The baffling benthos: unraveling the mysteries of mountain lake benthic algae with underwater photogrammetry

The benthic region of lakes plays a significant role in whole lake ecosystem processes. Over the past couple decades, benthic algal blooms have been increasingly observed in mountain lakes across the globe. Global change drivers such as increased nutrient deposition and warming temperatures have been correlated with recent increases in total algal biomass and productivity in mountain lakes, but the spatial and temporal patterns of benthic algal dynamics are largely unknown. Our study aims to develop a method to both map and quantify the extent of benthic algae in mountain lakes using a combination of direct field measurements and underwater photogrammetry. Two lakes in Rocky Mountain National Park, Colorado - The Loch (a shallow subalpine lake) and Sky Pond (a deep alpine lake) - were sampled across the ice-free seasons in 2020 and 2021 for benthic, pelagic and littoral (nearshore) zone algal biomass (as chlorophyll a, chl a) with target areas mapped for benthic algae using stitched imagery from an underwater remotely operated vehicle. Preliminary mapping results suggest that benthic algae have variable spatial and temporal dynamics across both lakes during the ice-free season. Comparison of early summer 2021 benthic chlorophyll a measurements from The Loch to pelagic and littoral zone chlorophyll a measurements suggest that benthic algal biomass is a significant portion of whole lake algal biomass (benthic = 8.9 mg/m2, pelagic = 0.39 mg/m2 littoral = 3.6 mg/m2). Our data from The Loch suggest that clear-water systems, characterized by low dissolved organic matter and phytoplankton concentrations, can support highly productive benthic algal productivity. Data from Sky Pond are forthcoming. Quantifying algae in the benthos in addition to the pelagic and littoral zones allows for a better understanding of whole lake algal dynamics and allow these previously unobserved phenomena to be monitored going into the future.