Vol. 418: 25–45, 2010 MARINE ECOLOGY PROGRESS SERIES Published November 18 doi: 10.3354/meps08797 Mar Ecol Prog Ser OPENPEN ACCESSCCESS Biotic and abiotic factors affecting distributions of megafauna in diffuse flow on andesite and basalt along the Eastern Lau Spreading Center, Tonga Elizabeth L. Podowski1, Shufen Ma3, 4, George W. Luther III3, Denice Wardrop2, Charles R. Fisher1,* 1Biology Department and 2Geography Department, Pennsylvania State University, University Park, Pennsylvania 16802, USA 3College of Marine and Earth Studies, University of Delaware, Lewes, Delaware 19958, USA 4Present address: Department of Earth and Planetary Sciences, University of California, Berkeley, California 94720, USA ABSTRACT: Imagery and environmental data from 7 diffuse flow hydrothermal vent sites along the Eastern Lau Spreading Center (ELSC) are used to constrain the effects of lava type, temperature, chemistry, and biological interactions on faunal distributions. Of the species with chemoautotrophic endosymbionts, the snail Alviniconcha spp. occupies habitats with the greatest exposure to vent fluids. Temperatures exceeding 45°C define its upper limit of exposure to vent flow, and minimum sulfide requirements constrain its lower limits. The mussel Bathymodiolus brevior experiences the least exposure to vent flow; temperatures of about 20°C determine its upper limit, while its lower limit is defined by its minimum sulfide requirements. The snail Ifremeria nautilei inhabits areas with intermediate exposure to vent fluids and biological interactions are likely the most important factor shaping this snail’s realized niche. Microhabitats of non-symbiont-containing fauna were defined in terms of symbiont-containing faunal distributions. The crab Austinograea spp. occupies areas with the greatest exposure to vent flow; shrimp, the snail Eosipho desbruyeresi, and anemones inhabit intermediate zones of vent flow; and the squat lobster Munidopsis lauensis dominates the periphery of diffuse flow areas, with little exposure to vent fluids. The physical structure of different lava types along the ELSC differentially affects the diffusion of vent fluids, which has a variety of implications for fauna, particularly distributions of zoanthids, anemones, and mixed communities of I. nautilei and B. brevior. KEY WORDS: Lau back-arc basin · Faunal distributions · Diffuse flow · Lava type · In situ voltammetry · GIS Resale or republication not permitted without written consent of the publisher INTRODUCTION proximity to a subduction zone, geophysical processes and geochemical properties change systematically The 400 km-long Eastern Lau Spreading Center along a north to south gradient. For example, mag- (ELSC) is located in the Lau back-arc basin, bordered matic robustness (Martinez & Taylor 2002, Martinez et on the west by the remnant Lau arc and on the east by al. 2006) and crustal thickness (Martinez & Taylor the active Tofua volcanic arc and the Kingdom of 2002, Taylor & Martinez 2003) increase from north to Tonga (Parson et al. 1990, Taylor et al. 1996). The south, while spreading rate (Martinez & Taylor 2002), northernmost segment of the ELSC is 110 km from the plume incidence (Martinez et al. 2006), concentrations active arc, while the southernmost segment (the Valu of dissolved H2 and H2S in end-member vent fluids Fa Ridge) is separated from the volcanic arc by only (Seewald et al. 2005), and depth all decrease. Lava 40 km (Martinez et al. 2006). As a result of variable type also changes along a north to south gradient: the *Corresponding author. Email: [email protected] © Inter-Research 2010 · www.int-res.com 26 Mar Ecol Prog Ser 418: 25–45, 2010 northern portion of the ELSC is basalt-hosted and the spp. (3 different species, including Alviniconcha hess- southern portion is andesite-hosted (Vallier et al. leri [Okutani & Ohta 1988], that are not differentiable 1991). Andesitic lavas have a higher silica content than in images) and Ifremeria nautilei (Bouchet & Warén basaltic lavas (Press & Siever 1974), and as a result 1991), and the bathymodiolin mussel Bathymodiolus of hydrous melting, andesitic lavas in the Lau Basin brevior (von Cosel et al. 1994), all of which require tend to be more permeable (Hawkins 1995) and vesic- exposure to hydrothermal vent fluids in order to ular (Vallier et al. 1991) compared to more basaltic maintain nutritional symbioses with chemoauto- lavas. trophic microbes housed in their gills (Endow & Ohta The ultimate source of energy for primary produc- 1989, Windoffer & Giere 1997, Dubilier et al. 1998, tion at hydrothermal vents is derived from reduced Suzuki et al. 2005a, Urakawa et al. 2005). Unlike the chemicals (particularly sulfide) in vent effluent, which vestimentiferan tubeworms that dominate many