Diadema and its relationship to coral spat mortality: Grazing,competition, and biological disturbance

The benthic grazer Diadema antillarum Philippi (Echinoidea) has been demonstrated experimentally to contribute to the control of coral community structure in shallow water. In Discovery Bay, Jamaica, West Indies, Diadema densities were manipulated over a range of 0-64/m² with the aid of enclosures. Grazing by Diadema under primary and post-primary succession conditions were compared.Algal percent-cover decreased as Diadema density was increased. Despite the presence of high algal cover. highest coral recruitment and diversity occurred at lowest Diadema densities, with planular settlement occurring predominantly in openly exposed micro-habitats. However, since algal growth rates greatly exceeded those of corals, space was rapidly monopolized by the former, resulting in intense competition and high coral mortality. This was particularly evident in Agaricia and Porites spp. At high Diadema densities, coral recruitment was greatly depressed in at least a genus-specific manner by intense levels of biological disturbance resulting from the echinoid's abrasive grazing activities. Favia Fragum (Esper) was especially susceptible to this perturbation. The surviving coral spat were found generally in cryptic, protected areas. Here they suffered some competitive losses to other sessile epifauna and -flora, particularly coralline algae, polychaetes and forams, which were well adapted to these physical and biological conditions. Increased sedimentation also depressed coral recruitment, replacing grazing as a limiting factor for successful settlement.Optimal conditions for coral survival, competitive success, and possibly growth were found at intermediate densities due to a balance between competition for space and biological disturbance. Diadema antillarum plays an important role in controlling the distribution and abundance of coral spat in the shallow reef community.     

Effects of grazing by Diadema antillarum Philippi (Echinodermata: Echinoidea) on algal diversity and community structure

In Discovery Bay, Jamaica W.I., densities of Diadema antillarum Philippi were experimentally manipulated over a range of 0–64 individuals · m^?2 with the aid of enclosures in shallow water. The effects of grazing by this echinoid on the algal community structure of a coral reef were compared under conditions of primary and post-primary succession.Algal biomass decreased as Diadema density was increased, particularly under primary succession conditions. A similar response was noted under post-primary conditions, but here biomass was also dependent on species composition and abundance of algae prior to experimental alterations of Diadema density. Peyssonnelia sp. was found to be particularly resistant to echinoid grazing, surviving even the most intense grazing. Within 11 months, algal diversity decreased significantly in an exponential fashion as Diadema density increased. This occurred under conditions of both primary and post-primary succession. Diversity, irrespective of index, was not maximized at intermediate grazing pressures. No single species of alga dominated the benthic community at even the lowest Diadema densities.Species composition of algae in the benthic community prior to changes in grazing pressure and lack of successful recruitment of a potential dominant were found to have a major influence on the response of the algal community to the treatments. Each of these factors can influence both the relationship between algal diversity and grazing pressure and the rate at which the community approaches an initial plateau of diversity in its earliest stages of succession.     

Environmental factors influencing urchin spatial distributions on disturbed coral reefs (New Caledonia, South Pacific)

Few works have examined the relative contributions of habitat variables to the distribution of coral reef urchins. In the present study, the spatial distribution of two common urchin species (Diadema setosum and Echinometra mathaei) was studied in the fringing reefs of two urban bays in New Caledonia (South Pacific). Urchins were surveyed at 105 stations with contrasted habitat structure/anthropic disturbance levels; 32 environmental variables (water/sediment characteristics, reef structuring species) were considered. Moderate densities were generally observed at station scale (mean 0.5 individuals m^− 2). The combination of univariate and multivariate techniques highlighted patchy distributions for Diadema as well as Echinometra, with distinct species/habitat associations; environmental gradients occurring within the bays did not seem to influence the species patterns. For Diadema, the spatial variability was better explained by sediment type than by biotic cover; increasing densities occurred across habitats with larger sediment sizes and decreasing coral complexity/macrophytes cover. In contrast, the distribution of E. mathaei exhibited weak relationships with habitat variables. In coral reefs, small-scale heterogeneity may thus be responsible for most of urchins spatial variability.     

