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.     

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.     

Limited Functional Redundancy in a High Diversity System: Single Species Dominates Key Ecological Process on Coral Reefs

Herbivory is a key process structuring plant communities in both terrestrial and aquatic ecosystems, with variation in herbivory often being related to shifts between alternate states. On coral reefs, regional reductions in herbivores have underpinned shifts from coral to dominance by leathery macroalgae. These shifts appear difficult to reverse as these macroalgae are unpalatable to the majority of herbivores, and the macroalgae suppress the recruitment and growth of corals. The removal of macroalgae is, therefore, viewed as a key ecological process on coral reefs. On the Great Barrier Reef, Sargassum is a dominant macroalgal species following experimentally induced coral–macroalgal phase-shifts. We, therefore, used Sargassum assays and remote video cameras to directly quantify the species responsible for removing macroalgae across a range of coral reef habitats on Lizard Island, northern Great Barrier Reef. Despite supporting over 50 herbivorous fish species and six macroalgal browsing species, the video footage revealed that a single species, Naso unicornis, was almost solely responsible for the removal of Sargassum biomass across all habitats. Of the 42,246 bites taken from the Sargassum across all habitats, N. unicornis accounted for 89.8% (37,982) of the total bites, and 94.6% of the total mass standardized bites. This limited redundancy, both within and across local scales, underscores the need to assess the functional roles of individual species. Management and conservation strategies may need to look beyond the preservation of species diversity and focus on the maintenance of ecological processes and the protection of key species in critical functional groups.     

Density and height of Sargassum influence rabbitfish (f. Siganidae) settlement on inshore reef flats of the Great Barrier Reef

Macroalgal beds have been suggested to be an important settlement habitat for a diversity of reef fishes, yet few studies have considered how the composition or structure of macroalgal beds may influence fish settlement. The aim of this study was to investigate how the physical characteristics of Sargassum beds, a common macroalga on inshore coral reefs, influence the abundance of recently-settled rabbitfishes (Siganidae) on Orpheus Island, Great Barrier Reef. The abundance of recently-settled rabbitfish (< 3 cm total length), the density and height of Sargassum thalli, and benthic composition were quantified within replicate 1-m² quadrats across 15 mid-reef flat sites. A total of 419 recently-settled rabbitfish from three species (Siganus doliatus, S. lineatus and S. canaliculatus) were recorded across 150 quadrats (range 0–16 individuals m⁻²), with S. doliatus accounting for the majority (85.2%) of individuals recorded. The abundance of S. doliatus and S. lineatus was greatest at moderate Sargassum densities (ca. 20–30 holdfasts m⁻²) and generally increased with Sargassum height and the cover of ‘other’ macroalgae. These findings demonstrate the potential importance of the physical characteristics of macroalgal beds to the settlement of rabbitfishes on inshore reef flats.

     

Local variation in herbivore feeding activity on an inshore reef of the Great Barrier Reef

Threats to coral reefs may be manifested through an increase in macroalgae. Across the globe, phase-shifts from coral to macroalgal dominance have been reported from the Caribbean, Indian and Pacific Oceans. While the Great Barrier Reef (GBR) is in relatively good condition, inshore reefs may exhibit over 50% macroalgal cover. However, our understanding of the processes preventing the macroalgal expansion remains uncertain. Using a remote video bioassay approach, this study quantified herbivory in three bays along the leeward margin of Orpheus Island. Despite significant with-in bay variation in herbivory there was no detectable statistical difference in the rates of herbivory among bays. Furthermore, of the 45 herbivore species recorded from the island, only three played a significant role in bioassay removal, Siganus canaliculatus, Siganus javus and Kyphosus vaigiensis, with only one species predominating in each bay. Reefs of the GBR may therefore be more vulnerable than previously thought, with the removal of macroalgae depending on just a few species, which exhibit considerable spatial variability in their feeding behaviour.     

