PHYTODETRITUS AT THE ABYSSAL SEA-FLOOR ACROSS 10-DEGREES OF LATITUDE IN THE CENTRAL EQUATORIAL PACIFIC

Fresh phytoplankton detritus (or phytodetritus) has been reported from numerous deep seafloor sites in the temperate North Atlantic and Paci fic Oceans following seasonal phytoplankton blooms. Here we report the first strong evidence for abyssal accumulations of phytodetritus in t he tropics. in the central equatorial Pacific. In November-December 19 92 we obtained photographs and/or sediment-core samples from 61 abyssa l stations (water depths of 4280-5012 m) between 12 degrees S and 9 de grees N along similar to 140 degrees W. Greenish flocculent material w as recovered from the top of multiple-core samples from 5 degrees S to 5 degrees N; this material was most abundant from 2 degrees S to 2 de grees N, in some areas forming continuous layers at least 5 mm thick, and individual aggregates > 1 cm in diameter. The greenish material wa s clearly visible in bottom photographs as a green veneer that covered > 95% of the seafloor near the equator, and as individual cm-scale ag gregates covering < 1% of the seafloor. Occasionally, thick accumulati ons of cm-scale aggregates occurred in biogenic pits. Cleared trails a nd feeding traces suggest that surface-deposit-feeding holothurians an d echiurans grazed the greenish material. Microscopic examination of g reenish material recovered from core tops and a burrow lumen revealed relatively intact diatoms (including Rhizosolenia sp.) and other micro algae with chloroplasts containing chlorophyll. The greenish material was 1-12.5% organic carbon by weight, i.e. 5-39 times richer than asso ciated seafloor sediments. It also contained high excess activities of Th-234, suggesting arrival from the water column in the previous 100 days. Samples of the greenish flocculent material from 0 degrees and 5 degrees N incubated at simulated environmental pressure and temperatu re with C-14-labeled glutamate exhibited greater than or equal to 5-fo ld higher rates of microbial activity than underlying sediments or bro wn flee from 9 degrees N. Surface-sediment samples (which included the greenish flocculent material) from 5 degrees S to 5 degrees N also co ntained significant concentrations of chlorophyll a and other chloropi gments; the chloropigment concentrations were roughly comparable to de ep-sea phytodetritus collected in the North Atlantic. We conclude that fresh, organic-rich phytodetritus was present on the seafloor from 5 degrees S to 5 degrees N along 140 degrees W in November-December 1992 , with highest concentrations within 2-3 degrees of the equator. This material is likely to be a concentrated, high-quality food resource fo r deep-sea microbes and metazoans. We estimate an upper limit for the standing stock of this phytodetritus to be similar to 2.6 mmol C-org/m (2); this corresponds to similar to 3% of the annual flux of organic c arbon to the seafloor at these latitudes in 1992. Because the degradat ion rate of this material appears to be very high, its presence at the seafloor for several months per year could yield significant phytodet rital contributions to the annual seafloor organic-carbon budget. We a lso suggest that the phytodetrital aggregates are formed at intense co nvergence zones resulting from seasonal passage of tropical instabilit y waves within 5 degrees of the equator; if so, phytodetrital accumula tions are likely to recur seasonally over broad areas of the abyssal e quatorial Pacific. Copyright (C) 1996 Elsevier Science Ltd.