Drivers and Dynamics of Phytoplankton Communities and Harmful Algal Blooms in Mountain Lakes

Abstract

Harmful cyanobacterial blooms (cyanoHABs) are a complex and widespread disturbance in freshwater water bodies, impacting water quality for wildlife and human populations. While cyanobacteria often bloom in warm lakes impacted by human development like agriculture, blooms are increasingly reported in cooler waters with limited development in the surrounding watershed. As much of cyanoHAB research has focused on lakes in highly developed watersheds, the understanding of factors leading to cyanobacteria dominance and blooms in the absence of major development remains limited. Mountain lakes can serve as ideal systems to study bloom-forming cyanobacteria in watersheds with minimal development. In addition, mountain lakes span natural gradients of productivity and climate due to the varied elevation and topography. Mountain lakes are also highly valued for recreation and habitat for sensitive species due to perceived pristine conditions, but are vulnerable to multiple stressors such as climate warming and increased atmospheric deposition of nutrients, which limit cyanobacteria growth. Therefore, the potential for cyanoHABs in mountain lakes is likely to rise in the near future, making it crucial to understand the drivers of cyanobacteria dominance and blooms in these sensitive lakes.

The goal of this dissertation is to improve the understanding of drivers and dynamics of bloom-forming cyanobacteria in mountain lakes, including factors across lake to watershed scales, to temporal and spatial patterns in planktonic communities. I conducted field surveys to examine the physical and chemical conditions as well as the phytoplankton and other key biological communities in a set of Cascade Mountain lakes located in Oregon, USA. In Chapter 2, I used survey data and existing geospatial data to identify food web, lake, and watershed drivers, as well as interactions among drivers across scales on nitrogen (N)-fixing cyanobacteria, the common cyanoHAB taxa in the region. In Chapter 3, I examined the intra- and inter-annual variation in cyanoHABs and cyanotoxins relative to meteorological conditions and the bacterioplankton communities. Based on genetic sequencing and time integrated toxin monitoring, I determined possible temporal relationships and biological indicators of blooms and bloom toxicity. In Chapter 4, I used survey data as along with existing and interpolated trait data to assess how the top-down and bottom-up effects of grazing zooplankton and nutrients could have shaped phytoplankton communities functionally. I discuss the resulting implications for bloom-forming cyanobacteria dominance over other phytoplankton.

I identified watershed size and hypolimnetic dissolved oxygen as key drivers of N-fixing cyanobacteria biomass likely through the regulation of limiting nutrients such as phosphorus. Temperature and rainfall did not correspond strongly to the blooms and toxins of N-fixing cyanobacteria across lakes, suggesting other meteorological factors as well as lake productivity may be more important. Yet further study is needed with high-frequency sampling and better characterization of meteorological conditions. In terms of the food web, I found grazing cladocerans were a significant factor for N-fixing cyanobacteria, likely via consumption of other phytoplankton. These grazers may be in turn influenced by fish populations. I also found additional biological indicators for cyanobacteria blooms as well as bloom toxicity in heterotrophic bacteria. Finally, I found high grazer abundances favor fast growing and small unicellular phytoplankton at relatively low nutrients, while increasing phosphorus favors colonial cyanobacteria capable of toxin production, such as N-fixing taxa. At high levels of grazers and phosphorus, toxigenic cyanobacteria likely dominate phytoplankton communities in mountain lakes.

This dissertation sheds light on the key factors and indicators for N-fixing cyanobacteria dominance and cyanoHABs in mountain lakes. My results suggest that lake management agencies must consider how both landscape and within-lake factors may affect the risk for cyanoHABs, and how fish stocking increases risk via their impact on grazing zooplankton biomass and body size. As mountain lake monitoring is limited, my results suggest genetic identification of bacterial communities may provide a more time-integrated way to assess bloom development and toxicity. Determining how abiotic and biotic factors interact is needed to broaden management approaches since strict nutrient reduction programs have had mixed success in reducing cyanoHABs in lakes.