Phytoplankton biomass in the western Ross Sea: January and ...
Phytoplankton biomass in the western Ross Sea: January and February 1994 and MARY L. GARRErF, Botany Department and Graduate Program in Ecology, University of Tennessee, Knoxville, Tennessee 37996
WALKER 0. SMITH, JR., ANN-MAREE WHITE, JAMES RICH, AMY SCI-IAuER,
enhanced levels of phytoplankton would be encountered. Ice he southern Ross Sea has been shown repeatedly to be the cover was unusual because ice covered a large portion of the T site of large blooms of phytoplankton by both field studies western Ross Sea. This area usually becomes ice free in late (Smith and Nelson 1985; DeMaster et al. 1992) and remote December, but in 1994, the ice persisted throughout the sensing (Comiso et al. 1993). The bloom is advected into entire austral summer. McMurdo Sound, where it supports an active benthic food web. Despite the potential for spatial variability, a bloom in • The hydrographic conditions were also different from those encountered in previous years. For example, the density the southern Ross Sea has been observed in all years in which stratification was not as pronounced (figures 2A and 3A), and data were collected. the nitrate concentrations were nearly always greater than 20 During January and February 1994, we conducted a cruise on the U.S. Coast Guard Cutter Polar Sea to the south-• micromolar (lAM) (figures 2B and 3B). Chlorophyll concentrations were low and never exceeded 1 microgram per liter (.tg ern Ross Sea (figure 1). The purpose of the cruise was to inves • L- 1) (figures 2C and 3C). Chlorophyll values greater than 20 ig tigate the nitrogen and carbon dynamics of the phytoplankL' have been detected by remote sensing (Comiso et al. ton, and we sampled along 76°30'S. This is the same transect 1993), and discrete measurements have measured levels occupied in 1983, 1990, and 1992, so we were confident that greater than 12 jAg L-1 (DeMaster et al. 1992); therefore, the 1700180° concentrations we observed were much lower than had 160°E 71 0S been observed previously during this time of year. Similarly, the surface nitrate concentrations were greater than observed in 1983 and 1990 (approximately 10 and 15 lLMat ( the surface vs. more than 20 KM in 1994). ANTARCTICA 730 The nutrient levels suggest that substantial production, which persisted for the entire austral summer, had resulted in particulate-matter genesis, but little of that material remained in the water column. Similarly, the pro750 duction and biomass accumulation was less than had been observed in other years. Interannual variability 910111213141516 17181930 occurs in all marine systems, and the interannual variaa U U U U U Us, • • • 514 26 2 • 7 2829 30 31 tions in southern ocean productivity have been hypothe770 sized to be extreme (Smith, Keene, and Comiso 1988, pp. Ice ShiV 131-139). Our results show that the variations in nutrient removal and biomass accumulation at one location in the Ross Sea are indeed substantial. Such variations may affect the food webs dependent on organic material from the - T9os surface layer and the local biogeochemical cycles, but their Figure 1. Location of the stations sampled during the Polar Sea-94 c ruise. I .....I. ..-1 . . ..1 ...e...4...... !3t1. +...+) t.....I. ,A4 13A_131 . causes remain elusive. LQLII • IIt.L I uIuI.,IUUtJ LaLIJII ,rtJ, I lQIIO .,I I I
24
Station Number 25 26 27 28 29 30 31
24
Station Numbs, 25 20 27 2829 30 SI
24
Station Number 25 26 27 28 20 30 31
20
20 40
:
60 Ic
so
so
120
120 140 100
200 300 400 600 600 Distance From Coast (km)
so
b)
140 200 300 400 800 800 too 100 Distance From Cod (km)
200 300 400 500 Distance From Canal )km)
Figure 2. The distribution along transect 1 of (A) at (an expression of density), (B) nitrate (in .tM), and (C) chlorophyll (in tg L1) ANTARCTIC JOURNAL - REVIEW 1994 144
500
6 7
Station Numb., 6 7 8 9 10 11 12 13 14 1516 17 19 Ii 20
Station Numb.,
8 8 10 II 12 13 14 1016 17 18 18 20
0
Station Numb., 6 7 8 9 10 11 12 13 14 1516 17 16 19 20
0
20
20
40
40
so
00
so
80
so
100
100
120
120
120
140
140
100
240 300 400 500 000 Distance From Coast (km)
100
20 40 so
140 200300 400 500 600 Distance From Coast (km)
100
200 300 400 500 600 Distance From Coast (km)
Figure 3. The distribution along transect 2 of (A) at (an expression of density), (B) nitrate (in pM, and (C) chlorophyll (in IAg L-1). We thank W. Sutherland, G. Taroncher, M. White, and the officers and crew of the Polar Sea for their assistance in the field. This research was supported by National Science Foundation grant OPP 91-16872.
