Primary productivity in Antarctic coastal waters
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T. Jickells, K. Weston, R. Chance
School of Environmental Sciences, University of East Anglia |
A. Clarke
British Antarctic Survey, Cambridge |
ECG Bulletin July 2007
Production of phytoplankton in the oceans
Primary production in the oceans is dominated by microscopic phytoplankton living in the upper layers (about the top 100 m) of the oceans. Their rate of growth depends on the availability of light and key nutrients, of which nitrogen (as nitrate, nitrite, ammonium and various dissolved organic nitrogen compounds: DON), phosphorus (as dissolved inorganic phosphorus, the dissociation product of phosphoric acid, and dissolved organic phosphorus), dissolved silicon (silicic acid) and dissolved iron are recognised as particularly important.
Primary production in the oceans is dominated by microscopic phytoplankton living in the upper layers (about the top 100 m) of the oceans. Their rate of growth depends on the availability of light and key nutrients, of which nitrogen (as nitrate, nitrite, ammonium and various dissolved organic nitrogen compounds: DON), phosphorus (as dissolved inorganic phosphorus, the dissociation product of phosphoric acid, and dissolved organic phosphorus), dissolved silicon (silicic acid) and dissolved iron are recognised as particularly important.
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The importance of these nutrients reflects their vital role in biological reactions and their relatively low abundance in natural waters, meaning that their supply can become limiting for rates of growth. Silicon is unique among these nutrients in being used to form the skeletons of one major group of phytoplankton, the diatoms, while the other nutrients are used for cellular functions by all algae. Hence silicon limits the type of algae that grow, but not the overall productivity.
The role of oceanic phytoplankton Phytoplankton and associated bacteria form the basis of the food web within the oceans, being consumed by larger zooplankton, which are in turn eaten by other zooplankton, fish, birds and marine mammals. The breakdown of their cells also fuels the bacteria, the action of which contributes to the regeneration of the nutrients. |
SEM image of a diatom
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In addition to ultimately sustaining life within the oceans, primary production is also responsible for the exchange of very large amounts of CO2 across the air-sea interface. The sinking of the organic carbon produced by primary production into the deep ocean represents an important mechanism for removing CO2 from the atmosphere and hence influencing climate.
Influence of light on phytoplankton production
Over the tropical regions of the oceans light is available throughout the year, but the warming of surface waters by the sun creates a stable stratification which restricts mixing with deep water reservoirs of inorganic nutrients. Here primary productivity removes almost all of the inorganic nutrients, and primary production rates are then limited by rates of nutrient supply by mixing from below or from external inputs − the rivers or the atmosphere. At high latitudes in the Arctic or Antarctic, productivity is limited in winter by lack of light, but cooling of surface waters during winter allows deep mixing and the supply of inorganic nutrients from deep water which are ultimately the regeneration products of sinking organic matter. As the light climate improves in spring, algae begin to grow rapidly in the absence of light or nutrient limitation, creating the so called “spring bloom”. This tends to continue for a period of a few weeks until nutrient utilisation exceeds supply and nutrient limitation sets in and persists through the summer.
Influence of iron on phytoplankton production
Productivity in the Southern Ocean is relatively low compared to some regions of the high latitude northern hemisphere and concentrations of inorganic nitrogen, phosphorus and silicon do not fall to limiting levels. Over the last two decades we have discovered that this is because in these regions productivity becomes limited by the supply of iron. Iron is rather insoluble in seawater and is rapidly removed to sediments compared to the other metals and nutrients. Complexation by organic ligands (siderophores) acts to retain the iron in the water column to some extent, but the supply from deep water is often inadequate to allow utilisation of the other nutrients. Over many parts of the oceans, this deficit of iron is made up by supply from the atmosphere via the transport of soil dust. This dust is derived mainly from the great deserts of the world and these are concentrated in the northern hemisphere, for instance the North African, Arabian and Chinese deserts. Dust supply to the remote Southern Ocean is very limited, contributing to the iron limitation in these waters.
The paradigm of iron limitation of the Southern Ocean is now well established and applies to much of this vast ocean. However, it is clear from satellite imagery of ocean colour (a measure of phytoplankton abundance) that there are exceptions to this with some regions of high productivity within this area, particularly associated with land masses such as islands and sea mounts. There is a particularly striking feature in the region between the Antarctic Peninsula and South Georgia, which is probably associated with the South Scotia Ridge. It seems likely that the supply of iron in these regions is enhanced by supplies from sediments by resuspension, dissolution and redox processes. However, oceanographic data from this region to confirm this is limited because of the remoteness and difficulty of working in the Southern Ocean, hence the value of satellite imagery although persistent cloud limits its applicability.
Phytoplankton production in the coastal waters of Antarctica
Along the Antarctic Peninsula region itself, satellite imagery suggests even higher phytoplankton abundance. Access to this region is again very difficult due to sea ice persisting throughout a significant part of the year. However, near to major research bases along the peninsula, for example the British Antarctic Survey (BAS) base at Rothera, it is possible to have rather good regular access to the Antarctic coastal waters, allowing the seasonal cycle of productivity in these waters to be investigated.
