Douglas C. Biggs

Utilization of ammonium and nitrate during austral summer in the Scotia Sea

Abstract The nitrogenous nutrition of the phytoplankton in the Scotia Sea was investigated with 15 N tracer techniques during the austral summer of 1979. On a regional scale, ambient NH 4 + concentrations in the upper hundred meters were variable (0.1 to 2.5 μg-at. 1 −1 ) and one or more orders of magnitude lower than ambient NO 3 − concentrations (17 to 31 μg-at. 1 −1 ). Despite the abundance of NO 3 − , late summer phytoplankton showed a consistent preference for NH 4 + utilization relative to NO 3 − . This was determined by use of a relative preference index and the patterns are similar to those of other marine, estuarine, and fresh waters to which it has been applied. The ratio of NO 3 − utilization to total nitrogen utilization indicated that when ambient NH 4 + concentrations were > 1.0 μg-at. 1 −1 , NO 3 − accounted for s40% of the total nitrogen utilized. Thus, even when NO 3 − is present in quantities that approach the upper limits for near-surface open-ocean waters, the processes that recycle NH 4 + locally can make a major contribution to the nutrition of the phytoplankton. Data for NO 3 − uptake from the 0.1% light level at several of the southernmost stations appeared anomalous. The observed values were higher than would have been expected solely from phytoplakton uptake.

Phytoplankton standing crop, primary productivity, and near-surface nitrogenous nutrient fields in the Ross Sea, Antarctica

Abstract In austral summer 1978 a study was made of the phytoplankton standing crop, primary productivity, and nutrient chemistry of the waters along a north-south transect between New Zealand and the Ross Ice Shelf and along two east-west transects in the Ross Sea. The area was characterized by a high degree of spatial biological variability, with pronounced differences in phytoplankton biomass, primary productivity, and near-surface nitrate and ammonium fields between the northern, central, and southern regions of the Ross Sea. An extensive bloom of the colonial flagelate Phaeocystis pouchetiii (Prymnesiophyceae) was encountered along the barrier edge of the Ross Ice Shelf and extended to depths of 100 to 150 m at some stations. Chlorophyll a throughout the euphotic zone along the ice shelf barrier edge averaged > 1 mg m−3, and primary productivity locally reached nearly 1 g C m−2 day−1 there. In contrast, both chlorophyll a concentration and rates of 14C uptake averaged two- to four-fold less some 200 km offshore of the Ross Ice Shelf, where P. pouchetii generally was rare. Where P. pouchetii was abundant, more than 25% of the water column primary production occurred at depths below the 1% light penetration level. Near-surface nitrate concentrations were about a third lower at the stations where P. pouchetii was abundant than at those with few or no P. pouchetii. Regionally there was a strong negative correlation between NH4+ and chlorophyll a, suggesting that the biogenic uptake of NH4+ by the phytoplankton controlled the distribution of NH4+ within the euphotic zone.

Comparison of ship and satellite bio-optical measurements on the continental margin of the NE Gulf of Mexico

Surface flow-through data were collected three times per year from November 1997 to August 1999 from the continental margin of the NE Gulf of Mexico on hydrographic cruises that surveyed 11 cross-margin transects from the 10 m to the 1000 m isobath. These data, calibrated using standard water sampling and filtration methods, show seasonal and spatial patterns in riverine outflow and bio-optical properties. The ship data are compared with satellite estimates of sea surface temperature (Advanced Very High Resolution Radiometer (AVHRR), National Oceanic & Atmospheric Administration (NOAA)) and ocean colour (Sea-viewing-Wide-Field-of-View Sensor (SeaWiFS), Orbview II). For autumn-winter cruises, the mean relative error of satellite estimates were < - 35% for chlorophyll and coloured dissolved organic matter (CDOM) and the mean error was < - 0.5°C for sea surface temperature. For spring and summer cruises, the error was larger but still < - 50% and < - 1°C. The flow-through method has been found effective in estimating large-scale CDOM absorption. Correlation between CDOM and sea surface salinity was consistently high (R L m 20.85 or higher for > 7500 data points), suggesting that CDOM is a better indicator than chlorophyll to trace sea surface salinity dilution by river plumes in remotely sensed ocean colour data.

