160 NOTES AND COMMENT

Transcription

1 NOTES AND COMMENT 159 THE DISTRIBUTION AND CHEMICAL COMPOSITION OF ORGANIC PARTICULATE MATTER IN THE SEA AND SEDIMENTS OFF THE EAST COAST OF SOUTH AMERICAS Menzel (1967) showed that concentrations of particulate organic carbon suspended in the sea below about 100 m, adjacent to the continental rise off Peru, ranged between 6 and 15 pg/liter. The mean and range of the values were smaller than those reported by Hobson ( 1967)) Menzel (1964), Menzel and Goering (1966), Parsons and Strickland ( 1962)) Riley, Van Hemert, and Wangersky ( 1965)) and Szekielda ( 1967) from comparable depths in other offshore areas. The purpose of this study was to obtain samples of particulate organic carbon ( POC) off th e east coast of South America ( Fig. 1) to compare with measurements from the Pacific Ocean ( Menzel 1967). The mass and distribution of suspended particulate matter ( PM ), nitrogen ( PN ), and phosphorus (PP), and the organic carbon and nitrogen contents of the sediment, were also studied and are compared with relevant data in the literature. METHODS AND MATERIALS Samples were taken with a lo-liter glass and Teflon water bottle at 50-m intervals from m; 100-m intervals from 200-1,000 m; from 2,500 m; and from m above the sea bottom. The seawater was drained into a glass reservoir using Tygon plastic tubing and immediately filtered in a closed glass system. Samples of 6-7 liters were filtered through metal membrane filters ( Selas Flotronics, 1.2-p-pore diam), and POC and PN were determined using a Perkin-Elmer No. 240 Elemental Analyzer. The filters were combusted before use at 340C in a stream of O2 for 1 hr and contained ca. 10,ug of carbon and <l pg of nitrogen. Samples for PM (2 liters) and PP ( liter) were filtered through RA Millipore filters (1.2-p-pore diam). The method of Banse, Falls, and Hobson ( 1963) was used to determine PM using a Mettler microbalance to weigh filters. Particulate phosphorus was determined by the method outlined in Menzel and Ryther (1964) ex- cept that no MgCO, was placed on filters before filtration, and the entire filter was digested. Replicate samples were taken on AtZantis II cruise 31 to the South Atlantic Ocean and cruise 30 to the east of the Gulf Stream off New York. These data were trans- formed to logarithms and a one-way analysis of variance used (at the 95% confidence level) to estimate total errors. These were +45, +30, and +-42% for POC, PN, and PM for concentrations measured below a depth of 200 m and were used to select contour intervals. The error for PP was not determined and contour intervals were chosen arbitrarily. Sediment samples were taken with a gravity corer equipped with a 60- x 4-cmdiam barrel, then frozen and stored. Sub- - 0 IO l Contribution No from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Supported in part by National Science Foundation Grant GZ 259, Office of Naval Research Contract Nonr-C0241, and U.S. Atomic Energy Commission Contract AT(30-l)-3862 ( Ref. NYO ). 60 FIG. 1. Track and station locations of Atlantis II, February cruise

2 160 NOTES AND COMMENT z-9 50 STATION NUMBER S JATION NUMBER FIG. 2. Spatial distribution of particulate car- FIG. 3. Spatial distribution of particulate nibon ( pg/liter ). trogen ( pg/liter ). samples of about 2 g were taken from the sediment surface and at various depths below the surface. They were dried, weighed, ground, and mixed with ml of hot 0.3 N trichloroacetic acid to remove CaC03. After CO, evolution had ceased, the subsamples were rinsed three times with lo-15 ml of distilled water, dried, reweighed, and contents of organic carbon and nitrogen determined using the Elemental Analyzer. RESULTS AND DISCUSSION The concentrations of POC, PN, and PM were not homogeneous at depths shallower than 1,000 m ( Figs. 24). The largest gradients occurred between 100 and 200 m where POC, PN, and PM were pg/liter, pg/liter, and loo-225 pg/liter. Below a depth of 200 m, POC decreased with depth from to <2.0 pg/liter, and PN decreased from to <0.08 pg/liter. Values of PM were generally less than 100 pug/liter. Particulate organic carbon, PN, and PM increased close to the sea bottom when the depth was less than 4,000 m (Figs. 2-4). The increases indicate that particulate matter may be resuspended into the water column from the bottom or is concentrated in close proximity to the bottom (Ewing and Thorndike 1965; Jerlov 1953; Lisitsin 1962). None of the variations of the distributions could be correlated with water mass distribution, estimated using temperature-salinity data of Defant ( 1961). The concentrations of POC are smaller than those reported by Menzel ( 1967) in the sea off the coasts of Peru and Ecuador. Most of his samples were taken near the precipitous continental rise off Peru and ranged between 6 and 15 pg/liter (Fig. 5). Those taken at depths greater than 1,200 m (at stations between the coastal area and the Galapagos Islands) contained slightly less than 6 pg/liter (Fig. 5) -about equal to concentrations observed during this study. Particulate phosphorus was apparently

