Abstract:
The Ross Sea, a marginal Antarctic sea south of New Zealand, has been studied since the days of the Scott/Amundsen expeditions. It is the site of extensive penguin, bird and mammal colonies, and also the home of two significant scientific bases, McMurdo Station (US) and Stazione Baia de Terra Nova (Italy). Its oceanography is relatively well known, including the ... physical oceanography and hydrography, nutrient concentrations (nitrate and silicic acid), and phytoplankton biomass and species composition. However, marked variations in these variables occur spatially and temporally. Indeed, the variations with time are the greatest factor in the Ross Sea habitat. For example, throughout much of the winter the Ross Sea is ice covered, and no incident radiation is available to drive photosynthesis. In contrast, during the summer months the Ross Sea is ice-free, and large accumulations of phytoplankton biomass occur.
In addition to these seasonal changes, variations among years occur in all oceanographic variables, whether they be current velocities and directions, ice concentration, winds, or phytoplankton productivity. The causes and consequences of these interannual variations, however, are poorly known. They likely have both local and remote drivers; that is, ice concentrations are likely controlled by the Antarctic circumpolar wave observed by White et al. (1999), as well as by basin-wide changes induced by El Nino. Regardless of the causes, the degree of variation among years in biological variables is unknown, and this is what IVARS seeks to explore. Ultimately we hope to understand not only the causes, but also the consequences to the food web of the region.
How will we assess the interannual differences in biological processes? We first will collect as much historical data from the region from various cruises conducted in the past three decades and generate a climatology or long-term mean of both nutrients (nitrate and silicic acid) and chlorophyll concentrations. This will enable us to compare results collected during our cruises to assess the degree of variation from those observed earlier. The second approach is to collect, from a pre-defined sampling grid, nutrient and phytoplankton information using standard ship-board sampling procedures. Two cruises per year are planned: one in mid- to late-December (the period of maximum productivity and phytoplankton biomass), and one in mid-February (the end of the growing season). A total of five field seasons will be undertaken, with the first in 2001-2002 having been already completed. By comparing the nitrate data from each cruise to that of "pre-bloom" water (found prior to phytoplankton growth early in the growing season), seasonal productivity can be calculated and compared with that of previous years, hence quantifying the interannual variations in seasonal productivity.
The third component of IVARS is the deployment of two moorings, each with an elaborate suite of instruments designed to investigate the proximate causes of the limitation of phytoplankton growth. The locations of the moorings are placed where historically there are two different phytoplankton assemblages. The first (Calinectes), located north of Ross Island, is normally dominated by diatoms, whereas the second (Xiphias) is dominated by the haptophyte Phaeocystis antarctica. These phytoplankton types have extremely different roles in the local food web and biogeochemical cycles. Each mooring will have the following instruments on it: a fast repetition rate fluorometer (FRRF), a nitrate analyzer, a silicic acid analyzer, a whole water sampler, two additional fluorometers, a sediment trap, thermistors, a CTD, and current meters. The aim of each mooring is to provide continuous sampling of the surface layer properties to get a closely spaced measurement of the variations within each growing season. In addition, the FRRF gives an assessment of the degree of limitation by inorganic nutrients. It has been shown that iron becomes limiting in the austral summer (e.g., Olson et al., 2000), although the extent and onset of this limitation is unknown. Using the FRRF will give a sensitive measure of the degree of iron limitation through time. The nitrate data will allow for continuous estimates of seasonal production (and its short-term variations), and the silicic acid measurements will allow for the estimation of diatom productivity (as Phaeocystis antarctica does not use silica). Diatoms are often considered to be more strongly iron limited than other species, but it is uncertain if this is true in the Ross Sea. Furthermore, the ratio of nitrate:silicate uptake is another indicator of iron stress in diatoms, and will provide a separate estimate from the bulk community measurements provided by the FRRF. Finally, the sediment trap will allow estimates of the losses of organic material through the water column, which can be compared to productivity and biomass estimates to construct a one-dimensional budget of nitrogen (Smith and Asper, 2000).
Using this three-tiered approach, we hope to accurately assess the variations in several important variables of the Ross Sea, and their relationships to the food web of the region as well as to large-scale forcing. Hopefully this record will also allow for an assessment of subtle changes that might occur in future years in response to global climate change. Such changes have already been observed in the hydrographic conditions of the region (e.g., Jacobs et al., 2002), but this would be the first attempt to relate biological responses of the lower portion of the food web to large-scale changes in surface layer properties that occur as a result of climate changes.
