A REVIEW OF THE EFFECTS OF ON BIOLOGICAL PROCESSES

George W. Boehlert

Southwest 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 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 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 the water column. Two of the observed chlorophyll maximum from the first visits, one month apart, to Pulkovskaya Seamount survey, however, suggest a minimum residence time in the South Pacific suggested that vortices of the hypothesized Taylor column on the order of existed around the twin peaks and "satellite" days [Genin and Boehlert, 19851. vortices remained in surrounding waters. Again, The vertical extent and residence time of signals were apparent in salinity, temperature, seamount-induced upwelling will determine the and silicate: silicate concentration at the summit magnitude of its effect on local biological depth of 500 m extended to the surface, with processes in overlying waters. Unfortunately, the values nearly double that in surrounding waters temporal sampling scale of the studies described [Kozlov et al.. 19821. Darnitsky et al. 119841 above is inadequate to determine the temporal studied nutrients over Wanganella Bank near New dynamics clearly, and knowledge of regional Zealand during six visits from 1974 to 1977. currents is frequently lacking; moreover, the time Although upwelling and the resultant nutrient scales will vary from seamount to seamount, which enrichment were clearly observed twice, the water will have an effect on the manner in which any was stratified with no apparent seamount effect enhanced primary productivity may reach higher during four other transects. trophic levels. In oligotrophic oceans. A shorter time scale was addressed in a study phytoplankton production would increase if the by Genin and Boehlert [19851. who conducted a uplifted isotherms penetrated into the euphotic series of transects to describe temperature and zone. replenishing its depleted water with chlorophyll distribution over Minami-Kasuga Sea- nutrients, as noted by Kozlov et al. [I9821 and in mount. On the first of three surveys made within the first survey by Genin and Boehlert I19851. a month, uplifted isotherms formed a subsurface Entrapment on the order of days would probably cold dome above the seamount (Fig. 1). The verti- affect only the primary producers, and hence. a cal displacement of the uplifted isotherms gradu- patch of relatively high chlorophyll concentra- ally decreased with distance above the seamount. tions would be associated with the seamount. A from a 50 m uplift of the 17O isotherm close to longer residence time. on the order of several the substratum, to a decay of the cold anomaly at weeks, may locally affect the growth and abundance about 80 m depth (180 m above the seamount top). of zooplankton species: months would be necessary Different deflection trends of isotherms in the for micronekton [Pudyakov and Tseitlin. 19861. vicinity of the substratum around the seamount Lagrangian current observations made above the slope formed a "boundary zone" comprising three Emperor Seamounts [Cheney et al.. 19801 and the distinctive layers composed of downward deflected Corner Rise Seamounts [Richardson. 19801 suggested isotherms from approximately 500 m to about 420 m. entrapment periods up to several weeks within a relatively well mixed "transition zone" (Fig. 1) seamount-generated anticyclonic eddies. Much between the 14.5O and 15O isotherms (approximately longer periods (on the order of several months) 50 m above the previous layer). and an upwelling were inferred from hydrographic and Eulerian CUP layer shallower than 340 m. These layers may be rent measurements above a deep seamount in the related to energy dissipation in the benthic boun- North Atlantic [Owens and Hogg. 19801. If local dary layer and agree with the statement by Bezru- enrichment persists for long periods and is a kov and Natarov [19761 that there is typically a recurrent phenomenon. nektonic organisms may be change in the sign of vertical velocity between attracted to or aggregated in these habitats [Uda 300 and 600 m on the flanks of seamounts. On this and Ishino. 1958; Boehlert and Seki. 1984: Uchida first survey, calculations based upon seamount and Tagami. 19841. An analogy may be drawn to morphology suggested that conditions favoring demersal plankton in nearshore systems [Hob- maintenance of a Taylor column were present. son and Chess, 1978; Alldredge and King, 19851. 322 SEAMOUNT BIOLOGICAL OCEANOGRAPHY

13 March 1984

CHLOROPHYLL a (mg/m3) Oili 0;2 0;3 Oi4 0;s

, i/ Seamount

Fig. 2. Chlorophyll 5 profiles above Minami-Kasuga Seamount taken concurrently with the temperature transects shown in Figure 1. Three stations were occupied over the top of the seamount (right) and at control stations which were 10 km from the seamount summit (left). (From Genin and Boehlert 1985.)