It is possible to sample through the ice in winter and via boat in the summer allowing a seasonal cycle to be investigated, although the weather and environmental conditions regularly severely restrict sampling. BAS have been engaged in measuring the seasonal cycle of phytoplankton and nutrients in the coastal waters off Rothera for many years, and with Antarctic Funding Initiative (AFI) support this activity has been enhanced over recent years. This work is ongoing but several important results are already available. First the work confirms that the waters are indeed very rich in algae and that this persists throughout the ice free season. Secondly, the phytoplankton abundance is much greater than seen during the spring bloom in the North Sea for comparison, and also persists throughout the season, rather than lasting just for a few weeks as it does in the North Sea.
We still have much to learn about the coastal waters of Antarctica. They are not just an excellent laboratory in which to test out ideas about how ocean productivity is regulated. They are also ecologically very important; the high productivity helps sustain the large seabird and marine mammal populations in this area that first lured humans to this stunning area, at that time primarily for commercial whaling. In addition, the Antarctic Peninsula is one the fastest warming parts of the earth, and climate change will profoundly affect this region with possible ramifications throughout the whole Earth System.
Further reading
P. G. Falkowski, R.T. Barber, V. Smetacek. Biogeochemical Controls and Feedbacks on Ocean Primary Production. Science, 1998, 281, 200-206.
T. D. Jickells, Z. S. An, K. K. Andersen [+ 16 co-authors]. Global Iron Connections between Desert Dust, Ocean Biogeochemistry, and Climate. Science, 2005, 308, 67-71.
P. W. Boyd, T. Jickells, C. S. Law [+ 20 co-authors]. Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions. Science, 2007, 315, 612-617.
A. Clarke et al. Deep Sea Research Part II: Topical Studies in Oceanography, 2007. http://www.sciencedirect.com/science/journal/09670645 in press
A. Clarke, E. J. Murphy, M. P. Meredith, J. C. King, L. S. Peck, D. K.A. Barnes, R. C. Smith. Climate Change and the Marine Ecosystem of the Western Antarctic Peninsula. Philos. Trans. R. Soc. London, Ser. B, 2007, 362, 149-166.
O. Holm-Hansen, M. Naganobu, S. Kawaguchi et al. Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters. Deep Sea Research Part II: Topical Studies in Oceanography, 2004, 51, 1333-1350.
M. P. Meredith, J. C. King. (2005), Rapid Climate Change in the Ocean West of the Antarctic Peninsula during the second half of the 20th century. Geophys. Res. Lett., 2005, 32, Article No. L19604.
Acknowledgement
We thank our colleagues Mark Brandon, Damian Carson, Harry Elderfield, Raja Ganeshram, Kate Hendry, Mike Meredith, Ros Rickaby and Margaret Wallace and for their help with this project. We gratefully acknowledge the excellent work of UKORS and the crew of the JCR. None of this work would have been possible without the dedication of a series of excellent Marine Assistants working at Rothera – Alice Chapman, Jenny Beaumont, Rayner Piper, Andrew Miller, Paul Mann, Helen Rosetti and Alsion Massey. We thank NERC/BAS and AFI for funding supporting this work
Web link Iron and phytoplankton production http://www.atmosphere.mpg.de/enid/1vw.html
T. JICKELLS, K. WESTON, R. CHANCE
School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ;
A. CLARKE
British Antarctic Survey, Cambridge, Madingley Road, Cambridge CB3 OET
This article is based on a presentation by Dr Jickells at the ECG’s 2007 Distinguished Guest Lecture and Symposium ‘Environmental Chemistry in the Polar Regions’.
Over the tropical regions of the oceans light is available throughout the year, but the warming of surface waters by the sun creates a stable stratification which restricts mixing with deep water reservoirs of inorganic nutrients. Here primary productivity removes almost all of the inorganic nutrients, and primary production rates are then limited by rates of nutrient supply by mixing from below or from external inputs − the rivers or the atmosphere. At high latitudes in the Arctic or Antarctic, productivity is limited in winter by lack of light, but cooling of surface waters during winter allows deep mixing and the supply of inorganic nutrients from deep water which are ultimately the regeneration products of sinking organic matter. As the light climate improves in spring, algae begin to grow rapidly in the absence of light or nutrient limitation, creating the so called “spring bloom”. This tends to continue for a period of a few weeks until nutrient utilisation exceeds supply and nutrient limitation sets in and persists through the summer.
Influence of iron on phytoplankton production
Productivity in the Southern Ocean is relatively low compared to some regions of the high latitude northern hemisphere and concentrations of inorganic nitrogen, phosphorus and silicon do not fall to limiting levels. Over the last two decades we have discovered that this is because in these regions productivity becomes limited by the supply of iron. Iron is rather insoluble in seawater and is rapidly removed to sediments compared to the other metals and nutrients. Complexation by organic ligands (siderophores) acts to retain the iron in the water column to some extent, but the supply from deep water is often inadequate to allow utilisation of the other nutrients. Over many parts of the oceans, this deficit of iron is made up by supply from the atmosphere via the transport of soil dust. This dust is derived mainly from the great deserts of the world and these are concentrated in the northern hemisphere, for instance the North African, Arabian and Chinese deserts. Dust supply to the remote Southern Ocean is very limited, contributing to the iron limitation in these waters.
