‘Ten Years After’—a long-term settlement and bioerosion experiment in an Arctic rhodolith bed (Mosselbukta,Svalbard)

Rhodolith beds and bioherms formed by ecosystem engineering crustose coralline algae support the northernmost centres of carbonate production, referred to as polar cold-water carbonate factories. Yet, little is known about biodiversity and recruitment of these hard-bottom communities or the bioeroders degrading them, and there is a demand for carbonate budgets to include respective rates of polar carbonate build-up and bioerosion. To address these issues, a 10-year settlement and bioerosion experiment was carried out at the Arctic Svalbard archipelago in and downslope of a rhodolith bed. The calcifiers recorded on experimental settlement tiles (56 taxa) were dominated by bryozoans, serpulids and foraminiferans. The majority of the bioerosion traces (30 ichnotaxa) were microborings, followed by attachment etchings and grazing traces. Biodiversity metrics show that calcifier diversity and bioerosion ichnodiversity are both elevated in the rhodolith bed, if compared to adjacent aphotic waters, but these differences are statistically insignificant. Accordingly, there were only low to moderate dissimilarities in the calcifier community structure and bioerosion trace assemblages between the two depth stations (46 and 127 m), substrate orientations (up- and down-facing) and substrate types (PVC and limestone), in that order of relevance. In contrast, surface coverage as well as the carbonate accretion and bioerosion rates were all significantly elevated in the rhodolith bed, reflecting higher abundance or size of calcifiers and bioerosion traces. All three measures were highest for up-facing substrates at 46 m, with a mean coverage of 78.2% (on PVC substrates), a mean accretion rate of 24.6 g m^?2  year^?1 (PVC), and a mean bioerosion rate of ?35.1 g m^?2 year^?1 (limestone). Differences in these metrics depend on the same order of factors than the community structure. Considering all limestone substrates of the two platforms, carbonate accretion and bioerosion were nearly in balance at a net rate of ?2.5 g m^?2 year^?1. A latitudinal comparison with previous settlement studies in the North Atlantic suggests that despite the harsh polar environment there is neither a depletion in the diversity of hard-bottom calcifier communities nor in the ichnodiversity of grazing traces, attachment etchings and microborings formed by organotrophs. In contrast, microborings produced by phototrophs are strongly depleted because of limitations in the availability of light (condensed photic zonation, polar night, shading by sea ice). Also, macroborings were almost absent, surprisingly. With respect to carbonate production, the Svalbard carbonate factory marks the low end of a latitudinal gradient while bioerosion rates are similar or even higher than at comparable depth or photic regime at lower latitudes, although this might not apply to shallow euphotic waters (not covered in our experiment), given the observed depletion in bioeroding microphytes and macroborers. While echinoid grazing is particularly relevant for the bioerosion in the rhodolith bed, respective rates are far lower than those reported from tropical shallow-water coral reefs. The slow pace of carbonate production but relatively high rates of bioerosion (both promoted by low carbonate supersaturation states in Arctic waters), in concert with high retention of skeletal carbonates on the seafloor and no calcite cements forming in open pore space created by microborers, suggest a low fossilisation potential for polar carbonates, such as those formed in the Mosselbukta rhodolith beds.     

Sponge bioerosion accelerated by ocean acidification across species and latitudes?

In many marine biogeographic realms, bioeroding sponges dominate the internal bioerosion of calcareous substrates such as mollusc beds and coral reef framework. They biochemically dissolve part of the carbonate and liberate so-called sponge chips, a process that is expected to be facilitated and accelerated in a more acidic environment inherent to the present global change. The bioerosion capacity of the demosponge Cliona celata Grant, 1826 in subfossil oyster shells was assessed via alkalinity anomaly technique based on 4 days of experimental exposure to three different levels of carbon dioxide partial pressure (pCO₂) at ambient temperature in the cold-temperate waters of Helgoland Island, North Sea. The rate of chemical bioerosion at present-day pCO₂ was quantified with 0.08–0.1 kg m^?2 year^?1. Chemical bioerosion was positively correlated with increasing pCO₂, with rates more than doubling at carbon dioxide levels predicted for the end of the twenty-first century, clearly confirming that C. celata bioerosion can be expected to be enhanced with progressing ocean acidification (OA). Together with previously published experimental evidence, the present results suggest that OA accelerates sponge bioerosion (1) across latitudes and biogeographic areas, (2) independent of sponge growth form, and (3) for species with or without photosymbionts alike. A general increase in sponge bioerosion with advancing OA can be expected to have a significant impact on global carbonate (re)cycling and may result in widespread negative effects, e.g. on the stability of wild and farmed shellfish populations, as well as calcareous framework builders in tropical and cold-water coral reef ecosystems.     

