Seasonal radiolarian flux at the VERTEX North Pacific time-series site

Abstract

Fluxes of radiolarians were measured at several depths from 50 to 2000 m at 33°N. 139°W with free-floating particle interceptor traps in a series of 3-month collections from October 1986 to May 1988. Fluxes of spumellarians, nassellarians and phaeodarians were generally highest and were most variable in the upper 100 m. Patterns of flux differed among the three groups, and no clear seasonal patterns were evident, within the resolution of the methods. Standing stocks, sampled with large-volume water samples, varied little during the study period. Fluxes of each group from the base of the euphotic zone (150 m) were similar throughout the study period, as generally were fluxes from 150 to 2000 m. Flux patterns of two species of nassellarians and of several species of phacodarians were examined. Flux of the symbiont-bearing nassellarian Lithopera bacca from the euphotic zone peaked in summer, but at 2000 m there was no seasonal pattern to its flux. Among the phacodarians. Lirella bullata showed spring peaks in flux at 2000 m. The lack of clear seasonal patterns for the groups and for many individual species suggests that the seasonal variability in factors, such as primary productivity and nitrogen dynamics, at this oligotrophic site was too weak to affect radiolarians, which are at least one trophic step removed from these factors. Variability in radiolarian fluxes at all depths over the 18 month period ranged from undetectable to a factor of 10; this probably represents normal variability at this oligotrophic site. The majority of polycystines trapped in the upper 100 m were alive at the time of capture, and the percentage of living specimens decreased rapidly as depth increased. In contrast, living phacodarians dominated the fluxes of phacodarians at all depths. Several species of phacodarians with shallower depth distributions were absent from the deepest traps, whereas species with deeper distributions were present in the 2000 m traps. Carbon flux of living radiolarians, estimated from volume: carbon conversions, showed no clear seasonal pattern. Radiolarians were estimated to contribute from <0.1 to > 80% of the total carbon flux, and they generally contributed < 10%. The low radiolarian carbon flux at 2000 m throughout the year suggests that radiolarians contribute an insignificant amount of carbon to the deep sea in this oligotrophic regime. Analysis of reproduction of phacodarians showed no evidence that formation of swarmers was causing phacodarians to sink at any time during the study. Multinucleate phcodarians were very rare. Organisms undergoing binary fission were more common, but there was no convincing evidence of seasonal patterns of asexual reproduction for any of the phacodarians.