PHYSICAL AND BIOLOGICAL OBSERVATIONS OF COASTAL SQUIRTS UNDER NONUPWELLING CONDITIONS

Offshore-directed plumes of inshore water, marked by a shallow mixed l ayer, low salinity, and low temperature during winter, were observed s outh of the Hokitika Canyon on the South Island west coast, New Zealan d. Hydrographic and satellite observations are presented which show th at 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 Can yon, perhaps as a consequence of coastal-trapped wave activity, is a p ossible mechanism for plume formation. Biologically, the plumes were i mportant because they stabilized the water column and prevented the mi xing of phytoplankton below the photic zone. Although the plumes were found to transport productive nearshore waters offshore, the major imp act of the plumes was to lower total water column chlorophyll a in out er shelf regions. The relationship between the Hokitika plumes and the filaments and squirts identified off the Californian coast is discuss ed, and it is concluded that the plume dynamics resemble those of the squirts found inshore of the seasonal filaments off California. The re sults of this study indicate that squirts not only influence biologica l variability in shelf waters but are important mechanisms for the acr oss-shelf exchange of coastal and oceanic waters.