Stratification triggered by winter rain events: is there enough light for primary production?

Light is the primary limiting factor for winter phytoplankton growth at temperate latitudes, and is therefore a control on spring bloom seeder populations and subsequent succession. In continental shelf seas, a combination of net cooling and high winter winds result in turbulent mixing that distributes phytoplankton cells over the entire depth of the water column, resulting in light limitation. Here we present observations from winter 2015 that show sporadic rain events to initiate ephemeral periods of stratification that may persist up to 2 days, coinciding with increased chlorophyll florescence. Rain-induced stratification is more likely to occur during late winter, when a combination of positive buoyancy from increased heating, freshwater input and a decrease in winter winds is more likely to outcompete the mixing effects of wind, waves and convection. The time that phytoplankton cells spend in the euphotic zone is derived from the turbulent eddy diffusivity under varying euphotic depth conditions (10m, 23m and 35m). We investigate phytoplankton dynamics under two mixed layer depth (z_sml) conditions: a highly dynamic z_smlthat is representative of sporadic storm events, and a fixed z_sml, derived from observed daily averages. The classification of the z_smlis shown to be critical when determining mixing timescales during short-lived stratification events. A highly dynamic, time-varying estimate for the z_smlresults in phytoplankton cells spending up to 4 hours longer in the euphotic zone than a fixed z_sml. Crucially, sporadic rain events allow phytoplankton cells to remain for several hours longer in the euphotic zone during periods of high turbidity. We demonstrate that rainfall is a critical mechanism for promoting ephemeral winter stratification, helping to maintain winter phytoplankton populations by reducing light limitation.