Improving our understanding of climate variability effects on Weddell seals

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Circulation and water masses

The Southern Ocean is one of the main drivers of the global thermohaline circulation (THC), which contributes to the world meridional redistribution of heat (Orsi et al. 1999; Marshall et al. 2008). The Southern Ocean therefore plays a fundamental role in the regulation of the global climate (Orsi et al. 1999; Marshall et al. 2008). This is dependent on a complex zonal and meridional hydrological circulation dominated by the Antarctic circumpolar current (ACC); the major feature in the Southern Ocean (Orsi et al. 1995, 1999; Marshall et al. 2008)(Fig. 1.3). The ACC flows clockwise (from west to east) far offshore (between 40°S and 65°S) around Antarctica (Orsi et al. 1995). It is itself sub-divided in three major fronts: the sub-Antarctic front (which sets the northern boundary of the Southern Ocean according to Deacon 1933), the Polar front and the southern boundary of the ACC (SB-ACC) (Orsi et al. 1995) (Fig. 1.3). In contrast, the inshore cold Antarctic coastal current circulates from east to west between the coast and the SB-ACC (Orsi et al. 1995; Nicol et al. 2006). The currents and water masses are separated by a series of frontal zones and the Antarctic coastal current is composed of a series of complex interlinked gyres rather than being a coherent zonal current (Orsi et al. 1995; Nicol et al. 2006). Moreover, the position and complexity of these hydrological boundaries are often defined by regional topographic and bathymetric features. This results in considerable meridional variations in hydrological regimes around Antarctica (Nicol et al. 2006).

