Marine Nutrient Cycling and Productivity

Transcription

1 Marine Nutrient Cycling and Productivity Phytoplankton, as the primary producers of the ocean, form the basis of the marine food web, converting energy and nutrients into usable form for consumption by all other organisms. Their growth patterns determine the possibilities for marine life. They, in turn, depend on an adequate supply of light and nutrients to grow namely, carbon, nitrogen, and phosphorus, and to some extent, iron and silicate. Phytoplankton are primarily present in the lit surface layer of the ocean, known as the euphotic zone, and derive necessary nutrients both from the atmosphere and the deep sea. Diatoms and dinoflagellates are the primary phytoplankton groups. Diatoms are larger and require silicate to construct their cell walls. They grow faster and are the first to colonize a phytoplankton bloom, but also quickly die out because they are not able to recycle nutrients in the surface layer very efficiently. Dinoflagellates, on the other hand, are smaller and do not need silicate. Since they have greater surface area per unit volume and flagella to facilitate movement, they are more efficient nutrient recyclers. Both groups, however, require carbon, nitrogen, phosphorus, and trace amounts of oxygen. The mechanisms by which nutrients are cycled through the world s oceans are relatively unknown and are a topic of current research. These mechanisms determine what is available to fuel phytoplankton growth. Nutrient availability determines what and how much can grow in the ocean.. In situations where nutrients are limiting, phytoplankton will not be present in large numbers and neither will the many carnivorous organisms that depend on them for food. Generally, upwelling zones, such as those located off the coast of Chile, South Africa, Australia, and California are areas of great productivity due to the abundance of nutrients brought from the deep sea to the euphotic zone. The open ocean, in contrast, tends to be less productive, especially in calm tropical areas where the water becomes stratified and nutrients are quickly depleted. Without constantly replenishing nutrient supplies, continued phytoplankton growth cannot occur, crippling the marine food web in that particular region. Much current research focuses on understanding nutrient cycles on both global and local levels simply because little is known about them and their impacts on primary production. It is thought that the carbon, nitrogen, and phosphorus cycles are coupled, but the specifics of this relationship are vague at best. Increased knowledge about these cycles will lead to heightened understanding of the impacts of human activity on their continued functioning. Greater understanding of nutrient cycles and specifically the ways in which nutrient availability has limited primary productivity has led to increased interest in researching ways to combat nutrient depletion and therefore increase productivity, which could increase the oceans potential to act as a carbon sink. Most of this research has concentrated on the impacts of relieving nutrient stress, specifically that induced by iron. Oceans are currently highly significant carbon sinks, but it is thought that artificially increasing productivity, and in particular, diatom productivity, could amplify this ability. However, the current state of knowledge is not yet extensive enough to contemplate such extensive human manipulation of the system. For now, increased knowledge about marine nutrient cycles is the goal. Global Nutrient Availability Controlled by High-Latitude Upwelling Nutrients cycle through the oceans to be used and reused. They are incorporated into phytoplankton, pass through the food chain, and eventually filter down to the deep ocean as biological matter. However, the process by which nutrients return to the ocean surface to be used again by phytoplankton is not well known. The classical explanation hypothesized that the downward movement of biological matter was balanced by the upwelling of nutrients, a process that was driven by vertical mixing. It has since been demonstrated that vertical mixing is not strong enough to account for the amount of nutrients that are recycled up to the surface. Sarmiento et al. (2004) hypothesize that waters with high nutrient concentrations are brought to the surface of the Southern Ocean and incorporated into Subantarctic Mode Water (SAMW), which then distributes nutrients around the globe. SAMW is a layer of water of relatively uniform density that forms in a ring around the Southern Ocean during the winter. It is characterized by high nitrate concentrations and low silicic acid levels. The difference between silicic acid and nitrate concentrations provided a relatively stable marker for the scientists to trace the movement of SAMW and the nutrients it carried around the world.

