Walker O. Smith, Jr.
Dept. of Ecology and Evolutionary Biology
University of Tennessee
Knoxville, TN 37996
The Southern Ocean, here defined as the entire region below the sub-tropical convergence, is a physically heterogeneous region that cannot be characterized simply. For example, it is partially seasonally ice-covered, which greatly influences air-sea interactions and modifies the temporal impacts and nature of vertical mixing processes. It exhibits a vast range of physical properties that vary seasonally (e.g., surface temperature, solar irradiance, wind stress), and hence is highly dynamic. Furthermore, we now are beginning to observe and understand that these large-scale variations also have counterparts on the small- and mesoscale.
Because temperature limits the absolute rates of biological reactions, biological processes in polar regions are expected to be more strongly impacted by physics than temperate or tropical waters. That also implies that the biological effects often can be more difficult to experimentally quantify than in other regions. For example, if phytoplankton doubling times are 0.15 d-1 and grazing rates of 0.06 d-1, the resultant growth rate (0.09 d-1) is difficult to measure over appropriate time scales without introducing additional potential error from other experimental artifacts (e.g., adaptation to experimental irradiance field, bottle effects, etc.). Certainly grazing rates via the use of current JGOFS dilution techniques are problematic. Therefore temperature is of primary importance to both modeling efforts and experimentalists, and its effects and problems should never be overlooked.
It also has been observed that the Q10 values at very low temperatures in the Southern Ocean are often extreme. If that finding is a general characteristic of Antarctic organisms, some areas like those seasonally covered by ice, may experience a temperature effect due to solar heating of a shallow surface layer. Such an effect has been suggested previously, but it quantitative influence has never been adequately addressed. Heating of surface layers occurs over broad areas (e.g., the Ross Sea) during the austral summer, and hence any heating effect might have wide impacts. It is also noteworthy that Eppley's (1972) treatment of temperature effects on phytoplankton doubling rates did not include any data below 2°C, and despite its application to colder waters, the strength of the relationship has never been thoroughly tested.
Productivity in the Southern Ocean can be controlled by either irradiance, nutrients, micronutrients (e.g., trace metals), or/and grazing. A variety of authors have provided evidence for specific controls at some points in time and space by each. The following is an incomplete listing of recent papers on each:
A detailed whole-ocean investigation using available CZCS data was conducted by Sullivan et al. (1993), and a corollary study was conducted by Comiso et al. (1992). Sullivan et al. (1993) concluded that the pigments surrounding Antarctica were asymmetrically distributed, whereas most geophysical properties (bathymetry, hydrography, wind stress, eddy kinetic energy; see also McClain et al., 1991) were distributed symmetrically around the continent. One parameter, silicic acid concentration, was more highly correlated with phytoplankton biomass, and it was suggested that low levels limited the growth of diatoms. However, this conclusion was based on early data that suggested that diatoms had high affinity constants for Si(OH)4, and these high constants have not been verified by more recent field work (Nelson and Tregeur, 1992). Sullivan et al. (1993) also suggested that large blooms occurred downflow of continental shelf regions, and hence were areas of substantial iron input into surface waters. Experimental evidence for iron concentration increases is lacking, however. Taken as a whole, the data presented by Sullivan et al. (1993) suggest a system dominated at all scales by "bottom-up" processes.
The detailed correlations between various parameters suggests potential factors influencing phytoplankton growth in the Southern Ocean, but just as often the correlations do not appear to be causal in nature. For example, the Geosat-derived eddy kinetic energies are highly correlated with bottom topography in many locales, which presumably suggests that local upwelling and nutrient injection are occurring which stimulates phytoplankton growth. However, given the high levels of nutrients present initially in many locations, the cause of such a stimulation remains elusive. Furthermore, in other areas the correlation is negative, suggesting increased mixing and decreased phytoplankton growth. There are also areas that have extensive mixing and are correlated with low phytoplankton biomass; conversely, there are areas that are traversed by receding ice edges but have generally elevated phytoplankton biomass. These relationships suggest control via the irradiance/mixing regime over broad spatial scales. Field studies, specifically the British JGOFS work, have failed to demonstrate a close relationship between phytoplankton biomass, meltwater input, and vertical stratification over periods of weeks to months, and their results suggested that for their study area (the Bellingshausen Sea) frontal enhancement of phytoplankton biomass and growth was critical. Studies in the Weddell Sea marginal ice zone also have documented the importance of frontal zones to phytoplankton biomass accumulation.
The case for limitation of phytoplankton biomass and hence production by grazing is less direct. Much of the work has been either through modeling using simple N-P-Z formulations or analogy to other high nutrient, low biomass regions. Few direct experiments have been conducted on the role of microzooplankton in pelagic systems, and experiments using macrozooplankton/krill are difficult to interpret over broader scales due to the heterogeneous distribution of the organisms.
Given the rates of ingestion of particles that have been observed, the potential for grazing limitation exists, but the actual experimental description of such a limitation is lacking.
