Project Summary

Richard G. Fairbanks and Robert W. Houghton


Modes of circulation and mixing on Georges Bank: Quantifying Source Water Using the Oxygen isotope Tracer

Circulation over Georges Bank can be described by two modes: (i) the canonical mode is an anticyclonic gyre with source waters originating in the coastal Gulf of Maine and, (ii) the "shunt mode" which is characterized by direct injection of Scotian Shelf Water across Northeast Channel onto the south flank of Georges Bank. The primary objective of this study is to make a quantitative assessment of the amount of the biological variance explained by these two circulation modes.

This project is to continue use of the oxygen isotope tracer H218O which has demonstrated the unique ability to distinguish local and upstream freshwater sources. It has revealed significant interannual variations in the percent of freshwater on the Bank derived from local river sources. Although the details of the mixing processes responsible for exchanging water onto the Bank are not yet clearly understood, these results demonstrate the existence of distinct modes of circulation and mixing. Assessing the sensitivity of these modes to variations in external forcing and the consequences for the flux of particle and dissolved constituents onto Georges Bank is our primary objective. Water samples from GLOBEC broad-scale surveys and contemporaneous Canadian surveys upstream will be analyzed creating time series exceeding 6 years in some places.

The significance of this project is to bring a quantitative assessment of the influence of local and far-field water sources on the Georges Bank environment. The water molecule isotope composition provides a tracer linking modern climate processes (e.g., runoff, sea ice, ice sheet, and marine precipitation) and Georges Bank source waters. The water molecule isotope tracer is among the best monitors of the impact of changing climate on Georges Bank mass origins and their variability. The proposed research will contribute directly to modeling studies to understand the physical and biological processes controlling the early life stages and recruitment of fish and zooplankton stock on Georges Bank as well as provide a baseline for monitoring the influences of any future climate change.