Algal Crusts Research Articles

Taphonomic differentiation of Acropora palmata facies in cores from Campeche Bank Reefs, Gulf of México

AbstractA common assumption in the geological analysis of modern reefs is that coral community zonation seen on the surface should also be found in cores from the reef interior. Such assumptions not only underestimate the impact of tropical storms on reef facies development, but have been difficult to test because of restrictions imposed by narrow‐diameter cores and poor recovery. That assumption is tested here using large‐diameter cores recovered from a range of common zones across three Campeche Bank reefs. It is found that cores from the reef‐front, crest, flat and rubble‐cay zones are similar in texture and coral composition, making it impossible to recognize coral assemblages that reflect the surface zonation. Taphonomic signatures imparted by variations in encrustation, bioerosion and cementation, however, produce distinct facies and delineate a clear depth zonation. Cores from the reef‐front zone (2–10 m depth) are characterized by sections of Acropora palmata cobble gravel interspersed with sections of in‐place (but truncated) A. palmata stumps. Upper surfaces of truncated colonies are intensely bioeroded by traces of Entobia isp. and Gastrochaenolites isp. and encrusted by mm‐thick crustose corallines before colony regeneration and, therefore, indicate punctuated growth resulting from a hurricane‐induced cycle of destruction and regeneration. Cores from the reef crest/flat (0–2 m depth) are also characterized by sections of hurricane‐derived A. palmata cobble‐gravels as well as in‐place A. palmata colonies. In contrast to the reef front, however, these cobble gravels are encrusted by cm‐thick crusts of intergrown coralline algae, low‐relief Homotrema and vermetids, bored by traces of Entobia isp. and Trypanites isp. and coated by a dense, peloidal, micrite cement. Cores from the inter‐ to supratidal rubble‐cay zone (+0–5 m) are only composed of A. palmata cobble gravels and, although clasts show evidence of subtidal encrustation and bioerosion, these always represent processes that occurred before deposition on the cay. Instead, these gravels are distinguished on the basis of their limited bioerosion and marine cements, which exhibit fabrics formed in the intertidal zone. These results confirm that hurricanes have a major influence on facies development in Campeche Bank reefs. Instead of reflecting the surface coral zonation, each facies records a distinctive, depth‐related set of taphonomic processes, which reflect colonization, alteration and stabilization following the production of new substrates by hurricanes.

The vertical microdistribution of cyanobacteria and green algae within desert crusts and the development of the algal crusts

Substantial amounts of algal crusts were collected from five different desert experimental sites aged 42, 34, 17, 8 and 4 years, respectively, at Shapotou ( China) and analyzed at a 0.1 mm microscale of depth. It was found that the vertical distribution of cyanobacteria and microalgae in the crusts was distinctly laminated into an inorganic-layer (ca. 0.00 - 0.02 mm, with few algae), an algae-dense-layer ( ca. 0.02 - 1.0 mm) and an algae-sparse-layer ( ca. 1.0 - 5.0 mm). It was interesting to note that in all crusts Scytonema javanicum Born et Flah ( or Nostoc sp., cyanobacterium), Desmococcus olivaceus (Pers ex Ach., green alga) Laundon and Microcoleus vaginatus Gom. ( cyanobacterium) dominated at the depth of 0.02 - 0.05, 0.05 - 0.1 and 0.1 - 1.0 mm, respectively, from the surface. Phormidium tenue Gom. ( or Lyngbya cryptovaginatus Schk., cyanobacterium) and Navicula cryptocephala Kutz.( or Hantzschia amphioxys (Ehr.) Grun. and N. cryptocephala together, diatom) dominated at the depth of 1.0 - 3.0 and 3.5 - 4.0 mm, respectively, of the crusts from the 42 and 34 year old sites. It was apparent that in more developed crusts there were more green algae and the niches of Nostoc sp., Chlorella vulgaris Beij., M. vaginatus, N. cryptocephala and fungi were nearer to the surface. If lichens and mosses accounted for less than 41.5% of the crust surface, algal biovolume was bigger when the crust was older, but the opposite was true when the cryptogams other than algae covered more than 70%. In addition to detailed species composition and biovolume, analyses of soil physicochemical properties, micromorphologies and mineral components were also performed. It was found that the concentration of organic matter and nutrients, electric conductivity, silt, clay, secondary minerals were higher and there were more micro-beddings in the older crusts than the less developed ones. Possible mechanisms for the algal vertical microdistribtion at different stages and the impact of soil topography on crust development are discussed. It is concluded that biomethods ( such as fine species distribution and biovolume) were more precise than mineralogical approaches in judging algal crust development and thus could be a better means to measure the potentiality of algal crusts in desert amelioration.