Diet and cross-shelf distribution of rabbitfishes (f. Siganidae) on the northern Great Barrier Reef: implications for ecosystem function

Herbivorous fishes are a critical functional group on coral reefs, and there is a clear need to understand the role and relative importance of individual species in reef processes. While numerous studies have quantified the roles of parrotfishes and surgeonfishes on coral reefs, the rabbitfishes (f. Siganidae) have been largely overlooked. Consequently, they are typically viewed as a uniform group of grazing or browsing fishes. Here, we quantify the diet and distribution of rabbitfish assemblages on six reefs spanning the continental shelf in the northern Great Barrier Reef. Our results revealed marked variation in the diet and distribution of rabbitfish species. Analysis of stomach contents identified four distinct groups: browsers of leathery brown macroalgae (Siganus canaliculatus, S. javus), croppers of red and green macroalgae (S. argenteus, S. corallinus, S. doliatus, S. spinus) and mixed feeders of diverse algal material, cyanobacteria, detritus and sediment (S. lineatus, S. punctatissimus, S. punctatus, S. vulpinus). Surprisingly, the diet of the fourth group (S. puellus) contained very little algal material (22.5 %) and was instead dominated by sponges (69.1 %). Together with this variation in diet, the distribution of rabbitfishes displayed clear cross-shelf variation. Biomass was greatest on inner-shelf reefs (112.7 ± 18.2 kg.ha^?1), decreasing markedly on mid- (37.8 ± 4.6 kg.ha^?1) and outer-shelf reefs (9.7 ± 2.2 kg.ha^?1). This pattern was largely driven by the browsing S. canaliculatus that accounted for 50 % of the biomass on inner-shelf reefs, but was absent in mid- and outer-shelf reefs. Mixed feeders, although primarily restricted to the reef slope and back reef habitats, also decreased in abundance and biomass from inshore to offshore, while algal cropping taxa were the dominant group on mid-shelf reefs. These results clearly demonstrate the extent to which diet and distribution vary within the Siganidae and emphasise the importance of examining function on a species-by-species basis.     

Damselfish territories as a refuge for macroalgae on coral reefs

Herbivory is widely accepted as a key process determining the benthic community structure and resilience of coral reefs. Recent studies have mostly focused on the importance of roving herbivorous fishes in ecosystem processes. Here, we examine the role of territorial damselfish in shaping patterns of macroalgal distribution based on benthic surveys and macroalgal bioassays. The territory composition and effect of resident damselfish on the removal of Sargassum bioassays were quantified for six species of damselfish on Lizard Island, a mid-shelf reef in the northern Great Barrier Reef (GBR). The functional composition of algal communities within territories varied markedly among species. The territories of four species (Dischistodus perspicillatus, Dischistodus pseudochrysopoecilus, Plectroglyphidodon lacrymatus, and Stegastes nigricans) were characterized by algal turfs, while the territories of two species (Dischistodus prosopotaenia and Hemiglyphidodon plagiometopon) were characterized by foliose and leathery brown macroalgae. Sargassum, a generally rare alga on mid-shelf reefs, was a particularly common alga within D. prosopotaenia territories on the leeward side of the island but absent within their territories on the windward side of the island. D. prosopotaenia was the only species to retain the transplanted Sargassum, with only a minimal reduction in Sargassum biomass (1.1%) being recorded within their territories at both leeward and windward sites over a 24-h period. In contrast, reductions in Sargassum biomass were high in areas adjacent to D. prosopotaenia territories (83.8%), and within and adjacent to the territories of the five remaining damselfish species (76.2–92.5%). Overall, only one of the six damselfish species provided a refuge for leathery brown macroalgae and may facilitate the development of this macroalgae on mid-shelf reefs of the GBR.     

