Abstract

Two short biophysical surveys were carried out in order to assess how the decapod crustacean larvae vertical distribution and circulation patterns in an equatorward facing embayment (Gulf of Arauco, 37° S; 73° W) influenced larval transport into and out of the Gulf. The embayment is located at the upwelling area of south central Chile and features a deep (∼60 m) and a shallow (∼25 m) pathway of communication with the adjacent coastal ocean. Profiles of zooplankton, temperature, salinity, dissolved oxygen and current velocity were measured during a 22-h period at the shallow entrance to the gulf. In addition, three zooplankton profiles were carried out at the deep entrance to the gulf on the basis of observed changes in hydrographic structure. At the shallow entrance to the gulf, current velocities showed a clear semidiurnal tidal signal, with stronger ebb than flood at the surface and stronger flood at depth. Decapod larval abundances showed a bimodal distribution through time, with a maximum during night-time ebb, and a smaller, second peak at day-time ebb. Larval transport in and out of the Gulf depended on larval vertical distribution and the timing of higher densities. Export was reduced when larvae deepened during ebbing tides or when larvae remained deeper throughout the day. Export was enhanced when larger numbers of individuals occupied ebbing surface waters. At the deep entrance, two groups of crustacean larvae were found associated with two different water types: a surface high oxygen water and a deeper suboxic equatorial subsurface water. A group of larvae that originates in the shelf area appeared entering the Gulf in the deeper waters of this entrance but was absent from the shallower one. Larval vertical distribution in the two layer circulation at the embayment entrances can enhance or preclude retention within the bay. The motion of these layers was determined remotely by upwelling dynamics and modified by local wind (sea breeze) and tides. Overall, high frequency processes (semidiurnal tides, diurnal fluctuations) superimposed onto others of lower frequency (3-8 d period of wind-driven upwelling), may contribute significantly to variations in the transport of individuals from bays in upwelling areas. © The Author 2005. Published by Oxford University Press. All rights reserved.