Oxygen demands of San Diego Trough sediments: an in situ study1

In situ total oxygen demand of San Diego Trough sedirncnts measured at a depth of 1,230 m was 2.40 -t 0.10 ml O2 m-” hr-I. Community respiration and chemical demand were 1.31 and 1.09 ml O9 in-’ hr-’ and constituted 54.G and 45.4% of the total demand. Compartmentalization of community respiration with antibiotic treatment yielded a “bacterial respiration” mcasurcmcnt of 0.21 ml OQ m-’ hr-I. Surface water enrichment due to seasonal upwelling over the San Diego Trough is manifested in the sctlimcnt carbon demands, which are significantly higher (3.5Ob of the estimated annual primary production) than demands at comparable depths in more oligotrophic arcas. Little information is available on the metabolic rates of deep-sea populations and communities. Jannasch et al. (1971) and Jannasch and Wirsen (1973) have shown that bacterial activity rates in the deep sea are much lower than those measured in shallow water areas (Wirsen and Jannasch 1974). In situ respiration measurements of benthic communities at 1,850 m indicate a metabolic rate two orders of magnitude less than at shallow shelf depths ( Smith and Teal 1973). However in an area of richer surface productivity, the Peru upwelling, Pamatmat ( 1971) mcasured oxygen uptake at the surface on sediment cores taken from 2,800 and 2,900 m and found consumption rates comparable to shallow water rates obtained with the same method; his samples were subjected to pressure and temperature changes before measurements. Another highly productive area is the California Current upwelling sys tcm. A relatively deep basin beneath this seasonal upwelling area is the San Diego Trough, which reaches depths of 1,500 m near the short (Emery 1954). The descriptive geology (Emery and Hiilsemann 1963; Shepard and Einsele 1962) and biology (Barnard 1954; Hartman 1963; Schultz 1 Contribution No. 3329 from the Woods Hole Oceanographic Institution. This research was supported by National Science Foundation Grants GA 39910 and GA 40322. This study is No. 6 of a series of reports on the results of Expedition Quagmire. 1966) oE this benthic environment have received considerable attention. With this extensive background available, workers from Scripps Institution of Oceanography, Woods 1101~ Oceanographic Institution, Allan IIancock Foundation, and the Univcrsity of Hamburg made a study of the San Diego Trough benthos using the remote underwater manipulator ( RUM ) . In situ mcasuremen ts of sediment oxygen uptake and its components cnablcd us to study the effects of increased surface productivity on deep-sea benthic sediments and associated organisms. I thank R. R. IIcsslcr, G. D. Wilson, and the crews of the ;Rum and RI? Orb for assistance and discussions throughout this study and II. W. Jannasch, G. T. Rowe, I-1. L. Sanders, J. M. Teal, and S. W. Watson for critically reading the manuscript. Materials and methods In situ measurements of sediment oxygen uptake were made at 10 locations within a 500-m sampling triangle ( 32”35’N, 117’29’W) in the San Diego Trough. Twenty measurements were made over a 2-week period (15-29 October) at a depth of 1,230 m. Positioning was maintained using a three-point moored platform, ORB ( Ocean Research Buoy), from which the RUM was lowcrcd daily to the bottom on a coaxial cable. Exact positioning within this area was accomplished by interrogating three transponders on the bottom. LIMNOLOGY AND OCEANOGRAPHY 939 NOVEMBER 1974, V. 19 (6)