Community structure across a large-scale ocean productivity gradient: Marine bird assemblages of the Southern Indian Ocean

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Abstract

Our objective was to understand how marine birds respond to oceanographic variability across the Southern Indian Ocean using data collected during an 16-day cruise (4–21 January 2003). We quantified concurrent water mass distributions, ocean productivity patterns, and seabird distributions across a heterogeneous pelagic ecosystem from subtropical to sub-Antarctic waters. We surveyed 5155 km and sighted 15,606 birds from 51 species, and used these data to investigate how seabirds respond to spatial variability in the structure and productivity of the ocean. We addressed two spatial scales: the structure of seabird communities across macro-mega scale (1000 s km) biogeographic domains, and their coarse-scale (10 s km) aggregation at hydrographic and bathymetric gradients. Both seabird density and species composition changed with latitudinal and onshore–offshore gradients in depth, water temperature, and chlorophyll-a concentration. The average seabird density increased across the subtropical convergence (STC) from 2.4 birds km−2 in subtropical waters to 23.8 birds km−2 in sub-Antarctic waters. The composition of the avifauna also differed across biogeographic domains. Prions (Pachyptila spp.) accounted for 57% of all sub-Antarctic birds, wedge-tailed shearwaters (Puffinus pacificus) accounted for 46% of all subtropical birds, and Indian Ocean yellow-nosed albatross (Thallasarche carteri) accounted for 32% of all birds in the STC. While surface feeders were the most abundant foraging guild across the study area, divers were disproportionately more numerous in the sub-Antarctic domain, and plungers were disproportionately more abundant in subtropical waters. Seabird densities were also higher within shallow shelf-slope regions, especially in sub-Antarctic waters, where large numbers of breeding seabirds concentrated. However, we did not find elevated seabird densities along the STC, suggesting that this broad frontal region is not a site of enhanced aggregation.

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.

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