Irradiance and nutrient limitation of Dicytota spp. populations on Conch Reef, Florida Keys, USA

The photosynthetic performance, pigmentation and response to nutrient enrichment of a Dictyota community were studied over a 32 m depth gradient on Conch Reef, Florida Keys, USA. Dictyota spp. was the dominant space occupier on Conch Reef. During summer months from 1994 to 2001, mean percent cover was 43% at 7 and 21 m depths. Percent cover of Dictyota spp. was markedly lower at 32 m relative to shallower sites. The metabolism of Dictyota menstrualis and Dictyota pulchella were negatively impacted by attenuation of irradiance with increased depth such that ETR_max and P/R ratio decreased by 85% and 47%, respectively in samples from 7 to 32 m. Decreased cover of Dicytota spp. at 32 m relative to shallower sites may be the result of the inability of this species complex to acclimate to low irradiance levels as indicated by a lack of change in α and photosynthetic pigment content with increased depth.The response of D. menstrualis populations on Conch Reef to nutrient enrichment was variable. During August 2000, a natural enrichment experiment was conducted as D. menstrualis photosynthetic performance was surveyed both during and for days after a period of cool, nutrient-rich internal tidal bores bathed the 21 m site. No changes in in situ photosynthetic performance were observed either during or for 2 to 4 days after this natural event suggesting no nutrient limitation. In August 2001, a manipulative nutrient enrichment experiment was conducted with D. menstrualis from 7, 21 and 32 m. Increases in Φ_II were found in individuals exposed to nutrient enrichment from all depths, indicating at that time D. menstrualis was nutrient-limited on Conch Reef. Variation in the physiological response to nutrient enrichment may be the result of the frequency of internal tidal bores on Conch Reef in the months prior to our experiments. Variation in the responses by Dictyota spp. to irradiance and nutrient enrichment suggests that long-term monitoring over relevant temporal and spatial scales is necessary to accurately characterize limits on productivity and spread of this weedy species assemblage.     

Asexual propagation in the coral reef macroalga Halimeda (Chlorophyta, Bryopsidales): production, dispersal and attachment of small fragments

Siphonous, green macroalgae of the genus Halimeda are ubiquitous and ecologically important in tropical and subtropical marine environments. It has been hypothesized that the abundance of Halimeda on coral reefs is in part due to the ability of this genus to propagate asexually via vegetative fragmentation. However, vegetative fragmentation has only been documented for H. discoidea in a laboratory setting. To test the hypothesis that vegetative fragmentation contributes to field populations of Halimeda, we examined three aspects of fragmentation by H. tuna (Ellis and Solander) Lamouroux, H. opuntia (Linneaus) Lamouroux and H. goreaui Taylor on Conch Reef in the Florida Keys: (1) short-term (8 days) and long-term (14 weeks) fragment survival and rhizoid production in the laboratory and field (7 and 21 m), (2) size of the fragment pool and (3) influences of herbivory and water motion on production and dispersal of fragments. Although morphologically similar to H. discoidea, only a small percentage of H. tuna fragments survived. Fragments of H. opuntia and H. goreaui were more robust, and survival and rhizoid production were positively correlated with size in short-term trials. In 14-week field trials, one-third or fewer fragments of any species survived at 7 m, potentially because fragments were covered by large amounts of sediment. Survivors included some buried, seemingly dead individuals that turned green when exposed to light, highlighting the remarkable ability of this genus to survive disturbances. There was much less sediment accumulation at 21 m, where more fragments survived. Most (93%) eight-segment fragments of H. opuntia produced attachment rhizoids by the end of the 14-week trial. Overall, a range of 4.7-9.4 fragments of Halimeda m⁻² day⁻¹ were found on Conch Reef; most fragments were generated by H. goreaui. Fish bite marks were evident on 75-85% of the individuals of H. tuna and the number of bites per thallus ranged from 1 to 23. Herbivorous reef fish commonly fed on all three species of Halimeda. Some fish consumed the biomass, while others rejected most bites. For example, 83% of bites were rejected by the blue-striped grunt. Dispersal distances for rejected bites ranged from 0 to 31 m. Water motion was also responsible for fragment dispersal; experimentally produced fragments moved up to 48 cm day⁻¹. Results presented here suggest that asexual propagation of fragments of Halimeda is an important component of the life-history of this genus and vegetative fragmentation contributes to the abundance of this genus on coral reefs.     

