Macroalgal Removal Research Articles

Sea‐weeding: Manual removal of macroalgae facilitates rapid coral recovery

Abstract Coral reef ecosystems globally are under threat, leading to declining coral cover and macroalgal proliferation. Manually removing macroalgae (i.e. ‘sea‐weeding’) may promote local‐scale coral recovery by reducing a biological barrier, though the impact of removal on community composition of benthic reef organisms has not been quantified. In this three‐year study (2018–2021), fleshy macroalgae (predominantly Sargassum spp.) were periodically removed from 25 m2 experimental plots on two inshore fringing reefs of Yunbenun (Magnetic Island) in the central Great Barrier Reef. By the end of the study, coral cover in removal plots (n = 12 plots) assessed through in‐field transects increased by at least 47% (2019 mean: 25.5%, 2021 mean: 37.4%), and macroalgal cover decreased by more than half. In contrast, in control plots (n = 12 plots), there was no change in macroalgal cover while coral cover remained stable (2019 mean: 16.4%, 2021 mean: 13.6%). Changes in benthic cover were supported by photoquadrat data, with Bayesian probability modelling indicating a 100% likelihood that coral cover more than doubled in removal plots over the study period, compared to only a 29% chance of increased coral cover in control plots. Synthesis and applications. Manual macroalgal removal can provide rapid benefits and enhance inshore coral reef recovery. Through involvement of community groups and citizen scientists, larger scale removal of macroalgae is a low‐tech, high‐impact, and achievable method for local reef management.

Comparative image analysis approaches to assess ecological effects of macroalgal removal on inshore reefs of Magnetic Island, Australia

Macroalgae removal is a proposed management option in the GBR to reverse declines in inshore coral reef health. Automated image analysis (AIA) is a valuable tool to assess benthic community assemblages. This study compared the accuracy of benthic community assemblages assessed through the AIA program CoralNet to manual image analysis. The ecological effect of macroalgae removal on benthic community composition was also investigated on established permanent quadrats (5x5 m) for reefs at Florence and Arthur Bay, Magnetic Island. Control and treatment quadrats (n=3 respectively) were photographed before and after macroalgae removal over 6 months. The results obtained by AIA and manual approaches were consistent, with macroalgae cover is approximately 77%-87% in all quadrats before macroalgal removal. Through the monitoring period, a small increase in coral cover in the macroalgal removal quadrats was observed in Florence and Arthur Bay (an increase of 1.8% and 0.1%, respectively). CoralNet was demonstrated to be robust for assessing reef benthic cover with no significant difference in recorded benthic categories when compared to the manual approach. CoralNet was accurate for identifying broad benthic categories, but less effective than manual image analyses for lower taxonomic categories (i.e., genus or species level).

Recovery patterns of the coral microbiome after relief of algal contact

Interactions between macroalgae and corals are omnipresent on eutrophied and overfished reefs worldwide. Contact with macroalgae can disrupt corals and their microbiomes through diverse mechanisms, including shading, abrasion, and the release of algal exudates. However, changes in the coral microbiome after algal contact ceases have not been studied. We investigated the recovery of the microbiome of massive reef-building Porites corals following experimental removal of the overgrowing green macroalga Halimeda macrophysa. We followed changes in the microbiome of macroalgal-removed and adjacent healthy-looking tissue of coral colonies over 40 days. Coral tissue was predominantly bleached underneath the macroalgae but regained almost its full pigmentation by day 40. Despite this recovery in pigmentation, the bacterial microbiome of macroalgal-removed coral tissue did not return to that of adjacent healthy-looking tissue (control). Overall, macroalgal contact led to the suppression of Gammaproteobacteria and increased diversity and dominance of Alphaproteobacteria, a shift that persevered for 40 days after algal removal. Causal effect analysis showed a positive effect of influential OTUs in healthy-looking tissue assigned to Gammaproteobacteria and Bacteroidia on the relative abundance of other OTUs within these classes. The effect of influential OTUs assigned to Alphaproteobacteria in macroalgal-removed tissue on the relative abundance of other OTUs was more diverse. Despite the high heterogeneity of coral microbiomes, differences in the relative abundance of main bacterial classes and orders between control/healthy and macroalgal-removed tissue showed temporal patterns. Differences in the Alpha-, Gamma-, Deltaproteobacteria and Bacteroidia between control/healthy and macroalgal-removed tissue increased after cessation of macroalga contact and stabilized or declined towards day 40. Acidimicrobiia, Deltaproteobacteria, Rhodospirillales and Rhodovibrionales returned to average relative abundances in the adjacent control/healthy tissue after 40 days. Nevertheless, Rhizobiales and Rhodobacterales (Alphaproteobacteria) still dominated the macroalgal-removed microbiome on day 40. We conclude that macroalgal overgrowth induces changes in the coral microbiome, and that algal removal did not lead to full recovery of the microbiome in 40 days. Return of pigmentation and distinct shifts in bacterial groups over time appear a possible pathway to the recovery of the coral microbiome after macroalgal removal.