A physico-chemical comparative study on extracellular carbohydrate polymers from five desert algae

Hydrodynamic properties of five newly isolated algal extracellular polysaccharides with putative adhesive properties are described, using a combination of size exclusion chromatography, total or ‘multi-angle’ laser light scattering and analytical ultracentrifugation. The respective polysaccharides had been extracted from four filamentous cyanobacteria: Microcoleus vaginatus, Scytonema javanicum, Phormidium tenue and Nostoc sp. and a coccoid single-cell green algae Desmococcus olivaceus that had been separated from desert algal crusts of the Chinese Tegger Desert. SEC/MALLS experiments showed that the saccharides had diverse weight average molecular weights ranging from 4000 to 250,000 g/mol and all five showed either bi-modal or tri-modal molecular weight distribution profiles. Use of the Mark–Houwink–Kuhn–Sakurada (MHKS) scaling relationship between sedimentation coefficient and (weight average) molecular weight for the five samples, assuming a homologous conformation series revealed an MHKS b exponent of (0.33±0.04), suggesting a conformation between that of a stiff rod ( b∼0.18) and a random coil ( b∼0.4–0.5), i.e. a ‘ flexible rod’ or ‘stiff coil’.

Extracellular carbohydrate polymers from five desert soil algae with different cohesion in the stabilization of fine sand grain

Extracellular polymeric substances (EPS) from four filamentous cyanobacteria Microcoleus vaginatus, Scytonema javanicum, Phormidium tenue and Nostoc sp. and a coccoid single-cell green alga Desmococcus olivaceus that had been separated from desert algal crusts of Tegger desert of China, were investigated for their chemical composition, structure and physical properties. The EPS contained 7.5–50.3% protein (in polymers ranging from 14 to more than 200 kD, SDS-PAGE) and 16.2–40.5% carbohydrate (110–460 kD, GFC). 6–12 kinds of monosaccharides, including 2- O-methyl rhamnose, 2- O-methyl glucose, and N-acetyl glucosamine were found. The main carbohydrate chains from M. vaginatus and S. javanicum consisted mainly of equal proportion of Man, Gal and Glc, that from P. tenue consisted mainly of arabinose, glucose and rhamnose. Arabinose was present in pyranose form, mainly α- l 1→3 linked, with branches on C4 of almost half of the units. Glucose was responsible for the terminal units, in addition of having some units as β1→3 and some as β1→4 linked. Rhamnose was mainly 1→3 linked with branches on C2 on half of the units. The carbohydrate polymer from D. olivaceus was composed mainly of β-1→4 linked xylose, galactose and glucose. The galactose part was present both in β-pyranose and -furanose forms. Arabinose in α- l-furanose form was mainly present as 1→2 and 1→2, 5 linked units, rhamnose only as α 1→3 and xylose as β 1→4. The backbone of the polysaccharide from Nostoc sp. was composed of β-1→4 linked xylose, galactose and glucose. Most of the glucose was branched on position C6, terminal glucose and 2- O-methyl glucose units are also present. The relationship between structure, physical properties and potential biological function is discussed.

