Potential reversal of a phase shift: the rapid decrease in the cover of the invasive green macroalga <Emphasis Type="Italic">Dictyosphaeria cavernosa</Emphasis> Forsskål on coral reefs in Kāne‘ohe Bay,Oahu, Hawai‘i

The native green macroalga Dictyosphaeria cavernosa dominated most of the reef slope habitat in Kāne‘ohe Bay, Hawai‘i for 40 years prior to 2006 and had displaced corals from the habitats they created. This has been one of the most oft-cited examples of a phase shift occurring on a coral reef. After decades of relatively constant, high abundance of the alga, percent cover declined dramatically throughout the bay between February and June 2006. The sudden decrease in cover of this alga appears to be the result of an unusually protracted cloudy, rainy period in March 2006, which may have reduced irradiance and caused the alga to lose weight. Corals and red macroalgae living at the same depths and in some of the same habitats were apparently not affected by this 42-day period of rain and overcast skies. Competition between corals and D. cavernosa for space on reef slopes has been virtually eliminated by the death of this alga, but the unstable rubble formations, which remain in much of the area formerly covered by D. cavernosa may not be conducive to rapid increase in cover by the remaining corals or to establishment by coral recruits. Two years later, there was still no recovery of D. cavernosa. This represents a rare example of decline in macroalgal dominance on a reef and a partial reversal, possibly only temporary, of a phase shift.     

Newly dominant benthic invertebrates reshape competitive networks on contemporary Caribbean reefs

Competition is a fundamental process structuring ecological communities. On coral reefs, space is a highly contested resource and the outcomes of spatial competition can dictate community composition. In the Caribbean, reefs are increasingly dominated by non-scleractinian species like sponges, gorgonians, and zoanthids, yet there is a paucity of data on interactions between these increasingly common organisms and historically dominant corals. Here, we investigated interactions among these groups of sessile benthic invertebrates to better understand the role of spatial competition in shaping benthic communities on Caribbean reefs. We coupled surveys of competitive interactions on the reef with a common garden competition experiment to determine the frequency and outcome of interference competition among eight focal species. We found that competitive interactions were pervasive on Florida reefs, with 60% of sessile benthic invertebrates interacting with at least one other invertebrate. Increasingly common non-scleractinian species were some of the most abundant taxa and consistently outcompeted the contemporarily common scleractinian species Porites porites and Siderastrea siderea. The encrusting gorgonian, Erythropodium caribaeorum, was the most aggressive species, reducing the live area of its competitors on average 42% ± 7.04 (SE) over the course of 5 months. Surprisingly, the most aggressive species declined in size when competing, while some less aggressive species were able to increase or maintain area, suggesting a trade-off between aggressiveness and growth. Our findings suggest that competition among sessile invertebrates is likely to remain an important process in structuring coral reefs, but that the optimal strategies for maintaining space on the benthos may change. Importantly, many non-scleractinian species that now dominate reefs appear to be superior competitors, potentially increasing the stress on corals on contemporary reefs.

     

Impacts of a population outbreak of the urchin Tripneustes gratilla amongst Lord Howe Island coral communities

Subtidal reef surveys within the Lord Howe Island Marine Park revealed that populations of the sea urchin Tripneustes gratilla underwent an explosive outbreak in some regions of the park over a 2-year period. This urchin was rare or absent during 2006 surveys at 33 sites studied, but at sites off northern Lord Howe Island in 2008, densities averaged >1.3 m⁻². Dramatic increases in T. gratilla density (exceeding 4 m⁻²) were observed at some sites. We quantify community-level impacts of T. gratilla using ‘before-after’ and ‘control-impact’ data. Zones closed to fishing exhibited similar increases in T. gratilla density to zones open to fishing. Although not previously reported as a keystone species affecting coral habitat, T. gratilla was found to possess an ‘ecosystem engineer’ function. Outbreak sites were characterised by significant declines in cover of foliose algae, including red algae, which decreased from 11.2% in 2006 compared to 2.5% in 2008. Brown foliose algae also declined at sites where T. gratilla outbreaks occurred, averaging 20.4% in 2006 compared to 1.8% in 2008. By contrast, crustose coralline algal cover increased at sites where high T. gratilla densities were observed, from 2.7% in 2006 to an average of 42.6% in 2008. We found no clear indication of impacts on sessile invertebrates or flow-on effects to other levels of the food web, with no significant change in coral cover or densities of mobile invertebrates or fish populations associated with the T. gratilla outbreak.     