Winter Antarctic environment

Figure 1.3. Southern Ocean geography and principal fronts from Talley et al. 2011 (Descriptive physical oceanography: an introduction, chapter 13). The Subtropical Front (STF) is the oceanographic northern boundary for the region. The eastward Antarctic Circumpolar Current (ACC) includes several fronts: Subantarctic Front (SAF), Polar Front (PF), Southern ACC Front (SACCF), Southern Boundary (SB). Front locations are taken from Orsi et al. (1995). The westward Antarctic Slope Front (ASF) (thin) follows the continental slope.
For instance, because there is little geographic variability along the coastline of the East-Antarctic region it is dominated by circumpolar circulation, unlike the Weddell and Ross seas, which are both characterized by a large embayment, and therefore influenced by large gyres (Nicol et al. 2006) (Fig. 1.3). This hydrological circulation appears to drive annual regional sea-ice extent, the position of oceanic boundaries, and biological productivity, resulting in the structuration of the pelagic Antarctic ecosystem (Nicol et al. 2000). Indeed, a winter survey conducted in East Antarctica revealed a positive relationship between the offshore distance of the SB-ACC and the maximal extent of winter sea ice (Nicol et al. 2000). Moreover, they found productivity at all levels (e.g. primary productivity, zooplankton, whales and seabirds) is also influenced and delimited by the SB-ACC. For instance, productivity occurs in a wider band where SB-ACC is located further offshore (i.e. western section of survey area [80-115°E] close to Davis), whereas productivity is concentrated nearer to the coast as the SB-ACC approaches the coast (115-150°E encompassing DDU) (Nicol et al. 2000).
The ACC also plays a crucial role in the formation of Antarctic Bottom Water (AABW). The AABW is the cold, dense and oxygen rich water mass laying in the abyssal layer, accounting for 30-40 % of the global ocean mass (Johnson 2008). AABW production is a key process of the THC, responsible for the ventilation and supply of nutrients to abyssal layers of the world’s major oceans (Orsi et al. 1999; Williams & Bindoff 2003; Marshall et al. 2008; Ohshima et al. 2013) (Fig. 1.4). First, the warm and deep saline waters that originated in the northern hemisphere upwells at the south of each of the three global ocean basins and are transported around Antarctica by the ACC (Schmitz 1995) (Fig. 1.4). While surfacing, these circumpolar deep waters (CDW) mix along their path with the colder Antarctic surface water (ASW) thereby forming denser intermediate waters, known as modified circumpolar deep waters (MCDW) (Williams et al. 2008, 2010) (Fig. 1.5). Then, AABW formation involves the formation of high salinity shelf waters (HSSW) from the pole-ward intrusions of MCDW trough deep bathymetric canyons and depressions over the continental Antarctic shelf (Williams et al. 2008, 2010) (Fig. 1.5). Brine rejection from sea-ice formation during winter is the major process of HSSW formation (Williams et al. 2008, 2010) (Fig. 1.5). Also involved, is the mixing with cold, fresh ice shelf waters (ISW) from ocean/ice interactions beneath ice shelves which increase the density of the HSSW enough that they sink (Williams et al. 2008, 2010) (Fig. 1.5). The newly formed AABW sinks to the abyssal layers, crosses the continental shelf break at specific locations and mix down the continental slope flowing equator-wards in each of the ocean sectors (Williams et al. 2008, 2010) (Fig. 1.4 and 1.5). The conversion of MCDW into the cold, saline AABW only occurs in several unique locations around Antarctica (Fig. 1.4). These include the Weddell sea (71%) (Foster & Carmack 1976; Fahrbach et al. 1995; Foldvik et al. 2004), the Ross sea (6%) (Jacobs et al. 1970; Whitworth & Orsi 2006), as well as two locations that encompass the two focal study sites within East Antarctica: the Adelie Land coastline (23%) (where DDU is located; (Williams et al. 2008, 2010) and the Prydz bay (where Davis is located) which could account for 6-13 % of AABW production (Ohshima et al. 2013). Thus, East Antarctica is a major contributor to AABW formation.
Figure 1.4. The global thermohaline circulation around Antarctica from Schmitz (1995), highlighting in particular the circulation of Antarctic Bottom Water.
Figure1.5. Antarctic bottom water formation on the Antarctic continental shelf from Williams et al.
(2010).
In some locations such as the Prydz bay and Adelie land, the formation of dense water is tightly linked to large coastal polynya systems (i.e. areas of open water within the sea-ice; Mertz Glacier polynya in Adélie Land and Cape Darnley polynya in Prydz bay) that results in intense ice production, and therefore, enhanced brine release into the water column (Fig. 1.6) (Williams et al. 2008, 2010; Ohshima et al. 2013). Another dominant factor is the onshore flow of MCDW on the shelf, allowed by the regional bathymetric features (i.e. presence of deep canyons and depressions), which increases shelf water salinity, and potentially supplies sufficient heat to help maintain the polynya (Rintoul 1998 p. 199; Williams & Bindoff 2003) (Fig. 1.6). In combination, these processes greatly increase the salinity of the shelf water, enhancing HSSW production (Williams et al. 2008; Ohshima et al. 2013). Over time, the volume of dense shelf water increases and accumulates in bathymetric depressions (i.e. Adélie depression in Adelie Land) until it spills over the shelf break (e.g. the Adelie sill in Adelie Land) and descends to the abyssal layer (Fig 1.6) (Williams et al. 2008, 2010; Ohshima et al. 2013).

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Sea-ice environment

Sea-ice is a substrate, which after initial freezing of sea water, is profoundly modified by interactions between physical, biological and chemical processes (Dieckmann & Hellmer 2010). Antarctic sea-ice is highly dynamic that extends from hundreds to thousands of kilometres from the land in winter, before melting and receding back towards the shore in summer. Indeed, its annual expansion varies from ~ 19 million km2 in September (end of winter) to only ~ 4 million km2 remaining in February (end of summer) (Comiso & Nishio 2008) (Fig. 1.7). This represents one of the greatest seasonal changes in physical properties anywhere on Earth (Nicol et al. 2006; Massom & Stammerjohn 2010). However, in some locations, such as the Weddell and the Amundsen sea, the sea-ice persists over years (Fig. 1.7).