VARIABILITY IN THE ECOPHYSIOLOGY OF HALIMEDA SPP. (CHLOROPHYTA,BRYOPSIDALES) ON CONCH REEF,FLORIDA KEYS,USA1

The photosynthetic performance, pigmentation, and growth of a Halimeda community were studied over a depth gradient on Conch Reef, Florida Keys, USA during summer–fall periods of 5 consecutive years. The physiology and growth of H. tuna (Ellis & Solander) Lamouroux and H. opuntia (L.) Lamouroux on this algal dominated reef were highly variable. Maximum rate of net photosynthesis (P_max), respiration rate, and quantum efficiency (α) did not differ between populations of either species at 7 versus 21 m, even though the 21‐m site received a 66% lower photon flux density (PFD). Physiological parameters, as well as levels of photosynthetic pigments, varied temporally. P_max, saturation irradiance, compensation irradiance, and growth were greatest in summer months, whereas α, chl a, chl b, and carotenoid concentrations were elevated each fall. Halimeda tuna growth rates were higher at 7 m compared with 21 m for only two of five growth trials. This may have arisen from variability in light and nutrient availability. Individuals growing at 7 m received a 29% greater PFD in August 2001 than in 1999. In August 1999 and 2001 seawater temperatures were uniform over the 14‐m gradient, whereas in August 2000 cold water regularly intruded upon the 21‐m but not the 7‐m site. These results illustrate the potentially dynamic relationship between nutrients, irradiance, and algal productivity. This suggests the necessity of long‐term monitoring over spatial and temporal gradients to accurately characterize factors that impact productivity.     

Two species of Halimeda, a calcifying genus of tropical macroalgae, are robust to epiphytism by cyanobacteria

Cyanobacteria, an increasingly important epiphyte on macroalgae and seagrass, have been shown to have strong effects on its hosts; this association has been identified as a driving mechanism that maintains algal blooms on coral reefs. We examined both the costs and benefits of epiphytism on 2 algal congeners of Halimeda (H. tuna and H. opuntia), both of which are abundant members of tropical reef communities in the Caribbean. To evaluate potential benefits of an associational defense as well as costs to growth, we manipulated herbivore access to (uncaged/caged) and cyanobacteria presence on (epiphytized/cleaned) 2 species of Halimeda on shallow patch reefs in Belize and measured change in branch length and segment number after 10 (H. tuna) and 5 (H. opuntia) days. Cyanobacterial epiphytes did not serve as an associational defense from herbivory as there were no differences between caged and uncaged treatments for either response variable. The presence of cyanobacterial epiphytes did not affect the growth of branches or net generation of new segments, demonstrating there was also no cost to growth. The robustness of both species of Halimeda to epiphytism contrasts strongly with recent research that found strong effects of epiphytes on several other species of tropical algae. Our results may be attributed to the unique characteristics of Halimeda, a heavily physically and chemically defended algal genus, and the shallow nature of the patch reefs reducing the potential for significant light limitation. These findings suggest that close interactions such as epiphytism may not be as generalizable as originally assumed; studies must consider differences among host species, as this may lead to a better understanding of community-wide effects.     

Halimeda biomass,growth rates and sediment generation on reefs in the central great barrier reef province

