Walker O. Smith

Stocks and dynamics of dissolved and particulate organic matter in the southern Ross Sea, Antarctica

Abstract Dissolved and particulate organic matter was measured during six cruises to the southern Ross Sea. The cruises were conducted during late austral winter to autumn from 1994 to 1997 and included coverage of various stages of the seasonal phytoplankton bloom. The data from the various years are compiled into a representative seasonal cycle in order to assess general patterns of dissolved organic matter (DOM) and particulate organic matter (POM) dynamics in the southern Ross Sea. Dissolved organic carbon (DOC) and particulate organic carbon (POC) were at background concentrations of approximately 42 and 3xa0μM C, respectively, during the late winter conditions in October. As the spring phytoplankton bloom progressed, organic matter increased, and by January DOC and POC reached as high as 30 and 107xa0μM C, respectively, in excess of initial wintertime conditions. Stocks and concentrations of DOC and POC returned to near background values by autumn (April). Approximately 90% of the accumulated organic matter was partitioned into POM, with modest net accumulation of DOM stocks despite large net organic matter production and the dominance of Phaeocystis antarctica. Changes in NO3 concentration from wintertime values were used to calculate the equivalent biological drawdown of dissolved inorganic carbon (DICequiv). The fraction of DICequiv drawdown resulting in net DOC production was relatively constant (ca. 11%), despite large temporal and spatial variability in DICequiv drawdown. The Cxa0:xa0N (molar ratio) of the seasonally produced DOM had a geometric mean of 6.2 and was nitrogen-rich compared to background DOM. The DOM stocks that accumulate in excess of deep refractory background stocks are often referred to as “semi-labile” DOM. The “semi-labile” pool in the Ross Sea turns over on timescales of about 6 months. As a result of the modest net DOM production and its lability, the role DOM plays in export to the deep sea is small in this region.

Relationship between dimethylsulfide and phytoplankton pigment concentrations in the Ross Sea, Antarctica

Dimethylsulfide (DMS), particulate dimethylsulfoniopropionate (DMSPp) and phytoplankton pigments were measured on three transects in the Ross Sea, Antarctica during February 1992. Although relatively low chl a levels (mean of 0.7 μg l−1) were encountered, the DMS inventory in the photic zone ranged from 81 to 3484 μol m−2. In comparison, the DMSOp standing stock was very low (range of 13–298μol m−2) and accounted for only 11% of the average integrated DMS concentration. DMS:chl a ratios (58–78 nmol μg−1) were significantly higher in waters dominated by Phaeocystis antarctica compared to diatom-dominated waters (2–12 nmol μg−1). A strong DMS gradient was observed from south to north with the highest DMS concentrations (123 nM) measured in the southern Ross Sea and the lowest (<1 nM) observed in the northern Ross Sea. In general, this gradient was similar to that observed for the Ph. antarctica pigment, 19-hexanoyloxyfucoxanthin (HEX), normalized to chl a. Areas with high fucoxanthin: chl a ratios were associated with relatively low DMS concentrations as compared to areas with high HEX: chl a ratios. There was no significant linear relationship, however, observed between DMS and HEX, chl a, or primary productivity in waters where Ph. antarctica dominated. In contrast, we observed a significant correlation between DMS and chl a in the northern Ross Sea where diatoms dominated and where the DMSPp:DMS ratio was the highest (0.43). We hypothesize that short-term DMS accumulation and the presence of a large pool of dissolved DMSP were responsible for such high DMS concentrations during post-bloom conditions in the southern and central Ross Sea.

The Oceanography and Ecology of the Ross Sea

The continental shelf of the Ross Sea exhibits substantial variations in physical forcing, ice cover, and biological processes on a variety of time and space scales. Its circulation is characterized by advective inputs from the east and exchanges with off-shelf regions via the troughs along the northern portions. Phytoplankton biomass is greater there than anywhere else in the Antarctic, although nitrate is rarely reduced to levels below 10 μmol L(-1). Overall growth is regulated by irradiance (via ice at the surface and by the depths of the mixed layers) and iron concentrations. Apex predators reach exceptional abundances, and the worlds largest colonies of Adélie and emperor penguins are found there. Krill are represented by two species (Euphausia superba near the shelf break and Euphausia crystallorophias throughout the continental shelf region). Equally important and poorly known is the Antarctic silverfish (Pleuragramma antarcticum), which is also consumed by most upper-trophic-level predators. Future changes in the Ross Sea environment will have profound and unpredictable effects on the food web.

