Turbulent Plumes From Submarine Groundwater Discharge

Turbulent plumes are a ubiquitous phenomenon in the ocean; point sources of buoyancy and momentum tend toward a plume structure in a stratified fluid. A potentially pervasive but little studied source of buoyancy at depth in the coastal ocean is the offshore discharge of fresh water from confined aquifers. These groundwater systems are often called subterranean estuaries, as they are known through tracer studies to be globally important sources of nutrients, carbon, and metals (Moore, Annu. Rev. Mar. Sci., 2010). Research on submarine groundwater discharge to date has been limited to locations where the presence of the discharge is revealed by some manifestation at the sea surface. Advancing our knowledge of the oceanographic and ecologic impacts of these hidden buoyancy sources requires state of the art surveying and modelling tools. To this end, we investigate the structure of a submarine freshwater discharge plume at turbulent eddy-resolving time and space scales using an open-source Large Eddy Simulation ocean model, acoustic backscatter imagery, and theory. The observations are from a submarine spring at 45m depth at the head of Cambridge Fiord, Nunavut. Both the acoustic imagery and the model results reveal the presence of intermittent O(10 m) scale boluses of buoyant water rising at ca. 30 cm/s velocities. The estimates of buoyancy flux at the seafloor and entrainment of ambient seawater into the plume are used to determine the effects of the discharge on mixing rates and vertical stratification in the immediate vicinity of the discharge.