Significance of <Emphasis Type="Italic">Halimeda</Emphasis> bioherms to the global carbonate budget based on a geological sediment budget for the Northern Great Barrier Reef,Australia

Since the correlation between carbon dioxide (CO₂) levels and global temperatures was established in the ice core records, quantifying the components of the global carbon cycle has become a priority with a view to constraining models of the climate system. The marine carbonate budget is still not adequately constrained and the quantitative significance of the calcareous green alga Halimeda still remains particularly poorly understood. Previously, it has been suggested that Halimeda bioherms on the shelf of the Great Barrier Reef may contain a volume of carbonate equal to or greater than that contained within the shelf edge coral reefs. This study uses published datasets to test this hypothesis in the Northern Great Barrier Reef (NGBR) province. It is estimated that Halimeda bioherms on the outer shelf of the NGBR contain at least as much (and up to four times more) CaCO₃ sediment as the adjacent ribbon reef facies. Globally, if these findings are even only partially applicable, the contribution of shallow water carbonate sediments to the global carbon budget based on coral reefs alone is currently substantially underestimated.     

The Messinian reef complex of the Salento Peninsula (southern Italy): Stratigraphy, facies and paleoenvironmental interpretation

Summary An integrated study of the early Messinian reef complex cropping out along the eastern coast of the Salento Peninsula (southern Italy), including stratigraphy, facies analysis and paleoecological aspects, is here presented. Fourteen facies types belonging to three main facies associations (back reef and shelf, shelf-edge, slope) have been recognized. They document a wide spectrum of depositional environments, reef building organisms and growth fabrics, in response to depth and other environmental factors in different parts of the reef complex. The biotic structure of the reef is also described and discussed in detail. It consists of different types of reef building organisms and of their bioconstructions (mainlyPorites coral reefs,Halimeda bioherms and vermetidmicrobial “trottoirs”), that differ in composition and structure according to their position on the shelf edge-toslope profile. Results indicate that the reef complex of the Salento Peninsula has strong similarities with the typical early Messinian reefs of the Mediterranean region. However, the recognition of some peculiar features, i.e. the remarkable occurrence ofHalimeda bioherms and of vermetid-microbial “trottoirs”, gives new insights for better understanding reef patterns and development of the reef belt during the Late Miocene in the Mediterranean.     

Sedimentary environments of the Central Region of the Great Barrier Reef of Australia

The sediments and calcareous organisms on the outer reefal shelf of the Central Region of the Great Barrier Reef were collected and observed by SCUBA diving and research vessel techniques (including underwater television) to understand the production and processes of deposition of the sediment. The carbonate grains are mainly sand and gravel size and solely of skeletal origin. Over the whole area the major CaCO₃ producers, in order of decreasing importance are: benthic foraminiferans (chiefly Operculina, Amphistegina, Marginopora, Alveolinella and Cycloclypeus), the calcareous green alga Halimeda, molluscs and corals. Coral abundance is high only close to reefs and submerged rocky substrates. Benthic foraminiferal sands dominate the inter-reef areas i.e. the bulk of the shelf, and Halimeda gravels form an outer shelf band between 60 and 100 m depths. Seven distinct facies are recognised after quantitative analyses of the sediments. These are: A. Shelf edge slope (>120 m depth); B. Shelf edge (with rocky outcrops); C. Outer shelf with high Halimeda (>40%); D. Inter-reef I; E. Inter-reef II ( 100 m depth but >2% pelagics); F. Lee-ward reef talus wedge (<2 km from sea level reefs); G. Lagoonal.     

