Feeding and diel vertical migration cycles of <i>Metridia gerlachei</i> (Giesbrecht) in coastal waters of the Antarctic Peninsula

Abstract

Diel vertical migration and feeding cycles of adult female <i>Metridia gerlachei</i> in the upper 290 m of a 335-m water column were measured during a total of 65 h in two periods of early summer (Dec 20–21 and Dec 25–26, 1991). Samples collected in eight depth strata by 35 MOCNESS tows (333-μm mesh) were analyzed for abundance and mean individual gut pigment content. Most of the copepod population was concentrated in a 50-m depth interval at all times. Feeding began simultaneously with nocturnal ascent from a depth of 200–250 m at ≈ 18:00 h (local time), when the relative change in ambient light intensity was greatest. Ingestion rate increased exponentially (ki = 0.988 h−1) at double the gut evacuation rate (ke = 0.488 h−1) as the population moved upward at 22.3–26.5 m h−1 through increasing concentrations of particulate chlorophyll-a. Although the bulk of the population did not move to depths shallower than 50 m, and began its downward migration at a rate of 20.8–31.7 mh−1 in complete darkness, individual females continued to make brief excursions into chlorophyll-rich surface waters (4–8 μg l−1) during the first few hours of population descent. Ingestion rate diminished abruptly by one order of magnitude (ki = 0.068 h−1) at dawn (≈ 03∶30 h). Within four more hours, the population had reached its daytime depth and gut pigment content remained constant at a minimum value until the next migration cycle. No feeding appeared to take place at depth during the day. Ingestion by <i>M. gerlachei</i> females removed < 4% of daily primary production, with only ≈ 20% of this amount being removed from surface waters by active vertical transport.