Microhabitat and associated biota of abalone juveniles, Haliotis fulgens and H. corrugata, in Bahía Tortugas, Baja California Sur, Mexico

Abalone is an important economical resource on the west coast of Baja California. The most important species in this fishery are green and pink abalone (Haliotis fulgens and H. corrugata, respectively). Overfishing has been implicated in the decline of the Mexican abalone fishery. Among the population parameters, recruitment is a highly variable event. To obtain a better understanding of the ecology of abalone juvenile recruitment, the physical and biological characteristics of the microhabitat of juveniles of green and pink abalone were studied at Bahía Tortugas, Baja California Sur (Mexico), during autumn and spring from 1996 to 1998. Two abalone reefs with different depth, relief and wave exposure were surveyed: Clam Bay, with depths of 0.5 to 5 m, and Morro de Adentro, with depths of 6 to 12 m. Four different bottom types were recognized: (a) sheltered and low relief bottom, (b) semiexposed and moderate relief bottom, (c) exposed and high relief bottom, and (d) exposed and moderate relief bottom. Juveniles of green and pink abalone from 3 to 13 mm shell length were observed under small rocks, pebbles and flat stones no longer than 15|00E2|??20 cm, always covered with crusts of coralline algae. Several tiny juveniles of Fisurella, Stenoplax, and Crepipatella of similar sizes (5–10 mm) shared the microhabitat. Juveniles between 15 and 90 mm shell length were found under flat, rhombic or irregular-shaped rocks (20–110 mm length), most of them deployed in imbricate arrangement. Abalone juveniles shared this microhabitat with bryozoans, sponges, ascidians, chitons, and annelids. Bottoms with imbricate arrangement, low wave exposure, and the low depth of Clam Bay were favorable for the high occurrence of juveniles of H. fulgens. Juveniles of H. corrugata showed a high occurrence in exposed and deeper zones of Morro de Adentro.

Salt tolerance of Microcoleus vaginatus Gom., a cyanobacterium isolated from desert algal crust, was enhanced by exogenous carbohydrates

Microcoleus vaginatus isolated from a desert algal crust of Shapotou was cultured in BG-11 medium containing 0.2 mol l −1 NaCl or 0.2 mol l −1 NaCl plus 100 mg l −1 sucrose, extracellular polymeric substances (EPS) or hot water-soluble polysaccharides (HWP), respectively. Photosynthetic oxygen evolution rates, photosystem II activity ( Fv/ Fm) and dark respiration of NaCl-stressed cells were enhanced significantly by the added sucrose or EPS under salt stress conditions (0.2 mol l −1 NaCl). Compared with cells treated with salt alone, sodium contents in cells reduced significantly; the content of cellular total carbohydrate did not change, and intracellular sucrose, water-soluble sugar increased significantly following the addition of exogenous carbohydrates. Sucrose synthase (SS) activity of NaCl-stressed cells increased following the addition of sucrose, and sucrose phosphate synthase (SPS) activity of NaCl-stressed cells increased following the addition of exogenous sucrose, EPS or HWP compared with cells stressed with NaCl only. The results suggested that the extruded EPS might be re-absorbed by cells of M. vaginatus as carbon source, they could increase salt tolerance of M. vaginatus through the changes of carbohydrate metabolism and the selective uptake of sodium ions.

A Methylobacterium-like organism from algal crusts covering silicone rubber electric insulators in Africa.

The primary goals of this study were to isolate, identify and characterize culturable bacteria living in a close association with microalgae within green crusts covering silicone rubber electric insulators in Tanzania. Twenty-four bacterial colonies were isolated from an Apatococcus crust. Characterization by statistical analyses of total cellular protein profiles demonstrated that they were highly similar to one another. Final identification was achieved using 16S rDNA sequencing and fatty acid methyl ester profiling. These analyses revealed the presence of microbes with high similarity to Methylobacterium radiotolerans. The selected isolate, A1, displayed strong inhibitory activity against Rhizoctonia solani and was found to be resistant to relatively high concentrations of zinc in the growth medium. This study revealed the presence of M. radiotolerans bacteria in a novel environment--within algal crusts formed on electrical insulators in Africa. Moreover, this bacterium was found to be a predominant culturable species within those complex algal-microbial associations. The isolate also shared some traits of biotechnological importance with other members of the Methylobacterium genus. The data presented provide a valuable contribution concerning the formation and function of associations between green microalgae and bacteria. This study also provides some information about the utility of bacteria from the genus Methylobacterium in biotechnological applications, such as biocontrol of rhizoctoniosis and bioremediation of heavy metal-contaminated soils.