The Behavioural Response of Mayfly Nymphs (Stenonema sp.) to Chemical Cues from Crayfish (Orconectes rusticus)

Artificial reef systems play an important role in the increase of natural production of biological marine resources and they have been deployed worldwide. In Portugal, seven artificial systems have been deployed along the southern coast of the Algarve. Research to date has focussed mainly on fish populations, particularly those of economical importance. The present work aims to study the macrobenthic communities of the artificial reef structures, as these will enhance the food resources and shelter, making the reefs more attractive to fish. In particular, we experimentally analysed the sequence of colonisation of macrobenthic communities of the Ancão artificial reef system, which was deployed in the summer of 2002. The study of the colonisation of benthic communities was done using samples of concrete cubic units (15 × 15 cm) that were suspended at the reef modules at a depth of 20 m, at the time of the reef immersion. Four replicate samples were collected by SCUBA diving from two groups of the Ancão reef every three months from the starting date. Sampling was done using essentially non-destructive methods to assess the percentage cover of macrobenthic organisms in both vertical and horizontal surfaces. The percentage cover of the taxonomic groups was compared within the different surfaces of the samples and between the two reef groups. The bottom surface of cubic samples had a significantly higher colonisation related to the dominance of barnacle cover, probably due to lower sedimentation levels. Samples from both reef groups showed a similar pattern of colonisation. Barnacles, bryozoans and serpulids dominated the samples three months immediately after the beginning of the experiment. Other invertebrates groups, such as Porifera, Hydrozoa, Anthozoa, other sessile Polychaeta, Decapoda, Gastropoda and Bivalvia, were more abundant after six months of colonisation.     

Experimental separation of effects of consumers on sessile prey in the low zone of a rocky shore in the Bay of Panama: direct and indirect consequences of food web complexity

The effects of predation by a diverse assemblage of consumers on community structure of sessile prey was evaluated in the low rocky intertidal zone at Taboguilla Island in the Bay of Panama. Four functional groups of consumers were defined: (1) large fishes, (2) small fishes and crabs, (3) herbivorous molluscs, and (4) predaceous gastropods, (l) and (2) included fast-moving consumers and (3) and (4) included slow-moving consumers. Experimental treatments were: no consumers deleted (all groups present), most combinations of deletions of single groups (i.e., one group absent, three present), pairs of groups deleted (two absent, two present), trios of groups deleted (three absent, one present), and the entire consumer assemblage deleted (all groups absent). Changes in abundance (percent cover) of crustose algae, solitary sessile invertebrates, foliose algae, and colonial sessile invertebrates were quantified periodically in 2–4 plots of each treatment from February 1977 to January 1980 after the initiation of the experiment in January 1977.

Space on this shore is normally dominated by crustose algae; foliose algae, solitary sessile invertebrates, and colonial sessile invertebrates are all rare. After deletion of all consumers, ephemeral green algae increased from 0 to nearly 70% cover. Thereafter, a succession of spatial dominants occurred, with peak abundances as follows: the foliose coralline alga Jania spp. by July 1977, the barnacle Balanus inexpectatus by April 1978, and the rock oyster Chama echinata by January 1980. Although no longer occupying primary rock space, Jania persisted as a dominant or co-dominant turf species (with the brown alga Giffordia mitchelliae and/or the hydrozoan Abietinaria sp.) by colonizing shells of sessile animals as they became abundant instead of the rock surface.

