Charles N. Flagg

The shelf edge exchange processes experiment, SEEP-II: an introduction to hypotheses, results and conclusions

Abstract The SEEP (Shelf Edge Exchange Processes)-II experiment was the second of two that took place in the Middle Atlantic Bight (MAB) of the eastern U.S. continental shelf and slope. The experiment included an array of 10 multi-instrumented moorings deployed for 15 months and 10 oceanographic cruises, all designed to address the problem of the fate of continental shelf particulate matter in general, and organic carbon in particular. This paper provides the setting and background for the SEEP Program, the SEEP-II experiment and an introduction to the 18 papers constituting the subject of this special volume. Because those papers lack one of a general nature on the physical oceanographic setting of the experiment, that aspect is treated in somewhat more detail here. The results of the experiment overwhelmingly show that the working hypothesis on which the SEEP Program was undertaken and sponsored by the Department of Energy is not valid. That is, there is not an export to the adjacent slope and open ocean of a large proportion of the particulate matter introduced to and biologically generated in the waters of the continental shelf; most of the biogenic particulate matter is recycled by consumption (bacterial and otherwise) and oxidation on the shelf, and only a small proportion (of order ⪡5%) is exported to the adjacent slope. The small amount that is exported appears to be deposited preferentially in the sediments of an area of the slope centered at about 1000 m, and the export and sedimentation to that depocenter appears to increase from the northern to the southern MAB.

On the use of the acoustic Doppler current profiler to measure zooplankton abundance

Abstract A pilot intercalibration study was performed to evaluate the ability of RD Instruments 307 kHz acoustic Doppler current profiler to measure zooplankton abundance. The intercalibration was between a bottom-mounted acoustic profiler deployed in 150 m of water at the edge of the New England shelf and zooplankton samples collected with a MOCNESS net system which was towed near the profiler. The results of the study showed significant correlations between backscattered signal intensity and total zooplankton volume, cross-sectional area, and dry weight. From the correlations it appears to be possible to predict zooplankton biomass to approximately ± 15 mg m −3 over the range of abundance spanned by our data with enormous resolution in time and space. However, experience with processing the acoustic data clearly indicates that significant results are only possible after careful attention to the calibration of the transducers/electronics and to the quality of the data produced by the individual profiler beams.

The fate of a spring phytoplankton bloom: export or oxidation?

Abstract The hypothesis that a large fraction of the primary production on continental shelves is exported to the deep ocean basins was investigated during the spring of 1984 off the coast of Long Island, New York. Using data collected aboard ship and from moored instruments, we constructed a carbon budget to account for the production and utilization of the spring bloom. We observed a strong cross-shelf gradient in the species composition of the phytoplankton and in chlorophyll a throughout the spring. Shelf waters were separated from slope water by a front, which was located between the 80- and 100-m isobaths. Between mid-February and mid-April chlorophyll concentrations increased from 10 μg l−1 in waters shallower than 70 m. This increase is typical of a spring bloom on the shelf. However, records from moored fluorometers near the shelf-slope front did not document an increase in chlorophyll, and it appears that very little of the bloom from the shelf was transported across the front, even during storms. Grazing pressure by zooplankton on the shelf increases throughout the spring, and copepods cropped about 34% of the daily production in early spring. About 51% of the daily production sank and formed a nepheloid layer which was oxidized on the shelf. Despite the lack of vertical density stratification on the shelf, there was a vertical gradient in dissolved oxygen. The oxidation and decomposition of organic matter regenerated nutrients that sustained high productivity until the onset of stratification in late spring. We conclude that the export of shelf-derived production to the deep ocean is small, averaging only 10–20% of the spring bloom.

Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

Abstract The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the highest and lowest concentrations of plant biomass in the world’s ocean. During the US JGOFS Process Study in the Arabian Sea (September 1994–January 1996), the absolute maximum in biomass of epipelagic zooplankton in the entire study was observed during the Southwest Monsoon season inshore of the Findlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monsoon, as was the strongest onshore–offshore gradient in biomass. Lowest biomass throughout the study was observed at the most offshore station (S15), outside the direct influence of the monsoon forcing. The greatest day/night contrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exceeding nighttime in the Southwest Monsoon season. The diel vertical migration patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of zooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppress migration. Based on the distribution of biomass, we hypothesize that inshore of the Findlater Jet, zooplankton grazing on phytoplankton is the dominant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous, conditions that result in reduction of the carbon flux mediated by the zooplankton. Predation by mesopelagic fish, primarily myctophids, may equal daily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integrated, may have evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Monsoons boost biological productivity in Arabian Sea

Monsoons over the Arabian Sea—the oceanic basin that separates the Arabian peninsula from the Indian subcontinent—follow seasonal cycles, reversing directions twice a year, in summer and winter. Rather than spreading across the expanse of the sea, the southwest (summer) monsoon is often concentrated into a jet over the central Arabian Sea. Evidence suggests that variations in wind stress force substantial upwelling in the ocean to the west of the jet, and weaker upwelling or even downwelling to the east. This upwelling provides nutrients to the euphotic zone and enhances biological productivity.

