So far, I have made little or no reference to global or regional models of production processes that involve Fe inputs from aerosol or upward mixing, largely because no consensus has been reached by modeling studies. They have not greatly advanced our understanding of the problem, although their indications cannot be ignored.
Because it directly supports the concept that I believe has become mythic, it will be useful to address at once the model of Erickson et al. (2003). These authors merge SeaWiFS data with the output from the global model of dust-deposition of Ginouxet al.
(2001) that, incidentally, they describe as a “state-of-the-art geophysical data set.” They obtain fields of anomaly correlation between surface chlorophyll and dust deposition at 2×25 resolution over the Southern Ocean, where the anomalies are the time-series mean subtracted from each monthly value. Correlations of >06 are obtained for a zonal swath from 40 to 55S across the entire ocean, but principally in the Atlantic and western Indian Ocean. The location of high correlation is commonly displaced by 2–5 of latitude from locations of chlorophyll maxima, and so Ekman transport is invoked to explain the resultant bloom. Their analysis, they suggest, elucidates “the spatial response of chlorophyll to iron flux” and identifies “those regions where ocean biology is possibly tightly coupled with atmospheric Fe deposition.”
Really, of course, it does neither of these things. Statistical correlation between two factors that have been selected from among the very many that are involved in a com- plex ecological process cannot, of itself, demonstrate causation: that much we learn in elementary statistics courses. So I remain unimpressed by apparent correlation between dust deposition and those places where phytoplankton growth and accumulation is most
84 Chapter 5: Nutrient Limitation: The Example of Iron
active, because it is quite certain that many other factors, not considered, are involved in the complicated processes that lead to an algal bloom. In the Southern Ocean blooms are largely restricted to meandering frontal zones and are possibly enhanced downstream from shallow topography, as I have already discussed.
Then, the authors find it informative that both dust deposition and surface chlorophyll have seasonal maxima in the Southern Ocean in midsummer. They did not, apparently, consider that each of these cycles is an independent response to sun angle. The algal response is obvious at high latitudes, but it is not so well known that the dust-carrying winds from Patagonia are “monsoonal,” so that dust flies NW in winter and SE in summer. Nevertheless, the Ericksonet al. model has been influential.
The wide range of assumptions used for the input field of Fe at the sea surface indicates the level of uncertainty that is still inherent in geochemical nutrient models.
To obtain this input field in models, extrapolation has been made from observations of haze at sea (Duce and Tindale, 1991), from global vegetation cover data (Mahowald et al., 1999), from vegetation, soil texture, and land surface modification data (Tegen and Fung, 1994), and from the location of topographic depressions (Ginouxet al., 2001). If Fe is 3.5% of deposited dust (the usual assumption), the estimates of Fe deposition on the ocean ranges sixfold from 15 to 100 Tg y−1 so the global pattern of deposition flux cannot yet be narrowly constrained. Yet it is probable that only between 5 and 20N in the eastern Atlantic, and seasonally in the extreme NW Arabian Sea, is flux greater than 1–5 g m−2yr−1. In the southern hemisphere, it seems to exceed 025 m−2yr−1only patchily, in some small regions, and seasonally.
We remain uncertain even of the relative magnitude of the fluxes of labile iron across the sea surface and across the nutricline, although this must be critical to any quantitative understanding of Fe limitation in the ocean. For instance, the suggestion of de Baar et al. (1995) that an aerosol flux of 30 mg m−2 y1 dominates over vertical flux across the nutricline in the North Pacific is based on what they called an “order of magnitude assessment” of Duce and Tindale’s classical map. Modeling of the atmospheric flux has given equally diverse results: Funget al. (2000) simulated a deposition rate almost twice that estimated by the model of Mahowaldet al. (1999) and almost four times what was suggested by Duce and Tindale’s data (263 and 132×109mol y−1, respectively).
Funget al. remarked that uncertainty concerning aeolian flux to the photic zone is as large as a factor of 5–10, with an order of magnitude uncertainty in determining the soluble fraction. Despite this, they asserted that entrainment and upwelling deliver only 07×109mol Fe/y−1 across the nutricline, as against 960×109mol Fe/y−1across the surface: the consequences of entrainment and upwelling flux of Fe are trivial for these authors. On the other hand, Archer and Johnson (2000) compare aeolian flux from the sources quoted earlier and conclude that 70–80% of global carbon export production by phytoplankton can be supported by the “upwelling of iron in seawater rather than by atmospheric deposition.” These authors go on to propose that “ocean recycling of Fe appears to play a major role in determining the strength of the biological pump in the ocean and the pCO2of the atmosphere.”