Studies on Halimeda

Large areas of the inter-reefal seabed in the Great Barrier Reef are carpeted with vegetation composed almost entirely of the green calcareous alga Halineda. These meadows occur principally in the northern sections between 11°30 and 15°35S at depths of 20 to 40 m, but there are also some in the central and southern sections, where they have been found at depths down to 96 m. The vegetation is dominated by the same sprawling Halimeda species that are common on coral reefs in this region. However, on reefs these species grow on solid substrata, not soft sediments like the Halimeda-rich gravels that underlie the meadows. A total of 12 Halimeda species, together with two Udotea and one Penicillus species, are characteristic components of the shallow meadows. Below 50 m depth, species composition is restricted to only two major components. One, H. copiosa, is also important shallower, but the other is an unusually large and heavily calcified form of H. fragilis, a species that is normally a minor, fragile component of the shallow meadows. The maximum biomass found in these meadows was 4637 gm² of calcareous algae, although the thean for vegetated areas was 525 gm². These meadows are confined to the nutrient-depleted waters of the outer continental shelf just inside the outer barrier reefs, and are usually associated with distinct shoaling of the seabed caused by accumulation of thick deposits of calcareous Halimeda segments. The meadows are probably supported by very localized upwelling of nutrients from the adjacent Coral Sea onto the shelf, where they enrich the otherwise nutrient-depleted waters.Contribution No.367 from the Australian Institute of Marine Science     

Halimeda bioherms along an open seaway: Miskito Channel,Nicaraguan Rise,SW Caribbean Sea

A recent research cruise to examine small, detached carbonate platforms situated on the Nicaraguan Rise in the SW Caribbean Sea has revealed the presence of numerous Halimeda bioherms. Based upon interpretations from seismic reflection data some exceed 140 m in relief. This is the first documented occurrence of these green-algal buildups in the Caribbean/Bahama Bank region. The Halimeda bioherms form a nearly continuous band that borders the margins of the Miskito Channel—a shallow, open, 125 km long seaway. This 220 m deep channel bisects the Miskito Bank which is a major carbonate shelf. In seismic profile the bioherms appear acoustically soft and reveal a local relief of 20–30 m. Tops of these features lie in about 40–50 m of water. Samples from dredge hauls are coarse, poorly cemented packstones/grainstones which are dominated by largely unbroken, disarticulated Halimeda segments set in a poorly sorted sandy matrix. Exposed surfaces were stained brown. Very little living material was brought up in the dredges. The significance of these bioherms and their full extent in the Caribbean are not understood. Undoubtedly, further study will provide important answers concerning their role in the geologic development of Caribbean carbonate platforms.     

Morphology and sedimentology of Halimeda bioherms from the eastern Java Sea (Indonesia)

Halimeda bioherms, occurring primarily along the western and southern margins of Kalukalukuang Bank in the eastern Java Sea, display a wide variety of thicknesses and shapes. In general, high-frequency forms of the northern bank are replaced by thicker and lower frequency forms along the deeper southern margin. Sidescan sonar data suggest aperiodic reworking of shallow bioherm crests of the northern bank into features suggestive of bedforms. These features are not associated with deeper bioherms of the southern bank. Cores from the bioherms indicate that they consist mostly of disarticulated Halimeda plates set in a lime mud matrix composed largely of Halimeda fragments and foraminifera tests. Carbon-14 dating shows that, with exception of some deep southern bank examples, bioherms are actively accreting. Results of mineralogy and elemental chemistry on piston core PC-12 suggest cyclic variations in Mg-calcite (cement in Halimeda utricles), which may be related to periodic excursions of cold Pacific throughflow water onto the bank. Composition of the Mg-calcite (8.6 mole-%) suggests a temperature of formation of about 22 °C, which is 7 °C below average surface water temperatures. The carbon and oxygen isotope compositions of both aragonite and Mg-calcite phases are remarkably homogeneous, but were inconclusive with regard to the cold-water intrusion hypothesis. However, a lack of reef-building corals below a depth of 15 m, abundance of Halimeda bioherms on the western margin of K-Bank, where upwelling is predicted, extensive boring of sedimentary particles by endolithic boring algae, and high nutrient values of water at the thermocline (50–70 m deep) all support the incursion of cold, nutrient-rich water onto the bank. Upwelling and nutrient overloading are suggested as explanations for remarkable algal growth at the expense of reef-building corals.     

