PLANKTONIC PRODUCTIVITY AND LOCAL OCEANOGRAPHIC VARIABILITY IN GORGONA ISLAND, EASTERN TROPICAL PACIFIC OF COLOMBIA

Abstract

Gorgona Island is the vastest insular area in the Colombian Pacific Ocean. This island, located 30 Km offshore, has a high biological and ecosystem diversity; Gorgona´s coral reef is considered the best developed in the eastern tropical Pacific, being declared Natural National Park (PNN) in 1980. Despite its relevance in Colombian conservation policy, a reduced research effort has been focused to describe and to evaluate the local variability in the biological, physical and chemical condition of the pelagic environment. In order to define the oceanographic conditions of the PNN Gorgona during the warm season, the local variability of plankton productivity (Chlorophyll-a and zooplankton biomass) and physcial (temperature, transparency, trubidity) and chemical (salinity, dissolved oxygen, nitrate) conditions of pelagic system were evaluated at 24 sampling stations during July 2006. The mean surface temperature and salinity were 27.81 ± 0.08 °C and 31.75 ± 0.01 respectively. A strong vertical stratification was detected, with a permanent thermocline between 45-55 m depth. Water column transparency was 10.37 ± 0.33 m, and turbidity was highly variable (14.13 ± 2.60 NTU). Surface dissolved oxygen was 4.29 ± 0.03 mL L-1. Surface Chlorophyll-a was 0.15 ± 0.015 μg Clo-a L-1, however the highest records were at 30 m depth (0.27 ± 0.03 μg Clo-a L-). Zooplankton biomass was highly variable in the study zone (16.59-311.53 mg 100 m-3). Phytoplankton was dominated by diatomeas (91.17 %) of the genera Rhizosolenia (22.4 %) and Odontella (21.6 %). Futhermore, 31 taxonomic groups of zooplankton were identified, with dominance of copepods (56.8 %) and apendicularians (16.9 %).