60 



ATKINSON 



(a) 



(b) 



Fig. 3 Phosphate (a) and nitrate (b) contours for lagoon 

 water. Concentrations in mmole m^'. 



windward cross-reef currents is shown in Fig. 2 by horizon- 

 tal lines along the eastern boundary of the atoll. These 

 currents are a result of breaking waves on the windward 

 reef; they vary in response to surf height (and therefore 

 regional wind patterns) and tide height. From Enjebi to 



Enewetak Island (Fig. 2), water crosses the reef from the 

 ocean to the lagoon in a direction approximately perpen- 

 dicular to the reef front. The windward cross-reef currents 

 do not reverse direction, flowing from lagoon to ocean. 

 The current speed ranges from 10 to 150 cm s ^'. These 

 currents range in volume transport from 0.05 m s ' per 

 meter of reef front during low tide and low surf to about 

 1.5 m'^ s ' during high tide and high surf. A mean volume 

 transport value of 0.56 m s~ m~ was calculated; this is 

 equivalent to 6.6 X 10 m per tidal cycle across the 

 windward reef (a tidal cycle is used to facilitate comparison 

 with other volume transports). The volume transport of the 

 entire windward cross-reef current probably varies by a fac- 

 tor of 2 to 3. Winter tropical storms drive water over the 

 reef in massive amounts, building and eroding atoll islands. 



The area of leeward cross-reef currents is represented 

 by vertical lines in Fig. 2. These currents do not flow in 

 any well-developed pattern. Transport along the leeward 

 reef, rather than across it, is common. During a period of 

 high surf from a north swell, S. V. Smith and 

 E. D. Stroup (January 1976, unpublished data) measured 

 inward flowing currents along the northwest leeward reef. 

 Current speeds and volume transports at 10 different loca- 

 tions ranged from about 15 to 50 cm s~ and from 0.15 

 to 0.57 m'^ s^'m"\ respectively. Significant inflowing and 

 outflowing currents were measured in the region north of 

 Kildrenen Island and south of the Southwest Passage 

 (Fig. 2). Noshkin et al. (1974), using surface concentra- 

 tions of 2^*Pu, 239pu, ^'*°Pu, and ^^^Cs, have also shown 

 that, during winter high tide periods, significant amounts of 

 oceanic water enter the lagoon across the northwest and 

 southwest leeward reef. 



Dye releases on the leeward reef flat demonstrate a 

 slow drift either oceanward or lagoonward over the lee- 

 ward reef margin A maximum value for oceanward flow 

 might be the speed of the net oceanic drift of the lagoon 

 surface current. A characteristic current speed over the 

 entire leeward reef might be about 50% of the lagoon sur- 

 face current speed. Much of the reef margin bares at low 

 tide, so an estimate of the average depth of the reef is 

 near 50% of the mean tidal range, or 0.42 m. The net 

 transport of water out of the atoll over the leeward reef 

 margin is estimated to be 0.4 X 10^ m'' per tidal cycle 

 (i.e., only 6% of the windward reef input). 



Channel Currents 



The Deep Entrance current (Fig. 2) reverses approxi- 

 mately every 6.2 hours, with the tide. The current speed 

 ranges from to 80 cm s~\ increasing from zero to a 

 maximum in about 3 hours and decreasing to slack water 

 in another 3 hours. The period of slack water in the chan- 

 nel is no more than a few minutes. The direction and 

 speed of the current are nearly constant throughout the 

 water column. The volume transport of the Deep Entrance 

 current varies between neap and spring tides. On May 31, 

 1979, near maximum spring tide, the current transported 

 3.0 X 10* m'^ per 6.2 hours over an entire tidal cycle. 