Seismic characteristics and accretion history of Halimeda bioherms on Kalukalukuang Bank,eastern Java Sea (Indonesia)

Extensive areas of Halimeda bioherms similar to those described by Orme et al. (1978), Orme (1985), Davies and Marshall (1985), and Phipps et al. (1985) from Australia's Great Barrier Reef have formed on Kalukalukuang Bank (K-Bank) 50 km east of the Sunda Shelf margin in the easter Java Sea. K-Bank is an isolated limestone platform whose top slopes southward from a water depth of about 20 m at the north to about 100 m at the south (30 km). It occurs in a bidirectional monsoonal wind regime and a predominantly southerly flowing current from Makassar Strait. The water column around K-Bank has a well developed shallow thermocline (50 m to 150 m). K-Bank has a relatively flat top with marginal banks of suspected Pleistocene origin as interpreted from seismic relationships. A reconnaissancelevel survey grid of high-resolution seismic profiles indicates that Halimeda bioherms are restricted to the bank margins with the exception of the eastern margin. Bioherms either extend to the steep margin of the platforms or are separated from the platform edge by banks of coral and coralline algae. The morphology of the Halimeda bioherms varies from steep-sided, elongate ridges in the northern bank area, through coalescing symmetrical mounds with partly infilled valleys, to broad undulating areas similar to those described by Orme (1985) from the Great Barrier Reef. High-resolution seismic records indicate erosional episodes in the high-relief areas, an interpretation that seems to be supported by accumulation rates calculated from C¹⁴ dates of cores. Thicknesses of Halimeda accumulation above a prominent reflector considered as Pleitocene, vary from around 20 m in the north to a maximum of over 50 m in the southwest. Accumulation rates obtained from dating of two cores to-ward the north of K-Bank, average 0.294 m/100 yr for core VC4 and a maximum of 0.59 m/100 yr in a part of core PC12. Dating of material from the top 30 cm of a deep bioherm (100 m) in the southwest of K-Bank, indicates that the growth rate of these bioherms has slowed markedly, presumably because of increasing water depth (decreasing light) over the Holocene transgression.     

Comparison of deep-water coral reefs and lithoherms off southeastern USA

Two types of deep-water coral bioherms occur off the coast of southeastern United States: Oculina and Lophelia/Enallopsammia. The deep-water Oculina bioherms form an extensive reef system at depths of 70–100 m along the shelf edge off central eastern Florida. These reefs are comprised of numerous pinnacles and ridges, 3–35 m in height. Each pinnacle is a bank of unconsolidated sediment and coral debris that is capped on the slopes and crest with living and dead colonies of Oculina varicosa, the ivory tree coral. In comparison, deep-water reefs of Lophelia pertusa and Enallopsammia profunda corals occur at depths of 500–850 m (maximum 150-m relief) along the base of the Florida-Hatteras slope in the Straits of Florida. On the western edge of the Blake Plateau off South Carolina and Georgia, 54-m high banks of Enallopsammia and Lophelia occur at depths of 490–550 m, whereas on the eastern edge of the plateau the reefs form structures 146 m in height and at depths of 640–869 m. The geomorphology and functional structure of both the Oculina and Lophelia reefs are similar. North of Little Bahama Bank, at depths of 1000–1300 m, a region of bioherms is dominated by the coral Solenosmilia sp.; Lophelia is reportedly absent. This paper summarizes 25 years of submersible studies on the deep-water Oculina reefs, describes submersible reconnaissance of deep-water Lophelia reefs off the southeastern United States, and contrasts these types of bioherms with the deep-water lithoherms in the Straits of Florida west of the Bahamas.     

Form and seismic stratigraphy of Halimeda banks in part of the northern Great Barrier Reef Province

Twenty-six percent of the total shelf area in the Northern Region of the Great Barrier Reef Province between latitudes 14°27 and 15°02S is occupied by algal (Halimeda) lithofacies. Sea-bed relief over this bankforming algal lithofacies, which dominates the outer shelf, is generally complex and variable over relatively short distances, but lateral continuity of morphological features near Petricola and Stewart shoals has been demonstrated by parallel profiling. The most prominent subbttom reflector is a pre-Holocene erosion surface, regarded as the Holocene/Pleistocene unconformity. In this area, the maximum thickness of Holocene Halimeda banks above the main subbottom reflector is 19 m. Seismic response suggests three main sequences in the Halimeda bank depostis, which probably relate to changes in environmental conditions, and the recognition of similar seismic characteristics in the deposits beneath the Holocene/Pleistocene unconformity indicates that Halimeda banks may have been a feature of the outer shelf of the Great Barrier Reef in Plesitocene times.     

