Elizabeth Johns

Ocean acidification refugia of the Florida Reef Tract.

Ocean acidification (OA) is expected to reduce the calcification rates of marine organisms, yet we have little understanding of how OA will manifest within dynamic, real-world systems. Natural CO2, alkalinity, and salinity gradients can significantly alter local carbonate chemistry, and thereby create a range of susceptibility for different ecosystems to OA. As such, there is a need to characterize this natural variability of seawater carbonate chemistry, especially within coastal ecosystems. Since 2009, carbonate chemistry data have been collected on the Florida Reef Tract (FRT). During periods of heightened productivity, there is a net uptake of total CO2 (TCO2) which increases aragonite saturation state (Ωarag) values on inshore patch reefs of the upper FRT. These waters can exhibit greater Ωarag than what has been modeled for the tropical surface ocean during preindustrial times, with mean (± std. error) Ωarag-values in spring = 4.69 (±0.101). Conversely, Ωarag-values on offshore reefs generally represent oceanic carbonate chemistries consistent with present day tropical surface ocean conditions. This gradient is opposite from what has been reported for other reef environments. We hypothesize this pattern is caused by the photosynthetic uptake of TCO2 mainly by seagrasses and, to a lesser extent, macroalgae in the inshore waters of the FRT. These inshore reef habitats are therefore potential acidification refugia that are defined not only in a spatial sense, but also in time; coinciding with seasonal productivity dynamics. Coral reefs located within or immediately downstream of seagrass beds may find refuge from OA.

The Deep Western Boundary Current in the tropical North Atlantic Ocean

Abstract Tracer and CTD data collected on four cruises to the western tropical North Atlantic during 1987–1989 are used to describe the water mass properties and geostrophic transport of the Deep Western Boundary Current (DWBC). The study are extends along the boundary and east to the mid-Atlantic Ridge from 14.5°N to the equator. Two cores of recently ventilated (with respect to the chlorofluorocarbon F11) northern hemisphere water are advected through the area. A shallow core is centered at about 1500 m and a deeper core at about 3500 m. The upper core of high F11 (bounded by the 3.2 and 4.7°C potential temperature isotherms) is typically located inshore of the deeper core (bounded by the 1.8 and 2.4°C isotherms). Geostrophic currents and transports were computed relative to a zero reference velocity on the 4.7°C potential temperature surface. Total transport below the 4.7°C surface for the most intense portion of the DWBC is 26 Sv (1 Sv = 106 m3 s−1). Of this total, 17 Sv are contained in the two recently ventilated high F11 cores. The Ceara Rise blocks equatorward flow in the DWBC below the 1.8°C potential temperature surface, causing at least the coldest waters to recirculate back to the north.

Upper layer circulation in the western tropical North Atlantic Ocean during August 1989

Shipboard acoustic Doppler current profiler velocity measurements and profiles of temperature, salinity, and dissolved oxygen measurements are used to map the flow field above the σθ = 26.8 isopycnal (approximately the upper 300 m of the water column) in the North Brazil Current (NBC) retroflection region (0° to 14°N, 60° to 40°W) during August of 1989. The water column is divided into a near-surface, upper thermocline layer (above σθ = 24.5) and a main to subthermocline layer (σθ = 24.5 to σ0 = 26.8). In the upper layer the eastward flowing North Equatorial Countercurrent (NECC) is composed of 16×106 m3/s of NBC transport that has retroflected from the coast between 6°N and 8°N and 8×106 m3/s of North Equatorial Current (NEC) transport returning eastward. An anticyclonic eddy with a 10×106 m3/s transport lies northwest of the NBC retroflection. No throughflow is observed along the boundary, and only a small portion of the observed NEC transport (1.5×106 m3/s) enters the Caribbean Sea. In the lower layer the NBC transports 8×106 m3/s into the subthermocline North Equatorial Undercurrent (NEUC). In addition, 15×106 m3/s joins the NEUC from the north. At 44°W the subsurface core of the NEUC lies south of the near-surface core of the NECC. Nearly half of the NEUC transport is made up of a mixed water type with salinity-oxygen (S-O2) characteristics intermediate to the characteristics of the original component transports. There is no evidence of continuous NBC flow into the Caribbean Sea in the lower level. The closed eddy to the north of the retroflection, however, contains water masses with South Atlantic S-O2 properties.

The Annual Cycle of Meridional Heat Flux in the Atlantic Ocean at 26.5°N

Abstract Total meridional heat flux through a zonal oceanic section at 26.5°N in the Atlantic Ocean is computed from hydrographic, direct current and surface wind observations. The oceanic current and temperature fields are decomposed into depth-averaged and depth-dependent (including Ekman and geostrophic) components to perform the calculation. The mean annual boat flux is estimated to be 1.21 ± 0.34 PW. Mean monthly values of net heat flux are also computed from the data. The annual cycle of net heat flux determined from these values ranges from a minimum of 0.69 PW in February to a maximum of 1.86 PW in July. Thus, in contrast to an earlier estimate of the annual cycle of oceanic heat flux derived indirectly from surface energy fluxes and upper-layer heat content changes, there is no net southward heat flux during the Call. Results from a simulation of the circulation of the North Atlantic give an annual cycle of heat flux similar to our calculations with a summer maximum and winter minimum. However, t...

Upper layer temperature structure of the western tropical Atlantic

Mean monthly topographies of the 20°C and 10°C isothermal surfaces are used to describe the vertical displacements of the upper and lower thermocline in the western tropical Atlantic. The isotherm topographies are generated from expendable bathythermograph data collected between 1966 and 1993. The topographies confirm, and extend closer to the coast, earlier findings that demonstrate large spatial and temporal variability in the region. For example, the ridge and trough systems observed previously in the interior are shown, and their extension to the western boundary is described. In particular, it is shown that the ridge associated with the North Equatorial Countercurrent (NECC) extends from the interior northwestward along the western boundary, reaching farther north along the boundary in the upper thermocline than in the lower thermocline. South of the equator the northwestern corner of the countercurrent trough is apparent on the lower surface but not on the upper. The annual and semiannual harmonics of the vertical isotherm displacements account on the average for about 60% of total variance on both surfaces. The horizontal structure of the first harmonic amplitude is similar for both surfaces, showing maximum amplitude along the axis of the NECC ridge. Minimum amplitudes are observed to the north along the axis of the countercurrent trough. These distributions are similar to the pattern of the first-harmonic amplitude of the wind stress curl, supporting earlier studies of curl forcing of near-surface current features.

Deep flow along the western boundary south of the Blake Bahama Outer Ridge

Abstract In June–July 1990, hydrographic, chlorofluorocarbon (CFC), and velocity observations were taken along the western boundary of the North Atlantic south of the Blake Bahama Outer Ridge from 30° to 24°N between the northern Bahamas and 71°W. The deep flow in the region, associated with the deep western boundary current, forms a pattern of strong, narrow currents and cyclonic gyres close to the continental slope with broad, slower southward flow offshore. The CFCs reveal that the most recently “ventilated” water (i.e., having the highest CFC concentrations due to more recent contact with the atmosphere in the northern North Atlantic) is found along the western boundary in two distinct cores between potential temperatures 4°–6°C and 1.9°–2.4°C. Geostrophic transport streamlines were constructed for the deep flow, referenced using direct velocity observations at 26.5°N and assuming mass conservation between closed areas bounded by the hydrographic sections. The tracers and transports are used together ...

Satellite-Observed Black Water Events off Southwest Florida: Implications for Coral Reef Health in the Florida Keys National Marine Sanctuary

A “black water” event, as observed from satellites, occurred off southwest Florida in 2012. Satellite observations suggested that the event started in early January and ended in mid-April 2012. The black water patch formed off central west Florida and advected southward towards Florida Bay and the Florida Keys with the shelf circulation, which was confirmed by satellite-tracked surface drifter trajectories. Compared with a previous black water event in 2002, the 2012 event was weaker in terms of spatial and temporal coverage. An in situ survey indicated that the 2012 black water patch contained toxic K. brevis and had relatively low CDOM (colored dissolved organic matter) and turbidity but high chlorophyll-a concentrations, while salinity was somewhat high compared with historical values. Further analysis revealed that the 2012 black water was formed by the K. brevis bloom initiated off central west Florida in late September 2011, while river runoff, Trichodesmium and possibly submarine groundwater discharge also played important roles in its formation. Black water patches can affect benthic coral reef communities by decreasing light availability at the bottom, and enhanced nutrient concentrations from black water patches support massive macroalgae growth that can overgrow coral reefs. It is thus important to continue the integrated observations where satellites provide synoptic and repeated observations of such adverse water quality events.

On the Accuracy of SeaWiFS Ocean Color Data Products on the West Florida Shelf

ABSTRACT Cannizzaro, J.P.; Hu, C.; Carder, K.L.; Kelble, C.R.; Melo, N.; Johns, E.M.; Vargo, G.A., and Heil, C.A., 2013. On the accuracy of SeaWiFS ocean color data products on the West Florida Shelf. Despite the importance of the West Florida Shelf (WFS) on regional ecology and local economy, systematic shelf-wide assessment of the ocean biology has not been conducted, primarily because of budgetary limitations for routine field campaigns and unknown accuracy of satellite-based data products. Here, using shipboard spectral normalized water-leaving radiance (nLw[λ]) data and chlorophyll-a concentrations (Chl-a) collected regularly during two multiyear field programs spanning >10 years, the accuracies of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) standard data products were evaluated. The in situ data covered a wide dynamic range, with about one order of magnitude in nLw(490) (0.47 to 4.01 mW cm−2 μm−1 sr−1) and two orders of magnitude in Chl-a (0.07 to 10.6 mg m−3). Near-concurrent in situ and satellite nLw(λ) data showed absolute percent differences (APD) increasing from 7–9% to 10–14% when data with elevated aerosol optical thicknesses at 865 nm (τa865) were included. Most of this uncertainty, however, canceled in the maximal blue-to-green reflectance band ratios traditionally used for estimating Chl-a. SeaWiFS OC4 Chl-a showed a root mean square (RMS) uncertainty of 0.106 for log-transformed data in waters offshore of the 20-m isobath that increased to 0.255 when all data were considered. The increased likelihood for nearshore SeaWiFS Chl-a greater than ∼0.5 mg m−3 to be overestimated was shown to be caused by a variety of factors (colored dissolved organic matter [CDOM], suspended sediments, and bottom reflectance) that varied in both time and space. In the future, more sophisticated algorithms capable of taking these factors into consideration are required to improve remote determinations of Chl-a in nearshore waters of the WFS.