Multivariate analysis variance (MANOVA) indicated that the effect of each group was as follows. Molluscan herbivores grazed foliose algae down to the grazer-resistant, but competitively inferior algal crusts, altered the relative abundances of the crusts, and inhibited recruitment of sessile invertebrates. Predaceous gastropods reduced the abundance of solitary sessile animals. Small fishes and crabs, and large fishes reduced the cover of solitary and colonial sessile animals and foliose algae, although they were incapable of grazing the foliose algae down to the rock surface. Many of the effects of each consumer group on prey groups or species were indirect; some effects were positive and some were negative. The variety of these indirect effects was due to both consumer-prey interactions among the consumers, and competitive or commensalistic interactions among the sessile prey. Comparison of the sum of the effects of each of the single consumer groups (i.e., the sum of the effect observed in treatments with one group absent, three present) with the total effects of all consumers (i.e., the effect observed in the treatment with all groups absent) indicates that a “keystone” consumer was not present in this community. Rather, the impacts of the consumer groups were similar but, due to dietary overlap and compensatory changes among the consumers, not readily detected in deletions of single consumer groups. The normally observed dominance of space by crustose algae is thus maintained by persistent, intense predation by a diverse assemblage of consumers on potentially dominant sessile animals and foliose algae. The large difference in structure between this and temperate intertidal communities seems due to differences in degree, not kind of ecological processes which produce the structure.     

Fine-scale time series surveys reveal new insights into spatio-temporal trends in coral cover (2002–2018), of a coral reef on the Southern Great Barrier Reef

Reef monitoring programmes often focus on limited sites, predominantly on reef slope areas, which do not capture compositional variability across zones. This study assessed spatial and temporal changes in hard coral cover at four hierarchical spatial scales. ~ 55,000, geo-referenced photoquadrats were collected annually from 2002 to 2018 and analysed using artificial intelligence for 31 sites across reef flat and reef slope zones on Heron Reef, Southern Great Barrier Reef, Australia. Trends in hard coral cover were examined at three spatial scales: (1) “reef scale”, all data; (2) “geomorphic zone scale”—north/south reef slope, inner/outer reef flat; and (3) “site scale”—31 sites. Coral cover trajectories were also examined at: (4) “sub-site scale”—sub-division of sites into 567 sub-sites, to estimate variability in coral cover trajectories via spatial statistical modelling. At reef scale coral cover increased over time to 25.6 ± 0.4 SE % in 2018 but did not recover following disturbances caused by disease (2004–2008), cyclonic conditions (2009) or severe storms (2015) to the observed pre-disturbance level (44.0 ± 0.7 SE %) seen in 2004. At geomorphic zone scale, the reef slope had significantly higher coral cover than the reef flat. Trends of decline and increase were visible in the slope zones, and the southern slope recovered to pre-decline levels. Variable coral cover trends were visible at site scale. Furthermore, sub-site spatial modelling captured eight years of coral recovery that occurred at different times and magnitudes across the four geomorphic zones, effectively estimating variability in the trajectory of the reef’s coral community. Derived spatial predictions for the entire reef show patchy coral recovery, particularly on the southern slope, and that recovery hotspots are distributed across the reef. These findings suggest that to fully understand and interpret coral decline or recovery on a reef, more accurate assessment can be achieved by examining sites distributed within different geomorphic zones to capture variation in exposure, depth and consolidation.

     

Near-shore bathymetry and reef biotopes of Henderson Island, Pitcairn Group

The coral reef which surrounds two thirds of Henderson Island, Pitcairn Group, consists of a reef flat, reef margin and fore-reef which slopes gradually into deeper water. The range of sublittoral habitats provided by this topography is limited, reflecting a low level of coral diversity. Spur and groove formations of the fore-reef are present around the northern end of the island which contrasts with the south-western corner, where, in the absence of a fringing reef platform, subaerial cliff faces are fronted by a cliff foot submarine 'trench'. In general, live coral cover on the fore-reef was estimated at between 10–30%, though at sites off the east coast it reached 80%. The associated communities, particularly of sessile filter-feeding groups, were found to be impoverished. The two factors of biogeographical isolation and of local bioerosion processes are proposed as being the main reasons for the depauperate nature of the island's reef. Brief notes are included on the lagoon and fore-reef bathymetry and habitats of Ducie and Oeno Atolls for comparative purposes.     

