Deep Sea Research Part I: Oceanographic Research Papers
Community structure across a large-scale ocean productivity gradient: Marine bird assemblages of the Southern Indian Ocean
Introduction
Oceanic species distributions are influenced by physical and biological variability over multiple spatio-temporal scales (Haury et al., 1978; Hunt and Schneider, 1987). Over large (meso-mega, 100 s to 1000 s km) spatial scales, the ranges of large predatory fishes, marine birds, mammals, and turtles mirror oceanographic domains and current systems (Griffiths et al., 1982; Sund et al., 1981; Tynan, 1998; Polovina et al., 2004). Smaller-scale (coarse, 10 s km) physical processes enhance localized ocean productivity and concentrate nektonic predators and their prey at hydrographic (e.g., fronts) and bathymetric (e.g., shelf-slope) features (Franks, 1992; Hunt et al., 1999; van Franeker et al., 2002).
The southern Indian Ocean (20–60°S) is characterized by strong latitudinal gradients in physical, chemical, and biological properties (Metzl et al., 1991; Park et al., 1993; Park and Gambéroni, 1995; Fiala et al., 2003). A major oceanographic feature in this region is the subtropical convergence (STC), a broad (∼100 km) frontal region separating warm and salty subtropical waters to the north, from cold and fresh sub-Antarctic waters to the south (Park et al., 1993, Park et al., 2002; Park and Gambéroni, 1995; Kostianoy et al., 2004). Between 30°S and 45°S, surface waters are brought together by the prevalent wind patterns, thereby creating a zone of enhanced surface gradients (Lutjeharms, 1985; Furuya et al., 1986; Park and Gambéroni, 1995; Rio and Hernández, 2003). The STC is recognizable by a strong latitudinal decline in water temperature (sea-surface temperature (SST)) of ∼7 °C (summer: 18–10 °C, winter: 15–8 °C), concurrent with a drop in salinity, and an increase in nitrate and phosphate (Lutjeharms et al., 1985; Longhurst, 1998). This permanent frontal zone delineates three biogeographic provinces with distinct physical and biological properties: the south subtropical convergence (SSTC), the Indian south subtropical gyre (ISSG) to the north, and the sub-Antarctic water ring (SAWR) to the south (Longhurst, 1998).
This study focuses on marine birds because these numerous and conspicuous predators respond to spatial and temporal oceanographic variability across the southern Indian Ocean (Pocklington, 1979; Stahl et al., 1985; Inchausti et al., 2003; Pinaud et al., 2005). We hypothesized that the marine avifauna of the Southern Indian Ocean is structured by the large-scale latitudinal gradients in hydrographic properties and ocean productivity patterns described above. The objectives of this study are to (1) characterize spatial patterns of seabird density across a large and heterogeneous oceanic ecosystem; and (2) determine whether distinct seabird assemblages inhabit specific ocean domains defined by distinct physical (e.g., bathymetry, SST) and biological (e.g., chlorophyll-a, productivity) properties. To quantify differences in seabird density and community structure across large biogeographic domains, we also had to consider small-scale spatial variability (e.g., fronts, shelves-slopes). Thus, our analysis addresses seabird aggregation at coarse-scale (∼10 s km) bathymetric and hydrographic features, as well as mega-macro scale (∼1000 s km) dispersion patterns.
Section snippets
Study area
We sailed on an 18-day (4–21 January 2003) cruise from La Réunion (21°06′S; 55°6′E), visited the French sub-Antarctic territories of Crozet (46°30′S; 51°00′E), Kerguelen (49°30′S; 69°30′E), St. Paul (38°43′S; 77°29′E), and Amsterdam (37°52′S; 77°32′E), and finished in Perth, Western Australia (32°01′S; 115°47′E). This cruise track, from subtropical to sub-Antarctic waters, allowed us to examine the avifauna within three biogeographic domains with distinct physical and biological characteristics
Oceanographic observations
Our cruise track spanned subtropical to sub-Antarctic waters, and crossed four frontal systems (Fig. 1). During the southward transect from La Réunion to the Crozet Basin, wind speed increased (Fig. 2C), SST declined (Fig. 2E), and CHL increased above 0.2 μg Chl m–3 (Fig. 2F). These conditions reversed during the northward transect from Kerguelen to St. Paul Island. We sailed through subtropical water (18–20 °C) east of Amsterdam Island, and encountered the warmer SSTs (20–22 °C) characteristic of
Discussion
Our study suggests that the avifauna of the southern Indian Ocean is structured by large-scale (∼1000 s km) gradients in physical and biological properties. We surveyed a 3000-km north–south transect spanning subtropical to sub-Antarctic waters, and documented a shift in seabird communities along a large-scale (22–50°S) latitudinal gradient in surface (SST and CHL) and sub-surface (MLD) water properties. Cooler sub-Antarctic waters of higher ocean productivity and phytoplankton standing stocks
Acknowledgments
We are grateful to the officers and crew of the R.V. Marion Dufresne, to the French Polar Institute Paul-Emile Victor (IPEV, program no. 109) for facilitating our participation in this cruise, and to LOCEAN-IPSL for supporting the Ocean Indien Service d’Observation (OISO) program. The US National Science Foundation Office of Polar Programs supported GLH and KDH (Grant OPP-0234570) and RRV (Grant OPP-9983751). The Pacific Fisheries Environmental Laboratory and the Coriolis program provided the
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Present address: School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195, USA.