Sedimentation of acantharian cysts in the Iceland Basin: Strontium as a ballast for deep ocean particle flux, and implications for acantharian reproductive strategies

Abstract

Acantharian cysts were discovered in sediment trap samples from spring 2007 at 2000 m in the Iceland Basin.\rAlthough these single-celled organisms contribute to particulate organic matter flux in the upper mesopelagic,\rtheir contribution to bathypelagic particle flux has previously been found negligible. Four time-series sediment\rtraps were deployed and all collected acantharian cysts, which are reproductive structures. Across all traps, cysts\rcontributed on average 3–22%, and 4–24% of particulate organic carbon and nitrogen (POC and PON) flux,\rrespectively, during three separate collection intervals (the maximum contribution in any one trap was 48% for\rPOC and 59% for PON). Strontium (Sr) flux during these 6 weeks reached 3 mg m22 d21. The acantharian\rcelestite (SrSO4) skeleton clearly does not always dissolve in the mesopelagic as often thought, and their cysts can\rcontribute significantly to particle flux at bathypelagic depths during specific flux events. Their large size\r(, 1 mm) and mineral ballast result in a sinking rate of , 500 m d21; hence, they reach the bathypelagic before\rdissolving. Our findings are consistent with a vertical profile of salinity-normalized Sr concentration in the Iceland\rBasin, which shows a maximum at 1700 m. Profiles of salinity-normalized Sr concentration in the subarctic\rPacific reach maxima at # 1500 m, suggesting that Acantharia might contribute to the bathypelagic particle flux\rthere as well. We hypothesize that Acantharia at high latitudes use rapid, deep sedimentation of reproductive\rcysts during phytoplankton blooms so that juveniles can exploit the large quantity of organic matter that sinks\rrapidly to the deep sea following a bloom.