Differences of Macroalgal Consumption by Eight Herbivorous Coral Reef Fishes From the Xisha Islands, China

Herbivorous fishes play an important role in preventing the overabundance of macroalgae on coral reefs. Understanding the feeding selectivity and consumption of macroalgae by herbivorous fishes can be challenging in studies of their ecological role in the preservation and recovery of coral reefs. Coral reef decline, macroalgal overgrowth and overfishing are clearly visible in the Xisha Islands, China. However, there are seldom studies on the feeding behaviors of herbivorous fishes in this area. We used microscopy, 18S rRNA high-throughput sequencing, and stable isotope analyses to comprehensively examine the diet of eight herbivorous reef fish species common in the Xisha Islands, including one parrotfish, two chub, two unicorn fish, and three rabbitfish. Based on microscopic examination,Siganus argenteusfed on the highest number of macroalgae species (five species), followed byNaso unicornis(four species).Kyphosus cinerascens,K. vaigiensis,N. unicornisandS. punctatissimusfed on the entire macroalgal thallus, indicating their greater ecological importance compared with species that only consume algal fronds. According to the 18S rRNA high-throughput sequencing of fish intestinal contents, cluster analysis revealed that consumed macroalgae composition from the eight fishes always grouped together based on the fish species, but with low similarity.K. vaigiensisremoved the highest diversity of macroalgae species as well as the greatest quantity of macroalgae.Calotomus carolinuscan consume the red algaPneophyllum conicum, which is widely distributed on Indo-Pacific coral reefs and can overgrow and kill live corals.N. unicorniswas found to occupy the lowest trophic position based on stable isotope analysis. Multi-technique analyses revealed thatK. vaigiensis,N. unicornisandS. argenteusshowed a high consumption potential of macroalgae, suggesting that they are the key browsers that should receive priority protection in the Xisha Islands. A diverse herbivorous fish fauna is also very important in the Xisha coral reefs. These results not only demonstrated the various functions of different herbivorous fish species in macroalgal removal, but also provided insights into the management of herbivorous fishes on the coral reefs of the South China Sea.

Compounded effects of sea urchin grazing and physical disturbance on macroalgal canopies in the lagoon of Moorea, French Polynesia

Release from herbivory is a factor underpinning coral replacement by macroalgae. Once macroalgae have achieved dominance, shifts back to the coral-dominated state can be hindered by stabilizing feedbacks. Thus, restoring herbivore assemblages alone can be insufficient to trigger coral recovery. However, herbivores could control macroalgal recovery in the aftermath of physical disturbances removing macroalgae. Diadematid urchins at Moorea (French Polynesia) have collapsed in the last decade. By means of a manipulative field experiment, we tested the interactive effects of physical disturbance and increased diadematid densities on macroalgae inside the lagoon. Massive Porites colonies, referred to as ‘bommies’, were assigned to 3 different macroalgal removal treatments (removal of stipes and fronds of the canopy-forming macroalgae Turbinaria ornata and Sargassum pacificum, total removal of erect macroalgae or untouched) and exposed to 3 different urchin densities (absent, low [~0.5 ind. m-2], and intermediate [~1 ind. m-2]). After 1 yr, sea urchins had no effect on the covers of S. pacificum and T. ornata when macroalgal canopies were left untouched. Urchins could control the recovery of S. pacificum on total macroalgal removal bommies, but not that of T. ornata. However, urchins, even when at intermediate densities, did not generate major changes in the structure of benthic assemblages on experimental bommies. Our study indicates that a moderate increase in diadematid densities is unlikely to reduce the extent of macroalgal stands in Moorea back reefs unless associated with the recovery of other herbivore guilds able to remove adult macroalgae (i.e. browsers).