2 As the upwelled water moves north to form the SAMW around the Southern Ocean, the nutrients are utilized by organisms. Though there is initially more silicic acid relative to nitrogen, silicic acid is gradually depleted by diatom growth. Studies conducted by other researchers have shown that diatoms tend to take up more silicic acid relative to nitrogen in situations of iron limitation. In the Southern Ocean, though the iron in sea ice and the upwelling water is enough to maintain an intense diatom bloom, it is not sufficient to prevent silicic acid depletion. Thus, the upwelled water from the Southern Ocean is rapidly depleted of silicic acid, limiting the amount available for diatoms in other areas of the ocean that depend on upwelled water to fulfill their nutrient requirements. The movement of SAMW helps explain why diatom production is so low in much of the world. Under ideal conditions of adequate light, temperature, and nutrients, diatoms contain silicon and nitrogen in a 1:1 ratio. However, because silicon-deprived SAMW provides much of the nutrients for the world s oceans, diatom production is constrained. The authors found that the Southern Ocean nutrient supply accounted for 75 percent of biological marine production north of 30 S. Since Southern Ocean water processes, especially the formation of SAMW, is so important for distributing nutrients in oceans around the world, climate change could have a devastating impact on this means of dispersal. Upwelling in the North Pacific also provides nutrients for the world s oceans, and the authors found that its impact on productivity was stronger than expected. Strong vertical mixing upwells water and brings nutrients to the surface of the North Pacific. Because vertical mixing does not expose the water to the surface, nutrients are not depleted and thus retain high concentrations of silicic acid. The influence of this upwelled water extends to the equator and accounts for 70 percent of the silicic acid supply, even though half of the nitrogen supply is derived from the Southern Hemisphere. The authors found that nutrient cycling in the North Pacific was more important than previously anticipated, because it was critical in determining productivity in the low latitudes and equatorial Pacific. Horizontal Nutrient Dispersion Within the nutrient availability constraints provided by global recycling and redistribution, there are also local particularities that further determine regional nutrient supply and therefore primary productivity. Nutrient availability in surface water has conventionally been attributed to vertical processes. In the case of nitrogen, it is fixed by certain kinds of marine bacteria into a usable form, taken up by the phytoplankton, used, released to fall below the productivity zone, and then is either brought back to the surface waters to be used again or released to the air. Phosphorus is already present in the water and once used, falls below the productivity zone, remineralizes, and is eventually carried back up to the surface by currents. Thus, these two nutrients move through the water vertically in a complex process of recycling and exchange with the water s surface. However, Palter et al (2005) found that horizontal movement in deeper layers of the ocean also had an effect on nutrient availability. The authors studied the North Atlantic subtropical gyre, which is the area around the Caribbean islands extending north to the southeastern United States. Here, low nutrient waters known as subtropical mode water (STMW) form just south of the Gulf Stream every winter and slowly move southward, becoming covered with warmer waters in the process. STMW is characterized by a temperature of 18 C. Its waters remain low in nutrients because nitrogen and phosphorus are not regenerated fast enough to significantly raise concentrations. The strength STMW production varies by year. When it is strong, the low-nutrient STMW maintains the low nutrient zone below the productivity zone where phytoplankton are present and thus stifles growth. However, when STMW production is weak, nutrient concentration grows linearly with increasing depth between the bottom of the euphotic zone and the deep remineralized nutrients. This linear growth is expected because there are fewer phytoplankton found at depth due to lack of light. The presence of low-nutrient STMW can provide some explanation for variations in phytoplankton activity in a given area. Phytoplankton activity is measured by chlorophyll concentrations because the organisms power themselves with chlorophyll. Thus, high chlorophyll concentrations would indicate the presence of many phytoplankton. The subtropical North Atlantic is not uniformly low in chlorophyll. Rather, there is a ringed pattern of chlorophyll concentration, with the chlorophyll minimum occurring where the STMW and the deepest area of low nutrient concentration are. This coincidence supports the hypothesis that the cold water, low nutrient water wedge of STMW does limit phytoplankton activity by limiting nutrient availability.

