Florida Shelf Research Articles

Comparison of the X-TRACK altimetry estimated currents with moored ADCP and HF radar observations on the West Florida Shelf

The performance of coastal altimetry over a wide continental shelf is assessed using multiple-year ocean current observations by moored Acoustic Doppler Current Profilers (ADCP) and high-frequency (HF) radar on the West Florida Shelf. Across-track, surface geostrophic velocity anomalies, derived from the X-TRACK along-track sea level anomalies are compared with the near surface current vector components from moored ADCP observations at mid shelf. The altimeter-derived velocity anomalies are also directly compared with the HF radar surface current vector radial components that are aligned perpendicular to the satellite track. Preliminary results indicate the potential usefulness of the along-track altimetry data in contributing to descriptions of the surface circulation on the West Florida Shelf and the challenges of such applications. On subtidal time scales, the root-mean-square difference (rmsd) between the estimated and the observed near surface velocity component anomalies is 8–11cm/s, which is about the same magnitude as the standard deviations of the velocity components themselves. Adding a wind-driven Ekman velocity component generally helps to reduce the rmsd values.

Evaluation of trajectory modeling in different dynamic regions using normalized cumulative Lagrangian separation

[1] The Lagrangian separation distance between the endpoints of simulated and observed drifter trajectories is often used to assess the performance of numerical particle trajectory models. However, the separation distance fails to indicate relative model performance in weak and strong current regions, such as a continental shelf and its adjacent deep ocean. A new skill score is proposed based on the cumulative Lagrangian separation distances normalized by the associated cumulative trajectory lengths. This skill score is used to evaluate surface trajectories implied by Global HYCOM hindcast surface currents as gauged against actual satellite-tracked drifter trajectories in the eastern Gulf of Mexico during the 2010 Deepwater Horizon oil spill. It is found that the new skill score correctly indicates the relative performance of the Global HYCOM in modeling the strong currents of the Gulf of Mexico Loop Current and the Gulf Stream and the weaker currents of the West Florida Shelf. In contrast, the Lagrangian separation distance alone gives a misleading result. The proposed dimensionless skill score is particularly useful when the number of drifter trajectories is limited and neither a conventional Eulerian-based velocity nor a Lagrangian-based probability density function may be estimated.

Sound production of red grouper Epinephelus morio on the West Florida Shelf

Passive acoustic and digital video record- ings were used to investigate sonic activity and behav- ior of red grouper Epinephelus morio on the West Florida Shelf. Red grouper were found to produce a unique series of low-frequency (180 Hz peak) pulses consisting of 1 to 4 brief (0.15 s) broadband pulses and a 0.5 to 2 s growl (short call); occasionally these were followed by a rapid series of 10 to 50 broadband pulses (pulse train). Sound production was ob served through- out the day and night, but most sounds occurred between sunrise and sunset, with a noticeable increase during late afternoon. Behaviors asso ciated with sound production included solitary male activity and court - ship interactions, indicating that sound production is likely related to spawning activity. Thus, passive acoustics could be an effective tool in monitoring red grouper reproduction and defining critical habitat of a keystone species.

Satellites, models combine to track Deepwater Horizon oil spill

Figure 1. The 3.5-day surface oil trajectory forecast using the (a) West Florida Shelf (WFS) model, (b) Gulf of Mexico Hybrid Coordinate Ocean Model (HYCOM), (c) South Atlantic Bight–Gulf of Mexico (SABGOM) model, and (d) Global HYCOM. Black denotes virtual particles and purple their swept-out areas. Background fields are sea surface temperatures (SST) and currents. of South Florida (USF) by marshaling numerical modeling, observational, and satellite resources from existing coastal ocean observing systems.2 The fate of oil in the ocean depends on many factors, including transport and dispersion by circulation, chemical transformations, and biological consumption. Important information, such as the oil flow rate, was unknown throughout the event. Mitigation activities (dispersants, burning, and skimming) added further uncertainties. To reduce these uncertainties along with inherent model error growth, we used surface oil locations from satellite imagery to frequently reinitialize various models.

