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.     

Monitoring the coral disease, plague type II, on coral reefs in St. John, U.S. Virgin Islands

In July 1997, conspicuous white patches of necrotic tissue and bare skeleton began to appear on scleractinian corals in several bays around St. John, US Virgin Islands. Analysis of diseased coral tissue from five different species confirmed the presence of a Sphingomonas-like bacterium, the pathogen for plague type II. To date, 14 species of hard corals have been affected by plague type II around St. John. This disease was monitored at Haulover and Tektite Reefs at depths of 7-12 meters. The study site at Tektite Reef has > 50% cover by scleractinian corals with 90% of hard corals being composed of Montastraea annularis. Monthly surveys at Tektite Reef from December 1997 to May 2001 documented new incidence of disease (bare white patches of skeleton) every month with associated loss of living coral and 90.5% of all disease patches occurred on M. annularis. The frequency of disease within transects ranged from 3 to 58%, and the area of disease patches ranged from 0.25 to 9000 cm2. The average percent cover by the disease within 1 m2 ranged from 0.01% (+/- 0.04 SD) to 1.74% (+/- 9.08 SD). Photo-monitoring of 28 diseased corals of 9 species begun in September 1997 at Haulover Reef revealed no recovery of diseased portions with all necrotic tissue being overgrown rapidly by turf algae, usually within less than one month. Most coral colonies suffered partial mortality. Very limited recruitment (e.g., of Agaricia spp., Favia spp. and sponges) has been noted on the diseased areas. This coral disease has the potential to cause more loss of live coral on St. John reefs than any other stress to date because it targets the dominant reef building species, M. annularis.     

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.     

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.     

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.     

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.     

High, but localized recruitment of Montastraea annularis complex in St. John, United States Virgin Islands

This study describes relatively high recruitment of Montastraea annularis complex (hereafter, MAC) on the shallow reefs (≤9 m depth) along 4 km of the south coast of St. John that have been censused for 16 years. Before 2008, the density of juvenile MAC along this stretch of coast was <0.068 colonies m⁻² (and often <0.032 colonies m⁻²), but in 2008 it increased to 0.152 colonies m⁻² and remained at 0.116 colonies m⁻² in 2009. These densities were driven by only 7–9 colonies year⁻¹, but against the long-term context of weak recruitment and declining cover of this taxon, the densities are notable. In 2009, the spatial extent of the surveys was expanded to 47 km, along the north, east, and south coasts of St. John. This larger effort demonstrated that MAC recruitment was localized to the initial 4 km surveyed; outside this area, the mean density was 0.010 juvenile colonies m⁻². These results demonstrate that MAC is still capable of recruitment at rates similar to those of the 1970s, but the highly limited spatio-temporal extent of the effect is unlikely to appreciably alter the declining population trajectories of this taxon.     

Recent settlement trends in Panulirus argius (Decapoda: Palinuridae) pueruli around St. Thomas, U.S. Virgin Islands

Puerulus settlement of the western Atlantic spiny lobster, Panulirus argus, was monitored using modified Witham collectors from December 1996 to March 1998 at seven sites around St. Thomas, U.S. Virgin Islands. A total of 605 pueruli were collected from 553 samples for a catch per unit effort (CPUE) for all sites of 1.09 pueruli. The greatest settlement occurred on sites within a recently declared marine reserve, which had an overall CPUE of 1.77 pueruli. Settlement in non-reserve sites was much lower with an overall CPUE of 0.31 pueruli. Pueruli recruitment declined 67% at inshore sites and 53% at offshore sites between July 1992 - April 1994 and February 1997 - March 1998. Also, only 10% of months sampled in 1997-98 had a CPUE > 0.5 compared to 55% in a previous study in 1992 - 1993. Despite the decline in pueruli CPUE in 1997-98 compared to 1992-94, the commercial lobster catch in the 2000-01 fishing season, and by inference the adult lobster population (legal lobster size in the US Virgin Islands is > or = 3.5 cm carapace length), remained stable.     

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.     

Eastern Caribbean Circulation and Island Mass Effect on St. Croix,US Virgin Islands: A Mechanism for Relatively Consistent Recruitment Patterns

The northeastern Caribbean Sea is under the seasonal influence of the Trade Winds but also of the Orinoco/Amazon freshwater plume. The latter is responsible for intensification of the Caribbean Current in general and of its eddy activity in the northern part of the Caribbean Sea. More importantly, we show in this study that the front of the freshwater plume drives a northward flow that impinges directly on the island of St. Croix in the United States Virgin Islands. The angle of incidence of the incoming flow controls the nature of the wake on both sides and ends of the island, which changes from cyclonic to anticylonic wake flow, with either attached or shed eddies. Using an off-line bio-physical model, we simulated the dispersal and recruitment of an abundant Caribbean coral reef fish, the bluehead wrasse (Thalassoma bifasciatum) in the context of the wake flow variability around St. Croix. Our results revealed the role played by the consistent seasonal forcing of the wake flow on the recruitment patterns around the island at the interannual scale. The interannual variability of the timing of arrival and northward penetration of the plume instead controls the nature of the wake, hence the regional spatial recruitment patterns.     

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.     

Sedimentation as a control of reef development: St. Croix,U.S.V.I.

Sediment-transport rates in and around Salt River submarine canyon, St. Croix, U.S.V.I., were measured over a 2-year period using bedload traps and Legrangian sediment-tracer experiments. Sodiments generatly move in a westerly direction along the north shore of St. Croix. Consequently, sediment moves into the canyon at a higher rate over the eastern margin (47,000 kg/m-year) than over the west wall (19,000 kg/m-year). As a result, extensive reef growth is limited to the west wall, and the inner portion of the eastern margin is a cobblecovered slope inhabited primarily by gorgonians and a few sediment-tolerant corals. Sedimentation exhibits similar controls on other reefs around the island. At Cane Bay, sedimentation rates are lower than at Salt River, and reef development is accordingly greater. Along the east side of Christiansted canyon and the downdrift margin at Sandy Point, extreme sedimentation has completely buried the reefs. Sediment transport was an order of magnitude higher during storms than during fair weather. Under all conditions measured, the amount of sediment exported from the canyon was considerably below that entering over the canyon walls. It is hypothesized that major hurricanes periodically flush the excess, and keep long-term sediment influx and export in balance.     

Benthic structure and cryptic mortality in a Caribbean mesophotic coral reef bank system, the Hind Bank Marine Conservation District, U.S. Virgin Islands

Coral reef banks may form an important component of mesophotic coral ecosystems (MCEs) in the Caribbean, but remain poorly explored relative to shallower reefs and mesophotic habitats on slopes and walls. Consequently, the processes structuring mesophotic coral reef communities are not well understood, particularly the role of disturbance. A large and regionally important mesophotic system, the Hind Bank Marine Conservation District (MCD), St. Thomas, USVI, was systematically surveyed. Data were used to construct a comprehensive benthic habitat map for the MCD, describe the abiotic and biotic components of the benthos among habitats, and investigate patterns of coral health among habitats. Two-thirds of the MCD (23.6 km²) was found to be dense coral reef (Coral Cover = 24.1%) dominated by the Montastraea annularis species complex. Coral reef ecosystems were topographically complex, but could be classified into distinct habitat types, including high coral banks (35.8% of the MCD) and two large novel coral reef habitat types corresponding to an extremely flat basin (18%) and a highly rugose hillock basin (6.5%), containing thousands of coral knolls (2–10 m high). An extreme disease event with undescribed signs of mortality occurred on 47% of coral reefs and reached a high prevalence in affected areas (42.4% ± 6.3 SE, N = 26). The disease was significantly clustered in the basin habitats of the western MCD (global Moran’s I = 0.32, P < 0.01). Observations of the spatial pattern suggested that the driver was specific to the basin habitats and may have been caused by a coherent abiotic event.     

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...     

Cleaning behavior ofBodianus rufus,Thalassoma bifasciatum,Gobiosoma evelynae,andPericlimenes pedersoni along a depth gradient at Salt River Submarine Canyon,St. Croix

Synopsis Cleaning activity at specific cleaning stations was monitored over a depth range of 15–45 m at Salt River Submarine Canyon, St. Croix using the HYDROLAB underwater habitat. We observed over 4600 cleaning events involving 32 host species. Cleaners included the fishesGobiosoma evelynae (cleaning goby), juvenileBodianus rufus (Spanish hogfish) and juvenileThalassoma bifasciatum (bluehead wrasse) and the shrimpPericlimenes pedersoni, often active simultaneously at the same stations. Although cleaning was observed to depths of 30 m, the greatest density of cleaning fishes occurred at a sharp break in the bottom contour at 15 m.Clepticus parrae was the dominant fish cleaned by fishes (80% of events), and the graysby,Petrometopon cruentatum, was most frequently cleaned byPericlimenes. Cleaning was initiated byGobiosoma at 0600 to 0630 h and continued throughout the day as intense bouts of cleaning activity were interspersed with periods of relative calm.Bodianus patrolled areas of several meters and would sometimes rise over a meter above the substrate to initiate cleaning. In contrast, gobies were much more restricted in both lateral and vertical movements.Gobiosoma andBodianus cleaned 3104 and 1346 hosts during the study compared to only 207 forThalassoma. This study of a deeper reef area shows significantly more cleaning activity byBodianus and much less byThalassoma than has been reported from other studies in shallower water     

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.