Christopher Garside

A nitrate-dependent Synechococcus bloom in surface Sargasso Sea water

Considerable debate exists concerning the magnitude of oceanic primary production, its rate of transfer to other trophic levels and turnover times of carbon and nitrogen1–5. In nitrogen-limited ocean systems, episodic increases in nitrate concentrations can support a significant fraction of annual phytoplankton production5. Yet little information is available regarding the distribution of nitrate in seasonally stratified oceanic surface waters, because concentrations are below the 0.03 μM detection limit of colorimetric methods6. We present the first evidence that high surface productivity in stratified Sargasso Sea water was supported by nanomolar changes in nitrate concentrations. This change was stoichiometrically consistent with the subsequent cellular production of a cyanobacterial (Synechococcus) bloom. Initially, cellular phycoerythrin and chlorophyll pigments increased, after which growth was enhanced to near maximum rates, and grazing was closely coupled to production. These observations suggest that Synechococcus occupies an important trophic position in the transfer of new nitrogen into the oceanic food web.

Biomass and nitrogen uptake by heterotrophic bacteria during the spring phytoplankton bloom in the North Atlantic Ocean

N biomass and N uptake by heterotrophic bacteria and phytoplankton were examined during the North Atlantic spring bloom (May 1989). Phytoplankton N had reached its zenith by the time the authors arrived at the station and increased only slightly during the first week of the study, while particulate nitrogen (PN) nearly doubled (increase of 450 mg N m−2), apparently due to the growth of heterotrophic organisms other than bacteria. During the second week, bacterial N doubled, increasing by 280 mg N m−2. These increases in biomass N and total PN were large compared to NO3− uptake and to N export from the upper 100 m. To examine the role of bacteria in greater detail, we estimated total N uptake and the uptake of free amino acids (DFAA), NH4+, and NO3−. The ratio of total N uptake by bacteria to total N uptake by phytoplankton varied greatly and was often high (0.2–0.9). When uptake was corrected for phytoplankton activity, heterotrophic bacteria accounted for 22–39% and 4–14% of NH4+ and NO3− uptake. DFAA, NH4+ and NO3− supplied 12–34%, 19–29% and 2–8% respectively, of the nitrogen needed for bacterial production. Preference indices further indicated that NO3− was the N source least preferred by heterotrophic bacteria. Nitrogen uptake fuelled a buildup of bacterial biomass and other suspended material that needs to be considered in reconciling new production and N export from the upper layers of the ocean during the spring bloom.

Distribution and composition of biogenic particulate matter in a Gulf Stream warm-core ring

Abstract We have characterized the biogenic particle field in Gulf Stream warm-core ring 82-B in June of 1982. Our observations include chlorophyll α and phaeopigments, ATP, particulate organic carbon and nitrogen, biogenic silica, total particle volume and size distribution, bacterial abundance and picoplankton biomass, and the abundances of diatoms, dinoflagellates and coccolithophorids in the upper 700 m along two transects of the ring. A distinct maximum in phytoplankton biomass occurred within the thermocline (20 to 40 m) at the rings center of rotation. This maximum had not been present in late April, and apparently developed within 3 to 4 weeks after the ring stratified in mid May. It exhibited a high degree of axial symmetry about the center of the ring, with biomass decreasing outward from ring center. A second biomass maximum associated with shelf surface water was being entrained into the anticyclonic flow field of the ring 60 to 70 km from its center. Maximum chlorophyll α and ATP concentrations observed in the two biomass maxima were similar, but the ring-center maximum was 2 to 10 times richer in particulate carbon, biogenic silica, particles > 5 μm in diameter, dinoflagellates, diatoms and estimated organic detritus, while the entrained shelf water had 2 to 5 times greater abundances of unicellular monads. Heterotrophic bacterial abundance and biomass, and the abundance of cocoid cyanobacteria were maximal in the region of highest rotational velocity 40 to 50 km from ring center. In this region the abundances of bacteria and cyanobacteria were 2 to 5 times as great as at the center of the ring. Two possible mechanisms can explain the development of an axially symmetrical maximum in biogenic particulate matter in the center of a warm-core ring: concentration by the flow field and in situ growth. Our data on the distribution and composition of biogenic material in ring 82-B indicate a greater likehood that this particular ring-center maximum developed in situ .

COASTAL SOURCE WATERS AND THEIR ROLE AS A NITROGEN SOURCE FOR PRIMARY PRODUCTION IN AN ESTUARY IN MAINE

: The Sheepscot Estuary, located in central Maine, has been the subject of a survey study comprising eighteen cruises from July 1976. The lower 25 kilometers of the estuary had a seasonally varying salinity range from 20 to 26 ‰ at Wiscasset to 33 ‰ at the mouth and had a two-layered “structure” defined by a halocline and a seasonally coincident thermocline. The seasonal temperature range was from 0 to 15°C. The 1% light level was between 5 and 15 m, and production versus intensity curves show no light limitation. Inorganic nitrogen concentrations were high (<3 μg at N 1-1) throughout the year, and during the summer months primary production and chlorophyll a concentrations were high compared with adjacent ocean waters; ammonia and nitrate contributed to the inorganic nitrogen pool in the surface layer. The ultimate source of nitrogen for this estuary is the inflow of deep water from the Gulf of Maine; the drainage basin has neither large population centers nor a large area of agricultural land. That the phytoplankton population did not consume all the available inorganic nitrogen, phosphate, or silicate indicated that population size was limited by some “cropping factor.” High ammonia concentrations and regeneration rates suggest a probable role of herbivorous filter feeders in cropping.