Entrainment and advection in an islands tidal wake, as revealed by light attenuance, zooplankton, and ichthyoplankton
Limnol. Oceanogr., 49(1), 2004, 283-296 | DOI: 10.4319/lo.2004.49.1.0283
ABSTRACT: Spatial and temporal patterns of light attenuance, zooplankton abundance, and larval fish assemblages observed at night in the flood tide wake of a 2-km-wide, steep-sided island within the Great Barrier Reef lagoon (40-45-m local depth) are compared with two simple models. Eddy upwelling is shown to be slow relative to erosion, vertical entrainment, and advection (EA) arising near the flanks of the island, where currents were accelerated to 1-2 m s-1, approximately twice that in the free stream. Turbidity (particles ,300- mm equivalent spherical diameter [ESD], inferred from increased water column light attenuance of an optical plankton counter) and medium-sized zooplankton (700-1,000- mm ESD) appeared to be entrained toward the surface, to form a V-shaped plume. The plume originated near the islands flanks and converged 4 km downstream. Here, light attenuance returned to the free stream conditions, presumably as sediments settled, leaving a patch of medium-sized zooplankton that had a three- to fourfold greater biomass concentration than the free stream. A decrease in the concentration of small zooplankton (300-500- mm ESD, generally found in surface waters) is also consistent with vertical mixing by EA. Neuston net collections across the wake revealed two larval fish assemblages that were correlated with either the small surface zooplankton or with the deeper, medium-sized zooplankton, which included epibenthic taxa. If EA is a common process for patch formation in tidal waters, then the geometry of the associated plumes may predict larval settlement (recruitment "hotspots") in shallow tidal waters.