George A. Knauer

VERTEX: carbon cycling in the northeast Pacific

Abstract Particulate organic carbon fluxes were measured with free-floating particle traps at nine locations during VERTEX and related studies. Examination of these data indicated that there was relatively little spatial variability in open ocean fluxes. To obtain mean rates representative of the oligotrophic environment, flux data from six stations were combined and fitted to a normalized power function, F = F 100 ( z /100) b ; e.g. the open ocean composite C flux in mol m −2 y −1 = 1.53 ( z /100 −0.858 with depth z in meters. It is shown that the vertical derivative of particulate fluxes may indicate solute regeneration rates, and accordingly regeneration rates for C, H and N were estimated. Oxygen utilization rates were also estimated under the assumption that 1.5, 1.0 and 0.25 moles of O 2 were used for each mole of N, C and H regenerated. Regeneration ratios of these elements were depth-dependent: i.e. N:C:H:−O 2 = 1.0 N: 6.2 ( z /100) 0.130 C: 10.0( z /100) 0.146 H: [1.5 + 6.2 ( z /100) 0.130 + 2.5 ( z /100) 0.146 ]− O 2 . Comparisons of our rates with those in the literature indicate that trap-derived new productivities in the open Pacific (≈1.5 mol C m −2 y −1 ) are substantially less than those estimated from oxygen utilization rates in the Sargasso Sea (≈4 mol C m −2 y −1 ). A hypothesis is presented which attempts to explain this discrepancy on the basis of the lateral transport and decomposition of slow or non-sinking POC in the Sargasso Sea. Data gathered during the VERTEX studies are also used for various global estimates. Open ocean primary productivities are estimated at 130 g C m −2 y −1 which results in a global open ocean productivity of 42 Gt y −1 . Organic C removal from the surface of the ocean via particulate sinking (new production) is on the order of 6 Gt y −1 . Fifty percent of this C is regenerated in the upper 300 m of the water column. The ratio of new production (measured with traps) to total primary production (measured via 14 C) is 0.14. It is concluded that the 14 C technique yields reasonable estimates of primary productivity provided that care is taken to prevent heavy metal contamination.

Role of large particles in the transport of elements and organic compounds through the oceanic water column

Abstract During the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare, large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea. This finding has important implications for understanding the transfer, distribution and fate of elements and organic compounds in marine waters. The “large” (> 100 μ m) detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small, discrete fecal pellets and plankton hard parts to large, amorphous aggregates or “snow” which contain both organic and inorganic constituents. Depending on size, shape and density, these particles sink at rates ranging from 1000 m day −1 . Several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport. Flux studies have highlighted the importance of marine heterotrophs in packaging fine, suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth. Large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column. Chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements, radionuclides and organic compounds (natural and anthropogenic) in various oceanic regimes. Pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested.

Sampling and analytical methods for the determination of copper, cadmium, zinc, and nickel at the nanogram per liter level in sea water

Abstract Sea-water samples collected by a variety of clean sampling techniques yielded consistent results for copper, cadmium, zinc, and nickel, which implies that representative, uncontaminated samples were obtained. A dithiocarbamate extraction method coupled with atomic absorption spectrometry and electrothermal atomization is described which is essentially 100% quantitative for each of the four metals studied, has lower blanks and detection limits, and yields better precision than previously published techniques. A more precise and accurate determination of these metals in sea water at their natural ng l -1 concentration levels is therefore possible. Samples analyzed by this procedure and by concentration on Chelex-100 showed similar results for cadmium and zinc. Both copper and nickel appeared to be inefficiently removed from sea water by Chelex-100. Comparison of the organic extraction results with other pertinent investigations showed excellent agreement.