Zooplankton and Nekton A specific component of the plankton studied above seamounts has been the ichthyoplankton. In Evidence concerning densities of planktonic the North Pacific, Borets and Sokolovsky [19781 organisms above seamounts is often conflicting. observed no differences in ichthyoplankton abun- Sorokin and Sorokina [198Sl noted no differences dance or species composition above seamounts as between waters above seamounts and surrounding compared to distant waters: Belyanina [19851 had waters with respect to bacterioplankton. Many similar results in the Indian Ocean. Nellen surveys in the regions of seamounts. however, [19731. however. in a study on the Great Meteor indicate two- to eightfold increases in zooplank- Seamount. noted increased abundance of neritic ton abundance in waters over seamounts [Fedosova species and depletion of others, most typically 19741 ; Zaika and Kovalev [19841 summarize results midwater fish larvae. above the seamount. Boeh- from a variety of Soviet papers. Zooplankton lert [1985l described ichthyoplankton densities samples were taken by Genin and Boehlert [198Sl in from winter and summer cruises at Southeast two of their three surveys, one during elevated Hancock Seamount. Fish larvae at this open-ocean and the other under normal chlorophyll levels. seamount are dominated by midwater rather than Zooplankton displacement volume was greater above neritic species. In the summer, daytime samples the seamount only on the first survey; in contrast showed no difference between the seamount and to the chlorophyll signal, however, zooplankton reference stations with the exception of the 50- volumes were higher within the cold dome and above 100 m stratum (Fig. 3A). At night, however, the it. possibly indicative of the more vertically densities at the reference station were signifi- mobile nature of zooplankton [Genin and Boehlert cantly increased as compared to daytime values. 19851. Whereas the studies referred to above have but not in the samples taken over the seamount. typically considered zooplankton biomass, it may Samples taken on a winter cruise showed a differ- be more appropriate to consider specific taxa. ent pattern from that in the summer (Fig. 3B): which may be either more abundant or relatively ichthyoplankton densities over the seamount were depleted in waters above seamounts [Hirota and much greater than at the reference station, and Boehlert. 19851. the pattern held for night and day. These data BOEHLERT AND GENIN 323

FISH LARVAE PER 103m3 A 1500 1200 900 600 300 0 300 600 I f I I I 1 0-25111

25-50111

50-100m

100-200m NIGHT DAY 128-29 JULY 19841 115-16 JULY 19841

L 25-501'11

50-100m

100-200m

NIGHT DAY 18-10 FEB. 1985) I (. FEB. 1985 011) 9 FEE 1985 on Fig. 3. Comparison of ichthyoplankton densities above Southeast Hancock Seamount (lat. 29O8'N. long. 178O5'E) with those at reference stations 20 km away. Samples were taken at discrete depths with an opening-closing Tucker trawl. A. Summer 1984. B. Winter 1985. Daytime densities are on the right, nighttime on the left, with each value the mean of duplicate samples. Densities above the seamount are represented by the upper (solid) bar. densities at the reference station by the lower (cross-hatched) bar. (Presented in Boehlert, 1985.) suggest that some phenomena differing between Maurolicus muelleri. typically associated with winter and summer (or an aliased time scale) continental shelf-slope breaks, has been observed impact the abundance of these taxa. in abundance over South Atlantic and North Pacific As compared to the passive planktonic species, seamounts [Linkowski. 1983; Boehlert and Seki. micronekton and nekton have more control over 19841. Kozlov et al. [19821 noted vertical their movements. Fewer studies over seamounts columns of scattering layers over two seamounts in have been conducted on these species, but Uda and the North Pacific but mistakenly considered them Ishino [19581 suggested that aggregations of such to be simply vertical manifestations of the animals may result in enhancement of fishing oceanic scattering layers arrayed vertically like grounds. Densities may be estimated using hydro- the "columnar" distributions of temperature and acoustic observations and net sampling. Some nutrients. Hydroacoustic observations at night organisms normally rare in open-ocean areas may be above Southeast Hancock Seamount [Boehlert and abundant on seamounts. The sternoptychid fish, Seki. 19841 showed pronounced scattering (Fig. 4). 324 SEAMOUNT BIOLOGICAL OCEANOGRAPHY