Biogenic volatile organic compounds from an invasive species: impacts on plant–plant interactions

Invasive plant species impact both ecosystems and economies worldwide, often by displacing native biota. Many plant species exude/emit compounds into the surrounding environment with minor consequences in their native habitat due to a long coevolutionary history. However, upon introduction to ecosystems naïve to these compounds, unpredictable interactions can manifest. The majority of the putative allelochemicals studied have been root exudates, despite the large number of plant species that emit volatile organic compounds. We quantified the concentrations and ecological consequences of volatile monoterpenes from the North American invasive perennial Artemisia vulgaris. Ambient monoterpene-mixing ratios inside an A. vulgaris canopy were 0.02–4.15 ppbv in May and 0.01–0.05 ppbv in August, but were negligible (below instrument detection limit of 0.01 ppbv) 10 m away. Foliar disturbance increased total monoterpene concentration to a maximum of 27 ppbv. However, this level remains 1,000-fold lower than that shown to be phytotoxic to sensitive species in laboratory assays. In contrast, soil monoterpene concentrations were >74-fold higher inside [≤35 ± 11 ng g^?1 (SDW)] and 19-fold higher at the edge [9 ± 3 ng g^?1 (SDW)], compared to outside the A. vulgaris stand [0.48 ± 0.05 ng g^?1 (SDW)]. A common native competitor species, Solidago canadensis, grown in pots and resident soil in situ yielded up to 50% less aboveground biomass inside as compared to outside the A. vulgaris stand. Activated carbon had no effect on greenhouse-grown S. canadensis performance when grown with A. vulgaris, suggesting root-derived exudates are not responsible for field observations. Results from this study suggest that A. vulgaris-derived monoterpenes have little direct activity in their volatile gaseous state, but are concentrated in the soil matrix within and bordering the A. vulgaris stand, thereby reducing interspecific performance and potentially fostering the subsequent local invasion of this species.     

Benthic Composition of a Healthy Subtropical Reef: Baseline Species-Level Cover,with an Emphasis on Algae,in the Northwestern Hawaiian Islands

The Northwestern Hawaiian Islands (NWHI) are considered to be among the most pristine coral reef ecosystems remaining on the planet. These reefs naturally contain a high percent cover of algal functional groups with relatively low coral abundance and exhibit thriving fish communities dominated by top predators. Despite their highly protected status, these reefs are at risk from both direct and indirect anthropogenic sources. This study provides the first comprehensive data on percent coverage of algae, coral, and non-coral invertebrates at the species level, and investigates spatial diversity patterns across the archipelago to document benthic communities before further environmental changes occur in response to global warming and ocean acidification. Monitoring studies show that non-calcified macroalgae cover a greater percentage of substrate than corals on many high latitude reef sites. Forereef habitats in atoll systems often contain high abundances of the green macroalga Microdictyon setchellianum and the brown macroalga Lobophora variegata, yet these organisms were uncommon in forereefs of non-atoll systems. Species of the brown macroalgal genera Padina, Sargassum, and Stypopodium and the red macroalgal genus Laurencia became increasingly common in the two northernmost atolls of the island chain but were uncommon components of more southerly islands. Conversely, the scleractinian coral Porites lobata was common on forereefs at southern islands but less common at northern islands. Currently accepted paradigms of what constitutes a “healthy” reef may not apply to the subtropical NWHI, and metrics used to gauge reef health (e.g., high coral cover) need to be reevaluated.     

Seascape-scale trophic links for fish on inshore coral reefs

It is increasingly accepted that coastal habitats such as inshore coral reefs do not function in isolation but rather as part of a larger habitat network. In the Caribbean, trophic subsidies from habitats adjacent to coral reefs support the diet of reef fishes, but it is not known whether similar trophic links occur on reefs in the Indo-Pacific. Here, we test whether reef fishes in inshore coral, mangrove, and seagrass habitats are supported by trophic links. We used carbon stable isotopes and mathematical mixing models to determine the minimum proportion of resources from mangrove or seagrass habitats in the diet of five fish species from coral reefs at varying distances (0–2,200 m) from these habitats in Moreton Bay, Queensland, eastern Australia. Of the fish species that are more abundant on reefs near to mangroves, Lutjanus russelli and Acanthopagrus australis showed no minimum use of diet sources from mangrove habitat. Siganus fuscescens utilized a minimum of 25–44 % mangrove sources and this contribution increased with the proximity of reefs to mangroves (R ² = 0.91). Seagrass or reef flat sources contributed a minimum of 14–78 % to the diet of Diagramma labiosum, a species found in higher abundance on reefs near seagrass beds, but variation in diet among reefs was unrelated to seascape structure. Seagrass or reef flat sources also contributed a minimum of 8–55 % to a fish species found only on reefs (Pseudolabrus guentheri), indicating that detrital subsidies from these habitats may subsidize fish diet on reefs. These results suggest that carbon sources from multiple habitats contribute to the functioning of inshore coral reef ecosystems and that trophic connectivity between reefs and mangroves may enhance production of a functionally important herbivore.     

