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 Calanus finmarchicus were limited by the lack of food on the southern flank of Georges 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 of 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 front where chlorophyll concentrations were very low; and at several stations along an initial survey grid in the same region. At the drifter site, measurements of size, condition, molting rate, and egg production rate declined over time, and growth rates were enhanced when ambient water was enriched with mixed phytoplankton cultures. RNA/DNA ratios, molting rates, and growth rates were much lower than laboratory measurements at high food at the same temperatures, and egg production rates were lower than predicted under non-limiting conditions. The effects of food limitation were more severe in naupliar and early copepodite stages. In comparison, in the higher chlorophyll region, younger copepodites were larger and in better condition, and RNA/DNA ratios and egg production rates were higher and near maximum predicted values. We evaluate these results with respect to the question of food limitation on the bank and explore the implications for understanding variability in C. finmarchicus abundance and its potential role in the trophodynamics of fish larvae.