3 NOTES AND COMMENT STATION NUMBER STATION NUMBER FIG. 4. Spatial distribution of particulate matter ( pg/liter ). homogeneous at 0.2 pug/liter at all depths greater than 300 m. This is probably an artifact due to the small amount of material used in the phosphorus analysis. Menzel and Ryther (1964) showed that there was no PP at depths greater than 200 m in the western North Atlantic Ocean, implying that phosphorus was rapidly remineralized. Our results and those of Holm-Hansen, Strickland, and Williams ( 1966) indicate that PP is present at all depths. The exact composition of the particulate matter of individual samples could not be determined accurately because the methods are not precise enough at the low concentrations present. But a general description can be made from ratios of the mean concentration values at a given depth from all stations ( Table 1). Percentages of POC in the PM decreased from 5-7% at depths shallower than 75 m to 3-6% at all other depths. These values are lower than maximum percentages of lo-20% estimated by Hobson ( 1967). Ratios of POC : PN FIG. 5. Spatial distribution of particulate carbon ( pg/liter ) and track of cruise 15 of the Anton Bruun off the northwest coast of South America (modified after Menzel 1967). increased from 5-7 at depths shallower than 175 m to 9-16 at all other depths. These ratios are high compared to values of 2-6 reported in the literature (Dal Pont and Newell 1963; Menzel and Ryther 1964; Parsons and Strickland 1962) but agree closely with values of lo-14 at depths deeper than 150 m measured by Holm- Hansen et al. ( 1966)) although their reported concentrations of POC and PN were 10 times greater than ours. The differences between our concentrations of POC and PN and POC : PN ratios and those in the literature cannot be explained. They may reflect sampling errors, or they may be real variations. Marine phytoplankton, zooplankton, bac- teria, and many organic molecules contain carbon and nitrogen in the ratio, by weight, of 3-7 (Beers 1966; Parsons, Stephens, and Strickland 19611)) although the ratio may increase in phytoplankton when their growth is limited by nitrogen ( unpublished results). If we assume that low ratios are

4 162 NOTES AND COMMENT TABLE 1. Percentage of particulate organic carbon (POC) by weight of the particulute mutter (PM) and the ratio of the particulate organic carbon to the particulate nitrogen (PN) ut selected depths (m) Depth (ml ,000 2,500 5,000 (PC/PM)XlOO g-2 4: Z-9" 5: ~~~ 5:3 PC:PN Y-78 7: indicative of living organisms and decomposable organic matter, our ratios indicate that most of the POC at depths shallower than 175 m was of this nature and that the ratio of this matter to organic material resistant to decomposition decreased with depth. Either the concentration of nitrogen-rich organic molecules decreases with depth or some compound, such as a mucopolysaccharide, containing carbon and nitrogen in the ratio, by weight, of 10-14, forms a large fraction of the particulate organic matter at depths deeper than 200 m. The percentage of organic carbon by weight at the sediment surface was in the deep water increasing to (Stas ) on the continental slope of South America (Table 2), agreeing with those reported by Gross (1967) for the Tufts Abyssal Plain ( <0.5%) in the northeast Pacific Ocean and the continental slope of the northwestern United States ( >2%), Apparently the carbon content of the sediment decreased by about 10% a short distance below the surface. This could be caused by dilution of organic by inorganic matter from sources other than TABLE 2. Percentage of organic carbon (OC) by weight (W) and the rutio of organic carbon to nitrogen (N) at the surface und 25 cm of the bottom sediment Surface 25 cm Station (OC/W)X 100 0C:N (OC/W)xlOO 0C:N ii ii z; suspended matter or by the oxidation of organic matter by organisms living on the sea bottom. The percentage was at 25 cm below the sediment surface at stations in deep water, increasing to at stations on the continental slope (Table 2). Ratios of organic carbon to nitrogen were 5-10 at the sediment surface decreasing to 2-6 at 25 cm below the sediment surface (Table 2). The value of the ratio of the surface sediment is lower than an average of 10 reported by Trask ( 1939). LOUIS A. HOBSON DAVID W. MENZEL Woods Hole Oceanographic Institution, Woods Hole, Massachusetts REFERENCES BANSE, K., C. P. FALLS, AND L. A. HOBSON A gravimetric method for determining suspended matter in sea water using Millipore filters. Deep-Sea Res., 10: BEEZS, J. R Studies on the chemical composition of the major zooplankton groups in the Sargasso Sea off Bermuda. Limnol. Oceanog., 11: DAL PONT, G., AND B. NEWELL Suspended organic matter in the Tasman Sea. Australian J. Marine Freshwater Res., 14: DEFANT, A Physical oceanography, v. 1. Macmillan, New York. 729 p, EWING, M., AND E. M. THORNDIKE Sus-