New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef,Australia

Halimeda bioherms occur as extensive geological structures on the northern Great Barrier Reef (GBR), Australia. We present the most complete, high-resolution spatial mapping of the northern GBR Halimeda bioherms, based on new airborne lidar and multibeam echosounder bathymetry data. Our analysis reveals that bioherm morphology does not conform to the previous model of parallel ridges and troughs, but is far more complex than previously thought. We define and describe three morphological sub-types: reticulate, annulate, and undulate, which are distributed in a cross-shelf pattern of reduced complexity from east to west. The northern GBR bioherms cover an area of 6095 km², three times larger than the original estimate, exceeding the area and volume of calcium carbonate in the adjacent modern shelf-edge barrier reefs. We have mapped a 1740 km² bioherm complex north of Raine Island in the Cape York region not previously recorded, extending the northern limit by more than 1° of latitude. Bioherm formation and distribution are controlled by a complex interaction of outer-shelf geometry, regional and local currents, coupled with the morphology and depth of continental slope submarine canyons determining the delivery of cool, nutrient-rich water upwelling through inter-reef passages. Distribution and mapping of Halimeda bioherms in relation to Great Barrier Reef Marine Park Authority bioregion classifications and management zones are inconsistent and currently poorly defined due to a lack of high-resolution data not available until now. These new estimates of bioherm spatial distribution and morphology have implications for understanding the role these geological features play as structurally complex and productive inter-reef habitats, and as calcium carbonate sinks which record a complete history of the Holocene post-glacial marine transgression in the northern GBR.

     

Paleoecology of Pennsylvanian phylloid algal buildups in south Guizhou,China

Pennsylvanian phylloid algal reefs are widespread and well exposed in south Guizhou, China. Here we report on reefs ranging from 2 to 8 m thickness and 30–50 m lateral extension. Algae, the main components, display a wide spectrum of growth forms, but are commonly cyathiform (cup-shaped) and leaf-like (undulate plates). The algal reef facies is dominated by boundstone. Algal thalli form a dense carpet whose framework pores are filled with marine cement and peloidal micrite. The peloidal matrix is dense, partly laminated or clotted with irregular surfaces and often gravity defying. Algal reefs in Guizhou differ from examples reported to date by the high biodiversity of organisms other than phylloids: e.g., the intergrowth of algae with corals (some of which are twice the size of algal thalli) and numerous large brachiopods. This contrasts to previous views that phylloid algal “meadows” dominated the actual seafloor, excluding other biota. Also, the pervasive marine cements (up to 50%) including botryoidal cement are noteworthy. Algal reefs developed at platform margins, a depositional environment similar to that of modern Halimeda mounds in Java, Australia and off Bahamas, and to that of time-equivalent examples reported from the Canadian Arctic Archipelago. Whereas nutrients appear decisive in the growth of Halimeda reefs, algal reefs reported herein seemingly grew under conditions of low nutrient levels. Overall, algal reefs in Guizhou challenge previous views on growth forms, diversity patterns, and depositional environments and add to the spectrum of these partly puzzling biogenic structures.     

Controls on reef development and the terrigenous-carbonate interface on a shallow shelf,Nicaragua (Central America)

