Collette Dick Burke

Coral diversity and mode of growth of lateral expansion patch reefs at Mexico rocks, northern Belize shelf, central America

Mexico Rocks is a large patch reef complex on the outer shelf of northern Belize, to the lee of the barrier reef. The complex consists of approximately 100 patch reefs, clustered on a topographic ridge of Pleistocene limestone, and is composed predominantly (83%) of head corals (Montastrea annularis). Biotic zonation is not apparent on any of the patch reefs. Concomitant with increasing area of individual reefs is an increase in surficial areas of dead coral, areas degraded by physical and bio-erosion, and cavities. Such areas enhance habitat complexity and provide zooid-free substrates that are colonized rapidly by more cryptic coral species. The result of such processes is an increase in spatial heterogeneity and species richness on larger reefs relative to smaller patch reefs in the complex.Initial colonization of the rocky substratum (Pleistocene limestone bedrock) by single head corals began about 3.5 KaBP, and the patch reefs mostly accreted vertically as growth kept pace with slowly rising sealevel. Continued growth involved lateral expansion and coalescence of adjoining patch reefs to form larger composite reef masses as a consequence of the rapid growth rates ofMontastrea annularis and lack of verticla accommodation space. Such a lateral-expansion style of reef growth, together with dominance of head corals, may mimic the biotic composition and geometry of give-up (drowned) reefs common in early transgressive systems tracts in modern and ancient shelf settings.

Environmental setting of Holocene sabellariid worm reefs, northern Belize

Communities of sabellariid worms (Polychaeta) occur as areally discontinuous, unlithified reefs on an irregular depositional topography of Holocene and older sediments at the mouth of the Northern River Lagoon, Belize, Central America. They are found in nearshore marine, moderate energy, tidally influenced environments of normal to low salinity. These colonies, as much as 30 cm thick, are composed of dense thickets of agglutinated worm tubes (1.0 mm diameter, 3.0 cm length) that trap and bind sand- to silt-sized bioclastic debris, microorganisms, and micrite

Coral mortality, recovery and reef degradation at Mexico Rocks Patch Reef Complex, Northern Belize, Central America: 1995–1997

The 1995 coral bleaching event in the western Caribbean was the first reported episode that significantly affected the Belize barrier and lagoonal patch reefs. Bleaching was attributed to a 2 mo period of warm water temperatures above 30°C. Near Ambergris Caye, barrier and patch reefs experienced up to 50% bleaching. At Mexico Rocks patch reef complex, the bleaching resulted in changes in reef health, community, and physical structure. Prior to the hyperthermal episode, patch reef surface area consisted of 47% healthy framework coral coverage, 12% secondarily colonized biotic coverage, 35% dead coral surfaces that were degraded by biological activity and physical erosion, and 6%cavities. six months after bleaching, most corals had regained their color, but, owing to coral mortality, areas of surface degradation had increased to an average 49% (p=0.029 based on Kruskal–Wallis analyses). Eighteen months after bleaching, degraded surface areas expanded to 53% (p=0.0366). Although re-coloring indicates rapid recovery for surviving corals, the persistence in dead coral surfaces suggests that reef skeletal structure recovery lags behind that of individual corals. Initial results of framework measurements indicate that bleaching events may result in an ‘imbalance’ in the carbonate production rate of coral reefs and produce mass wasting of the skeletal structure. Remapping of reef skeletal structure should establish quantitative measures for the long-term effects of bleaching on patch reef frameworks.

The effects of late-Quaternary climatic changes and glacioisostatic rebound on lake level fluctuations and benthos of Lake Michigan

Past lake stages of Lake Michigan are the result of water level stillstands and fluctuations initiated by the withdrawal of the Wisconsin glacial ice sheet (approximately 10,000-5000 years before present). Dominance diversity, evenness, and community profiles were used to determine the effects of these fluctuations on ostracode populations and to reconstruct the paleoenvironment of glacial Lake Michigan. Results indicate that ostracode community diversity was low, and evenness was high during this period. Few species of ostracodes could tolerate the cold temperatures and oligotrophic conditions of the prehistoric lake system. This assemblage consists of Candona subtriangulata and C. crogmaniana. Community profiles changed with fluctuations in water level. One significant change in diversity indices is recorded between the oldest lake stage (Algonquin) and the stratigraphically overlying, shallowest stage (Chippewa). Increase in diversity was probably a result of increasing nutrient availability and salinity, two by-products of erosional processes and evaporation initiated when the lake level declined and the climate warmed. These changes in diversity and abundance may imply an increase in primary productivity. As much as 32% of lake-bottom sediments were exposed during the Chippewa water level minimum. This exposure of land and the resulting decrease in lake surface may have affected local and regional climate. Glacioisostatic rebound of channel openings caused the lake level to rise to an extreme high (Nipissing stage). With increasing water levels, ostracode profiles changed and suggest that the hypolimnion decreased in salinity and dissolved 02. Although statistical analysis of diversity indices does not support significant changes in the environment between Winnetka and Lake Forest Members, this analysis may have been biased by small sample numbers.

Chemical Differences Among the Shells of Two Euryhaline Species of Fossil Ostracoda (Crustacea): A Preliminary Study

Microfossils were extracted from a Florida Bay sediment core, and brackish and marine environments were interpreted on the basis of fossil ostracode and foraminiferid assemblages. A total of 48 hand-picked specimens of two species of euryhaline ostracodes (Cyprideis salebrosa and Peratocytheridea setipunctata) were chemically analyzed for Ca, Mg, Sr, and Fe concentrations to determine the effect of salinity on bulk skeletal chemistry. Results indicate that adult specimens of the two species have similar Mg and Sr concentrations, but Fe is more concentrated in the shells of C. salebrosa. There are no differences in trace element concentrations in adult specimens from brackish or marine sections of the core. Nodose and non-nodose instars of P. setipunctata contain similar concentrations of Sr, but greater concentrations of Mg and Fe than conspecific adults or adults of C. salebrosa. The enrichment of Mg and Fe in instars may be the result of rapid shell growth rate. Rapid carapace calcification may represent an adaptive strategy for survival that is maintained throughout the ontogeny of an individual. Ostracodes (Cambrian-Recent) are microscopic bivalved crustaceans ubiquitous to most water bodies. After hatching from an egg, an ostracode grows by molting (ecdysis). As a result of this life cycle, a series of carapaces graded in size and shape are shed in the sediment. Each ostracode can potentially molt 7 to 8 times prior to development of the genitalia that characterize the adult stage. When fossilized, these shells provide a record of ontogenic development and abundant specimens for paleontological analysis primarily in the fields of paleoecology and biostratigraphy. The use of suites of fossil ostracodes for interpretation of paleoecology stems from their well documented sensitivity to diverse environments in the Recent and the extensive geologic ranges of some ostracode taxa. Ostracodes abundantly occupy fresh, brackish, and marine waters, and their fossilized carapaces can provide evidence to support paleoenvironmental

Erratum to: The jurassic of the circum-pacific

In volume9, no. 1. p, 113–114, BOOK REVIEWS section, the book “THE JURASSIC OF THE CIRCUM-PACIFIC” was reviewed by Collette D. Burke, Wichita State University, Wichita, Kansas.