Fear effects and group size interact to shape herbivory on coral reefs

AbstractFear of predators (‘fear effects’) is an important determinant of foraging decisions by consumers across a range of ecosystems. Group size is one of the main behavioural mechanisms for mitigating fear effects while also providing foraging benefits to group members. Within coral reef ecosystems, fear effects have been shown to influence the feeding rates of herbivorous fishes, a key functional group that prevents macroalgal overgrowth. Yet, how fear effects and group size interact to shape macroalgal removal on coral reefs remains unclear.Here, we conducted field‐based experiments using models of a common piscivorous fish, the leopard coral grouperPlectropomus leopardusand a series of macroalgalSargassum ilicifoliumassays positioned at increasing distances from the models (1, 2, 3 and 4 m) on two coral reefs in Singapore to investigate how acute fear effects shape the intensity of herbivory, and whether these effects were influenced by variation in the group size of herbivorous fishes feeding on the assays.We found acute fear effects strongly influenced the foraging behaviour of herbivorous fishes over small spatial scales. Rates ofSargassumbiomass removal, feeding rates and the total number of individual feeding events were all lower near the predator model. These effects dissipated rapidly with increasing distance from the predator model and were undetectable at a distance of 4 m. We also found generally larger group sizes of herbivorous fishes further from the predator model, presumably reflecting decreased risk. Furthermore, the number of individual bites/event increased significantly with increasing group size for two common browsing fishes,Siganus virgatusandSiganus javus.Our findings highlight that acute fear effects influence the distribution and intensity of herbivory over small spatial scales. Fear effects also interacted with herbivore group size resulting in changes in the number of individual feeding events and bite rates that collectively shape the realized ecosystem function of macroalgal removal on coral reefs. Group size is an important context‐dependent factor that should be considered when examining fear effects on coral reefs.A freePlain Language Summarycan be found within the Supporting Information of this article.

Temporal variation in macroalgal removal: insights from an impacted equatorial coral reef system

Macroalgal removal is a critical ecosystem function yet few studies have considered its temporal variability, especially on impacted reefs with limited herbivorous fish biodiversity. To address this, we quantified macroalgal removal and mass-standardised bite rates of herbivorous fishes monthly from July 2016 to June 2017 using a series of transplanted Sargassum ilicifolium assays and underwater video cameras on three degraded coral reefs in Singapore: Pulau Satumu, Kusu Island, and Terumbu Pempang Tengah. Our results revealed a distinct temporal pattern in macroalgal herbivory (proportion of biomass removed and mass-standardised bite rates) rates across all sites, increasing from July and decreasing from January, with the highest rates recorded in December (28.10 ± 3.05 g 3.5 h−1; 208.24 ± 29.99 mass-standardised bites 3.5 h−1) and the lowest in May (0.86 ± 0.17 g 3.5 h−1; 9.55 ± 3.19 mass-standardised bites 3.5 h−1). These coincided with the S. ilicifolium growth cycle, confirming previous evidence that herbivory rates are closely linked to macroalgal condition. Video analyses revealed nine species feeding over a year (31,839 bites; 8702.89 mass-standardised bites), with Siganus virgatus responsible for sim 80% of the total mass-standardised bites. Siganus virgatus took the largest proportion of bites monthly, except between April and June, when Scarus rivulatus was dominant, suggesting temporal constraints in functional roles.