Exposure to elevated carbon dioxide does not impair short‐term swimming behaviour or shelter‐seeking in a predatory coral‐reef fish

Adult bluespotted rockcod Cephalopholis cyanostigma, a coral‐reef grouper, were acclimated to either ambient (mean ± s.d. 406 ± 21 μatm; 1 atmos = 101325 Pa) or high pCO₂ (945 ± 116 μatm) conditions in a laboratory for 8–9 days, then released at the water surface directly above a reef (depth c. 5 m) and followed on video camera (for 191 ± 21 s) by scuba divers until they sought cover in the reef. No differences were detected between groups in any of the six measured variables, which included the time fish spent immobile after release, tail beat frequency during swimming and the time required to locate and enter the protective shelter of the reef.     

Microorganisms uniquely capture and predict stony coral tissue loss disease and hurricane disturbance impacts on US Virgin Island reefs

Coral reef ecosystems are now commonly affected by major climate and disease disturbances. Disturbance impacts are typically recorded using reef benthic cover, but this may be less reflective of other ecosystem processes. To explore the potential for reef water-based disturbance indicators, we conducted a 7-year time series on US Virgin Island reefs where we examined benthic cover and reef water nutrients and microorganisms from 2016 to 2022, which included two major disturbances: hurricanes Irma and Maria in 2017 and the stony coral tissue loss disease outbreak starting in 2020. The disease outbreak coincided with the largest changes in the benthic habitat, with increases in the percent cover of turf algae and Ramicrusta, an invasive alga. While sampling timepoint contributed most to changes in reef water nutrient composition and microbial community beta diversity, both disturbances led to increases in ammonium concentration, a mechanism likely contributing to observed microbial community shifts. We identified 10 microbial taxa that were sensitive and predictive of increasing ammonium concentration. This included the decline of the oligotrophic and photoautotrophic Prochlorococcus and the enrichment of heterotrophic taxa. As disturbances impact reefs, the changing nutrient and microbial regimes may foster a type of microbialization, a process that hastens reef degradation.     

The distribution of dimethylsulfoniopropionate in tropical Pacific coral reef invertebrates

Dimethylsulfoniopropionate (DMSP) is an important component of the global sulfur cycle and may be involved, via its cleavage product dimethylsulfide, in climate regulation. Although it is common in many algae, reports of DMSP in animals, particularly tropical invertebrates, are limited. This study examined the distribution of DMSP in a diverse group of coral reef invertebrates. DMSP was present in all 22 species of cnidarians and ranged from 9 to 723 μmol g⁻¹ of dry mass (DM) with a mean (± 1SD) of 110 ± 180 μmol g⁻¹ DM. It was not detected in a flatworm and an ascidian or in two of five sponges. Concentrations in sponges ranged from undetectable to 16 μmol g⁻¹ DM with a mean of 4 ± 7 μmol g⁻¹ DM. Within the cnidarians, DMSP concentrations did not differ among orders. Among cnidarian species, DMSP concentrations were correlated with symbiotic zooxanthellae densities. Within cnidarian species, DMSP concentrations of individuals were positively correlated with zooxanthellae densities in three of the four species examined. We speculate that DMSP is dietarily derived in sponges and derived from zooxanthellae in the cnidarians. The functions of DMSP in coral reef invertebrates are not known.     

