Flood discharges of a small river into open coastal waters: Plume traits and material fate

The dynamics of plumes from large rivers are relatively well known. Many estuaries are small, however, and discharge directly onto exposed, open shorelines and presumably produce smaller plumes that may have different properties. Therefore this study measured traits of a small estuary on the Australian East Coast as a model system, focusing on (a) plume size, (b) distinctness of plume edges, and (c) imprints on the seafloor. Although plumes were found to be limited in spatial extent (ca. 1 km offshore × 2.4 km longshore) and were constrained near the shore by onshore winds, they exported high nutrient loads from an urbanised watershed. The small plumes were shallow (<2 m) and strongly buoyant, with sharp vertical and horizontal clines similar to much larger plumes. The edges of the plumes were highly distinct, clearly separating disparate water masses that trapped significantly higher amounts of nutrients inside the plume. Some particulate material exported from the estuary in the plumes reached the benthos of the nearshore zone, as evidenced by increases in copper concentrations in sediments under the plume. By contrast, the amount of land-sourced carbon delivered by small plumes to the seafloor was minor in comparison to larger inputs from marine sources (e.g. onshore advection of phytoplankton blooms or algae dislodged from reefs) that swamped any contribution from plumes. Overall, small plumes can be important in land-ocean coupling, but their zone of influence may be limited.