The oceanographic structure of the eastern Scotia Sea—III. Distributions of euphausiid species and their developmental stages in 1981 in relation to hydrography

Abstract

Euphausia superba (krill), E. frigida, and Thysanoessa macrura are the dominant euphausiids inhabiting the region (33 to 50°W) where water from the Weddell Sea mixes with the Antartic Circumpolar Current flowing out of the Drake Passage into the Scotia Sea. Five north-south transects during January to February 1981 showed that the earliest larval stages (calyptopes) of E. superba in the 0 to 200 m layer were throughout the mixing region, but most of the high densities were associated with subsurface thermal domes. This suggests that these were places where ascent of larvae from greater (hatching) depths was associated with mixing processes. Vertical transport may have aided larvae, resulting in high local survivorship. High chlorophyll a concentrations tended to be near, but not coincident with, high abundances of youngest E. superba larvae. Adult and sub-adult krill were widely dispersed. Swarms were found only near islands. A month later, in mid-March, dense concentrations of later larvae (furciliae), presumed to have developed from January to February calytopes, were still/again near the persistent front northeast of the South Orkneys (59 to 60°S). Regional groupings of larvae, previously distinguished by differing stage-frequency structures, occur in relation to segments of fronts in March. E. frigida is, numerically, a minor species. Calytopes were also concentrated near the front during January to February. The younger the stage, the farther south was its limit of distribution, indicating either southward progression in recruitment, or northeasterly dispersal of developing stages. T. macrura was the most broadly distributed species; high abundances of calyptopes 2 and 3—but not other larval stages—were associated with the frontal zone. Recruitment in T. macrura seems to have been more continuous, of longer duration, and more widespread than in the other two euphausiids. Relative abundances of E. superba larvae, E. superba adults, and T. macrura (larvae and adults) were examined along several March series of closely spaced samples. Highest numbers of E. superba adults usually co-occured with low numbers both of its larvae and of T. macrura. Maxima of E. superba larvae were associated with few or no adult E. superba. T. macrura were in nearly uniform proportion to E. superba larvae, along a transect of chlorophyll-depleted open ocean. Nearshore, abundances and relative proportions of these two species were less consistent, evidently reflecting variability in the coastal environment.