3 The horizontal movement of mode water, demonstrated here through an examination of STMW in the subtropical North Atlantic, thus establishes the strength of the nutrient reservoir from which all vertical recycling processes draw. When STMW formation is strong, it limits the amount of phosphorus and nitrogen which can be recycled to the surface to fuel further phytoplankton growth. Understanding how mode water can further limit nutrient concentrations by forming a thick wedge of low nutrient water extending below the productivity zone can help to develop a more thorough understanding of marine nutrient cycles and how they impact the ecological community. Carbon Exports to the Deep Sea Carbon, nitrogen, and phosphorus cycles through the ocean are thought to be coupled. They are generally present in a constant ratio known as the Redfield ratio: 106 atoms carbon to 16 atoms nitrogen to 1 atom phosphorus. This ratio corresponds to the needs of phytoplankton growth. The assumption for cycling of these nutrients from the surface to the deep sea and back is that the amount of carbon exported to the deep sea is related to the amount of new nitrogen and phosphorus brought to the surface through remineralization and addition from the atmosphere, keeping the ratio constant. Organic matter export is the means by which carbon is sequestered in the deep sea. Through the process of carbon export to the deep sea, carbon from the atmosphere can be effectively sequestered, thereby removing some of the effects of increased carbon dioxide production from anthropogenic sources. Overall organic matter export upholds the assumption of the Redfield ratio, cycling nutrients at expected concentrations. It operates by two mechanisms. Particulate organic matter, which is the bacteria and phytoplankton biomass that constitute 80 percent of export, adheres to the Redfield ratio. However, dissolved organic matter (DOM), accounting for the remaining 20 percent of organic matter in the oceans, radically departs from the expectations of the Redfield ratio, leading to questions about its operational mechanisms and the implications for carbon export. Hopkinson and Vallino (2005) investigated dissolved organic carbon (DOC) cycling in the ocean to determine how it deviated from the expected Redfield ratio and what the implications of this deviation were for carbon sequestration. DOC is thought to be coupled with dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) cycles, but the exact mechanisms are poorly understood. The authors examined DOM levels in all different ocean environments, separating them into refractory and labile DOM. Refractory DOM is all old DOM that has DOC concentrations at or below deep sea levels and is rich in carbon but has few other nutrients. Labile DOM is young and consists of all DOM that has DOC concentrations above that of the deep sea. It is at an intermediate level, meaning that it is still rich in carbon compared to the Redfield ratio but not to the extent of refractory DOM. Labile DOM is replenished by surface waters and therefore still contains essential nitrogen and phosphorus. The predictions of global climate change led the researchers to conclude that increased temperature and the resulting ocean stratification could increase labile DOM export, which would enhance its ability to sequester carbon. However, increased UV radiation and increased temperature would also promote refractory decomposition, decreasing carbon sequestration. With the current data, it is not entirely clear what could happen to DOC sequestration with climate change yet. However, the researchers did find that the dissolved organic cycles of carbon, nitrogen, and phosphorus are coupled, but not in the expected way. For a given mass of new nitrogen or phosphorus introduced to the surface, 62 to 82 percent more DOC can be exported than predicted by the Redfield ratio. This result suggests that DOM is more efficient than particulate organic matter in exporting carbon to the deep sea, allowing more accurate estimates of carbon export and nutrient balances in the oceans. Phosphorus: Recycled from Extracellular DNA The deep ocean floor is largely inhabited by bottom-dwelling prokaryotes that is, tiny one-celled organisms that lack a distinct nucleus, such as bacteria. This community plays an important role in phosphorus

4 cycling throughout the ocean, but the mechanisms of this cycle are not well understood. Marine organisms usually take in phosphates to assist in cell functioning when they are available. However, for unknown reasons, the prokaryotes of the deep sea preferentially recycle organic phosphorus instead, even though phosphates are present in sufficient quantities. Dell Anno and Danovaro (2005) explored the role DNA, an organic phosphorus-rich molecule, might play in this process and determined that extracellular DNA provided a good portion of the available organic phosphorus in marine sediments and stimulated prokaryotic growth. DNA contains 10 percent phosphorus by weight, but up to the authors study had been virtually ignored as a participant in phosphorus cycling. It is generally only thought of as the genetic material of living organisms, not as a nutrient source. The authors found that the concentration of DNA in deep-sea sediments worldwide is very high, and that around 90 percent of the DNA in these sediments is extracellular, meaning that it is located outside the cell and readily available to be cycled through nutrient systems. In fact, extracellular DNA in these deep-sea sediments represents the largest reservoir of DNA in the world ocean. It is six to eight times higher than the amount of DNA contained in all prokaryotes in the top ten centimeters of the world s