Depth distribution of Calanus finmarchicus nanplii on the Georges Bank during 1995 and 1996

Calanus finmarchicus nauplii were sampled on the Georges Bank and in adjacent waters as part of the US GLOBEC Georges Bank program. Depth-stratified samples Mere collected by plankton pump between January and June of each year. beginning in 1995, and data are presented here on depth distribution of nauplii in 1995 and 1996. We use the index Weighted Mean Depth/Sampling Depth (WMD/D) to normalize for variations in sample depth. No day/night differences in weighted mean depth (WMD/D) were detected for any of the stages. There Mere differences in WMD/D between stages, however, with non-feeding nauplii, N1 and N2. not significantly different from each other, but significantly deeper than N3-6. In turn. the WMD/Ds of these older nauplii were not significantly different from each other. There Mas a significant negative relation between degree of stratification as measured by the difference in sigma-t between the surface and 30 m, and the WMD/D of the nauplii. There was also a significant positive relationship between the WMD/D of the older nauplii and the depth-distribution of chlorophyll tr. It is concluded that the depth distribution of NI-NZ is determined by their passive sinking rate and the degree of vertical mixing. Older nauplii have a strong preference for residing in the surface layer, but this may be overcome by strong vertical mixing. (C) 2000 International Council fur thr Exploration of the Sea.