Interference behavior and ecology of threespot damselfish (Eupomacentrus planifrons)

Summary Interference behavior by threespot damselfish directed at the sea urchins, Diadema antillarum and Echinometra viridis, differed in levels of aggression and discrimination. Higher aggression towards and recognition of Diadema by threespots was correlated with the distribution of Diadema along edges of coral patches. Lower aggression levels combined with lower recognition levels of Echinometra were correlated with a distribution of Echinometra closer to damselfish algal lawns. Differences in behavior of the urchins stimulated differing levels of aggression by the damselfish directly affecting the distribution of the urchins in the back-reef environment.This is contribution number 167 from the Discovery Bay Marine Laboratory of the University of the West Indies, Jamaica     

The effects of Hurricane Allen on Back Reef populations of Discovery Bay,Jamaica

Sixteen months after Hurricane Allen, an assessment of the condition (living vs. dead and encrusted) and volume of staghorn coral, Acropora cervicomis Lamarck, patches within the East Back Reef of Discovery Bay, Jamaica was made. Data generated by this assessment were compared with similar data collected in 1975–1976 prior to the storm. Densities of two urchins, Diadema antillarum Philippi and Echinometra viridis A. Agassiz, and the threespot damselfish, Eupomacentrus planifrons Cuvier, within the coral patches were also measured.Although staghorn coral patches were significantly smaller (? 65%) in mean volume in 1981 compared to 1975–1976, 22% of the patches were unchanged since 1976 or had increased in volume and only 9% were reduced to piles of rubble. Diadema and threespot densities were significantly higher than in 1976. Mortality of damselfish and larger Diadema appeared to have been reduced. Coral patches with both damselfish and Diadema present exhibited a high proportion of living coral tissue, while those patches dominated by either damselfish or Diadema were overgrazed with < 5% of the substrata covered by living coral. Similarly, the fore reef exhibited high urchin and low damselfish densities, possibly contributing to its low proportion of living coral.     

A 3000 year record of Caribbean reef urchin communities reveals causes and consequences of long‐term decline in Diadema antillarum

Urchins are the last abundant grazers of macroalgae on most Caribbean reefs following the historical overexploitation of herbivorous fishes. The long‐spined urchin Diadema antillarum was particularly effective at controlling macroalgae and facilitating coral dominance on Caribbean reefs until its ecological extinction from a catastrophic disease epidemic in the early 1980s. Despite their important role in the structure and functioning of Caribbean reef ecosystems, the natural dynamics of Caribbean reef urchin communities are poorly known due to the paucity of ecological survey data prior to large‐scale human disturbances and the Diadema dieoff. To help resolve the baseline abundances and ecological roles of common urchin taxa, we track changes in urchin abundance and composition over the past 3000 yr from analysis of subfossil urchin spines preserved in reef matrix cores collected in Caribbean Panama. Echinometra consistently dominated the subfossil spine assemblage, while Diadema was consistently rare in the subfossil record in this region. Rather than increasing during a period of heightened human exploitation of their fish competitors and predators, Diadema began declining over a millennium ago. Convergent cross mapping (CCM) causality analyses reveal that Diadema abundance is causally related to coral community composition. Diadema is negatively affected by Acropora cervicornis dominance, likely due to the tight association between this coral and the threespot damselfish, an effective Diadema competitor. Conversely, Diadema positively affects the abundance of the coral Madracis mirabilis, possibly via its control of macroalgae. Causal relationships were not detected among abundances of individual urchin taxa, indicating that inter‐specific echinoid competition is not a factor limiting Diadema recovery. Our detailed record of prehistorical and historical urchin community dynamics suggests that the failure of Diadema to recover over 30 yr after its mass mortality event may be due in part to the prey release of damselfish following the long‐term overfishing of piscivorous fishes.     

