Ian G. Macintyre

Acropora palmata reef framework: A reliable indicator of sea level in the western atlantic for the past 10,000 years

SummaryA minimum sea-level curve for the past 10,000 years has been constructed on the basis of radiocarbon dates of Acropora palmata (Lamarck) samples from the shallow-water framework of both relict and modern reefs of the tropical western Atlantic. A. palmata framework is a reliable reference for reconstructing the history of late Quaternary sea levels owing to its restricted depth range (<1 to 5 m), the lack of postdepositional transport of A. palmata framework, the ease of obtaining uncontaminated samples, and the minimal compaction of A. palmata reef facies. The minimum sea-level curve constructed in this study is useful not only in evaluating the reliability of present and future Holocene sea-level curves for the western Atlantic, but also in estimating paleo-water depths in the study of Holocene reef history of this area.

THE EXPANDING SCALE OF SPECIES TURNOVER EVENTS ON CORAL REEFS IN BELIZE

Beginning in the late 1980s, white-band disease nearly eliminated the stag- horn coral Acropora cervicornis from reefs in the central shelf lagoon of Belize. The lettuce coral Agaricia tenuifolia replaced Acropora cervicornis in the early 1990s, but anomalously high water temperatures in 1998 caused severe bleaching and catastrophic mortality of Agaricia tenuifolia. The short-lived transition in dominance from Acropora cervicornis to Agaricia tenuifolia left an unambiguous signature in the fossil record of these uncemented lagoonal reefs. Analysis of 38 cores, extracted from 22 sampling stations in a 375-km2 area of the central lagoon, showed that Acropora cervicornis dominated continuously for at least 3000 years prior to the recent events. Agaricia tenuifolia occasionally grew in small patches, but no coral-to-coral replacement sequence occurred over the entire area until the late 1980s. Within a decade, the scale of species turnover increased from tens of square meters or less to hundreds of square kilometers or more. This unprecedented increase in the scale of turnover events is rooted in the accelerating pace of ecological change on coral reefs at the regional level.

PHASE SHIFTS, ALTERNATIVE STATES, AND THE UNPRECEDENTED CONVERGENCE OF TWO REEF SYSTEMS

Initial conditions can generate differences in the biotic composition of spatially disjunct communities, but intense, large-scale perturbations have the potential to reduce or eliminate those historical differences. The latter possibility is of particular concern with respect to coral reefs, which have undergone dramatic changes in the last 25–30 years. This paper reports a case in which two reef systems with different biotic histories were recently perturbed to a single, novel state. n nWe compared millennial-scale records of species dominance from reefs in Bahia Almirante, a coastal lagoon in northwestern Panama, to previously published records from reefs in the shelf lagoon of Belize. Reef cores extracted from Bahia Almirante at 5–10 m water depth revealed that the Panamanian reefs were persistently dissimilar from the Belizean reefs for at least 2000–3000 years prior to the last several decades. The Panamanian reefs were dominated continuously by branching finger corals, Porites spp. (primarily P. furcata). Shifts from the Porites-dominated state to dominance by other coral species were rare, were restricted to small areas, and lasted for decades to centuries. The Belizean reefs were dominated continuously by the staghorn coral Acropora cervicornis in the same depth range during the same period. Excursions from the Acropora-dominated state were again rare and spatially localized. Populations of Ac. cervicornis in the Belizean lagoon were nearly extirpated by an outbreak of white-band disease in the late 1980s, and changes in water quality were apparently detrimental to branching Porites in Bahia Almirante in recent decades. These large-scale perturbations caused the two reef systems to converge on a third, historically unprecedented state: dominance by the lettuce coral Agaricia tenuifolia. Ag. tenuifolia possesses life-history attributes and environmental tolerances that enabled it to become dominant in both disturbed ecosystems. Although the two phase shifts to Ag. tenuifolia differed in both their general mechanisms and specific causes, they had the effect of eliminating the salient difference in benthic composition between the Panamanian and Belizean reefs. The changes in species composition thus obliterated the influence of several thousand years of reef history.

