NORTHERN SANDY BEACHES AS SEASONAL REDOX OSCILLATORS

Sandy beaches represent a key environment of biogeochemical dynamic at the land-sea interface where fresh and salt waters mix. In northern regions, where the hydrology and the organic carbon dynamics of wetlands are changing in response to seasons, little is known about the biogeochemical transformations of terrestrial carbon, nutrients, and metabolic products that transfer through sandy beaches to the ocean. In this presentation, we will explore the link between seasonal conditions and the complex biogeochemical redox environment and non-conservative behaviour of nutrients and metabolites in the pre-discharge beach groundwater. The study was done in a northern sandy beach system setting at the Îles-de-la-Madeleine (Gulf of St-Lawrence, Canada) where there have been significant groundwater discharges. Seasonal variations of nutrients and metabolic products were examined down to 3 m depth in pore-waters collected at low and high tide in a cross-shore section. In early summer, increase in phosphorus and dissolved iron were observed along the discharge pathway beyond that expected from mixing with marine water. A slow groundwater velocity, high dissolved organic carbon and suboxic conditions would be suitable for anaerobic respiration to occur and subsequently alter groundwater borne nutrients. In mid fall and winter, nutrients and metabolites concentrations are low along the discharge pathway. Low temperatures and the presence of ice foot that freezes on the shore down to ~0.8 m depth probably limit the pore water flows and decrease metabolic processes. Redox oscillations influence the biogeochemical environments in the beach aquifer and alter groundwater inputs to the coastal environment. Because northern coastal aquifer rapidly response to hydroclimatic changes due to global warming, the relative importance of seasonal redox oscillations on biogeochemical transformations at the beach faces harbouring groundwater discharges should be clarified.