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Warming-induced changes in benthic redox as a potential driver of increasing benthic algal blooms in high-elevation lakes


Nearshore blooms have been observed in oligotrophic lakes worldwide. These blooms are particularly puzzling in lakes that are relatively well protected from human disturbance such as remote mountain lakes. In a paper published in Limnology and Oceanography Letters, we suggest that nearshore blooms are triggered by warming, mediated by redox associated mobilization of nutrients from sediments under warmer and lower oxygen conditions. The released nutrients are quickly scavenged by periphyton before they reach the water column, such that water column nutrients may not change. Once a periphyton mat becomes established, the biofilm and accumulating detritus below the mat can create anoxic conditions that encourage nutrient release from the sediment, particularly at night when photosynthesis does not produce oxygen. The phenomenon may be particularly noticeable in high-elevation lakes that are warming faster than lowland counterparts, receive higher solar radiation, and hold less oxygen at saturation due to lower atmospheric pressure.

Littoral greening of clear lakes: the mystery of benthic filamentous algae blooms

Over the last few years, Dr. O was involved in a series of working groups of researchers and natural resource managers who have united around the theme of understanding the proliferation of filamentous algal blooms (FABs), emerging phenomenon impacting seemingly pristine lakes around the globe. Initially the workshops were limited to mountain lake limnologists, but we eventually expanded our reach to scientists who work on some of the world's largest lakes (e.g., Lake Tahoe, Lake Baikal, Lake Taupo).

One outcome of our group was the synthesis of the drivers of FABs in clear-water lake. The environmental stressors leading to FABs are diverse, ranging from direct and localized (e.g., point source nutrient pollution, non-native species introductions) to indirect and  regional or even global (e.g., atmospheric nutrient deposition, climate change). While the stimulus that leads to FABS may differ across locations and conditions, the responses converge to produce a proliferation of FABs in lakes traditionally viewed as relatively pristine.

To advance our understanding of FABs standardized monitoring approaches need to be implemented to assess the spatiotemporal variation in littoral zone attached algae. To that end, Dr. O co-authored a methodological review paper with a group of early-career scientists where we collated periphyton and groundwater sampling and modeling techniques, with the hope of motivating researchers and managers to integrate these techniques into their programs. 


Diagram Credit: Thomas Shannon


Because of a warming climate, changes in snowpack, and chronic nutrient deposition, mountain lake algal ecology is changing; however, the exact reasons leading to increased green algae in seemingly pristine lakes are still largely unknown. Using a series of experiments, we explored the implications of temperature and nutrient availability for benthic algal assemblages and ecosystem processes in a Colorado mountain lake. In a field experiment, we found that nutrient additions of both nitrogen and phosphorus favored green algae and produced the highest overall algal biomass. In the absence of nutrient enrichment, the relative abundance of diatoms was significantly greater than green algae and cyanobacteria. In laboratory assays, nitrogen uptake increased substantially with warming, but net ecosystem production decreased, resulting in reduced N use efficiency. Our results demonstrate that nutrient availability favors filamentous green algae in benthic algal communities and that the increase in green algae may alter ecosystem processes like nitrogen cycling and metabolic functions in mountain lake near shore habitats.​ You can read more about these experiments in our paper published in early 2021 in Freshwater Science.

Nutrients and warming alter mountain lake benthic algal structure and function

Blue Waters, Green Bottoms: ​Benthic Filamentous Algal Blooms Are an Emerging Threat to Clear Lakes Worldwide

Many of the world’s most iconic clear lakes are degrading at an alarming rate – shallow, nearshore lake bottoms are being carpeted by bright green fronds of slimy algae, especially during the summer. Scientists are unsure why FABs are suddenly showing up in remote mountain lakes, as well as in some large lakes such as Lake Tahoe (USA), Lake Baikal (Russia) and Lake Wakatipu (New Zealand), but I worked as part of an international group of lake scientists to try to tackle the problem. In an open-access article published in BioScience, we explored how nutrient pollution, climate change, loss of aquatic animals that eat algae, and invasive species contribute to the increased occurrence of green bottoms. 


Seasonal Shifts in the Importance of Top-Down and Bottom-Up Factors on Periphyton Community Structure


A number of studies have assessed how top-down or bottom-up factors influence stream periphyton community structure, but investigations over time are rare. Whitney Beck led these experiments in the headwaters of the Poudre River watershed and I assisted with the pigment analyses that we used to characterize community change. We found that top– down grazing effects were strongest in late summer when grazers were abundant. The effects of phosphorus additions on algal biomass decreased over time because temperature became more limiting to growth than nutrients, and because reduced current velocity decreased nutrient uptake rates. This paper is accepted in Oikos.

Photo credit: Whitney Beck

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