Variability in the vertical distribution of chlorophyll in a spill-managed temperate reservoir

Abstract  Phytoplankton form the base of pelagic food webs, and as aquatic systems come under increasing pressure from environmental stressors, managers are becoming more concerned about how these pressures may result in changes to phytoplankton distribution and abundance. The vertical distribution of phytoplankton, including the phenomenon of subsurface chlorophyll a (Chl-a) maxima (SCM), is known to vary widely, with a combination of biotic and abiotic processes hypothesized to be driving this variation. We examined how the vertical distribution of Chl-a generally, and SCM specifically, varied over 4 years (2013–2016) in a managed temperate reservoir (Lacamas Lake, Washington, USA) that experiences rapid (2–3weeks) release of water from within the epilimnion each autumn. Seasonally, a strong thermocline developed each spring at depths of 5–10m, and persisted throughout summer and early autumn. Although temperature varied somewhat between years, elevated concentrations of Chl-a occurred in spring and summer of each year. In addition, pronounced SCM of 30–60mg/L at depths of 2–8m were observed each year, and were nearly always above the thermocline. There was no “seasonal deepening” of SCM in Lacamas Lake, as has been observed in several other lakes and oceans, and annual drawdown events in early fall had little to no effect on SCM in Lacamas Lake. We recommend that more process-oriented studies be undertaken to fully understand which biotic and abiotic processes are driving the formation and maintenance of SCM in lakes and reservoirs, and how SCM are modulated by different reservoir drawdown practices.