Host specificity of the endophytic sporophyte phase of Acrosiphonia (Codiolales, Chlorophyta) in southern British Columbia, Canada

We investigated the host specificity of Acrosiphonia’s (Codiolales, Chlorophyta) endophytic sporophyte phase, previously identified as Chlorochytrium inclusum and Codiolum petrocelidis. In southern British Columbia, Canada, ‘Chlorochytrium inclusum’ is abundant in the foliose red alga, Mazzaella splendens, whereas ‘Codiolum petrocelidis’ densely colonizes the red algal crust, Petrocelis franciscana. Examination of foliose and crustose algae revealed a wide range of alternate hosts for Acrosiphonia sporophytes. Three foliose red algae, M. heterocarpa, M. sanguinea and Sparlingia pertusa, and two crustose algae, Raifsia pacifica and Hildenbrandia occidentalis, are newly reported hosts for ‘Chlorochytrium’ and ‘Codiolum’. This is the first study to report ‘Codiolum’ colonization of a brown alga (= Raifsia), and foliose brown and green algae have never been reported to be colonized by ‘Chlorochytrium’. ‘Codiolum’ demonstrated a greater affinity for Petrocelis than for other coexisting crustose algae, and ‘Chlorochytrium’ colonized some foliose red algal species more frequently than others. Endophyte density data suggest that variations in structural composition among host species (i.e. how closely appressed the filaments are in crusts, and the compactness in the cells of the cortex and medulla of foliose red algae) may be important in determining the ease of penetration by the endophyte. Host cell wall composition is suspected to play a role in preferential colonization by ‘Chlorochytrium’. Carrageenophytes and carragar-producing hosts were colonized by higher densities of ‘Chlorochytrium’ than were agarophytes. The hypothesis that host life history phases might be differentially susceptible to colonization by endophytes was not supported in our study; among host species with an alternation of generations neither the gametophytes nor the sporophytes seemed to be consistently more heavily colonized.

Cementing mechanism of algal crusts from desert area

34-, 17-, 4-, 1.5-year old natural algal crusts were collected from Shapotou Scientific Station of the Chinese Academy of Sciences, 40-day old field and greenhouse artificial algal crusts were in situ developed in the same sandy soil and the same place (37°27′N, 104°57′E). Their different cohesions both against wind force and pressure were measured respectively by a sandy wind-tunnel experiment and a penetrometer. On the basis of these algal crusts, the cementing mechanism was revealed from many subjects and different levels. The results showed that in the indoor artificial crusts with the weakest cohesion bunchy algal filaments were distributed in the surface of the crusts, produced few extracellular polymers (EPS), the binding capacity of the crusts just accomplished by mechanical bundle of algal filaments. For field crusts, most filaments grew toward the deeper layers of algal crusts, secreted much more EPS, and when organic matter content was more than 2.4 times of chlorophyll a, overmuch organic matter (primarily is EPS) began to gather onto the surface of the crusts and formed an organic layer in the relatively lower micro-area, and this made the crust cohesion increase 2.5 times. When the organic layer adsorbed and intercepted amounts of dusts, soil particles and sand grains scattered down from wind, it changed gradually into an inorganic layer in which inorganic matter dominated, and this made the crusts cohesion further enhanced 2–6 times. For crust-building species Microcoleus vaginatus, 88.5% of EPS were the acidic components, 78% were the acidic proteglycan of 380 kD. The uronic acid content accounted for 8% of proteglycan, and their free carboxyls were important sites of binding with metal cations from surrounding matrix.

