Physical dynamics on the continental shelf off southwestern Ireland and their influence on coastal phytoplankton blooms

Abstract

Distributions of physical, chemical and biological characteristics were recorded for shelf waters off southwestern Ireland between 1992 and 1995. The location of a haline front (the Irish Shelf Front; ISF) separating coastal and oceanic water had a horizontal scale of movement comparable to the distance of the front from the coastline (ca. 30 km). Current measurements from deployments of up to 120 days in the region revealed a residual northwestwards (325°) flow of 8 cm s -1 inshore of the front and a southeastwards flow (160°) of 3.5 cm s -1 offshore. The buoyancy-driven coastal flow inshore of the ISF was modified by wind direction. During periods of extended winds from the southwest quadrant a counterbalancing wind-driven flow to the southeast of similar magnitude to the measured residual flow was observed. Under these conditions the ISF approached the coastline. Winds blowing from the east resulted in a strong (20 cm s -1) clockwise coastal flow around southwestern Ireland and the front was found further offshore. Thus, under prevailing southwesterly winds, water in the northwestern Celtic Sea tended to become isolated and, as a result, dissolved oxygen levels below the pycnocline became markedly reduced (65–70% saturation) towards late summer. At the same time high concentrations of dinoflagellates associated with the pycnocline were frequently recorded in this water. These dinoflagellate communities were advected towards and into the bays of southwestern Ireland when the coastal current inshore of the ISF flowed as a result of changes in wind direction; this often resulted in discoloured water, or ‘red tides’, along the shore.