Macroalgae removal on coral reefs: realised ecosystem functions transcend biogeographic locations

Coral reef ecosystems are at the forefront of biodiversity loss and climate change-mediated transformations. This is expected to have profound consequences for the functioning of these ecosystems. However, assessments of ecosystem function on reefs are often spatially limited, within biogeographic realms, or rely on presumed proxies such as traits. To address these shortcomings and assess the effects of biogeography and fish presence on the critical ecosystem function of macroalgal removal, we used assays of six algal genera across three reef habitats in two biogeographically distinct locations: Little Cayman in the Caribbean and Lizard Island on the Great Barrier Reef (GBR). Patterns of fish feeding and realised ecosystem function were strikingly similar between the two geographic locations, despite a threefold difference in the local diversity of nominally herbivorous fishes, a 2.4-fold difference in the diversity of fishes feeding and differences in the biogeographic history of the two locations. In both regions, a single species dominated the function: a surgeonfish, Naso unicornis, at the GBR location and, surprisingly, a triggerfish, Melichthys niger, at the Caribbean location. Both species, especially M. niger, were relatively rare, compared to other nominally herbivorous fishes, in censuses covering more than 14,000 m2 at each location. Our study provides novel insights into the critical function of macroalgal removal in hyperdiverse coral reef ecosystems, highlighting: (a) that function can transcend biogeographic, taxonomic and historical constraints; and (b) shortcomings in our assumptions regarding fish presence and realised ecosystem function on coral reefs.

Fear effects associated with predator presence and habitat structure interact to alter herbivory on coral reefs.

Non-consumptive fear effects are an important determinant of foraging decisions by consumers across a range of ecosystems. However, how fear effects associated with the presence of predators interact with those associated with habitat structure remain unclear. Here, we used predator fish models (Plectropomus leopardus) and experimental patches of the macroalga Sargassum ilicifolium of varying densities to investigate how predator- and habitat-associated fear effects influence herbivory on coral reefs. We found the removal of macroalgal biomass (i.e. herbivory) was shaped by the interaction between predator- and habitat-associated fear effects. Rates of macroalgal removal declined with increasing macroalgal density, likely due to increased visual occlusion by denser macroalgae patches and reduced ability of herbivorous fishes to detect the predators. The presence of the predator model reduced herbivory within low macroalgal density plots, but not within medium- and high-density macroalgal plots. Our results suggest that fear effects due to predator presence were greatest at low macroalgal density, yet these effects were lost at higher densities possibly due to greater predation risk associated with habitat structure and/or the inability of herbivorous fishes to detect the predator model.

Invertebrate herbivores: Overlooked allies in the recovery of degraded coral reefs?

A key question for coral reef conservation is whether reefs dominated by macroalgae can recover. Since the near-disappearance of the herbivorous urchin Diadema antillarum in the Caribbean, a prevalent management paradigm has focused on protecting herbivorous fishes to trigger shifts back to a coral-rich state. However, in the absence of D. antillarum, the contribution of other large macroinvertebrates to herbivory intensity has been largely overlooked. We used day and night field surveys and behavioural observations at 16 degraded reef patches in the Bahamas to measure the abundance of large herbivorous macroinvertebrates and their consumption of fleshy macroalgae. Tripneustes sea urchins and Maguimithrax crabs were the main herbivorous macroinvertebrates on our sites and were active mainly at night, with 97% of urchins and 45% of crabs observed consuming fleshy macroalgae. By comparison, < 5% of herbivorous fishes observed ate macroalgae. In the laboratory, Tripneustes sea urchins and Maguimithrax crabs readily consumed macroalgae (at rates of 0.19 g h−1 and 0.38 g h−1, respectively), but their low abundance on patch reefs (4 crabs and 2.3 urchins per reef, on average) translated into low overall rates of macroalgal removal. Perhaps for this reason, there was no relationship between the density of these large macroinvertebrates or their grazing rate and macroalgal cover on patch reefs. Nevertheless, we calculated that macroalgal consumption by Maguimithrax crabs alone could exceed macroalgae production with a doubling of their current low abundance; a 2.6-fold increase in Tripneustes urchin abundance would achieve the same result. Our results suggest that large herbivorous macroinvertebrates, some of which are currently the target of artisanal fishing in many Caribbean countries, could contribute greatly to the recovery of coral reefs with established macroalgal communities, at least in patch reef habitats.

