % diatoms and calories. Although percent mortality did not differ among chironomids fed SFPM from different sites, instantaneous growth rates (IGRs) of chironomids fed SFPM from the most downstream site were significantly higher than those fed SFPM from the most upstream site, implying higher food quality at the downstream site. IGRs were not significantly different among seasons for any site. The traditional measures individually and in combination (using principal components analysis) were not related to IGRs. IGRs are a more realistic indicator of food quality than measures of individual attributes, because IGRs integrate the consumer’s response. The quality of a food resource is not merely the sum of its measurable parts (% lipids, calories, % inorganic, and diatoms), and one or a combination of measures is not adequate to predict food quality. The Little Tennessee River has very high secondary production of filtering invertebrates at the 7th order site. The quality of SFPM as a food resource and extensive favorable habitat of macrophytes on bedrock, support the high secondary production of filtering invertebrates at this site.