Day and night ichthyoplankton assemblages and zooplankton biomass size spectrum in a deep ocean island wake

Zooplankton size and larval fish assemblages were compared between locations in the incident free stream and the island wake of Cato Reef, in the south Coral Sea over 6 d in February 1993. Weak incident northward flow of 0.3 m s(-1) generated a wake on the lee side of the reef. Total abundance or larval fish diversity did not differ between the lee side and the wake. Myctophids and gonostomatids dominated the catch (85 %), while reef-associated taxa represented < 2 % of the total. There were significantly fewer reef fish larvae in the wake (similar to 25 %) than in the free stream, suggesting that reef-related settlement and/or predation may explain the difference. Most of the ichthyo-plankton (58 families) were associated with the thermocline (ca. 50 m depth) during day and night. We found significantly steeper slopes of the normalised biomass size spectrum (NBSS, derived from an optical plankton counter) in the wake compared to in the free stream and near the thermocline. The slope was significantly correlated with the shape parameter of the Pareto distribution (r = 0.90), with local light attenuation and with smaller (318 to 883 pm equivalent spherical diameter) zooplankton biomass, consistent with the NBSS slope being a proxy for secondary production. The slope did not vary significantly between day and night despite a 30 % daytime reduction in total zooplankton biomass. Steeper slopes were significantly correlated with a greater abundance of larval myctophids and gonostomatids, possibly by fish seeking productive areas and by top-down removal of larger zooplankton.