Competition between herbivourous fishes and urchins on Caribbean reefs

Summary When the common sea urchin Diadema antillarum was removed from a 50 m strip of reef in St. Thomas, US Virgin Islands, cover of upright algae and the grazing rates and densities of herbivorous parrotfish and surgeonfish increased significantly within 11–16 weeks when compared to immediately adjacent control areas. Sixteen months after removal, Diadema had recovered to 70% of original density, abundance of upright algae no longer differed between removal and control areas, and the abundance and grazing activity of herbivorous fish in the removal was approaching equivalence with control areas. On a patch reef in St. Croix that had been cleared of Diadema 10–11 years earlier (Ogden et al. 1973b), urchins had recovered to only 50–60% of original density. This reef still showed significantly higher rates of grazing by fish and a significantly greater density of parrotfish and surgeonfish than a nearby control reef where Diadema densities had not been altered. These results indicate that high Diadema densities (7–12/m² for this study) may suppress the densities of herbivorous fish on Caribbean reefs.     

Caractérisation de la production particulaire par l’oursin Echinometra mathaei sur les récifs Indo-Pacifiques : influence des peuplements coralliens et implications sur la dynamique sédimentaire

The characteristics of the sedimentary grains produced by the sea urchin Echinometra mathaei were described from two reef sites: a fringing Acropora-dominated reef at La Reunion island (Indian Ocean) and a barrier Porites-dominated reef at Moorea island (French Polynesia). The composition of the sediment produced by Echinometra was determined from SEM observations. The size and shape of the particles were measured by using image analysis method. The grain diameters range between a few micrometres and 2 mm, with a large predominance (more than 80 %) of particles smaller than 400 μm. The grain size distribution is dependent on the nature of the grazed substratum. Echinometra individuals collected at La Reunion on branching Acropora colonies produce a higher proportion of particles smaller than 200 μm compared to those collected at Moorea on massive Porites colonies. At Moorea, more grains having a diameter comprised between 200 and 500 μm are produced. The microstructure of coral substrata affects the mean particle diameter, which is 192,17 μm for a Acropora substratum and 244,69 μm for a Porites substratum. Since the sediment derived from Acropora erosion is finer, the proportion of suspended material that is exported from the reef is greater at La Reunion than at Moorea. We estimate that, for similar erosion rates and hydrodynamic conditions, the production of sands by Echinometra mathaei is higher and the retention of this erosional sediment more effective on reefs dominated by massive Porites than on Acropora-dominated reefs. This result is in accordance with the proportions of suspension-moving grains that have been previously measured on Moorea and La Reunion reefs. This study highlighted the effect of coral communities on the production of particles related to the bioerosion and on the sedimentary budget.     

Indo-Pacific echinoids in the tropical eastern Pacific

The existing literature reports that only one species of Indo-Pacific echinoid (Echinometra oblonga), occurs in the eastern Pacific. In this study we confirm the presence of this species at Islas Revillagigedo and also report the presence of two species ofEchinothrix (a genus hitherto unknown outside the Indo-Pacific) at Isla del Coco and at Clipperton Island. We also present evidence from isozymes and from mitochondrial DNA sequences indicating that at least one individual ofDiadema at Clipperton may belong to a maternal lineage characteristic of the west Pacific speciesD. savignyi. These data are consistent with the hypothesis that the observed populations of Indo-Pacific echinoid species are recent arrivals to the eastern Pacific, as opposed to the view that they are relicts of Tethyan pan-tropical distributions.Echinothrix diadema, in particular, may have arrived at Isla del Coco during the 1982-1983 El Nifio. In addition to Indo-Pacific species, Clipperton, Isla del Coco and the Revillagigedos contain a complement of eastern Pacific echinoids. The echinoid faunas of these islands should, therefore, be regarded as mixtures of two biogeographic provinces. Though none of the Indo-Pacific species are known to have reached the coast of the American mainland, their presence at the offshore islands of the eastern Pacific suggests that, for some echinoids, the East Pacific Barrier is not as formidable an obstacle to migration as was previously thought.     

