SHENKER; OCEANOGRAPHIC ASSOCIATION OF NEUSTONIC MEROPLANKTON 



the Manta net in the daytime were lower than at 

 night for each station (Fig. 7). The differences 

 between day and night were particularly appar- 

 ent on bright sunny days and at the offshore sta- 

 tions. Only one of 13 samples of megalopae col- 

 lected when light intensity exceeded 4,000 lux 

 had a density over 200/1,000 m^, while 12 of 24 

 day samples collected at lower light levels ex- 

 ceeded this density. Highest daytime densities 

 (1,600-2,400/1,000 m^ in neuston trawl samples) 

 were observed on 19 May when a sudden shift in 

 weather before daybreak resulted in very dark, 

 overcast and rainy skies, with 40-45 km/hour 

 winds and 2-3 m breaking waves. 



Water clarity may also affect the daytime 

 abundance estimates of megalopae in the neuston 

 along the transect. Daytime densities were al- 

 ways low seaward of 30 km where Secchi depths 

 exceeded 9 m, while higher densities were ob- 

 served in the more turbid near-coastal waters 

 with Secchi depths of 4-9 m. 



The 27-h occupancy of the 50 km station on 8-9 

 June provided additional evidence of a distinct 

 diel pattern in utilization of the neustonic habitat 

 by megalopae (Fig. 8). Abundance of megalopae 

 peaked during the twilight periods of dawn and 

 dusk. A significant decrease in abundance was 

 noted in the middle of the night, while midday 

 samples did not collect any megalopae. 



In addition to the wide distribution of megalo- 

 pae along the transect, sampling occasionally de- 

 tected the aggregation of very large numbers of 

 megalopae (denoted by letters over station blocks 



n 



1,000 



o 

 o 

 o 



0) 

 D 



a. 

 o 



o 



0) 



0) 



E 



100 -- 



10 



o o Manta Net 



• • Neuston // 



Trawl // 



I 

 // 



1 -: . 



I ' ' f^m^^^^^m=^ I i I . ii I T-rr '•' i 



18:00 24:00 06:00 12:00 18:00 



Time of Day 



Figure 8. — Density of megalopae collected by h)oth nets during 

 a 27 h sampling p>eriod 50 km ofTshore on 8-9 June. 



in Figure 7). These aggregations were typically 

 associated with hydrodynamic or biological struc- 

 tures in the surface layer. 



Event A: Simultaneous tows with the Manta 

 net and neuston trawl caught megalopae in densi- 

 ties of 373 and 325/1,000 m^. The neuston trawl 

 was then redeployed and towed along a foam line 

 marking a surface convergence. An estimated 

 60,000 megalopae were collected from the conver- 

 gence zone (24,000/1,000 m^). 



Event B: A series of five consecutive tows 

 with the trawl net was made. Each tow covered a 

 linear distance of approximately 1 km in a west- 

 ward direction, with an estimated distance of 300 

 m between tows. Megalopae were very sparse in 

 the simultaneous Manta and trawl tows at the 

 beginning of the series, with densities of and 

 1.9/1,000 m"^, respectively. The subsequent neu- 

 ston trawl tows produced density estimates of 79, 

 3,555, 2,414 and 187/1,000 m^. Apparently, the 

 megalopae were aggregated over an area extend- 

 ing at least 5 km in the east-west direction. 



Event C: Following a period of southwest 

 winds, a large raft of Velella velella was found 

 around the inshore station. Velella velella were 

 generally found at offshore stations but at densi- 

 ties far lower than the 36 kg/1,000 m^ level taken 

 here. Although the overall extent of the raft could 

 not be determined at night, the density of megalo- 

 pae was estimated by both nets at 21,500/1,000 



Event D: The density of megalopae sampled 

 by the Manta net was 790/1,000 m^, while a 

 simultaneous neuston trawl tow along a conver- 

 gence zone only 30 m away collected 150,000 

 megalopae (46,400/1,000 m^). 



Census estimates of megalopal abundance were 

 computed from nighttime samples for each date 

 (Fig. 9). A modification of Smith's regional census 

 estimate (1972) was used to compute the total 

 number of megalopae occurring in a 1 m 

 wide X 1 m deep track along the surface from the 

 coast to 55 km offshore: 



S (^A) 



j=i 



where C^ = estimate of abundance of megalopae 

 per meter of coastline during cruise 

 k 

 Aj = volume of water in the track sur- 

 rounding station i, computed from 



311 