marine sediments. When DNA filters down from the photic, or lit, layer to the deep sea, the production of these bottomdwelling prokaryotes increases. Extracellular DNA provides approximately 13 percent of the total organic phosphorus flux to the deep sea, importing 12 million metric tons per year. Because extracellular DNA provides carbon and nitrogen as well as phosphorus, it is a vital nutrient source for prokaryotic growth. The use of extracellular DNA provides four percent of carbon, seven percent of nitrogen, and 47 percent of phosphorus required daily for prokaryotes. The actual contribution of phosphorus from extracellular DNA to the total organic phosphorus pool is about 3 percent. The residence time of organic phosphorus in the top centimeter of sediment is 40.3 years; for extracellular DNA it is 9.5 years. Thus, phosphorus from extracellular DNA is present in sediments for an extended period of time, allowing it to remain in the nutrient cycle. Remineralization is the process by which nutrients are made available for use and a look at DNA remineralization rates shows that it is a large supplier of organic phosphorus. Extracellular DNA remineralization in the top ten centimeters of deep-sea sediments provides up to 17 percent of the total organic phosphorus regeneration. This figure suggests that extracellular DNA is selectively remineralized within the organic phosphorus pool and rapidly degraded for use over other phosphorus sources, which are not remineralized at such a high rate. Based on this evidence, it is thought that extracellular DNA plays a key role in the functioning of the deepsea ecosystem. It is preferentially taken up by bottom-dwelling prokaryotes to fuel their growth and contributes significantly to the amount of phosphorus cycled through the deep sea. Since this is a new research focus, the contributions of extracellular DNA to nutrient cycling may perhaps still be underestimated and poorly understood. Nitrogen Fixation Limited by Iron and Phosphorus in the Eastern Tropical North Atlantic Nitrogen is, on average, 16 times as abundant in the world s oceans as phosphorus (Arrigo 2005) and the coabundance of both influence phytoplankton productivity. Some phytoplankton are capable of fixing nitrogen from the atmosphere in situations where it is scarce, but this process in turn is limited by the amount of available phosphorus. It has recently been hypothesized that iron plays a similar role in limiting nitrogen fixation, but the relative importance of each has not yet been determined for all areas. Phosphorus generally is recycled from the deep sea, and iron is also recycled as well as deposited via dust. Mills et al. (2004) experimented in the eastern tropical north Atlantic, an area rich in nitrogen fixing bacteria known as diazotrophs and with extensive Sahara dust input, to test whether nitrogen, phosphorus, iron, or some combination of these limited nitrogen fixation in these oligotrophic, or low nutrient, waters. Here, nitrogen fixation may provide the means by which up to 50 percent of export production is made. There was a lot of iron present due to the dust transported from the Sahara, which was thought to exceed the growth requirements of Trichodesmium, a prevalent cyanobacteria that fixes nitrogen. This knowledge, along with the fact that rate of nitrogen fixation was strongly correlated with the phosphorus content of Trichodesmium and that the area had low concentrations of dissolved inorganic phosphorus, was used to argue that the whole region was phosphorus-limited.

5 The authors set up three stations to assess the effects of nitrogen, phosphorus, iron, and combinations of these on the phytoplankton community and diazotrophs in particular. They found first of all that the eastern tropical north Atlantic was nitrogen-limited. Carbon dioxide fixation increased two- to threefold and chlorophyll concentrations rose fold with the artificial addition of nitrogen to the water, but did not change at all when just phosphorus, iron, or both were added. The largest change was observed when all three nutrients were added, suggesting that growth was limited by nitrogen concentrations in the water. The researchers found that nitrogen fixation was also co-limited by both iron and phosphorus, and with their addition, nitrogen fixation increased two- to threefold. Both nutrients were necessary to increase nitrogen fixation rates at two of the three sites, meaning that nitrogen fixation was largely co-limited in this area. It is surprising that iron is also a limiting factor in these waters, considering the area was thought to be iron-rich. If iron limits nutrient fixation here, when it is present in relatively high concentrations, its low concentrations may be more significant than previously thought in other iron-poor areas lacking continual dust input. Additionally, adding more Sahara dust, thought to be the primary source of iron, stimulated nitrogen fixation as much as twofold, implying that perhaps it might be a significant source of phosphorus as well. Phosphorus is approximately 30 times less prevalent in the Earth s crust as iron, while phytoplankton require times as much for nitrogen fixation. It is surprising that the amount of phosphorus contained in the dust would be adequate to further stimulate nitrogen production, given that phytoplankton need much greater amounts of it in comparison to iron and there was also much less of it initially available in the dust. This result may be due to the incidental addition of another trace nutrient, however, and further research is needed to determine the specific nutrients that cause this increase in nitrogen fixation. The authors pointed out that their data could be significantly