IA)

c E u I I- n w 0

Fig. 4. Acoustic transects over Southeast Hancock Seamount on 17-18 July 1984 taken with a 38 kHz echo sounder. Each transect. from west to east (left to right) near the central axis of the seamount, took approximately 25 min. The distance across the flat portion of the seamount is approximately 1.8 km. A. 1931 h. Note the scattering layers rising off the seamount flanks to a depth.of 40-60 m; net samples shov these early layers to be predominantly Maurolicus muelleri. Sunset vas at 1907. B. 0330 h. The layer has developed over the seamount extending from the flanks upwards to near 100 m; it developed this configuration at approximately 2130 and remained in a similar configuration throughout the night. until dispersing in early morning. These scattering targets may have been larger fishes or squids not sampled by our midvater trawl. (Presented in Boehlert and Seki. 1984.)

Scatterers typically remained on the flanks of the responsible for the acoustic traces. Subsequent seamount by day. but began streaming vertically hauls with midvater trawls demonstrated differ- upvard to depths near 50 m early in the evening ences in abundance and species composition bemeen (Fig. &A). followed by consolidation of the shal- waters above the seamount and at the reference low layers, a slight sinking of the top layer. and stations. Maurolicus muelleri and the lophogas- expansion downwards until the scattering layer trid mysid. Gnathophausia longispine. dominated extended from the summit of the seamount upwards the catch over the seamounts. A third species. to approximately 100 m depth (Fig. 4B). By con- the sepiolid squid. Iridoteuthis iris. vas charac- trast. no deep scattering layers in surrounding teristic of the deeper portions of the seamount oceanic waters displayed either such high density scattering layer (Fig. 5A). In waters away from or this type of behavior, suggesting that orga- the seamount, oceanic taxa were generally more nisms specific to the region of the seamount were abundant and the three seamount taxa were either BOMLERT AND GENIN 325

