Seabird patchiness in tropical oceanic waters: the influence of Sargassum reefs

-The relationships of seabirds to the holopelagic macroalga Sargassum were studied during a 2-yr survey in the South Atlantic Bight off the southeastern United States. Seabirds were significantly more abundant in waters with large (>5 m2) patches of the alga, and mean avian density was 32-43 times greater in waters where Sargassum was present than in adjacent waters without the alga. Seabirds aggregate at Sargassum patches formed by Langmuir circulation, convergence fronts, Gulf Stream eddies, and Gulf Stream warm core rings. Twenty-three seabird species foraged at Sargassum. Significantly more than 50% of White-tailed Tropicbirds (Phaethon lepturus), Masked Boobies (Sula dactylatra), and Bridled Terns (Sterna anaethetus) were observed at algal patches. Species that use aerial-dipping and plunge-diving foraging behaviors displayed the greatest affinity for Sargassum. Large-bodied seabirds (large shearwaters, tropicbirds, and boobies) generally were found at large patches, while small-bodied species (phalaropes and Puffinus Iherminieri) occurred at smaller patches. The seasonal abundance of the pelagic Bridled Tern corresponded to seasonal variation in Sargassum abundance. Most seabirds associated with Sargassum for foraging, but Bridled and Black terns (Chlidonias niger) also used large algal mats for roost sites. The highly localized biomass of the pelagic Sargassum community and its associated motile macrofauna (zooplankton and fish) may allow seabirds to forage efficiently for prey in the oligotrophic surface waters of tropical marine environments. The patchy occurrence of Sargassum "reefs" may explain part of the local and mesoscale variation in seabird distribution and abundance in portions of the western North Atlantic Ocean. Received 19 November 1984, accepted 21 August 1985. PATCHINESS in seabird distribution has been little studied, due in part to the difficulty of sampling quantitatively at small scales (Bailey and Bourne 1972, Devillers 1978). The patchy distribution and abundance of seabirds may arise from local concentrations of prey (Brown 1980), or from social interactions critical for successful foraging (Bailey and Bourne 1972, Schneider 1982, Duffy 1983). Interspecific differences in the breeding ecology of seabirds also may affect the patches used for feeding (Erwin 1977). Variability in the physical marine environment of seabirds may indirectly influence the patchiness of prey (Ashmole 1971). Physical features governing seabird patchiness at one scale may account for much of the temporal and spatial variation in seabird patchiness at larger and smaller time/space scales (Haney and McGillivary 1985a). Seabird patchiness occurs in tropical and subtropical oceans (Haney and McGillivary 1985a, b), characterized by their seasonally stable water masses. The permanent thermocline prevents widespread vertical enrichment of the euphotic zone by nutrient-rich deeper waters, and little variation in primary productivity occurs as a result (Russell-Hunter 1970). Because the food available to higher trophic groups is limited, any factor that elevates the food biomass within this oligotrophic system may be expected to affect seabird distribution and abundance. The Sargassum community and associated fauna are one example of locally elevated biomass in the nutrient-poor waters of the North Atlantic Ocean. Both Sargassum natans and S. fluitans are widely distributed, holopelagic macroalgae and are restricted to the ocean surface, where gas-filled bladder floats maintain their buoyancy (Parr 1939). The animals inhabiting Sargassum have been characterized as a "displaced benthos" (Hedgpeth 1957). Because most of the fauna are derived from populations of littoral genera and species, the fauna and flora together form a pseudobenthic or "reef" habitat surrounded by the pelagic ecosystem (Adams 1960, Weis 1968, Dooley 1972, Fletemeyer 1978, Carr and Meylan 1980, Kracht and Tesch 1981, Bulter et al. 1983). Although several seabird species have been 141 The Auk 103: 141-151. January 1986 This content downloaded from 207.46.13.76 on Fri, 09 Sep 2016 04:28:12 UTC All use subject to http://about.jstor.org/terms 142 J. CHRISTOPHER HANEY [Auk, Vol. 103