Effect of desert soil algae on the stabilization of fine sands

Four filamentous cyanobacteria, Microcoleus vaginatus, Phormidium tenue, Scytonema javanicum (Kutz.) and Nostoc sp., and a single-celled green alga, Desmococcus olivaceus, all isolated from Shapotou (Ningxia Hui Autonomous Region of China), were batch cultured and inoculated onto unconsolidated sand in greenhouse and field experiments. Their ability to reduce wind erosion in sands was quantified by using a wind tunnel laboratory. The major factors related to cohesion of algal crusts, such as biomass, species, species combinations, bioactivity, niche, growth phase of algae, moisture, thickness of the crusts, dust accretion (including dust content and manner of dust added) and other cryptogams (lichens, fungi and mosses) were studied. The best of the five species were M. vaginatus and P. tenue, while the best mix was a blend of 80% M. vaginatus and 5% each of P. tenue,S. javanicum,Nostoc sp. and D. olivaceus. The threshold friction velocity was significantly increased by the presence of all of the cyanobacterial species, while the threshold impact velocity was notably increased only by the filamentous species. Thick crusts were less easily eroded than thin crusts, while biomass was more effective than thickness. Dust was incorporated best into Microcoleus crust when added in small amounts over time, and appeared to increase growth of the cyanobacterium as well as strengthen the cohesion of the crust. Microbial crust cohesion was mainly attributed to algal aggregation, while lichens, fungi and mosses affected more the soil structure and physico-chemical properties.

Soil Algae in Brown Coal and Lignite Post‐Mining Areas in Central Europe (Czech Republic and Germany)

AbstractAlgal communities were investigated in two contrasting chronosequences established on reclaimed spoils in the west Bohemian brown coal mining district near Sokolov (Czech Republic) and in the Lusatian lignite mining district near Cottbus (Germany). The Sokolov chronosequence was characterized by tertiary cypric clay substrate, afforestation with Alnus glutinosa (L.) Gaertn., and high pH of deposited spoil material, Cottbus chronosequence by tertiary carboniferous and pyritic sand of extremely low pH ameliorated by fly ash, and afforestation with Pinus sylvestris L. and P. nigra Arnold. A total of 122 species of algae was found in both areas. Although the same species number (80) was identified from Sokolov and Cottbus, both proportion of individual algal groups and species composition were different. Green algae prevailed in both areas, but in Sokolov cyanobacteria and diatoms were also quite diverse, and in younger sites they were also abundant. Total abundance of algae ranged mostly between 104–107 cells/g dry soil, and was one order higher in Cottbus than in Sokolov. Species number was high in young sites, decreased with increasing age, and was the lowest in control forests. In Sokolov, the highest abundance was recorded in the youngest alder plantation. In Cottbus, sludge and compost fertilization used in the youngest pine plantations resulted in rapid formation of visible algal crusts dominated by Klebsormidium crenulatum (Kütz) Lokhorst.

Life history of Acrosiphonia (Codiolales, Chlorophyta) in southwestern British Columbia, Canada

This study establishes the phenology of the alternate life history phases of the green alga Acrosiphonia in British Columbia, Canada. Free-living, filamentous plants are seasonal, March-July, with peak percent cover (10%) in April. Plants are fertile immediately after establishment. The unicells, previously identified as Chlorochytrium inclusum and Codiolum petrocelidis, are the sporophyte phase of Acrosiphonia. 'Chlorochytrium,' spherical and 160-280 μm in diameter, colonizes the foliose red alga Mazzaella splendens 1 mo after filamentous Acrosiphonia plants appear. Maximum density (53 'Chlorochytrium' cells/cm(2) of blade) was recorded in May. 'Codiolum,' on the other hand, is stalked (the vesicle measures 150 × 50 μm) and colonizes the red algal crust Petrocelis. Peak density (22 400 'Codiolum' cells/cm(2) of crust) was recorded 2 mo after 'Chlorochytrium' density peaked. The endophytes survive high summer temperatures, which correlate with death of the free-living plants, and overwinter in their hosts. Zoospore release in late winter corresponds to decreased host abundance, suggesting the endophytes have evolved a strategy whereby duration in the host is synchronized with host seasonality. A bet-hedging strategy is proposed for Acrosiphonia's life history: two morphologically different phases have adapted to a seasonally variable environment, with the sporophyte phase capable of colonizing two different hosts.