DENSITY (Ind./WM3) A Ecology of Seamount Benthos

Evidence that seamounts are inhabited by unique, and sometimes rich. benthic communities was first obtained in the surveys of Vema Seamount Maurollcur muellerl (South Atlantic) and Bowie and Cobb Seamounts (northeast Pacific). all shallow seamounts where IF-----L scuba diving was used [Simpson and Heydorn. 1965; Vlnclguerrla spp. Scagel. 1970; Birkeland. 19711. Developments in deep-sea photography allowed the first visual examination of deeper seamounts. Some of the first photographs [Heezen and Hollister. 19711 Myclophldab showed current-swept beds inhabited by , sea pens, anemones. , crinoids, and other sessile suspension feeders on Eltanin Seamount Other Fishes (southeast Pacific, 162 m), Kelvin Seamounts (western North Atlantic, 1.419 m, 903 m). and Ampere Seamount (North Atlantic, 143 m, 529 m). Other studies have revealed dense populations of Iridoleulhis iris the gorgonian , Ellisella flaxellum. seen in photographs taken on Josephine and Great Meteor Seamounts (east Atlantic, 200-300 m; Grasshof f. 19721 and the high abundance of sponges, hydroids. bryozoans, and serpulid tubworms on the top of B Patton Seamount (Gulf of Alaska, 433 m; Raymore. 19821. Most of the photographic investigations noted above were rather limited in spatial coverage, I 1. with photographs taken at only a few locations on Gnrlhophausla longlsplna each seamount. Only with the use of more advanced photographic instruments could the distributional patterns of deep seamount benthos be studied. Euphaurlldr Genin et al. 119861 used the Deep Tow [Spiess and Tyce. 19731 and Grigg et al. [19871 used towed sleds to take hundreds of photographs along several transects on different parts of seamounts. Penaelds Manned submersibles have also been used recently in seamount studies, allowing detailed surveys of megafauna as well as small meiofauna [Levin et Carldeanr al.. 1986; Lissner and Dorsey, 19861. Submersible investigations, which allow detailed observations and even experimental manipulations, hold great Fig. 5. Comparison of densities of selected promise for understanding seamount benthos. micronekton taxa captured above Hancock Seamount Seamounts are highly diverse habitats. A with those taken at reference stations 5 to 20 km single seamount usually extends over a large depth away in summer, 1984. Twelve samples were taken interval, sometimes from the euphotic zone to the above the seamount and six at reference stations abyss. Substrata of seamounts vary from exposed with a 6-ft Isaacs-Kidd midwater trawl. A. Fishes rocky bottom to a continuous thick layer of and squid. B. Crustacea. Two values are repre- sediments. The latter is usually found on the top sented for each taxon; the upper (solid) bar indi- of deep and in topographic depressions, cates mean density (22 S.E.) from the tows over whereas rocky outcrops and extensive hard-bottom the seamount while the lower (cross-hatched) bar areas characterize the steep flanks as well as the indicates mean densities in off-seamount tows; tops of shallow guyots [Karig et al.. 1970; Lons- note that abundances are on a log scale. (Pre dale et el.. 1972; Raymore. 1982; Genin et al. sented in Boehlert and Seki 1984.) 19861. Sediments on seamounts can vary in grain size and can be rippled at one site and smooth at another. The sediment distribution pattern can be absent or in low abundance (Fig. 5B). It would greatly affected by the topographically-induced appear that this seamount, and others described current regime, creating sometimes a moat around above, have important effects upon the pelagic the seamount base [Roberts et al.. 19741. 'Qpes ecosystem. A better understanding of the local of hard bottom on seamounts are highly variable, physical and biological oceanography will be ranging from carbonate rocks and pillow to necessary to determine the nature and cause of rocks encrusted with a hard ferromanganese layer. these effects. Remarkably different substrata can even be found 326 SEAMOUNT BIOLOGICAL. OCEANOGRAPHY

