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Community respiration rates were measured in unfiltered seawater collected in the upper 75 m of the water column along a transect in the Sargasso Sea and at the Bermuda Atlantic Time-series Study (BATS) station (31° 50' 00 N; 64° 10' 00 W) during a cruise in June and July 2001. Community respiration rates in the upper 75 m of the water column averaged 1.1 ± 0.4 µM O2 d -1 and exhibited significant spatial and temporal variability. Concurrent determination of the heterotrophic and autotrophic community revealed no relationship between community respiration and the abun- dance of any of the major metabolic groups. Addition of inorganic nutrients (NO3 and PO4) and organic carbon (glucose) indicated that community respiration was P-limited in the surface mixed layer. Size-fractionation experiments indicated that the abundance of heterotrophic bacteria in the

The cycling of nutrients was studied over a 16 mo period to determine how processes occurring between the water column and benthos influenced nutrient dynamics in a Thalassia testudinum dominated seagrass meadow. Nutrient concentrations were low and ranged from below detection to 0.59 μM ammonium (NH 4 + ), 0.04 to 0.29 μM nitrate plus nitrite (NO 3 - + NO 2 - ), and below detection to 0.22 μM soluble reactive phosphate (SRP). Water column and benthic fluxes of NO 3 - + NO 2 - and SRP were usually below detection. The benthic fluxes of NH 4 + ranged from an uptake of -228 μmol N m -2 d -1 to a release of 363 μmol N m -2 d -1 . Positive fluxes (i.e. directed out of the sediment) occurred primarily in light incubations and from seagrass-dominated sediments. Water column fluxes of NH 4 + ranged from a net uptake of -145 μmol N m -2 d -1 to a net regeneration of 643 μmol N m -2 d -1 . The net regeneration of NH 4 + in the water column usually exceeded the release of NH 4 + from the benthos. There was a significant correlation between the regeneration of NH 4 + in the water column and the light-mediated release of dissolved organic matter (DOM) from the benthos, indicating that benthic-derived DOM supported the regeneration of NH 4 + in the water column. Bacterioplankton growth efficiencies were significantly and positively correlated to the regeneration of NH 4 + in the water column, possibly resulting from changes in the composition of DOM released from the benthos. The C:N ratios of the organic matter remineralized in the water column were variable and ranged from 14 to 81, with lowest values occurring in late summer and highest values in spring. The results of this study indicated that temporal variations in the source and composition of DOM significantly influenced the cycling of nutrients in the water column of this seagrass meadow.

The photochemical reactivity of dissolved lignin and photobleaching of dissolved organic matter (DOM) were examined in riverine and open-ocean water samples. Approximately 75% of the total dissolved lignin in Mississippi River water was lost during 28 d of incubation in sunlight, mostly due to photooxidation. The remaining fraction of dissolved lignin was much less susceptible to photooxidation. About 90% of the dissolved lignin in river water was present as high-molecular-weight (HMW, >1,000 Dalton) DOM. However, after exposure to sunlight, about 80% of the remaining lignin was present as low-molecular-weight (LMW

The cycling of dissolved organic matter (DOM) and its significance to ecosystem metabolism was studied over a 16 mo period in a Thalassia testudinum dominated meadow. The benthos was usually net autotrophic (annual gross primary production to respiration ratio [P:R] = 1.3) while water column respiration (R) exceeded gross primary production (annual P:R = 0.3). Net fluxes of dissolved organic carbon (DOC) from the benthos primarily occurred in the light (0 to 18 mmol C m -2 d -1 ) and from seagrass-dominated areas, suggesting that release of DOC was mainly due to seagrass exudation. Net benthic DOC fluxes measured in the light were significantly correlated (p 50 %) of the DOC released from the benthos on a daily basis. Annual estimates of bacterioplankton C demand, based on water column R (∼8 mol C m -2 yr -1 ), represented >50% of the benthic NPP (∼14 mol C m -2 yr -1 ) These measurements indicate a stronger linkage between benthic and water column processes than previously believed, and it appears that water column heterotrophic processes are largely dependent upon seagrass exudation.

Carbon flow through bacterioplankton can be evaluated only if both growth and respiration are known. Measurements of community and bacterial respiration (oxygen consumption) and bacterial production ([3H]leucine incorporation) were made in highly productive shelf and less productive slope waters in the northern Gulf of Mexico. Rates of bacterial production and community respiration, as well as bacterial abundance and dissolved organic C concentrations, declined with depth at both locations. Water-column bacterial production ranged from 0.1 to 3.1 hg C liter- I h-l, and community respiration rates ranged from 0.05 to 0.45 PM 0, h-l. In comparison to the slope, the shelf was characterized by 2-fold higher bacterial abundance and bacterial production but similar community respiration rates. Estimated production per bacterium values decreased logarithmically with depth (1.4-O. 15 fg C cell-’ h-l) and were similar at both locations. Estimated respiration per bacterium values for the surface water ranged from 0.10 to 0.36 fmol O2 cell-l h-l and were higher on the slope than the more densely populated shelf. A selective suppression of bacterial respiration occurred under both natural and experimentally (tangentialflow ultrafiltration) enhanced bacterial abundances. Measured growth efficiencies fell between 26 and 55%, with higher efficiencies occurring on the shelf (50%) than the slope (26%). Bacterioplankton at the less productive slope station processed a larger daily share of local primary production (69%) than bacteria at the highly productive shelf station (25%). Metabolism represents the sum of two fundamental and complementary life processes: growth and respiration. While there is a considerable body of information on marine productivity (including growth rates for autotrophs and heterotrophs), there is a paucity of data regarding respiration. This disparity is glaring considering that, unlike photosynthesis, respiration is common to all aerobic organisms and occurs at all depths in the ocean. According to Williams (1984a, p. 357), who reviewed the available information before 1984, “there are remarkably few direct measurements of marine respiration.” This general paucity of marine respiration data continues

The Arctic system is moving toward a new state that falls outside the envelope of glacialinterglacial fl uctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any, processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this “super interglacial” state.