Spatial variability of phytoplankton in relation to the distributional patterns of Krill (Euphausia superba)

Abstract

The role of krill (Euphausia superba) in influencing the spatial variability of phytoplankton is examined. Data were collected during a cruise of the S.A. Agulhas (10 February - 20 March l981) in the area between 15(DEGREES)- 30(DEGREES) east and 60(DEGREES)- 70(DEGREES) south. Krill density and swarm characteristics (number, size, and biomass) were estimated acoustically. Data on phytoplankton biomass, productivity and species composition, together with data on the physico-chemical environment were obtained from discrete water samples collected from either 7 or 8 depths between 0-100 m at a total of 37 stations. Correlation, multiple regression, and canonical analyses are applied to the station data to examine interrelationships between the variables. Phytoplankton biomass is inversely correlated with mean krill density but positively correlated with the biomass of individual swarms. A positive correlation between krill density and the relative amounts of phaeo-pigments in the > 20- m size fraction of phytoplankton suggests that krill selectively feed on larger cells. The application of spectral analysis to continuous horizontal profiles of integrated krill density, surface seawater temperature, and in vivo fluorescence reveals that, over the range of 4-20 km, the variability in phytoplankton biomass is largely determined by physical processes. However, the steepness of the fluorescence variance spectrum relative to the temperature spectrum, together with consistent coherence between the phytoplankton and krill profiles suggest that predator-prey interactions are of importance in determining the distributional patterns of the phytoplankton.