Conclusions

The shelf break is a highly dynamic environment where oceanic and coastal waters meet; therefore the dissolved iron distribution was expected to be influenced by a multitude of processes induced by these two different environments.

Results are consistent with the main source of dissolved iron near seafloor being POM remineralisation, but other processes including mixing and removal complicated the interpretation. Dissolved iron concentrations were highest (5.4 nM) on shelf, and pore water resuspension was likely an additional source of iron to these bottom waters. Transport of dissolved iron was evident. Horizontal advection of dissolved iron (~ 3.2 nM) associated with an intermediate nepheloid layer propagating along an isopycnal was identified, and dissolved iron was possibly also transported within the along-slope pole-ward flowing current. A second weaker deeper INL did not show enhanced dissolved iron concentrations relative to background values (~ 1.3 nM), which may be due to variations in the scavenging efficiency or in the magnitude of the sources of dissolved iron. There was also evidence of vertical advection of nutrient-rich waters underlying the thermocline to the surface at the shelf break front, driven by the internal tide and shallowing topography. In the seasonal thermocline, the biology and nutrient distributions were typical of summertime in the northern hemisphere, and dissolved iron

uptake was suggested at the chlorophyll a maximum at two stations on the upper slope.

Nitrate appeared to be limiting phytoplankton growth in most of the seasonal thermocline; however, the phytoplankton population may become iron-stressed at some upper slope stations. Other forms of iron limitation, stress, or co-limitation were considered, and should be further investigated in the future.

Potential consequences of enrichment of shallow waters with dissolved iron were examined along an additional transect at the North Scotia Ridge between the Falkland Islands and South Georgia in the Southern Ocean. It was suggested that benthic sources may alleviate iron-limitation downstream of South Georgia, and lead to increased biological activity and photo-physiological efficiency. These results therefore support the theory of the “island mass effect” in HNLC waters of the Southern Ocean as already

shown at the Kerguelen Islands (Blain et al., 2001), and is under investigation at the

Crozet islands.

Implications of these results reside in the improvement in our understanding of the iron cycle in shelf break environments (see model Figure V.1). Initially dissolved iron, nitrate, phosphate, and silicon for diatoms are taken up by phytoplankton in the nutrient- rich surface waters during the spring bloom. Sinking POM is then partially remineralised below the thermocline releasing nutrients. These shallow nutrient-rich waters may then be advected vertically, especially at the shelf break front, and fertilise nutrient-depleted surface waters. This recycling likely sustains a bloom at the shelf edge and allows growth of larger cells. When reaching the seafloor, the remaining fraction of POM is remineralised releasing dissolved iron and nutrients. On shelf, POM remineralisation in sediments will intensify if more detritus reaches the seafloor, and this may lead to micro-reducing zones where iron oxides could be dissolved through this bacterial respiration. Resuspension of sediments or mixing through bio-turbation may then release dissolved iron from pore waters into bottom waters in addition to that released by POM oxidation. Dissolved iron is likely organically complexed or colloidal when released from sediments therefore stabilising it when entering oxic waters, but a significant portion is eventually lost from solution by precipitation and/or adsorption onto particles. Iron- and SPM-enriched bottom waters may then be transported laterally as intermediate nepheloid layers or within the along-slope current. During wintertime, the mixed layer deepens towards the seafloor leading to enrichment of surface waters in iron and macro-nutrients, which are then consumed during the spring bloom.

As the aim of this work was to give a conceptual framework for discussing processes, several questions are raised, which could not be answered in the scope of this study given the limited data, but they do provide a basis for future work:

1. Sources of dissolved iron to bottom waters

We clearly need a better understanding of release processes near the seafloor in order to determine fluxes of dissolved iron from benthic sources, and fluxes that actually reach surface waters, and thus allow them to be included in the global budget of oceanic iron

(Elrod et al., 2004). Additionally, it is still unknown whether dissolved iron is

organically complexed when released from pore waters or from POM oxidation. This point is important in understanding how high dissolved iron concentrations may be sustained in oxic shelf waters and possibly transported offshore. The source and stability of these organic ligands also remains unknown and could potentially be of biological or terrestrial origin. The importance of inorganic colloids in the dissolved

iron fraction and their role in the iron cycle is also still largely unclear. Hong et al.

