Yrene Astor

Comparison of Vertical Distributions of Prokaryotic Assemblages in the Anoxic Cariaco Basin and Black Sea by Use of Fluorescence In Situ Hybridization

ABSTRACT Individual prokaryotic cells from two major anoxic basins, the Cariaco Basin and the Black Sea, were enumerated throughout their water columns using fluorescence in situ hybridization (FISH) with the fluorochrome Cy3 or horseradish peroxidase-modified oligonucleotide probes. For both basins, significant differences in total prokaryotic abundance and phylogenetic composition were observed among oxic, anoxic, and transitional (redoxcline) waters. Epsilon-proteobacteria, Crenarchaeota, and Euryarchaeota were more prevalent in the redoxclines, where previous studies reported high rates of chemoautotrophic production relative to those in waters above and below the redoxclines. Relative abundances of Archaea in both systems varied between 1% and 28% of total prokaryotes, depending on depth. The prokaryotic community composition varied between the two anoxic basins, consistent with distinct geochemical and physical conditions. In the Black Sea, the relative contributions of group I Crenarchaeota (median, 5.5%) to prokaryotic communities were significantly higher (P < 0.001; n = 20) than those of group II Euryarchaeota (median, 2.9%). In contrast, their proportions were nearly equivalent in the Cariaco Basin. Beta-proteobacteria were unexpectedly common throughout the Cariaco Basins water column, accounting for an average of 47% of 4′,6′-diamidino-2-phenylindole (DAPI)-stained cells. This group was below the detection limit (<1%) in the Black Sea samples. Compositional differences between basins may reflect temporal variability in microbial populations and/or systematic differences in environmental conditions and the populations for which they select.

Seasonal and interannual variation in the hydrography of the Cariaco Basin: implications for basin ventilation

The hydrography of the Cariaco Basin (temperature, salinity, density, dissolved oxygen concentration) was studied using monthly observations collected between November 1995 and August 1998 at the CArbon Retention In A Colored Ocean (CARIACO) time-series station (10.51N, 64.661W). Satellite scatterometer wind estimates showed that changes in the wind preceded changes in hydrography by 1–2 weeks. Upward migration of isopleths within the upper 150 m was observed between November and May each year, when the Trade Wind was more intense. A seasonal deepening of the isopleths was observed when winds relaxed. A secondary upwelling event was observed every year between July and August, in response to an intensification of the southward component of the Trade Wind. Interannual variation in the upwelling cycle was driven in part by variations in wind intensity and in part by other events at time scales of 1–3 months. The latter were associated with 90–140 m deep intrusions of Caribbean Sea water that forced waters above them to the surface. Satellite-derived sea surface height anomaly maps demonstrated that these events were related to cyclonic and anticyclonic eddies moving along the continental shelf. Waters deeper than 1200 m showed small temperature and salinity increases of 0.00751 Cy r � 1 and 0.0016 yr � 1 , consistent with previous estimates.

Si cycle in the Cariaco Basin, Venezuela: Seasonal variability in silicate availability and the Si:C:N composition of sinking particles

result of rapid utilization. In most years, the upper water column during winter and spring is marked by Si(OH)4 :NO3 and Si* values of less than 1. This indicates that silicate limitation in Cariaco Basin is most severe during upwelling and may restrict diatom production. Conversely, during the summer and fall when upwelling is reduced, Si(OH)4 :NO3 ratios in the upper 50 m of the water column exceed 10, implying that nitrate rather than silicate is acting to limit production during this time of year. On average, sinking particles collected at 150-m depth in the Cariaco Basin have Si:C and Si:N values of 0.17 ± 0.01 and 1.14 ± 0.10, respectively. These ratios increase with depth to 400 m and then remain relatively constant, suggesting minimal selective removal of elements with remineralization in the anoxic portion of the water column. Similar depth-dependent changes in these ratios are seen in surface sediments from the basin. Seasonally, particulate Si:C and Si:N are highest during the early part of the year when upwelling is most intense, while both ratios decrease to their lowest values during summer and fall. The observed seasonal variability in these ratios is due to changes in both nutrient utilization by diatoms and the contribution of diatoms to the total phytoplankton. The high ratios during upwelling suggest enhanced export of Si relative to C and N during this time of year.