Macroalgal browsing on a heavily degraded, urbanized equatorial reef system

The removal of macroalgal biomass is critical to the health of coral reef ecosystems. Previous studies on relatively intact reefs with diverse and abundant fish communities have quantified rapid removal of macroalgae by herbivorous fishes, yet how these findings relate to degraded reef systems where fish diversity and abundance are markedly lower and algal biomass substantially higher, is unclear. We surveyed roving herbivorous fish communities and quantified their capacity to remove the dominant macroalga Sargassum ilicifolium on seven reefs in Singapore; a heavily degraded urbanized reef system. The diversity and abundance of herbivorous fishes was extremely low, with eight species and a mean abundance ~1.1 individuals 60 m−2 recorded across reefs. Consumption of S. ilicifolium varied with distance from Singapore’s main port with consumption being 3- to 17-fold higher on reefs furthest from the port (Pulau Satumu: 4.18 g h−1; Kusu Island: 2.38 g h−1) than reefs closer to the port (0.35–0.78 g h−1). Video observations revealed a single species, Siganus virgatus, was almost solely responsible for removing S. ilicifolium biomass, accounting for 83% of the mass-standardized bites. Despite low herbivore diversity and intense urbanization, macroalgal removal by fishes on some Singaporean reefs was directly comparable to rates reported for other inshore Indo-Pacific reefs.

Macroalgal herbivory on recovering versus degrading coral reefs

Macroalgal-feeding fishes are considered to be a key functional group on coral reefs due to their role in preventing phase shifts from coral to macroalgal dominance, and potentially reversing the shift should it occur. However, assessments of macroalgal herbivory using bioassay experiments are primarily from systems with relatively high coral cover. This raises the question of whether continued functionality can be ensured in degraded systems. It is clearly important to determine whether the species that remove macroalgae on coral-dominated reefs will still be present and performing significant algal removal on macroalgal-dominated reefs. We compared the identity and effectiveness of macroalgal-feeding fishes on reefs in two conditions post-disturbance—those regenerating with high live coral cover (20–46 %) and those degrading with high macroalgal cover (57–82 %). Using filmed Sargassum bioassays, we found significantly different Sargassum biomass loss between the two conditions; mean assay weight loss due to herbivory was 27.9 ± 4.9 % on coral-dominated reefs and 2.2 ± 1.1 % on reefs with high macroalgal cover. However, once standardised for the availability of macroalgae on the reefs, the rates of removal were similar between the two reef conditions (4.8 ± 4.1 g m−2 h−1 on coral-dominated and 5.3 ± 2.1 g m−2 h−1 on macroalgal-dominated reefs). Interestingly, the Sargassum-assay consumer assemblages differed between reef conditions; nominally grazing herbivores, Siganus puelloides and Chlorurus sordidus, and the browser, Siganus sutor, dominated feeding on high coral cover reefs, whereas browsing herbivores, Naso elegans, Naso unicornis, and Leptoscarus vaigiensis, prevailed on macroalgal-dominated reefs. It appeared that macroalgal density in the surrounding habitat had a strong influence on the species driving the process of macroalgal removal. This suggests that although the function of macroalgal removal may continue, the species responsible may change with context, differing between systems that are regenerating versus degrading.

Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 422:239-251 (2011) - DOI: https://doi.org/10.3354/meps08916 Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes Carine D. Lefèvre*, David R. Bellwood Australian Research Council Centre of Excellence for Coral Reef Studies and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia *Email: carine.lefevre@jcu.edu.au ABSTRACT: Despite the widely accepted importance of fish herbivory on coral reefs, few studies have considered the temporal variability in the nature of algal–herbivore interactions. We therefore quantified monthly feeding intensity on Sargassum sp. bioassays for 12 mo with remote underwater video cameras deployed to identify the herbivores responsible for macroalgal removal on an inshore island of the Great Barrier Reef, Australia. Significantly higher removal rates were observed during the summer months whereas winter months were characterized by 4 times lower removal rates. However, rather than being simply changes in the feeding activity of a single species, this temporal pattern in herbivory also incorporated changes in the species responsible for the removal of Sargassum. Video analyses revealed that, of the 43 herbivore species recorded from the bay, only 3 played a significant role in Sargassum removal: Kyphosus vaigiensis, Naso unicornis and Scarus rivulatus. K. vaigiensis, a rudderfish, was primarily responsible for the removal of Sargassum during the summer months (83% of the total recorded bites; 85553 bites). There was almost no feeding activity on Sargassum by K. vaigiensis during the winter months (82 bites). However, there was a reciprocal increase in feeding intensity on Sargassum by parrotfishes in the winter months, particularly S. rivulatus (71 bites during summer versus 2884 bites in winter). This temporal variability in herbivore functional roles suggests that functional redundancy on reefs may be less than previously assumed in that the feeding activities of fishes may be both spatially and temporally constrained. KEY WORDS: Regime shift · Kyphosus vaigiensis · Trophic status · Phase shift · Resilience · Functional redundancy · Macroalgae · Sargassum · Great Barrier Reef Full text in pdf format Supplementary material PreviousNextCite this article as: Lefèvre CD, Bellwood DR (2011) Temporal variation in coral reef ecosystem processes: herbivory of macroalgae by fishes. Mar Ecol Prog Ser 422:239-251. https://doi.org/10.3354/meps08916 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 422. Online publication date: January 31, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Salt marsh stabilization affects algal primary producers at the marsh edge

As sea level rise and human activities erode coastal wetlands, managers rebuild or preserve wetlands that can perform the ecosystem services of a natural system. One increasingly common mitigation activity is the construction of rock sills in the low marsh zone to stabilize marsh elevation. Sills dramatically alter the physical structure of marshes by changing elevation, adding hard substrate and potentially altering the spatial structure of benthic algal communities in and adjacent to the low marsh. We documented differences in benthic algal abundance at the seaward marsh edge in silled and unsilled marshes in North Carolina. We found that sills were associated with reduced standing stocks of benthic algal primary production and reduced macroalgal taxonomic richness, and this difference was driven primarily by differences in macroalgal abundance. We experimentally tested the effect of macroalgal abundance on cordgrass (Spartina alterniflora) growth in the low zone of an unmanipulated marsh, and found that macroalgal removal had no effect on final cordgrass abundance. Our study suggests that salt marsh management through the construction of sills in low marsh zones impacts benthic primary production in the low marsh zone, but that benthic algal production does not affect cordgrass growth over a growing season.

Seasonality and dynamics in coral reef macroalgae: variation in condition and susceptibility to herbivory

Seasonal variation in coral reef macroalgal size and condition is well documented, yet seasonal variability of herbivory on macroalgae by coral reef fishes is unknown. Herbivore feeding intensity was quantified monthly on an inner-shelf reef on the Great Barrier Reef, using Sargassum bioassays. Removal rates of transplants displayed high levels of variation with significantly higher rates of removal during the summer months. Differences in Sargassum plant size and condition suggest that the variability in herbivore feeding intensity is attributed primarily to the variation in the condition of the macroalgae, especially epiphyte loads. The dramatic changes in macroalgal removal reveal a considerable decrease in herbivore activity in the winter. This highlights the clear distinction between ‘summer’ and ‘winter’ months in terms of reef processes, emphasizing the high seasonal variation in macroalgal removal rates at different time of the year.

