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We investigated the rates of the main microbiological processes (primary production, aerobic and anaerobic carbon degradation) and transport phenomena in an intertidal sand plate with a combination of in situ microsensor measurements and incubations. The sand was coarse, organically poor (0.6-1 mg of total organic carbon per gram dry weight of sediment), and highly permeable to water flow (k ϭ 1.5-7 ϫ 10 Ϫ11 m 2 ). Aerobic respiration rates ranged from 105 to 175 mmol m Ϫ2 d Ϫ1, sulfate reduction rates from 0.08 to 13.7 mmol m Ϫ2 d Ϫ1, and net primary production Ͻ35 mmol m Ϫ2 d Ϫ1 . In situ microsensor measurements showed large changes in oxygen and sulfide concentrations in the top 10 cm, depending on tides and waves. The observed dynamics and high aerobic degradation rates imply that pressure gradients drive advective influx of oxygen and organic material from the water column into the sediments. Our results show that intertidal porous sand plates have high aerobic degradation rates, despite having an organic matter content that is one to two orders of magnitude lower than that of fine-grained deposits with similar decomposition rates.

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Large-scale spatial heterogeneity of macrofaunal and microbial commun~ties was examined on the continental slope off North and South Carolina, USA, by comparing 3 sites, separated by 130 to 150 km and all at 850 m water depth. Significant variation was found among macrofaunal assen~blages at all 3 sites, and between microbial counts at 2 sites. We investigated the hypothesis that 100 km scale heterogeneity was driven by variation in organic C flux to the sea floor. The northernmost site (Site 111, off Cape Hatteras, NC) was found to have macrofaunal abundances (> 55 000 m-2) higher than any previously recorded from this depth, and significantly hlgher than those at Site I1 (off Cape Lookout, NC) (21 319 m-2) or Site I (off Charleston, SC) (9438 m-2). Trends in macrofaunal abundance did not follow those of sediment TOC (total organic carbon), but agreed well with estimates of total carbon flux for the 3 sites. Mixing coefficients determined from profiles of naturally occurring 234Th (half life 24 d ) indicate that the sediments at Site I11 are mixed 2 to 6 times faster than at the other 2 sites, which is consistent with the trends in macrofaunal abundance and biomass. Using ',C-based sedimentation rates and sediment carbon content, we estimated carbon flux to be 0.6. 20 and >?0 g C m-2 yr-l at Sites I, I1 and 111, respectively. Inventories of 2 3 4~h and downcore concentration profiles of dissolved SO,,' , ZC02 and CH, within the sediment provided evidence that the flux of metabolizable carbon was greater at Site I11 than at the other sites. Polychaetes, which comprised 43, 74 and 65 % of the fauna at Sites I, I1 and Ill, respectively, exhibited lower diversity, higher dominance, and a completely different species composition at Site I11 than at the other 2 sites. Scalibregma inflatum and Aricidea quadrilobata comprised 33 % of total macrofauna at Site 111, but were absent at Sites I and 11. The species composition, high dominance, and prevalence of juveniles among polychaetes at Site I11 is suggestive of a response to organic enrichment. Enrichment of the Site I11 benthos is attributed to physical oceanographic and geophysical causes, including Gulf Stream-induced upwelling, a confluence of currents focused by bottom topography, and lateral inputs resulting from mass wasting processes. Despite significant differences in macrofaunal abundance, Sites I and I1 exhibited considerable overlap in microbial counts, polychaete species composition, dominance and diversity patterns.

Although open ocean time-series sites have been areas of microbial research for years, relatively little is known about the population dynamics of bacterioplankton communities in the coastal ocean on kilometer spatial and seasonal temporal scales. To gain a better understanding of microbial community variability, monthly samples of bacterial biomass were collected in 1995-1996 along a 34-km transect near the Long-Term Ecosystem Observatory (LEO-15) off the New Jersey coast. Surface and bottom sampling was performed at seven stations along a transect line with depths ranging from 1 to 35 m (n=178). Microbial populations were fingerprinted using ribosomal 16S rRNA genes and terminal restriction fragment length polymorphism analysis. Results from cluster analysis revealed distinct temporal patterns among the bacterioplankton communities in the Mid-Atlantic Bight rather than grouping by sample location or depth. Principal components analysis models supported the temporal patterns. In addition, partial least squares regression modeling could not discern a significant correlation from traditional oceanographic physical and phytoplankton nutrient parameters on overall bacterial community variability patterns at LEO-15. These results suggest factors not traditionally measured during oceanographic studies are structuring coastal microbial communities.

Submillimeter depth distributions of total dissolved inorganic carbon (DIC) were derived from pH and PcO, profiles measured with microelectrodes in an organic-rich, laboratory-maintained sediment. The DTC profiles were used to calculate diffusive fluxes of DIC across the sediment-water interface. In two experiments, the calculated diffusive fluxes fell within 250% of the total flux of DIC determined by core incubation. An assessment of errors suggests that the microelectrode-derived estimates are not significantly different from measured total DIC fluxes (P = 0.05). It is concluded, therefore, that pH and PcO, microelectrode measurements can be paired to determine finescale pore-water DIC profiles and DIC diffusive fluxes. Problems will arise only in situations in which pH and P,,, gradients are extremely steep or spatially heterogeneous; this is because these conditions can cause mismatching of pH and PcoZ measurements or CO, system disequilibrium.Microelectrodes have been used to measure in situ oxygen and pH pore-water profiles in a wide variety of marine environments in recent years. These new data are being used to calculate diffusive benthic fluxes of 0, (Reimers et al. 1986(Reimers et al. , 1992Archer and Devol 1992;Glud et al. 1994a) and applied as constraints for diagenetic models that estimate rates of organic matter degradation and calcium carbonate dissolution (Archer et al. 1989;Cai et al. 1995; Hales and Emerson 1996, 1997). However, one shortcoming of these studies is that 0, and pH data need to be accompanied by an additional measurable parameter of the CO, system to fully describe this system in pore solutions. P,.,, is the only other parameter of the CO, system that is readily measurable by microelectrodes. Therefore, Cai and Reimers (1993) and Cai et al. (1995) attempted to use a potentiometric Pco2 microelectrode to measure the first in situ PVo2 profiles in marine sediments. These PcO, profiles, however, did not compare well with models constrained by the pH and oxygen microelectrode data at several key depths within the sediment. Cai and Reimers (1993) and Cai et al. (1995) attributed these deviations to the slow response characteristics of the potentiometric P,,, sensor.The goal of this study was to verify that potentiometric PH and PO, microelectrodes, given adequate response times, produce pH and Pcoz data that can be used to derive (1) reliable total dissolved inorganic carbon (DIC) concentrations in pore waters and (2) accurate DIC fluxes across the sediment-water interface. Diffusive fluxes of DIC across the sediment-water interface were calculated using pH and Pco, microelectrode profiles and compared with total fluxes determined directly using laboratory core incubations as the primary test of microelectrode performance. It was assumed AcknowledgmentsWe thank Wei Wang for the construction of oxygen microelectrodes and assistance in the field. S. Seitzinger and G. Taghon helped improve early versions of the manuscript. We also thank B. Boudreau, B. Hales, and W. Martin for i...