Distribution and factors influencing on structure of reef fish communities in Nha Trang Bay Marine Protected Area, South-Central Vietnam

Visual censuses of coral reef fishes in Nha Trang Bay Marine Protected Area (MPA) were conducted during September–October 2005. Nha Trang Bay MPA is relatively rich in reef fishes compared to other areas in Vietnam and the Pacific Ocean outside the ‘Coral Triangle,’ consistent with its biogeographic location in the western South China Sea. A total of 266 species of 40 families of coral reef fishes formed five distinct assemblages. Spatial variations in distribution and structure of the assemblages were associated with eight significant biological and physical variables which were cover of living hard corals, encrusting corals, branching corals, Acropora, Millepora, Montipora, depth and distance from the coast of the mainland. The six factors in front are likely related to provision of shelter and nutrition, while the distance factor is likely to represent a gradient in disturbance and impacts from various mainland sources including sedimentation and pollution discharge from nearby rivers. Local species richness ranged from 35 to 70 species 500 m⁻² (mean: 51 ± 2 SE) for reef flat stations and from 23 to 68 species 500 m⁻² (mean: 48 ± 4 SE) for reef slope stations. Total species richness at each site averaged 76 species (±4 SE), ranging from 56 to 110 species, dominated by wrasses, damselfishes, butterflyfishes, parrotfishes, surgeonfishes, groupers and goatfishes. Density of total fishes at each station ranged from 348 to 1,444 individuals 500 m⁻² (mean: 722 ± 302 SE) for the reef flat stations and from 252 to 929 individuals 500 m⁻² (mean: 536 ± 215.7 SE) for the reef slope stations. Overall mean density at each site averaged 628.9 (±238.4 SE) individuals 500 m⁻². The highly protected sites supported higher mean density of fishes per site (ranged: 904.5–1,213 individuals 500 m⁻² for Hon Mun and 1,167.5 individuals 500 m⁻² for Hon Cau) compared to other sites (<800 individuals 500 m⁻²). Of the families included in the census, densities were dominated throughout the MPA by damselfishes and wrasses. Many target species, particularly groupers, snappers and emperors, were rare or absent and the low abundance of big fishes was consistent with over-harvesting. Similarly a low density of butterfly fishes and angelfishes is likely related to the supply for marine aquaria in Vietnam and overseas. This study provides an important baseline against which the success of present and future MPA management initiatives may be assessed.     

Spatial variation of reef fishes and the relative influence of biotic and abiotic habitat traits

Patterns of distribution of reef fishes were examined across three spatial scales and related to habitat traits along 25 km of the northern Portuguese coast. Response variables included the multivariate assemblage structure, the total number of taxa and individuals, and the abundance of single groups categorized according to their preference for the benthic, proximo-benthic or pelagic environment, feeding and reproductive behaviour. Habitat traits included topographic elements (small and large ‘drops’ like cracks and crevices) and the extent of dominant morpho-functional types of macroalgae (kelp, large foliose, small erect, turf-forming filamentous, and encrusting). All fish responses were characterized by the largest variance at the smallest scale (among transects tens m apart), followed by that among reefs (hundreds m to 1 km apart) and almost null variance among sites (some km apart). Small and large ‘drops’ of the substratum explained, respectively, considerable variation of assemblage structure and the total abundance of individuals, while the extent of bare rock influenced the richness of taxa and that of benthic fishes, fishes feeding on sessile invertebrates and fishes laying benthic eggs or having nesting behaviour. Combinations of abiotic and biotic structural attributes of reefs influenced proximo-benthic fishes, the predators of mobile animals and fishes releasing pelagic eggs. The here reported associations between patterns of distribution of reef fishes and habitat traits have implications for the design of future protection schemes suitable to guarantee the conservation of reef fish communities and of the processes responsible for their variation. Within the SLOSS (single-large vs. several-small) debate in the design of marine reserves, for example, effective protection to the studied reef fishes would be provided by a set of small reserves, rather than a single large which might be appropriate for fishes having wider home ranges.     