Abundance, distribution and sinking rates of aggregates in the Ross Sea, Antarctica

The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica, during two field seasons, one in austral spring 1994 and one in early summer, 1995/96. Aggregate abundance, size and potential sinking rates were determined by photographic techniques. Measurements ofwater column parameters, including particulate organic carbon concentrations, were assessed simultaneously, as was the flux oforganic matter with floating sediment traps. The numbers ofaggregates (and to a lesser extent their size) increased with time, although there was substantial spatial variability in these variables at any point in time. Some aggregates appeared to sink extremely rapidly, and for these, our photographic measurements were able to estimate only a minimum sinking rate, which equaled 288 m d � 1 . Estimates ofaggregate organic carbon flux were compared to those determined by floating sediment traps. From these results, aggregate fluxes appear to have dominated the vertical export oforganic matter f the euphotic zone. The genesis and flux ofaggregates in the Ross Sea are the critical processes controlling the export of biogenic material from the surface layer. r 2003 Elsevier Science Ltd. All rights reserved.

Primary productivity and in situ quantum yields in the Ross Sea and Pacific Sector of the Antarctic Circumpolar Current

We determined the in situ primary productivity, particle absorption, pigment concentrations, and photosynthetic quantum yields during two Southern Ocean cruises in 1997 and 1998 in the Pacific sector of the Antarctic Circumpolar Current (ACC), and in the Ross Sea. Integrated primary productivity in the Ross Sea during the austral summer averaged 762xa0mg Cxa0m−2xa0d−1 (range: 413–1200xa0mg Cxa0m−2xa0d−1), and mean integrated chlorophyll a levels were 80.6xa0mgxa0m−2 (range: 47.8–155xa0mgxa0m−2). The mean (±standard deviation) quantum yield was 0.09 (±43%)xa0mol Cxa0Ein−1 during summer in the Ross Sea. Less than 2% of the depth variability in quantum yield in the Ross Sea was attributable to light absorption by photoprotective pigments. There was an inverse relationship found between the value of the in situ maximum quantum yield (φ[max]) and the ratio of particulate organic carbon to nitrogen (POC:PON), indicating that variations in quantum yields were controlled largely by nutrient availability. POC:PON ratios near Redfield ratio of 6.6 were found to have in situ φ(max) values near the theoretical maximum of 0.12xa0mol Cxa0Ein−1. POC-to-particulate iron values were also found to have been lower in high φ(max) regions. Integrated primary productivities and chlorophyll a levels in the ACC during the early summer were similar to those in the Ross Sea. The mean integrated primary productivity was 734xa0mg Cxa0m−2xa0d−1 (range: 606–839xa0mg Cxa0m−2xa0d−1), and the mean integrated chlorophyll equaled 56.2xa0mgxa0m−2 (range: 33.3–71.9xa0mgxa0m−2). The mean (±standard deviation) quantum yield was 0.04 (±28%)xa0mol Cxa0Ein−1 and was less than the mean value measured for the Ross Sea. As in the Ross Sea, light absorption by photoprotective pigments was responsible for <2% of the variability in quantum yield. There was no discernable relationship found between in situ φ(max) and elemental ratios of the particulates, although the sample size for the ACC was smaller (n=4) compared to the Ross Sea (n=10). Overall, the phytoplankton of the ACC required higher irradiance than the phytoplankton of the Ross Sea to saturate photochemistry.

The US Southern Ocean Joint Global Ocean Flux Study: Volume two

This is the second volume of manuscripts comprising results from the process studies in the Southern Ocean conducted by the US Joint Global Ocean Flux Study (US JGOFS). The overall objectives of the study were to better constrain the fluxes of carbon in the Southern Ocean, to identify the factors and processes that regulate the magnitude and variability of primary productivity (as well as the fate of biogenic material), and to gain a sufficient understanding to model past and present carbon fluxes with sufficient accuracy to predict their response to future global changes. Field work was conducted both on the continental shelf of the Ross Sea, and within the Antarctic Circumpolar Current (ACC) near 170°W. Results from both regions described in this volume significantly address the goals and objectives of the entire program.

US Southern Ocean JGOFS Program (AESOPS): Part III

Abstract This is the third and final volume of manuscripts comprising the results from a series of process studies conducted as part of the US Joint Global Ocean Flux Study. The objectives of this study were to provide insights into the quantities and controls of fluxes in the Southern Ocean, to elucidate the processes that control primary production in both space and time, and to use these results to model the processes that were dominant in past environments. The results demonstrate the importance of the Southern Ocean with regard to both the present and past productivity patterns and vertical fluxes from the surface layer.