Terry E. Whitledge

Production and transport of phytoplankton biomass over the continental shelf of the new york bight

Abstract Seasonal and event scale variations in the distribution and growth of phytoplankton in different hydrographic regions of the continental shelf are compared and evaluated in terms of floristic composition and the evolution of density and nutrient structure across the shelf. Annual cycles of phytoplankton biomass inshore of the 1000-m isobath are characterized by a March maximum and a July minimum. Cross-shelf biomass gradients usually increase in an offshore direction, a phenomenon that is most pronounced during March and April when biomass is high, diatoms dominate, and growth rate is light limited. This is a consequence of the combined effects of growth along the stratified side of the shelf-break front and offshore transport of biomass produced nearshore. We estimate that about 90% of the diatom biomass produced during the February to April bloom period (35% of annual production) is exported from shelf to slope water. Similar but less-pronounced gradients develop during summer due to the development of a chlorophyll maximum layer below the pycnocline where growth rate is also light limited. Production and loss are more tightly coupled under these conditions and about 9% of the biomass produced during May to October appears to be exported (5% of annual production). Export during the diatom bloom period is balanced mainly by nitrate inputs from the Gulf of Maine and adjacent slope water while summer export may be balanced by anthropogenic nitrogen input. The latter could be coupled with biomass export by ammonium remineralization and nitrification in the cold pool of the mid-shelf region. In general, export is greatest when diatoms dominate, growth is light limited, and biomass distributions are physically forced. Export is lowest when nanoplankton dominate, growth is nitrogen limited, and biomass distributions are controlled by grazing. The shelf-break front plays a key role, influencing patterns of phytoplankton growth, biomass distributions, and shelf export. During the diatom bloom period, the development of stratification in nutrient-rich offshore water between storm events results in high growth rates and biomass near the surface on the shelf side of the front. Under these conditions, biomass accumulates in the mid-shelf region on a time scale of days to weeks. Export occurs during wind events with net export from the shelf occurring on a time scale of weeks to months. Blooms also develop along the shelf side of the front during summer but below the pycnocline. Most of the summer export of biomass probably takes place here with accumulation and export occurring on a time scale of hours to days. While this export is small compared to export during the diatom bloom period, it may be critical to the prevention of anoxic events such as that of 1976.

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 inorganic nitrogen distributions and dynamics in the southeastern Bering Sea

Abstract The annual cycle of the distribution of nitrate and ammonium concentrations in the PROBES area of the southeastern Bering Sea was highly interactive with the physical and biological processes. Nitrate concentrations were replenished over the shelf during the autumn and winter at a very uniform rate until the spring bloom commenced. In the middle shelf after nitrate concentrations were depleted in the upper mixed layer during the spring bloom, large quantities of ammonium were produced in the bottom layer. Cross-shelf diffusion, vertical diffusion, vertical mixing by storms, benthic release, and possibly nitrification interacted to supply nitrogen utilized by primary production. Nitrogen uptake by photosynthetic processes as estimated by nitrate depletion and nitrogen isotope measurements agreed with primary production estimates made by radiocarbon uptake rate measurements and ∑CO 2 budgets.

Do continental shelves export organic matter

It has been suggested that biological production and consumption of organic matter is not balanced in coastal marine ecosystems1,2. If, as suggested, 90% of the phytoplankton produced during the spring bloom period were exported, excess organic carbon would be sequestered on the continental slope below the permanent thermocline. Here we summarize the shelf-edge exchange processes (SEEP) experiment, designed to test the export hypothesis. The absence of a positive imbalance in the organic carbon budget, reinforced by modest sediment deposition and biomass on the continental slope, led us to reject the concept. Only a small fraction of continental shelf phytodetritus is exported; that not consumed in the spring is for the most part used on the continental shelf during the ensuing stratified season. The original hypothesis failed to recognize the contribution of pelagic microbial consumption and the lag in coupling between seasonal production and consumption processes.

