Hydrographic measurements from two cruises that took place on the western Antarctic Peninsula continental shelf from April to June and July to September of 2001, provide a description of changes in water mass distributions and circulation patterns that occurred in the Marguerite Bay region as a result of seasonal variability and offshore forcing by the southern boundary of the Antarctic Circumpolar Current (ACC). The primary seasonal changes in water mass properties is the reduction in Antarctic Surface Water and replacement by a thick Winter Water layer. The primary effect of the ACC is to pump Circumpolar Deep Water (CDW) onto the continental shelf below 200 m at specific sites that correspond to bathymetric features, such as the Marguerite Trough. The pulse of CDW that was observed in austral fall had entered Marguerite Bay by the austral winter and a second CDW intrusion was beginning at the shelf edge. These observations suggest that onshelf intrusions of CDW may be a frequent occurrence in this region, which has implications for heat and salt budgets. The hydrographic distributions also show a narrow southwestern flowing coastal current that enters Marguerite Bay around Adelaide Island and exits around Alexander Island. This current was well developed in austral fall but appeared to be absent in the austral winter. This current may be the result of seasonal buoyancy forcing .