22 3.5 Environmental Aspects of Bikini Atol] 3.5-1 Geography and Hydrology S*kini Atol] is Tocated in the Central Pacific Ocean rear TISN latitude, 165°E longitude. The coralline atoll rises 4600 meters off the sea floor in the Northern Marshall Islands. The 629-square kilometer lagoon area averages 47.5 meters in depth and is enclosed on all sides by an exposed reef. An 18.3 meter lagoon terrace divides the deeper lagoon from tne reef perimeter. The hydrology of Bikini Lagoon has been the subject of papers by Von Arx (1948, 1954), Munk and Sargent (1954), Munk, Ewing and Revelle (1949), Johnson (1949) and Ford (1949). The model of the winter lagoon circulation shcvn in Figure 3 resembles a typical wind-driven, two-layer system of inland labes. The circulation is driven by the action of wind, waves, tides and the Ncrth Equatorial Current (Von Arx, 1954). Year-round entrance of seawater to the lagoon occurs through passes on the southern perimeter of the reef and is surplenented during the winter by the tradewinds which force water over the inter-island reefs during high tide. The surface waters that do not exhaust Cyer southern and western passes or reefs sink and form a slower moving return fic, wnich bifurcates at the eastern portion of the lagoon, forming two deep meter spirals. Von Arx (1954) placed the limits of the upwelling zone in the fertern lagoon at about 2500 meters from the midpoint of the east reef and as art £4°7-3399 meters wide. eee . He also found that the direction and speed of te surface and deep-water components is nearly constant at 3% of the - Ye-hour average wind speed. e7Br loos This results in an average 9-13 meter wer Surface layer, with average speeds of 15 and 25 cm/s in the summer LOtl tae! ; : . wvssran, and winter (tradewind) seasons, respectively (Von Arx, 1954). seh wee The me current speed is generally thought to be approximately one-third