(1986) showed that a significant fraction of iron released from sediments was Fe(II) at the Peru upwelling system. No additional studies were carried out in non-upwelling systems so that the fate of dissolved Fe(II) in oxic waters such as the Celtic Sea shelf edge is unknown. If dissolved Fe(II) were to be transported in oxic waters, it should be stabilised by organic complexation before its oxidation to Fe(III). Additionally if it were to reach the euphotic zone its almost immediate removal by biological uptake would be expected.

2. Transport / export of dissolved iron

In this study, high dissolved iron concentrations were measured only within one of the two observed intermediate nepheloid layers. This result implies that dissolved iron can survive particle scavenging in some conditions, and that it can be decoupled from particles, and therefore clearly needs to be investigated further. Additionally, since dissolved iron can potentially be transported within intermediate nepheloid layers, it would be interesting to determine how far offshore enhanced dissolved iron concentrations can be measured, as this would give an indication of removal kinetics within these layers providing that the velocity and mixing of these waters can be established. However, following an intermediate nepheloid layer may be a real challenge (R. Lampitt, 2005, personal communication).

Vertical advection of dissolved iron was also observed in this study. Since this mixing was likely induced by internal tide propagation, it would be interesting to monitor dissolved iron concentrations and its speciation for several tidal cycles at stations with different degree of stratification. This experiment may allow determination

of a vertical flux of dissolved iron, and its fate, but may be difficult to study due to the dynamics of the system, and the work load involved.

3. Biological influence on dissolved iron distribution in surface waters

Additional studies are needed at the shelf break front to determine the potential for iron stress, limitation, or co-limitation of phytoplankton. Incubation experiments could be

carried out at stations with different water column stratification (i.e. on shelf, at the

shelf break, at the upper slope, and offshore), with iron and/or other nutrients additions while monitoring physiological parameters, species composition, and zooplankton grazing. This limitation would be expected to occur only at the end of the summer when recycling may not be sufficient to provide nutrients in the Fe:N ratio required for minimum growth of coastal species.

The role of the zooplankton community could also be examined in terms of their participation in recycling or export of dissolved iron from the euphotic zone, which potentially can increase the iron stress for the phytoplankton population (Wang and Dei, 2001).

CHAPTER

VI.

VI.1. Initial objectives

The aim of this project was to improve our understanding of the marine iron cycle by investigating the processes influencing dissolved iron distributions in different environments. The two major objectives were: 1) to develop an analytical method to determine dissolved iron in seawater at sub-nanomolar concentrations, and to ensure the quality of the data obtained; and 2) to use this method to determine dissolved iron in samples collected in different environments: the Celtic Sea shelf and shelf edge, and the open Atlantic Ocean.

The implementation of the analytical method using recent published methods proved difficult, and was not a trivial exercise. Given the difficulties in optimising the initial method chosen (see Chapter II), an alternative technique was developed, which also proved difficult but was in the end successfully used (see Chapter III). The quality of the analyses of main samples was found satisfactory for specific samples based on current means of assessment (see Chapter IV). A summary of main findings during this analytical exercise, and comments on future work are given in Section VI.2.

Two sets of samples were collected using careful trace metal techniques, as contamination risks are high when sampling for iron (see Chapter IV). Unfortunately despite all precautions, one set of samples was contaminated apparently through diffusion of iron from the walls of the storage bottles into some of the samples analysed from the AMT-12 cruise in the open Atlantic Ocean (see Chapter IV and Section VI.2). Despite uncertainties in the quality of the analysis, samples collected during the JR98 cruise at the Celtic Sea shelf edge were generally of good quality (see Chapter IV), and the data was interpreted in terms of processes (i.e. sources, removal, and transport) influencing dissolved iron distribution at the Atlantic Ocean – Celtic Sea shelf edge. This data was used in association with ancillary information to provide a conceptual framework for future studies in these highly dynamic environments (see Chapter V). An additional set of samples from the North Scotia Ridge between the Falkland Islands and South Georgia (Atlantic sector of the Southern Ocean, not collected within this project) was analysed using the newly developed technique for total dissolvable iron (see Chapter V). This additional study gave insights into the importance of benthic sources of iron for enhancing primary production and the physiological impact on algal cells of the alleviation of iron-stress in regions of the ocean where atmospheric inputs are low.

A summary of main findings from the study of the Celtic Sea samples and Southern Ocean samples, and suggestions for future work are given in Section VI.3.