Cross-frontal distributions of zooplankton on the Northeast Peak of Georges Bank, March 1999: GLOBEC Phase III preliminary results

K. Wishner, D. Outram, C. Gelfman, and S. Hurley

Previous GLOBEC studies have shown that the Northeast Peak of Georges Bank is an important location of Calanus inflow to the Bank during the early spring. GLOBEC Phase III programs at this site during spring 1999 examined cross-frontal processes affecting Calanus transport, with special emphasis on the influence of the tidal cycle and tidal front.

Work reported here was conducted on the RV Endeavor in March - April 1999. Three stations were selected along a north - south transect from about 80 - 200 m and crossing the tidal front on northeast Georges Bank. The Front station was located near the center of the frontal zone, while the On-Bank and Off-Bank stations were located near the maximum extent of the frontal oscillation, such that frontal characteristics were present at these stations during only part of the tidal cycle. Two sampling strategies were employed. During the Eulerian mode, CTD rosette casts (Gifford and Sieracki) and MOCNESS tows were done every 4 hrs at a single station. During the Lagrangian mode, rapid transects of the 3 stations (CTD rosette cast and MOCNESS tow at each station in quick succession) were done for 4 combinations of the tidal and day / night cycles. These combinations were the Maximum On-Bank Slack tide (day and night) and the Maximum Off-Bank Slack tide (day and night). The former was the time of maximum water and plankton penetration on-bank, at the end of maximum on-bank (southwest) flow. The latter was the time of maximum water and plankton penetration off-bank, at the end of maximum off-bank (northeast) flow. Predictions of tides were generated from Flagg's model by Ullman and Belkin and used to target sampling times. This project was closely coordinated with those of Gifford / Sieracki / Belkin (microzooplankton, feeding, and hydrography) and Hebert / Barth (SeaSoar and float tracking on the R/V Oceanus).

Preliminary results showed that the hydrographic structure along the transect varied with the tidal phase. During Maximum On-Bank Slack tide, a thin low salinity surface feature extended south onto the Bank from the Off-Bank to the Front station. During the Maximum Off-Bank Slack tide, well-mixed surface water extended all the way from the On-Bank to the Off-Bank station. Calanus was usually associated with the near-surface low salinity feature and interface. It was present at all 3 stations with peak abundances off-bank during these 2 transects. Abundances and vertical distributions changed little between tidal phases. Metridia was a deep-living off-bank copepod. However, during the Maximum On-Bank Slack tide, it also occurred at the Front and On-Bank stations. It also migrated to the surface at night and was carried further on-bank at night than during the day in the fast surface tidal currents.