influenced by the time period in which it was collected. Temporal variability can change the relative amounts of nutrients present, and since most data on this subject had been collected during the seasonal high of dust deposition in the spring whereas this study was done in the autumn low, the data may simply reflect these differences. Nonetheless, this study shows that dust deposition has a discernable impact on nitrogen fixation. Increased deposition as a result of climate change could supply the limiting nutrients to fix more nitrogen in this area of the eastern tropical north Atlantic, thereby increasing productivity. Climate Change and Productivity in the Arabian Sea Climate change has the potential to drastically affect wind and current patterns, which in turn would impact global nutrient flows. Goes et al. (2005) looked at the impact that snow cover in Eurasia had on productivity in the Arabian Sea, and found that in this case, decreased snow cover caused primarily by global warning led to increased productivity. The Arabian Sea is the world s only ocean basin that completely changes flow direction on a semiannual basis. This phenomenon is largely attributable to temperature variation on the Eurasian landmass. In the summer months of June through September, Eurasia heats up considerably, creating a low-pressure zone over land. The ocean remains relatively cool and thus is capped by a high pressure zone. As air moves from areas of high to low pressure, a strong southwesterly wind forms. Its strength is dependent on the temperature differences between land and sea the larger the difference in temperature, the larger the difference in pressure, which creates a stronger wind. The Findlater Jet, a northeasterly surface current, also forms as a result of these processes and causes strong upwelling on the coasts of Somalia, Yemen, and Oman. In the winter, the wind and sea currents reverse, reflecting the shift in temperature differences between land and sea. From November to February, the Eurasian landmass becomes very cold and thus becomes the high pressure zone. A low pressure zone forms over the warmer Arabian Sea. As a result, wind direction switches and northeasterly monsoon winds form.

6 These wind and current patterns have dramatic implications for life in the Arabian Sea. They facilitate the upwelling of nutrients on the western edge of the ocean basin, allowing phytoplankton growth in waters that are normally nutrient-poor. With phytoplankton present as the first organisms in the food chain, other marine animals can survive there as well. To test for a correlation between wind strength, productivity, and temperatures on the Eurasian landmass, Goes et al. (2005) measured phytoplankton biomass, sea surface temperatures, and Eurasian snow cover from 1997 to They found that there has been a consistent increase in phytoplankton biomass during this period. The authors measured concentrations of chlorophyll a to determine approximate measures of phytoplankton productivity, because chlorophyll a is the active powerhouse of the organisms, converting light to energy. By the summer of 2003, the concentration of chlorophyll a was more than 350 percent higher than it had been initially in the upwelling zone. This increase in phytoplankton biomass was accompanied by a decline in sea surface temperatures and Eurasian snow cover. Lower sea temperatures indicate that upwelling has intensified, bringing more cold water to the surface. They also suggest that the temperature differential between land and sea may have increased significantly, especially when declining snow cover in Eurasia is taken into account, and thus winds would have been stronger than they had been in previous years. The southwestern monsoon winds drive the upwelling system, which brings nutrients from the ocean bottom to the productive but nutrient-deprived surface. The wind strength, in turn, is determined by the temperature difference between the land and the sea. The land tends to be cooler when it is covered with snow later in the season, because the sun s energy is expended in melting and evaporating the snow rather than heating the land. Land also tends to be cooler when early snowfall sticks, for this increases reflection and resistance to melting by the sun. Reduced snowfall, on the other hand, increases land temperature. The authors found that there has been a decline in Eurasian snow cover since 1997, which creates favorable conditions for strong winds and greater upwelling and productivity in the western Arabian Sea. These phenomena are all strongly correlated, suggesting there is in fact a strong relationship. With continued global warming, then, it is expected that the winds and the affected upwelling system will continue to increase in strength, raising the overall productivity of the western Arabian. This research thus raises questions of what impacts climate change could potentially have for other ocean basins. Silicate Additions Enhance Carbon Sequestering in the Northeastern Atlantic Diatoms, as discussed above, are unicellular phytoplankton that use silicon in their cell wall production. Unlike many phytoplankton, their growth is dependent on the supply of dissolved silicate as well as the availability of nitrogen and phosphorus. Diatoms are the first phytoplankton to bloom in the spring and they rapidly utilize available nutrients. Following nutrient (usually silicate) exhaustion, they quickly die and their remains fall to the deep ocean. Thus, the diatom life cycle provides a mechanism for carbon to be exported from the atmosphere to the ocean atmospheric carbon in the form of carbon dioxide is incorporated into the diatoms during photosynthesis and then falls to the deep