Physical and biological observations of coastal squirts under nonupwelling conditions

Abstract

Offshore‐directed plumes of inshore water, marked by a shallow mixed layer, low salinity, and low temperature during winter, were observed south of the Hokitika Canyon on the South Island west coast, New Zealand. Hydrographic and satellite observations are presented which show that plume time scales were of the order of a few days, and across‐shelf length scales were approximately 50 km. The low‐salinity plumes were found to be associated with the southern edge of an offshore‐directed jet that followed the Hokitika Canyon bathymetry. Decay of the plumes was rapid and appears to be associated with mixing of plume water into offshore surface waters. The occurrence of the plumes is nonseasonal, and they do not appear to be associated with upwelling. Observations suggest that strong southward flow around the head of the Hokitika Canyon, perhaps as a consequence of coastal‐trapped wave activity, is a possible mechanism for plume formation. Biologically, the plumes were important because they stabilized the water column and prevented the mixing of phytoplankton below the photic zone. Although the plumes were found to transport productive nearshore waters offshore, the major impact of the plumes was to lower total water column chlorophyll a in outer shelf regions. The relationship between the Hokitika plumes and the filaments and squirts identified off the Californian coast is discussed, and it is concluded that the plume dynamics resemble those of the squirts found inshore of the seasonal filaments off California. The results of this study indicate that squirts not only influence biological variability in shelf waters but are important mechanisms for the across‐shelf exchange of coastal and oceanic waters.