The effects of turbulence on light exposure in a large-lake surface mixed-layer: a preliminary in-situ dual-dye study.

Light exposure is a critical variable for chemical and biological processes in aquatic systems, and much effort has been expended in measuring and modeling light fields. However, these approaches have almost entirely relied upon Eulerian rather than Lagrangian measurements. Such approaches, while giving good estimates of the instantaneous light field, may not accurately reflect light doses experienced by chromophores passively transported by turbulence within a lake or ocean surface-layer. Here a dual-dye approach, used as a Lagrangian light dosimeter, was applied in Lake Superior’s surface mixed layer to determine the importance of mixing in attenuating light dosage in this layer. In this approach, two fluorescent dyes were deployed in the water column in a known ratio; one of the dyes (fluorescein) was sensitive to light exposure and one (rhodamine WT) was relatively photostable. Fluorescence-photodecay equations for fluorescein and rhodamine WT were determined from controlled irradiations of dye solutions with natural sunlight. The mixed-layer light dosage (from 460 to 510 nm) was then traced over two dye deployments in Lake Superior, the world’s largest lake by area. The dye results were compared with model results assuming: 1. no turbulent mixing within the mixed layer and 2. rapid homogenization of the surface mixed layer. The dye results fall within these end-member modeling studies and show that vertical mixing is important and variable in its effects on light dose within Lake Superior’s surface layer on time scales of hours. Such approaches, in conjunction with fine-scale physical measurements , should lead to a much better understanding of the effects of turbulence in large-lake and ocean mixed-layers.