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Approximately half of the sedimentation flux of particulate phosphorus in the Laurentian Trough in the Gulf of St. Lawrence is mobilized within the sediment and returned to the water column. In the oxidizing surface sediment, a major portion of the sedimentation flux of organic phosphorus is mineralized, and the released phosphate is partitioned between the pore water and surface adsorption sites. Surface-adsorbed phosphate is released to the pore water as needed to replace dissolved phosphate that escapes to the overlying water. Most of the phosphate is released deeper in the sediment column from iron oxides undergoing reduction. The nonmobilized phosphorus, which is buried with the accumulating sediment, appears to consist mostly of stable minerals such as apatite.The concentration of dissolved phosphate in sediment pore waters increases sharp!y across the sediment-water interface from 2 pmol PO, liter-l in the bottom water to 6+3 pmol PO, liter-' in the top centimeter, remains almost constant at this value down to 5-l 5-cm depth, and then increases rapidly with further depth. In the region of constant concentration, phosphate is buffered by adsorption-desorption equilibria with the sediment. The production rate of phosphate, the buffering capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface control the shape of the pore-water profile.In the aquatic environment, dissolved phosphate is consumed during the growth of phytoplankton and is regenerated during bacterial decomposition of organic matter. Much of the regeneration takes place in the water, but in relatively shallow environAcknowledgments

320sediment cores collected at three soft bottom stations; two brackish-marine and one freshwater. One of the marine stations was reduced and azoic, whereas the freshwater and the other marine station had well oxygenated conditions in the bottom waters. Positive redox-turnovers, including anaerobic incubation followed by reaeration, were generated in the cores and the supernatant water.In cores from the oxygenated freshwater and marine stations, dissolved phosphate and ferrous ions were released from the sediment during the anaerobic incubation. At the positive redox-turnover, the concentration of dissolved phosphate in the supernatant water decreased sharply due to scavenging by rapidly formed colloidal ferric hydroxide. Dissolved phosphate was also released during the incubation of the marine sediment from the reduced station. However, in these cores the concentration of iron in the supernatant water was low throughout the experiment and after the redox-turnover phosphate remained dissolved. In a parallel experiment in which iron was added to the supernatant water, dissolved phosphate was scavenged by ferric hydroxide at the positive redox-turnover in a similar way as observed for the two oxygenated stations. The low abundance of dissolved iron in the reduced marine system could be due to a rich supply of sulphide.In freshwater systems the concentration of dissolved phosphate is effectively diminished after a positive redox-turnover due to interaction with iron. In marine systems, which have had prevailed reduced conditions in the bottom waters, iron is immobilised. Consequently, a potent retention mechanism for phosphorus is eliminated. Our results imply that the cycling of phosphorus, in this aspect, differs in fresh and saltwater systems. This difference might have large effects on the availability of phosphorus as a nutrient.Manganese showed a consistent redox-dependent behaviour in all systems, but it did not interact with iron or phosphorus.The concentration of dissolved phosphate in sediment box-cores from the Laurentian Trough in the Gulf of St. Lawrence increased sharply across the sediment-water interface from 2.0 #mol/PO 4 /1 in the bottom water to 6 + 3 tmol PO 4 /1 in the top cm, remained almost constant at this value down to 5-15 cm depth, and then increased rapidly with further depth. In the region of constant concentration, phosphate is buffered by sorption equilibria with the sediment. The production rate of phosphate, the sorption capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface appear to control the shape of the pore water profile. Even though the buffering places an upper limit on the concentration gradient across the sediment-water interface, and hence on the flux, the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment. A model is proposed to relate sediment chemistry to phosphorus fluxes.Approximately half of the net sedimentation flux of phosphorus is not buried but is mobil...