Percentage cover,biomass, distribution,and potential habitat mapping of natural macroalgae,based on high-resolution satellite data and in situ monitoring,at Libukang Island,Malasoro Bay,Indonesia

In this study, we combined remote sensing data and in situ observations to explore the potential habitats of macroalgae at Libukang Island, Indonesia. High-resolution satellite images from the GeoEye-1 were used to estimate and to map the geomorphological structures together with macroalgal species in the study area. Seasonal variations of percentage cover and biomass of macroalgae associated with substrates were investigated in May and November 2014, and June 2015, using quadrats as sampling unit. A total of nine common genera were found in the study area with three dominant genera: Sargassum, Padina, and Turbinaria. Most of macroalgae was observed in the eastern part of the Island, on several substrate types and particular oceanographic conditions (wave and current). Mean biomasses of Sargassum and Padina were high in May (1189.6 ± 455 and 166.7 ± 15.4 g DW.m^?2, respectively), while the biomass of Turbinaria was high in November (3245 ± 599.8 g DW.m^?2). The map accuracy of image classification for all typology substrates was 74.19%. Overall, approximately 62.3% of the total study area can be considered as potential for natural macroalgae habitats. Spectral response characteristic of shallow water substrates at study area based on GeoEye-1 is also presented. The results of this study exhibit a potential utilization of natural macroalgae in the study area, and provide information for a possible diversification of the use of macroalgae in Indonesia. The method could be useful for habitat management and future biomonitoring in the study area or other similar areas in Indonesia.     

Seasonal growth rate of Sargassum species at Teluk Kemang, Port Dickson, Malaysia

The seasonal growth rates of three Sargassum species were studied along two reef flats of Teluk Kemang, located at Port Dickson, Malaysia from September 2009 to September 2010. Systematic quadrat and line transects were sampled monthly. Nondestructive sampling was conducted, whereby Sargassum plants were tagged and monitored for a 13-month period. The majority of the tagged Sargassum samples belonged to lower length classes (<200 mm), especially in 0–99 mm (Sargassum polycystum, 64.20 %; Sargassum binderi, 68.29 %; Sargassum siliquosum, 56.80 %). Analysis of the monthly mean thallus length (MTL) revealed a bimodal pattern in growth rates, with two periods of high growth rates (January–February 2010 and June–July 2010) and two periods of higher degenerative rates (April 2010 and September 2010). The highest growth rates were recorded in February 2010 (4.08 mm day^?1) for S. siliquosum, and in June for S. polycystum (2.54 mm day^?1) and S. binderi (1.89 mm day^?1). Redundancy analysis (RDA) was employed to test for the overall correlation between monthly variation in MTL and the environmental parameters measured; S. binderi was correlated with ambient temperature (r = 0.5395), while S. siliquosum was correlated with seawater salinity (r = 0.5419) and ammonia (r = ?0.4603). This study reviews the seasonality of Sargassum species on two reefs of Teluk Kemang and their correlation with the selected environmental parameters.     

