Evidence of an unproductive coastal front in Baía d'Abra, an embayment on the south east of Madeira Island, Portugal

Five oceanographic cruises were carried out in Baía D'Abra, Madeira Island (27 July–22 September 1994) in collaboration with the Madeira Fisheries Directorate. The aim of the study was to investigate how the distribution of various plankton communities was determined by physical oceanographic variables (e.g., temperature, salinity, surface currents). Five sampling stations (A, B, C, D and E) were chosen on a linear transect extending 9 km offshore. Depths varied from 20 m at the inshore station (A) to 500 m at the offshore station (E). A Conductivity-Temperature-Depth (CTD - SBE seabird electronics) device was deployed at each station. A 335 μm net was used to collect plankton samples. The samples were analyzed at the Madeira Fisheries Laboratory and at the Plymouth Marine Laboratory (PML) facilities (UK). Results revealed retention of fish eggs in Baía D'Abra probably due to the formation of an anticyclonic eddy. The eddy formed a thermo-haline front with the offshore waters around station B where a surface convergence seemed to decreased mesozooplankton productivity. The overall pattern of biological productivity was low at the coastal front (B) and high at its margins (C or D). Six to 9 km offshore oceanic waters were stratified. Different plankton communities were characteristic of the inshore and offshore waters. Fish eggs were commonly found within the inshore stations suggesting that either the bay served as an 'ecological safe' place or that the eddy was responsible for such aggregations. Furthermore, biodiversity seemed to increase with distance offshore. Typical coastal taxa included fish eggs and Noctiluca sp. whereas siphonophora and chaetognaths constituted the main oceanic groups.