Cross-shelf differences in the pattern and pace of bioerosion of experimental carbonate substrates exposed for 3 years on the northern Great Barrier Reef, Australia

Patterns of bioerosion of dead corals and rubbles on the northern Great Barrier Reef were studied by using blocks of the massive coral Porites experimentally exposed at six sites, located on an inshore–offshore profile, for 1 year and 3 years. Rates of microbioerosion by microborers, grazing by fish, and macrobioerosion by filter-feeding organisms were simultaneously evaluated using image analysis. Microbioerosion, grazing, and total bioerosion were lower at reefs near the Queensland coast than at the edge of the continental shelf (1.81 kg m⁻² and 6.07 kg m⁻² after 3 years of exposure respectively, for total bioerosion). The opposite pattern was observed for macrobioerosion. Bioaccretion was negligible. These patterns were evident after 1 year of exposure, and became enhanced after 3 years. Microborers were established and were the main agent of bioerosion after 1 year of exposure, and as the principal support for grazing, continued to be the main cause of carbonate loss after 3 years. Full grazing activity and establishment of a mature community of macroborers required more than 1 year of exposure. After 1 year, macroborers and grazers were the second most important agents of bioerosion on both inshore and offshore reefs. However, after 3 years, grazers became the main agents at all sites except at the inshore sites, where macroborers were the principal agents. Because the contribution of microborers, grazers, and macroborers to bioerosion varies in space and time, we suggest that the estimation of reef carbonate budgets need to take in account the activities of all bioerosion agents.     

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A total of 59 taxa of epibionts and endobionts occurred on experimentally deployed gastropod shells within one year of emplacement at depths ranging from 15 m to 260 m in the Bahamas. Most of the diversity occurred within 73 m of water depth. The experimental shells at the deepest sites (210 m, 260 m) were essentially pristine. Differences in experimental treatment affected the results: shells in bags contained more bionts than tethered shells, suggesting the bags had more protective areas for biont settlement. Soft‐bodied encrusters were restricted to the upper 73 m while foraminiferans and bryozoans exhibited bathymetric trends to the deepest sites. While boring algae and cyanobacteria were ubiquitous on the shells to 73 m, other bioeroders (e.g., clionid sponges) were rare. Bioaccumulation, rather than bioerosion, is the predominant process affecting mollusc shells during the first year of taphonomic exposure in carbonate systems to depths of 260 m.     

Coral reef encruster communities and carbonate production in cryptic and exposed coral reef habitats along a gradient of terrestrial disturbance

Encrusting calcareous organisms such as bryozoans, crustose coralline algae (CCA), foraminiferans, and serpulid worms are integral components of tropical framework-building reefs. They can contribute calcium carbonate to the reef framework, stabilise the substrate, and promote larval recruitment of other framework-building species (e.g. coral recruits). The percentage cover of encrusting organisms and their rates of carbonate production (g m⁻² year⁻¹) were assessed at four sites within a coastal embayment, along a gradient of riverine influence (high-low). As the orientation and type of substrate is thought to influence recruitment of encrusting organisms, organisms recruiting to both natural (the underside of platy corals) and experimental substrates were assessed. The effect of substrate exposure under different levels of riverine influence was assessed by orientating experimental substrates to mimic cryptic and exposed reef habitats (downwards-facing vs upwards-facing tiles) at each site. Cryptic experimental tiles supported similar encruster assemblages to those recruiting to the underneath (cryptic side) of platy corals, suggesting that tiles can be used as an experimental substrate to assess encruster recruitment in reef systems. Encruster cover, in particular CCA, and carbonate production was significantly higher at low-impact (clear water), high wave energy sites when compared to highly riverine impacted (turbid water), low wave energy sites. Cryptically orientated substrates supported a greater diversity of encrusting organisms, in particular serpulid worms and bryozoans. The inverse relationships observed between riverine inputs and encrusters (total encruster cover and carbonate production) have implications for both the current and future rates and styles of reefal framework production.     

Calcium carbonate budgets for two coral reefs affected by different terrestrial runoff regimes, Rio Bueno, Jamaica

A process-based carbonate budget was used to compare carbonate framework production at two reef sites subject to varying degrees of fluvial influence in Rio Bueno, Jamaica. The turbid, central embayment was subjected to high rates of fluvial sediment input, framework accretion was restricted to ≤30 m, and net carbonate production was 1,887 g CaCO₃ m⁻² year⁻¹. Gross carbonate production (GCP) was dominated by scleractinians (97%), particularly by sediment-resistant species, e.g. Diploria strigosa on the reef flat (<2 m). Calcareous encrusters contributed very little carbonate. Total bioerosion removed 265 g CaCO₃ m⁻² year⁻¹ and was dominated by microborers. At the clear-water site, net carbonate production was 1,236 g CaCO₃ m⁻² year⁻¹; the most productive zone was on the fore-reef (10 m). Corals accounted for 82% of GCP, and encrusting organisms 16%. Bioerosion removed 126 g CaCO₃ m⁻² year⁻¹ and was dominated by macroborers. Total fish and urchin grazing was limited throughout (≤20 g CaCO₃ m⁻² year⁻¹). The study demonstrates that: (1) carbonate production and net reef accretion can occur where environmental conditions approach or exceed perceived threshold levels for coral survival; and (2) although live coral cover (and carbonate production rates) were reduced on reef-front sites along the North Jamaican coast, low population densities of grazing fish and echinoids to some extent offset this, thus maintaining positive carbonate budgets.     

Effects of Water Depth,Seasonal Exposure,and Substrate Orientation on Microbial Bioerosion in the Ionian Sea (Eastern Mediterranean)

The effects of water depth, seasonal exposure, and substrate orientation on microbioerosion were studied by means of a settlement experiment deployed in 15, 50, 100, and 250 m water depth south-west of the Peloponnese Peninsula (Greece). At each depth, an experimental platform was exposed for a summer period, a winter period, and about an entire year. On the up- and down-facing side of each platform, substrates were fixed to document the succession of bioerosion traces, and to measure variations in bioerosion and accretion rates. In total, 29 different bioerosion traces were recorded revealing a dominance of microborings produced by phototrophic and organotrophic microendoliths, complemented by few macroborings, attachment scars, and grazing traces. The highest bioerosion activity was recorded in 15 m up-facing substrates in the shallow euphotic zone, largely driven by phototrophic cyanobacteria. Towards the chlorophyte-dominated deep euphotic to dysphotic zones and the organotroph-dominated aphotic zone the intensity of bioerosion and the diversity of bioerosion traces strongly decreased. During summer the activity of phototrophs was higher than during winter, which was likely stimulated by enhanced light availability due to more hours of daylight and increased irradiance angles. Stable water column stratification and a resulting nutrient depletion in shallow water led to lower turbidity levels and caused a shift in the photic zonation that was reflected by more phototrophs being active at greater depth. With respect to the subordinate bioerosion activity of organotrophs, fluctuations in temperature and the trophic regime were assumed to be the main seasonal controls. The observed patterns in overall bioeroder distribution and abundance were mirrored by the calculated carbonate budget with bioerosion rates exceeding carbonate accretion rates in shallow water and distinctly higher bioerosion rates at all depths during summer. These findings highlight the relevance of bioerosion and accretion for the carbonate budget of the Ionian Sea.     

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.     

Changing dynamics of Caribbean reef carbonate budgets: emergence of reef bioeroders as critical controls on present and future reef growth potential

Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate production and reef growth. Using a cross-regional dataset, we show that widespread reductions in bioerosion rates—a key carbonate cycling process—have accompanied carbonate production declines. Bioerosion by parrotfish, urchins, endolithic sponges and microendoliths collectively averages 2 G (where G = kg CaCO₃ m⁻² yr⁻¹) (range 0.96–3.67 G). This rate is at least 75% lower than that reported from Caribbean reefs prior to their shift towards their present degraded state. Despite chronic overfishing, parrotfish are the dominant bioeroders, but erosion rates are reduced from averages of approximately 4 to 1.6 G. Urchin erosion rates have declined further and are functionally irrelevant to bioerosion on most reefs. These changes demonstrate a fundamental shift in Caribbean reef carbonate budget dynamics. To-date, reduced bioerosion rates have partially offset carbonate production declines, limiting the extent to which more widespread transitions to negative budget states have occurred. However, given the poor prognosis for coral recovery in the Caribbean and reported shifts to coral community states dominated by slower calcifying taxa, a continued transition from production to bioerosion-controlled budget states, which will increasingly threaten reef growth, is predicted.     

Temperate bioerosion: ichnodiversity and biodiversity from intertidal to bathyal depths (Azores)

In the temperate Azores carbonate factory, a substantial fraction of the calcareous skeletal components is recycled by a remarkable biodiversity of biota producing bioerosion traces (incipient trace fossils). To study this biodiversity, experimental carbonate substrates were exposed to colonisation by epilithic and endolithic organisms along a bathymetrical gradient from 0 to 500 m depth, during 1 and 2 years of exposure. The overall bioerosion ichnodiversity is very high and comprises 56 ichnotaxa and ichnoforms attributed to cyanobacteria, chlorophytes, fungi, other micro-chemotrophs, macroborers, grazers and epilithic attachment scars. In the intertidal, hydrodynamic force, partial emersion and strong temperature fluctuations lead to the lowest ichnospecies richness. This contrasts with the highest ichnodiversity found at 15 m under the most favourable environmental conditions. Towards aphotic depths, a gradual depletion in ichnodiversity is observed, most probably because of the restricted light availability and a slowdown in ichnocoenosis development. Analysis of similarity (ANOSIM), in combination with non-metrical multidimensional scaling (NMDS), was used to highlight variability in the relative abundance of traces among depths, substrate orientations and exposure times. Ichnodiversity and abundance of traces decrease significantly with depth and are higher on up-facing versus down-facing substrates, whereas differences between years were not as pronounced. This study demonstrates that statistical methods of biodiversity analysis are not per se restricted to biotaxa but may well be applied also to ichnotaxa. In the analysis of trace fossil assemblages, this approach supports the recognition of diversity patterns and their relation to environmental gradients.     

Experimental studies of rapid bioerosion of coral reefs in the Galápagos Islands

Experimental carbonate blocks of coral skeleton,Porites lobata (PL), and cathedral limestone (LS) were deployed for 14.8 months at shallow (5–6 m) and deep (11–13m) depths on a severely bioeroded coral reef, Champion Island, Galápagos Islands, Ecuador. Sea urchins (Eucidaris thouarsii) were significantly more abundant at shallow versus deep sites.Porites lobata blocks lost an average of 25.4 kg m^–2yr^–1 (23.71 m^–2yr^–1 or 60.5% decrease yr^–1). Losses did not vary significantly at depths tested. Internal bioeroders excavated an average of 2.6 kg m^–2 yr^–1 (2.41 m^–2 yr^–1 or 0.6% decrease yr^–1), while external bioeroders removed an average of 22.8 kg m^–2 yr^–1). (21.31 m^–2 yr^–1). or 59.9% decrease yr^–1). few encrusting organisms were observed on the PL blocks. Cathedral limestone blocks lost an average of 4.1 kg m^–2 yr^–1). (1.81 m^–2 yr^–1). or 4.6% decrease yr-'), also with no relation to depth. Internal bioeroders excavated an average of 0.6 kg m^–2 yr^–1). (0.31 m^–2 yr^–1). or 0.7% decrease yr^–1). and external bioeroders removed an average of 3.5 kg m^–2 yr^–1). (1.51 m^–2 yr^–1). or 3.9% decrease yr^–1). from the LS blocks. Most (57.6%) encrustation occurred on the bottom of LS blocks, and there was more accretion on block bottoms in deep (61.4 mg cm^–2 yr^–1). versus shallow (35.0 mg cm^–2 yr^–1) sites. External bioerosion reduced the average height of the reef framework by 0.2 cm yr^–1). for hard substrata (represented by LS) and 2.3 cm yr^–1). for soft substrata (represented by PL). The results of this study suggest that coral reef frameworks in the Galápagos Islands are in serious jeopardy. If rates of coral recruitment do not increase, and if rates of bioerosion do not decline, coral reefs in the Galápagos Islands could be eliminated entirely.     

Experimental studies on microbial bioerosion at Lee Stocking Island, Bahamas and One Tree Island, Great Barrier Reef, Australia: implications for paleoecological reconstructions

Different kinds of experimental calcareous substrates were exposed at Lee Stocking Island (Bahamas) and One Tree Island (Great Barrier Reef, Australia) to study which endolithic bacteria, algae and fungi contribute to bioerosion and what their bioerosion rates are. The sites at Lee Stocking Island were several leeward shallow water and several windward shallow and deep-water positions (from the Acropora palmata reef at 2 m down to 275 m depth). At One Tree Island, the experiments were conducted in patch reefs treated with P and N to study the influence of mineral nutrients on bioerosion. The exposure periods ranged from 1 week to 2 years. The micritic carbonate substrates exposed on Lee Stocking Island contained 6 genera with 15 species of cyanobacteria, green and red algae, and different kinds of microendolithic heterotrophs. The mean values of bioerosion rates measured between 1 to 2 g/m²/y at 275 m and 520 g/m²/y at one of the leeward sites. The composition of the endolithic community and the bioerosion rates changed over time. At One Tree Island, shell pieces of Tridacna were used as substrate exposed for 5 months to endolith activity. Five genera and 6 species of cyanobacteria, green and red algae and different kinds of heterotrophic microendoliths were found with bioerosion rates of 20-30 g/m²/y. There are differences in abundance of taxa between Lee Stocking Island and One Tree Island. The introduction of nutrients had no apparent impact on the microborer community. Controlling factors for the distribution and abundance of microborers are mainly light, but also the kind of substrate and, possibly, the biogeographic position. The results support the paleoecological importance of microendoliths.     

Bioerosion experiments at Lizard Island,Great Barrier Reef

The rates at which dead coral substrates are modified by bioerosional processes were determined by exposing recently killed corals for up to four years in a variety of reef environments at Lizard Island (northern Great Barrier Reef). Grazers were the major croding agents of these coral substrates and exhibited differences between sites that varied between sampling periods. Subtidal reef slopes and lagoon environments of water depths < 20 m were subjected to higher average rates of grazing erosion (0.30–1.96 kg/m²/y) than shallow depths less than 1 m (0.07–0.26 kg/m²/y). A deep site at 20 m experienced low average rates of grazing (0.08–0.29 kg/m²/y). Boring rates by worms (polychaetes and sipunculans), sponges and molluscs were relatively low and varied between sites, but increased with length of sampling period as larger borers succeeded the initial colonizing small polychaete worms. We hypothesize from these experiments that the extent of boring in reef substrates will be influenced by the interaction between the succession of the boring community and the rate at which the substrate is destroyed by grazing. We suggest that the level of grazing modifies the successional pattern of borers by removing the surface substrate and continually exposing bare substrate that can be colonized by early boring colonists. Thus, constant high levels of grazing may maintain the boring community at an early successional stage and prevent the development of a mature boring community. In order to establish large borer populations, reef substrates must be protected from extensive grazing bioerosion. This interaction of grazing and boring has important implications for the way dead coral is preserved in different reef environments.     

Bioerosion rates of the sponge Cliona orientalis Thiele, 1900: spatial variation over short distances

We studied bioerosion rates and tissue growth of the sponge Cliona orientalis Thiele, 1900. Experimental blocks grafted with sponge tissue were deployed at three sites in Moreton Bay, QLD, Australia, which have different environmental conditions. Bioerosion rates varied between 4, 5, and 10 kg m⁻² year⁻¹ when related to final tissue area and between 4, 7, and 16 kg m⁻² year⁻¹ when related to initial tissue area of the graft, which supports findings of earlier studies. Comparing results between the sites, eutrophication appeared to have the most stimulating effect and is most likely to have caused the measured differences. However, slight differences between shading and current speeds may also have played a role. Variation may have masked spatial differences of sponge growth, which were insignificant between study sites. Growth and bioerosion nevertheless followed the same trend and were weakly correlated. Habitat quality itself had no influence. Overall, the twofold difference in sponge bioerosion over a distance as short as 10 km suggests that when estimating bioerosion rates, subsamples should be tested at different locations.     

Epiphytes from a deep-water characean meadow in an oligotrophic New Zealand lake: species composition,biomass and photosynthesis

1. The epiphytic flora of a characean meadow in Lake Coleridge, a deep, oligotrophic lake on the South Island of New Zealand, was dominated by diatoms, particularly Eunotia pectinalis and Achnanthes minutissima. The meadows occupied a depth range from 5 to 30 m. Adnate taxa predominated at all depths below 5 m, while increased taxonomic diversity at 5 m resulted from an increased abundance of erect taxa, including chlorophytes and stalked diatoms. 2. Seasonal changes in epiphyte biomass were followed using artificial substrata and by estimating epiphyte chlorophyll a concentration on host plants. The latter required development of a novel technique utilizing the consistent relationship between fucoxanthin and chlorophyll a concentrations in the epiphyton. Epiphyte chlorophyll a on host plants varied with depth and host species between 0.1 and 0.3 mg g^–1 dry weight. Maximum epiphyte biomass was at 10–15 m depth. At depths of 15 m and less, epiphyte chlorophyll a reached a maximum of ≈ 200–300 mg m^–2 in mid-summer, while at greater depths maximum biomass was less and coincided with a period of clear water in spring. 3. Photosynthetic carbon fixation was estimated from photosynthesis–radiation curves and estimates of radiation flux at sampling depths. At depths greater than 10 m, variability of the vertical extinction coefficient of lake water rather than seasonal fluctuations in incident radiation were responsible for determining the temporal pattern of production. Chlorophyll a-specific photosynthesis was estimated to peak in summer at 5 m (8 mg mg^–1 day^–1), and in spring at all other depths. 4. Annual epiphyte production was estimated as 27 g C m^–2 year^–1 at 5 m depth, falling to 15 g C m^–2 year^–1 at 15 m and 1 g C m^–2 year^–1 at 30 m. Areal biomass changes tended to be temporally but not quantitatively coupled to estimated in situ photosynthesis, and we hypothesize that epiphyte biomass may have been controlled by grazing gastropod snails.     

Spatial variation in sea urchins, fish predators, and bioerosion rates on coral reefs of Belize

Although sea urchins are critical for controlling macroalgae on heavily fished coral reefs, high densities threaten reefs, as urchins are also prodigous bioeroders. This study examined urchin population characteristics, bioerosion rates, their fish predators (Labridae), and potential competitors (Scaridae) on unprotected reefs and a reef within a marine protected area (MPA) in the lagoonal regions off Belize. Urchin density (<1 m⁻²) and bioerosion rates (∼0.2 kg CaCO₃ m⁻² year⁻¹) were lowest and members of the Labridae were the highest (∼20 fish 200 m⁻³) within the MPA, while several unprotected reefs had higher (∼18–40 m⁻²) urchin densities, lower Labridae abundances (1–3 fish 200 m⁻³), and bioerosion rates ranging from ∼0.3–2.6 kg CaCO₃ m⁻² year⁻¹. Urchin abundances were inversely related to Labridae (wrasses and hogfish) densities; however, on reef ridges, low algal cover (∼15%), small urchin size (∼14 mm), and low proportion of organic material in urchin guts suggested food limitation. Both top–down (predation) and bottom–up factors (food limitation) likely contribute to the control of urchins, predominantly Echinometra viridis, off Belize, thereby potentially diminishing the negative impacts of bioerosion activities by urchins.     

Bioerosion in the Miocene Reefs of the northwest Red Sea,Egypt

Macroborings provide detailed information on the bioerosion, accretion and palaeoenvironment of both modern and fossil reefs. Dolomitized reefal carbonates in the Um Mahara Formation exhibit an outstanding example of spatially distributed, well‐preserved bioerosion structures in tropical to subtropical syn‐rift Miocene reefs. Ten ichnospecies belonging to five ichnogenera are identified; three belonging to the bivalve‐boring ichnogenus Gastrochaenolites, three attributed to the sponge‐boring ichnogenus Entobia, and four ichnospecies assigned to three worm‐boring ichnogenera Trypanites, Maeandropolydora and Caulostrepsis. The distribution of the reported borings is strongly linked to the palaeo‐reef zones. Two distinctive ichnological boring assemblages are recognized. The Gastrochaenolites‐dominated assemblage reflects shallower‐marine conditions, under water depths of a few metres, mostly in back‐reef to patch‐reef zones of a back‐reef lagoon. The Entobia‐dominated assemblage signifies relatively deeper marine conditions, mostly in reef core of the fringing Miocene reefs. These ichnological assemblages are attributed herein to the Entobia sub‐ichnofacies of the Trypanites ichnofacies. This ichnofacies indicates boring in hard carbonate substrates (such as corals, rhodoliths, carbonate cements and hardgrounds) during periods of non‐sedimentation or reduced sediment input.     

The effects of eutrophication-related alterations to coral reef communities on agents and rates of bioerosion (Reunion Island,Indian Ocean)

This study investigated the variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) is subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers was measured using experimental substrata exposed for 1 year in three sites characterized by different levels of nutrient input and benthic community response. The relationship between bioerosion and epilithic algal cover of hard substrata and the interactions between the various agents of bioerosion were analyzed with parametric statistics. Significant variations in bioerosion were found among sites, ranging from 1.63 to 3.52 kg CaCO₃ m^-2 year^-1 for grazing rates, from 6.73 to 32.25 g m^-2 year^-1 for macroboring rates, and from 43.78 to 67.56 g m^-2 year^-1 for microboring rates. One of the major factors controlling these variations appeared to be changes in the epilithic algal cover on substrata in response to changes in reefal water chemistry. In low nutrient areas, where dead corals were colonized mainly by algal turfs, erosion by microorganisms was low (43.78 g m^-2 year^-1) due to intense grazing (3.52 kg m^-2 year^-1). In reef zones receiving high nutrient inputs, the development of encrusting calcareous algae and macroalgae was associated with the lowest grazing (1.63 kg m^-2 year^-1) and macroboring (6.73 g m^-2 year^-1) rates recorded among sites. In contrast, high microboring rates (57.54 and 67.56 g m^-2 year^-1) were found in enriched areas in association with high macroalgal cover.     

Syn-Vivo Bioerosion of Nautilus by Endo- and Epilithic Foraminiferans (New Caledonia and Vanuatu)

A variety of syn-vivo bioerosion traces produced by foraminiferans is recorded in shells of Nautilus sampled near New Caledonia and Vanuatu. These are two types of attachment scars of epilithic foraminiferans and two forms of previously undescribed microborings, a spiral-shaped and a dendritic one, both most likely being the work of endolithic ''naked'' foraminiferans. Scanning electron microscopy of epoxy-resin casts of the latter revealed that these traces occur in clusters of up to many dozen individuals and potentially are substrate-specific. The foraminiferan traces are the sole signs of bioerosion in the studied Nautilus conchs, and neither traces of phototrophic nor other chemotrophic microendoliths were found. While the complete absence of photoautotrophic endoliths would be in good accordance with the life habit of Nautilus, which resides in aphotic deep marine environments and seeks shallower waters in the photic zone for feeding only during night-time, the absence of any microbial bioerosion may also be explained by an effective defence provided by the nautilid periostracum. Following this line of reasoning, the recorded foraminiferan bioerosion traces in turn would identify their trace makers as being specialized in their ability to penetrate the periostracum barrier and to bioerode the shell of modern Nautilus.     

Biostratigraphic and paleoclimatic significance of a new Pliocene foraminiferal fauna from the central Arctic Ocean

A Pliocene benthic foraminiferal fauna containing a previously unknown species association was found in the basal section of a piston core collected from the crest of Northwind Ridge (NWR) in the central Arctic Ocean. The fauna is dominated by Epistominella exigua, Cassidulina reniforme, Eponides tumidulus, Cibicides scaldisiensis, Lagena spp., Cassidulina teretis, Eponides weddellensis, Bolivina arctica, and Patellina corrugata. The presence of Cibicides scaldisiensis in the assemblage and the occurrence of Cibicides grossus higher in the core are indicative of an early Pliocene age. The morphologically distinctive species Cibicidoides sp. 795 of McNeil (in press) which occurs in the NWR core sample was previously known only from Oligocene through Miocene deposits in the Beaufort-Mackenzie Basin of Arctic Canada. Ehrenbergina sp. A and Cibicidoides aff. C. sp. 795, also present in the core, are new and endemic to the Arctic late Miocene and early Pliocene. These species, and possibly others, are survivors of the late Miocene (Messinian) sea-level crisis, which caused a significant faunal turnover in the Arctic Ocean. The predominantly calcareous assemblage indicates deposition above the calcium carbonate compensation depth in an upper bathyal environment. Paleogeographic affinities for the bulk of the assemblage indicate probable connections between the Arctic and the North Atlantic Oceans, but the endemic species identify environmental differences or partial isolation of the western Arctic Ocean. The species association suggests a cold but milder paleoclimate than that which existed during Pleistocene glacial intervals.