Marine geology and physical oceanographic data collected during two field projects (∼4 months) on the Caribbean shelf of Nicaragua indicate a surprising dominance of carbonate deposition and reef growth on a shelf that is receiving an abnormally large volume of terrigenous sediments. High rainfall rates (∼400–500 cm/year), coupled with a warm tropical climate, encourage rapid denudation of the country’s central volcanic highland and transport of large volumes of terrigenous sediment and fresh water to the coast. Estimates suggest that three times more fresh water and fifteen times more sediment are introduced per unit length of coastline than on the east coast of the United States. Distribution of the terrigenous facies, development of carbonate sediment suites, and the location and quality of viable reefs are strongly controlled by the dynamic interaction near the coasts of highly turbid fresh to brackish water effluents from thirteen rivers with clear marine waters of the shelf. Oceanic water from the central Caribbean drift current intersects the shelf and moves slowely in a dominant northwest direction toward the Yucatan Channel. A sluggish secondary gyre moves to the south toward Costa Rica. In contrast, the turbid coastal water is deflected to the south in response to density gradients, surface water slopes, and momentum supplied by the steady northeast trade winds. A distinct two-layered flow is commonly present in the sediment-rich coastal boundary zone, which is typically 10–20 km wide. The low-salinity upper layer is frictionally uncoupled from the ambient shelf water and therefore can expand out of the normally coherent coastal boundary zone during periods of abnormal flooding or times when instability is introduced into the northeast trades. Reef distribution, abruptness of the terrigenous-carbonate interface, and general shelf morphology reflect the long-term dynamic structure of the shelf waters. A smooth-bottomed ramp of siliciclastic sands to silts and clays mantles the inner shelf floor in a linear belt paralleling the coast. This belt generally corresponds to the western flank of the coastal boundary zone. Occurrence of reefs is generally confined to areas outside this zone. Terrigenous clays and silts of the inner shelf are abruptly (<20 km from the coast) replaced byHalimeda-rich sediment of the middle and outer shelf. Within the carbonate facies belt, reef complexes thrive as small, isolated masses; large, reef-capped platforms; reef fringes around islands; and shelfedge structures with vertical relief that can exceed 25 m. In general, the frequency and proliferation of reefs increase away from the turbid coastal boundary layer and toward the cooler and saltier water that upwells at the shelf margin.     

Variation in reef associated assemblages of the Lutjanidae and Lethrinidae at different distances offshore in the central Great Barrier Reef

Synopsis Fish traps were used to quantify the distribution and abundance of the Lutjanidae and Lethrinidae on reefs across the central Great Barrier Reef. The assemblages of fishes on inshore reefs were distinctive from those on midshelf and outershelf reefs. There were significantly fewer individuals of the Lutjanidae and Lethrinidae inshore and all species examined displayed significant cross-shelf changes in abundance. These significant cross shelf changes in abundance were due to an absence or low abundance of individuals of a species at one or more cross shelf locations, with many species present in only one location on the continental shelf. The genera Aprion, Lutjanus, Macolor, Symphorichthys, Symphorus, Gnathodentex, Gymnocranius, Lethrinus and Monotaxis were all characteristic of the shallow shelf waters less than 100 m. In contrast, species of the genera Paracaesio, Pristipomoides and Wattsia were characteristic of the intermediate depths (100–200 m) and the deeper outer reef slope waters in excess of 200 m were characterised by species of the genus Etelis.     

Discovery of a “Living Dinosaur”: Globally unique modern hexactinellid sponge reefs off British Columbia, Canada

Summary Globally unique hexactinellid sponge reefs occur on the continental shelf off British Columbia, Canada. They cover about 425 km² of seafloor on the continental shelf off British Columbia (Canada) in water depths between 165 and 240 metres and occur on a low-angle deep shelf, iceberg scoured seafloor, characterized by very low sedimentation rates and very stable environmental conditions. The sponge bioherms are up to 19 metres high with steep flanks, whereas the biostromes are 2–10 metres thick and many kilometres wide. They all consist of dense populations of only seven hexactinellid species. Three of them, all hexactinosan species (Aphrocallistes vastus, Heterochone calyx, Farrea occa) are the main frambuilders, composing a true rigid framework of sponge skeletons encased in a organic rich matrix of modern clay baffled by the sponges. Growth rates of hexactinosan sponges range in the order of 0–7 centimetres per year. The base of the oldest sponge reefs date from approximately 9000 years b.p. Different invertebrate and fish faunas occupy the reefs than occur on adjacent seafloor areas and some species appear to use the sponge reef complex structures as refugia where they can hide. Sidescan sonar data and direct observation by manned submersible clearly show that large areas of sponge reefs have been heavily damaged by seafloor trawling in the past decade. These unique extant siliceous sponge reefs can be used as a modern analogue for a better understanding and interpretation of fossil siliceous sponge reefs, known from many ages and many locations world wide.     

