Influence of Environmental Factors on the Vertical Distribution of Phytoplankton in Lacamas Lake, WA

Influence of environmental factors on the vertical distribution of phytoplankton in Lacamas Lake, WA Kaitlin Perkins Washington State University Vancouver Spring 2017 Gretchen Rollwagen-Bollens Aquatic Ecology Lab Abstract: Urbanization in watersheds has led to nutrient enrichment (eutrophication) and dissolved oxygen depletion (hypoxia) in many freshwater systems. These conditions impact species diversity, availability of habitat, and organism behavior within these systems. Lacamas Lake is a managed reservoir in Camas, WA that experiences seasonal stratification, hypoxia in bottom waters, and is highly eutrophic, sometimes resulting in harmful algal blooms. Lacamas Lake also undergoes a drawdown for dam maintenance purposes each autumn. To better understand the impacts of hypoxia and management actions on the phytoplankton community in Lacamas, we pursued three research questions: 1) How is phytoplankton biomass vertically distributed in relation to dissolved oxygen levels? 2) Are there differences between day and night vertical distributions of phytoplankton? 3) How does phytoplankton vertical distribution vary before and after lake drawdown. 4) What is the relationship between phytoplankton size and vertical distribution? During August (pre-drawdown) and October (post-drawdown) 2015, phytoplankton biomass was measured at six depths from surface to bottom and a weighted mean depth was calculated for each sampling time. We found that phytoplankton biomass was consistently concentrated above the hypoxic zone, indicating these organisms were avoiding the low oxygen water. There was a significant difference in vertical distribution between day and night in one size fraction, as well as a significant difference in the vertical distribution between pre- and post-drawdown. These results highlight the need for strategies that manage run-off flowing into the watersheds in urban areas and further research into the ecological implications of the annual drawdown at Lacamas Lake. Keywords: Phytoplankton ecology, diel vertical distribution, hypoxia, eutrophication, reservoir Introduction: Urbanization and agriculture have altered the ecology of aquatic systems globally. Livestock grazing, the burning of fossil fuels, and paved surfaces increase pollutant loading, and dams have created reservoirs where previously rivers and lakes dominated (Carpenter et al 2011). Dams are most frequently constructed for power production, and their associated reservoirs often take on social significance as recreational sites (Lehman et al 2014). Reservoirs differ from other aquatic ecosystems in their hydrology, nutrient loading, and seasonal response, among other factors. In turn, they harbor phytoplankton communities with specific spatiotemporal dynamics and life strategies for success (Tornés et al 2014). Approximately fifty percent of lakes in the United States are classified as eutrophic, meaning they are impacted by excess nutrients, primarily nitrogen and phosphorus (Smith et al 1999). Despite efforts to mitigate and prevent eutrophication through programs to eliminate nutrient runoff, internal phosphorus loading may still be a problem as nutrients stored in reservoir and lake sediments re-enter the water column. Phosphorus is typically released from sediments when the bottom of the lake becomes anoxic during periods of stratification (Nurnberg et al 2013). Eutrophication is commonly linked to low phytoplankton community diversity, and often only one or two dominant species are seen in eutrophic system (Izaguirre et al 2012). Eutrophication is also linked to harmful algal blooms (Conley et al 2009) and hypoxia (Vanderploeg et al 2009) in freshwater lake and reservoir systems. Harmful algal blooms (HABs) occur when water conditions spur the rapid reproduction of phytoplankton in marine or freshwater systems. In freshwater lakes, cyanobacterial blooms are the most common type of HAB and a sign of eutrophication (Conley et al 2009). HABs may be detrimental to human health, fish stocks, and ecosystem structure and function (Anderson et al 2002). Hypoxia is a common indicator of eutrophication as well (Vanderploeg et al 2009). Hypoxia changes trophic dynamics in aquatic environments by altering migratory patterns of motile phytoplankton, as well as habitat availability (Zhang et al 2015). The stress of low oxygen levels leads to a decrease in trophic diversity and increases the populations of opportunistic organisms such as cyanobacteria (Friedrich et al 2012). Cyanobacteria are more adapted to low light conditions that often occur in turbid, eutrophic lakes, and therefore may dominate over other species or genera of phytoplankton (Sinistro et al 2015). Nutrient and light availability, predation, and the mixing conditions