68 



ATKINSON 



water because the weather was rainy during the collection 

 period. Relatively high salinity ocean water cascades over 

 the windward reefs and flows in through the Deep 

 Entrance. 



Northeast trade winds blow less saline surface water 

 downwind with a buildup in the northwest region of the 

 lagoon. Because water is trapped in the leeward side of 

 the lagoon, return flow develops in the deeper water. The 

 low salinity return flow is mixed with surface water, creat- 

 ing a relatively vertically well-mixed water column with low 

 salinity downwind and high salinity upwind. During long 

 dry periods, opposite salinity gradients might be expected, 

 with high salinities downwind and relatively low salinities 

 upwind. Only a small portion of downwind surface water 

 escapes out of the Southwest Passage. The excess water 

 must move south toward the South Channel; consequently 

 isohalines bend toward the south (Fig. 13). There is no evi- 

 dence of a discrete water mass sinking on the downwind 

 side of the lagoon, flowing upwind as deep water, and 

 upwelling on the leeward side of the lagoon (as reported 

 by Ford, 1949 at Bikini). The water column appears 

 vertically well mixed (Table 1 and Fig. 13). There is also 

 no suggestion that water can maintain vertical structure for 

 5 to 10 days at Encwetak. As ocean water pours over the 

 windward reefs and into the lagoon, it mixes ver- 

 tically and horizontally as it moves downwind. Conse- 

 quently the salinity gradient is low to high, west to east, 

 regardless of depth. The water on the windward side of 

 the lagoon is predominately ocean water, but water on the 

 western side of the lagoon reflects net processes in the 

 lagoon. Scuba divers can observe strong mixing on the 

 upper vertical wall of the West Spit. Lower salinity lagoon 

 water mixes with high salinity ocean water in this region. 

 Phosphate and nitrate are lower in the western lagoon 

 water than in eastern water. Because water on the eastern 

 side in general reflects net lagoon processes, low nutrients 

 in that water suggest net uptake of these nutrients into the 

 ecosystem. Net organic production of benthic ecosystems 

 has been estimated by net uptake of nutrients (Smith and 

 Jokiel, 1976; Atkinson, 1981; Smith and Atkinson, 1983). 

 The observed decrease in these nutrients indicates a rea- 

 sonably low, net organic production for the atoll. 



Ford suggested oceanic eddies might move through the 

 broad open channel at Bikini, the Enyu Channel. Perhaps 

 this process might occur in open lagoons; however, it docs 

 not appear to occur at Enewetak. Large eddies would be 

 destroyed when flowing into the lagoon by strong tidal 

 currents in the channels. Although a large eddy could not 

 be maintained, large oceanic eddies moving by the atoll 

 could influence the chemical and biological composition of 

 inflowing water. 



Tide 



Tidal currents directly influence the flow of water 

 within several kilometers of the passes, especially in the 

 southern part of the lagoon. These tidal currents can 

 overwhelm the wind-driven circulation, leading to such 



local effects as the "left-hand" spiral observed two kilome- 

 ters north of Enewetak Island (Fig. 6c). 



RESIDENCE TIMES 



In the most elementary analysis, the average residence 

 time of water in the lagoon can be estimated by dividing 

 the lagoon volume by the net rate of water input. The cal- 

 culation yields a residence time of 33 days. Clearly there 

 is a variation of actual residence time from one part of the 

 lagoon to another because: (1) the water is introduced all 

 along the windward reef, but exists primarily through the 

 South Channel; and (2) there is no major north-south 

 recirculation mixing northern waters with southern water. 

 Thus, the residence time for water entering the north end 

 of the lagoon will be relatively long; water entering across 

 the southern reef will have a short residence time. 



Because the water entering the northern lagoon must 

 transit the entire lagoon before exiting and because it 

 undergoes mixing by the superimposed wind-driven circula- 

 tion during that transit, a very simple estimate of the 

 residence time for that part of the inflow will have at least 

 qualitative validity. If it is estimated that the northern part 

 of the lagoon receives one-quarter of the total inflow, then 

 the residence time for this water (under the same very sim- 

 ple assumptions) will be four times longer than that for the 

 lagoon water as a whole, or 132 days. 



Water entering the system in the north is of particular 

 interest because it flows across the areas with high 

 bottom-sediment concentrations of transuranic radionu- 

 clides (Nelson and Noshkin, 1972). Figure 14 is a general- 

 ized plot of sediment radionuclide activity; it indicates that 

 if release into the water column is proportional to the con- 

 centration in the sediment then most of the radionuclides 



E 

 >- 

 > 



< 



o 



20 



DISTANCE (km) 



40 



Fig. 14 Decrease of sediment radionuclide activity as a 

 function of distance from the north end of the lagoon. 

 Radionuclides include "Sr, ^^u, '^Cs. "Co. 