What do the empty stomachs of northern pike (<Emphasis Type="Italic">Esox lucius</Emphasis>) reveal? Insights from carbon (δ<Superscript>13</Superscript>C) and nitrogen (δ<Superscript>15</Superscript>N) stable isotopes

The frequency of individuals with empty stomachs (FES) can vary greatly among northern pike populations. However, the FES has only seldom been analyzed in this species and its meaning is still not fully understood. It has been suggested that a high FES may reflect a strongly piscivorous behaviour while low FES could reflect a higher utilization of invertebrates. We compared the stomach contents and the trophic position of northern pike in 16 populations of individuals feeding mainly on fish or benthic invertebrates. We tested the hypothesis that northern pike with empty stomachs or with fish in their stomachs have a higher trophic position than individuals feeding on invertebrates. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope signatures were used to estimate the trophic position of individuals. We found no significant difference in the trophic position among piscivores, invertebrate feeders, and northern pike with empty stomachs. The average trophic position of northern pike was high (mean ± SD = 4.3 ± 0.4, n = 66) and was correlated with total length. These results indicate that, although invertebrates could be an important part of the diet of northern pike in Canadian Shield lakes, fish are still the dominant prey. Hence, feeding on invertebrates in our study lakes would reflect an opportunistic rather than a specialized feeding strategy.     

Changes in the macrozoobenthos of the intertidal zone at Helgoland (German Bight, North Sea): a survey of 1984 repeated in 2002

Changes in the presence and absence of invertebrates as well as in species conspicuousness were documented in a rocky intertidal community based on surveys in 1984 and 2002. In 2002 six vertically and/or morphologically different stations of an intertidal platform were sampled. Five of these six habitats had already been surveyed in 1984. Replicating precisely the method of the first assessment, presence/absence changes as well as changes in species conspicuousness of 83 invertebrate species were documented, indicating that this intertidal community changed considerably during the 18-year interval. Compared with the study in 1984, 27 species newly appeared, whereas 32 species disappeared. Furthermore, 16 species increased in conspicuousness, whereas eight invertebrates decreased. The total number of species in 2002 was 154 versus 158 in 1984. Although algal species were not recorded as thoroughly as invertebrates, a massive decline in cover of Halidrys siliquosa was noted. Conversely, two invasive algal species became established after 1984, Sargassum muticum (since 1988), a cosmopolitan fucoid alga that prefers shallow subtidal areas for colonization, and Mastocarpus stellatus (introduction in the 1980s) that particularly colonized areas in the mid intertidal. In 1984 the mid intertidal zone was dominated by the brown alga Fucus serratus, whereas in 2002 the blue mussel Mytilus edulis and the periwinkle Littorina littorea were the most conspicuous organisms. Annual mean sea surface temperature (BAH measurements) warmed by 1.1°C over the past four decades. Range-related community shifts, introductions of non-indigenous species and the input of pollutants, are considered to explain long-term ecological changes in the invertebrate community at Helgoland.     

9. LIST OF MEMBERS (as at date of publication)

We studied the breeding ecology of the Chestnut-backed Sparrowlark Eremopterix leucotis over three years between 2008 and 2010. The breeding season was bimodal with a main peak in laying in autumn (March–April) and another smaller peak in spring (September–October). Nest microhabitat analyses showed they prefer nesting in open areas with lots of bare ground (median 67.5%). Nest entrance directions were biased towards the south (mean vector (µ) = 186.44°). The majority of nests (78.2%) had an apron at the nest entrance. The mean clutch size was 1.88 but there was geographic variation in clutch size between northern and southern races of the species. The mean incubation and nestling periods were 10.33 d (range 10–11 d) and 9.20 d (range 8–10 d), respectively. The results suggest that parental contributions during incubation are almost equal, but females made significantly more food deliveries during the nestling period compared to males. The diet of nestlings comprised mainly of invertebrates (50.2%), seeds (34.4%) and unidentified food items (15.4%). Breeding success was low, averaging 16.1% (range 8.1–20.6%), and the average number of fledged young per pair was 0.36 ± 0.71. Replacement broods were common and we also recorded repeat brooding attempts.     