Satellite information clearly provides the best spatial coverage of the Southern Ocean, but temporal aspects of specific regions remain elusive. This is in large part due to the logistic difficulty of having a ship (or platform) located in one region through a large part or all of the growing season of phytoplankton. Merging different years of data to produce a coherent temporal pattern is often difficult, given the known variations in ice conditions and other physical forcing functions. Indeed, the satellite composites used by Sullivan et al. (1993) are averages over long periods of time, and hence the error associated with any specific location is large.
THE ROLE OF IRON: Given the relatively deep nature of the continental shelf and slope region, how do vertical processes (e.g., mixing) contribute to the resuspension and regeneration of materials and species? Can the sediments of the continental shelf provide iron to supply the requirements of phytoplankton growth during the growing season? A priori the answer is yes, in that iron stimulation does not appear to be operative on shelves, but a coupled biological-physical model that includes the effects of iron might elucidate the role of iron cycling, turnover, vertical mixing, ice input of iron, etc. Conversely, in deeper (off-shelf) regions of restricted aeolian iron input, can vertical processes provide iron to support surface productivity, or would models suggest that productivity is iron limited?
SATELLITE PATTERNS: If the data provided by satellite composites are approximately correct, what causes the differences in phytoplankton biomass among regions with high eddy kinetic energies and those with low EKE's? How can these differences be resolved knowing what we know about mixing/irradiance regimes? Can large-scale models resolve the patterns of phytoplankton biomass?
GRAZING LIMITATION: In regions of the Southern Ocean that remain ice-free and are observed to be characterized by consistently low phytoplankton biomass, can N-P-Z models adequately address the dynamics of such systems? Do the loss terms have to be extensively modified and made more realistic in order to more closely reflect the conditions found in the ocean?
ICE-COVERED REGIONS: Large areas of the Southern Ocean are ice covered and hence are impacted by receding ice edges. However, not all regions are the sites of biomass accumulation as originally suggested. Can models accurately couple ice sub- models to realistic 3D coupled physical-biological models to predict the effect of receding ice edges? Can the effects of winds on the ice be incorporated to produce mesoscale models of coastal polynyas? Finally, because snow has the greatest attenuation of irradiance (relative to water or ice), can stochastic or probabilistic snow fall be incorporated into models of ice-covered regions?
SUB-TROPICAL CONVERGENCE: The sub-tropical convergence is known to be the site of biomass accumulation as well as the removal of silicic acid to low concentrations, but not of nitrate to the same degree. Can models suggest rates of nutrient input and regeneration (and phytoplankton losses) that are necessary to maintain this state over long (many weeks) time periods?
Comiso, J.C., C.R. McClain, C.W. Sullivan, J.P. Ryan and C.L. Leonard. 1992 coastal zone color scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features. J. Geophys. Res. 98: 2419-2451.
DiTullio, G.R. and W.O. Smith, Jr. Studies on dimethylsulfide in Antarctic coastal waters. In Proceedings of the Sixth SCAR Symposium on Antarctic Biology (ed. D. Walton), Elsevier (in press).
Eppley, R.W. 1972. Temperature and phytoplankton growth in the sea. Fish. Bull. 70: 1063-1085.
Frost, B.W. 1991. The role of grazing in nutrient-rich areas of the open sea. Limnol. Oceanogr. 36: 1616-1630.
Martin, J.H., R.M. Gordon and S.E. Fitzwater. 1991. The case for iron. Limnol. Oceanogr. 36: 1793-1802.
McClain, C.R., C.J. Koblinsky, J. Firestone, M. Darzi, E. Yeh and B.D. Beckley. 1991. Examining several Southern Ocean data sets. EOS 72: 345,351.
Miller, C.B., B.W. Frost, P.A. Wheeler, M. Landry, N. Welschmeyer and T. Powell. Ecological dynamics in the subarctic Pacific, a possibly iron-limited ecosystem. Limnol. Oceanogr. 36: 1600-1615.
Mitchell, B.G., E. Brody, O. Holm-Hansen, C. McClain and J. Bishop. 1991. Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean. Limnol. Oceanogr. 36: 1662-1677.
Mitchell, B.G. and O. Holm-Hansen. 1991. Observations and modeling of the Antarctic phytoplankton crop in relation to mixing depth. Deep-Sea Res. 38: 981-1008.
Nelson, D.M. and W.O. Smith, Jr. 1991. Sverdrup revisited: critical depths, maximum chlorophyll levels, and the control of Southern Ocean productivity by the irradiance-mixing regime. Limnol. Oceanogr. 36: 1650-1661.
Nelson, D.M. and P. Treguer. 1992. Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone. Mar. Ecol. Progr. Ser. 80: 255-264.
Sullivan, C.W., K.R. Arrigo, C.R. McClain, J.C. Comiso and J. Firestone. 1993. Distributions of phytoplankton blooms in the Southern Ocean. Science 262: 1832- 1837.