Context-Dependent Diversity-Effects of Seaweed Consumption on Coral Reefs in Kenya

Consumers and prey diversity, their interactions, and subsequent effects on ecosystem function are important for ecological processes but not well understood in high diversity ecosystems such as coral reefs. Consequently, we tested the potential for diversity-effects with a series of surveys and experiments evaluating the influence of browsing herbivores on macroalgae in Kenya’s fringing reef ecosystem. We surveyed sites and undertook experiments in reefs subject to three levels of human fishing influence: open access fished reefs, small and recently established community-managed marine reserves, and larger, older government-managed marine reserves. Older marine reserves had a greater overall diversity of herbivores and browsers but this was not clearly associated with reduced macroalgal diversity or abundance. Experiments studying succession on hard substrata also found no effects of consumer diversity. Instead, overall browser abundance of either sea urchins or fishes was correlated with declines in macroalgal cover. An exception was that the absence of a key fish browser genus, Naso, which was correlated with the persistence of Sargassum in a marine reserve. Algal selectivity assays showed that macroalgae were consumed at variable rates, a product of strong species-specific feeding and low overlap in the selectivity of browsing fishes. We conclude that the effects of browser and herbivore diversity are less than the influences of key species, whose impacts emerge in different contexts that are influenced by fisheries management. Consequently, identifying key herbivore species and managing to protect them may assist protecting reef functions.     

Seasonally changing habitat use patterns among roving herbivorous fishes in the southern Red Sea: the role of temperature and algal community structure

Coral reefs are characterized by intense herbivory. Spatial patterns in herbivory—particularly along the depth gradient—influence the distribution and abundance of algae. Depth gradients in herbivorous reef fishes are generally assumed to be temporally stable, but this assumption has rarely been questioned. Here, we use underwater visual census and herbivore exclusion experiments to study the community composition and temporal patterns in habitat use by roving herbivorous fishes in an environment characterized by profound seasonal changes in algal biomass and distribution and extreme summer temperatures. Among the 18 species of roving herbivores recorded, parrotfishes were dominant in species richness and biomass, while regional endemic species represented 77 % of the total biomass. During most of the year, roving herbivores aggregate in the shallow reef zones and their biomass declines with depth. The herbivore community on the reef flat is distinct from that in deeper zones. The former is characterized by Siganus rivulatus, Acanthurus gahhm and Hipposcarus harid, while the deeper reef zones are characterized by S. ferrugineus, Chlorurus sordidus and Ctenochaetus striatus. In summer, the distinct community structures among reef zones are lost as reef flat herbivores tend to exploit deeper reef zones and some reef crest species venture on to the reef flat. This summer change in herbivore distribution is also reflected in reduced turf biomass and increased yield to herbivores in the deeper reef zones. Habitat use is related to the feeding mode such that browsers dominate the reef flat and scrapers the reef crest, while the seasonal changes correspond to changes in availability of targeted algal resources. These seasonal changes appear to be driven by the extreme temperatures in summer, reaching 36 °C on the shallow reef flat.     

Impaired growth and survival of tropical macroalgae (Sargassum spp.) at elevated temperatures

Increasing ocean temperatures associated with ongoing climate change have resulted in regional reductions in the cover of live coral and increasing concerns that coral reefs will be overgrown by macroalgae. The likelihood of macroalgal overgrowth will, however, depend on the thermal sensitivities of the macroalgae themselves. We exposed recently settled propagules of the common canopy-forming macroalga Sargassum swartzii and adult thalli of three species of Sargassum (S. swatzii, S. cristaefolium, S. polycystum) to three experimental temperatures: ambient, + 2 °C, and + 3.5 °C, reflective of summer minimum, mean, and maximum temperatures for the region. Growth and survival of Sargassum swartzii propagules were assessed over 48 days, and the growth, physical toughness, elemental composition, and susceptibility to herbivory of adult thalli were assessed after short-term exposure (2-weeks) to experimental temperatures. Growth and survival of S. swartzii propagules were reduced by 43% and 84%, respectively, when cultured at the elevated (+ 3.5 °C) temperature compared to ambient temperature. Similarly, elevated temperature resulted in a 17–49% decline in the growth of adult Sargassum thalli relative to controls. Susceptability of S. swartzii and S. cristaefolium to herbivory (i.e. mass removed by herbivores) was 50% less for thalli cultured at elevated (+ 3.5 °C) compared to ambient temperature, but this pattern was not related to changes in the physical or chemical properties of the thalli as a result of elevated temperature. The negative effects of elevated temperatures on the growth and survival of both Sargassum propagules and adult thalli will likely restrict the capacity of Sargassum, and potentially other macroalgae, to establish in new areas, and may also threaten the persistence of existing macroalgal meadows under future ocean temperatures. The thermal sensitivities of tropical Sargassum, together with those of corals, suggest ongoing ocean warming may lead to novel reef ecosystems that are low in both coral cover and macroalgal cover.