Effects of herbivory, nutrient levels, and introduced algae on the distribution and abundance of the invasive macroalga Dictyosphaeria cavernosa in Kaneohe Bay, Hawaii

Since the 1960s, and possibly earlier, the macroalga Dictyosphaeria cavernosa has overgrown and displaced corals on reef slopes and outer reef flats in Kaneohe Bay, Oahu. This shift in reef community composition is generally attributed to nutrient enrichment resulting from sewage discharge. Following the diversion of most of the sewage effluent in 1977-1978, it was expected that D. cavernosa growth would become nutrient-limited and its abundance would consequently decline, but the alga remains abundant in much of the bay. One explanation for its persistence is that nutrients are once again high enough to support the alga's growth. An alternative explanation is that there has been a reduction in grazing intensity in the bay. In this study we resurveyed the distribution and abundance of D. cavernosa at 120 reef slope sites originally surveyed in 1969. We conducted additional surveys to estimate the biomass of herbivores and the areal coverage of D. cavernosa and other macroalgae on reef slopes and flats. Field experiments were used to determine spatial and temporal patterns of grazing intensity on and growth rates of D. cavernosa and the introduced macroalga Acanthophora spicifera. Laboratory experiments were used to examine preferences among herbivores for some of the most abundant macroalgae on Kaneohe Bay reefs. Twenty years after sewage diversion, D. cavernosa cover on reef slopes has decreased substantially in southern Kaneohe Bay, the site of most of the historical sewage discharge. D. cavernosa cover has changed less in other regions, remaining high in the central bay and low in the north bay. D. cavernosa thalli protected by grazer exclusion cages sustained positive growth rates on reef slopes and flats throughout the bay. Reduced nutrient concentrations may have caused a reduction in D. cavernosa growth rates, and a consequent reduction in D. cavernosa abundance in the south bay shortly after sewage diversion. Measurements of grazing intensity and surveys of herbivorous fish abundance suggest that the continued abundance of D. cavernosa is the result of a reduction in grazing intensity. Reduced grazing intensity on D. cavernosa may in turn be the result of a historical reduction in herbivore biomass or the establishment of several introduced macroalgae on reef flats. The introduced species are preferred by herbivorous fishes over D. cavernosa, as indicated by preference tests. The hypothesis that reduced grazing pressure on D. cavernosa is related to the establishment of introduced species is supported by the observation that D. cavernosa cover is highest on reef slopes where the cover of preferred introduced macroalgae on the adjacent outer reef flat is also high. Conversely, D. cavernosa cover is low or zero on reef slopes where the cover of introduced macroalgae on the adjacent reef flat is low or zero     

Nitrate Uptake by Reef Corals

In specimens of the hermatypic coral species Fungia scutaria and Montipora verrucosa and in the alga Ulva lactuca, nitrate uptake was measured in light and dark with a flow-through apparatus. The nitrate uptake was measurable in high-nitrate bay water of Kaneohe Bay and also in low-nitrate open ocean water. Nitrate consumption rates by the corals and the alga did not differ from light to dark. Neither the coral nor the alga showed measurable immediate nitrate uptake in open ocean water of low nitrate concentration when they had been held previously in the high-nitrate bay water. In low-nitrate open ocean water the uptake per unit time increases when the flow of the water increases. The uptake of nitrate by reef corals even from low concentrations indicates nonspecific nutrient sources for reef corals.     

