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A Review of the Effects of Seamounts on Biological Processes

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A Review of the Effects of Seamounts on Biological Processes A REVIEW OF THE EFFECTS OF SEAMOUNTS ON BIOLOGICAL PROCESSES George W. Boehlert Southwest Fisheries Center Honolulu Laboratory, National Marine Fisheries Service. NOAA. 2570 Dole St., Honolulu, HI 96822-2396 Amatzia Genin Scripps Institution of Oceanography, A-008, University of California, La Jolla, CA 92093 Abstract. Seamounts interacting with oceanic continental shelf or slope counterparts at similar currents create flow complexities which depend water depths [Hubbs, 19591. In the open ocean, upon current speed, stratification. latitude, and seamounts interact with ocean currents and create seamount morphology. Seamount effects. which variability in the physical flow field. Several include internal wave generation. eddy formation. studies have described these effects on the Gulf local upwelling. and closed circulation patterns Stream [Vastano and Warren, 19761 and the Kuroshio called Taylor columns. have important effects upon [Roden et al.. 1982; Roden. 19871. The physical pelagic and benthic ecosystems over seamounts. effects include local small- and mesosca7.e phe- The biological effects of these current-topography nomena including the shedding of mesoscale eddies interactions are poorly understood. Flow accel- which alter flow patterns for significant dis- eration on upper flanks of seamounts may lead to tances downstream of the seamounts [Royer. 19781. low sedimentation but areas of high standing Biological effects of these physical complexities stocks of benthic fauna, particularly filter feed- are not well understood [Genin and Boehlert 1985; ers. Other effects extend into the water column: Boehlert, 19861. Discovery of seamount fishery nutrient enrichment and enhanced primary produc- [Uchida and Tagami. 19841 and mineral resources tivity occur over some seamounts. Longer observa- [Manheim, 19861, however, has caused increased tional periods will be necessary to understand the interest in seamount oceanography and its effects time-varying nature of such enhanced productivity on biota [Darnitsky et al.. 1984: Genin and Boeh- and the extent to which it remains at the seamount lert. 1985; Uchida et al.. 19861. or is advected away. At higher trophic levels, The effects of sea floor topography on ocean unusual patterns of distribution and abundance currents have been a topic of interest to physical occur at some seamounts. Maintenance of high oceanographers for several decades. This area has standing stocks of seamount-associated micronekton recently been reviewed by Hogg l19801 and was the and demersal fishes suggests that seamounts are topic of a monograph by Kozlov [19831. Semi- locations for high rates of energy transfer. The stationary eddies or Taylor columns above sea- energy driving this biological productivity may mounts have been theoretically predicted and either be generated from in situ processes or be experimentally demonstrated in the laboratory advected from elsewhere and concentrated at the [Taylor. 1917: Huppert and Bryan, 19761 and have seamount; interdisciplinary studies will be neces- been observed over scme seamounts [Darnitsky. 1980; sary to better understand these ecosystems. Owens and Hogg. 1980; Richardson, 19801. Unfortu- nately. oceanographic surveys generally have sta- Introduction tion patterns inappropriate to detect these open- ocean, small-scale or mesoscale phenomena [Roden Seamounts represent a major physical feature of 19861. Still. past theoretical and observational all ocean basins. For marine biota, they may be studies on the physics of topographic effects are considered as islands separated by deep ocean available to serve as a background for biological areas: seamounts were thus the topic of many bio- studies. geographic studies [Wilson and Kaufman. 19871. Many studies have suggested that ecosystems at Biological communities at seamounts, however, may certain banks or seamounts are highly productive differ qualitatively and quantitatively from their [Uda and Ishino 1958; Fedosova. 1974; Zaika and Kovalev. 1984; Tseitlin. 19851. The ideas which explain such high productivity are typically based Copyright 1987 by the American Geophysical Union. upon either local enhancement and subsequent 31 9 320 SEAMOUNT BIOLOGICAL OCEANOGRAPHY 0 100 h E v 200 I I- n W n 300 400 500 Fig. 1. Temperature structure at Kinami-Kasuga Seamount (lat. 21°6'N. long. 143'8'E) along a west-east transect shoving uplifted isotherms aver the summit of the seamount. Note that the vertical scale is 50 times greater than the horizontal scale. (Prom Genin and Boehlert. 