Fluxes of particulate carbon, nitrogen, and phosphorus in the upper water column of the northeast Pacific

Abstract Concentrations of carbon, nitrogen and phosphorus were determined in particles that passively sank into multi-replicate collectors set at 50, 250, and 700 m in coastal waters, and 75, 575, and 1050 m in the open ocean. Fluxes as high as 36, 4.1, and 0.19 mmoles of C, N, and P m−2 day−1 were observed at 50 m under coastal upwelling conditions; at 700 m, upwelling period fluxes (9.6, 0.9, and, 0.053 mmoles of C, N and P m−2 day−1) exceeded those measured at 50 and 75 m when samplers were set under low productivity surface waters. 210Pb flux estimates were made on coastal trap particulates. The resulting values were close to the expected and suggest that overall flux estimates are representative of those occuring in the environment. Atomic ratios of C:N:P under upwelling conditions were similar to values reported for living plankton (∼180:18:1), while in the open ocean, atomic ratios of C and N in relation to P were markedly higher (400 to 900:30:1). Fecal pellet fluxes were two orders of magnitude higher under upwelling conditions (∼1 to 3 × 105m−2 day−1) than those in the open ocean (∼1000 m−2 day−1). Quantities of passively sinking particulate C, N, and P appeared to be equal to or in excess of the amounts required to meet the nutritional needs of the mid-water zooplankton. Rates of change for C, N, and P and inferred rates of oxygen change varied widely in relation to surface productivity. For example, oxygen utilization rates were as high as 790 μll−1 yr−1 in near-surface waters under upwelling conditions and as low as 4.4 μll−1 yr−1 at mid-depth in the open ocean. Our rates of change, determined by direct measurement, generally agree with previously published estimates from mathematical models.

Seasonal patterns of ocean biogeochemistry at the U.S. JGOFS Bermuda Atlantic time-series study site

Seasonal patterns in hydrography, oxygen, nutrients, particulate carbon and nitrogen and pigments were measured on monthly cruises at the Bermuda Atlantic Time-series Study site, 80 km southeast of Bermuda. Between October 1988 and September 1990, the annual cycle was defined by the creation of 160–230 m-deep mixed layers in February of each year and a transition to strong thermal stratification in summer and fall. The 230 m mixed layer in February 1989 resulted in mixed-layer nitrate concentrations of 0.5–1.0 μmole kg−1, carbon fixation rates over 800 mg C m−2 day−1, and a phytoplankton bloom with chlorophyll concentrations over 0.4 mg m−3. Chlorophyll a, particulate organic matter, inorganic nutrients and primary production had returned to prebloom levels the following month with the exception of a chlorophyll maximum layer at 100 m. Particle fluxes at 150 m in February 1989 reached 56 mg C m−2 day−1 and 11 mg N m−2 day−1 (0.77 mmole N m−2 day−1). Estimates of new production during the bloom period calculated from changes in oxygen and nitrate profiles ranged from 100 to 240 mmoles N m−2, significantly higher than the sediment trap fluxes and approaching the measured total production rates. In spring of 1990, mixed layer depths did not exceed 160 m, nitrate was rarely detectable in the upper euphotic zone, chlorophyll a concentrations were similar to 1989, and particulate organic matter concentrations were lower. The period of elevated biomass lasted for 3 months in 1990, and phytoplankton pigment composition varied between cruises. The average rates of primary production and particle flux were higher in 1990 than those measured in the spring of 1989, despite the differences in mixed layer depth. Throughout both years, NO3 : PO4 ratios in the upper thermocline exceeded Redfield ratios. The maintenance of this pattern requires a net uptake of PO4 between 150 and 250 m, a depth range usually associated with net remineralization. The exact mechanism that maintains elevated PO4 uptake and its implication for the nutrient supply to the euphotic zone remain unknown.

Zinc in north-east Pacific water

ALMOST all lead data for the marine environment are inaccurate, contends Patterson1, because of gross contamination from faulty sampling and analytical procedures. Most marine chemists assume that similar problems are associated with other trace elements as well. Hence, clean sampling and analytical techniques have been adopted. These procedures, in conjunction with the improvement of analytical instrumentation, have resulted in reports on Cu, Ni and Cd (refs 2–4; 3, 5; and 3, 6–8 respectively) levels in seawater that are at least an order of magnitude lower than those previously thought to exist. We report here that Zn concentrations (10–600 ng l−1) are also considerably lower than previously published estimates of 1–30 µg l−1 and that its vertical distribution (surface depletion, deep enrichment) is very similar to that of a major plant nutrient; that is, silicate.