5 NOTES AND COMMENT 163 pended matter in deep ocean water. Science, 147 : GROSS, M. G Organic carbon in surface sediment from the northeast Pacific Ocean. Intern. J. Oceanol. Limnol., 1: HOBSON, L. A The seasonal and vertical distribution of suspended particulate matter in an area of the northeast Pacific Ocean. Limnol. Oceanog., 12 : HOLM-HANSEN, O., J. D. H. STRICKLAND, AND P. M. WILLIAMS A detailed analysis of biologically important substances in a profile off southern California. Limnol. Oceanog., 11: JERLOV, N. G Particle distribution in the ocean. Swedish Deep-Sea Exped. Rept., 3: LISITSIN, A. P Distribution and composition of suspended materials in seas and oceans, p Zn Recent sediments of the seas and oceans [In Russian]. USSR Acad. Sci., Comm. Sediments, Div. Geol. Geograph. Sci., Moscow. MENZEL, D. W The distribution of dissolved carbon in the western Indian Ocean. Deep-Sea Res., 11: Particulate organic carbon in the deep sea. Deep-Sea Res., 14: AND J. J. GOERING The distrib&on of organic detritus in the ocean. Limnol. Oceanog., 11: AND J. H. RYTHER The composition of particulate organic matter in the western North Atlantic. Limnol. Oceanog., 9 : PARSONS, T. R., AND J. D. H. STRICKLAND Oceanic detritus. Science, 136: K. STEPHENS, AND J. D. H. STRICKLAND On the chemical composition of eleven species of marine phytoplankters. J. Fisheries Res. Board Can., 18: RILEY, G. A., D. VAN HE-T, AND P. J. WANGERSKY Organic aggregates in surface and deep waters of the Sargasso Sea. Limnol. Oceanog., 10: SZEKJXLDA, K Some remarks on the influence of hydrographic conditions on the concentration of particulate carbon in seawater, p Zn H. L. Golterman and R. S. Clymo [eds.], Chemical environment in the aquatic habitat. North Holland Publ. Co., Amsterdam. TRASK, P. D Organic content of recent sediments, p Zn P. D. Trask [ed.], Recent marine sediments. Am. Assoc. Petrol. Geologists, Tulsa, Okla. OBSERVATIONS ON CUMULATIVE BOTTOM DRIFT IN MONTEREY BAY USING SEABED DRIFTERS~ The technique of determining nontidal bottom-current flow patterns on the Continental Shelf using Woodhead seabed drifters has been successfully used on the western Atlantic shelf and the seas around Great Britain (Woodhead and Lee 1960, unpublished manuscript). Bumpus ( 1964) and others concerned with nontidal drift on the western Atlantic shelf are using seabed drifters to learn how bottom currents influence fish. Bottom-current studies off the Middle Atlantic States may show how larval and juvenile stages of such species as summer flounder and menhaden use the prevailing bottom drift for their migration from offshore deep-water spawning areas to coastal estuarine areas. In the Irish and North seas, seabed drift- l The U.S. Naval Air Facility at Monterey, California, in cooperation with the Office of Naval Research, San Francisco, California, provided the aircraft used in the first drop of drifters. ers have been used to determine large-scale seasonal direction of bottom-water drift in the southern North Sea and bottom drift patterns in certain spawning areas (Corlett, unpublished manuscript). Other studies in Solway Firth and the northeast Irish Sea have been concerned with bottom drift as an indicator of the long-term movement of radioactive silt (Perkins, Williams, and Bailey, unpublished manuscript). In the study reported here, I used Woodhead type seabed drifters to investigate bottom drift in Monterey Bay, C aliforma. METHODS AND RESULTS Seabed drifters ( Geodyne Corporation, Waltham, Mass.) were obtained through the cooperation of D. F. Bumpus. Each drifter had attached to its top a plastic label requesting return information and promising a reward. At each air-drop station a cluster of five