Cálculo del Uso del ER

Marine predators play a fundamental role in mantaining the function of marine ecosys- tems and they are sentinels of the their ecosystem health. Their study however presents a challenge due to complexity of their behaviour and of their turbulent pelagic environ- ment. Recently, progress in bio-logging technology, remote sensing and modelling have opened novel possibility of addressing these inter-disciplinary questions. My work has explored how (sub-)mesoscale turbulence structures marine predators habitat by affect- ing their movement, their foraging behaviour, and the trophic chain they depend on. The (sub)mesoscale is indeed a regime which is expected to have a twofold structuring role on the ecology of top predators: Firstly, through bottom-up effects (because of its impact on lower trophic levels); and secondly, by direct means, because the (sub)mesoscale oc- curs on temporal scales of days to weeks, which are the same of the behavioral switches of predators. I focused on the case of the Kerguelen region, which is an ideal end-to-end case study because (i) several marine predators species have large colonies on the island, (ii) the area is located in a highly dynamical ocean regime dominated by the Antarctic Circumpolar Current, with strong (sub)mesoscale activity, (iii) the trophic web in the area is relatively simple and (iv) production is dominated by iron limitation, making it possible to disentangle physical and ecological effects. I combined bio-logging, remote sensing, in-situ (samples from ships and autonomous platforms like ”bio-argo” profilers) with ecological and Lagrangian modelling to study how (sub)mesoscale features affects movements and foraging behaviours of marine predators from a mechanical and ecolog- ical point of view. My work had two main axes: a first one that relates marine predator to the mechanical effect of (sub-)mesoscale turbulence (partII) and a second one where I combined ecology and stirring to describe the “biological” mesoscale landscape explored by Kerguelen’s marine predators (partIII).

The conclusions of this study can be summarised as:

1. The two species I studied (foraging elephant seals and Macaroni penguins) exhibit “quasi-planktonic” bouts where they horizontally drift together with the water par- cel in which they dive for finding their food resource. Locations of quasi-planktonic behaviour statistically correspond to more intense foraging and high frontal activ- ity, suggesting another possible and non-exclusive mechanism to interlace animal trajectories and (sub-)mesoscale fronts apart from bottom-up mechanisms. The re- sults of Chapter1and2challenge the assumption – until now common among ma- rine ecologists - that the mechanical effect of (sub-)mesoscale currents’ on marine predator is negligible. A direct consequence is that models that aim at inferring differences in foraging behaviour from movement patterns are likely to improve

their accuracy if the trajectories are corrected for the effect of the currents. Also, this approach allows to identify qualitatively which sectors along a predator’s tra- jectory are likely to be characterised by intense foraging behaviour, as shown in Chapter1. The findings presented in Chapter1emerge from the specific case study of Kerguelen’s elephant seals, but they are confirmed by observations on Macaroni penguins as detailed in Chapter 2. Furthermore, my results are not specifically related to the study region and the same principles and broad findings are likely to be relevant in other highly dynamical regions such as the Gulf Stream, the Kuroshio Current, the Agulhas and the East Australian Current [Cheney et al., 1983, Olson, 1991].

2. The Lagrangian approach “water age”, that represents how long before water parcels have been in contact with the Kerguelen Plateau (i.e. before the estimated iron enrichment), positively correlates with diatom-dominance inferred from in- situ pigment sampling and optical re-analysis of ocean color images. This result provides a proxy for favourable conditions for diatom dominance of the phyto- planktonic community that is based on horizontal transport, a variable that is almost synoptically quantified thanks to altimetry. Although this work is focused on the Kerguelen region, the Lagrangian approach is appropriate for application to other iron-limited regions (e.g. the Crozet archipelago, the Galapagos Islands), or even more generally in cases where the distribution of a resource (e.g. nutrients, waters with favourable temperature, etc.) is largely driven by horizontal transport [Coles et al., 2016,Kubryakov et al., 2016].

3. A simple ecological model describing variations in phytoplankton communities in relationship with iron (inferred from altimetry-derived pathways from the plateau) shows that bistable ecological states may occur. This means that, besides being affected by the concentrations of available iron, whose effect is studied in the “threshold model”, the initial phytoplankton inoculum that is picked up and ad- vected within a water parcel from the plateau can determine which branch of the trophic web develops. In order to estimate the effect of phytoplankton bistabil- ity on the trophic chain, I introduced a qualitative ecological modelling approach, and found that waters hosting a bloom of large diatoms are able to support the growth of crustaceans and fish larvae that constitute a large part of the diet of penguins, seals and large fish. These preliminary results show that iron forcing and inter-specific competition regulate the structure of the base of the Kerguelen trophic web and their effect qualitatively propagate across the trophic web. A preliminary analysis shows that elephant seals’ foraging behaviour suggests that particularly profitable foraging areas correspond to water parcels that have been in contact with the Kerguelen plateau approximately 3-4 months before. Figure

2.9 a) shows three simultaneous trajectories of elephant seals that left the colony in the beginning of November 2011, swam and foraged for about 2 months before turning back towards the colony (25th December 2011). Areas characterised by intensive foraging are highlighted in grey, and occurs in the proximity of water parcels of water age around 100-120 days Figure 2.9 b), that are likely to have hosted blooming phytoplankton. While this is a tantalising result, we acknowl- edge that it is qualitative and preliminary, and that a more systematic analysis of several trajectories would be more conclusive. Also, the relationship is diffi- cult to quantify given that the accuracy on the location of water parcels decreases as time of advection increases [Ozg¨¨ okmen et al., 2000]. However, the result is nonetheless promising and pushes further the findings by Cotte’ and co-authors [Cott´e et al., 2015] who showed that elephant seals preferred water parcels that had hosted the spring bloom on both their post-moult and post-bloom foraging trips (February-September).