The average biomass ofHalimeda per m² of solid substratum increased progressively on a series of reefs situated at increasing distances from the shore in the central Great Barrier Reef. There was none on a reef close inshore, increasing to nearly 500 g m^?2 total biomass (?90% calcium carbonate) on an oceanic atoll system in the Coral Sea. The biomass measured contained 13 species ofHalimeda but was dominated by only two species,H. copiosa andH. opuntia, except on the atoll whereH. minima was dominant. Three sand-dwelling species were also present but did not occur anywhere in substantial quantities. Growth rates of the dominant species were measured bv tagging individual branch tips. A mean value of 0.16 segments d^?1 was recorded but 41% of the branch tips did not grow any new segments whilst only 1% grew more than one per day. The number of branch tips per unit biomass was very constant and has been used in conjunction with growth rates and biomass to calculate productivity rates, and thence sedimentation, in the lagoon of one of the reefs. Biomass doubling time of 15 d and production of 6.9 g dry wt m^?2 d^?1 are considerably higher than previously reported values forHalimeda vegetation and there was little seasonal change detected over a whole year. Those values indicate annual accretion of 184.9 g m^?2 year^?1 ofHalimeda segment debris over the entire lagoon floor (5.9 km²) of Davies Reef, equivalent to 0.13 mm year^?1 due toHalimeda alone, or 1 m every 1,892 years when other contributions to that sediment are taken into account.     

Field biology of Halimeda tuna (Bryopsidales, Chlorophyta) across a depth gradient: comparative growth, survivorship, recruitment, and reproduction

Growth, survivorship, recruitment, and reproduction of Halimeda tuna, a dominant green alga in many reef systems of the Florida Keys, were monitored at a shallow back reef (4–7m) and deep reef slope (15–22 m) on Conch Reef. Despite lower light intensities and similar grazing pressures, amphipod infestations, and epiphyte loads at both sites, the deeper site exhibited significantly higher growth rates in summer months over a 4-year period than found for the shallow population, possibly because of higher nutrient levels at depth and photoinhibition of shallow plants. Sexual reproductive events occurred simultaneously across the entire reef, with up to 5% of the population at both sites developing gametangia. New upright axes formed from zygotes, asexual fragmentation, or vegetative runners. Plants appear to have persistent basal stumps that survive harsh environmental conditions, even if upright, photosynthetic axes are removed. Sexual reproduction and smothering by epiphyte overgrowth are hypothesized to be two causes of death for individuals.     

Photoinhibition of photosynthesis on a coral reef

Photoinhibition of macroalgae in the epilithic algal community (KAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. F_v/F_m (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low-light habitats were more photoinhibited (2h at 1400μimol m^?2 s^?1) than those from high-light habitats. The sensitivity of Lobophora variegata (Phacophyta) and Chlorodesmis fastigiata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas. Photoinhibition and recovery were depth- and species-specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of Q _A oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non-photochemical fluorescence quenching (NPQ), but in Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.     

Interaction between benthic algae (Lyngbya bouillonii, Dictyota dichotoma) and scleractinian coral Porites lutea in direct contact

Competition between the massive scleractinian coral Porites lutea and two benthic algal species, thin-filamentous blue-green Lyngbya bouillonii (Cyanophyta) commonly observed growing as mats and fleshy brown Dictyota dichotoma (Phaeophyta), was investigated. Experiments were designed to expose coral fragments to different treatments to test the role of abrasion, shading and allelopathy by Lyngbya and Dictyota on coral growth and physiology in direct physical contact. The growth rates of coral fragments were significantly lower in both the algal/coral and the net control (only plastic net touched the corals) treatments than in the manipulation control (contact with algae and plastic net was prevented), demonstrating the importance of abrasion in Porites-Lyngbya and Porites-Dictyota interactions. Furthermore, coral fragments exposed to Lyngbya grew significantly slowly than net controls, but this effect was not statistically significant for P. lutea maintained in contact with Dictyota. Light levels were reduced equally in the algal/coral and shading mimic (plastic net touched the corals shaded with neutral-density filters) treatments. However, there were no significant differences in the growth rates between the shading mimic and the net control treatments, suggesting that shading had no measurable effect on coral growth. Thus, the growth of P. lutea in contact with Dictyota was reduced by abrasion whereas in direct contact with L. bouillonii, abrasion was supplemented by additional factors unique to Lyngbya in mediating coral-algal competition. Physical contact with L. bouillonii induced a significant reduction in photochemical efficiency (F_v / F_m) of PSII and chlorophyll concentration of in hospite zooxanthellae of P. lutea fragments, as well as a decrease of the symbiotic dinoflagellate density. Analysis of the growth rate and F_v / F_m of the investigated algae revealed a significant reduction in both parameters for D. dichotoma but not for L. bouillonii when in direct contact with P. lutea fragments. Thus, the competitive inhibition by the coral P. lutea and the brown alga D. dichotoma was mutual, while L. bouillonii acted as a one-sided inhibitor for scleractinian corals inducing bleaching and severe damage of live coral tissue. The fact that mats-forming blue-green alga L. bouillonii acts as a poison for scleractinian corals and is able to kill live coral tissue is reported for the first time. Allelochemical mechanism of the effect on live coral tissue by this alga is suggested. Possible mechanisms of competitive interactions for substrate between the coral polyps of scleractinians and algal species investigated are discussed.     

