Holocene reef building on eastern St. Croix, US Virgin Islands: Lang Bank revisited

New core and seismic data suggest that widespread reef building started on Lang Bank by 8,900 CalBP and was dominated by Acropora palmata for the next three millennia. Accretion rates averaged 5.81 m ky^?1, a rate that was sufficient for reefs to keep pace with rising sea level on the bank throughout their history. Seismic data show a deep platform interior that was flooded well in advance of reef building along the elevated rim. As a result, those reefs were buffered from sediment stress by their higher positions and active water flow to the west. A. palmata disappeared from the shallow margin by 6,350 yr ago, and reef building on Lang Bank largely ceased by 5,035 CalBP. The reasons for these dramatic events are unclear. Water depth over the reefs was generally shallower than when they started to build, and sea level was slowing dramatically. The new data described here show that reefs flourished on Lang Bank throughout the hiatus suggested by earlier studies (10–7 kyrs BP), and the ultimate demise of shelf-edge reefs is clearly not associated with either poor water quality or sudden sea-level rise. In addition, accretion rates from eastern St. Croix and throughout the Caribbean were well below the high values (≥10 m ky^?1) that have been widely assumed. These data collectively argue against models that require extreme environmental or oceanographic phenomena to drown reefs on Lang Bank where reef building was too fast to be outpaced by Holocene sea-level rise. This also bears on more generalized Caribbean models that depend on the presumed reef history on eastern St. Croix.     

An outbreak of dengue fever in St. Croix (US Virgin Islands), 2005

Background

Periodic outbreaks of dengue fever occur in the United States Virgin Islands. In June 2005, an outbreak of dengue virus (DENV) serotype-2 with cases of dengue hemorrhagic fever (DHF) was detected in St. Croix, US Virgin Islands. The objective of this report is to describe this outbreak of DENV-2 and the findings of a case-control study examining risk factors for DHF.

Methodology/Principal Findings

This is the largest dengue outbreak ever recorded in St. Croix, with 331 suspected dengue cases reported island-wide during 2005 (62.2 cases/10,000 population); 54% were hospitalized, 21% had at least one hemorrhagic manifestation, 28% had thrombocytopenia, 5% had DHF and 1 patient died. Eighty-nine laboratory-positive hospitalized patients were identified. Of these, there were 15 (17%) who met the WHO criteria for DHF (cases) and 74 (83%) who did not (controls). The only variable significantly associated with DHF on bivariate or multivariable analysis was age, with an adjusted odds ratio (95% confidence interval) of 1.033 (1.003,1.064).

Conclusions/Significance

During this outbreak of DENV-2, a high proportion of cases developed DHF and increasing age was significantly associated with DHF.     

Miocene pelagic biogenic sediment production and diagenesis,St. Croix,U.S. Virgin Islands

The Middle Miocene Kingshill Marl of St. Croix, Virgin Islands, affords an opportunity to reconstruct ancient island-margin calcareous plankton communities and to determine their contribution to the accumulation of island-slope sediments. Because of the present outcrop pattern of this unit, both lateral and vertical changes in organism/sediment relationships may be investigated.Subsidence of a NE-SW trending grabenal structure on St. Croix during the latest Early Miocene produced the Kingshill Seaway, which was flanked on the northwest and southeast by island masses of Cretaceous volcanogenic sediments, and on the northeast and southwest by the insular shelf edges. Hydrographic conditions in the shallow Seaway promoted high rates of pelagic biogenic skeletal production, resulting in the accumulation of thick pelagic calcareous oozes composed of a framework of calcitic planktonic foraminiferal tests in a matrix of calcareous nannoplankton, planktonic foraminiferal debris and fine aragonite needles. Minor siliceous components included diatom frustules and sponge spicules. Turbidity currents and debris flows transported terrigenous detritus and reef-tract skeletal rubble into the Seaway from the shallow basin margins.Comparison of the pelagic chalks and marls of the Kingshill Marl with modern sediments accumulating on the northwest St. Croix island slope establishes valuable guidelines to infer the total biogenic composition of the original ooze accumulating on the Kingshill Seaway floor. Comparison of the diagenetic processes affecting island-slope calcareous oozes with those affecting their deep-sea counterparts underscores the necessity of considering the range and intensity of differential solution as a factor in the ooze → chalk diagenetic continuum. The major diagenetic event in the Kingshill Marl ooze → chalk process was the solution of aragonitic skeletal sediment, probably during flushing by fresh water.     

Distribution and abundance of elkhorn coral, Acropora palmata, and prevalence of white-band disease at Buck Island Reef National Monument, St. Croix, US Virgin Islands

In the 1970s and 1980s elkhorn coral, Acropora palmata, declined dramatically throughout the Caribbean primarily due to white-band disease (WBD). In 2005, elkhorn coral was proposed for listing as threatened under the US Endangered Species Act. WBD was first documented at Buck Island Reef National Monument (BIRNM). Together with hurricanes WBD reduced live elkhorn coral coverage by probably over 90%. In the past decade some recovery has been observed at BIRNM. This study assessed the distribution and abundance of elkhorn coral and estimated the prevalence of WBD at the monument. Within an area of 795 ha, we estimated 97,232–134,371 (95% confidence limits) elkhorn coral colonies with any dimension of connected live tissue greater than one meter, about 3% of which were infected by WBD. Despite some recovery, the elkhorn coral density remains low and WBD may continue to present a threat to the elkhorn coral population.     

Scleractinian coral recruitment patterns at Salt River submarine canyon,St. Croix,U.S. Virgin Islands

Scleractinian coral recruitment patterns were studied at depths of 9, 18, 27 and 37 m on the east and west walls of Salt River submarine canyon, St. Croix, U.S. Virgin Islands, by censusing coral juveniles which settled on experimental settling plates placed on the reef for 3–26 months as well as coral juveniles within quadrats on the reef. The most common species in the juvenile population within quadrats were Agaricia agaricites, Porites astreoides, Madracis decactis, Stephanocoenia michelinii, and A. lamarcki. The only species settling on settling plates were Agaricia spp., Madracis decactis, Porites spp., Stephanocoenia michelinii and Favia fragum. A total of 271 corals settled on 342 plates, with 51% of the juveniles on the east wall and 49% on the west wall. Of these 34% settled on horizontal surfaces and 66% on vertical surfaces. Based on results from quadrats, Agaricia agaricites and Porites astreoides had high recruitment rates relative to their abundance on the reef. In contrast, Agaricia lamarcki, Montastraea annularis, M. cavernosa and Siderastrea siderea had high amounts of cover compared to their abundance as juveniles within quadrats. The mean number of juveniles per m² within quadrats ranged from 3 to 42. In general, there was a decrease in the mean number of juveniles and the number of species with depth. Total number of juveniles on settling plates was highest at 18 m on both walls. The largest number within quadrats was at 18 m on the east wall, followed by 9 m and 18 m on the west wall. High rates of coral recruitment tended to be associated with low algal biomass and relatively high grazing pressure by urchins and fishes.     

Spatio-temporal variability of coral recruitment on shallow reefs in St. John, US Virgin Islands

In this study, coral recruitment was measured on a kilometer-wide scale over two years on shallow (5-6 m depth) fringing reefs in St. John, US Virgin Islands, with the objective of determining the extent to which variation in recruitment was affected by biophysical coupling involving temperature and flow. Coral recruitment was measured using settlement tiles deployed at 10 sites along 10 km of shore. The tiles were first deployed in August 2006, and thereafter replaced every ≈ 6 months to sample from either August to January, or January to August over 2 years. Seawater temperature was recorded at the 10 sites using logging thermistors, and flow was quantified using drogues. Overall, corals recruited at a rate equivalent to 76 corals m^− 2 6 months^− 1, and were represented mostly by poritids (43% of recruits), agaricids (29%), faviids (17%) and siderastreids (7%). Although the density of recruits differed among sites in a pattern that varied among periods and years, there was a consistent trend for mean density to decline from ≈ 4 corals tile^− 1 at eastern sites, to ≤ 1 coral tile^− 1 at western sites. One aspect of seawater temperature - the daily range - differed among sites and was greater at western compared to eastern sites, and while it was related inversely to recruitment over one of the sampling periods, it was equivocal as a physical process affecting recruitment. Instead, our results are consistent with biophysical coupling involving patch depletion and downstream filtering, whereby patches of coral larvae are delivered to the south shore of St. John and depleted of larvae through settlement as the water progresses westward.     

Contribution of Dead Wood to Biomass and Carbon Stocks in the Caribbean: St. John, U.S. Virgin Islands

Dead wood is a substantial carbon stock in terrestrial forest ecosystems and hence a critical component of global carbon cycles. Given the limited amounts of dead wood biomass and carbon stock information for Caribbean forests, our objectives were to: (1) describe the relative contribution of down woody materials (DWM) to carbon stocks on the island of St. John; (2) compare these contributions among differing stand characteristics in subtropical moist and dry forests; and (3) compare down woody material carbon stocks on St. John to those observed in other tropical and temperate forests. Our results indicate that dead wood and litter comprise an average of 20 percent of total carbon stocks on St. John in both moist and dry forest life zones. Island-wide, dead wood biomass on the ground ranged from 4.55 to 28.11 Mg/ha. Coarse woody material biomass and carbon content were higher in moist forests than in dry forests. No other down woody material components differed between life zones or among vegetation categories ( P > 0.05). Live tree density was positively correlated with fine woody material and litter in the moist forest life zone ( R = 0.57 and 0.84, respectively) and snag basal area was positively correlated with total down woody material amounts ( R = 0.50) in dry forest. Our study indicates that DWM are important contributors to the total biomass and, therefore, carbon budgets in subtropical systems, and that contributions of DWM on St. John appear to be comparable to values given for similar dry forest systems.     

A spatially aggressive peyssonnelid algal crust (PAC) threatens shallow coral reefs in St. John,US Virgin Islands

Coral Reefs - In recent decades, many Caribbean reefs have experienced large declines in abundance of scleractinian corals, and blooms of fleshy macroalgae have often accompanied these trends. In...     

The effects of storms,heavy rain,and sedimentation on the shallow coral reefs of St. John,US Virgin Islands

Efforts to limit plant growth in streams by reducing nutrients would benefit from an understanding of the relative influences of nutrients, streamflow, light, and other potentially important factors. We measured macrophytes, benthic algae, nutrients in water and sediment, discharge, and shading from 30 spring-fed or runoff-influenced streams in the upper Snake River basin, ID, USA. We hypothesized that in hydrologically stable, spring-fed streams with clear water, macrophyte and benthic algae biomass would be a function of bioavailable nutrients in water or sediments, whereas in hydrologically dynamic, runoff-influenced streams, macrophyte and benthic algae biomass would further be constrained by flow disturbance and light. These hypotheses were only partly supported. Nitrogen, both in sediment and water, was positively correlated with macrophyte biomass, as was loosely sorbed phosphorus (P) in sediment. However, P in water was not. Factors other than nutrient enrichment had the strongest influences on macrophyte species composition. Benthic algal biomass was positively correlated with loosely sorbed sediment P, lack of shade, antecedent water temperatures, and bicarbonate. These findings support the measurement of bioavailable P fractions in sediment and flow histories in streams, but caution against relying on macrophyte species composition or P in water in nutrient management strategies for macrophytes in streams.     

Recharting the Caribbean: Land, Law, and Citizenship in the British Virgin Islands

Recharting the Caribbean: Land, Law, and Citizenship in the British Virgin Islands. Bill Maurer. Ann Arbor: University of Michigan Press, 1997. 301 pp.     

Bleaching increases likelihood of disease on Acropora palmata (Lamarck) in Hawksnest Bay, St John, US Virgin Islands

Anomalously high water temperatures may enhance the likelihood of coral disease outbreaks by increasing the abundance or virulence of pathogens, or by increasing host susceptibility. This study tested the compromised-host hypothesis, and documented the relationship between disease and temperature, through monthly monitoring of Acropora palmata colonies from May 2004 to December 2006, in Hawksnest Bay, St John, US Virgin Islands (USVI). Disease prevalence and the rate of change in prevalence showed a positive linear relationship with water temperature and rate of change in water temperature, respectively, but only in 2005 during prolonged periods of elevated temperature. Both bleached and unbleached colonies showed a positive relationship between disease prevalence and temperature in 2005, but the average area of disease-associated mortality increased only for bleached corals, indicating host susceptibility, rather than temperature per se, influenced disease severity on A. palmata.     

Physical Disturbance and Community Structure of Exposed and Protected Reefs: A Case Study from St. John, U.S. Virgin Islands

Major physical disturbances impacted fringing reefs at 4 to12 m depth off the south coast of St. John, at intervals of28, 14 and 12 months over a 6 year period (1985–1991).The most wave exposed habitats were dominated by the hydrocoralMillepora alcicornis and the encrusting gorgonian Erythropodiumcaribaeorum. Both of these species displayed high resistancestability in response to the 1987 bleaching event and to HurricanesGilbert (1988) and Hugo (1989). Although Hurricane Hugo wasranked as the most severe hurricane of the past century, itcaused comparatively little damage to the shallow exposed reef,illustrating that the effects of physical disturbance may notalways conform to a priori predictions based on wave exposure.This counter-intuitive result is explicable when the chronologyof disturbance is considered. For example, the shallow exposedcommunity was more heavily damaged than the protected reef byHurricane Gilbert occurring one year prior to Hurricane Hugo.Sponges and thick mats of E. caribaeorum had accumulated onthe protected reef during a 54 month period of little disturbanceand were vulnerable to disturbance by virtue of their elevationabove the substratum. These organisms were dislodged by extremewater motion generated by Hurricane Hugo. In contrast, the shallowexposed community was still recovering from Hurricane Gilbertwhen Hurricane Hugo struck. Monitored populations of two coralspecies, Agaricia agaricites and Tubastrea coccinea, declinedover the six year period, but recruitment in 1990 partiallycompensated for high mortality caused byHurricane Hugo. Therewere significant increases in the abundance of algal turf onthree out of the four study reefs (4 m exposed and protected,12 m protected) in response to the creation of patch space bythe multiple physical disturbances. This study demonstratesthe need to interpret recent disturbances in light of the historyof past disturbance on the reef.