Table of contents :

PART I : GENERAL INTRODUCTION
A – WINTER ANTARCTIC ENVIRONMENT
1. Antarctica and study sites
2. Circulation and water masses
3. Sea-ice environment
B – SEA-ICE DEPENDENT PELAGIC ECOSYSTEM
1. Sea-ice community
2. Upper trophic levels depending on sea-ice habitat
2.1 The Antarctic continental shelf assemblages
2.2. The Antarctic shelf-break assemblages
C – STUDYING THE FORAGING ECOLOGY OF TOP PREDATORS
1. Foraging in a heterogeneous environment
2. Detection of foraging behaviour
2.1 Inferring foraging activity from tracking data
2.2 Direct measurements of foraging success
2.3 Inferring foraging activity from diving behaviour
3. Habitat use of top predators
D – THE WEDDELL SEAL
E – CONTEXT, OBJECTIVES AND THESIS OUTLINE
PART I I : METHODOLOGICAL DEVELOPMENTS
A – INTRODUCTION
B – PAPER 2
1. Abstract
2. Introduction
3. Materials and methods
3.1 Tagging procedure
3.2 Fine scale analysis of foraging behaviour
4. Results
4.1 General diving behaviour
4.2 Foraging behaviour
5. Discussion
5.1 Detection of intensive foraging activity within dives
5.2 Fine scale foraging strategy of Weddell and southern elephant seal
6. Conclusion
7. Acknowledgements
8. Appendix
C – PAPER 3
1. Abstract
2. Introduction
3. Material and Methods
3.1 Fine scale analysis of foraging behaviour
3.2 From high-resolution to low-resolution dive datasets
4. Results
4.1 General diving behaviour
4.2 From high-resolution to low-resolution dives: estimation of foraging effort
5. Discussion
5.1 Foraging effort in low-resolution dives
5.2 Ecological applications
6. Conclusion
7. Aknowledgements
8. Appendix
D – CONCLUSION
PART I I I : WEDDELL SEALS HABITAT USE DURING
ANTARCTIC WINTER
A – INTRODUCTION
B – PAPER 4
1. Abstract
2. Introduction
3. Materiel and methods
3.1 Animal handling and tagging
3.2 Data collected from the tags
3.3 Argos Kalman filtering
3.4 Bathymetry and sea-ice data
3.5 Hydrological data
3.6 Behavioural data
2.7 Statistical analysis
4. Results
4.1 Tag performance, foraging areas and diving features of Weddell seals in winter
4.2 Use of hydrographic environment
4.3 Influence of environmental and temporal factors on foraging behaviour
4.3.3 Bottom time residuals
5. Discussion
5.1 Effect of winter advancement and circadian light cycle on diving patterns
5.2 Habitat selection and influence of the environment on foraging behaviour
5.3 Future studies
6. Conclusion
7. Ackowledgement
8. Appendix
C – PAPER 5
1. Abstract
2. Introduction
3. Material and methods
3.1 Instrumentation
3.2 Argos locations filtering and track simulations
3.3. Environmental data
3.4 Diving behaviour
3.5. Movement pattern analyses
3.6 Statistical analysis
4. Results
4.1 Tag performance
4.2 Movement patterns
4.3 Diving behaviour
4.4 Identification of Area-restricted search
4.5 Area-restricted search behaviour
5. Discussion
5.1 Methodological discussion
5.2 Foraging strategies of the focal Weddell seals
5.3 Habitat use
6. Conclusion
7. Acknowledgements
8. Appendix
D – CONCLUSION
PART IV: GENERAL DISCUSSION, CONCLUSIONS AND PERSPECTIVES
A – METHODOLOGICAL DISCUSSION
1. Analysis of diving behaviour to infer foraging activity
1.1 The development of a new method
1.2 Detection and prediction of foraging success
2. Track analysis and implementation of the foraging effort index
B – ECOLOGY OF THE WEDDELL SEAL DURING WINTER
1. Context
2. The fast-ice: a primary habitat for Weddell seals
2.1 Movement patterns of Weddell seals during winter
2.2 Overwintering in fast-ice allows predator and inter-specific competition avoidance
2.3 Assessment of important sea-ice features to Weddell seals
3. Foraging strategies of Weddell seals
3.1 Optimal foraging from a breathing hole
3.2 Inference on Weddell seals’ diet from diving behaviour
3.3. Weddell seals’ adaptation to winter conditions
3.4 Environmental parameters influencing the behaviour of Weddell seals
C – CONCLUSION AND PERSPECTIVES
1. Main conclusions
2. Climate change and Weddell seals
2.1 Observed changes in Antarctic sea-ice
2.2 Assessment of potential climate change effects on Weddell seals
3. Perspectives
3.1 Information on prey: the missing link between top predators and their environment
3.2 Improving our understanding of climate variability effects on Weddell seals
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