Benthic carbon budgets for the continental shelf south of New England

Abstract The total benthic standing stock in terms of organic carbon (macrofauna, meiofauna, and bacteria) on the continental margin south of New England provides little evidence that substantial organic matter escapes from the continental shelf to the upper continental slope. Measurements of the metabolism of the total benthos allowed calculation of turnover times for both the organic detritus and the total biota. The turnover time of detritus increased as grain size decreased, suggesting that fine-grained deposits contain mostly refractory, non-reactive organic compounds, especially on the deep continental slope. Turnover times of the total biota were about the same in the coarse and fine-grained shelf deposits, but a far larger fraction of the turnover was attributed to the bacteria in the fine sediments on the shallow shelf than in the coarse. On average, about 25% of the primary production appeared to be utilized by the benthos on the continental shelf in the SEEP area.

The spawning habitat of the Peruvian anchovy, Engraulis ringens

Individual larvae of the anchovy Engraulis ringens appear to survive better off the northern coast of Peru during the austral winter season when a motile algal food supply is available. Concentrations of the dinoflagellate food of the first feeding anchovy larvae may be reduced less often by wind events in the north than in the south. Under El Nino conditions, more of the adult anchovy are found south within a less favorable spawning habitat, in terms of dilution and food quality, where there is evidently less larval survival and subsequent recruitment to adult stocks. Increased mortality of the larvae during 1965, 1972, and 1976 El Ninos and of the adults during the 20-year fishery has reduced the present stock to less than 20% of its biomass in times of peak yield. Such a change in food chain dynamics is reflected in transients of oxygen demand at the decadal scale of variability. Associated with an increase in zooplankton biomass and a decline of the anchovy fishery, the sardine species of the clupeid pair off Peru may also be increasing in number, implying that natural oscillations of clupeoids may be accelerated by overfishing.

Mesoscale response of diatom populations to a wind event in the plume of the Hudson River

Abstract Effects of a southwest wind event on distributions of dissolved inorganic nitrogen and diatom biomass are described and evaluated in terms of interactions between circulation, static stability of the water column, and the suspension and growth of diatom populations. A diatom bloom, dominated by Skeletonema costatum , developed in response to upwelling as a consequence of the vertical transport of biomass from the aphotic zone (deduced from distributions and rate measurements), a decrease in sinking rate from 1.0 to 0.3 m d −1 (from sediment trap collections), and lower dilution rates (from hydrography). Carbon-specific growth rate of diatoms showed little variability based on measurements made before and during the bloom, i.e., variations in diatom production were primarily due to variable loss rates rather than to growth. The influence of diatom production associated with the coastal plume of the Hudson River (areas 1000 km 2 ) was observed to extend ca. 100 km seaward of the zone of most active production. Episodes of cross-shelf transport and onshore accumulation of phytoplankton biomass appear to alternate with periods of high surface production in the plume.

Fate of nutrient enrichment on continental shelves as indicated by the C/N content of bottom sediments

Publisher Summary This chapter discusses the fate of nutrient enrichment on continental shelves as indicated by the C/N content of bottom sediments. The ratio of carbon to nitrogen (C/N) content in most marine organisms is less than six , unlike that of land plants which use more carbohydrates for their support structures and have C/N ratios >15. Detrital particles in the sea also have a C/N ratio greater than 10 as a result of the increased recycling of nitrogen compounds compared to slower decomposition of refractory carbon compounds. Although C/N ratios of >20 can be induced in laboratory cultures of phytoplankton under nitrogen starvation, individual phytoplankton cells are probably not nitrogen limited in the ocean. At close to maximal growth rates, laboratory cultures of phytoplankton have, in fact, C/N ratios

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.

Biological Processes Associated with the Pycnocline and Surface Fronts in the Southeastern Bering Sea

Publisher Summary The factors such as light and nutrient concentration conducive to primary production processes in the ocean environment are well known in the qualitative sense but only an approximation of these essential ingredients can be quantified especially when the absolute amounts are often not important. The availability of nutrients and light are subject to a wide range of environmental factors that vary widely in both space and time. It is useful to review an oceanic area that has high primary production and a small amount of relative water movement and mixing processes that render biological production measurements difficult. The chapter reviews the persistence of surface fronts and strength of the pycnocline that is coupled to the primary production and the phytoplankton biomass.