Annual flux of radiolaria and other shelled plankters in the eastern equatorial atlantic at 853 m: seasonal variations and polycystine species-specific responses

Abstract

Polycystine radiolarians, phaeodarians, tintinnids, tintinnid cysts and mollluscs (chiefly pteropod protoconchae) were counted in 20 time-series sediment trap samples retrieved in the eastern equatorial Atlantic (01°47.5′N, 11°07.6′W) at 853 m, between 1 March 1989 and 16 March 1990. In addition, polycystine species were identified. Mean annual fluxes, in ind. m −2 day −1, were: polycystines: 28,446; tintinnids: 27,275; foraminifers: 17,816; tintinnid (and probably other ciliates') cysts: 14,632; phaeodarians: 1370; and molluscs: 1192. These yields are noticeably higher than most previous data from various areas of the World Ocean, which in part is attributed to the inclusion of particles <10 μm. Only 3% of the polycystines and 30% of the phaeodarians were represented by “live” cells (i.e. with protoplasm). Indirect evidence suggests that foraminifer and radiolarian fluxes probably reflect fairly well their abundances in the plankton; for the tintinnids, however, only about 0.1% of the loricae 853 m intact. With the exception of tintinnid cysts and molluscs, fluxes for the groups counted were clearly correlated with total mass and organic carbon flux and, presumably, with primary production; in the study area both peak in July-September and in January-April. It is concluded that tintinnid high reproduction rates are responsible for tightest associations between the output of their loricae and total flux. The fluxes of Foraminifera and Radiolaria, which have longer life spans ( ca 1 month), are more loosely correlated with total flux, and often show peaks approximately 20–40 days after those of the latter. Molluscs, with life spans of ca 1 year, do not correlate with total flux. Juvenile Nassellaria comprised up to 44% of all polycystine shells (mean 25%). In total, 187 polycystine taxa were recorded, yet only six accounted for more than 50% of all the individuals identified. Although some species did vary in relative abundance in association with total and polycystine fluxes, these changes in assemblage composition were of very minor importance. Radiolarian equitability and the relative contribution of the Spumellaria were significantly higher at times of lower polycystine flux. These fluctuations, however, were also relatively small, and their effect woult not be expected to bias the sedimentary record toward restricted periods of higher radiolarian output. Comparison of the present data with detailed previous information for the Gulf of Alaska shows that both total radiolarian flux and its rate of accumulation throughout the year are very similar in the two areas.