Effects of accumulations of sediments and drift algae on recruitment of sessile organisms associated with oyster reefs

Increases in sediment and drift algae accumulations have caused degradation of coastal lagoons worldwide. It is well known that these factors are stressors of seagrass beds, sediment fauna and coral reefs. However, little is known about the impacts on temperate hard-bottom assemblages within soft-bottom lagoons. To test if accumulations of sediment and drift algae (stress) affected recruitment of sessile oyster reef organisms, we constructed cages in Hog Island Bay, Virginia that trapped drifting macroalgae (≈ 2.7 kg WW m²) and facilitated sedimentation (≈ 7 mm per 2-3 month). The stress treatments and unstressed controls were placed in front, between, and behind reefs (position) to represent wave exposed (≈ 0.3 m, windy conditions), current exposed (≈ 0.2 m s^− 1, peak tide) and protected (≈ 0 m, 0.0 m s^− 1) habitats. The percentage cover of recruited taxa onto bricks was mapped 4 times during a 1-year period. There were strong significant effects of stress on the total assemblage, plant (but not animal) richness, total plant and animal cover, and cover of the most common taxa. Unstressed bricks had high plant richness, high animal and plant cover, and high cover of the oyster Crassostrea virginica, the alien algae Gracilaria vermiculophylla and Codium fragile, the alga Agardhiella subulata, and high to medium cover of the opportunistic algae Ulva curvata and Enteromorpha spp. In comparison, sediment-stressed bricks had low plant richness, low animal and plant cover, and low cover of C. virginica, G. vermiculophylla, C. fragile, A. subulata, U. curvata and Enteromorpha spp. Similarly, algae-stressed bricks also had low cover of animals, C. virginica, G. vermiculophylla, C. fragile, and A. subulata, but intermediate plant richness and plant cover and high cover of U. curvata and Enteromorpha spp. Although reef position caused significant multivariate results, this factor was clearly less important than the stress factor. Our study shows that accumulations of sediments and drift algae have an adverse impact on sessile temperate reef organisms, reducing richness and abundance, but favoring a few small opportunistic taxa. As the reef-generating oysters themselves performed poorly under these stressors, the long-term impact of the causes of these stressors, eutrophication and urbanization, is likely to be diminished reefs with cascading adverse effects on sessile reef organisms.     

Community Structure of Fishes and Habitat Complexity on a Tropical Rocky Shore

The relationship between the variables of reef fish community structure (fish richness, fish diversity and total number of fishes) and those of habitat complexity (total surface area, substratum diversity, topographic complexity, number of holes, percent cover of turf algae, zoanthids, millepores, massive corals, bare rock, encrusting calcareous algae, urchins, other sessile organisms and Sargassum) were examined on three different rocky shores (FA, FB and FT) at Arraial do Cabo, a tropical region located on Brazilian southeastern coast (23° S, 42° W). Fish abundance and vertical distribution were assessed by a visual census technique using strip transects. Percentage cover of benthic organisms and other substratum types were calculated by replicated transects using a chain laid down on the substratum. Topographic complexity was determined by the chain link method and number of holes were estimated by direct counts on replicated transects. More than 91 fishes belonging to 37 families were seen in all study sites during one year of visual census effort. FA and FB sites had similar fish community structure and habitat complexity characteristics, while FT showed different habitat characteristics and higher fish diversity and richness, plus total number of fishes. Vertical distribution of fishes along the rocky shores studied seemed to be predictable and was determined by factors such as feeding habits and behavior, availability of refuges and social interactions. The habitat variables that best explained the higher diversity and number of fishes observed in FT site were total surface area of rocky shores and the abundance of benthic sessile invertebrates; these conditions were typical of rocky shores more exposed to wave surge.     

Fish Colonization of an Artificial Reef in the Gulf of Elat, Northern Red Sea

A small near shore artificial reef was constructed in the Gulf of Elat, northern Red Sea at a depth of 22–24m. The colonization of fishes was monitored for a period of 728 days and a total of 94 species was recorded. Colonization was initially rapid. The first species to appear were Dascyllus trimaculatus and Chaetodon paucifasciatus (day 2). In the first seven months, a gradual increase in the number of species was observed, after which it leveled off. Subsequently, a reduction in the number of individuals increased diversity of the community, as measured by the Shannon & Weaver index. The low complexity of the major components of the artificial reef, in addition to its location on a muddy, silty substrate, resulted in a constant cover of fine grain particles which presumably discouraged settlement of invertebrates and small cryptic fish species on the artificial reef.