Seasonal Progression of Algal Development and Quality in Streams that Vary in Timing of Springtime Peak Flow

Abstract

Climate driven shifts in stream hydrology from early, rapid snowmelt may have implications for algae growing within high altitude montane streams. Peak flow scours the benthic layer and clears senesced algal growth and sediment, effectively priming the substrate for new algal growth following peak flow. An earlier peak flow allows for a longer season for algae to accrue biomass, but the implications for algal resource quality are unknown. Algae are crucial components of aquatic ecosystems because they are the basal food resource for animals, and algal nutrient uptake can dominate stream nutrient cycling. This study considers how variation in the timing of peak streamflow of different streams in the southern Rocky Mountains could impact the resource quality and development of algae. Algal samples from these streams were analyzed for biomass, chlorophyll-a, community composition, and percent phosphorous. Baetis bicaudatus, a grazing mayfly, was also sampled as they can selectively feed on nutrient dense algae and therefore their developmental stage and body chemistry may characterize among stream variation in algal resource quality more effectively than bulk algal samples. I hypothesized that with shorter and earlier peak flows, algae will (1) accrue more biomass, (2) have higher amounts of chlorophyll, and (3) have poorer elemental resource quality than algae in a stream with a later peak flow. These hypotheses were tested along seven streams in the southwestern part of Colorado, USA. Analysis of variance (ANOVA) was used to see if there were differences in each variable among streams and dates. Biomass, phosphorus content, chlorophyll-A, and cell counts of algal phyla differed among streams, but only cell counts differed amongst dates. Although concrete conclusions about the effects of peak flow cannot be evaluated until streams have all been sampled to base flow this autumn, the differences detected among streams thus far suggest that an unknown factor is influencing algal development, and patterns in biomass and chlorophyll accrual are consistent with predictions based on these rivers’ melt order.

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