Coral bleach-out in Belize

The highest sea surface temperatures ever recorded, related both to the 1997–98 El Niño/Southern Oscillation and to global warming, caused severe bleaching of corals worldwide in 1998 (ref. 2). This thermal anomaly induced mass mortality of scleractinian corals on lagoonal reefs in Belize, the first time that a coral population in the Caribbean has collapsed completely from bleaching. Cores extracted from the Belizean reefs showed that these events were unprecedented over at least the past 3,000 years.

Thickest recorded Holocene reef section, Isla Pérez core hole, Alacran Reef, Mexico

The Isla Perez core hole (Alacran Reef, Mexico) records the thickest Holocene section (33.5 m) known from either Atlantic or Indo-Pacific reefs. The high rate of deposition of reef material—a maximum of 12m/l,000 yr—is attributed to accumulation of the open framework constructed by the rapidly growing coral Acropora cervicornis . Mineralogic and radiocarbon analyses indicate that in areas protected from frequent high-energy agitation, this fragile branching coral, which thrives in moderate to shallow water depths, is capable of constructing extensive Holocene biohermal structures having more than 15m relief.

The zonation patterns of Caribbean coral reefs as controlled by wave and light energy input, bathymetric setting and reef morphology: computer simulation experiments

The computer model COREEF was used to simulate variations in the zonation patterns of Caribbean reefs in relation to parameters that affect the magnitude and distribution of wave and light energy. We first developed a simulated standard reef by exposing a simplified profile of the reef at Discovery Bay, Jamaica, to the known wave and light energy conditions to establish a reference coralgal and sedimentological zonation pattern. We then varied 13 parameters related to the wave and light energy input, bathymetric setting, and gross morphology of this reef to determine the effects of each parameter on the zonation pattern. Analysis of the simulation results indicates that submerging the reef or altering the wave or light energy input to the reef produces the greatest modifications of the zonation pattern. Morphological structures that alter a reefs horizontal dimensions only minimally affect the zonation pattern, but those structures that alter a reefs vertical dimensions-particularly steep-sided, wave reflecting structures-can significantly modify the zonation of the structure itself and that of more leeward areas. The more seaward the location of a morphological structure, the more profoundly it can affect the overall reef zonation. If waves break at the reef crest, wave energy conditions in the back reef are greatly reduced and the bottom consists of lower wave energy zones than those found at the same depths in the fore reef. If waves do not break at the crest, the back reef is subjected to almost the same wave conditions that exist in the fore reef, and the zones tend to be similar. The zonation patterns of some existing reefs resemble those of our simulated reefs, but other zonation patterns cannot be reproduced accurately because our simulation experiments do not consider the interactions between multiple parameters found on many existing reefs.

Crystal Alteration in a Living Calcareous Alga (Halimeda): Implications for Studies in Skeletal Diagenesis

ABSTRACT Although calcification processes and the carbonate deposits in Halimeda have been described in many papers, previous workers have failed to recognize that a reorganization of crystals occurs within living individuals of common species of this alga. Detailed SEM observations of live Halimeda incrassata collected in Biscayne Bay, Florida, show that calcification occurs in two basic stages: (1) uniform-sized, small aragonite needles, 1-3 µm long, are precipitated in inter-utricle spaces and (2) these needles alter to anhedral equant aragonite approximately 0.1-0.5 µm in size. In addition, large aragonite needles, up to 15 µm long, are precipitated in open spaces, typically in the inner cortical and medullary regions during the second stage of calcification. Micritization of original skeletal needles in H. incrassata is extensive but is not readily detected with the petrographic microscope. Preliminary observations of another common species, H. opuntia, suggest that similar alteration processes also occur in this alga. Early micritization in live algae may be an important process of skeletal alteration that has not previously been recognized. This early alteration must be distinguished from postdepositional diagenetic processes in the sediment.

Growth History of Stromatolites in a Holocene Fringing Reef, Stocking Island, Bahamas

ABSTRACT A stromatolite and algal ridge reef complex 2.1 m thick fringes the east coast of Stocking Island, Exuma Cays, Bahamas. This reef was established on a Pleistocene calcarenite terrace about 4500 yr B.P. Stromatolites, which occur in back-reef and reef-flat zones, are up to 1 m thick and were constructed by cyanobacterial-dominated communities. Study of the growth history of these stromatolites, ranging in scope from facies analyses to details of microfabric construction, presents new perspectives on stromatolite formation. Lithologies identified in eight cores from across the Stocking Island reef complex, together with plots of 13 radiocarbon dates in relation to a Bahamian sea-level curve, indicate that this reef began as an intertidal vermetid gastropod buildup. Subsequent flooding of the Pleistocene terrace allowed the branching coralline alga Neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago. Shifting sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores, thereby promoting growth of cyanobacterial mats that formed stromatolite buildups. With a decrease in wave energy over the last 500 years, possibly due to the growth of offshore patch reefs, the urchin Echinometra lucunter coloni ed the algal ridge. Resultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite buildups. Lamination in the Stocking Island stromatolites results from early lithificafion processes in cyanobacterial mats, possibly in response to biogeochemical changes in the mats during hiatuses in sediment accretion. These processes, which create partially indurated laminae with a distinct microstructure, involve precipitation of thin micrite crusts, intense microboring along a surface below this crust, micritization of sediment grains, and precipitation of point-contact cement between micritized grains. Introduction of turf algae to the cyanobacterial mat community disrupts formation of the lithified laminae, thereby inhibiting stromatolite development.

Physiographic Features on the Outer Shelf and Upper Slope, Atlantic Continental Margin, Southeastern United States

Both erosional and constructional processes appear to have formed physiographic features near the shelf break along the southeastern United States, as indicated by extensive echosounder profiles, rock-dredge material, and bottom photographs. Between Cape Hatteras, North Carolina, and Fort Lauderdale, Florida, four distinct physiographic areas are delineated, each having characteristic morphologies and lithologies. The ridges and well-defined troughs on the outer shelf and upper slope (depths of about 50 to 150 m) between Cape Hatteras and Cape Fear may be related largely to earlier Gulf Stream erosion, and the rocks (algal limestones and sandstones) and sediments dredged from these features probably are mainly Holocene, relict shallow-water deposits forming a thin veneer over this erosional surface of the sea floor. Relatively rapid accumulation of pre-Holocene sediments may account for the general absence of pronounced physiographic features on the outer shelf and upper slope from Cape Fear to Cape Kennedy . Ledges, small terraces, and rises (depths of 50 to 110 m) in this area are probably Holocene features eroded into, or constructed on the pre-Holocene sediments, which are covered by transgressive Holocene algal limestones and sandstones similar to those collected to the north. The lithology, together with radiocarbon dates of rock material, indicate that well-defined ridges in depths of 70 to 90 m between Cape Kennedy and Palm Beach are relict oolitic ridges or “dunes” formed during the Holocene transgression; these features are now covered by modern Oculina sp. coral debris. From Palm Beach to Fort Lauderdale, where the continental shelf is narrow and shallow, a small ridge present at the shelf break (15 to 30 m) is thought to be an “inactive” coral reef.

EMERGENT ZONATION AND GEOGRAPHIC CONVERGENCE OF CORAL REEFS

Environmental degradation is reducing the variability of living assemblages at multiple spatial scales, but there is no a priori reason to expect biotic homogenization to occur uniformly across scales. This paper explores the scale-dependent effects of recent perturbations on the biotic variability of lagoonal reefs in Panama and Belize. We used new and previously published core data to compare temporal patterns of species dominance between depth zones and between geographic locations. After millennia of monotypic dominance, depth zonation emerged for different reasons in the two reef systems, increasing the between-habitat component of beta diversity in both taxonomic and functional terms. The increase in between-habitat diversity caused a decline in geographic-scale variability as the two systems converged on a single, historically novel pattern of depth zonation. Twenty-four reef cores were extracted at water depths above 2 m in Bahia Almirante, a coastal lagoon in northwestern Panama. The cores showed...