The ploys of sex: relationships among the mode of reproduction,body size and habitats of coral-reef brittlestars

Observations were made of 33 species of brittlestars (3980 specimens) from specific substrata collected in four zones on the Belize Barrier Reef, Caribbean Sea. The body size of most species of brittlestars with planktonic larvae differs significantly among different substrata. Generally, individuals from the calcareous alga Halimeda opuntia are smallest, those found in corals (Porites porites, Madracis mirabilis, and Agaricia tenuifolia) are larger, and those from coral rubble are the largest. This suggests that brittlestars with planktonic larvae move to new microhabitats as they grow. In contrast, most brooding and fissiparous species are relatively small and their size-distributions are similar among all substrata. Halimeda harbours denser concentrations of brittlestars and more small and juvenile individuals than the other substrata. Juveniles of the brooding and fissiparous species are most common in Halimeda on the Back Reef whereas juveniles developing from planktonic larvae are most common in Halimeda patches in deeper water. Fissiparity and brooding may be means for individuals (genomes) of small, apomictic species to reach large size (and correspondingly high fecundities) in patchy microhabitats that select for small body sizes. Small brittlestar species and juveniles are most numerous in the microhabitats called refuge-substrata, such as Halimeda, which may repel predators and reduce environmental stress. Whether young brittlestars are concentrated in refuge-substrata through settlement behavior, migration, or differential survival remains unknown. Experiments revealed that coral polyps kill small brittlestars, perhaps accounting for the rarity of small and juvenile brittlestars in coral substrata.     

Macroalgal Extracts Induce Bacterial Assemblage Shifts and Sublethal Tissue Stress in Caribbean Corals

Benthic macroalgae can be abundant on present-day coral reefs, especially where rates of herbivory are low and/or dissolved nutrients are high. This study investigated the impact of macroalgal extracts on both coral-associated bacterial assemblages and sublethal stress response of corals. Crude extracts and live algal thalli from common Caribbean macroalgae were applied onto the surface of Montastraea faveolata and Porites astreoides corals on reefs in both Florida and Belize. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene amplicons was used to examine changes in the surface mucus layer (SML) bacteria in both coral species. Some of the extracts and live algae induced detectable shifts in coral-associated bacterial assemblages. However, one aqueous extract caused the bacterial assemblages to shift to an entirely new state (Lobophora variegata), whereas other organic extracts had little to no impact (e.g. Dictyota sp.). Macroalgal extracts more frequently induced sublethal stress responses in M. faveolata than in P. astreoides corals, suggesting that cellular integrity can be negatively impacted in selected corals when comparing co-occurring species. As modern reefs experience phase-shifts to a higher abundance of macroalgae with potent chemical defenses, these macroalgae are likely impacting the composition of microbial assemblages associated with corals and affecting overall reef health in unpredicted and unprecedented ways.     

The deep-water species of Halimeda Lamouroux (Halimedaceae,Chlorophyta) from San Salvador Island,Bahamas: species composition,distribution and depth records

Caribbean species of Halimeda from the steep slopes of San Salvador Island, Bahamas, were collected along vertical transects between 25 and 255 m, on the northeast, northwest, west, southwest and south sides of the island, using the Harbor Branch Oceanographic Institution's submersible Johnson Sea Link I. The characteristics delineating species (segment and utricle size) were assessed for selected species, and the depth and western Atlantic distributions of the species reviewed. Halimeda copiosa was found to show a variation of surface utricle diameter with depth, with the deeper plants having 15% larger diameter utricles than the shallower plants. Of the seven species, one variety and one form of Halimeda studied from our submersible dive sites, H. copiosa, H. cryptica and H. gracilis grew to the greatest depths, from 120 to 150-m depths. H. lacrimosa var. globosa was found growing to 91 m, and H. tuna f. platydisca and H. discoidea to a depth of 73 m. Those of lesser depth, to 61 m, were H. goreauii, H. tuna f. tuna and H. lacrimosa var. lacrimosa. These represent substantial increases in depth distribution of the nine taxa found.     

