128 



COLIN 



(Ogden, 1976). Sea urchins are abundant and conspicuous 

 elements of the reef fauna. At Enewetak and much of the 

 Indo-West Pacific, sea urchins are considerably less abun- 

 dant. Diademnids are particularly less conspicuous, being 

 small and deeply hidden in the reef. One possible explana- 

 tion for this difference is a higher population of fishes 

 which prey on sea urchins in the Indo-West Pacific (Fricke, 

 1971). in general, western Pacific fish faunas are consider- 

 ably more diverse and "highly evolved" than the Atlantic, 

 and more species may be adapted for exploiting sea 

 urchins (among other things) as food. 



Gilmartin (1960) indicated that herbivores have a much 

 smaller influence on benthic algal communities at 19 to 

 63 m at Enewetak than they do on shallower communities. 

 Bakus (1967) felt that grazers influence the benthic biota 

 most in water <10 m deep. 



With heavy grazing pressure from herbivores, the pres- 

 ence of dense abundant algae implies some reason for its 

 avoidance by herbivores. For example, the filamentous 

 strands of the blue-green algae, L[^ngbia sp., occur 

 extremely abundantly on projections, particularly corals; on 

 many reefs at depths below 6 to 9 m on windward lagoon 

 reefs; and as shallow as 1 m in protected areas. The alga 

 covers large areas of the substratum, streaming from corals 

 and resembling long reddish hair. Often it virtually covers 

 the coral with a tangle of filaments that is extremely diffi- 

 cult to remove. Lf^ngbia sp. often seems to have detrimen- 

 tal effects on the live coral with the coral tissue beneath 

 the algae appearing unhealthy. Often coral areas beneath 

 the alga are dead, but whether the alga is the causative 

 agent or simply grows on available substratum is not 

 known. At some lagoon pinnacle reefs, such as Medren 

 Pinnacle, Liingbia sp. appears to have a significant impact 

 on the total reef and may be significant in coral mortality 

 there. Li/ngbia sp. is also abundant on some lagoon margin 

 patch reefs below 6 m depth but is absent on the shal- 

 lower portions of the same reefs. 



The small sea hare, Sfylocheilus longicauda, occurs 

 abundantly on the Li^ngbia sp. Sarver (MPRL, 1976) 

 reported it feeds almost exclusively on Liingbia sp. and 

 spends its life, exclusive of larval stages, on the alga. The 

 sea hare accumulates an antitumor agent, debromoaplysia- 

 toxin, from L^ingbia sp. and Oscillatoria sp. at Enewetak. 

 This poisonous lipid was first isolated from the digestive 

 tract of S. longicauda but has its origin from the blue-green 

 algae (Moore, MPRL, 1976). Sarver found that adult 

 S. longicauda (about 3 to 7 g) reproduce rapidly, at about 

 30 days age, and consume about 75 to 100 g of 

 the alga during their lifetime. 



Bioturbation in the Deep Lagoon 



A high level of bioturbation in sediment bottoms 

 throughout the deep lagoon has been verified by recent 

 work (Suchanek and Colin, 1986; Suchanek et al., 1986). 

 Gilmartin (1960) first noted, based on in-situ observations, 

 significant bioturbation of deep lagoon bottoms, but several 



other authors have commented on it Emery et al. (1954) 

 noted, in shallow lagoon photographs, disturbance of the 

 bottom and burrowing. Hillis Colinvaux (1980) noted a 

 "relative prominence of animal mounds and castings on the 

 lagoon floor near the base of pinnacles in 40 m." 



Bioturbation of the deep lagoon is evidenced by the 

 ubiquitous presence of "lebensspuren," a term designating 

 any sedimentary structure produced by a living organism 

 (Hantzschcl, 1962). A wide variety of lebensspuren occurs 

 on sediment bottoms at Enewetak, but the conical mounds 

 of ghost shrimps (Callianassids) that are as much as 1 m in 

 diameter and 30 to 40 cm high are the most apparent 

 type. The conical mounds represent the excurrent open- 

 ings of complex burrow systems which penetrate deep into 

 lagoon sediments and underlie nearly all the sediment 

 bottoms. 



Photographs from the Enewetak Lagoon benthic sur- 

 vey, observations from the submersible Makali'i, and scuba 

 diving on the lagoon margin have confirmed the near pan- 

 lagoon (below 10 m depth) distribution of callianassid 

 mounds. The basic morphology of the burrow system, 

 pumping rates, and sediment processing have been exam- 

 ined and will be discussed subsequently. Since the lagoon 

 sediments are the major repository of remaining radionu- 

 clides at Enewetak, an understanding of the mixing and 

 resuspension abilities of lebensspuren-producing organisms, 

 particularly callianassids, is of basic relevance in any con- 

 sideration of the future fate of long-lived radionuclides in 

 the marine environment. 



Callianassid mounds are often referred to as "vol- 

 canoes" because of their conical shape with steeply sloping 

 sides, a small apical depression (crater), and the resulting 

 eruption when water and sediment are pumped out of the 

 apical depression at irregular intervals. These volcanoes 

 can be so dense that essentially no level substratum can be 

 found in a large area, the bottom being comprised solely of 

 volcanoes and incurrent depressions of the callianassid bur- 

 row systems. It is estimated, based on the photographic 

 survey, submersible work, and diving observations that 

 about one volcano per square meter occurs overall in the 

 lagoon below 10 m depth. Since approximately 85% of the 

 lagoon bottom is soft substratum and covers about 8 X 

 10 m , on the order of 10 callianassid volcanoes occur in 

 the Enewetak Lagoon. Densities may vary from place to 

 place by a factor of 10, and several species of callianassids 

 are certainly involved. 



A typical callianassid burrow system at Enewetak con- 

 sists of three major elements: (1) conical depressions on 

 the surface where sediment enters the system, (2) a com- 

 plex network of horizontal and vertical burrows, and (3) 

 conical mounds (volcanoes) where sediment and water are 

 discharged. The incurrent openings to the system 

 (Suchanek and Colin, 1986), in which sediment and water 

 are drawn into the system, appear as a conical depression 

 many centimeters in diameter. The excurrent side of the 

 system is represented by the volcanoes, each of which is 

 fed by a vertical tube at its center through which sand and 

 water are pumped by the action of ghost shrimp in the 