Polychaetes in intertidal rocky and sedimentary habitats in the region of Tromsø, northern Norway

Abstract Polychaetes were collected from 10 rocky and 10 sandy and muddy shore habitats in fjords and coastal areas near Tromso, northern Norway, in 1994. The main emphasis was on qualitative samples, but quantitative samples were collected at six sedimentary shore sites. In the quantitative samples, all macrofaunal species (> 1 mm) were recorded. Altogether, 62 polychaete species belonging to 21 families were found. In rocky shore habitats, the most abundant species were the small fan worm Fabricia stellaris, spirorbids, and the orbiniid Naineris quadricuspida. In many shore sites, Fabricia tubes and filamentous green and brown algae formed densely interwoven mats, housing a highly diverse fauna of worms, molluscs and insect larvae. Crusts of coralline algae from exposed shore areas had been excavated by boring Polydora and Dodecaceria. In sandy and muddy shore habitats, species assemblages were dominated by polychaetes, oligochaetes, and bivalves. The most abundant polychaetes were the spionid Pygospio ...

Seasonality of the red algal crust ‘Petrocelis franciscana’(Gigartinales, Rhodophyta) on boulder‐strewn shores of southern British Columbia, Canada

SUMMARYThe red algal crust,‘Petrocelis franciscana’ Setchell et Gardner [= tetrasporophytic phase of Mastocarpus papillatus (Agardh) Kützing] exists as discrete patches on boulders throughout the intertidal zone at Burrard Inlet in Vancouver (mainland British Columbia) and at Sooke, Vancouver Island (British Columbia). This study photographically quantifies monthly surface area of 60 individual crusts over 12‐ and 18‐month periods, respectively, at the two sites. Conspicuous seasonal fluctuations in ‘Petrocelis’ abundance and relatively low survivorship of crusts sampled suggest that not all crusts are as static and as long‐lived (> 25 years) as previously suggested. Herbivory, sloughing of senescing tissue and abiotic factors visibly reduce ‘Petrocelis’ abundance in fall and winter; regeneration of lost crust tissue and relatively rapid crust growth occur from spring to early fall.

Diet, movement, and covering behavior of the sea urchin Toxopneustes roseus in rhodolith beds in the Gulf of California, México

The density, diet, movement, and covering behavior of Toxopneustes roseus (Agassiz) were investigated in rhodolith beds in the Gulf of California. Densities varied from a mean of 0.4 to 1.8/20 m2 with most urchins occurring in aggregations. Spatial patterns of urchins varied with depth, with greatest abundance at intermediate depths (7.5–9.4 m) in the middle of the rhodolith bed. Urchins ate rhodoliths and nongeniculate coralline algal crusts almost exclusively, despite the availability of other algae. The mean amounts ingested were 3.87 and 7.96 g carbonate/individual per day. Even when food was abundant, animals were highly mobile, moving an average of 6.6–11.7 cm/h depending on site and time of day. Diel movement may be a behavioral adaptation to avoid surge, which is greatest during the day. Covering behavior may also be related to surge, as the ratio of covering material:body weight and the percent cover of material held were highest at the site with the most surge. While an urchin consumed rhodoliths, its movement spread the grazing impact over large areas. Bioturbation resulting from urchin feeding, movement, and covering activity probably benefits the rhodoliths by turning them, which maintains rhodolith integrity, prevents fouling, and contributes to bed persistence.

