Biologically mediated sediment fluxes on coral reefs: sediment removal and off-reef transportation by the surgeonfish 'Ctenochaetus striatus'
Off-reef sediment transport by the surgeonfish 'Ctenochaetus striatus' (Acanthuridae) was quantified on the reef crest at Lizard Island, Great Barrier Reef. Three independent methods were implemented to estimate sediment ingestion rates. These considered (1) the bite rate and bite volume, (2) the defecation rate and faecal pellet size, and (3) the average gut contents and throughput rate. The 3 methods provided a broad range of estimates of sediment ingestion from 8.8 ± 2.4, to 66.1 ± 14.4 g fish-1 d-1 (mean ± SE). Nevertheless, these estimates were comparable to rates of sediment ingestion by parrotfishes (Labridae), the other major sediment-moving group on reefs. Overall, 36.5% of all sediment ingested was transported from the upper reef crest into deeper water, equating to a removal rate of 28.6 ± 6.2 kg 100 m-2 yr-1 at the study site. By brushing the reef, 'C. striatus' reduces the sediment loading in the epilithic algal matrix (EAM) while causing little damage to the algal turf. Reducing sediments in EAMs provides favourable settlement surfaces for benthic organisms and increases the palatability of the EAM to herbivorous reef fishes, thus supporting reef resilience. The ecological importance of 'C. striatus', which is abundant on reefs throughout the IndoPacific, appears to have been underestimated, particularly when considering reef sediment dynamics.
Ecological Consequences of Sediment on High-Energy Coral Reefs
Sediments are widely accepted as a threat to coral reefs but our understanding of their ecological impacts is limited. Evidence has suggested that benthic sediments bound within the epilithic algal matrix (EAM) suppress reef fish herbivory, a key ecological process maintaining reef resilience. An experimental combination of caging and sediment addition treatments were used to investigate the effects of sediment pulses on herbivory and EAMs and to determine whether sediment addition could trigger a positive-feedback loop, leading to deep, sediment-rich turfs. A 1-week pulsed sediment addition resulted in rapid increases in algal turf length with effects comparable to those seen in herbivore exclusion cages. Contrary to the hypothesised positive-feedback mechanism, benthic sediment loads returned to natural levels within 3 weeks, however, the EAM turfs remained almost 60% longer for at least 3 months. While reduced herbivore density is widely understood to be a major threat to reefs, we show that acute disturbances to reef sediments elicit similar ecological responses in the EAM. With reefs increasingly threatened by both reductions in herbivore biomass and altered sediment fluxes, the development of longer turfs may become more common on coral reefs.
Reef-scale partitioning of cryptobenthic fish assemblages across the Great Barrier Reef, Australia
Onshore to offshore gradients in marine assemblages have been well documented on coral reefs, with most studies showing a distinct separation between onshore and offshore locations. Here we use enclosed anaesthetic sampling of small, cryptobenthic reef fishes to assess changes in assemblage composition across the Great Barrier Reef continental shelf. The cryptobenthic fishes exhibited fine-scale partitioning across the shelf. Three dominant species of goby accounted for over 55% of all fishes collected, with 1 species characterising each of the 3 key shelf positions: inner-, mid- and outer-shelf. Multivariate analyses of assemblage composition revealed further separation of reefs within the inner- and mid-shelf positions, highlighting the exceptional sensitivity of cryptobenthic reef fish assemblages to shelf position, with a progressive separation of individual reef assemblages with distance from the shore. These among-reef patterns contrast markedly with other reef fish taxa which invariably show 2 broad assemblages across the continental shelf (inner- vs. a composite mid- and outer-shelf community). As a result of this exceptional sensitivity to environmental conditions, cryptobenthic reef fish communities may represent good subjects for high-resolution monitoring of disturbances on coral reefs.
The Roles of Dimensionality, Canopies and Complexity in Ecosystem Monitoring
Canopies are common among autotrophs, increasing their access to light and thereby increasing competitive abilities. If viewed from above canopies may conceal objects beneath them creating a 'canopy effect'. Due to complexities in collecting 3-dimensional data, most ecosystem monitoring programmes reduce dimensionality when sampling, resorting to planar views. The resultant 'canopy effects' may bias data interpretation, particularly following disturbances. Canopy effects are especially relevant on coral reefs where coral cover is often used to evaluate and communicate ecosystem health. We show that canopies hide benthic components including massive corals and algal turfs, and as planar views are almost ubiquitously used to monitor disturbances, the loss of vulnerable canopy-forming corals may bias findings by presenting pre-existing benthic components as an altered system. Our reliance on planar views in monitoring ecosystems, especially coral cover on reefs, needs to be reassessed if we are to better understand the ecological consequences of ever more frequent disturbances.
Composition and temporal stability of turf sediments on inner-shelf coral reefs
Elevated sediment loads within the epilithic algal matrix (EAM) of coral reefs can increase coral mortality and inhibit herbivory. Yet the composition, distribution and temporal variability of EAM sediment loads are poorly known, especially on inshore reefs. This study quantified EAM sediment loads (including organic particulates) and algal length across the reef profile of two bays at Orpheus Island (inner-shelf Great Barrier Reef) over a six month period. We examined the total sediment mass, organic load, carbonate and silicate content, and the particle sizes of EAM sediments. Throughout the study period, all EAM sediment variables exhibited marked variation among reef zones. However, EAM sediment loads and algal length were consistent between bays and over time, despite major seasonal variation in climate including a severe tropical cyclone. This study provides a comprehensive description of EAM sediments on inshore reefs and highlights the exceptional temporal stability of EAM sediments on coral reefs.
Quantifying Relative Diver Effects in Underwater Visual Censuses
Diver-based Underwater Visual Censuses (UVCs), particularly transect-based surveys, are key tools in the study of coral reef fish ecology. These techniques, however, have inherent problems that make it difficult to collect accurate numerical data. One of these problems is the diver effect (defined as the reaction of fish to a diver). Although widely recognised, its effects have yet to be quantified and the extent of taxonomic variation remains to be determined. We therefore examined relative diver effects on a reef fish assemblage on the Great Barrier Reef. Using common UVC methods, the recorded abundance of seven reef fish groups were significantly affected by the ongoing presence of SCUBA divers. Overall, the diver effect resulted in a 52% decrease in the mean number of individuals recorded, with declines of up to 70% in individual families. Although the diver effect appears to be a significant problem, UVCs remain a useful approach for quantifying spatial and temporal variation in relative fish abundances, especially if using methods that minimise the exposure of fishes to divers. Fixed distance transects using tapes or lines deployed by a second diver (or GPS-calibrated timed swims) would appear to maximise fish counts and minimise diver effects.