     

Disturbance and reef topography maintain high local diversity in Ecklonia radiata kelp forests

Disturbance of competitive‐dominant plant and algae canopies often lead to increased diversity of the assemblage. Kelp forests, particularly those of temperate Western Australia, are habitats with high alpha diversity. This study investigated the roles of broad‐scale canopy loss and local scale reef topography on structuring the kelp‐dominated macroalgal forests in Western Australia. Eighteen 314 m² circular areas were cleared of their Ecklonia radiata canopy and eighteen controls were established across three locations. The patterns of macroalgal recolonisation in replicate clearances were observed over a 34 month period. Macroalgal species richness initially increased after canopy removal with a turf of filamentous and foliose macroalgae dominating cleared areas for up to seven months. A dense Sargassum canopy dominated cleared areas from 11 to 22 months. By 34 months, partial recovery of the kelp canopy into cleared areas had occurred. Some cleared areas did not follow this trajectory but remained dominated by turfing, foliose and filamentous algae. As kelp canopies developed, the initial high species diversity declined but still remained elevated relative to undisturbed controls, even after 34 months. More complex reef topography was associated with greater variability in the algal assemblage between replicate quadrats suggesting colonising algae had a greater choice of microhabitats available to them on topographically complex reefs. Shading by canopies of either Sargassum spp. and E. radiata are proposed to highly influence the abundance of algae through competitive exclusion that is relaxed by disturbance of the canopy. Disturbance of the canopy in E. radiata kelp forests created a mosaic of different patch types (turf, Sargassum‐dominated, kelp‐dominated). These patch types were both transient and stable over the 34 months of this study, and are a potential contemporary process that maintains high species diversity in temperate kelp‐dominated reefs.     

Age structure of massive Porites lutea corals at Luhuitou fringing reef (northern South China Sea) indicates recovery following severe anthropogenic disturbance

Luhuitou fringing reef at Hainan Island (northern South China Sea) has experienced severe anthropogenic disturbance, with live coral cover declining by > 80 % since the 1960 s. To assess the size structure of Porites lutea, we measured the sizes of 1,857 colonies from the reef flat (0 m) and slope (2–4 m depth). Both populations were positively skewed and leptokurtic in shape, indicating a high abundance of smaller colonies (averaging 21.4 ± 2.3 cm on the flat and 31.9 ± 2.8 cm on the slope). Age structure of populations was determined through growth rates extracted from X-rays of P. lutea cores. The majority of colonies (> 95 %) were < 50 yr old, with 55 % of P. lutea on the reef flat having recruited following the establishment of a marine reserve in 1990. The abundance of younger colonies indicates significant recovery of P. lutea following the removal of chronic anthropogenic disturbance.     