Ecosystem responses in the southern Caribbean Sea to global climate change

Over the last few decades, rising greenhouse gas emissions have promoted poleward expansion of the large-scale atmospheric Hadley circulation that dominates the Tropics, thereby affecting behavior of the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO). Expression of these changes in tropical marine ecosystems is poorly understood because of sparse observational datasets. We link contemporary ecological changes in the southern Caribbean Sea to global climate change indices. Monthly observations from the CARIACO Ocean Time-Series between 1996 and 2010 document significant decadal scale trends, including a net sea surface temperature (SST) rise of ∼1.0 ± 0.14 °C (±SE), intensified stratification, reduced delivery of upwelled nutrients to surface waters, and diminished phytoplankton bloom intensities evident as overall declines in chlorophyll a concentrations (ΔChla = −2.8 ± 0.5%⋅y−1) and net primary production (ΔNPP = −1.5 ± 0.3%⋅y−1). Additionally, phytoplankton taxon dominance shifted from diatoms, dinoflagellates, and coccolithophorids to smaller taxa after 2004, whereas mesozooplankton biomass increased and commercial landings of planktivorous sardines collapsed. Collectively, our results reveal an ecological state change in this planktonic system. The weakening trend in Trade Winds (−1.9 ± 0.3%⋅y−1) and dependent local variables are largely explained by trends in two climatic indices, namely the northward migration of the Azores High pressure center (descending branch of Hadley cell) by 1.12 ± 0.42°N latitude and the northeasterly progression of the ITCZ Atlantic centroid (ascending branch of Hadley cell), the March position of which shifted by about 800 km between 1996 and 2009.

Increased marine sediment suspension and fluxes following an earthquake

Earthquakes are commonly cited as one possible triggering mechanism for turbidity flows—dense sediment–water plumes that can transport large volumes of sediment great distances down slope—in both marine and lacustrine settings. Heezen and Ewing were the first to make such a suggestion, attributing breaks in a sea-floor telephone cable in the North Atlantic Ocean to turbidity flows generated by the 1929 Grand Banks earthquake. Anumber of workers have consequently used sedimentary turbidite records to reconstruct the earthquake histories of various regions. Here we present direct observations of a seismically induced turbidity flow. Measurements of light scattering and sediment fluxes in the Cariaco basin indicate that the earthquake that occurred along the coast of northern Venezuela on 9 July 1997 resulted in considerable downslope displacement of sediments—probably >105 tonnes into the deep part of the basin. In such a seismically active region, this mechanism of sediment transport may be responsible for a significant component of the long-term sediment accumulation in the basin. Furthermore, this process may result in the sequestration in deep sea sediments of large amounts of carbon initially deposited at shallow depths.

Seasonal variability in the salinity and oxygen isotopic composition of seawater from the Cariaco Basin, Venezuela: implications for paleosalinity reconstructions.

Oxygen isotope measurements (δ18Ow) were made on seawater samples collected monthly between May 1996 and February 1997 and between December 2005 and May 2006 at various water depths at the Cariaco Basin ocean time series station (10°30N, 64°40W). The δ18Ow values are compared with concurrent salinity measurements to assess the δ18Ow:salinity relationship in this tropical region and to determine if significant seasonal variability exists in the relationship. The δ18Ow values range from 0.88 to 1.19‰ SMOW in the upper 250 m. Our results indicate that the strongest positive linear correlation between δ18Ow and salinity in the upper 250 m occurred during the February and April 2006 upwelling season (R2 = 0.95 and 0.94, respectively). The salinity: δ18Ow relationship displays significant seasonal variability which is attributed to seasonal changes in freshwater input from the Tuy, Neveri, and Unare rivers into the Cariaco Basin. Specifically, an inverse correlation (R2 = 0.77) exists between monthly Neveri River discharge and sea surface salinity. Our results demonstrate that significant seasonal changes in the δ18Ow:salinity relationship occur in the tropics. The data also show a distinct difference between the surface water δ18Ow:salinity relationship during the upwelling season (R2 = 0.96) and the nonupwelling season (R2 = 0.93) revealing zero-salinity end-members of −28.53 (SE ± 3.04) and −8.77 (SE ± 1.33), respectively. The seasonal mixing lines are an important consideration when utilizing the salinity:oxygen isotope relationship for paleosalinity reconstructions. The oxygen isotope composition (δ18Oc) was also measured in two surface-dwelling planktonic foraminiferal species, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in the Cariaco Basin between November 1996 and February 1997 and May 2003 through May 2006. The large range in δ18Oc during the study period, 1.4‰ for G. ruber and 1.5‰ for G. bulloides, is attributed to changes in calcification depths of the species from 1996–1997 to 2005–2006. Using the surface water δ18Ow:salinity equations generated for the upwelling and nonwelling seasons in the Cariaco Basin, we compare measured seawater salinity with the calculated seawater salinity at various depths of calcification. The δ18Ow:salinity equation generated from surface waters during the upwelling months yields salinity estimates that best agree with measured salinities.

TEMPORAL VARIABILITY IN THE NUTRIENT CHEMISTRY OF THE CARIACO BASIN

Nutrient data have been collected monthly at the CARIACO time series site in the Cariaco Basin since 1995, providing a unique picture of the cycling of NO3 − , NO2 − ,N H 4 + ,P O 4 3− and SiO2 in this permanently anoxic system underlying a major coastal upwelling zone. Our data indicate that nutrients for phytoplankton growth are primarily supplied by upwelling of subsurface water on a seasonal basis. In addition, coastal runoff seems to supply important amounts of silica and ammonium to surface waters. We saw no indication of local nitrogen fixation in the Cariaco surface waters. In the suboxic zone, our data to date are not of sufficiently high enough resolution to resolve all important features. However, at least partial phosphate removal appears to occur in a zone above the first appearance of sulfide, and associated with intermittent intrusions of oxygenated water. In the suboxic zone, there appear to be thin layers where ammonium and nitrite coexist, potentially permitting anaerobic ammonium oxidation (anammox) to take place. In the deep waters, concentrations of ammonium, phosphate and silica continue to increase at a rate consistent with prior studies. However, in the upper part of the anoxic zone, there is evidence for sulfide removal, probably associated with oxygen intrusions.

Local and regional geochemical signatures of surface sediments from the Cariaco Basin and Orinoco Delta, Venezuela

We have analyzed the chemical compositions of 87 samples from the Cariaco Basin and 20 samples from the Orinoco Delta, Venezuela, in order to better characterize the fluvial sources of material to the Cariaco Basin. We observe distinctive regional patterns in composition; shelf sediments found near the Tuy River, Manzanares River, the Araya-Margarita region, and in the northern portion of the Unare platform record reproducible compositional differences. However, linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin. Average trap values are best approximated by mixing upper crust (30%–50%) with local river sources (50%–70% of either the Tuy, Unare, or Neveri Rivers) and eolian dust (

Denaturing gradient gel electrophoresis analyses of the vertical distribution and diversity of Vibrio spp. populations in the Cariaco Basin

The Cariaco system is the second largest permanently anoxic marine water body in the world. Its water column is characterized by a pronounced vertical layering of microbial communities. The goal of our study was to investigate the vertical distribution and diversity of Vibrio spp. present in the Cariaco Basin waters using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rDNA fragments. Representatives of the Vibrio genus were detected by nested and direct PCR in seawater at 10 depths. Sequence analyses of 55 DGGE bands revealed that only 11 different operational taxonomic units (OTU) are identified as Vibrio species. Between one and five OTUs were detected at each depth and the most common OTUs were OTU 1 and OTU 2, which phylogenetically clustered with Vibrio chagasii and Vibrio fortis, respectively. OTUs 3 and 4 were only found in the anoxic zone and were identified as Vibrio orientalis and Vibrio neptunius, respectively. Several Vibrio species detected are potentially pathogenic to human, prawns and corals such as Vibrio parahaemolyticus, Vibrio fischeri and Vibrio shilonii. In the Cariaco Basin, different Vibrio species were found to be specific to specific depths strata, suggesting that this genus is a natural component of the microbial communities in this marine redox environment.

The biogeochemistry of the suboxic and anoxic zones in the cariaco basin

Los estudios realizados en la Fosa de Cariaco durante una decada han demostrado que la interfaz sub-oxica en este sistema posee una variabilidad temporal y espacial, siendo el habitat de una comunidad microbiana diversa y activa. La circulacion en la cuenca, caracterizada por intrusiones ricas en oxigeno desde los umbrales oriental y occidental, controla aparentemente el espesor y la estructura de la zona sub-oxica. Las poblaciones microbianas, las cuales incluyen microorganismos heterotroficos y quimioautotroficos, crecen activamente a un ritmo que no es posible que se sostenga mediante el transporte vertical de oxidantes y reductores. La presente investigacion sugiere que el anillo microbiano depende de la utilizacion de especies del azufre con estados de oxidacion intermedio (sulfito, tiosulfato y S elemental) acoplada posiblemente con el ciclo de los oxidos metalicos. La produccion de estas especies se mantiene por procesos activos de intrusion y por el transporte desde el continente de hierro y oxidos metalicos durante los episodios lluviosos.