ocean, where it remains for up to 100 years. Allen et al. (2005) studied the diatom community in the northeastern Atlantic in May and June of 2001 to examine this mechanism and determine the effects that a steady supply of silicate might have on carbon export. They postulated that if new silicate were supplied after the old had been depleted, then diatom growth could be prolonged and more carbon could be exported to either the deep ocean or to higher trophic levels, enhancing the ocean s ability to act as a carbon sink. The northeastern Atlantic is richer in nitrogen than in silicate, leading researchers to believe that silicate availability ultimately limits growth. The authors found that maximum diatom growth occurred at the start of the spring bloom, coinciding with higher silicate concentration. As silicate concentration decreased, diatom production dwindled and they began to die in large numbers. When the authors increased silicate concentrations, the phytoplankton readily responded, implicating silicate as the nutrient limiting production. However, there was variability in rates of all phytoplankton production across the research site, particularly that of diatoms, which could not be accounted for by the rapid depletion of silicate. Some areas continued to house

7 prolonged diatom blooms past the point at which silicate was expected to be depleted. These areas of the most prolonged production rates were in the vicinity of the Iceland-Faeroes Front (IFF), the longest boundary between the northeast Atlantic and the Arctic. The area is inherently unstable because of the many differences in the two mixing oceans and this results in an unstable boundary characterized by eddies. The authors found that the instability of the IFF and the water movement it created mixed waters such that diatom blooms could be sustained. The IFF water effectively transports phytoplankton biomass from the surface down to below the euphotic zone, and refertilizes the euphotic zone with nutrient-rich upwelling waters. The vertical transport of nutrients and phytoplankton biomass appears to be the most probable mechanism for silicate refertilization in the northeastern Atlantic. With continuing import of silicate from deep waters, the existing diatom bloom could be prolonged against rising pressure from grazers and other species. Extended diatom growth, in turn, allows for increased carbon sequestering. With rising atmospheric carbon levels, this mechanism for temporary carbon storage may become increasingly important. Additionally, the mechanism is not limited to silicate, but could apply to nitrogen, phosphorus, or iron in frontal regions in situations where they are limiting nutrients. Global Iron Distribution and Importance Like silicate, iron availability restricts diatom growth, serving to limit the potential for the oceans to act as carbon sinks. Iron is essential for all organisms and is used in photosynthesis, respiration, and nitrogen fixation. Marine phytoplankton must meet their iron requirements through the iron available in the surface layer of the water, and because iron concentrations in many parts of the oceans are low, iron limits phytoplankton growth. It has been estimated that 30 percent of the ocean is iron-limited, which in turn limits primary productivity. The available information on iron was compiled and analyzed by Jickells et al. (2005) in an effort to understand the mechanisms by which iron is allocated to oceans globally and the impacts projected climate change could have on iron distribution and marine primary productivity. Most iron reaching the oceans is derived from rivers, but this source affects only coastal areas. The open ocean receives most of its iron from dust deposits carried by the wind from terrestrial drylands and deserts, which cover a third of the Earth. These environments are especially susceptible to global climate change, and it is thought that this could have implications for ocean productivity by changing the amount of dust available for transport to the open ocean. Predictions do not agree on either the direction or extent of change, but since dust depends on dry land devoid of vegetation, wetter climates with more vegetation could dramatically reduce the amount available, whereas increasing desertification could increase dust production. The authors estimate that 1.7 trillion kilograms of dust are produced each year, two thirds of which is derived from North Africa. Twenty-six percent eventually falls into the oceans. It has been hypothesized that changing land use practices have increased dust flux, but it is uncertain how important this has been on a global scale. Regional affects have been demonstrated, such as the 1930s Dust Bowl in the United States, but it is unknown what the total cumulative impact of all regional land use practices could look like. Once iron is deposited in the ocean via dust, it is rapidly taken up by phytoplankton or filters down to the deep ocean in unusable form. Iron is not very soluble in water and much of it falls to the deep ocean. If some of this iron dissolves at depth, iron concentrations will also gradually rise and productivity in upwelling regions, such as the Southern Ocean, should rise in the long term. Iron limitation reflects deep water iron:nitrogen ratios that are not conducive to phytoplankton growth. As organic matter falls to the deep ocean, iron is scavenged by organisms at a faster rate than other nutrients, so when the water is upwelled, bringing deep sea nutrients back to the surface, iron is present in lower concentrations relative to the other essential nutrients. Thus, additional iron input is necessary to fuel growth, and this usually comes from dust in the atmosphere.