A new bio-optical algorithm for the remote sensing of algal blooms in complex ocean waters

[1] A new bio-optical algorithm has been developed to provide accurate assessments of chlorophyll a (Chl a) concentration for detection and mapping of algal blooms from satellite data in optically complex waters, where the presence of suspended sediments and dissolved substances can interfere with phytoplankton signal and thus confound conventional band ratio algorithms. A global data set of concurrent measurements of pigment concentration and radiometric reflectance was compiled and used to develop this algorithm that uses the normalized water-leaving radiance ratios along with an algal bloom index (ABI) between three visible bands to determine Chl a concentrations. The algorithm is derived using Sea-viewing Wide Field-of-view Sensor bands, and it is subsequently tuned to be applicable to Moderate Resolution Imaging Spectroradiometer (MODIS)/Aqua data. When compared with large in situ data sets and satellite matchups in a variety of coastal and ocean waters the present algorithm makes good retrievals of the Chl a concentration and shows statistically significant improvement over current global algorithms (e.g., OC3 and OC4v4). An examination of the performance of these algorithms on several MODIS/Aqua images in complex waters of the Arabian Sea and west Florida shelf shows that the new algorithm provides a better means for detecting and differentiating algal blooms from other turbid features, whereas the OC3 algorithm has significant errors although yielding relatively consistent results in clear waters. These findings imply that, provided that an accurate atmospheric correction scheme is available to deal with complex waters, the current MODIS/Aqua, MERIS and OCM data could be extensively used for quantitative and operational monitoring of algal blooms in various regional and global waters.

Relative role of subinertial and superinertial modes in the coastal long wave response forced by the landfall of a tropical cyclone

A set of numerical experiments has been performed in order to analyze the long-wave response of the coastal ocean to a translating mesoscale atmospheric cyclone approaching the coastline at a normal angle. An idealized two-slope shelf topography is chosen. The model is forced by a radially symmetric atmospheric pressure perturbation with a corresponding gradient wind field. The cyclone's translation speed, radius, and the continental shelf width are considered as parameters whose impact on the long wave period, modal structure, and amplitude is studied. Subinertial continental shelf waves (CSW) dominate the response under typical forcing conditions and on the narrower shelves. They propagate in the downstream (in the sense of Kelvin wave propagation) direction. Superinertial edge wave modes have higher free surface amplitudes and faster phase speeds than the CSW modes. While potentially more dangerous, edge waves are not as common as subinertial shelf waves because their generation requires a wide, gently sloping shelf and a storm system translating at a relatively high (∼10 m s −1 or faster) speed. A relatively smaller size of an atmospheric cyclone also favors edge wave generation. Edge waves with the highest amplitude (up to 60% of the forced storm surge) propagate upstream. They are produced by a storm system with an Eulerian time scale equal to the period of a zero-mode edge wave with the wavelength of the storm spatial scale. Large amplitude edge waves were generated during Hurricane Wilma's landfall (2005) on the West Florida shelf with particularly severe flooding occurring upstream of the landfall site.

Imprudent fishing harvests and consequent trophic cascades on the West Florida shelf over the last half century: A harbinger of increased human deaths from paralytic shellfish poisoning along the southeastern United States, in response to oligotrophication?