Halimeda: paleontological record and palaeoenvironmental significance

Udoteacean algae, identical or related to Halimeda, have been recorded in shallow-marine carbonate rocks since Upper Triassic. About 30 species have been described, most of which occur in Lower Cretaceous shelf carbonates. These species are conventionally attributed to four “genera” (Arabicodium Elliott, Boueina Toula, Halimeda Lamouroux, Leckhamptonella Elliott), but the validity of these taxa is a matter of discussion (generic or subgeneric position, or synonyma of Halimeda?) owing to wide discrepancies in the classification of fossil and recent species of halimediform algae. The paleoenvironmental setting of the Mesozoic and the Tertiary is comparable with that of recent Halimeda: lagoonal as well as reefal environments are already known from Upper Triassic occurrences. A reinvestigation of Boueina limestones described from Norian-Rhaetian lagoonal carbonates of Western Thailand indicates the important role of the alga (Boueina marondei n. sp.) in sediment accumulation from its very beginnings.

     

Mapping and imaging deep-sea coral reefs off Norway, 1982–2000

The survey and mapping group (SMG) of Statoil is responsible for all seafloor mapping for pipelines and field development in Statoil. During numerous reconnaissance and pipeline route surveys over large portions of our contintental shelf, in the North, Norwegian and Barents Seas, we have only detected deep-water coral reefs in a few specific areas. The first reef we found was in 1982, using a combined side scan sonar and sub-bottom profiler system off Fugløy, northern-Norway. Thereafter, numerous reefs were mapped with similar systems off mid-Norway, during the period 1985–1990 for the reconnaissance and final route mapping of the Haltenpipe project. During 1997, we also mapped some previously known reefs in the Trondheimsfjord area while surveying a route for the Tjeldbergodden - Skogn pipeline project. Between 1997 and 2000, we have mapped more reefs along pipeline routes on the outer mid-Norway continental shelf. These reefs are of a smaller size (less than 5 m high) than those mentioned above (5–31 m high). Although side scan sonar and sub-bottom profilers provide a less ambiguous detection of reefs, we have found that modern multi-beam echosounder data can provide adequate remotely sensed data for deep-water coral reef mapping. The interpretation of the reefs is based on visual documentation by ROV (remotely operated vehicle) or by sampling with gravity corers and grabs (ground-truthing). Based on a limited amount of such ground-truthing, it has been possible to extrapolate and use the specific morphological characteristics of the reefs to map their density and distribution. For the Haltenpipe project, we mapped an offshore route corridor of about 200 km length at a width of 3 km with multi-beam echosounder. On the basis of ground-truthing 14 of the suspected coral reefs, we have found the total number of reefs to be 57 within the 600 km² mapped area. All these reefs are higher than 5 m (the highest is 31 m) and of diameters at their base of more than 50 m. Although they occur in local clusters with up to 10 reefs per km², the mean density of reefs along the entire (200 km long) transect is only 0.09 suspected reefs per km². However, there is a large regional density variation, with the highest regional density being 1.2 reefs per km² in an area of subcropping Palaeocene sedimentary rocks. A brief discussion of why the corals have constructed their reefs in the deep, cool, and generally `hostile' waters of the Norwegian continental shelf and fjords concludes with them probably subsisting on a reliable and steady nutrient source, independent of season and variations in the Atlantic Drift (`Gulf stream'). This positive environmental component is called `hydraulically active substrata'. It is thus speculated that micro-organisms, bacteria etc., utilizing the hydraulically activated chemical porewater gradients, cause a local enrichment on which the cnidarian organisms ultimately depend.     