on a single seamount. In spite of this great coastal areas of the northeast Pacific [Birkeland variability, some unique environmental conditions 19711. Tunicates, which frequently dominate rocky seem to lead to common biological characteristics areas on the coasts of Washington. are represented of Peamount benthos. These features include the by a single species on Cobb Seamount. and neither clarity of overlying waters and extended light hydroids nor barnacles have been observed on the penetration, the presence of deep rocky substra- seamount. On the other hand, the community on tum. and the exposure to strong currents. The Cobb Seamount differs from that on Bowie Seamount: effects of these conditions on the structure Of barnacles and hydroids are found on the latter but biological communities on seamounts are the focus not on the former, and different molluscs are of this section. dominant at each site. Such changes are probably caused by inter-site differences of ecological Shallow Seamounts--Ef fects of Water Clarity and conditions combined with differences of coloniza- Isolation tion history. The presence of brooding species (the asteroid Leptasterias) and species with no Far away from sources of terrigenous turbidity, planktonic larvae (the gastropod Searlesia mid-ocean seamounts (like many oceanic islands] on Cobb Seamount [Birkeland. 19711 suggests that are characterized by exceptionally clear overlying rare colonization events can affect the structure waters, where the attenuation of light is far of the community on each seamount. Propagules of lower than in coastal waters. Consequently, the brooding asteroids, for example. probably benthic autotrophs may occur in greater depths. reached Cobb Seamount with drifting kelp [Birke- The deepest known plant life, for example, has land, 19711. recently been discovered on San Salvador Seamount in the Bahamas, where an undescribed species of Deep Seamounts--Effec_ts of Substratum coralline alga was found at 268 m [Littler et al. and Currents 19861. The flat top of this seamount contains a rich and exceptionally diverse multi-layer commun- The most distinctive characteristic of deep ity of macroalgae with planar algal cover (under- seamounts is the occurrence of extensive areas of story and canopies) exceeding 100%. On Tanner and hard substratum. Unfortunately, most information Cortes Banks (off southern California). a dominant on the abundance of deep hard-bottom species on macroalga. Eisenia arborea. grows as deep as 40 m. seamounts is incomplete, largely due to the small whereas its lower limit at coastal sites in the number of detailed surveys conducted. Further- Southern California Bight is between 5 and 12 m more. most of the observations were obtained with depth [Lewbel et al.. 1981: Lissner and Dorsey. dredges or photography, so that very little is 19861. Macroalgal species on these and other known about the abundance of smaller organisms. banks exhibit similar depth extensions [Scagel. Large suspension feeders. including sponges. horny 1970: Lissner and Dorsey. 19861. Most of these corals (gorgonians). black corals (antipath- species are usually found in the lower intertidal arians), ahermatypic scleractinian corals. or upper subtidal zones. This extension was also anemones. tunicates, brisingid seastars. and the case with some intertidal animals, such as the crinoids, are the dominant taxa which have been mussel. Mytilus californianus. found in relatively observed. Their abundance generally decreases greater depths on Bowie Seamount [Scagel. 19701 with depth [Grigg et al., 19871. Photographs and on a submerged pinnacle off the northwest taken shallower than 1,000 m usually exhibit coast of Washington [Paine. 19761. Depth exten- several taxa in each frame. sometimes forming sions of animals were attributed to biological dense communities (Fig. 6. and Heezen and Hollis- factors. such as the rarity or absence of preda- ter. 1971; Grasshoff. 1972: Raymore. 1982: Genin tors [Paine. 19761. The asteroid, Pisaster et al.. 19861. Rich gorgonian fields occur on ochraceus. determines the lower limit of Mytilus several seamounts (depth <1.000 m) in the Emperor in coastal regions but is rare on the submerged Seamount chain (northwest Pacific). The discovery pinnacle due to the lack of an intertidal zone. of these fields in the late 1970's caused a sharp Small recruits of Pisaster are found primarily in decline in prices of precious corals on the world the lower intertidal zone where they feed on &all market [Grigg. 19841. barnacles [Paine. 19761. In oligotrophic oceans, even shallow seamounts, In addition to extensions of depth ranges. such as Cross Seamount near Hawaii (summit depth distinctive differences between animal communities 300 m). exhibit sparse communities [Grigg et al. on seamounts and those at adjacent coastal sites 19871. whereas relatively high densities of large at corresponding depths have been described on suspension feeders are sometimes found in greater several shallow seamounts [Birkeland. 1971: Lewbel depths on seamounts located in fertile waters et al.. 19811. Suspension feeders are typically (e.&. on Kelvin Seamount at 1419 m depth; Heezen in much greater abundance on Tanner and Cortes and Hollister, 19711. In addition to the fer Banks than in corresponding coastal regions tility of overlying waters, differences in the [Lewbel et al.. 19811. The scallop, Hinnites abundance of organisms on different seamounts can multirugosus. dominates the entire primary sub- be related to local environmental conditions and stratum on vertical surfaces of Cobb Seamount, to availability of sources of larvae (i.e., the whereas it exhibits a scattered distribution in presence of adult populations upstream of a sea- BOEHLERT AND GENIN 327

Fig. 6. Antipatharians. sponges, and other suspension feeders near a satellite peak at 700 m depth on Jasper Seamount in the northeast Pacific. Stichopathes sp.. the whip- like , is the most abundant megafaunal species between 600 and 1.000-m depth. mount; Lutjeharms and Heydorn. 1981b: Grigg et (central North Pacific, A. Genin and K. L. Smith. al.. 1987 I . Jr.. unpubl. manuscr.. 1986). Other distribu- The presence of several satellite peaks on tional patterns of large suspension feeders on Jasper Seamount allowed Genin et al. [19861 to seamounts further support the hypothesis that the separate the effects of depth and topography on abundance of animals on seamounts is determined by the abundance of animals. It was expected that factors related to the local topography. These passive suspension feeders would be generally more patterns can be separated into three spatial abundant at shallower depths. where the concentra- scales, namely patterns observed on the entire tion of particulate food in the impinging waters area of a seamount, patterns on topographic is greater. The observations, however. showed features such as knobs and pinnacles. and small- that within a certain depth range the distance scale patterns such as those seen within a photo- from a peak is a key factor in determining animal graph. The large-scale patterns include the densities. The densities of a dominant species on above-mentioned increased densities near peaks. Jasper Seamount, the black coral, Stichopathes They can be divided into patterns on narrow taper- sp.. were significantly higher near peaks than in ing peaks and those on vide or flat peaks. Densi- mid-slope areas at corresponding depths (Fig. 7). ties on narrow peaks are greatest near the crest. A similar increase in the densities of corals on whereas densities on flat tops of guyots and on the upper part of a slope, near a peak, has been gently sloping peak6 are higher near the rim than observed at ca. 2.000 m depth on Horizon near the center (Fig. 8 and Genin et al.. 1986. 328 SEAMOUNT BIOLOGIC& OCEANOGRAPHY