Comparative efficiency of algal biofilters in the removal of chromium and copper from wastewater

The performance of a macroalgae (Sargassum sp.), a laboratory-cultivated microalgae (Chlorococcum sp.) and a commercially available granulated activated carbon (GAC) for the removal of copper (Cu) and chromium (Cr) from aqueous solutions was evaluated using batch experiments. Kinetic and isotherm experiments were done at the optimal pH of 4.5 ± 0.1 for Cu (II) and 2.0 ± 0.1 for Cr (total). The equilibrium isotherms were determined and the results were analyzed using the Langmuir and Freundlich models. The best Cu removal performance was observed on Sargassum at a maximum removal of 87.3% obtained for an initial concentration of 20 mg L^?1 Cu. The maximum uptake capacities for Cu (II) were 71.4, 19.3 and 11.4 mg g^?1 of Sargassum, Chlorococcum and GAC, respectively. The biosorbents were also able to remove appreciable amounts of Cr, again with Sargassum showing maximum uptake capacity over the other materials. Kinetic studies also reveal that the removal rate is faster for both metals in Sargassum. Tests with an actual wastewater confirm the maximum uptake capacity of Cu by Sargassum. In all experiments the Sargassum biofilter outperformed GAC, which makes it a promising low-cost alternative to conventional filtration materials for wastewater treatment.     

Tracking and mapping sun-synchronous migrations and diel space use patterns of Haemulon sciurus and Lutjanus apodus in the U.S. Virgin Islands

The spatially explicit diel movement patterns of fish using coral reef ecosystems are not well understood, despite the widespread recognition that many common species undergo distinct migrations to utilize different resources during night and day. We used manual acoustic telemetry coupled with global positioning technology to track the detailed spatially explicit daily movements (24 h) of multiple individuals of two common Caribbean fish species, Haemulon sciurus (bluestriped grunt) and Lutjanus apodus (schoolmaster snapper). Movement pathways and day and night activity spaces were mapped and quantified in a Geographic Information System (GIS). Directional sun-synchronous migrations occurred close to astronomical sunset and sunrise. Site fidelity within day and night activity spaces was high. Nine of twelve individuals exhibited overlap of day and night activity spaces and three fish (L. apodus) exhibited complete spatial segregation. Night activity spaces (H. sciurus: 11,309?±?3,548 m²; L. apodus: 9,950?±?3,120 m²) were significantly larger than day activity spaces (H. sciurus: 2,778?±?1,979 m²; L. apodus: 1,291?±?636 m²). The distance between sequential position fixes (step lengths) was significantly greater at night than day, indicative of nocturnal foraging and day resting behavior. Integrating acoustic telemetry, GIS techniques and spatial statistics to study fish movement behavior revealed both individual variability and some broader generality in movement paths and activity spaces suggestive of complex underlying behavioral mechanisms influencing diel movements.     

Spatio-temporal distribution patterns of the invasive macroalga Sargassum muticum within a Danish Sargassum-bed

Sargassum muticum was first observed in Scandinavia in Limfjorden (Denmark) in 1984, where it is now the most abundant and conspicuous macroalga. Despite the ecological importance of Sargassum, few studies have described seasonal patterns within Scandinavian Sargassum beds. We quantified the dynamics of macroalgae among years and seasons along a depth transect through a typical Sargassum bed in Limfjorden. The annual investigations (summer transects 1989–1999) showed a gradual increase in the dominance of Sargassum, especially at the 2–4-m depth interval. Significant seasonal dynamics in macroalgal abundance and assemblage structure were observed in this depth interval; the mean cover of Sargassum varied from ca. 5% (autumn and winter) to 25% (mid-summer). In comparison, encrusting algae had high and relatively stable covers throughout the year (ca. 20%). Other perennial macroalgae had low mean covers (<2%) characterized by a few patches of higher abundances. Except from a spring bloom, filamentous algae had low covers throughout the year. Within this relatively uniform bed, Sargassum abundance was positively related to boulders >10 cm in diameter and species richness was negatively correlated to depth and stones <10 cm in diameter, and non-correlated to other algal form-groups or grazer densities. Thus, in Limfjorden, the distribution of Sargassum is determined by large- (>6 m) and small-scale (<1 m) depth differences where low light limits Sargassum at depth, physical disturbance and sediment stress limits Sargasum in shallow waters, and the presence of stable boulder substratum facilitate Sargassum. Competition for space from other macroalgae and herbivory are probably of minor importance.