Distributional patterns of mobile fauna associated with Halimeda on the Tiahura coral-reef complex (Moorea,French Polynesia)

This study deals with the mobile fauna living associated with the turfs of three Halimeda species [H. incrassata, H. opuntia (three forms) and H. macroloba] from the Tiahura Reef complex. Ten 0.05 m² test areas of each Halimeda form have been randomly sampled from each geomorphological reef unit in order to obtain the specific abundance and biomass of different faunistic groups and species living within Halimeda populations. Then the raw data have been classified using the correspondence factor analysis to highlight the relationships between faunistic groups, or species, and the different Halimeda forms studied. The distributional patterns of the faunal communities seems to be controlled by the morphological features of the host-alga. The complex arborescent structure of the rhyzophytic H. incrassata species shelters fauna three times higher in abundance than other Halimeda heads. This fauna is mostly composed of a highly rich community of microcrustaceans and a very diversified community of Syllidae polychaetes. H. macroloba living on the outer reef flat retains a lot of small Nereidae polychaetes and a rich microgastropod assemblage. Dense H. opuntia tussocks (type A) on the fringing-reef and isolated H. opuntia fronds of pendulous chains (type B) on the barrier reef as well as beds of scattered H. opuntia (type C) on the outer reef flat provide environment for larger animals, and according to their zone of life, they retain very different faunal associations. Some ophiuroids, and some fish, collected in H. opuntia A and C, are newly reported from the Society Islands, French Polynesia, and the Pacific province. One Brachyurid species is presumably undescribed.     

Occurrence and paleocologic significance of Halimeda in late Miocene reefs,southeastern Spain

Facies mapping of a late Miocene reef complex near the town of Níjar (Almería Province, southeastern Spain) demonstrated that Halimeda-rich beds compose about 20% of the proximal-slope sedimets. Halimeda segments are unbroken, preserved as molds, randomly oriented to layered, and concentrated in beds that commonly contain few fossils other than Halimeda. The associated biota (a laminar form of the coral Porites, articulated bivalves, small gastropods, and in-situ branching coralline algae) and sediment texture suggest possible insitu formation of the Halimeda.Repetitive stratigraphy characterizes the proximal reef-slope sediments at Níjar. Each repetition consists of the following idealized succession: an eroded base, mixed-fossil hash, Halimeda-rich beds, and mixed-fossil beds that contain little if any Halimeda. Although Halimeda beds do not dominate in the proximal-slope environment, their local abundance may signify changed environmental conditions.The concentration of Halimeda in beds suggests spatial segregation of Halimeda from many reef-dwelling organisms. The repetitive stratigraphy suggests temporal segregation as well.Episodic upwelling may have been responsible for the repetitive stratigraphy. The occurrence of Halimeda-rich beds in reef complexes of similar age throughout the Spanish Mediterranean region, and the occurrence of possibly correlative cyclic basinal sequences, is consistent with an upwelling mechanism. If responsive to upwelling episodes, Halimeda beds may represent event strata of regional significance.     

Leeward bank margin Halimeda meadows and draperies and their sedimentary importance on the western Great Bahama Bank slope

Bryopsidalean algal meadows in water depths of 20–40 m on the leeward side of western Great Bahama Bank (WGBB) lie between non-skeletal-dominated sand flats on the bank top to the east and a cemented steep escarpment to the west. The meadows contain dense populations of rhipsalian Halimeda species, as well as Udotea and Rhipocephalus. Extensive populations of other Halimeda species (opuntioids) occur at greater depths on the cemented rocky escarpment, growing as drapes or vines rather than as upright thalli. These meadows and draperies are important sources of coarse-grained carbonate sediments. This is shown by (1) deeper bank-edge sediments (30–60 m) containing considerably more Halimeda fragments than do the bank top, non-skeletal sands, and (2) the coarser fraction of slope sediments (down to 200 m) dominated by Halimeda plates, partly or extensively altered and internally cemented by magnesian calcite and aragonite. A transect across the bank margin from bank top (<10 m) to lower slope (300 m) provides a useful comparison for the locus of sediment production and accumulation. The production of Halimeda in these bank-edge habitats approximates that in the Great Barrier Reef or off Indonesia and Nicaragua in similar water depths. The apparent lack of thick sediment accumulation in WGBB compared to that seen elsewhere may reflect the high rates of downslope transport off Great Bahama Bank.     

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.