The paradigm of iron limitation of the Southern Ocean is now well established and applies to much of this vast ocean. However, it is clear from satellite imagery of ocean colour (a measure of phytoplankton abundance) that there are exceptions to this with some regions of high productivity within this area, particularly associated with land masses such as islands and sea mounts. There is a particularly striking feature in the region between the Antarctic Peninsula and South Georgia, which is probably associated with the South Scotia Ridge. It seems likely that the supply of iron in these regions is enhanced by supplies from sediments by resuspension, dissolution and redox processes. However, oceanographic data from this region to confirm this is limited because of the remoteness and difficulty of working in the Southern Ocean, hence the value of satellite imagery although persistent cloud limits its applicability.
Phytoplankton production in the coastal waters of Antarctica
Along the Antarctic Peninsula region itself, satellite imagery suggests even higher phytoplankton abundance. Access to this region is again very difficult due to sea ice persisting throughout a significant part of the year. However, near to major research bases along the peninsula, for example the British Antarctic Survey (BAS) base at Rothera, it is possible to have rather good regular access to the Antarctic coastal waters, allowing the seasonal cycle of productivity in these waters to be investigated.
It is possible to sample through the ice in winter and via boat in the summer allowing a seasonal cycle to be investigated, although the weather and environmental conditions regularly severely restrict sampling. BAS have been engaged in measuring the seasonal cycle of phytoplankton and nutrients in the coastal waters off Rothera for many years, and with Antarctic Funding Initiative (AFI) support this activity has been enhanced over recent years. This work is ongoing but several important results are already available. First the work confirms that the waters are indeed very rich in algae and that this persists throughout the ice free season. Secondly, the phytoplankton abundance is much greater than seen during the spring bloom in the North Sea for comparison, and also persists throughout the season, rather than lasting just for a few weeks as it does in the North Sea.
We still have much to learn about the coastal waters of Antarctica. They are not just an excellent laboratory in which to test out ideas about how ocean productivity is regulated. They are also ecologically very important; the high productivity helps sustain the large seabird and marine mammal populations in this area that first lured humans to this stunning area, at that time primarily for commercial whaling. In addition, the Antarctic Peninsula is one the fastest warming parts of the earth, and climate change will profoundly affect this region with possible ramifications throughout the whole Earth System.
Further reading
P. G. Falkowski, R.T. Barber, V. Smetacek. Biogeochemical Controls and Feedbacks on Ocean Primary Production. Science, 1998, 281, 200-206.
T. D. Jickells, Z. S. An, K. K. Andersen [+ 16 co-authors]. Global Iron Connections between Desert Dust, Ocean Biogeochemistry, and Climate. Science, 2005, 308, 67-71.
P. W. Boyd, T. Jickells, C. S. Law [+ 20 co-authors]. Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions. Science, 2007, 315, 612-617.
A. Clarke et al. Deep Sea Research Part II: Topical Studies in Oceanography, 2007. http://www.sciencedirect.com/science/journal/09670645 in press
A. Clarke, E. J. Murphy, M. P. Meredith, J. C. King, L. S. Peck, D. K.A. Barnes, R. C. Smith. Climate Change and the Marine Ecosystem of the Western Antarctic Peninsula. Philos. Trans. R. Soc. London, Ser. B, 2007, 362, 149-166.
O. Holm-Hansen, M. Naganobu, S. Kawaguchi et al. Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters. Deep Sea Research Part II: Topical Studies in Oceanography, 2004, 51, 1333-1350.
M. P. Meredith, J. C. King. (2005), Rapid Climate Change in the Ocean West of the Antarctic Peninsula during the second half of the 20th century. Geophys. Res. Lett., 2005, 32, Article No. L19604.
Acknowledgement
We thank our colleagues Mark Brandon, Damian Carson, Harry Elderfield, Raja Ganeshram, Kate Hendry, Mike Meredith, Ros Rickaby and Margaret Wallace and for their help with this project. We gratefully acknowledge the excellent work of UKORS and the crew of the JCR. None of this work would have been possible without the dedication of a series of excellent Marine Assistants working at Rothera – Alice Chapman, Jenny Beaumont, Rayner Piper, Andrew Miller, Paul Mann, Helen Rosetti and Alsion Massey. We thank NERC/BAS and AFI for funding supporting this work
Web link Iron and phytoplankton production http://www.atmosphere.mpg.de/enid/1vw.html
T. JICKELLS, K. WESTON, R. CHANCE
School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ;
A. CLARKE
British Antarctic Survey, Cambridge, Madingley Road, Cambridge CB3 OET
This article is based on a presentation by Dr Jickells at the ECG’s 2007 Distinguished Guest Lecture and Symposium ‘Environmental Chemistry in the Polar Regions’.
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