Radiolarian fluxes, stocks, and population residence times in surface waters of the central equatorial Pacific

Abstract

Radiolarian settling fluxes and standing stocks from the central equatorial Pacific are more indicative of physical process than the living environment of the organism. Polycystine radiolaria were collected from plankton and in moored sediment traps along longitude 140°W during both warm and cold phases of the 1992 ENSO as part of the US JGOFS Equatorial Pacific program (EqPac). Highest shell fluxes occurred at the 5°N and 5°S trapsites, roughly the boundary between equatorial and subtropical environments. The sediment trap samples collected at these sites were composites of different living assemblages that live equatorward, poleward and, in some cases, directly within the overlying water. Stock to flux ratios indicate an average radiolarian residence time of around one week within the 5°N to 5°S equatorial band. The population residence time for Didymocyrtis tetrathalamus, which dominated the plankton during El Niño, is 3 weeks. Key indicator species of the cold tongue period, Tetrapyle octacantha (subtropical) and Lophophaena hispida (equatorial), have residence times slightly less than one week. This suggests environmental forcing can fundamentally alter community composition by limiting which species populations can be maintained under certain advective and mixing regimes. In this way, seasonal to interannual oscillations in the meridional current can control the composition of radiolarian communities. T. octacantha exhibited the highest stocks and fluxes of any radiolaria, which implies high sediment abundance of this species reflects high input versus preferential preservation. Correlations between fluxes of radiolaria and organic carbon indicate the radiolaria play a larger role in the plankton community within the gyre margin environment than at the lower latitude sites.