Effects of Disturbance, Life Histories, and Overgrowth on Coexistence of Algal Crusts and Turfs

Coexistence of species is generally attributed to the interacting roles of competition, predation, and disturbance. Overgrowth is considered to be an important mechanism of competition for space, and species are often ranked in hierarchies based on their abilities to overgrow. In some marine habitats, however, encrusting algae dominate primary substrata despite a dense permanent cover of epiphytes, suggesting that factors other than competition could be important in influencing their distributions. The spatial relationships and competitive interactions between encrusting algae and overgrowing filamentous, turf-forming algae were investigated on a subtidal rocky reef (Mediterranean Sea, Italy). Quantitative observations and field experiments were done from 1992 to 1998: (1) to investigate the relative patterns of distribution and abundance of crusts and turf and how they differed across space and time, (2) to test whether spatial relationships between crusts and turf were influenced by various characteristics of the substratum and by disturbance from wave action, (3) to investigate spatial and temporal patterns of recruitment and the mechanisms by which crusts colonize space, and (4) to test whether crusts and turf compete for space. Crusts were always among the first colonizers of available bare rock and were subsequently overgrown by turf. Despite variations in spatial and temporal recruitment of crusts, this pattern was never reversed. Covers of crusts and turf were not significantly affected by disturbance. There was some evidence for competition, but this did not result in the local exclusion of either crusts or turf. Despite extensive and persistent cover of turf, encrusting algae were abundant and were able to live, grow, and reproduce beneath the turf over long periods with little adverse effect. Similarly, crusts did not limit the distribution of turf, although they slowed its rate of growth. The capabilities of turf and crusts to overgrow and tolerate overgrowth, respectively, were identified as the major determinants of the structure of this two-layered assemblage and the probable basis of the success of these species in coexisting as the dominant algal forms. Overgrowth was not equivalent to competitive subordination and displacement, emphasizing the need for caution when interpreting competitive abilities from observed patterns of overgrowth. Overall, results suggested that, in some habitats, tolerance may be more important than competition in maintaining coexistence of species and influencing community structure.

Reef coralgal assemblages as recorders of paleobathymetry and sea level changes in the Indo-Pacific province

The coralgal framework within the outer reef margin of many Indo-Pacific reefs exhibits three main shallow-water communities, the environmental significance of which can be inferred by comparison with their modern counterparts. A community dominated by tabular Acropora gr. hyacinthus/ cytherea with branching Pocillopora damicornis, P. eydouxi, Montipora digitata, occasional domal faviids and mm-thick crusts of the coralline algae Lithophyllum and Mesophyllum (mainly), typical of the 6 – 15 m paleodepth range; a community including robust-branching Acropora gr. danai/ robusta, A. humilis, A. digitifera and subordinate Favia stelligera, Echinopora gemmacea, associated to vermetid gastropods and thick coralline crusts of Hydrolithon cf. onkodes and Neogoniolithon cf. fosliei flourishing in depths less than 6 m; in medium-to-high water-energy settings, a community composed of domal Porites cf. lutea and P. cf. lobata with occasional Acropora gr. danai/ robusta and cm-thick crusts of coralline algae in sheltered habitats in depths less than 10 m. These biological assemblages allow us to determine relationships between reef growth and paleobathymetry and, consequently, to reconstruct regional relative sea-level curves. High water-energy reefal assemblages provide stronger evidence for reconstructing sea-level curves than low-energy buildups, because they have generally been controlled by a keep-up growth mode. Subsiding reef sites seem to be more reliable indicators of sea-level variations because they usually present expanded reef sequences.

