Rationale

The work contained within this thesis deals entirely with open-ocean plankton ecosystems. For modelling purposes the surrounding waters of the Kerguelen Plateau are treated as an open ocean system. The effect of coastal margins is not taken into account.

Photosynthesis uses CO2 as its raw material. Over the past two three decades, the

scientific community has become concerned about the level of CO2 in the atmosphere

because of its rise in concentration due to the actions of human civilization (particularly the combustion of carbon-rich fossil fuels and the concurrent clearance of large areas of forested land).

Greenhouse gases influence the thermal balance of the Earth’s atmosphere by being transparent to shorter wavelength, visible electromagnetic radiation, but opaque to longer wavelength, infrared radiation. This essentially allows these gases to “trap” thermal energy in the atmosphere, and to warm the surface of the Earth. This effect has been a feature of the atmosphere for billions years, and in the past the concentrations of greenhouse gases have been considerably different to those at present. Past variations in the CO2 level of the atmosphere have been associated with

climate change [Adams et al., 1990; Falkowski et al., 2000] and this, plus the fact that the anthropogenic changes to the atmosphere at present are occurring at rates considerably faster than the natural rates measured from ice-core records [Monnin et al., 2001; Petit et al., 1999], suggests that increasing atmospheric CO2 is likely to

substantially affect both the natural biosphere and human activity.

There are many possible consequences that global warming through an enhanced greenhouse effect may result in changes to sea-level, weather patterns, ocean circulation, species distribution and the geographical range of diseases such as malaria [Boning et al., 2008; Martens et al., 1997; Poloczanska et al., 2007; Riebesell, 2004]. For example, in the context of plankton systems, Roemmich & McGowan [1995] report that, concurrent with a 1.5°C rise in surface water temperatures (probably caused by greenhouse warming), zooplankton populations in the California Current have fallen by 80% over a period of 40 years (they suggest that the mechanism is a shallower mixed layer and reduced phytoplankton production). Joos et al. [1999] and

Cox et al. [2000] draw the attention to several potential pathways of positive feedbacks which may occur as result of global warming (e.g. increases in atmospheric water vapour, decreases in reflective snow and ice, methane from wetlands, etc.). Hardin [1985] underlines the potential significance of such positive feedbacks by contrasting the relatively mild climate of the Earth at present with that of its sister planet, Venus, apparently a victim of an over-active greenhouse effect.

Given the potential for climate change to induce large-scale changes in the biosphere, researchers are interested in mechanisms which control or affect the quantity of CO2 in the atmosphere. For obvious reasons, mechanics which promote

“carbon burial” (the removal of CO2 from the atmosphere to long-term sinks) are of

particular interest. A major carbon burial (~80% of the total) route is via the carbonate-silicate cycle, where the weathering of silicate rocks results in the formation of carbonates which are subsequently buried by natural geological processes [Emerson & Hedges, 2008]. The remaining carbon burial occurs via biological activity (i.e. organic material which is not oxidized and is buried geologically).

In this context, plankton systems are important, since a fraction of their annual production is exported to the deep ocean where it may ultimately be buried [Adams et al., 1990]. Raven [1995] estimates that 0.3% of annual marine production is preserved in deep ocean sediments by this mechanism. While terrestrial production may also bury carbon, the carbon it consumes comes mostly from the atmosphere. Since almost all (95%; Siegenthaler & Sarmiento [1993]) of the global carbon pool exists in the

ocean (partially as dissolved CO2, but mostly as bicarbonate, HCO3 -

), any removal of CO2 directly from the atmosphere by, for instance, terrestrial primary production, may

merely shift the ocean-atmosphere equilibrium and lead to the replacement of the removed atmospheric CO2 by oceanic CO2. Consequently, the direct removal of

carbon form the oceans by plankton systems is of interest to researchers. Note nonetheless that there is still uncertainty about the potential importance of this carbon sink [Le Quéré et al., 2009].

In relation to a limitation on primary production, and of current interest, is the suggestion by Martin & Fitzwater [1988] that plankton in certain regions of the world ocean are growth-limited by the lack of availability of iron (which is an important micro-nutrient in certain photosynthetic and nutrient reductive enzymes; Geider &

LaRoche, 2004). While this suggestion has been vindicated by mesoscale iron enrichment experiments [Boyd et al., 2007], these short-term fertilizations are not able to assess all the ecosystem processes that follow from iron addition, and for this reason it was very useful to explore the role of persistent natural iron inputs in structuring ecosystems and transferring carbon. The Kerguelen Ocean and Plateau compared Study (KEOPS) was designed to examine the area of persistently high phytoplankton biomass that forms over the Kerguelen plateau each year. Shipboard observations in January-February 2005 revealed that the development of the largest phytoplankton bloom in the Subantarctic region [Sullivan et al., 1993] was due to natural iron enrichment [Blain et al., 2007; 2008].

Understanding the mechanisms causing such iron-fuelled blooms, and quantifying their associated primary production (and the fate of this production), is of fundamental importance in assessing global carbon budgets and modelling scenarios of global climate change. Here the aim of this thesis is to combine satellite observations of ocean properties with models of primary production and transport to explore the mechanism by which surface iron fertilization can affect phytoplankton biomass in the Southern Ocean.

For this study, a zero-dimensional biogeochemical model, based on the NPZD

model developed by Oschlies & Garçon [1999], has been modified. The thesis will focus on the modification of the biogeochemical model to its application in the Southern Ocean surface waters. Since 1999 there has been several iron fertilisation programs [see Boyd et al., 2007 for a comprehensive review] and two natural iron fertilisations studies [Blain et al., 2007; Pollard et al., 2009] in Southern Ocean waters. Data from the first Southern Ocean Iron Release Experiment (SOIREE) and from the first natural iron fertilization study in the Southern Ocean, KEOPS, are used to evaluate the model.

As well as for our understanding of the role of iron in the control of primary production, understanding the mechanisms that drive the plankton dynamics under different iron inputs scenarios are important for climate change studies [Boyd et al., 2008], geoengineering studies [Lampitt et al., 2008], iron biogeochemistry studies [Bowie et al., 2009] and paleo-climate studies of past atmospheric composition [Watson et al., 2000]. Acknowledging that an iron-tracking model would be able to

address additional issues such as iron supply and recycling, emphasis was on the biomass response which does not require and explicit iron model.

In summary given the significance of global warming, and the importance of plankton ecosystems in influencing the global carbon cycle, understanding the physical and biological dynamics which govern plankton communities is a key step towards improving our understanding of the processes which may ultimately weigh heavily on the fate of human civilisation.

This chapter has attempted to lay the foundation for the biology and physics which underlie the model that is applied and modified in this thesis. Additionally, sections have aimed to explain the simplification of this foundation by models.