Changes in the coral reefs of San Blas,Caribbean Panama: 1983 to 1990

Between 1983–1990 large changes in abundances of corals and macroalgae occurred on shallow (1–5m) lagoonal reefs in the San Blas Islands of Panama. In 1983 these reefs were dominated by the vertical plate forms of the coral generaAgaricia andMillepora. By 1990 we observed the following major changes: (1) loss of approximately one-half of the initial live coral cover, primarily during 1983–1986, and almost completely due to a decline in the abundance of Agaricia. Corals only occupied 12–26% of the reef area by 1990. (2) Macroalgae (mostlyDictyota andHalimeda) increased from 2% cover in 1983 to 28% cover in 1990. (3) Microalgal cover increased two to ten-fold between 1983 and 1986, then declined to 50% greater than the initial values by 1990. There are at least three contributors to these changes in the benthic community: (1) a coral bleaching event in 1983; which disproportionately affectedAgaricia; (2) the mass mortality ofDiadema antillarum in 1983, which led to decreases in grazing pressure on algae; and (3) possible increases in sediment and nutrient loads due to runoff from deforested mountainsides. Temporal patterns and observations of interactions suggest that the decrease inDiadema herbivory is a major factor in this shift in coral and algal populations.     

The relative impacts of grazing by caribbean coral reef fishes and Diadema: effects of habitat and surge

The effects of grazing on fleshy algal turfs by large herbivorous fishes and by the black spiny urchin, Diadema antillarum (Philippi), were investigated in three shallow (<3 m), moderately exposed, reef flat habitats in the San Blas Islands on the Caribbean coast of Panama. Grazing by Diadema was found to be a primary determinant of the biomass of fleshy algae in this shallow reef environment, and its impact was exposure-dependent. Feeding activity of Diadema was inhibited during the protracted periods of heavy wave action that characterize the dry season when strong north winds (24–27 kph) are typical. In the shallowest, most exposed elkhorn bluff habitats urchin grazing was minimal even during calm periods due to the effects of waves generated by unpredictable, light winds. In a relatively protected backreef habitat, water movement was inadequate to discourage grazing significantly, even during the turbulent dry season. As a result, algal biomass (decalcified dry weight) was typically three-fold greater in the elkhorn bluff habitat than on the ackreef. In habitats of intermediate exposure (elkhorn fields) algal biomass was reduced during calm periods, but increased to the level characteristic of the elkhorn bluff habitat during protracted rough periods that inhibited grazing by Diadema.A caging experiment in the backreef habitat demonstrated a relatively minor effect of grazing by herbivorous fishes in comparison to that of grazing by Diadema. Observations on the distributions and abundances of herbivorous fishes also indicated a relatively minor role for these grazers as determinants of algal biomass in the more exposed habitats.     

Herbivory on coral reefs: community structure following mass mortalities of sea urchins

The community structure of Jamaican coral reefs has undergone drastic change since mass mortalities of the long-spined black sea urchin Diadema antillarum Philippi occurred in 1983. In the absence of Diadema, algal abundance has increased enormously, up to a mean of 95% cover or 4.6 kg wet weight · m ⁻². Coral cover, which was already low on some reefs following Hurricane Allen in 1980, has been further reduced by as much as 60% since 1983 by competition with algae. Densities of D. antillarum at 10 sites in 1986 ranged from 0 to 12% of pre-1983 levels. Other echinoids, which might potentially compensate for the lack of herbivory from D. antillarum, have not increased significantly in density. Numbers of herbivorous scarids and acanthurids also remain at relatively low levels, because of overfishing. In the absence of high densities of fish and sea urchins, it is likely that recent changes in community structure will continue, resulting in further replacement of corals by algae in shallow water. The impact of the urchin mass mortalities is qualitatively similar to previous experimental removals of this species. In both cases, removal of echinoids resulted in substantial increases in macroalgae. However, quantitatively, the responses of algal and coral communities to the natural die-off were significantly greater, probably due to wide differences in spatial and temporal scales of the respective perturbations.     