Three recent intermediate-complexity ecosystem models appear to support, inadver- tently, the primacy of physical processes in determining the pattern of productivity in the ocean. Each is coupled to an OGCM and each comprises several classes of phytoplank- ton with appropriate herbivore classes and sinking terms. The models are distinguished principally by their nutrient assumptions. The first has three macronutrients with no Fe input, the second by the same team (Gregget al., 2003) is similar, but has Fe input from the GOCART model of Ginoux et al. (2001), and the third (Mooreet al., 2002) has a full range of nutrients (NO3 NH4 SiO3 PO4, and Fe). In this case, Fe input is from the models of Tegen and Fung (1994) and Mahowaldet al. (1999), with solubility set at 2%. A fourth model (Christianet al., 2002) is structured like that of Gregget al. (2003),
Models of Regional Nutrient Flux and Limitation 85
but is applied only to the tropical Pacific domain and uses Tegen and Fung (1994) for Fe input.
What is remarkable is that each of this wide range of models tracks rather well the CZCS or SeaWIFS chlorophyll fields, with believable interyear variability. Gregg and Conright (2001) remark that the model lacking Fe input “was able to represent the seasonal distribution of chlorophyll during the SeaWIFS era and was capable of distinguishing the widely different processes” that occur globally in Niño and Niña years.
The model presented global monthly maps of sea surface chlorophyll that adequately matched SeaWIFS maps for both pattern and values except near coasts, and SeaWIFS monthly means in 12 domains were tracked quite closely, excepting only the equatorial domains where the lack of river input (Amazon) and coastal influences (upwelling coasts) produced values that were overall too low. A similar model, with Fe input and a more complex representation of phytoplankton functional groups (Gregget al., 2003), performed very similarly, with very similar deviation from observations in the equatorial domains.
The model of Moore et al. (2002) likewise presents monthly global patterns of sur- face chlorophyll that closely match SeaWIFS observations, and global patterns of derived properties. Standard runs were compared with runs having no Fe input or having sat- urating Fe input globally; both produce global maps of primary production that are realistic and represent all the major features seen in maps of production rates computed directly from SeaWIFS surface chlorophyll. Yentsch’s effect of geostrophy is remarkably well preserved. The Fe-saturated model notably retains the high chlorophyll in the frontal zones of the Southern Ocean, strongly isolated from the more oligotrophic conditions between; the run lacking Fe input produced unrealistically low chlorophyll accumulation, unlike the model of Gregg (2002) that also lacks an Fe input but simulates reasonable chlorophyll biomass fields. Depending on the solubility assumption, the standard runs of the Mooreet al. model indicated annual global primary production of 42–47 Gt C y−1. This range was extended to 35–70 Gt C y−1 by including the runs with no Fe and with saturating Fe input; direct computations from satellite data all fall within this range.
Now, what would happen if dry deposition patterns of Fe were significantly to change in some way, so that deposition rates over the eastern Pacific resembled those over the eastern Atlantic? If indeed skies everywhere became as dusty as under West African harmattanwinds then, after a brief period of adjustment, each regional ecosystem would adopt a pattern of primary production and cell accumulation rather closely resembling what we see today. Further, there would be very little change in the specific composition of the characteristic phytoplankton communities, although the greatest uncertainty might be in the distribution of N-fixing organisms. The eastern Pacific would continue to accumulate chlorophyll in a manner typical of equatorial regions but quite different from what we expect in higher latitudes, or in low-latitude oligotrophic regions. I doubt if the sea-surface chlorophyll field of a dusty Southern Ocean would differ greatly, once equilibrium was established, from that of the present day. The new field would continue to reflect the simple fact that where Sverdrup does not permit growth of cell populations, none will occur, no matter what the ratio and concentration of available nutrients.
Obviously, where Sverdrup does permit cell population growth, but this does not occur because of lack of an essential element, then its addition—as in the iron-enrichment experiments—must induce a bloom. That such events occur naturally is not in doubt and there are now many observations that confirm this. For example, a response to dry deposition of Fe-rich terrestrial dust particles was observed in the ADIOS project in the oligotrophic, high-SNorth Pacific at 26N (Younget al., 1991). Here, in an oligotrophic ocean, when mixed-layer NO3 was in very low concentration, each deposition event created a brief, descending pulse (as the aerosol particles sank) of increased production rates of about 50% over ambient. Both iron and nitrogen were delivered in the dust,
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so that the near-surface concentration of NO3 increased from 0.2 to 07 nM kg−1 after one episode. Nevertheless, it was thought that the dust-fall released the autotrophic community from Fe limitation, rather than from NO3limitation. Similarly, on the West Florida Shelf, deposition pulses of African dust may provide an explanation of blooms of Trichodesmiumwhose nitrogenase enzyme system has a high iron demand; after a dust event, background iron levels increase from <05 to 16 nM kg−1, and Trichodesmium colonies increase a hundredfold (Leneset al., 2001).