Ship and satellite observations of chlorophyll stocks in interacting cyclone‐anticyclone eddy pairs in the western Gulf of Mexico

When anticyclonic eddies shed by the Loop Current of the Gulf of Mexico reach the western margin of the gulf, they influence the surface circulation over the continental slope and rise. Of particular interest is the generation of cyclone (cold-core)-anticyclone (warm-core) pairs when aging Loop Current eddies interact with the continental margin. In this paper we describe the physical and biological characteristics of these cyclone-anticyclone pairs. Our objective was to determine how eddy pairs affect the distribution of phytoplankton in the region and how satellite ocean color measurements are applicable to tracing of the eddies. We present shipboard data collected between 1980 and 1982 on the hydrography, chlorophyll stocks, and nutrient concentrations of eddy pairs in the western Gulf of Mexico and compare these data with coastal zone color scanner (CZCS) images collected during the time frame of the cruises. Surface pigment concentrations followed a seasonal cycle, with low concentrations (0.05–0.1 mg m−3) found within cyclones and anticyclones from April through early November and higher concentrations (>0.1 mg m−3) found in the winter. CZCS pigment concentrations were locally high in the flow confluence of cyclone-anticyclone pairs. The CZCS imagery shows that some cyclone-anticyclone geometries transport high-chlorophyll shelf water seaward at least 100–200 km off-shelf.

The Great Flood of summer 1993: Mississippi River discharge studied

In the summer of 1993, the Mississippi River basin in the midwestern United States experienced anomalously high rainfall. Record flooding resulted from an abnormally persistent atmospheric weather pattern consisting of a quasi-stationary jet stream positioned over the central part of the nation, where moist, unstable air flowing north from the Gulf of Mexico converged with unseasonably cool, dry air moving south from Canada. In concert with the persistent weather pattern over the United States, highly anomalous circulation patterns were observed over much of the Northern Hemisphere [Richards, 1994]. The rainfall anomalies over the central United States produced abnormally high river discharges along the Louisiana coastline from the Mississippi and Atchafalaya Rivers during July and August, traditionally months of low river discharge. Some of the river water discharged into the northern Gulf of Mexico reached the Straits of Florida by September 1993 [Lee et al., 1994].

Patterns of distribution of sound‐scattering zooplankton in warm‐ and cold‐core eddies in the Gulf of Mexico, from a narrowband acoustic Doppler current profiler survey

The acoustic backscatter intensity (ABI) reflected from epipelagic zooplankton communities in the central Gulf of Mexico was measured during June 1995 with a vessel-mounted, narrowband-153-kHz acoustic Doppler current profiler (ADCP). Horizontal and vertical variations in ABI were documented in three kinds of mesoscale hydrographic features commonly found in the Gulf of Mexico: the warm-core Loop Current (LC), a warm-core Loop Current eddy (LCE), and a cold-core region that separated the two warm-core features. Since new nitrogen domes close to surface waters in cold-core features whereas surface waters of warm-core features are nutrient depleted, the cold-core region was expected to have higher biological stocks as a result of locally higher primary production. Both ABI and net tow data confirmed that the cold-core region was in fact a zone of local aggregation of zooplankton and micronekton. During both day and night, ABI when integrated for the upper 50 and 100 m in the cold-core region was significantly greater than in the LC or in the LCE, and ABI was positively correlated with standing stock biomass taken by the net tows. Further investigations into the biological differences between Gulf of Mexico divergence and convergence regimes are warranted, and the ADCP will be a useful tool for examination of the distribution of sound scatterers in such features.

