Variations in aquatic macrophyte phenology across three temperate lakes in the Coeur d’Alene Basin

Abstract

We studied the phenology of submerged macrophytes within three temperate lakes in 2017. Phenology was examined with macrophyte biomass, sonar biovolume, and water quality data. The primary objective was to determine the timing and duration of macrophyte senescence. The secondary objective was to investigate the feasibility of using sonar biovolume to measure seasonal changes in macrophytes. Our results indicated that Black and Thompson lakes had longer growing seasons and higher species richness than Chatcolet Lake likely due to differences in hydrologic connectivity. Macrophyte senescence was more distinct in Black and Chatcolet lakes. The prolonged and indistinct senescence at Thompson Lake was likely due to the dominance of the macrophyte community by Myriophyllum spicatum. Biovolume results revealed complex spatial patterns in macrophyte senescence at Chatcolet Lake. Macrophyte biovolume was positively correlated with biomass at all sites. Spearman’s rank correlation was highest at Chatcolet Lake (⍴ = 0.75, p-value < 0.001) followed by Black Lake (⍴ = 0.59, p-value < 0.001) and Thompson Lake (⍴ = 0.49, p-value < 0.001). The relationship between biovolume and biomass is likely non-linear and variation in the relationship can be attributed to site specific differences such as species composition in addition to sampling error. Sonar biovolume provided a convenient and nondestructive way to measure seasonal changes in macrophytes.