in a lake system may directly and indirectly influence the composition of its phytoplankton community (Becker et al 2010). Higher latitude reservoirs are often thermally stratified in the summer, which coincides with vertical gradients of nutrients, light availability, and dissolved oxygen. Phytoplankton are impacted by thermal and density stratification, as it creates resource niches vertically in the water column (Cantin et al 2011). Access to these resources niches by phytoplankton is determined by their life strategies, such as size, motility, and buoyancy (Reynolds 2006). Small organisms such as cyanobacteria, for example, may capitalize on increased buoyancy to access light in the surface layer of the water column, while larger motile organisms such as flagellates may migrate deeper in the water column to access the nutrient-rich lower layers (Cantin et al 2011). Phytoplankton are plant-like organisms that are important globally, as they are responsible for approximately half of the world’s primary production (Kruk et al 2012). Phytoplankton communities are vertically heterogeneous, and their location in the water column informs their role as primary producers and a food source for other organisms (Mellard et al 2011). Their availability as a resource for grazing zooplankton can have bottom-up trophic cascade effects, and similarly zooplankton and fish community structure can impact phytoplankton community composition in a top-down manner (Reynolds 2006). Phytoplankton, due to their sensitive response to changes in abiotic factors such as nutrients, hydrology, and stratification, make them excellent indicators of ecological change (Paerl et al 2006). The visual appearance of water is impacted by phytoplankton, and knowledge of their community structure and function is a useful freshwater management tool (Kruk et al 2012). Biomass and size aggregation estimates, in conjunction with vertical distributions, are useful tools for investigating phytoplankton ecological response to environmental conditions (Kruk et al 2002). Phytoplankton have a crucial role in determining the usability and public perception of recreational waterways due to nuisance algal blooms and poor water quality, and Lacamas Lake has a history of monitoring and restoration projects due to negative public opinion of its “health” (Carlson 1985). Lacamas Lake has been monitored since the 1980s, and its seasonal hydrologic and nutrient dynamics are well documented, making it an ideal model reservoir system (Carlson 1985, Hutton and Schnabel 2004). The reservoir is managed through restoration programs and an annual drawdown, and is impacted by urbanization and upstream livestock grazing and agricultural land use practices (Hutton and Schnabel 2004). Reservoirs differ from lakes as they are often managed through controlled water release for maintenance, aquatic vegetation management, or water consumption (Cooke 1980). Water drawdown may have unintended effects, as water level reductions lead to soil desiccation, prompting nutrient release upon rewetting (Baldwin et al 2008). Drawdowns have also been linked to phytoplankton blooms, which occur when nutrients are released from the sediment (Klotz and Linn 2001). Increased urbanization means that anthropogenically-impacted lakes will become increasingly common, highlighting the importance of understanding their ecology and function. Phytoplankton are an important indicator of the ecological status of a body of water, and can inform water quality in freshwater systems. Investigation into the vertical distribution, seasonal dynamics, and size structure of the phytoplankton in Lacamas Lake provides insights into its ecological status. More broadly, we seek to answer: how does the phytoplankton community in Lacamas Lake respond to changes in season and resource availability? Research Questions: 1) How is phytoplankton biomass vertically distributed in relation to dissolved oxygen levels? 2) Are there differences between day and night vertical distributions of phytoplankton? 3) How does phytoplankton vertical distribution vary before and after lake drawdown? 4) What is the relationship between phytoplankton size and vertical distribution? We established a sampling program to measure phytoplankton biomass at multiple depths during the day and night in the summer and autumn 2015, both before and after the annual drawdown. The collections involved recording environmental factors that impact phytoplankton, including temperature gradients and dissolved oxygen levels. Methods: Study site: Lacamas Lake is a small reservoir (1.3 km2) located in Clark County, Washington (45.37N, 122.25W) (Fig. 1). The lake, which is as deep as 19.8 meters and has an average depth of 7.8 meters, was dammed in 1938 (Deemer et al Figure 1 Map of Lacamas

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