Unexpected richness of reef corals near the southern Amazon River mouth

Manoel Luis Reefs (00° 52’ S, 44° 15’W),located 180 km north of Sao Luis, offMaranhao State, represent the northernmostoutpost of the Brazilian reef coral fauna, andthe closest well-developed reefs to theAmazon River mouth (Figure 1). In July1998, during a reef fish survey along theBrazilian coast, we found a surprising richness of corals and other reef associatedorganisms in the area, including nine speciesof reef corals and two milleporans, six ofwhich were not previously reported for theNorthern coast of Brazil (Belem et al. 1986,Veron 1996). Despite its proximity to the Amazon Rivermouth, the West flowing Equatorial Currentprovides the region with clear and salinewater. The tidal range is about 6.5 m andvisibility ranged between 20 and 30 m duringour 12 day visit. The reef is approximately y-shaped, with about 10 km in its SE-NEaxis (Figure 1). It is constituted by approximately circular pinnacles, ranging from 50 to 300 m in diameter.The pinnacles rise from 25–30 m depth to near the surface, the tops of some being exposed on the lowesttides. Superimposed coralline algal crusts constitute the framework of the reef, which is sparsely covered withcolonies of Agaricia fragilis(Dana), Favia gravidaVerrill, Madracis decactis(Lyman), Meandrina brazilien-sisMilne Edwards & Haime, Montastraea cavernosa(Linne), Mussismilia hispida(Verrill), Poritescf. ast-reoidesLamarck, Scolymia wellsiLaborel, Siderastrea stellataVerrill, Millepora alcicornis(Linne) and abizarre sub-massive form of Millepora,possibly a new species. Fifty-five percent of the reef coral fauna surveyed iscomposed of species widely distributed in the Western Atlantic and 45 % of Brazilian endemics (Belem et al.1986, Hetzel and Castro 1994). At the time of our visit all coral species were bleached. The Manoel Luis Reefs region, due to its proximity to the Amazon River, may play a major role in geneflow between northern and southern Western Atlantic reef fauna. The presence and great abundance ofCaribbean reef organisms which do not occur along the eastern coast of South America, such as the purple reeffish Chromis scotti,provide additional evidence that the Manoel Luis Reefs may be one of the main faunalstepping stones between the Caribbean and the Brazilian coast (Collette & Rutzler 1977).

Spatial and temporal distribution of algae in soil crusts in the Sahel of W Africa: Preliminary results

Hahn, A. & Kusserow, H.: Spatial and temporal distribution of algae in soil crusts in the Sahel of W Africa: Preliminary results. − Willdenowia 28: 227-238. 1998. − ISSN 0511-9618. As part of an international project dealing with genetic diversity in wild forage species of the Sahelian Zone, a preliminary investigation of soil algal crusts in highly degraded environments was made. Soil sealing and the formation of crusts belong to the complex mechanisms of desertification processes. The kinds of soil algae present in the crusts and their temporal dynamics through the seasons are of relevance for the understanding of these processes. Surface crust samples from two wet and two dry seasons were examined for their algal flora by culturing and microscopy, in order to assess the role of algae in such processes. At least two different types of crusts were distinguished: dark crusts with a high algal contribution, dominated by filamentous blue-green algae (cyanobacteria), and light crusts with a far lesser algal contribution. The development of cyanobacterial crusts over two years suggests a seasonality, with a codominance of coccal green algae during the dry seasons. On light soils, a gradient was also apparent in the algal flora from sealed to sandy patches. In long-term development of fast changing arid ecosystems such as the Sahel, soil algae might prove a useful bio-indicator in screening for desertification and revegetation.

Effects of benthic biology on bottom boundary layer processes, Dry Tortugas Bank, Florida Keys

An instrumented tetrapod was deployed for three weeks on the Dry Tortugas Bank at a depth of 26 m in February 1995. Bottom roughness was dominated by shrimp burrows and worm mounds with rms roughness amplitudes ranging from 0.47 to 1.75 cm. Logarithmic velocity profiles show apparent total roughness heights ranging from 0.30 to 1.45 cm, values consistent with observed biological roughness. The bed sediments were weakly bound by an algal crust at the sediment–water interface. When this bound layer was scraped away by a mooring that was accidentally dragged, sharp-crested wave-induced ripples appeared within the resulting swath. We conclude that physically induced roughness is biologically suppressed, but if dominant, would be significantly higher than the prevailing biological roughness.