Creighton D. Wirick

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.

Natural Versus Anthropogenic Factors Affecting Low-Level Cloud Albedo over the North Atlantic.

Cloud albedo plays a key role in regulating Earths climate. Cloud albedo depends on column-integrated liquid water content and the density of cloud condensation nuclei, which consists primarily of submicrometer-sized aerosol sulfate particles. A comparison of two independent satellite data sets suggests that, although anthropogenic sulfate emissions may enhance cloud albedo immediately adjacent to the east coast of the United States, over the central North Atlantic Ocean the variability in albedo can be largely accounted for by natural marine and atmospheric processes that probably have remained relatively constant since the beginning of the industrial revolution.

High frequency sampling of the 1984 spring bloom within the mid-Atlantic Bight: Synoptic shipboard, aircraft, and in situ perspectives of the SEEP-I experiment

Abstract Moorings of current meters, thermistors, transmissometers, and fluorometers on the Mid-Atlantic shelf, south of Long Island, suggest a seaward export of perhaps 0.20 mg Chl m −3 day −1 at depths of 75–81 m, between the 80- and 120-m isobaths during February–April 1984. Using a C/Chl ratio of 45/1, such a horizontal loss of algal carbon over the lower third of the water column would be 19–67% of the March–April 1984 primary production within the overlying euphotic zone. This possible physical carbon loss is similar to daily grazing losses to zooplankton of 32–40% of the algal fixation of carbon. Metabolic demands of the benthos could be met by just the estimated fecal pellet flux, without direct consumption of the remaining algal carbon. Similarly bacterioplankton metabolism could be fueled by excretory release of dissolved organic matter during photosynthesis, rather than by consumption of particulate carbon. Sediment traps tethered 10 and 70 m off the bottom at the 120-m isobath caught as much as 0.10–0.16 g C m −2 during March–April 1984. This presumed vertical flux is about one-third to one-half of the horizontal flux of 0.30 g C m −2 day −1 estimated over the lower 33 m of the water column at the 100-m isobath. These estimates suggest that ∼50% of the carbon export at the shelf-break might be derived from the adjacent overlying water column, with the remainder from lateral injections of near-bottom particles originating on the inner shelf.

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.

Modeling vertical oxygen and carbon flux during stratified spring and summer conditions on the continental shelf, Middle Atlantic Bight, eastern U.S.A.

The Shelf Edge Exchange Processes II (SEEP-II) program was designed to examine the potential for export of organic carbon from the continental shelf to the deeper ocean. In the Middle Atlantic Bight of the east coast, U.S.A., a “cold pool” of relict winter water is isolated by the development of a strong seasonal thermocline on the shelf. Oxygen concentrations were monitored in and above the cold pool from March 1988 to May 1989, with electrodes moored at 19 and 38 m at a 42-m station off the Delmarva Peninsula, eastern U.S.A. An oxygen-flux simulation model was constructed to describe long-term changes in oxygen concentration and saturation. The model utilized biological rate and biomass measurements obtained at the mooring location during cruises. Vertical eddy diffusion was constrained by comparison with the redistribution of chlorofluorocarbons and heat after stratification, and by sensitivity analyses. Model predictions of the average daily change in oxygen concentration and saturation at 38 m were in good agreement with average changes recorded by moored oxygen sensors, when biological generation of oxygen was approximately equal to the thermotrophic consumption. Strong, but transient, fluctuations concentration and saturation were clearly associated with specific advective events, and had little lasting impact on the overall long-term trends. Consequently, model parameters derived from intermittent, cruise-based observations yielded satisfactory predictions of long-term trends. A carbon budget was constructed for the stratified summer period from data that largely overlapped with those used for the oxygen model. The continental shelf ecosystem operates in approximate balance during the summer, with a potential export of no more than 4% of primary production.

Observations of chlorophyll concentrations off Long Island from a moored in situ fluorometer

Abstract A fluorometer fitted with high-pressure cuvettes was placed at 10.5-m depth as part of a moored telemetry system off Long Island in August and September 1977. A nearby array with electromagnetic current meters, conductivity and temperature sensors (∼4, 8, 16, and 25 m) collected simultaneous data while shipboard measurements of in situ fluorescence and extracted chlorophyll were used for calibration. The chlorophyll concentrations from the mooring varied between 1 and 5 μg l −1 during the 10- and 11-day mooring periods, the dominant cause being tidal oscillations. Diel changes, probably of biological origin, were also observed.

Exchange of phytoplankton across the continental shelf-slope boundary of the Middle Atlantic Bight during spring 1988

A moored array of current meters, transmissometers and fluorometers was deployed off the Delmarva Peninsula, United States, from 8 February to 9 June 1988 to measure the exchange of particles, phytoplankton and water between the continental shelf and the slope sea. The measurements were made at the 90-m isobath and near the shelf-slope front, which is the natural physical boundary between the shelf- and slope-water. The highest standing stocks of phytoplankton (>4 mg Chl a l−1) occurred during April and May directly above the shelf-slope front. Although the shelf-slope front and associated phytoplankton populations made large cross-shelf excursions (>40 km), the vertically averaged advective transport of phytoplankton was onshore at the 90-m isobath. Compared with the estimated primary production on the inner shelf, the advective transport across the 90-m isobath was −2.60% ± 5.52% of the spring production, with the negative values indicating import. These calculations suggest that primary production occurring landward of the 90-m isobath is not exported laterally, as phytoplankton or phytodetritus, to the ocean basin. This places a limit on the possible export from the shelf, but does not exclude shelf export because roughly half the volume of shelf water from Cape Cod to Cape Hatteras lies in a thin surface wedge outside the 100-m isobath [Wright and Parker (1976) Limnology and Oceanography, 21, 563–571]. Carbon and nitrogen budgets of the Middle Atlantic Bight should acknowledge the limited exchange of phytoplankton between the inner and outer portions of the shelf.