Environmental factors influencing diurnal distribution of zooplankton and ichthyoplankton

Abstract

The diurnal vertical distribution of a large number of species of zooplankton, ichthyo- plankton and micronekton were determined in the top 150 m in three locations in the Shelf Water, on the Nova Scotia Shelf, and Slope and on Georges Bank during spring and fall periods. Species were categorized as to their trophic level and their type of diurnal migration behaviour. The influence of temperature, salinity, and water density on the diurnal vertical distribution of the species was exam- ined. Temperature was found to have the greatest influence on the distribution of the largest number of species. Diurnal migration behavior of the same species in Shelf and Slope water and at different times of the year was examined. Results showed that species changed their behavior in the two water masses, while some species changed their migration behavior at different times of the year. During the night in April the most abundant copepod species, Calanus finmarchicus, making up about 80ft of the biomass, was found concentrated above the thermocline and the main chlorophyll layer. The majority of the less abundant species of copepods were found below the thermocline and the chloro- phyll layer. At night in August the two most abundant copepod species, Centropages typicus and Paracalanus parvus, making up at least 80ft of the zooplankton biomass, were also concentrated above the thennocline and the main chlorophyll layer. Three species of copepods were concentrated at the depth of the main chlorophyll layer and two species were concentrated below the chlorophyll layer and thermocline. The vertical distribution of other zooplankton and ichthyoplankton species was examined in relation to the thennocline and chlorophyll layer. Relationships between concentrations of six species of fish larvae and all species of copepods in the same samples showed a general increase in the numbers of larvae m1 as the numbers of copepods m1 increased in a range of 500- 4000 m1. However, the concentration of Merluccius bilinearis decreased as the concentration of copepods exceeded 4000 m • suggesting that high concentrations of copepods may not be a favour- able environment for the larvae.