Within the context of ubiquitous overfishing of piscivores, recent consequent increments of jellyfish and clupeids have occurred at the zooplanktivore trophic level in the eastern Gulf of Mexico (GOM), after overfishing of one of their predators, i.e. red snapper. Initiation of a local trophic cascade thence led to declines of herbivore stocks, documented here on the West Florida shelf. These exacerbating world-wide trophic cascades have resulted in larger harmful algal blooms (HABs), already present at the base of most coastal food webs. Impacts on human health have thus far been minimal within nutrient-rich coastal regions. To provide a setting for past morbidities, consideration is given to chronologies of other trophic cascades within eutrophic, cold water marine ecosystems of the Scotian Sea, in the Gulf of Alaska, off Southwest Africa, within the Barents, White, and Black Seas, in the Gulf of Maine, and finally in the North Sea. Next, comparison is now made here of recent ten-fold increments within Florida waters of both relatively benign and saxitoxic HABs, some of which are fatal to humans. These events are placed in a perspective of other warm shelf systems of the South China and Caribbean Seas to assess prior and possible future poison toxicities of oligotrophic coastal habitats. Past wide-spread kills of fishes and sea urchins over the Caribbean Sea and the downstream GOM are examined in relation to the potential transmission of dinoflagellate saxitoxin and other epizootic poison vectors by western boundary currents over larger “commons” than local embayments. Furthermore, since some HABs produce more potent saxitoxins upon nutrient depletion, recent decisions to ban seasonal fertilizer applications to Florida lawns may have unintended consequences. In the future, human-killing phytoplankton, rather than relatively benign fish-killing HABs of the past, may be dispersed along the southeastern United States seaboard.

Isolation on the West Florida Shelf with implications for red tides and pollutant dispersal in the Gulf of Mexico

Analysis of year-long drifter trajectories and records of simulated surface Lagrangian Coherent Structures (LCSs) have suggested the presence of a resilient Cross-Shelf Transport Barrier (CSTB) on the West Florida Shelf (WFS). The CSTB was conjectured to provide a large degree of isolation, which is consequential for the fueling of red tides on the southern WFS by nutrients possibly released by rivers and canals directly on the region. Here this conjecture is thoroughly tested by identifying LCSs as well as performing tracer advection calculations based on seven-year-long records of surface and subsurface currents produced by a HYbrid-Coordinate Ocean Model (HYCOM) simulation of the Gulf of Mexico (GoM). The identified LCSs suggest that the CSTB extends downward in the water column. The tracer calculations suggest that, while the majority of the nutrients possibly released by rivers and canals directly on the southern WFS are retained within the region for long times, only a small fraction of the nutrients possibly released by rivers outside the WFS reach the southern WFS, mainly accompanying shoreward excursions of the CSTB. These results add importance to the role played by the CSTB in controlling red tide development on the WFS. Implications of the results for the dispersal of pollutants, such as oil, in the GoM are discussed.

Spatial and temporal patterns of dolphin distribution and relative abundance on the West Florida Shelf based on autonomous acoustic recorder detections and visual survey data.

Determining the abundance, movements, and distribution of animals can be accomplished using a variety of methods. Here we use boat‐based visual surveys and detection of sounds with autonomous acoustic recorders to determine the presence of dolphins in the coastal waters of the West Florida Shelf in the eastern Gulf of Mexico. The autonomous acoustic array consisted of 12 bottom‐mounted digital spectrogram (DSG) recorders deployed from June to September 2008. Whistles and echolocation trains were identified in spectrograms for each recorder. Mean active space of each recorder was determined using the Bellhop propagation model with inputs appropriate for the location, including bathymetry, frequency, bottom loss, sound velocity profiles, and weather conditions. Acoustic detections per km2 per hour were calculated for each recorder. Visual surveys spatially overlapped the acoustic array from June to August 2008. Visual detections were analyzed per unit effort in grid cells overlaying the DSG locations. The overall relative abundance of dolphin acoustic detections in recorder active spaces and visual detections in corresponding grid cells are compared. In addition, spatial and temporal patterns of the acoustic presence of dolphins are discerned from the continuous time series of acoustic recordings.