Cleavage of a Gulf of Mexico loop current eddy by a deep water cyclone

Eddy Triton, an anticyclonic eddy shed by the Loop Current in late June 1991, drifted SW across the central Gulf of Mexico in the first 6 months of 1992, along the “southern” of the three characteristic drift paths described by Vukovich and Crissman [1986] from their analyses of 13 years of advanced very high resolution radiometer sea surface temperature data. An expendable bathythermograph (XBT) and conductivity-temperature-depth (CTD) transect of opportunity through Triton at eddy age 7 months in January 1992 found that eddy interior stood 23 dyn. cm higher than periphery; this gradient drove an anticyclonic swirl transport of 9–10 Sv relative to 800 dbar. At eddy age 9–10 months and while this eddy was in deep water near 94°W, it interacted with a mesoscale cyclonic circulation and was cleaved into two parts. The major (greater dynamic centimeters) piece drifted NW to end up in the “eddy graveyard” in the NW corner of the gulf, while the minor piece drifted SW and reached the continental margin of the western gulf off Tuxpan. This southern piece of Eddy Triton then turned north to follow the 2000-m isobath to about 24°N and later coalesced with what remained of the major fragment. Because Eddy Tritons cleavage took place just before the start of marine mammals (GulfCet) and Louisiana-Texas physical oceanography (LATEX) field programs, the closely spaced CTD, XBT, and air dropped XBT (AXBT) data that were gathered on the continental margin north of 26°N in support of these programs allow a detailed look at the northern margin of the larger fragment of this eddy. Supporting data from the space-borne altimeters on ERS 1 and TOPEX/POSEIDON allow us to track both pieces of Eddy Triton in the western Gulf and follow their spin down in dynamic height, coalescence, and ultimate entrainment in January 1993 into another anticyclonic eddy (Eddy U).

RESPIRATION AND EXCRETION WITHIN A MASS AGGREGATION OF EUPHAUSIA SUPERBA: IMPLICATIONS FOR KRILL DISTRIBUTION

ABSTRACT The oceanographic environment within a mass aggregation of krill encountered north of Elephant Island in March 1981 was sampled with a submersible pump/CTD combination. Estimates of acoustic biomass made concurrently at 50 kHz were used to infer krill population density. At depths between 50-100 m, differences in dissolved oxygen and ammonium ion between two pumped hydrocasts made within the krill school and two made at adjacent sites but outside the school averaged -0.15 ml oxygen 1-1 and +0.75 μg-atoms NH4+ 1-1. Using previously reported data on the respiration and excretion rates of individual krill, we demonstrate that these differences could have been generated in 16-18 h. From information on the mean velocity of near-surface currents in the vicinity of Elephant Island, we speculate on how population density of krill schools may be constrained in time and space.

The role of sulfate exclusion in buoyancy maintenance by siphonophores and other oceanic gelatinous zooplankton

1. 1. In order to determine if a sulfate exclusion buoyancy system is operating in siphonophores, representative species were collected and analyzed for their sulfate content so that a calculated lift could be determined. 2. 2. All the gelatinous Zooplankton examined had sulfate concentrations that were significantly (P < 0.005) lower than local seawater samples. 3. 3. Protein concentrations were determined in a few species and compared with the calculated lifts in order to quantitate the importance of this kind of buoyancy compensation. 4. 4. Buoyancy maintenance mediated by the partial exclusion of sulfate appears to be important in offsetting the protein-induced negative buoyancy of gelatinous Zooplankton. 5. 5. The ecological importance of this type of buoyancy system is discussed in light of the behavior patterns of these animals.

The influence of advection on the spatial variability of nutrient concentrations on the Texas-Louisiana continental shelf

Abstract Water column nutrient concentrations are presented for winter 1989 over the continental margin of the northwest Gulf of Mexico. Three physical processes advect waters of different nutrient concentrations onto the Texas-Louisiana shelf. These advection processes: (1) river discharge; (2) bay discharge; and (3) shelf-edge upwelling, influence the large-scale spatial variations of nutrient concentrations observed over the shelf. On the inner shelf near Atchafalaya Bay, river discharge injects high concentrations of silicate, phosphate, nitrate and nitrite into near surface water. Farther west, near Galveston and Port Aransas, inner shelf waters have high silicate and phosphate concentrations due to nutrient enrichment in the discharges from Galveston Bay and Corpus Christi Bay. Finally, upwelling at the shelf edge can introduce high nutrient midwater near the base of the mixed layer over the outer shelf. This upwelled water usually has higher silicate, phosphate and nitrate concentrations than those in the overlying surface water. However, if upwelled from below the nitrite maximum, the upwelled water can have quite low nitrite concentrations. The shelf edge upwelling is a result of bottom Ekman upwelling caused by a northeastward current on the outer shelf. At the shelf edge off Galveston the data show that upwelling was not occurring at the time of the study but dissolved oxygen saturation levels and nutrient concentrations show that it had occurred.