A solid-state AufHg voltammetric microelectrode was used to measure, simultaneously and with millimeter spatial resolution, the vertical distributions of O,, Mn'+, Fe 2+ HS-, and I , in the pore water of sediments from the Canadian continental shelf and slope. The electrode was used shipboard to analyze undisturbed sediment cores and in a sediment mesocosm to determine the three-dimensional distribution of redox species in the sediment surrounding an actively irrigated worm burrow. In cores from Emerald Basin and Cabot Strait, 0, disappeared and Mn' ' , FeL , and HS appeared in the vertical sequence predicted on the basis of thermodynamics. In a core from the Scotian slope, Oz disappeared at -14-mm depth but Mn'+ was not detected over the 50-mm depth examined with the probe. In all three cores, I was detected in the pore water below the oxygen-penetration depth. In none of the cores did the distributions of OL and Mn'+overlap. The thickness of the layer within which neither Oz nor Mn" could be detected ranged from 8 to 40 mm; we suggest that nitrate, not oxygen, is used to oxidize Mn". The upwarddirected Mn" gradients of 3.5 and 10 mol mm4 in the Emerald Basin and Cabot Strait sediments, respectively, drive manganese fluxes that are more than one order of magnitude lower than the O2 fluxes, which are estimated at 4-S mmol mm' d I. The three-dimensional distributions of solutes in the pore water in the sediment surrounding an irrigated worm burrow demonstrate that both the Oz-penetration depth and the depth where Mn" is first detected are deeper in the vicinity of the worm burrow than away from it. This is consistent with the notion that irrigation brings oxygenated water into the worm tube, allowing oxygen to diffuse across the burrow wall into the sediment and react with Mn?+ and other reduced porewater constituents. Three-dimensional heterogeneity in porewater composition cannot be detected by conventional one-dimensional coring and slicing techniques. Because these latter techniques average the porewater composition in a large volume of sediment, they may indicate that the distributions of individual species overlap when in fact they do not.Microelectrodes were first applied to the study of sediment properties by Revsbech et al. (1980Revsbech et al. ( , 1983 who used them to describe the distribution of dissolved oxygen in sediment pore water with sub-millimeter resolution. Because of their small size, 0, microelectrodes can be used without destroying the sediment sample; therefore, they can be used to observe how the 0, concentration in the pore water varies in response to changes imposed at the sediment-water interface (Revsbech et al. 1983(Revsbech et al. , 1986.

Models of biogenic carbon (BC) flux assume that short herbivorous food chains lead to high export, whereas complex microbial or omnivorous food webs lead to recycling and low export, and that export of BC from the euphotic zone equals new production (NP). In the Gulf of St. Lawrence, particulate organic carbon fluxes were similar during the spring phytoplankton bloom, when herbivory dominated, and during nonbloom conditions, when microbial and omnivorous food webs dominated. In contrast, NP was 1.2 to 161 times greater during the bloom than after it. Thus, neither food web structure nor NP can predict the magnitude or patterns of BC export, particularly on time scales over which the ocean is in nonequilibrium conditions.

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Diagenetic modeling and mass-balance calculations were applied to box-core and sediment-trap data from three stations at 300-400-m depth in the Laurentian Trough to estimate downward and upward fluxes of manganese across the sediment-water interface, fluxes across the redox boundary in the sediment, rates of dissolution and precipitation of manganese, and rates of manganese accumulation.At all stations the cycling of manganese between the oxidizing and the reducing zone of the sediment was quantitatively more important than the cycling between the sediment and the water column. The redox boundary was the site of the largest fluxes. Downward fluxes across this boundary (0.45, 1.23, and 13.9 mmol mm2 d-l) were 3-50 times the rates of sedimentation or accumulation of manganese. The production of dissolved manganese turns over the inventory of particulate manganese in the sediment surface layer in 43-207 days. A small proportion of the dissolved manganese produced (13-29%) escapes the sediment, reprecipitates in the water column and, in part, returns to the sediment. Increased rates of bioturbation increase the rate of internal manganese cycling more than they do the rate of cycling across the sediment surface.