HF Radar Performance in a Low-Energy Environment: CODAR SeaSonde Experience on the West Florida Shelf*

AbstractThree long-range (5 MHz) Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde HF radars overlooking an array of as many as eight moored acoustic Doppler current profilers (ADCPs) have operated on the West Florida Shelf since September 2003 for the purpose of observing the coastal ocean currents. HF radar performance on this low-energy (currents and waves) continental shelf is evaluated with respect to data returns, the rms differences between the HF radar and the ADCP radial currents, bearing offsets, and radial velocity uncertainties. Possible environmental factors affecting the HF radar performance are discussed, with the findings that both the low-energy sea state and the unfavorable surface wave directions are the main limiting factors for these HF radar observations of currents on the WFS. Despite the challenge of achieving continuous backscatter from this low-energy environment, when acquired the data quality is good in comparison with the ADCP measurements. The rms differences range from 6 to 10 cm s−1 for hourly and from 3 to 6 cm s−1 for 36-h low-pass-filtered radial currents, respectively. Bearing offsets are in the range from −15° to +9°. Coherent variations of the HF radar and ADCP radial currents are seen across both tidal and subtidal frequency bands. By examining the HF radar radial velocities at low wave energy, it is found that the data returns decrease rapidly for significant wave heights smaller than 1 m, and that the rms differences between the HF radar and ADCP radials are degraded when the significant wave height is smaller than 0.3 m.

Long-term evaluation of three satellite ocean color algorithms for identifying harmful algal blooms ( Karenia brevis) along the west coast of Florida: A matchup assessment

We present a simple algorithm to identify Karenia brevis blooms in the Gulf of Mexico along the west coast of Florida in satellite imagery. It is based on an empirical analysis of collocated matchups of satellite and in situ measurements. The results of this Empirical Approach is compared to those of a Bio-optical Technique – taken from the published literature – and the Operational Method currently implemented by the NOAA Harmful Algal Bloom Forecasting System for K. brevis blooms. These three algorithms are evaluated using a multi-year MODIS data set (from July, 2002 to October, 2006) and a long-term in situ database. Matchup pairs, consisting of remotely-sensed ocean color parameters and near-coincident field measurements of K. brevis concentration, are used to assess the accuracy of the algorithms. Fair evaluation of the algorithms was only possible in the central west Florida shelf (i.e. between 25.75°N and 28.25°N) during the boreal Summer and Fall months (i.e. July to December) due to the availability of valid cloud-free matchups. Even though the predictive values of the three algorithms are similar, the statistical measure of success in red tide identification (defined as cell counts in excess of 1.5 × 10 4 cells L −1) varied considerably (sensitivity— Empirical: 86%; Bio-optical: 77%; Operational: 26%), as did their effectiveness in identifying non-bloom cases (specificity— Empirical: 53%; Bio-optical: 65%; Operational: 84%). As the Operational Method had an elevated frequency of false-negative cases (i.e. presented low accuracy in detecting known red tides), and because of the considerable overlap between the optical characteristics of the red tide and non-bloom population, only the other two algorithms underwent a procedure for further inspecting possible detection improvements. Both optimized versions of the Empirical and Bio-optical algorithms performed similarly, being equally specific and sensitive (~ 70% for both) and showing low levels of uncertainties (i.e. few cases of false-negatives and false-positives: ~ 30%)—improved positive predictive values (~ 60%) were also observed along with good negative predictive values (~ 80%).

Evidence for the production of marine fluorescent dissolved organic matter in coastal environments and a possible mechanism for formation and dispersion

A positive linear relationship between salinity and fluorescent dissolved organic matter (FDOM) was observed on several occasions along the West Florida shelf at salinities greater than 36.5. This represents a departure from the typical inverse relationship between FDOM and salinity observed in most coastal regions caused by the mixing of riverine FDOM with clear oceanic water. Three‐dimensional excitation‐emission matrices showed that the high‐salinity, high‐FDOM water had blue‐shifted spectra characteristic of autochthonous, marine FDOM, with peak M concentrations eight times higher than previously reported for seawater. The blue‐shifted fluorescence endmember at high salinity was clearly distinguishable from a photobleached FDOM endmember. A high‐resolution time series collected in a shallow embayment with significant Gulf of Mexico influence supplemented cruise data and provided a possible mechanism for the formation of high‐salinity, high‐FDOM water. During a dry period of spring tides, high‐salinity, high‐FDOM water was exported at ebb tide and lower‐salinity, low‐FDOM water was imported during flood tide. During neap tide, FDOM and salinity demonstrated no evidence of either export or dilution from incoming seawater. After a significant rain event, a more typical inverse relationship between salinity and FDOM was observed. Production of FDOM‐rich water in shallow embayments has not been observed previously. This is likely an important source of organic matter and dispersion of this material may explain observations of a high‐salinity, high‐FDOM water in adjacent coastal regions.

