Temporal and spatial patterns of UV penetration in Lakes Michigan and Huron

Expansion of dreissenid mussel populations and decreased nutrient loading have led to a decrease in seston concentration and an increase in water clarity documented by Secchi depth and photosynthetically active radiation (PAR) in Lakes Michigan, Huron and Ontario. We hypothesized that the increase in water clarity also led to increased penetration of ultraviolet (UV) wavelengths that may affect diurnal vertical migrations and survival of zooplankton and larval fish. To predict UV penetration from standard limnological variables and how they may vary across habitats, we measured penetration of ultraviolet wavelengths and PAR across seasons in nearshore and offshore areas of Lakes Michigan and Huron from 2012 to 2021. Offshore and nearshore areas of Lake Huron showed little difference in UV and PAR penetration among sites; however, UV and PAR penetration in Lake Michigan increased with distance from shore owing to dilution of high concentrations of chromophoric dissolved organic matter (CDOM), total suspended solids, and high chlorophyll a associated with loading from major rivers. A comparison with data from the mid-1980s in Lake Michigan indicated that UV-A and PAR penetration has increased in spring before and immediately after stratification, likely related to removal of particulate material by dreissenid mussel grazing when they are connected with the total water column during isothermal periods. In contrast, UV-B penetration, because of its strong association with CDOM, has remained unchanged or even decreased during summer. Nearshore areas near major rivers may serve as refuges for UVsensitive organisms. Potential impacts on plankton and larval fishes are discussed.