Seasonal variability in primary production and particle flux in the northwestern Sargasso Sea: U.S. JGOFS Bermuda Atlantic time-series study

The relationship between primary production and sediment trap-derived downward flux of particulate organic matter was characterized over a 2 year period at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site to evaluate the importance of temporal variations in upper ocean biogeochemical processes. Water column-integrated primary production (∫PP), determined once each cruise using 14C incubations (in situ dawn-to-dusk), peaked in late winter/early spring of both 1989 and 1990. Smaller increases in ∫PP also occured in July 1989 and October–December 1990. Annual ∫PP was 9.2 mol C m−2 y−1 in 1989 and 12 mol C m−2 y−1 in 1990. This was higher than the 1959–1963 annual average (6.8 mol C m−2 y−1) determined at Station “S” located approximately 50 km northwest of the BATS site. n nFluxes associated with sinking of total particulate mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured at 150, 200, 300 and 400 m using a free-floating sediment trap array generally deployed once each cruise for 72 h. Fluxes varied seasonally, and within our ability to resolve differences (i.e. monthly sampling), there was no distinguishable time offset between peaks in ∫PP and corresponding peaks in elemental flux. Fluxes generally decreased with increasing depth, and fluxes of POC and PON were positively correlated with particulate mass flux at all depths. POC/PON (C/N) ratios at 150 m during periods of high ∫PP were generally characteristics of live planktonic biomass. Higher C/N ratios in material collected by the deeper traps were consistent with more rapid losses of PON than POC from sinking particles. POC and PON fluxes at 150 m, nominally the base of the euphotic zone, were positively correlated with ∫PP. The fraction of ∫PP leaving the euphotic zone in the form of sinking particles (i.e. collected in traps) varied seasonally and was inversely proportionato ∫PP. Surface export of organic matter estimated by sediment traps at 150m was 0.78 mol C m−2y−1 (0.10 mol N m−2y−1) in 1989 and 0.77 mol C m−2y−1 (0.11 mol N m−2y−1) in 1990.

Microbial production and particle flux in the upper 350 m of the Black Sea

A field experiment, designed to evaluate several aspects of the microbially mediated organic carbon cycle, was conducted in the western basin of the Black Sea during May 1988. Measurements included standing stocks of particulate and dissolved carbon, rates of microbial production and downward particle flux. n nPhotoautotrophic production was restricted to the upper 55 m of the water column. Values ranged from 0.7–15 mg Cm−3 day−1; total integrated primary production was 575 mg C m−2 day−1. Euphotic zone heterotrophic bacterial production was estimated to be 260 mg C m−2 day−1, a value equivalent to 45% of the contemporaneous photoautotrophic production. Beneath the euphotic zone, and across the oxic-to-anoxic boundary (55–95 m), bacterial production rates were low but measurable (≤6 mg C m−3 day−1). Heterotrophic bacterial productivity was approximately equal to the rate of chemoautotrophy in the 55–95-m depth interval. No bacterial biomass or production peaks were observed in the chemocline region. POC and PON fluxes from the base of the euphotic zone (60 m) were 140 and 11.6 mg m−2 day−1, respectively. Beneath 60 m, POC and PON fluxes decreased rapidly with depth to 39 and 3.3 mg m−2 day−1 at 80 m, followed by a more gradual decline to 24 and 2.0 mg m−2 day−1 at 350 m. The fluxes of particulate ATP and fecal pellets also exhibited order of magnitude decreases in the 60–80-m depth interval with minimal losses thereafter. Our results indicate a rapid and efficient recycling of particulate organic matter in the sub-oxic portion of the water column (60–80 m) and relatively low rates of decomposition in the permanently anoxic zone.

