Influence of the biological and physical environment on the vertical distribution of mesozooplankton and micronekton in the eastern tropical Pacific

The day/night vertical distributions of mesozooplankton and micronekton biomass and that of a large number of copepod species to a depth of 1 000 m are described and contrasted in detail from two eastern tropical Pacific stations, the DOME station, in a region of continuous upwelling and the BIOSTAT station, in a non-upwelling area. The effects of various biological parameters such as the zones of primary production and chlorophyll maxima plus physical parameters, such as temperature, salinity and oxygen concentrations, on the distributions of the species and mesozooplankton and micronekton biomass are examined. The thermocline depth appeared to have the greatest influence of all the physical factors on the vertical distribution of the copepod species. The vertical distribution of many species was truncated in the region of the oxygen minimum layer with very few species found below this layer. The vertical distribution of copepod molts in the top 1 000 m showed that the molts probably originated at the depths at which they were found and that the molts from the euphotic zone did not reach the deep water. Copepod species association at different depths showed that few species had common depth distributions during both day and night. The highest degree of association occurred at night in the region of the thermocline. At depths below the thermocline, the degree of similarity in vertical distributions was greatly reduced. The biomass of mesozooplankton at these two stations is compared to other regions of the tropical oceans described in the literature and shown to be among the highest values recorded. The oxygen minimum layer and the thermocline had the greatest impact on species vertical distribution. The biomass of mesoplankton was about twice as high and the number of copepods between three and four times as high at the DOME site as compared to the BIOSTAT site, and evidence suggested that this was influenced by the higher primary production in this region. The difference in micronekton biomass between sites was less, with DOME having about 1.2 to 2.2 the biomass. This suggests that the influence of the higher level of primary productivity at the DOME was evidence of the higher trophic levels.