ATOLL RE3EARCH BULLETIN ---.. 9. The Coral Reefs of Urno Atoll., Marshall Islands Issued by THE PncIiw SCIENCE EO~D National Academy of Sciences - National Research Council Rashington, D. 2. December 15, 1951 SCIINTIE'IC I>JVF;STIGATI(!NS IN i.iIC1JUPESIA - P:?cii'ic Science bard An atoll. has 8. top tha%'s S1n.t And fe,s.tureless extremely. Corals and. algae make a mat Where mountains are not seemly. John 1. l'ells Cornell University December 1951 The aufJno? of this report participated in the 1950 SIM (Sc-entific Investigations in Micronesj.a) Project of the Pacific Science Board of the National Research Council. This project ties support,ed bf funds granted to the Nstionai 4ca3emy of Sciences the Office of N~valResearch, and the fie16 work was carried 0u.t with the active assistance of the Navy Lie?artment, the Military Air Transport Service, and the officials of the Civil lidminjstrative Staff of the Trust Territory (Navy). To all of the above who helped to make possibie the research described in this report I wish to express sy gratitude and sppreciction. Special thanks are due to Mr. 3onald F'. Squires (Cornell '50) without whose able assistance in the field much less would have been accomplished. Page I F:.elcj, -----.------.---.-------------- 1 A. Reefs ?. B. Lagoon 3A. C. Collections i 11. Stri-ct~reand F'nysical Processes ------ 2 A. (;eneral. Sta terne?.t -- 2 B. Structure of Che Atoll Islands ---- 3 C. Surface Processes 2nd Chacgee ----- 5 D. Reef Types " E. Lanoon Reefs ...................... 0 i P. Corc1 Knolls ---- -.-- 9 ZII. neer zonatjon 10 A. SeavarG Reefs ..................... LO 1. Tvpe IA ....................... 10 2. Type IB 10 3. Type IIh ...................... 10 B. Lagoorl Reefs ...................... 11 1. Leeward Type ------------------ il 2. Windward Type ----------------- 1.1 I:T. Reef Tempelatures ..................... 12 V. P-ppendix: List of Co-a1 Genera ------- ?4 FIGURES Map of Arno, showing distribction of reef types Physiogra~hicdiagram of Arno Cross-section of Arno, true scale De::ail map: North Horn, 1:8000 Generalized section of island cnd reef Section of island swept by typhoon Typhoon destruction, and reconstruction, at Arno Deteii mop: Vicinity of Ine ilillege, Ine Islnnd, 1:8000 Section: sehward ree?, Lype IH, East of Ine Villege Section: seaward reef, type IB, Takleb Island Section: seewsrd reef, type IIA, Ine Ancharage Section: lagoon reef, leewurd t,:@e, Takleb 1sl.and Section: lzgoon reef, leeward type, IQorthHorn Section: lagoon reef, windward type, heVillage Kaxi:num-minimun serial temperatures of water across Ine ;:nchorsge reef, June, 1950 High md low water tenpracures dong ine Anchor~gereef, at inner margin of hcropora Zone iii I. Field Work A. Beefs The coral reefs were the main object of study during the field period, and all types of reefs represented at Arno were investigated in more or less detail: Ine Island - Seaward-leeward reef; windward-legoon reef. A three-mile stretch on either side of Ine Village was mapped in detail on a scale of 1:8000. Ta.kleb Island - Seaward-windward reef; leeward-lagoon reef. Mapped. Aotle and associated islets - Seawzird-windward reef; lagoon reef. Mapped. East Horn (Male1 to Lonar I.) - Seaward-windrjard reef; lagoon reef (enclosed lagoon). Mapped. North iforn (beman-I3ikarej-Namwi) - Seaward-windward reef; seaward- leeward reef; enclosed lagoon reef; reef l'damll at north end. Mapped. Studies were made of the water temperature fluctuations on the reef --.- ? tract on the seavard. side of Ine ?::. L :L' ?is. Following the arrival on 19 July of dredging gear from 'Washington, dredging was carried out nearly every day until 1 August, when the gear was re-packed for shipment. Earlier attempts to dredge with a crude dredge made from material at hand produced results, but were less thm satisfactory. The regular dredges were more successful, but the great difficulties in operating from sailing outrigger canoes emphasize that systematic dredging, a necessary part of any scientific study of an atoll, is possible only from a sizeable powered boat. Nevertheless, as a result of Mr. Squires' per- sistence, a number of bottom samples and zoological specimens were obtained from soqe 25 hauls in the lagoon, even though nearly half of them were dry runs. C. Collections Corals. About 600 specimens, mostly cured, of reef and lagoon corals wele collected. -Other, Insofar as possible, with only 2 gallons of alcohol, specimens of mollusca, echinoderms, alcyonariens, foraminifera, crustaceans, etc., were collected for the U.S. National Museum, as well as sone dried material. k number of specimens were preserved but later had to be discarded or dried because of lack of sufficient preservative. Ten gallons of alcohol requested from Majuro late in June was finally delivered at lYejuro on the return journey. Some was aaded to tne one tank, but the rest by that time was of no use and was returned. 