1985.) retention of productivity or advection and concen- interactions, which have been summarized in this tration of food produced elsewhere. Some support volume by Roden 119871. Where seamounts act as exists for each of these hypotheses. Differences obstacles to current flow, compression of stream- often exist between the pelagic ecosystems of lines will occur due to flow acceleration. Local waters above seamounts and adjacent. oceanic deflections of isotherms. usually in the form of waters, including nutrient concentrations [Kozlov uplifting, have been observed above several sea- et al.. 19821. chlorophyll [Genin and Boehlert. mounts at different locations and depths [Meincke. 19851. plankton biomass [Bezrukov and Natarov. 1971; Vastano and Warren, 19761. Taylor columns 19761. ichthyoplankton [Nellen. 19731. and micro- represent an interesting phenomenon pertinent to nekton [Boehlert and Seki. 19841. In the case of seamounts; associated theory [Taylor. 1923: advected productivity. however. seamount popula- Huppert and Bryan. 19761 predicts such deflections tions may not be limited by local production, but as a result of the encounter between a current and rather by physical aggregation mechanisms [Isaacs a seamount. Under certain conditions of current, and Schwartzlose. 1965; Darnitsky et al., 19841. stratification. and topography, a closed stream- which have been shown to exist in gyres and near lined anticyclonic vortex, or Taylor column, is reefs and coastal headlands [Alldredge and expected to remain trapped above the aeamount Hamner. 1980; Olson and Backus. 19851. Benthic [Hogg. 1973; Huppert. 19751. Such trapping may ecosystems on seamounts and islands may similarly enhance nutrients shallower in the water column differ from corresponding systems on continental and if the residence time of a water mass above shelves and slopes but have typically been less the seamount is sufficiently long, result in well studied. In many cases, unusual or even enhanced primary productivity and transfer into unique faunas exist [Simpson and Heydorn. 1965; higher trophic levels. In this section we Littler et al.. 19861. In addition. more recent consider evidence for such enhancement and the studies have investigated the relationship of role it plays in different trophic levels. benthic fauna co current speeds and seamount- induced physical variability [Genin et al.. 19861. Nutrients and Primary Productivity In this paper we review the effects of seamounts on biological processes, particularly as they The uplifting of isotherms over seamounts is affect pelagic and benthic ecosystems and fish- distinct and occurs with sufficient frequency to e r i e s p r odu c t ivit y. provide support for Taylor column dynamics. As an example. isotherms Over Minami-Kasuga Seamount in The Pelagic Ecosystem at Seamounts the Mariana Archipelago [Genin and Boehlert. 19851 showed a clear uplift (Fig. 1) which did not reach Much of the impact of seamounts on pelagic the surface. Such upwelling at seamounts. like ecosystems may be traced to current-topography coastal upwelling, will transport nutrients into BOEHLERT AND GENIN 321 the euphotic zone where the primary production is Three chlorophp91 profiles baken within the cold nutrient-limited; an analogous situation exists dome showed a distinctive maximum between 80 and around islands in stratified seas, where increased 100 m depth, whereas the chlorophyll maximum layer tidal mixing may stimulate primary productivity at the four control stations was comparatively [Simpson et al.. 19821. Observations on a variety diffuse (Fig. 2). The causal relationships of seamounts support this contention; Bezrukov and between the localized upwelling and the biological Natarov [I9761 suggested that vertical velocities response are corroborated by the confinement of on the order of 0.00003 to 0.0008 cm-sec-l exist the chlorophyll increase to depths below 80 m. the over various seamounts and that differences in the uppermost edge of the cold dome. The chlorophyll magnitude of upwelling may explain variability in concentrations at shallower depths above the sea- seamount productivity. Vortices associated with mount and at the control stations varied little seamounts can produce physical structures much throughout the area. These results clearly con- like open-ocean cold core rings. except they trast with data from the second and third surveys remain centered over the seamount. Kozlov et al. in which neither cold dome nor chlorophyll [I9821 describe a "columnar distribution" of tem- increases were detectable along the same tran- perature, salinity. silicate, and phosphate over sects. These observations suggest that varying one of the summits of Milwaukee Seamount in the strength of oncoming currents result in a varying southern Emperor Chain, in which elevated nutrient time scale for presence of the uplifted isotherms. levels corresponding to the values at the seamount Estimates of the time necessary for the formation flanks reach high into
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