Comparative analysis of bacterioplankton assemblages from Karenia brevis bloom and nonbloom water on the west Florida shelf (Gulf of Mexico, USA) using 16S rRNA gene clone libraries

The brevetoxin-producing dinoflagellate, Karenia brevis, forms nearly annual blooms off the Florida west coast, severely impacting the region's ecology and economy. Bacteria are often cited as either promoting or interfering with the development of algal blooms, and thus a detailed study of the bacterioplankton assemblages associated with K. brevis was undertaken. We developed sixteen 16S rRNA gene clone libraries from K. brevis bloom and adjacent nonbloom water to determine the bacterial groups present and assess the influence of K. brevis cell number and/or depth on bacterioplankton community composition. Most notably, bacterial groups such as Rhodobacterales (Alphaproteobacteria) and Cytophagales/Sphingobacteriales (Bacteroidetes), reported previously to be associated with other harmful algal species, were often abundant in the presence of K. brevis. Cyanobacteria frequently dominated surface samples containing no detectable K. brevis, consistent with earlier work suggesting that these photosynthetic organisms may be important in promoting the proliferation of these blooms by conditioning the water. Moreover, differences in the abundance/diversity of traditionally more rare and often undocumented phylogenetic groups (e.g. Betaproteobacteria, Deltaproteobacteria, Chloroflexus, Firmicutes) were apparent in bloom vs. nonbloom water. This is the first study to document the association of these phylogenetic groups with natural K. brevis populations and suggests a potential role for these microorganisms in K. brevis bloom dynamics.

A historical analysis of the potential nutrient supply from the N2 fixing marine cyanobacterium Trichodesmium spp. to Karenia brevis blooms in the eastern Gulf of Mexico

Blooms of Karenia brevis co-occur with populations of the N2 fixing cyanobacterium Trichodesmium on the West Florida shelf where iron (Fe) and phosphorus (P) supplies have been shown to control the growth potential of these cyanophytes. Historical data records of Trichodesmium presence and abundance (1960–2008) were analyzed in relation to the nutrient requirements of annually coincident K. brevis blooms, focusing on the potential direct supply of nitrogen (N) via N2-fixation with subsequent excretion of recently fixed N2 as NH4+ and dissolved organic nitrogen and indirect transfer from Trichodesmium biomass (as particulate N and P) during cell death. While release of “new” N from biological N2-fixation can supply up to 40% of the 2001 large red tide (9.5 × 106 cells L−1), estimates of the total N available, including the regeneration of additional nutrients supplied from particulate stocks aggregated within surface populations of observed Trichodesmium biomass during cell death, suggest that up to 100% of N and P requirements of such large blooms can be met from N2-fixation sources.

Remote detection of Trichodesmium blooms in optically complex coastal waters: Examples with MODIS full-spectral data

Remote detection of the Trichodesmium spp. cyanobacteria blooms on the west Florida shelf (WFS) has been problematic due to optical complexity caused by sediment resuspension, coastal runoff, and bottom interference. By combining MODIS data measured by the ocean bands and land bands, an approach was developed to identify surface mats of Trichodesmium on the WFS. The approach first identifies possible bloom patches in MODIS FAI (floating algae index) 250 m resolution imagery derived from the Rayleigh-corrected reflectance at 667, 859, and 1240 nm. Then, spectral analysis examines the unique reflectance characteristics of Trichodesmium at 469, 488, 531, 551, and 555 nm due to specific optical properties (absorption, backscattering, and fluorescence) of the unusual pigments in Trichodesmium. These spectral characteristics (i.e., high–low–high–low–high reflectance at 469-488–531–551–555 nm, respectively) differentiate Trichodesmium mats unambiguously from other features observed in the FAI imagery, such as Sargassum spp. Tests in other coastal locations show that the approach is robust and applicable to other optically complex waters. Results shown here can help study Trichodesmium bloom dynamics (e.g., initiation and bloom formation) and may also help design future sensors to better detect and quantify Trichodesmium, an important N 2 fixer in the global oceans.