Species diversity and distribution of the calcareous green macroalgae Halimeda in Taiwan,Spratly Island,and Dongsha Atoll,with the proposal of Halimeda taiwanensis sp. nov.

The calcified green algal genus Halimeda is one of the most ecologically important but morphologically diverse seaweeds in sub-tropical and tropical waters. Because of its high morphological plasticity, the identification of Halimeda species based on morphological characters is challenging without the assistance of molecular analysis. To date, the species diversity of Halimeda in Taiwan and its overseas territories has not been investigated with the assistance of DNA sequencing, and this taxonomic knowledge gap should be filled. The present study initiates a systematic examination of the species diversity and distribution of Halimeda in Taiwan, Spratly Island, and Dongsha Atoll in the South China Sea, using DNA sequence data (plastid tufA gene and rbcL) and morphological data. Our DNA analyses revealed the presence of 10 Halimeda species (Halimeda borneensis, Halimeda cylindracea, Halimeda discoidea, Halimeda distorta, Halimeda macroloba, Halimeda minima, Halimeda opuntia, Halimeda renschii, Halimeda taiwanensis sp. nov., and Halimeda velasquezii) in the waters around Taiwan, Spratly Island, and Dongsha Atoll. The majority of the species could be readily distinguished by their morphological and anatomical characters. The proposed new species, H. taiwanensis, was differentiated not only by our algorithmic species delimitation analyses (statistical parsimony network analysis and automated barcode gap discovery), but also by its morphological features. The proposed new species differs from two externally resembled species, H. cuneata and H. discoidea, in having an undulated segment margin, the complete fusion of medullary siphons at the node, the lack of segment stalk, and the presence of a large primary utricle. Here, we present the up-to-date taxonomic account, molecular diversity, and geographical distribution of Halimeda spp. in Taiwan and associated areas of the South China Sea. Environmental factors that might drive the occurrence and latitudinal distribution of the species are also discussed.     

Trace and minor element ratios inHalimeda aragonite from the Great Barrier Reef

The calcareous green algaHalimeda can be a substantial contributor to aragonite sediment in reef ecosystems. In contrast to coral aragonite, little is known about the trace and minor element composition ofHalimeda aragonite, so it is difficult to test oceanographic hypotheses about factors controlling its past growth. We investigated adapting trace element cleaning protocols for modern and HoloceneHalimeda aragonite, modern and HoloceneHalimeda trace and minor element compositions, and the potential utility ofHalimeda aragonite for paleoceanographic investigations. We successfully adapted and applied sample treatment protocols developed for measuring trace elements in coral aragonite (generally less than 500 y old) toHalimeda aragonite (modern to approximately 5000 y old in this study). ModernHalimeda aragonite from John Brewer Reef in the Central GBR had mean Cd/Ca ratios of 5.19 ± 1.68 nmol/mol forHalimeda micronesica and 2.35 ± 0.38 nmol/mol for three closely related species important in bioherm accumulationHalimeda copiosa, Halimeda hederacea, andHalimeda opuntia. Mn/Ca ratios, with means from 89–239 nmol/mol for these four species, showed both intra-and inter-specific variability. Sr/Ca ratios (10.9 ± O.1 mmol/mol) and Mg/Ca ratios (1.35 ± 0.26 mmol/mol) were similar for all samples. HoloceneHalimeda aragonite samples from cores of two bioherms in the northern GBR seemed well preserved on the basis of mineralogy and Sr/Ca and Mg/Ca ratios similar to those in modernHalimeda aragonite. Cd/Ca ratios (overall mean 0.96 ± 0.15 nmol/mol) were lower than those measured in the modernHalimeda from the central GBR location. However, Mn/Ca ratios in both cores were substantially higher than in modernHalimeda aragonite. While it may be possible to extract paleoceanographic information fromHalimeda aragonite, substantial care is needed to evaluate and avoid the effects of post-depositional alteration.     

