Mesozooplankton grazing in the Southern California Bight. I. Population abundances and gut pigment contents

Gut pigment content of the mesozooplankton community in Santa Monica Basin, California, USA was examined on 3 winter-spring and 3 autumn cruises between October 1985 and February 1988. Mean chlorophyll concentrations for the upper 70 m varied from 0.30 to 0.32 mg m-3 for autumn cruises and from 0.35 to 1.7 mg m-3 for winter-spring cruises. Larger crustacean zooplankton species, particularly the copepod Calanus pacificus, were more abundant in the winter-spring, and this trend was also evident in size structure of pelagic tunicates. Gut pigments of larger, migratory taxa (C. pacificus, Metridia lucens, Pleuromamma spp., large euphausiids, and ostracods) and often some of the smaller, non-migratory forms (Clausocalanus spp., Acartia spp , and appendicularians) indicated die1 periodicity in feeding intensity with highest gut pigments generally at night or in the early morning. Feeding periodicity was weakest when chlorophyll concentration and mean gut pigment content (ng chl equ. ind:') was highest in the spring. The night:day ratio of community gut pigment (CGP) varied from 1.02 to 1.95 for the winter-spring period and from 1.77 to 3.39 for the autumn period. Migrating taxa explained most of the day-night difference in the winter-spring but relatively little of the difference in the autumn. Small species and developmental stages (c1.5 mm body length) dominated daytime (95.1 to 99.8%) and nighttime (63.6 to 96.2 %) CGP during the autumn and were important, though relatively less so, in the winter-spring cruises (day 58.2 to 76.9%, night 37.7 to 53.7%). Copepods accounted for the majority of CGP in May 1986 and April 1987. Thaliaceans dominated in February 1988 and were generally more important in winter-spring than autumn, as were euphausiids. Pteropods made a significant contribution to CGP in October 1985. Appendicularians (Oikopleura sp.) were major grazers during all cruises, but their contribution to CGP surpassed that of copepods on the 3 autumn cruises.