Beyond Eutrophication: Vancouver Lake, WA, USA as a Model System for Assessing Multiple, Interacting Biotic and Abiotic Drivers of Harmful Cyanobacterial Blooms

Gretchen Rollwagen-Bollens,Stephen Bollens,Vanessa Rose,Tammy Lee

doi:10.3390/w10060757

Abstract

Eutrophication of lakes and reservoirs has contributed to an increase in the magnitude and frequency of harmful cyanobacterial blooms; however, the interactive effects of nutrient availability (eutrophication) and other abiotic and biotic drivers have rarely been comprehensively studied in the field. We undertook an eight-year (2005–2013) research program that assessed the interaction of multiple factors driving cyanobacterial blooms in Vancouver Lake, a large, shallow eutrophic lake in Washington, USA. Our program consisted of nearly continuous monthly or weekly monitoring of water quality and plankton community composition over eight years, as well as multiple zooplankton grazing experiments over three years. We found a relatively consistent seasonal succession of phytoplankton and zooplankton assemblages, and a pattern of interacting factors influencing cyanobacterial bloom dynamics. Typically, a combined effect of decreased dissolved inorganic nitrogen (N), a sudden increase of dissolved inorganic phosphorus (P), and a cascading effect of zooplankton grazing created a ‘perfect storm’ of conditions that promoted the rapid proliferation of cyanobacteria over the two to three weeks before a bloom. At the blooms’ peaks, cyanobacterial carbon biomass reached as high as 20 µg L−1, with total [chl a] often exceeding 750 µg L−1. In the weeks following the blooms’ peaks, [PO4-P] and [NH4-N] dropped and copepod feeding rates fell to near zero, whereas microzooplankton grazing rates reached their maxima. Microzooplankton grazing impact, combined with low nutrient availability, then drove down cyanobacteria abundance. Vancouver Lake serves as a model for understanding multiple, interacting drivers of cyanobacterial bloom dynamics in shallow, temperate lakes, and is therefore an important system in which to investigate new questions related to the science and management of harmful algal blooms.

Highlights

Cyanobacteria are globally ubiquitous in aquatic systems and form an important component of planktonic food webs, as biogeochemical mediators of nitrogen and phosphorus [1].Cyanobacterial blooms are historically natural occurrences and not inherently problematic [2].harmful cyanobacterial blooms—those causing major negative impacts on ecosystems—are a critical threat to freshwater systems in the 21st century.Cyanobacterial blooms may result in diminished water quality and low dissolved oxygen levels, leading to fish kills, decreased biodiversity, and the disruption of food webs and ecosystemWater 2018, 10, 757; doi:10.3390/w10060757 www.mdpi.com/journal/waterWater 2018, 10, 757 dynamics [3,4,5]
Copepods were incubated in unfiltered lake water before, during, and following cyanobacterial blooms that occurred in the August–September of both 2008 and 2009. [Note: in 2008–2009 we identified the dominant cyclopoid copepod species to be Diacyclops thomasi
20th century, has increased development in the watershed surrounding the lake since the early 20th century, has resulted resulted in substantial changes in theofecology the coincided lake, and coincided with the frequency increased of frequency in substantial changes in the ecology the lake,ofand with the increased harmful of harmful cyanobacterial blooms

Summary

Introduction

Cyanobacteria are globally ubiquitous in aquatic systems and form an important component of planktonic food webs, as biogeochemical mediators of nitrogen and phosphorus [1].Cyanobacterial blooms are historically natural occurrences and not inherently problematic [2].harmful cyanobacterial blooms—those causing major negative impacts on ecosystems—are a critical threat to freshwater systems in the 21st century.Cyanobacterial blooms may result in diminished water quality and low dissolved oxygen levels, leading to fish kills, decreased biodiversity, and the disruption of food webs and ecosystemWater 2018, 10, 757; doi:10.3390/w10060757 www.mdpi.com/journal/waterWater 2018, 10, 757 dynamics [3,4,5]. Cyanobacterial blooms are historically natural occurrences and not inherently problematic [2]. Harmful cyanobacterial blooms—those causing major negative impacts on ecosystems—are a critical threat to freshwater systems in the 21st century. Cyanobacterial blooms may result in diminished water quality and low dissolved oxygen levels, leading to fish kills, decreased biodiversity, and the disruption of food webs and ecosystem. Numerous cyanobacterial species produce hepatotoxins, neurotoxins, or contact irritants [6]. These cyanotoxins may cause illness or fatalities from exposure to contaminated water [1,6,7], which poses an especially challenging problem when drinking water supplies are affected

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