Fragment generation, survival, and attachment of Dictyota spp. at Conch Reef in the Florida Keys, USA

During the past decade, the relative abundance of the brown macroalgae Dictyota spp. has been high in the Florida Keys. Recent studies have shown that members of this genus successfully reproduce via vegetative fragmentation. To investigate the importance of fragmentation on the reef community, this study examined: (1) the degree of epiphytism on benthic organisms, (2) the rate of fragment production through fish foraging activities, (3) the likelihood of fragment entanglement, and (4) the fragment attachment and success rate. It was found that reef fish contributed substantially to the fragment pool; furthermore, most fish-produced fragments produced rhizoids and attached to sand grains within 24 h in the field. Fragments of Dictyota spp. most commonly became entangled around and then attached themselves to the green alga Halimeda tuna, and other Dictyota spp. These results suggest that vegetative fragmentation of Dictyota spp. plays an important role in the changing community structure on the Florida Keys reef tract.     

Calcification in the Green Alga Halimeda: IV. THE ACTION OF METABOLIC INHIBITORS ON PHOTOSYNTHESIS AND CALCIFICATION

The effects of a number of metabolic inhibitors on calcificationand photosynthesis in Halimeda tuna, H. discoidea, and H. macrolobaare described. The inhibitors used are CCCP, DNP, DCMU, azide,cyanide, chloramphenicol, cycloheximide, and Diamox. The effectsof these inhibitors, although complex, are consistent with ourmodel of calcification in Halimeda. Inhibition of photosyntheticCO₂ uptake inhibits calcification as does stimulation of respiratoryCO₂ evolution (i.e. uncoupling). There is also indirect evidencefor the presence of a possible light stimulated H⁺ efflux whichinhibits calcification. The observed calcification rate is thereforethe result of a number of factors which affect the concentrationof CO^–and the pH in the intercellular space of the Halimedathallus. The results obtained with the carbonic anhydrase inhibitor Diamoxprovide further evidence for the effective separation of theintercellular space from the external medium by the appressedperipheral utricles.     

Competition between macroalgae and corals: effects of herbivore exclusion and increased algal biomass on coral survivorship and growth

Recent declines in coral abundance accompanied by increases in macroalgal cover on Florida reefs highlight the importance of competition for space between these groups. This paper documents the frequency of coral-algal interactions on the Northern Florida Reef Tract and evaluates the effects of grazer exclusions and experimental algal addition on growth and tissue mortality of three coral species, Siderastrea siderea, Porites astreoides, and Montastraea faveolata. The frequency of interactions between corals and macroalgae was high as more than 50% of the basal perimeter of colonies was in contact with macroalgae; turf forms, Halimeda spp., and Dictyota spp. were the most common groups in contact with corals. Decreased grazing pressure resulted in significant increases in algal biomass within cages, and caged corals showed species-specific susceptibility to increased algal biomass. While no effects were detected for S. siderea, significant decreases in growth rates were documented for caged P. astreoides which had growth rates three to four times lower than uncaged colonies. When an algal addition treatment was included to duplicate maximum algal biomass levels documented for reefs in the area, colonies of P. astreoides in the algal addition treatment had growth rates up to ten times lower than uncaged colonies. High susceptibility to algal overgrowth was also found for the reef-building coral M. faveolata, which experienced significant tissue mortality under both uncaged (5.2% decrease in live tissue area per month) and caged (10.2% per month) conditions. The documented effects of increased algal biomass on coral growth and tissue mortality suggest a potential threat for the long-term survivorship and growth of corals in the Florida Reef Tract if present rates of algal growth and space utilization are maintained.     