8 Open ocean phytoplankton species require less iron than their coastal counterparts, most likely because they evolved to require less in their iron-limited environment. This is accomplished by reducing cell size or minimizing the number of iron-containing enzymes within the cell. Conversely, with the relief of iron stress, larger cells, especially diatoms, proliferate. Increasing primary productivity, and thus sequestering carbon, either through increased terrestrial vegetation or fertilizing iron-limited areas of the oceans, has been advanced as one way to offset anthropogenic increases in carbon dioxide levels, but not enough is known about the iron cycle in particular to begin tampering with engineering ecosystems. Iron Fertilization in the Southern Ocean: A Solution to Global Warming? There is high potential for iron fertilization in the Southern Ocean. This ocean has high nutrient concentrations but exhibits low phytoplankton growth. Growth is limited to some extent by light availability, but a lack of iron, an essential micronutrient, also plays a role. Iron is a catalyst in converting phosphorus into a usable form, which is essential for phytoplankton growth. Therefore, some have hypothesized that with the addition of iron, phytoplankton production should substantially increase. More phytoplankton translates to more carbon dioxide extracted from the atmosphere, converted into phytoplankton biomass, and eventually sequestered to the deep ocean. This sequestration process could play a role in reducing carbon dioxide levels in the atmosphere and alleviating the effects of global warming. A few iron fertilization studies have been conducted to date, most notably the Southern Ocean Iron Enrichment Experiment (SOIREE) in Though every study thus far has shown enhanced phytoplankton growth, carbon movement to the deep ocean has been more difficult to demonstrate. Buessler et al. (2004) performed iron fertilization experiments in January and February of 2002 to investigate whether iron enrichment and the resulting increase in phytoplankton growth did increase the amount of carbon moving to the deep ocean, and if so, whether the net change was significant in terms of providing a mechanism to sequester carbon. The researchers found that iron fertilization did in fact lead to an increase in carbon moving from the surface to the deep ocean. Particle movement was indirectly tracked by measuring levels of a naturally occurring radioactive compound, 234 Th, which has a natural affinity for particle surfaces. When the particles and the attached 234 Th fall below the surface layer, there is a measurable deficit in the expected amount 234 Th and the downward movement of particles can be calculated. Once below the surface layer, particles are sometimes remineralized, or transformed back into forms usable by bacteria. This results in an upward measurable movement of 234 Th. The net movement of particles can then be calculated, along with the movement of the elements associated with the particles. Using sensitive 234 Th detectors, the scientists were able to estimate the amount of carbon that sank into the deep ocean. Over the course of the experiment, the researchers found that more particles fell below the surface layer and fewer were remineralized at depth with the addition of iron. There were no obvious changes in the subsurface community, however, so it remains unclear why rates of remineralization declined. Regardless, the net result was greater carbon movement to the deep ocean than under normal conditions. However, carbon flux was relatively low in comparison to previous blooms. The authors hypothesized that perhaps the experiment did not last long enough to see the end of the bloom, for the phytoplankton community was not yet nutrient-stressed and grazing pressure was not strong enough to halt further growth. The authors estimate that this experiment sequestered approximately 900 tons of carbon. Even if estimate turns out to be low because the experiment did not measure the effects of the entire bloom, any revision will still sequester only minimal amounts of carbon when compared with the 6.5 billion tons of carbon emitted by human activity each year. Though iron fertilization may increase phytoplankton productivity in the Southern Ocean and lead to greater carbon sequestration, it will not provide a solution to global warming because it cannot sequester carbon at the amounts and rates necessary to offset anthropogenic carbon inputs. But What About Silicate?