Selective feeding by the echinoid,Evechinus chloroticus,and the removal of plants from subtidal algal stands in Northern New Zealand

Summary Feeding choice of the echinoid Evechinus chloroticus was examined for six fucoid and one laminarian species of algae. Three experiments were conducted to determine the algal choice by echinoids under controlled conditions. In the first experiment, the seven algal species were presented to echinoids in laboratory conditions. The second experiment had replicates of the algal species placed randomly on a subtidal rocky reef where echinoids were abundant and randomly dispersed. For the third experiment, which was also field-based, replicates of one highly-ranked species, Ecklonia radiata, were presented to naturally dispersed Evechinus. In addition, a series of controlled observations was used to examine the order in which echinoids removed algae from mixed species stands on subtidal boulders and to determine if this was related to the experimentally demonstrated choices of algal species.The results of the first two experiments showed that there were differences between algal species in the amount of material grazed by echinoids. Rankings of algal species from the field experiment were not correlated with rankings from the laboratory experiment. The order of removal of algal species from natural stands was correlated with the laboratory-based experimental rankings of algal species, but not with the rankings from the field-based experiment or with algal species availability. There were differences between algal species in their vulnerability to grazing by echinoids, as measured by regression analyses on the amount of material grazed from algal replicates vs. the number of attached echinoids. Within each species, echinoid numbers exerted a non-linear effect on the removal of algal material. In the third experiment, where only one species of algae was presented, the echinoids still distributed themselves non-randomly amongst replicates, aggregating on some samples.Data on the finer scale distribution of algal species over the entire subtidal reef on which these experiments and observations were conducted indicate that Evechinus are not often presented with a choice of adult plants of several different species in natural stands.The evidence from this study supports the conclusion that feeding preferences by echinoids are labile and do not clearly exert the major influence on the removal of plants from natural stands. Preference, as determined from experimental rankings of algal species, is only one of a number of factors which may affect the removal of algae by echinoids. Other important factors are the density of echinoids present, algal susceptibility to removal, and the distribution and abundances of the various algal species and echinoids relative to each other. It is suggested that algal life history characteristics may be unaffected by echinoids and that coevolutionary arguments are not appropriate for describing echinoid-algal interactions.     

Echinoid fauna from a Late Oligocene (Chattian) reef at Damon Mound,Brazoria County,Texas

The echinoid fauna collected from the Late Oligocene (Chattian) reef at Damon Mound, Brazoria County, Texas, includes at least seven species. The reef can be divided into four zones, in ascending order: reef drape, Porites thicket, reef core, and back-reef. The reef drape represents the deepest water and includes the echinoid species Clypeaster marinanus, Agassizia mossomi, and Lovenia alabamensis. The overlying zone consists of interbedded calcilutites and Porites coral thickets, and includes the species Echinometra prisca, C. marinanus, Clypeaster cf. oxybaphon, and Brissus exiguus. The reef core includes Prionocidaris cojimarensis and E. prisca. The shallow back-reef deposits include only C. marinanus. The Damon Mound fauna is more closely related to tropical Oligocene faunas of Mexico and the Antilles than to Gulf and Atlantic Coast faunas of the United States of the same latitude. During the Oligo-Miocene, Damon Mound and similar salt dome reef buildups in the northwestern Gulf of Mexico may have been havens for tropical species that could not have otherwise survived at that latitude.     