References
DeMaster, D.J., R.B. Dunbar, L.I. Gordon, A.R. Leventer, J.M. Morrison, D.M. Nelson, C.A. Nittrouer, and W.O. Smith, Jr. 1992. Cycling and accumulation of biogenic silica and organic matter in high latitude environments: The Ross Sea. Oceanography, 5(3), 146-153. Smith, W.O., Jr., N.K. Keene, and J.C. Comiso. 1988. Interannual variability in estimated primary productivity of the antarctic marginal
Comiso, J., C. McClain, C. Sullivan, J. Ryan, and C.L. Leonard. 1993. CZCS pigment concentrations in the southern ocean and their relationships to some geophysical parameters. Journal of Geophysical Research, 98(C2), 2419-2451.
Smith, W.O., Jr., and D.M. Nelson. 1985. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: Spatial coherence with the density field. Science, 227(4683), 163-166.
ice zone. In D. Sahrhage (Ed.), Antarctic ocean and resources variability. Heidelberg: Springer.
Dispersal of benthic invertebrates in the Scotia Arc by kelp rafting BRIAN S. HELMUTH, RICHARD
R. VEIT, and REBECCA HOLBERTON, Department ofZoology, University of Washington, Seattle, Washington 98195
he ability of organisms to disperse offspring can have a T profound influence on the genetic structure of populations and the geographic distribution of species (Scheltema 1977, pp. 74-109; Worcester in press). Populations in remote areas may be severely limited in the exchange of genetic material with other populations, and the extent to which this exchange occurs may be directly related to the likelihood that adults, larvae, or gametes survive transport between populations. Many sessile, benthic invertebrates such as mussels and barnacles, although restricted in their movement as adults, are able to disperse young through the production of pelagic larval stages that float in the plankton, often for extended periods. Many species, however, especially at high latitudes, instead retain (brood) larvae and early juveniles until their young crawl away or are deposited near the parent (Thorson 1950). Despite the lack of a defined pelagic dispersal stage, at least some brooding invertebrates exhibit a fairly wide geographic distribution (Jackson 1986). Highsmith (1985) has suggested that the comparatively large number of brooding species in high latitudes could be explained, at least in part, by the fact that sessile organisms may be transported on dislodged kelps, which are more prevalent in these regions
and can often form floating "rafts" of tangled kelp plants. With very few exceptions, however, direct evidence of rafting has been lacking, particularly in open-ocean environments. We examined the potential for long-distance dispersal of benthic invertebrates in the Scotia Arc in the vicinity of Cape Horn (South America), the Falkland Islands, and the antarctic island South Georgia on kelp (Macrocystis pyrifera) rafts. (More detailed results are presented in Helmuth, Veit, and Holberton, in press.) South Georgia (54 0 S 37 0W) is located approximately 2,000 kilometers (km) east of Cape Horn (approximately 53 0 S 68°W) and 1,300 km east-southeast of the Falkland Islands (52 0 S 59 0W; figure 1). Surface currents and winds in the region are dominated by the Antarctic Circumpolar Current ("West Wind Drift"), a predominantly unidirectional current running at 0.4-0.7 knots (0.75-1.3 kilometers per hour; Deacon 1982; U.S. Defense Mapping Agency 1988). As a result, kelps dislodged from the region of Cape Horn and the Falkland Islands could reach South Georgia but only if they remained intact and afloat during the necessary travel time, a minimum of 75-100 days at 0.5 knots (0.9 kilometers per hour). The potential for algal rafts to serve as a significant means of transport for benthic invertebrates, thus, depends on the abun-
ANTARCTIC JOURNAL - REVIEW 1994
145
WALKER 0. SMITH, JR., ANN-MAREE WHITE, JAMES RICH, AMY SCI-IAuER,