To what extent these observations can be generalized is unknown, but such events can be expected only in regions where the prevailing winds carry heavy dust loads:
this occurs commonly under the Atlantic trade winds and the North Pacific westerlies, but not elsewhere. Their frequency is unknown but probably follows the relative rate of dry deposition in each region. It has also been suggested that the flux of both iron and nitrogen at Midway 180W 30N and Bermuda 60W 32N may represent a major part of nutrient flux to the photic zone, and sufficient to induce a significant growth response (Donagheyet al., 1991). Suggestions have also been made that the strong increases in deposition of both desert dust and pollutant aerosols after the mid-20th century are currently modifying the structure of the base of the pelagic ecosystem by inducing anomalous population growth both of pathogenic microbes and of diazotrophic cyanobacteria (e.g., Hayeset al., 2001).
Observations such as these support the view that there must be regional consequences for phytoplankton growth of the deposition of aerosols at the sea surface and that we should expect that the most significant deposition occurs at the sea surface of aerosol particles, whether of desert dust or industrial haze. Accepting this view, we may conclude that the high productivity of parts of the Atlantic, especially in low latitudes, is one result of this process. Unfortunately, this is very difficult to demonstrate and, indeed, has yet to be done: nevertheless, I have already seen statements making this connection. The problem, of course, as discussed earlier is that it is difficult to isolate the deposition effect from the many other factors that control phytoplankton growth and accumulation. The physical forcing processes that are involved differ strongly between oceans, and between comparable biogeochemical provinces in each ocean: this makes meaningful comparison of aerosol deposition effects very difficult.
Recourse may be had to an estimate of phytoplankton productivity, partitioned among 50-odd biogeochemical provinces (Longhurstet al., 1995), that has been revised for this volume, using the same algorithms, with data both from SeaWiFS and from MODIS for 2002–2005. In only 8 provinces does total phytoplankton production exceed 400 gC m−2 y−1 and, of these, 3 are in Atlantic low latitudes: CNRY (Canary Current upwelling), GUIA (Amazon shelf and plume), and GUIN (tropical West African coast).
The other high-productivity provinces are the coast of China, the NE, SE and NW Atlantic shelf regions, the Alaska shelf, and the NW Arabian Sea. In each of these 8 provinces, as in the remainder, serial surface chlorophyll images clearly suggest that the pattern of productivity matches that of physical processes, rather than the more diffuse pattern of aerosol deposition.
It may be significant to the question of Fe deposition that the Canary Current upwelling province, lying directly below the Saharan dust plume, has almost twice the productivity of each of the other three eastern boundary current upwelling provinces: 710 gC m−2y−1 compared with 269–396 m−2y−1. However, these four upwelling provinces are not directly comparable, and the Canary Current region is unique in the great width of its shelf.
Then, tropical West Africa (GUIN) has the only coastline anywhere that is aligned close to, and parallel with, an equatorial current system. The Amazon shelf (GUIA) receives the total nutrient flux of the largest river on any continent. So, it is altogether too early to ascribe the relatively high production of some Atlantic regions to the effect of African dust with any confidence. For what it is worth, which isn’t much, statistical
Models of Regional Nutrient Flux and Limitation 87
correlation between regional production in all biogeochemical provinces and a “dust factor” (obtained from the interaction between contours in the Ginoux model and province boundaries) does not suggest a global relationship (y=29x−00003R=004) between deposition and productivity. Obviously, although many assume that such a relationship must exist, it cannot yet be demonstrated: isolated studies of only deposition processes and phytoplankton nutrient dynamics are insufficient for such a demonstration, because integration of holistic regional ecology and ocean physics is required.
Preoccupation with surface flux of a micronutrient was one part of our response to the possibility that the global CO2 flux budget might be manipulated by artificial inputs of particulate Fe, repeating what may have occurred naturally during glacial epochs. Of course, this possibility aroused intense interest and major funding, so it is not surprising that studies tended to place more emphasis on atmospheric flux than on more classical problems of nutrient flux into the photic zone from below. Nor would it be very surprising that results tending to demonstrate Fe limitation should have been emphasized, as has perhaps happened in recent years (e.g., Behrenfeld and Kolber, 1999).
So, if myths in science are “selective conjectures thereafter given privileged status”
lacking an “alternative suggestion” (Dickinson, 2003), then I suggest that when we think about nutrient limitation in high-S regions, we should remember the “alternative sug- gestion” of complex vertical flux of several nutrients across the nutricline. We should not think only about fluxes of iron across the sea surface, just because this happens to have been the fashionable topic of the recent past. However, in complex fields of natural science like geology and biological oceanography, where neither observations nor models can capture the true complexity of the dynamic system, it is true that myths—in the sense of geologist Dickinson—do have a role to play in arriving at scientific truth.