Transformation of algal turf by echinoids and scarid fishes on French Polynesian coral reefs

The respective roles of regular echinoids and scarid fishes in the transformation of turf algae, the main food resource for reef herbivores, were investigated on French Polynesian coral reefs. The role of one species of parrotfish (Scarus sordidus) was compared with that of four species of echinoids. The degree and ways of degradation of the algal matter were determined by the organic matter percentage, the composition of the sugar fraction, and the concentration and composition of chlorophylltype pigments as assayed by HPLC analysis. Chemical analyses were performed on anterior and posterior intestines for scarids, intestinal contents and faeces for echinoids, and on fresh algal turf as a control of initial food quality. A decrease in mean percentage of organic matter in gut content was observed from intestine (9.7%) to faeces (7%) in sea urchins, but not in parrotfishes. The total sugar fraction decreased from fresh algal turf (32% of total organic matter) to echinoid (28%) to scarid (18%) gut contents. The ratio of insoluble to soluble sugars (I/S ratios) was higher in echinoids (2.6) than in scarid gut contents (1.0). A decrease in the total pigment concentration was measured from fresh algal turf to echinoid and it was found to be even lower in scarid gut contents. Chromatograms showed that the composition of chlorophyll-type pigments in scarid intestines was very similar to fresh algal turf, with a dominance of native forms, mainly chlorophyll a and b. On the contrary, degraded pigment forms dominated in echinoids. The main degraded products were pheophorbides in sea urchins, and chlorophyllides in parrotfishes. These results provided evidence for differentiation in digestive processes occurring in the two types of grazers. Echinoids released higher degraded algal material than did scarids. Thus, these two types of grazers play different roles in the recycling of organic matter on coral reefs.     

The feeding activity of Echinostrephus molaris (de Blainville) in the central red sea

Echinostrephus molaris (de Blainville) is a small Indo‐pacific echinoid which burrows in coral reef limestone. Normally individuals do not leave the burrows so they cannot graze on algae growing around the burrow mouth. E. molaris catches floating algal particles with its long aboral spines. When a particle touches one of these organs or a tube‐foot in the area, the surrounding spines converge and grip it. Captured fragments are lowered to the test by further tube‐foot and spine action and are then passed across the ambitus towards the mouth. They are held by the oral tube‐feet and the shorter curved oral spines which aid ingestion. The behavioural and structural modifications shown for this habit are discussed. Burrowing and particle collecting have allowed E. molaris to occupy a particular niche on the reef. A similar method of food gathering is reported for Echinometra mathaei (de Blainville).     

Effects of competition and herbivory on interactions between a hard coral and a brown alga

Despite widespread acceptance of the negative effects of macroalgae on corals, very few studies have experimentally tested the competitive nature of the interaction, and most have ignored the potential effects of corals on algae. We report the effects of herbivory and competition on the growth of the branching scleractinian coral Porites cylindrica Dana and the creeping foliose brown alga Lobophora variegata (Lamouroux) Womersley, on an inshore fringing reef of the central Great Barrier Reef. L. variegata overgrows branches of P. cylindrica from the base up, forming a distinct boundary between the alga and the coral tissue. The experiment used exclusion cages to test for effects of herbivores, and removal of algae and coral tissue, at their interaction boundary, to test for inhibition of the competitors by each other. Comparisons of coral branches with the algae present or removed showed that the presence and overgrowth of the alga caused significant coral tissue mortality. Comparisons of branches with coral tissue unmanipulated or damaged showed that the coral inhibited the overgrowth by L. variegata, but that the algae were markedly superior competitors. Importantly, reduced herbivory resulted in faster algal growth and consequent overgrowth and mortality of coral tissue, demonstrating the critical importance of herbivory to the outcome of the competitive interaction.     