Satellite remote sensing of harmful algal blooms: A new multi-algorithm method for detecting the Florida Red Tide ( Karenia brevis)

In a continuing effort to develop suitable methods for the surveillance of harmful algal blooms (HABs) of Karenia brevis using satellite radiometers, a new multi-algorithm method was developed to explore whether improvements in the remote-sensing detection of the Florida Red Tide was possible. A Hybrid Scheme was introduced that sequentially applies the optimized versions of two pre-existing satellite-based algorithms: an Empirical Approach (using water-leaving radiance as a function of chlorophyll concentration) and a Bio-optical Technique (using particulate backscatter along with chlorophyll concentration). The long-term evaluation of the new multi-algorithm method was performed using a multi-year MODIS dataset (2002–2006; during the boreal Summer–Fall periods – July–December) along the Central West Florida Shelf between 25.75°N and 28.25°N. Algorithm validation was done with in situ measurements of the abundances of K. brevis; cell counts ≥1.5 × 10 4 cells l −1 defined a detectable HAB. Encouraging statistical results were derived when either or both algorithms correctly flagged known samples. The majority of the valid match-ups were correctly identified (∼80% of both HABs and non-blooming conditions) and few false negatives or false positives were produced (∼20% of each). Additionally, most of the HAB-positive identifications in the satellite data were indeed HAB samples (positive predictive value: ∼70%) and those classified as HAB-negative were almost all non-bloom cases (negative predictive value: ∼86%). These results demonstrate an excellent detection capability, on average ∼10% more accurate than the individual algorithms used separately. Thus, the new Hybrid Scheme could become a powerful tool for environmental monitoring of K. brevis blooms, with valuable consequences including leading to the more rapid and efficient use of ships to make in situ measurements of HABs.

Quasi-Operational Coastal Ocean Nowcast/Forecast Systems

For several years, quasi-operational (i.e., real-time, semi-autonomous, research-mode) nowcast/forecast systems have been run in two quite different regimes: (1) the Straits of Florida/East Florida Shelf, which includes the Florida Current, and (2) Prince William Sound, Alaska, which is a small, semi-enclosed sea with two major straits. For both regimes, the Princeton Ocean Model (POM) has been implemented with mesoscale resolution. Both implementations are forced by mesoscale numerical weather predictions, the US Navy's operational global ocean model (NCOM, which assimilates satellite altimetric sea surface height anomalies, MCSST, ARGO float temperature and salinity profiles, etc.) for open boundary conditions, and four diurnal and four semi-diurnal tides, also imposed on the open boundaries. Real-time observations are mainly used for model skill assessment, as a prelude to data assimilation. One of the benefits of this activity has been new understanding derived from diagnostics studies made possible by these numerical simulations. For example, the Florida Current Frontal (cyclonic) Eddies, which form weekly in the cyclonic shear zone along the shelfbreak, have been more fully characterized than had been possible by observations alone, and the prevalence of three-layered monthly mean flow in the straits of Prince William Sound has been determined in a highly variable regime that is difficult to observe comprehensively.

Variation in pelagic larval growth of Atlantic billfishes: the role of prey composition and selective mortality

Atlantic blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) larvae were collected from 10 monthly cruises (June–October 2003 and 2004) across the Straits of Florida to test (1) whether growth differed between the more productive western region near the Florida shelf, and the less productive eastern region toward the Bahamas, and (2) whether growth was related to prey consumption. Examination of larval sagittal otoliths revealed that instantaneous growth and daily growth during the first 2–3 weeks of life did not vary significantly between the two regions for either species. However, recent growth during the last two full days prior to collection was greater in the west for blue marlin larvae. Recent growth of blue marlin larvae <9 mm SL (primarily zooplanktivorous) was significantly related to prey composition (faster growth when higher proportions of Farranula copepods were consumed). Western larvae grew faster and had higher proportions of Farranula in their guts. Trends for sailfish larvae were not significant. In both species, comparison of early growth between <9 and ≥9 mm SL size groups indicated that growth trajectories diverged around 5–8 mm SL, the time when billfish larvae become capable of piscivory. Significantly faster growth of larger (older) larvae suggests that mortality was selective for fast growers and that the transition to piscivory may be a critical point in the early life of billfish.