enhanced levels of phytoplankton would be encountered. Ice he southern Ross Sea has been shown repeatedly to be the cover was unusual because ice covered a large portion of the T site of large blooms of phytoplankton by both field studies western Ross Sea. This area usually becomes ice free in late (Smith and Nelson 1985; DeMaster et al. 1992) and remote December, but in 1994, the ice persisted throughout the sensing (Comiso et al. 1993). The bloom is advected into entire austral summer. McMurdo Sound, where it supports an active benthic food web. Despite the potential for spatial variability, a bloom in • The hydrographic conditions were also different from those encountered in previous years. For example, the density the southern Ross Sea has been observed in all years in which stratification was not as pronounced (figures 2A and 3A), and data were collected. the nitrate concentrations were nearly always greater than 20 During January and February 1994, we conducted a cruise on the U.S. Coast Guard Cutter Polar Sea to the south-• micromolar (lAM) (figures 2B and 3B). Chlorophyll concentrations were low and never exceeded 1 microgram per liter (.tg ern Ross Sea (figure 1). The purpose of the cruise was to inves • L- 1) (figures 2C and 3C). Chlorophyll values greater than 20 ig tigate the nitrogen and carbon dynamics of the phytoplankL' have been detected by remote sensing (Comiso et al. ton, and we sampled along 76°30'S. This is the same transect 1993), and discrete measurements have measured levels occupied in 1983, 1990, and 1992, so we were confident that greater than 12 jAg L-1 (DeMaster et al. 1992); therefore, the 1700180° concentrations we observed were much lower than had 160°E 71 0S been observed previously during this time of year. Similarly, the surface nitrate concentrations were greater than observed in 1983 and 1990 (approximately 10 and 15 lLMat ( the surface vs. more than 20 KM in 1994). ANTARCTICA 730 The nutrient levels suggest that substantial production, which persisted for the entire austral summer, had resulted in particulate-matter genesis, but little of that material remained in the water column. Similarly, the pro750 duction and biomass accumulation was less than had been observed in other years. Interannual variability 910111213141516 17181930 occurs in all marine systems, and the interannual variaa U U U U U Us, • • • 514 26 2 • 7 2829 30 31 tions in southern ocean productivity have been hypothe770 sized to be extreme (Smith, Keene, and Comiso 1988, pp. Ice ShiV 131-139). Our results show that the variations in nutrient removal and biomass accumulation at one location in the Ross Sea are indeed substantial. Such variations may affect the food webs dependent on organic material from the - T9os surface layer and the local biogeochemical cycles, but their Figure 1. Location of the stations sampled during the Polar Sea-94 c ruise. I .....I. ..-1 . . ..1 ...e...4...... !3t1. +...+) t.....I. ,A4 13A_131 . causes remain elusive. LQLII • IIt.L I uIuI.,IUUtJ LaLIJII ,rtJ, I lQIIO .,I I I
24
Station Number 25 26 27 28 29 30 31
24
Station Numbs, 25 20 27 2829 30 SI
24
Station Number 25 26 27 28 20 30 31
20
20 40
:
60 Ic
so
so
120
120 140 100
200 300 400 600 600 Distance From Coast (km)
so
b)
140 200 300 400 800 800 too 100 Distance From Cod (km)
200 300 400 500 Distance From Canal )km)
Figure 2. The distribution along transect 1 of (A) at (an expression of density), (B) nitrate (in .tM), and (C) chlorophyll (in tg L1) ANTARCTIC JOURNAL - REVIEW 1994 144
500
6 7
Station Numb., 6 7 8 9 10 11 12 13 14 1516 17 19 Ii 20
Station Numb.,
8 8 10 II 12 13 14 1016 17 18 18 20
0
Station Numb., 6 7 8 9 10 11 12 13 14 1516 17 16 19 20
0
20
20
40
40
so
00
so
80
so
100
100
120
120
120
140
140
100
240 300 400 500 000 Distance From Coast (km)
100
20 40 so
140 200300 400 500 600 Distance From Coast (km)
100
200 300 400 500 600 Distance From Coast (km)
Figure 3. The distribution along transect 2 of (A) at (an expression of density), (B) nitrate (in pM, and (C) chlorophyll (in IAg L-1). We thank W. Sutherland, G. Taroncher, M. White, and the officers and crew of the Polar Sea for their assistance in the field. This research was supported by National Science Foundation grant OPP 91-16872.