The influence of habitat characteristics and conspecifics on attraction and survival of coral reef fish juveniles

In marine species with a pelagic larval stage, search behavior and selection of a suitable reef habitat can maximize the settlement success of recently settled juveniles and their subsequent performance (growth and survival of juveniles). Our objective was to test this hypothesis for a single target coral reef fish species (Chromis viridis) at Moorea Island. C. viridis settle on living coral colonies of Porites rus already populated with conspecifics. In the present study (conducted in experimental cages), we found that: 1) mortality rate of recently settled juveniles of C. viridis was lower in the settlement habitat (living coral colonies of P. rus) than in other habitats having physical structure different from those of P. rus colonies; 2) C. viridis juveniles preferentially colonized coral heads of P. rus with conspecifics present rather than uninhabited coral heads and they also preferentially colonized uninhabited coral heads rather than coral heads with heterospecifics; 3) mortality rate of C. viridis juveniles did not vary with the presence or absence of conspecifics or heterospecifics on P. rus colonies. Overall, the study allows us to highlight that site selection by juveniles for habitat containing conspecifics does not benefit their short term mortality rates, suggesting that in the short term at least, site selection has little importance.     

Predation and the Control of the Sea Urchin Echinometra viridisand Fleshy Algae in the Patch Reefs of Glovers Reef, Belize

The massive reduction in sea urchin Diadema antillarum populations since the mid-1980s has been associated with large increases in the abundance of fleshy algae on many Caribbean reefs despite the availability of other sea urchin and finfish grazers. This study examined the ecology of a grazer living sympatrically with D. antillarum, the common and abundant sea urchin Echinometra viridis. I examined the role that finfish and invertebrate predators play in controlling the distribution of E. viridis as well as the ability of this sea urchin to control exposed fleshy algae on the patch reefs of the Glovers Reef Atoll lagoon. I found that the major predators of this sea urchin were Calamus bajonado (jolthead porgy), Balistes vetula and Canthidermis sufflamen (queen and ocean triggerfish), Lachnolaimus maximus (hogfish), and a gastropod, probably Cassis madagascariensis. The abundance of E. viridis is constrained by predation, which restricts E. viridis to cryptic locations, such as crevices. Sea urchins bit a smaller percentage of experimental algal assays than finfish. Finfish herbivory was associated positively with patch reef topographic complexity. Unexpectedly, E. viridis abundance was positively correlated with fleshy algal abundance, but negatively correlated with the frequency of finfish bites. Predators restrict E. viridis to crevices and therefore reduce their influence on exposed fleshy algae, even at moderately high population densities (up to 10 per square meter). Since net benthic primary production of coral reefs is most strongly associated with herbivory on exposed surfaces, it would appear that E. viridis is unable to maintain the same production as reefs dominated by D. antillarum. Received 5 November 1998; accepted 2 June 1999.     

Evaluation of indicators of nitrogen limitation in deep prairie lakes with laboratory bioassays and limnocorrals

A comparison of Kenyan reefs of different historical and observed levels of fishing exploitation showed that more exploited reef lagoons had greater sea urchin densities and sizes, fewer and smaller fish and less coral cover. In the most exploited lagoon the biomass of the burrowing sea urchin Echinometra mathaei increased five fold during the previous 15 years. An ecological study of the three most common omnivorous sea urchin species inhabiting hard substrate within these reef lagoons (E. mathaei, Diadema savignyi and D. setosum) suggests that they are ecologically separated by predation and avoid predators and competitors by occupying different size burrows or crevices within the lagoon. Predator removal through fishing activities may result in ecological release of the sea urchins and result in competitive exclusion of weaker competitors. The most exploited reef had a nearly monospecific barren of E. mathaei living outside burrows suggesting that E. mathaei may be the top competitor. Its ecological release appears to lead to a decrease in live coral cover, increased substrate bioerosion and eventually a loss of topographic complexity, species diversity, fish biomass and utilizable fisheries productivity. Data from the outer reef edge were more difficult to interpret but may indicate similar patterns. Within this area, physical stresses such as waves and currents may be a greater controlling force in regulating fishing activities and coral reef community structure.