Direct evaluation of macroalgal removal by herbivorous coral reef fishes

Few studies have examined the relative functional impacts of individual herbivorous fish species on coral reef ecosystem processes in the Indo-Pacific. This study assessed the potential grazing impact of individual species within an inshore herbivorous reef fish assemblage on the central Great Barrier Reef (GBR), by determining which fish species were able to remove particular macroalgal species. Transplanted multiple-choice algal assays and remote stationary underwater digital video cameras were used to quantify the impact of local herbivorous reef fish species on 12 species of macroalgae. Macroalgal removal by the fishes was rapid. Within 3 h of exposure to herbivorous reef fishes there was significant evidence of intense grazing. After 12 h of exposure, 10 of the 12 macroalgal species had decreased to less than 15% of their original mass. Chlorodesmis fastigiata (Chlorophyta) and Galaxaura sp. (Rhodophyta) showed significantly less susceptibility to herbivorous reef fish grazing than all other macroalgae, even after 24 h exposure. Six herbivorous and/or nominally herbivorous reef fish species were identified as the dominant grazers of macroalgae: Siganus doliatus, Siganus canaliculatus, Chlorurus microrhinos, Hipposcarus longiceps, Scarus rivulatus and Pomacanthus sexstriatus. The siganid S. doliatus fed heavily on Hypnea sp., while S. canaliculatus fed intensively on Sargassum sp. Variation in macroalgal susceptibility was not clearly correlated with morphological and/or chemical defenses that have been previously suggested as deterrents against herbivory. Nevertheless, the results stress the potential importance of individual herbivorous reef fish species in removing macroalgae from coral reefs.

Scale of Disturbance and the Structure of a Temperate Fish Guild

Although it is acknowledged that the spatial scale of sampling and experimentation is important in assessing the degree of stability of a community, multiple scales are rarely incorporated into the same study. We compared the persistence and resilience of a temperate blennioid fish guild at three scales spanning an order of magnitude. Replicate permanent square quadrats of three sizes (4 m2, 25 m2, 100 m2) were delimited within contiguous tracts of reef in two habitats: broken reef and macroalgal forest. Two forms of disturbance were applied to the treatment quadrats. Direct disturbance (fish removal) was applied to treatments in the broken reef habitat. Indirect disturbance (macroalgal removal) was applied to treatments in the macroalgal forest. Both experiments identified several scale-dependent effects. Large quadrats were less variable than smaller quadrats, probably due to reduced edge effects due to movement of fish in and out of the quadrats. The smallest quadrats did not adequately represent the available substratum types within the reef zone. This interacted with species-specific behavioral factors such as mobility and habitat association to consistently bias the blennioid composition within the smallest quadrats toward dominance of the most sedentary blennioid species. Fish responded to disturbance in different ways depending on the stage of their life cycle. Adult fish responded at scales within their motility range of only a few meters. In contrast, newly settling fish responded to the habitat at larger scales. Quadrat size also determined which assemblage dynamics could be measured. Within-year changes in guild composition due to species-specific settlement timing and survivorship could be measured only at larger scales. We conclude that different scales of monitoring and manipulation will determine what can be perceived by the researcher as well as how the organism will respond to disturbance.

SCALE OF DISTURBANCE AND THE STRUCTURE OF A TEMPERATE FISH GUILD

Although it is acknowledged that the spatial scale of sampling and experimentation is important in assessing the degree of stability of a community, multiple scales are rarely incorporated into the same study. We compared the persistence and resilience of a temperate blennioid fish guild at three scales spanning an order of magnitude. Replicate permanent square quadrats of three sizes (4 m2, 25 m2, 100 m2) were delimited within contiguous tracts of reef in two habitats: broken reef and macroalgal forest. Two forms of disturbance were applied to the treatment quadrats. Direct disturbance (fish removal) was applied to treatments in the broken reef habitat. Indirect disturbance (macroalgal removal) was applied to treatments in the macroalgal forest. Both experiments identified several scale-dependent effects. Large quadrats were less variable than smaller quadrats, probably due to reduced edge effects due to movement of fish in and out of the quadrats. The smallest quadrats did not adequately represent the available substratum types within the reef zone. This interacted with species-specific behavioral factors such as mobility and habitat association to consistently bias the blennioid composition within the smallest quadrats toward dominance of the most sedentary blennioid species. Fish responded to disturbance in different ways depending on the stage of their life cycle. Adult fish responded at scales within their motility range of only a few meters. In contrast, newly settling fish responded to the habitat at larger scales. Quadrat size also determined which assemblage dynamics could be measured. Within-year changes in guild composition due to species-specific settlement timing and survivorship could be measured only at larger scales. We conclude that different scales of monitoring and manipulation will determine what can be perceived by the researcher as well as how the organism will respond to disturbance.