International Variability in the Ross Sea

Project Description
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.