t 16 Stichopathes

14 I - PEAK 12 0 - SLOPE 10 % 28 z 6

4

2

500 600 700 800 900 1000 1100 1200 1300

DEPTH (m) Fig. 7. Densities of Stichopathes (mean 2 6.d.) at different depth intervals on Jasper Seamount. The photographs were divided into those near peaks (solid bars) and those ,150 m below a peak (open bars). Stars indicate the intervals in which the mean near- peak density is significantly higher than the mean mid-slope density (P < 0.001, Mann- Whitney U test). The number of photographs taken at each depth interval is indicated above the corresponding bar. (From Genin et al.. 1986.)

Jasper Seamount: Grigg et al.. 1987. Cross Sea- stratification, and Coriolis parameter [Wunsch mount). A similar increase of coral densities was 1969: Eriksen. 1982. 19851. Such an intensifica- observed along the rim of Fieberling Guyot (east- tion is not expected to be distinctive on the flat ern North Pacific, ca. 500 m depth: A. Genin. or gently sloping areas at the center of guyots unpubl. data). On the scales of tens of meters, and wide peaks. where animal densities are low. animal densities are usually greater on topo- Existing physical theories do not predict the graphic prominences. such as pinnacles and knobs, occurrence of stronger currents near a crest of a than in surrounding areas (e.g.. Fig. 8). On the peak as compared with a mid-slope site at the same smaller scale, suspension feeders are frequently depth and on a similar slope angle. Hydrographic aggregated on upper parts of rocks, and small observations. however. suggest that the upwelling animals were found on protruding parts of larger above such peaks is confined to the proximity of organisms, such as sponges. the crest and does not occur over mid-slope areas The occurrence of similar distributional [Bezrukov and Natarov. 1976: Fukasawa and Nagata patterns on different seamounts. combined with 1978: Genin and Boehlert. 1985: Roden and Taft hydrodynamic theories and observations. suggests 19851. Short-term current measurements made by that a topographically induced current regime is a Genin et al. [19861 on Jasper Seamount also showed key factor in determining the abundance of suspen- differences between a peak and a mid-slope site: sion feeders on deep hard-bottom seamounts. lbo the average current speed near a peak was about different mechanisms may enhance currents near the twice the mean speed recorded at a mid-slope site edges of wide peaks. First. the flaw of waters at the same depth. The mid- and small-scale impinging on the flank may be upwelled above a increases of densities of suspension feeders on seamount's top. Due to conservation of potential knobs. pinnacles, and on the top of protruding vorticity, an anticyclonic motion is induced, rocks can be similarly explained by an exposure to resulting in an accelerated flow on the left side enhanced currents as these structures protrude to of a seamount (looking downstream) and decelera- higher elevations above the bottom and are there- tion near the center and on the right [Huppert. fore exposed to more energetic zones of the 1975: Huppert and Bryan. 19761. Alternating tidal benthic boundary layer [Butman. 1986: Grant and flows would thereby cause intermittent accelera- Madsen. 19861. Even on such small-scale features. tion periods near the edges and recurrent decel- however. variability in colonization rates may be eration near the center of wide peaks and guyots. related to small-scale flow structure [Nowell and The other mechanism is related to the reflection Jumars. 19841. Genin et al. [19861 proposed two of internal waves along a sloping bottom. where different mechanisms through which intensified intensification of the flow occurs. especially for currents can induce higher animal densities. In those waves with frequencies within an octave of a the "settlement pathway." a site is colonized by critical frequency defined by the bottom slope, relatively more recruits simply because more BOWLERT AND GENIN 329