18 In all/wooden packing cases were filled, weighing about 1000 pounds and shipped back by surface transport. The corals are still being studied, and a complete list at this time is impractical, but it is estimated that over 100 species representing about 45 genera of scleractiniawand hydro- zoan corals were procured, with ecological data. A list of these genera:is: appended. ,., . .., fibout 350 photographs (Kodachrome) were taken, mostly of reef phenomena, including a number of underwater views. Five hundred feet of 16-mm. color motion picture film were exposed, again mostly of reef features above and below water, with unexpectedly good results. The motion picture camera and tripod were a loan from the Navy through the Honolulu office of the Pacific Science Board, and grateful acknowled~~m~of this is here made. 11. Structure and Physical Processes A. General Statement Arno Atoll, in the southern Marshall Islands et approximately 172 deg. E. a~d7 deg. N., lies in the northeast trade wind belt end in the north equatorial current. The 1atter:is.deflected northwards during the summer months by the equatorial co-mter current. It is in that part of the Pacific where-the surface water tenperatui-es donotfluctuate more than 1 deg. above or below 28 deg. C. - the warmest part of the Pacific. The temperature,'-..::, clarity, and agitation or' the surface are at the optimum for reef coral development, but the planktonic food supply, on the other hand, despite the seasonal reversal of oceanic currents, is lower then in areas farther .tothe southwest and coral growth is below the maximum. Arno, like nany atolls, departs from the common ides of circular atol?:' . shape, being roughly rectangular with two extensions or "horns", one ex- . tending to the north about 5 miles from the north corner of the rectangl'e:': (lrNorth Horntt). The otiner extends about 7 miles northeastward froa the east:'. corner (*East Hornu) (Figs. 1 and 2). Both horns are peculiar in that each. encloses near its ti13 a small secondary lagoon separated from the main atoll lagoon. The total area enclosed by the seaward reefs and lagoons is about 147 sq. mi. The surface reefs cover about 16 sq. mi. and on them 133 islands and islets form about 5 sq. mi. of dry land. The main lagoon encloses about 125 sq. mi., the secondary lagoon in the E~stHorn about 4.5 sq. mi., while the little enclosed lagoon of the North Horn hes en area of only 1.5 sq. mi. (Fig. 4). The seaward reefs extend continuously around. the entire atoll, except where they are broken by Tutu and Takleb Passes, which have threshold de2ths (15-25 fms.) nearly equal to the lagoon depth, three shallow (4 fcs or less) passes ehst from Takleb and one shallow pass into the northwestpart of the East Horn lagoon. The total peripheral extent of the seeward reefs is abouk.65 miles, and except for ninor pass into the East Horn lagoon, all breaks in the reef are in the &mile windward stretch of the northeastern face of the atoll extending southeasterly from Tutu Pass toward Namej Isltind. The seairard slopes (Fig. 3) of the outer reefs are steeper to lee- ward than to windward. No precise dats are available, but it is obvious from an inspection of aerial photogr~phsof Amo, and indicated by H.O. Chart 6005, that there is a. decided difference. At Ine Anchorage a depth of 200 meters is reached about 150 meters out from the reef margin, a slope of 55 deg. At 1700 meters the depth is 1000 meters, about 27 deg. ,Thus the profile from reef edge sea.w&rd. is concave and steeper near the .. reefs, and gentler with increasing depth. On theseaward side to wind~ard off Takleb Island tine depth f+O0 meters out from the reef edge is only 40 meters, 2 7 deg. slope. In the next 350 meters the depth increases to 200 meters, a slope of 25 deg., nearly the saae as the downward slope from 200 meters on the leeward side. At 3500 meters from the reef margin a depth of 1000 meters is reached, a slope of 1.9 deg. from 750 meters. This slope is essentially convex near tine reefs, thence gently concave. Tne presence or absence of a 10-fathom terrace like thst at 3ikki is not determined, but comparison of aerial photographs of corresponding parts of the two atolls suggests strongly that it is developed on the windwerd side of Arno. The material of which the reef and islands of Arno iscomposed is wholly coral reef limestone and its derivatives: boulders, cobbles, pebbles,.sand, and finer silt-size particles. Srnall patches of phosphatic limestone occur in the interior of a few of the larger islands, but rock materiel.