Evidence for food limitation of Calanus finmarchicus production rates on the southern flank of Georges Bank during April 1997

We present here strong evidence that production rates of the copepod Calanu s finmarchicus were limited by the lack of food on the southern flank of Ge orges Bank during April 1997. Indices of production include size, condition , RNA/DNA ratio, and the rates of molting, growth, and egg production of C. finmarchicus. Observations were made at a site in well-mixed water north o f the tidal mixing front in a region of higher chlorophyll; a drifter site, occupied for 7 days, which was located in stratified water south of the fr ont where chlorophyll concentrations were very low; and at several stations along an initial survey grid in the same region. At the drifter site, meas urements of size, condition, molting rate, and egg production rate declined over lime, and growth rates were enhanced when ambient water was enriched with mixed phytoplankton cultures. RNA/DNA ratios, molting rates, and growt h rates were much lower than laboratory measurements at high food at the sa me temperatures, and egg production rates were lower than predicted under n on-limiting conditions. The effects of food limitation were more severe in naupliar and early copepodite stages. In comparison, in the higher chloroph yll region, younger copepodites were larger and in better condition, and RN A/DNA ratios and egg production rates were higher and near maximum predicte d values. We evaluate these results with respect to the question of food li mitation on the bank and explore the implications for understanding variabi lity in C. finmarchicus abundance and its potential role in the trophodynam ics of fish larvae. (C) 2000 Elsevier Science Ltd. All rights reserved.