dif- can also be toxic and hot. As a result, the vent envi- fuse flow vents on the East Pacific mid-ocean ridges, ronment is fundamentally different from most other the symbiont-containing snails and mussels of the environments because primary production is usually ELSC are mobile, an adaptation that enables each positively correlated with environmental stress. Gra- species to respond to local changes in hydrothermal dients of environmental change are also very steep in flow and seek the most suitable habitat available. areas of hydrothermal diffuse flow, where vent efflu- Previous studies (Desbruyères et al. 1994, Podowski ent mixes with the surrounding seawater, and can et al. 2009) have documented the distributional pat- vary on scales of centimeters and seconds. Thus in a terns and generally defined the microhabitats of the vent community, tolerance of extremes in chemistry abovementioned ELSC species. Aggregations of Alvi- and temperature may be a first order determinant of niconcha spp. are found in high-temperature and sul- a species’ ability to make use of the primary produc- fide-rich environments (Desbruyères et al. 1994, tion associated with these habitats. However, biologi- Podowski et al. 2009). I. nautilei occur in a fairly wide cal interactions will play an important role in defining range of intermediate chemical and thermal environ- patterns of animal distribution in areas where abiotic ments (Podowski et al. 2009) and B. brevior are found factors are less extreme. Understanding the relation in environments with the least exposure to vent fluids between different animals and the abiotic environ- (Podowski et al. 2009). Both Alviniconcha spp. and I. ment and the role of biological interactions in struc- nautilei occur as 2 different color morphotypes that turing the vent communities have been a primary can be distinguished in the mosaics. Prior to this focus of ecological research at vents for 2 decades. study, it was not known whether the distribution of Our understanding of the factors affecting hydrother- the different color morphotypes was correlated with mal vent community structure has been significantly differences in microhabitats. advanced as a result of intensive research efforts on In this study, we used new data sets collected from the Mid-Atlantic Ridge (Desbruyères et al. 2000, 7 diffuse flow communities distributed among 4 vent 2001),the East Pacific Rise (Hessler et al. 1985, 1988, fields and across 2 lava types to test hypotheses from Fisher et al. 1994, Johnson et al. 1994, Shank et al. previous studies and further define the relationships 1998, Luther et al. 2001, Van Dover 2003), and the between chemistry, lava type, and biology along the Juan de Fuca Ridge (Sarrazin & Juniper 1999, Sar- ELSC. Ratios of sulfide concentration to temperature razin et al. 1999, Govenar et al. 2002, Bergquist et al. anomaly (i.e. degrees above ambient temperature) 2007); however, relatively few studies have focused vary significantly among sites, enabling us to partially on vent communities in the Western Pacific, particu- separate the relative importance of these typically con- larly the ELSC (Desbruyères et al. 1994, Podowski et founded parameters and test the hypotheses from al. 2009). The mobility of the symbiont-containing Podowski et al. (2009) that temperature determines the fauna in this biogeographic province holds promise upper limits of vent exposure for Alviniconcha spp. for new insights into the fundamental processes that and Bathymodiolus brevior, while sulfide concentra- structure vent animal communities. tion determines the upper limit of exposure of Ifreme- The natural abiotic variation along the ELSC pro- ria nautilei to vent fluid. Furthermore, this larger data vides a unique opportunity to examine how chemistry set enabled statistical examination of the distributional and lava type affect local hydrothermal vent commu- patterns of other common back-arc basin vent nities. The diffuse flow vent fields in the ELSC host megafauna and the detection of differences in commu- high biomass communities (Desbruyères et al. 1994), nity composition on andesitic and basaltic lavas, similar in composition to other back-arc basins in the including the examination of how different lava types Western Pacific (Tunnicliffe & Fowler 1996, Van disperse hydrothermal fluid, which may explain in part Dover et al. 2002). These vent communities are domi- the observed differences between communities on the nated in biomass by provannid snails Alviniconcha 2 lava types. Podowski et al.: Distributions of megafauna along the Eastern Lau Spreading Center 27 MATERIALS AND METHODS Each assessment site was imaged twice using the re- motely operated vehicle (ROV) ‘Jason II’. One set of im- In situ imagery. The study