References
DeMaster, D.J., R.B. Dunbar, L.I. Gordon, A.R. Leventer, J.M. Morrison, D.M. Nelson, C.A. Nittrouer, and W.O. Smith, Jr. 1992. Cycling and accumulation of biogenic silica and organic matter in high latitude environments: The Ross Sea. Oceanography, 5(3), 146-153. Smith, W.O., Jr., N.K. Keene, and J.C. Comiso. 1988. Interannual variability in estimated primary productivity of the antarctic marginal
Comiso, J., C. McClain, C. Sullivan, J. Ryan, and C.L. Leonard. 1993. CZCS pigment concentrations in the southern ocean and their relationships to some geophysical parameters. Journal of Geophysical Research, 98(C2), 2419-2451.
Smith, W.O., Jr., and D.M. Nelson. 1985. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: Spatial coherence with the density field. Science, 227(4683), 163-166.
ice zone. In D. Sahrhage (Ed.), Antarctic ocean and resources variability. Heidelberg: Springer.
Dispersal of benthic invertebrates in the Scotia Arc by kelp rafting BRIAN S. HELMUTH, RICHARD
R. VEIT, and REBECCA HOLBERTON, Department ofZoology, University of Washington, Seattle, Washington 98195
he ability of organisms to disperse offspring can have a T profound influence on the genetic structure of populations and the geographic distribution of species (Scheltema 1977, pp. 74-109; Worcester in press). Populations in remote areas may be severely limited in the exchange of genetic material with other populations, and the extent to which this exchange occurs may be directly related to the likelihood that adults, larvae, or gametes survive transport between populations. Many sessile, benthic invertebrates such as mussels and barnacles, although restricted in their movement as adults, are able to disperse young through the production of pelagic larval stages that float in the plankton, often for extended periods. Many species, however, especially at high latitudes, instead retain (brood) larvae and early juveniles until their young crawl away or are deposited near the parent (Thorson 1950). Despite the lack of a defined pelagic dispersal stage, at least some brooding invertebrates exhibit a fairly wide geographic distribution (Jackson 1986). Highsmith (1985) has suggested that the comparatively large number of brooding species in high latitudes could be explained, at least in part, by the fact that sessile organisms may be transported on dislodged kelps, which are more prevalent in these regions
and can often form floating "rafts" of tangled kelp plants. With very few exceptions, however, direct evidence of rafting has been lacking, particularly in open-ocean environments. We examined the potential for long-distance dispersal of benthic invertebrates in the Scotia Arc in the vicinity of Cape Horn (South America), the Falkland Islands, and the antarctic island South Georgia on kelp (Macrocystis pyrifera) rafts. (More detailed results are presented in Helmuth, Veit, and Holberton, in press.) South Georgia (54 0 S 37 0W) is located approximately 2,000 kilometers (km) east of Cape Horn (approximately 53 0 S 68°W) and 1,300 km east-southeast of the Falkland Islands (52 0 S 59 0W; figure 1). Surface currents and winds in the region are dominated by the Antarctic Circumpolar Current ("West Wind Drift"), a predominantly unidirectional current running at 0.4-0.7 knots (0.75-1.3 kilometers per hour; Deacon 1982; U.S. Defense Mapping Agency 1988). As a result, kelps dislodged from the region of Cape Horn and the Falkland Islands could reach South Georgia but only if they remained intact and afloat during the necessary travel time, a minimum of 75-100 days at 0.5 knots (0.9 kilometers per hour). The potential for algal rafts to serve as a significant means of transport for benthic invertebrates, thus, depends on the abun-
ANTARCTIC JOURNAL - REVIEW 1994
145
