Identification of rain‐freshened plumes in the coastal ocean using Ra isotopes and Si

Abstract

Two transient plumes of freshened water in the South Atlantic Bight off the coast of South Carolina were measured and mapped during a continental shelf cruise in July 1994. Salinity in the plumes was depressed 1–2 parts per thousand compared to surrounding shelf waters. One plume was at least 160 km long, 20 km wide, and 5–7 m deep. The other was 75 km long, 25 km wide and 5–7 m deep. Both plumes disintegrated almost completely in 6 days due to mixing or advection away from the sampling area. Measurement and analysis of short‐lived Ra isotopes and Si indicated that the plumes were not caused by river discharge but were rather due to an intense rain event a few days prior to the sampling cruise. With only salinity measurements we would have incorrectly concluded that the freshened water was a result of river plumes having entered the shelf. The rain was associated with the system that spawned tropical storm Alberto. We estimate that 220–330 mm of rain fell on the coastal ocean immediately before the cruise. Accompanying the rain were pesticides and nutrients. We estimate that the flux of nitrate from the rain was twice the riverine flux of nitrate to the study area in the month preceding the cruise.