Cryptic zooplankton swimmers in upper ocean sediment traps

Abstract Sediment traps are the major oceanographic tool for collecting passively sinking particulate material (the “particle flux”) in the ocean. Sediment traps in the upper ocean also collect actively sinking zooplankton that are usually manually removed prior to analysis. Microscospic analysis of sediment trap samples collected over a 19-month period in the eastern North Pacific reveals that zooplankton “swimmers” are a larger problem than previously recognized. Zooplankton that are cryptic (i.e. difficult to see or distinguish from the detrital material) and difficult to remove (principally gelatinous zooplankton) may have contributed up to 20 mg C m −2 day −1 to the “particulate flux”, with the highest values in the upper 150 m. This swimmer problem is in addition to the previously recognized presence of crustaceans and other large metazoans in traps. Additionally, the detritus-laden, mucous-feeding structures (houses)of larvaceans probably enter the traps with the larvaceans and would be impossible to remove. We estimate that the contribution of the cryptic swimmers and larvacean houses could be as much as 96% of the measured carbon flux. The contribution is greatest in the euphotic zone and drops sharply below 200 m. Subtracting out this potential artifact at the VERTEX station results in vertical profiles of organic carbon flux that differ dramatically from the standard flux profile for carbon in the upper ocean: specifically, the implied “regeneration” rate is greatly reduced. Screened traps (300 μm screens below the baffles) contained numerous metazoans smaller than the screen mesh size. These traps also contained lower levels of other types of sinking particles, and it is unclear to what extent the screens reduced the relative contribution of swimmers to the trap-collected carbon. Although the expanded swimmer problem presented here is now documented at just the VERTEX site, we expect it exists elsewhere. The extent of this swimmer problem requires resolution before sediment traps, especially those deployed in the upper few 100 m, can be used to measure the “flux of particulate material.”

VERTEX: manganese transport through oxygen minima

Abstract Manganese transport through a well-developed oxygen minimum was studied off central Mexico (18°N, 108°W) in October–November 1981 as part of the VERTEX (Vertical Transport and Exchange) research program. Refractory, leachable and dissolved Mn fractions associated with particulates caught in traps set at eight depths (120–1950 m) were analyzed. Particles entering the oxygen minimum had relatively large Mn loads; however, as the particulates sank further into the minimum, total Mn fluxes steadily decreased from 190 nmol m −2 day −1 at 120 m to 36 nmol m −2 day −1 at 400 m. Manganese fluxes then steadily increased in the remaining 800–1950 m, reaching rates of up to 230 nmol m −2 day −1 at 1950 m. Manganese concentrations were also measured in the water column. Dissolved Mn levels −1 were consistently observed within the 150–600 m depth interval. In contrast, suspended particulate leachable Mn amounts were especially low at those depths, and never exceeded 0.04 nmol kg −1 . The combined water column and particle trap data clearly indicate that Mn is released from particles as they sink through the oxygen minimum. Rate-of-change estimates based on trap flux data yield regeneration rates of up to 0.44 nmol kg −1 yr −1 in the upper oxygen minimum (120–200 m). However, only 30% of the dissolved Mn in the oxygen minimum appears to be from sinking particulate regeneration; the other 70% probably results from continental-slope-release-horizontal-transport processes. Dissolved Mn scavenges back onto particles as oxygen levels begin to increase with depth. Scavenging rates ranging from −0.03 to −0.09 nmol kg −1 yr −1 were observed at depths from 700 to 1950 m. These scavenging rates result in Mn residence times of 16–19 years, and scavenging rate constants on the order of 0.057 yr −1 . Manganese removal via scavenging on sinking particles below the oxygen minimum is balanced by Mn released along continental boundaries and transported horizontally via advective-diffusive processes. Manganese appears to be very weakly associated with particulates. Nevertheless, the amounts of Mn involved with sinking biogenic particles are large, and the resulting fluxes are on the same order of magnitude as those necessary to explain the excess Mn accumulating on the sea floor. The overall behavior of Mn observed in this, and other, studies strongly suggests some type of equilibrium occurring between dissolved and particulate phases. This equilibrium appears to shift in direct or indirect response to dissolved oxygen levels.