Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension

Abstract

A free-fall benthic time-lapse camera and current meter system has been used to examine changes in the appearance of the sea bed in the Porcupine Seabight 50°N, 13°W — northeast Atlantic). Changes in the benthic environment due to rapid deposition of phytodetritus occur down to 4000 m. Photograhs taken every 8 h from 1 May to mid-August showed dramatic changes in the appearance of the sea bed between mid-June and mid-July. The sinking rate of the detritus was calculated from the time of the spring bloom until its arrival on the sea bed. In both 1982 and 1983, year which differed greatly in the timing of the spring bloom, the sinking rates were probably between 100 and 150 m d −1. Individual aggregates up to 12 mm diameter arrived between frames at all depths and up to 50 mm at 4000 m. Their subsequent disintegration was monitored over the few days following arrival on the sea bed. Some such aggregates were collected from the sea bed and a sinking rate experiment was carried out on them. Once on the sea bed, the detrital carpet moves over the sediment surface due to bottom currents; when currents exceed about 7 cm s −1 (at 1 m altitude), the material is resuspended. Increases in the quantity in suspension occur at the same time as decreases in the quantity visible on the sea bed. The tidal nature of the current gives a strong tidal component to the variation in suspended particles near the sea bed. The significance of a detrital critical erosion threshold to the structure of benthic and benthopelagic communities is discussed.