9 The iron fertilization experiments undertaken earlier did not take the availability of other nutrients, specifically silicate, into consideration. Because diatoms, thought to be the primary cause of carbon export, use silicate to construct their cell walls, silicate availability could also limit their growth, and therefore their ability to sequester more carbon. The SOIREE and EisenEx experiments were conducted in waters of moderate silicate concentration, in effect neglecting the potential impacts this nutrient could have on phytoplankton growth. Coale et al. (2004) attempted to address this shortcoming by conducting experiments to investigate the effects of silicate concentrations on photosynthesis and carbon export. The Atlantic Polar Front Zone (APFZ) divides the Southern Ocean into separate zones that markedly differ in silicate concentrations. The north is deficient, whereas the south has high concentrations. The researchers hypothesized that iron fertilization would result in a diatom bloom in the south, where silicate is available. The north, it was thought, would host other phytoplankton species that do not require silicate. If this increased growth resulted in increased export, then the addition of iron could potentially help control global climate. Iron did lead to increased phytoplankton growth, producing a diatom bloom in the south and a nonsilicious bloom in the north, as predicted. The limited availability of silicate hindered phytoplankton growth in the north, however. When silicic acid was added to samples of ocean water from the north, increased uptake of iron by phytoplankton resulted, indicating that phytoplankton growth was limited by silicate in the north and that this limitation determined what community structures resulted in the patch. The southern patch, in contrast, showed no response to the addition of silicic acid. Phytoplankton growth in the Southern Ocean was much slower than similar blooms observed at the equator, and this is thought to be mainly attributable to lower temperatures, which impede growth. Light limitation might also have slowed growth in the Southern Ocean, because as the bloom progressed, the areas receiving light were restricted to much shallower depths than had previously been observed. The rapid increase in primary production and phytoplankton biomass also resulted in heightened carbon export. Both sites experienced a downward flux of carbon, suggesting a stronger role for possible carbon sequestration with the addition of iron at these sites. Carbon also tended to stay at depth and was not often recycled, in contrast to nitrogen, which can be remineralized much more efficiently than carbon. Instead, as the amount of dissolved carbon dioxide declined in the water, it was replaced by carbon dioxide from the atmosphere, effectively enhancing the bloom s potential for carbon sequestration. The authors estimate that 2 billion tons of carbon could be exported to the deep sea each year through iron fertilization of the Southern Ocean from 50 S to 65 S. However, it is difficult to tell what effects large-scale enrichment would have on the atmosphere based on only a few experiments, and the logistics of consistently enriching 36 million square kilometers of ocean would be quite complex. Conclusion Phytoplankton growth is fundamentally limited by nutrient availability. Though the limiting nutrient depends on the circumstances, primary productivity is always restricted. There may be low-nutrient waters below that hinder nutrient uptake. The distance from upwelling centers may limit nutrient availability. A lack of iron may make an otherwise productive region record little primary productivity. Even though Sarmiento et al hypothesize that nutrient availability is largely determined by global nutrient cycling systems, locally specific conditions do have an influence, too. Much research has gone into gaining a better understanding of how nutrients circulate throughout the ocean, but current knowledge is far from comprehensive and a full picture of the dynamics that govern nutrient availability throughout the world s oceans is simply not available. One thing that seems certain, however, is that it is a complex but delicate process. Nutrient cycles are interdependent and related in mysterious ways that are still beyond our comprehension, highlighting the necessity of more research before attempting to alter the mechanisms of nutrient cycling. In this light, it would certainly be premature to consider iron fertilization as a solution for global warming. Even if it did work exactly as expected, sequestration time is limited and it is most likely that it could not soak up all of the excess carbon dioxide derived from anthropogenic sources. However, this does represent exciting new research that has the potential to increase

10 our understanding of nutrient cycling in the Southern Ocean. Indeed, it is only recently that the potential importance of iron as a limiting nutrient has been explored, with unexpectedly significant results. Several researchers were careful to point out the implications that climate change or other forms of anthropogenic influence had for continued functioning of nutrient cycles. Because they are so delicately balanced, changes in current or wind flow or even nutrient availability can have dramatic impacts on phytoplankton productivity, which has implications for all marine organisms and even terrestrial organisms through impacts on atmosphere air quality. This uncertainty highlights the need to proceed with caution, for our actions can have unforeseen consequences on ecological systems we as yet do not understand.

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