PEAK A PEAK C

DISTANCE (km) DISTANCE (km) Fig. 8. Bathymetry. densities of Stichopathes. and sediment cover across two satellite peaks on Jasper Seamount. Each dot indicates the animal density or sediment cover in a single photograph. Solid lines indicate 9-point running means. Note that the animal densities follow the bathymetric line on the sharp peak (C) whereas lower densities are found near the center of the wide peak (A) than near its edges. Animal densities are relatively higher on wall topographic prominences such as those at 1.4 and 3.9 km along the transect. Note the sharp increase of sediment cover in the topographic depression near peak C. (From Genin et al.. 1986.) water, and thereby more larvae. flow through per control sediments collected at distances of 1 m unit of time: in the "feeding pathway." more watez from those protozoans. Levin et al. 119861 pro- flows past suspension feeders at sites charac- pose that xenophyophores contribute to maintenance terized by stronger currents. resulting in of high benthic diversity by altering hydrodynamic increased feeding and growth rates and possibly conditions and by providing metazoans with sub- higher survival rates of small recruits. The stratum, food, and refuge. The diversity of soft- actual mechanisms involved are yet to be experi- bottom habitats on seamounts has not been compared mentally tested. with the surrounding deep sea: this would make an Very little is known about soft-bottom fauna on interesting investigation from a submersible, deep seamounts. Unlike hard-bottom epifauna, since a wide depth range occurs at a single site infauna in sedimentary substrata cannot be on soft-bottom seamounts. observed by photography. In a recent study of deep (1.000 to 3,000 m) seamounts in the Pacific The Role of Seamounts in Ocean off Mexico. Levin et al. [19861 used a Fisheries Productivity submersible to investigate the effects of giant protozoans (xenophyophores) on local sof t-bottom Seamounts and banks may aggregate resident communities. Xenophyophores are abundant on sea- demersal and transient, pelagic organisms which mounts where they agglutinate sediments to form can support fisheries [Uda and Ishino. 1958: large tests (up to 25 cm diameter) which protrude Uchida et al.. 19861. Polovina [1985] compared above the sediment. Sediments immediately sur- seamounts with bank and island systems and found rounding these organisms exhibit higher densities higher densities of the same species on the sea- and diversities of metazoan species relative to mounts. A variety of demersal resources in high 330 SEAMOUNT BIOLOGICAL OCEANOGRAPHY

