Spatial and temporal distributions of copepods to leeward and windward of Oahu, Hawaiian Archipelago

The spatial and temporal distributions of two island-associated copepod spe cies, Undinula vulgaris Dana and Labidocera madurae Scott, were compared to the distributions of two open ocean species, Cosmocalanus darwinii Lubbock and Scolecithrix danae Lubbock, along 28-km windward and leeward transects off the island of Oahu, Hawaii. Samples were taken in September and Decemb er 1985 and April and June 1986. A warm, low salinity pool on the leeward s ide was a prominent feature during all transects except December. The abund ances of the two oceanic species did not change significantly between leewa rd and windward stations, with distance from shore, or between September 19 85 and April 1986 samples. As expected, very high abundances of U. vulgaris occurred at some nearshore stations, up to 3 g dry wt m(-2) for adults alo ne. Calculations of respiratory loss at these densities (0.7 g C m(-2) d(-1 )) suggest a high local productivity would be required to meet these demand s. L. madurae, a surface-dwelling species normally restricted to within 1 k m of shore, was an effective indicator species of nearshore water movement. It was more common in offshore samples on the leeward transects, rarely be ing found offshore on the windward side, consistent with prevailing current s and the presence of the leeward warm, low salinity pool. The occurrence o f a strong mixing event in April 1986 resulted in L. madurae being distribu ted throughout the upper 100 m of the water column. The presence of oceanic species close to shore on the windward side also coincided with this wind- driven event. The primary environmental influence on vertical distributions was daytime cloud cover, with U. vulgaris tending to be found shallower on cloudy days. Of the two oceanic species, S. danae exhibited the most prono unced vertical migration,however, vertical distributions were not significa ntly correlated with environmental factors for either species. The abundant nearshore U. vulgaris population cannot be explained by differences in ver tical distribution between it and the two oceanic species that might allow a physical mechanism to concentrate U. vulgaris. A high population growth r ate is likely necessary to explain U. vulgaris' dominance.