Influence of extreme storm events on West Florida Shelf CDOM distributions

Colored Dissolved Organic Matter (CDOM) distribution and signatures provide vital information about the amount and composition of organic material in aquatic environments. This information is critical for deciphering the sources and biogeochemical pathways of organic carbon, and thus vital to the understanding of carbon cycling and budgets. Waters of the West Florida Shelf are heavily influenced by many river systems on Florida's Gulf Coast that, to the first order, control CDOM distributions on the shelf. Three storm events during 2004 and 2005 (Hurricane Charley, Hurricane Wilma, and a Winter Storm) profoundly altered the typical distribution of CDOM fluorescence and absorption properties on the Southern West Florida Shelf. Seasonal surveys revealed that changes in the underwater light field as a result of major hurricanes and resuspension events are linked closely with a number of factors prior to a storm's passing such as the presence of persistent blooms, rainfall and discharge. Additionally, storm track and wind direction were found to play a significant role in CDOM signatures.

Response to “Aerosol iron deposition to the surface ocean — Modes of iron supply and biological responses” by P.W. Boyd, D.S. Mackie, and K.A. Hunter

Boyd et al. (in press) propose a relatively simple tool that observationalists can use to constrain the extent to which biological events, such as observed episodic phytoplankton blooms, are driven by dust supply to the ocean environment. In their paper, they use this tool to examine four case studies from the literature, one of whichwas authored by Lenes et al. (2001). Boyd et al. (in press) definitively conclude that the 100-fold Trichodesmium biomass increase on the West Florida shelf (WFS) that occurred by July 1999 could not be attributed to the alleviation of iron (Fe) limitation by deposition of Saharan dust. Their calculations of iron deposition incorporate multiple assumptions related to atmospheric dust concentration, solubility, wet deposition, and air column height. In all iron deposition studies, large uncertainties are associatedwith each of these variables. For example, as acknowledged in their work, iron solubility can range over 5 orders of magnitude with changes in medium, pH, and particle size. While dissolution rates for soil dust are low (b1–2%; Jickells and Spokes, 2001), solubility can be substantially increased by organic complexation, and the proximity of the WFS region to industrial centers can greatly enhance the acidity of mineral aerosols due to mixing with SO2 and NOx (Jickells et al., 2005). Boyd et al. (in press) calculated that 600 μmol Fe m−2 was available for wet deposition in one day during the 27 June–4 July dust event. Given a 5-m surface mixed layer (SML) and a 2% dissolution rate, a net concentration increase of 2.4 nmol L−1 would result in the SML. This is similar to the ~3 nmol L−1 mean offshore iron concentration reported on 5–7 July (Lenes et al., 2001). In addition, this estimate does not account for increased SML iron concentrations from a prior dust event on 15–17 June. Similarly, Boyd et al. (in press) misunderstand the properties of the Saharan Air Layer (SAL). The SAL is an elevated layer of hot, dry, dust-laden air that originates over the Sahara (Prospero and Carlson, 1972). Over the Caribbean and southern Florida the layer is typically 2 km thick and extends to an altitude of 3–4 km, sometimes higher (Gatz and Prospero, 1996; Reid et al., 2003). Furthermore, Boyd et al. (in press) assume that the measured surface dust concentration is applicable to the SAL; in reality, dust concentrations in the layer are on average about three times higher than concentrations at the surface (Prospero and Carlson, 1972; Reid et al., 2003). Considering these