Assessing the use of artificial substrates to monitor Gambierdiscus populations in the Florida Keys

Four distinct coastal locations were sampled on a monthly basis near Long Key (Florida Keys, USA) over a 13-month period to study Gambierdiscus population dynamics on different substrates, including four macrophyte species (Dictyota spp., Halimeda spp., Laurencia spp., and Thalassia testudinum) and three artificial substrates (polyvinyl chloride (PVC) tiles, burlap, and fiberglass window screen). Cell densities of Gambierdiscus were generally lower on Dictyota versus Halimeda and Laurencia. Cell densities of Gambierdiscus were significantly correlated among macrophyte hosts in 54% of the comparisons, and between macrophyte hosts and artificial substrates in 72% of the comparisons. Predictive slopes determined from regression analyses between cell densities on artificial substrates and macrophyte hosts indicated that, on an areal basis, fewer cells were present on macrophytes versus artificial substrates (cells cm⁻²) and that slope variation (error) among the different macrophytes and sites ranged from 5% to 200%, averaging 61% overall. As the data required log-transformation prior to analyses, this level of error translates into two-orders of magnitude in range of estimation of the overall average abundance of Gambierdiscus cells on macrophytes (135 cells g⁻¹ wet weight); 20–2690 cells g⁻¹ ww. The lack of consistent correlation among Gambierdiscus cell densities on macrophytes versus artificial substrates, coupled with the high level of error associated with the predictive slope estimations, indicates that extreme caution should be taken when interpreting the data garnered from artificial substrate deployments, and that such deployments should be thoroughly vetted prior to routine use for monitoring purposes.     

Growth rate,ultrastructure and sediment contribution of Halimeda incrassata and Halimeda monile,Nonsuch and Falmouth Bays,Antigua, W.I.

Halimeda incrassata and Halimeda monile, the two dominant rhipsalian Halimeda, were evaluated behind a bank barrier reef, in a fringing reef lagoon and in an open lagoon. Growth was calculated in number of segments, weight of segments and turnover rate. More than 1800 plants were stained with Alizarin Red-S dye, yielding average number of segments/plant/day and g CaCO₃/m²/year for each of the above areas of 2.17/114, 1.43/65.7 and 1.6/56.9, respectively. Average weight CaCO₃/segment was 4 mg. SEM revealed ultrastructure of short and long unoriented aragonite crystals forming in new segments within 24 h and an effective holdfast system with filaments partially coated with carbonate fragments. Greatest growth occurred within thin to medium density grass beds. In Nonsuch Bay sediment production from these two species alone was 0.057 mm/year or 1 1/2 orders of magnitude less than estimates of the total production from all Halimeda species (1.01 mm/year) over the past 6745 years.     

Microenvironmental changes support evidence of photosynthesis and calcification inhibition in Halimeda under ocean acidification and warming

The effects of elevated CO₂ and temperature on photosynthesis and calcification of two important calcifying reef algae (Halimeda macroloba and Halimeda cylindracea) were investigated with O₂ microsensors and chlorophyll a fluorometry through a combination of two pCO₂ (400 and 1,200 μatm) and two temperature treatments (28 and 32 °C) equivalent to the present and predicted conditions during the 2100 austral summer. Combined exposure to pCO₂ and elevated temperature impaired calcification and photosynthesis in the two Halimeda species due to changes in the microenvironment around the algal segments and a reduction in physiological performance. There were no significant changes in controls over the 5-week experiment, but there was a 50–70 % decrease in photochemical efficiency (maximum quantum yield), a 70–80 % decrease in O₂ production and a threefold reduction in calcification rate in the elevated CO₂ and high temperature treatment. Calcification in these species is closely coupled with photosynthesis, such that a decrease in photosynthetic efficiency leads to a decrease in calcification. Although pH seems to be the main factor affecting Halimeda species, heat stress also has an impact on their photosystem II photochemical efficiency. There was a strong combined effect of elevated CO₂ and temperature in both species, where exposure to elevated CO₂ or temperature alone decreased photosynthesis and calcification, but exposure to both elevated CO₂ and temperature caused a greater decline in photosynthesis and calcification than in each stress individually. Our study shows that ocean acidification and ocean warming are drivers of calcification and photosynthesis inhibition in Halimeda. Predicted climate change scenarios for 2100 would therefore severely affect the fitness of Halimeda, which can result in a strongly reduced production of carbonate sediments on coral reefs under such changed climate conditions.