Increased methylmercury bioaccumulation and community shifts in zooplankton along a boreal multi-reservoir system.

New hydroelectric developments, built to meet rising energy demands, transform natural riverscapes into lentic-like reservoirs while flooding terrestrial systems. This has led to increased bacterial production of methylmercury (MeHg), a neurotoxin with high absorption and biomagnification potential, resulting in high concentrations in predatory fish. Still, lower trophic level processes governing this trophic increase, particularly zooplankton dynamics, are not well understood, hampering accurate predictions, especially for series of reservoirs along a river. We found that in a boreal multi-reservoir complex (Romaine River, QC), bulk zooplankton MeHg and total mercury (THg) bioaccumulation responded rapidly to upstream impoundments, with pronounced increases typically emerging the following year. Linear regression estimated MeHg increases of 25-63 ng⋅g-1⋅yr-1 in reservoirs (one run-of-river dam, two large reservoirs) and 43 ng⋅g-1⋅yr-1 downriver, highlighting their usefulness as indicators of short-term Hg dynamics. Synchronous year-to-year zooplankton MeHg and THg patterns across reservoirs and downriver suggested responses were driven by the timing of seasonal events (ice off, ecological succession), while transient variability between reservoir sub-environments led to higher baseline MeHg within warm, shallow bays. Dissolved MeHg (<0.45 μm) levels partially predicted (∼30 %) zooplankton MeHg and THg concentrations and although high bioaccumulation factors (BAFs) indicated an efficient bioaccumulation by algae and zooplankton, BAFs declined with higher dissolved MeHg. Rises in temperature were associated with decreased Cladocera abundance, increased copepod growth, and reduced MeHg aqueous exposure and bioaccumulation levels later in summer. We propose a conceptual model of zooplankton MeHg dynamics in a boreal reservoir series following impoundment, offering valuable insights for environmental managers monitoring recently impounded multi-reservoir systems.