Biological studies in the vicinity of a shallow-sea tidal mixing front. I. Physical and chemical background

Abstract

A study has been made of the distribution and activities of bacteria and zooplankton as they varied seasonally in 1980 and 1981 in the vicinity of a shallow-sea tidal mixing front in the western Irish Sea (approximate position 53° 20' N, 5° 45' W to 53° 50' N, 5° 0' W ). This paper presents the physical and chemical background to these studies as shown by the variations in temperature and salinity and concentrations of chlorophyll a , phaeopigments, cellular adenosine triphosphate (ATP), nitrate, nitrite and ammonium nitrogen, in sections normal to the front. Observations at drogue stations were made to establish the extent of diurnal variations in these properties but these appeared to be small relative to other variations. As the front developed, higher chlorophyll a concentrations appeared in the surface stratified water, in contrast to the bottom stratified water and mixed water, with highest concentrations at the surface at the stratified side of the front and in subsurface patches in the vicinity of the pycnocline. As the phytoplankton populations increased nitrate became depleted in the surface stratified water but nitrite and ammonium nitrogen concentrations remained at about the same levels. Cellular ATP concentration did not appear to be a useful measure of total biomass but indicated high biological activity in the surface stratified water.