Oceanography Vol. 23, No.1 206 BOX 12 | How Large Is the Seamount Biome? By Peter J. Etnoyer, John Wood, and homas C. Shirley Estimates of the number of seamounts occurring worldwide are high and increasing, largely because of improved remote-sensing capabilities. Numbers have grown from a baseline of 15,000 (Wessel, 2001; Marova 2002) to more than 45,000 seamounts worldwide. High-end estimates are in the hundreds of thousands (Hillier and Watts, 2007; Kitchingman et al., 2007; Wessel, 2007; Wessel et al., 2010). So, it is logical to ask: What is the total area of the seamount biome? If the world’s seamount features were assembled into a continuous region, how large would this place be? How would the area of the seamount biome compare to continents, and to other marine biomes? hese data would be informative, because terrestrial biomes are fairly well resolved and enumerated (Udvardy, 1975; Woodward, 2003), but marine biomes are less well mapped and understood. A biome is a major life zone characterized by similar biotic and physical characteristics (Woodward, 2003). Examples include coral reefs, tropical forests, savannas, deserts, and rocky intertidal zones. Seamounts are a marine biome because they occupy a large geographical area with similar physical attributes and similar biotic assemblages adapted to a similar environment— the deep sea. he environmental characteristics of seamounts are as uniform as those of any biome: they are predominantly aphotic, have a basaltic substrate, and are surrounded by consis- tent cold temperature and high-salinity water, and the biota primarily depend upon overlying surface waters for their food supply. Environmental gradients do exist on seamounts, but we assume one biome in the current treatment. he deinition of a seamount has been modiied over the years to include smaller features. his change is relected in high-end population estimates. By an early deinition, seamounts are isolated features of volcanic origin rising more than 1000 m from the sealoor (Menard, 1964). Individual seamounts of this size can have a large geographic footprint, on the order of 500–1000 km 2 . More recent deinitions include smaller topographic promi- nences, 100 –1000 m in height (see Staudigel and Clague, 2010). he global cumulative area of seamounts is unknown by either deinition. Here, we include seamounts larger than 1000 m in relief listed in a publicly available data set (Wessel, 2001) to generate our estimate of the seamount biome’s global “footprint.” Larger area estimates will result from using smaller features (Hillier and Watts, 2007). A global estimate of seamount area is possible because orbital satellites can detect and enumerate seamount features (Smith and Sandwell, 1997; Wessel, 2001; Etnoyer, 2005). Size is estimated from the vertical gravity gradient (VGG; Wessel and Lyons, 1997). VGG is the rate of change in the gravity ield in the vertical direction. Changes are highest close to a gravimetric source (e.g., over a seamount) and approach zero over featureless regions. Basal radius is derived from a conical approximation of seamount shape (Wessel, 2001), and basal area is calculated as a circular cross section (πr 2 ). he sum of basal areas provides a irst-order total global surface area estimate for seamounts. Satellites underestimate seamount size (Etnoyer, 2005), so the estimate is conservative. he data we used (Wessel, 2001) contain coordinates for 11,880 unique seamount features (Figure 1) with basal radius (in kilometers) derived from the VGG (Wessel and Lyons, 1997). Seamounts with greater than 1500 m of relief (n = 11,807) were categorized as large seamounts, and seamounts with 1000–1500 m of relief (n = 73) were categorized as small seamounts. Point coordinates were transformed to circular areas using seamount radius as a bufer in a geographic informa- tion system. Basal perimeters may overlap, so boundaries were dissolved to eliminate redundancy. he merged bufer zones were projected onto a spherical Mercator grid (WGS 84) using ArcGIS 9.2 software (ESRI, 2006). he total combined basal area of large seamounts from these calculations comes to 10,079,658 km 2 . We estimate that a seamount biome comprised of large seamounts closely approximates the size of Europe and Russia combined (9,938,000 km 2 ), a cumulative area larger than Australia Peter J. Etnoyer (peter.etnoyer@noaa.gov) is Schmidt Research Vessel Institute Research Fellow, Harte Research Institute for Gulf of Mexico Studies, Texas A&M University–Corpus Christi, Corpus Christi, TX, USA, and his current address is National Oceanic and Atmospheric Administration, Charleston, SC, USA. John Wood is PhD Candidate, Harte Research Institute for Gulf of Mexico Studies, Texas A&M University– Corpus Christi, Corpus Christi TX, USA. homas C. Shirley is HRI Endowed Chair, Harte Research Institute for Gulf of Mexico Studies, Texas A&M University–Corpus Christi, Corpus Christi, TX, USA. his article has been published in Oceanography, Volume 23, Number 1, a quarterly journal of he Oceanography Society. © 2010 by he Oceanography Society. All rights reserved. Permission is granted to copy this article for use in teaching and research. Republication, systemmatic reproduction, or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of he Oceanography Society. Send all correspondence to: info@tos.org or h e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA.