Nelson Melo

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.

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.

Cuban, Mexican, U.S. Researchers probing mysteries of Yucatan Current

The Yucatan Current has received very little biological or oceanographic attention in the past, but oceanographers from Cuba, Mexico, and the United States are changing that. They have assembled a multidisciplinary, multinational dataset and are beginning a detailed examination of the current, which has a large impact on all three countries. A series of theoretical and applied studies are using historical infrared and ocean color satellite data and some ship data. In general, western boundaries of large ocean gyres have intense currents along continental margins, and important physical and biological processes take place in such regions. In the northwest Caribbean Sea, such an intense flow is funneled through a constriction known as the Yucatan Strait, between Mexicos Yucatan Peninsula and Cuba (see Figure 1, inset).

Characterization of Available Light for Seagrass and Patch Reef Productivity in Sugarloaf Key, Lower Florida Keys

Light availability is an important factor driving primary productivity in benthic ecosystems, but in situ and remote sensing measurements of light quality are limited for coral reefs and seagrass beds. We evaluated the productivity responses of a patch reef and a seagrass site in the Lower Florida Keys to ambient light availability and spectral quality. In situ optical properties were characterized utilizing moored and water column bio-optical and hydrographic measurements. Net ecosystem productivity (NEP) was also estimated for these study sites using benthic productivity chambers. Our results show higher spectral light attenuation and absorption, and lower irradiance during low tide in the patch reef, tracking the influx of materials from shallower coastal areas. In contrast, the intrusion of clearer surface Atlantic Ocean water caused lower values of spectral attenuation and absorption, and higher irradiance in the patch reef during high tide. Storms during the studied period, with winds >10 m·s−1, caused higher spectral attenuation values. A spatial gradient of NEP was observed, from high productivity in the shallow seagrass area, to lower productivity in deeper patch reefs. The highest daytime NEP was observed in the seagrass, with values of almost 0.4 g·O2·m−2·h−1. Productivity at the patch reef area was lower in May than during October 2012 (mean = 0.137 and 0.177 g·O2·m−2·h−1, respectively). Higher photosynthetic active radiation (PAR) levels measured above water and lower light attenuation in the red region of the visible spectrum (~666 to ~699 nm) had a positive correlation with NEP. Our results indicate that changes in light availability and quality by suspended or resuspended particles limit benthic productivity in the Florida Keys.