Sediment resuspension over the continental shelf east of the Delmarva Peninsula

Abstract Resuspension of sediment over the continental shelf east of the Delmarva Peninsula has been examined using records of light-beam attenuation, near-bottom current speed and surface-wave height spectra collected during 1988 and 1989. These data give evidence of a factor of three variation in the bottom stress threshold required for sediment resuspension at the outer shelf. This appears to be related to resuspension history as the largest thresholds are observed after lengthy periods without resuspension. Episodes of shelf-wide sediment resuspension are evidenced only during very intense atmospheric storms. A 7-month-long set of records from the 90 m isobath show storm-induced sediment resuspension on only three occasions. The failure of storms of modest intensity to effect resuspension at the outer shelf is largely due the decline of surface-wave currents with depth. High-frequency currents, presumably due to internal waves, are shown to be an important agent in initiating sediment motion at the shelf edge. On a number of occasions, supertidal currents pushed the near-bottom current speed measured near the seafloor at the 131 m isobath above the estimated level required for sediment resuspension. Numerous clouds of turbid water detected by the light-beam attenuation records could not be attributed to local sediment resuspension. A probability analysis indicates that some, but not all, of these could have resulted from sediment resuspension by bottom fishing.

Hydrographic and Current Observations on the Continental Slope and Shelf of the Western Equatorial Atlantic

Abstract Hydrographic and current-profiling data from December 1980 and current-meter data obtained between September 1980 and November 1981 from the continental slope and shelf of the western equatorial Atlantic between 2° and 7°N are used to describe the character of the North Brazil Coastal Current (NBCC) system. Hydrographic data and the profiling results show that the NBCC has three distinct branches during the Boreal fall. In the near-surface a strong current flows northwestward parallel to the coast at least as far as 6°N. Current speed within the near-surface NBCC over a one-year period were between 75 and 250 cm s−1, while the transport in December 1980 was between 15 and 20 (×106 m3 s−1). There was a southeastward near-surface flow farther offshore, which appears to be the return of the NBCC, the ultimate destination of which is the North Equatorial Countercurrent. Offshore of the southeastward flow was a portion of a low-salinity lens of Amazon water. The waters within and below the thermocline...

The influence of a Gulf Stream meander on the distribution of zooplankton biomass in the Slope Water, the Gulf Stream, and the Sargasso Sea, described using a shipboard acoustic Doppler current profiler

Patterns in zooplankton biomass distribution in a Gulf Stream meander were docu- mented using a ship-mounted acoustic Doppler current profiler (ADCP) in fall 1988 as part of the BIOSYNOP program. The dominant signal in biomass was the regional variation between water masses, with greatest biomass recorded in the Slope Water, intermediate biomass at the Slope Water-Gulf Stream front, and lowest biomass in the Gulf Strearn/Sargasso Sea. Biomass was more variable in the Slope Water than in the Sargasso Sea. Diel variation, a consequence of diel vertical migration, was also observed. Comprehensive maps of the surveyed region documented meander associated enhancement of zooplankton biomass. Elevated biomass was documented in the region downstream of the meander crest, where entrainment of Slope Water and convergence of flow are hypothesized to occur. The ADCP was demonstrated to be an effective means of documenting patterns in zooplankton biomass, including estimates of the variability (patchiness).

The interaction of phytoplankton, zooplankton and currents from 15 months of continuous data in the Mid-Atlantic Bight

Abstract Fifteen months of data from an acoustic Doppler current profiler (ADCP) and three fluorometers obtained during the SEEP-II program in the southern Mid-Atlantic Bight provide a unique view of the seasonal progression of zooplankton and phytoplankton biomass and their responses to physical forcing. Phytoplankton and zooplankton biomass records were highly variable with a continuum of energy at all frequencies and substantial interannual variation. The zooplankton and phytoplankton spring blooms were coincident; that is, the spring increase in zooplankton biomass did not lag behind that of phytoplankton. The spring bloms were not the dominant events of the records, however; the largest fluctuations were linked to current fluctuations, although not always in the same manner. The seasonal succession of zooplankton and phytoplankton species, together with changes in stratification, led to significant differences in the vertical distribution of biomass and its response to physical forcing. There was about a factor of two difference in the maximum zooplankton biomass between two successive springs, while there was no difference in the phytoplankton blooms. Coherence between the phytoplankton, zooplankton and currents were all low. While individual events usually could be ascribed to along- or cross-shelf advective processes, the apparent extreme variability in horizontal biological gradients makes generalizations, aside from those on seasonal time scales, impossible from a single location. Examination of spring bloom data from two successive springs shows a fairly typical relation between primary and secondary production. Thus, the net daily chlorophyll increases ranged from 2.5 to 7.5%, and zooplankton daily ingestion was estimated at 30–55% of primary production, while 38–67% of the daily production was lost to micro-zooplankton, bacteria and the benthos. Zooplankton daily lossed were estimated to be between 25 and 33%. Time-scale estimates for phytoplankton increases agree with incubation values; however, those for zooplankton were much shorter than their reproduction rate, indicating active aggregation behavior.

Wavenumber Spectrum in the Gulf Stream from Shipboard ADCP Observations and Comparison with Altimetry Measurements

The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994‐2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy, and both indicate a k 23 slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k 22 slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k 23 slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence.