Chris T. Perry

Caribbean-wide decline in carbonate production threatens coral reef growth.

Global-scale deteriorations in coral reef health have caused major shifts in species composition. One projected consequence is a lowering of reef carbonate production rates, potentially impairing reef growth, compromising ecosystem functionality and ultimately leading to net reef erosion. Here, using measures of gross and net carbonate production and erosion from 19 Caribbean reefs, we show that contemporary carbonate production rates are now substantially below historical (mid- to late-Holocene) values. On average, current production rates are reduced by at least 50%, and 37% of surveyed sites were net erosional. Calculated accretion rates (mm year−1) for shallow fore-reef habitats are also close to an order of magnitude lower than Holocene averages. A live coral cover threshold of ~10% appears critical to maintaining positive production states. Below this ecological threshold carbonate budgets typically become net negative and threaten reef accretion. Collectively, these data suggest that recent ecological declines are now suppressing Caribbean reef growth potential.

Carbonate budgets and reef production states: a geomorphic perspective on the ecological phase-shift concept

Recent, region-scale estimates suggest that high levels of coral cover loss have occurred in both the Caribbean and Indo-PaciWc reef provinces (Gardner et al. 2003; Bruno and Selig 2007), and that some 60–70% of coral communities globally are directly threatened by anthropogenic activities (Wilkinson 1996; Hoegh-Guldberg 1999; Goreau et al. 2000). The ecological changes that have resulted from these disturbances have been widely discussed in terms of coral reef ‘health’ and resilience (Hughes et al. 2003; Bellwood et al. 2004) and often aligned with the ecological phase-shift concept originally outlined by Done (1992). A phase shift, in the context of a coral reef, has been deWned as a transition in the ecological state of the reef to conditions of low coral cover and persistent high Xeshy macroalgal cover (Done 1992; McManus and Polsenberg 2004). Whilst episodic natural disturbance events can be important drivers of such ecological transitions, a variety of both direct and indirect anthropogenic disturbances are also widely implicated (Done 1999). Such disturbances might modify natural reef processes, either by altering the ecological balance within individual reef systems (Jackson 1997; Jackson et al. 2001) or the environmental conditions under which reef growth occurs (Kleypas et al. 1999). Whilst earlier concerns over passive reef ‘drowning’ in response to greatly accelerated sea-level rise (e.g. Buddemeier and Smith 1988) have not been maintained (Spencer 1995), there remains considerable concern as to how more modest rates of sea-level rise, alongside climate change-related shifts in chronic stress from changing ocean temperatures and ocean chemistry, may lead to damage acute events occurring at ever more frequent intervals (e.g. Hoegh-Guldberg et al. 2007). The implications for reefs: as geomorphic structures, in terms of the extent of reef framework development and in terms of net carbonate accumulation (Edinger et al. 2000); the geomorphic integrity of reef associated sedimentary landforms; and the ecological eVects, in terms of changes in reef community structures (Chadwick-Furman 1996; Harvell et al. 2002; Hughes et al. 2003; Hoegh-Guldberg et al. 2007) remain, however, unclear. In the scientiWc literature, much of the discussion about the eVects of such ecological and environmental change has focused on quantifying the impacts upon coral communities, especially in terms of monitoring changes in coral cover and diversity, and in coral community structure. Central to many such discussions has been the issue of variations in the relative abundance of corals and macroalgae Communicated by Geology Editor Dr Bernhard Riegl

Marginal and non-reef-building coral environments

This special issue of Coral Reefs stems from a thematic session held at the European Meeting of the International Society for Reef Studies held in Cambridge in September 2002. A wide range of papers were presented, covering aspects of oceanography, sediment transport and accumulation, sedimentary settings, species assemblages, coral physiology, and geological evolution. These presentations emphasized the diversity and potential significance of the coral communities that occur in a wide range of what might be described as ‘‘marginal’’ settings, and demonstrated the geological and ecological significance of marginal and non-reefbuilding coral environments. Most papers published in this issue were submitted from researchers who presented work at the Cambridge meeting; however, some additional contributions were also accepted. When we were organizing the thematic session on ‘‘marginal’’ and non-reef-building coral environments at Cambridge, and then in editing this Special Issue, we wondered whether the title of this Special Issue, and specifically the word ‘‘marginal,’’ could be a little misleading (we expand a little on this thought in sections below). In this introductory paper, the term ‘‘marginal’’ marine is used in a broad sense, to describe settings where coral communities or framework reefs occur either close to well-understood (or strongly perceived) environmental thresholds for coral survival (sensu Kleypas et al. 1999) or in areas characterized by ‘‘suboptimal’’ or fluctuating environmental conditions. These include settings characterized by high or low temperatures, salinities, or nutrient levels, or by low light penetration or low aragonite saturation states. We are aware that much of this issue generally considers only one component of an ecological biota, so the use of the term ‘‘coral communities’’ here and in many of the papers in this issue might equally be read as ‘‘coral biotopes,’’ ‘‘coral assemblages,’’ or ‘‘ecological assemblages of corals.’’ Further, we use ‘‘framework reefs’’ here to explicitly denote those coral assemblages that have produced accretion through the production of reef framework, and we do not intend this as a contribution to any debate on what does or does not constitute a reef. The settings under consideration in this issue include settings where external factors act to change parameters such as temperature, salinity, nutrient load, and suspended sediment concentrations over a variety of magnitudes and timescales (e.g. ‘‘permanently,’’ seasonally, over lunar and other cycles, and episodically). Such settings may be considered by some as ‘‘marginal’’ in terms of reef-building potential, for example, the production of reef framework, but they are clearly important for maintaining a rich diversity of coral community and reef types, and as localized sites of coral and carbonate sediment production and, in some cases, accumulation.

Impacts of light penetration on the bathymetry of reef microboring communities: implications for the development of microendolithic trace assemblages

Abstract Sediment samples recovered from three proximate reef environments in north Jamaica, characterised by different light penetration regimes, reveal marked variations in the bathymetric range of sediment microboring communities. In contrast to clear water environments, assemblages developed in turbid water conditions undergo both bathymetric restriction and compression in their range. In clear water sites, the upper photic–lower photic zone transition occurs between depths of 20–30 m. This is marked by a shift from a predominance of cyanobacterial (e.g., Eurygonum nodosum, Scolecia filosa, Fasciculus frutex and Fasciculus dactylus) and chlorophyte (e.g., Rhopalia catenata, Fasciculus grandis, Reticulina elegans and Scolecia botulifera) traces, to assemblages dominated by the traces E. nodosum and S. filosa (cyanobacteria), F. grandis (chlorophyte), Palaeoconchocelis starmarchii (rhodophyte) and Orthogonum fusiferum and Saccomorpha sphaerula (fungi). At sites characterised by intermediate levels of turbidity this same assemblage transition occurs at around 15–20 m depth and at the most turbid site at depths of only 5–10 m. Integration of assemblage data and light readings indicates that this transition occurs where percentages of surface light illumination are reduced to around 10–15%. Compression of the lower photic zone is so extreme at the most turbid site, that microboring assemblages at only 30 m depth show evidence of a transition to an assemblage associated with dysphotic (

1200 year paleoecological record of coral community development from the terrigenous inner shelf of the Great Barrier Reef

Increased terrestrial sediment and nutrient yields are regarded as significant threats to coral reef health. Within the central Great Barrier Reef lagoon, where water quality has reportedly declined since European settlement (since ca. A.D. 1850), inner-shelf reef conditions have purportedly deteriorated. However, the link between reef decline and water-quality change remains controversial, primarily because of a lack of pre-European period ecological baseline data against which to assess contemporary ecological states. Here we present a high-resolution record of reef accretion and coral community composition from a turbid-zone, nearshore reef on the inner shelf of the Great Barrier Reef; the record is based on six radiocarbon date–constrained cores, and extends back to ca. 1200 calibrated yr B.P. Results demonstrate not only the potential for coral communities to initiate and persist in settings dominated by fine-grained terrigenous sediment accumulation, but also that a temporally persistent (but low diversity) suite of corals has dominated the reef-building community at this site for at least the past millennium. Furthermore, the coral assemblages exhibit no evidence of community shifts attributable to post-European water-quality changes. While extrapolation of these findings to other turbid-zone reefs must remain tentative, the study raises important questions about the resilience of inner-shelf reefs that are under terrestrial sediment influence and subject to elevated turbidity conditions, and demonstrates the potential to develop detailed, millennial time scale, coral community records from Holocene reef systems.

Evidence of very rapid reef accretion and reef growth under high turbidity and terrigenous sedimentation

Global-scale deteriorations in coral reef health are projected to lead to a progressive decline in reef-building potential and ultimately to states of net reef erosion. These transitions may be driven by various human disturbances and by climate change; however, increased terrestrial sediment and nutrient yields from anthropogenically modified coastal catchments are widely recognized as a major threat. As water quality deteriorates, reduced coral cover and species diversity are commonly inferred, and lower reef accretion rates and impaired reef development are assumed consequences. Here we present a detailed chronostratigraphic growth history, constrained by 40 accelerator mass spectrometry radiocarbon dates for Middle Reef, an inshore turbid-zone reef on Australias Great Barrier Reef, that challenges the assumption that high terrestrial sediment inputs inherently restrict reef accretion rates and inhibit reef development. We establish that Middle Reef has vertically accreted very rapidly for more than 700 yr, at an average rate of 8.3 mm yr⁻¹. Accretion rates varied across the reef at different times, but it is significant that the periods of most rapid accretion (averaging 13.0 mm yr⁻¹) coincide with phases of reef development dominated by fine-grained terrigenoclastic sediment accumulation. We suggest that this is in large part a function of a high rate of terrigenous sediment accumulation aiding the postmortem preservation of coral skeletal material. Both maximum and site-averaged accretion rates match or exceed those documented for most clear-water, mid- and outer-shelf reefs in the region over the past 9000 yr, and those determined for many reefs throughout the Indian and Pacific Oceans over the same period. While examples of inshore coral reefs that have been degraded in the short term by excessive terrestrial sedimentation clearly exist, others clearly tolerate high sedimentation and turbidity, and our data confirm that sustained and long-term rapid reef growth is possible in these environments.

Fish as major carbonate mud producers and missing components of the tropical carbonate factory

Carbonate mud is a major constituent of recent marine carbonate sediments and of ancient limestones, which contain unique records of changes in ocean chemistry and climate shifts in the geological past. However, the origin of carbonate mud is controversial and often problematic to resolve. Here we show that tropical marine fish produce and excrete various forms of precipitated (nonskeletal) calcium carbonate from their guts (“low” and “high” Mg-calcite and aragonite), but that very fine-grained (mostly < 2 μm) high Mg-calcite crystallites (i.e., MgCO3) are their dominant excretory product. Crystallites from fish are morphologically diverse and species-specific, but all are unique relative to previously known biogenic and abiotic sources of carbonate within open marine systems. Using site specific fish biomass and carbonate excretion rate data we estimate that fish produce ∼6.1 × 106 kg CaCO3/year across the Bahamian archipelago, all as mud-grade (the < 63 μm fraction) carbonate and thus as a potential sediment constituent. Estimated contributions from fish to total carbonate mud production average ∼14% overall, and exceed 70% in specific habitats. Critically, we also document the widespread presence of these distinctive fish-derived carbonates in the finest sediment fractions from all habitat types in the Bahamas, demonstrating that these carbonates have direct relevance to contemporary carbonate sediment budgets. Fish thus represent a hitherto unrecognized but significant source of fine-grained carbonate sediment, the discovery of which has direct application to the conceptual ideas of how marine carbonate factories function both today and in the past.

Factors Controlling Sediment Preservation on a North Jamaican Fringing Reef: A Process-Based Approach to Microfacies Analysis

ABSTRACT Compositional characteristics of limestones are fundamental to the interpretation of depositional environments, with an assumption that preserved grain assemblages preserve sufficient evidence of initial sediment input to enable delineation of environments. It is acknowledged, however, that a range of early diagenetic processes (including encrustation, fragmentation, abrasion, microboring, dissolution, cementation, and recrystallization) may influence grain preservation potential, and thus bias the composition of resultant grain assemblages. This study examines the effects and relative importance of a range of early diagenetic processes across a fringing reef at Discovery Bay, north Jamaica. Physical processes (abrasion and fragmentation) are most important at shallow fore-reef sites, whilst back-reef sites are dominated by biochemical processes, including microboring, recrystallization, biofilm-related calcification, and dissolution. Grain susceptibility to each process is highly variable and influenced by mineralogy and skeletal structure. Coral fragments, for example, are most susceptible to the effects of microboring, whereas Halimeda plates are most susceptible to dissolution. Extensive alteration of grain assemblages is predicted within the back-reef, where dissolution of coral and Halimeda occurs because of intense microboring and pore-water undersaturation. Fore-reef assemblages, by contrast, remain relatively unaltered. Results have implications both for improved ecological interpretation of carbonate sedimentary sequences and for understanding of how and why carbonate microfacies develop.

Evidence for the episodic "turn on" and "turn off" of turbid-zone coral reefs during the late Holocene sea-level highstand

Terrigenous sediment accumulation within nearshore marine environments is regarded as a major factor inhibiting carbonate production and coral reef accretion. While recent ecological and geological research into reef development under long-term terrigenous sediment influence questions the overly simplistic nature of such views, understanding of the time scales of reef initiation and growth and the morphodynamics of reef accretion in these settings remains limited. Here we present evidence to support recent suggestions that, once established, rapid reef accretion and progradation is possible, but that the restricted accommodation windows in which such reefs develop often result in short-lived (ephemeral) phases of reef building. Specifically, we describe two discrete periods of reef growth within one small (∼600 m wide) coastal embayment around a high island on the terrigenous sediment–dominated inner shelf of the central Great Barrier Reef, Australia. These reef-building phases occurred at either end of the Holocene sea-level highstand, the first during the late postglacial marine transgression and early highstand (∼6900–4500 calibrated (cal) yr B.P.), the second following the late Holocene regression and stillstand (∼1600 cal yr B.P. to present). An ∼3000 yr hiatus occurred between these events, probably as a function of subtle changes in sea level and associated shoreline morphodynamics.

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.