90 extremely high catch rates which declined drasti- cally in later years of the fishery. Some of the same physical mechanisms which alter the patterns of distribution and abundance of the taxa as 80 -UIMMEI SEAMOUNT described earlier for Hancock Seamount may be a-----I MILWAUUEE SEAMOUNTS invoked to explain the availability of the energy 0 - - - 0 COLAHAN SEAMOUNT .------* C -H SEAMOUNT necessary to maintain high densities of fish here 70 -HANCOCK SEAMOUNTS and at other seamounts. First, convergent flow e- - - - - OTHERS resulting in accumulation and greater flux of 1 oceanic plankton and micronekton may provide prey 5 60 [Isaacs and Schwartzlose. 1965; Darnitsky et al. 1984: Tseitlin. 19851. Secondly, as described P earlier, locally enhanced productivity may be (3 A 50 retained in the region of the seamount. Taylor 5 column or other stationary water mass could retain this productivity, but the residence times of such as features are unknown. Energy for the high biomass 40 demersal fish resources, however. appears to be z derived from oceanic rather than seamount-derived - sources. This assertion is supported by diet 30 studies of pelagic armorhead [Fedosova. 19761, P species on seamounts in the Indian Ocean [Parin 0 and Prutko. 19851, and elsewhere [Kashkin. 19841. 20 Simulation models of seamount fish populations [Tseitlin. 1985; Pudyakov and Tseitlin. 19861 suggest that such allochthonous energy inputs are necessary for population maintenance. 10 An intriguing question about these animal resources, given that most have pelagic larvae, is the mechanism of recruitment back to the seamount. 1970 1972 1974 1976 1978 1980 One of the first suggestions of such a mechanism invoked the concept of stationary Taylor columns YEAR Over seamounts for maintenance of pelagic larvae Fig. 9. Annual data on catch-per-unit-effort (an [Shomura and Barkley 19801. This hypothesis is an index of abundance) of Japanese trawlers for extension of the ideas on the conservation of pelagic armorhead on several of the southern insular plankton described by Boden [19521. Main- Ehperornorthern Hawaiian Ridge seamounts. 1969- tenance of pelagic larvae in closed circulations 81. These data show the sharp decline in stock above large bank systems has recently been demon- abundance. The Japanese catch was about one-f if th strated in several locations [Dooley 1984; Sundby. that of the Soviets; combined. they took, 1984; Smith and Morse, 19851. Others have sug- approximately one million metric tons of this gested that seamount populations are derived from species off these seamounts during this period. upstream source populations; the distances pro- (From Wetherall and Yong. 1986.) posed have been as great as 1,100 mi [Lutjeharms and Heydorn. 1981bl. In either of these cases, however. physical variability can lead to inter- abundance has been noted on seamounts including annual variability in recruitment strength; such fishes [Sasaki. 19861. lobsters [Lutjeharms and fluctuations may be characteristic of seamount Heydorn. 1981al. crabs [Hughes. 1981; Alton. 19861 resources [Lutjeharms and Heydorn 1981a; Wetherall and precious corals [Grigg. 1986; Genin et al. and Yong. 19861. Given the small geographic 19861. Pelagic species such as and squid extent of seamounts and the variability in may seasonally feed in waters above seamounts recruitment. great care must be taken to manage [Inoue. 1983; Yasui. 19861. as do some marine seamount resources and prevent overexploitation mammals [Hui. 19851. In the open ocean, seamounts [Boehlert. 1986: Sasaki. 19861. thus function as sites of increased production or aggregation of higher trophic level organisms. Conclusions and Suggestions for Future Research A specific example of high fisheries produc- tivity is provided in the southern Emperor and As we have described in this paper. seamounts northern Hawaiian Ridge seamounts. Between 1967 are sites where physical perturbations result in and 1975. nearly one million metric tons of development of unique ecosystems. Understanding pelagic armorhead. Pseudopentaceros wheeleri. were variability in the biological productivity of taken by Soviet and Japanese trawlers, and stand- seamounts is a challenging research problem which ing stocks were estimated at nearly 400,000 metric will require interdisciplinary research. Meso- tons [Borets. 1975; Sasaki, 19861. Japanese data scale physical oceanographic studies will be on catch per unit effort (Fig. 9) demonstrate necessary to define the conditions for development BOEHLERT AND GENIN 331

of eddies, Taylor columns. and other features of Acknowledgments. We thank the many people who flow complexity. Small-scale studies of upwell- helped in compiling the great deal of literature ing, turbulent mixing, and benthic boundary layer surveyed in this paper, particularly Vladimir effects will better characterize the local condi- Darnitsky, who provided much of the Soviet litera- tions near the seamount. Concurrent studies of ture, and Wilvan G. Van Campen. who tirelessly biological oceanography of the water column over provided translations. We also thank E. 0. Hart- seamounts can define the variability of nutrients wig. P. A. Jumars. and R. E. Young for providing and primary productivity and their residence reviews of the manuscript. Finally. it was Bill times. By understanding the seasonal and inter- Menard who first introduced the junior author to annual variability of these phenomena, we should the subject of "seamount biology" during a be able to better define the importance of class; his curiosity and enthusiasm have been enhanced productivity to higher trophic levels and inspirational. to determine the role of currents in concentrating or increasing the flux of allochthonous energy References sources. In addition to these temporal components of variability, comparative studies of seamounts Alldredge. A. L.. and W. M. Hamner. Recurring can provide an understanding of spatial varia- aggregations of zooplankton by a tidal current. bility. Given information on bottom topography Estuarine Coastal Marine Science 0.31-37, and ocean currents. perhaps we can develop gener- 1980. alities or predictive capabilities concerning Alldredge. A. L.. and J. M. King. The distance seamount productivity. 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