Frank E. Muller-Karger

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

Atmospheric Correction of SeaWiFS Imagery over Turbid Coastal Waters: A Practical Method

Abstract The current SeaWiFS algorithms frequently yield negative water-leaving radiance values in turbid Case II waters primarily because the water-column reflectance interferes with the atmospheric correction based on the 765-nm and 865-nm spectral bands. Here we present a simple, practical method to separate the water-column reflectance from the total reflectance at 765 nm and 865 nm. Assuming the type of aerosol does not vary much over relatively small spatial scales (∼50–100 km), we first define the aerosol type over less turbid waters. We then transfer it to the turbid area by using a “nearest neighbor” method. While the aerosol type is fixed, the concentration can vary. This way, both the aerosol reflectance and the water-column reflectance at 765 nm and 865 nm may be derived. The default NASA atmospheric correction scheme subsequently is used to obtain the aerosol scattering components at shorter wavelengths. This simple method was tested under various atmospheric conditions over the Gulf of Mexico, and it proved effective in reducing the errors of both the water-leaving radiance and the chlorophyll concentration estimates. In addition, in areas where the default NASA algorithms created a mask due to atmospheric correction failure, water-leaving radiance and chlorophyll concentrations were recovered. This method, in comparison with field data and other turbid water algorithms, was tested for the Gulf of Maine and turbid, posthurricane Gulf of Mexico waters. In the Gulf of Maine it provided more accurate retrievals with fewer failures of the atmospheric correction algorithms. In the Gulf of Mexico it provided far fewer pixels with atmospheric failure than the other methods, did not overestimate chlorophyll as severely, and provided fewer negative water-leaving radiance values. Background Since the launch of the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) aboard the SeaStar satellite in August 1997, global ocean color data are available to the science community on a regular basis. SeaWiFS is superior to the original Coastal Zone Color Scanner (CZCS, Hovis et al., 1980 ). It has much higher radiometric sensitivity (10 bits versus 8 bits) and additional spectral bands to aid in atmospheric correction and bio-optical applications. Specifications called for uncertainties less than ±5% in retrieved water-leaving radiance and less than ±35% in chlorophyll a concentration ([chl a]) in Case I waters ( Hooker et al., 1992 ; “Case I” defined in Morel and Prieur, 1977 ). SeaWiFS, however, generally fails to deliver such fidelity in turbid or shallow coastal waters (Case II waters). Turbid water constituents (suspended sediments, bubbles, etc., or bottom reflection) can contribute significant amounts of radiance to the atmospheric correction bands (765 nm and 865 nm). This will induce large errors when applying the standard atmospheric correction scheme since in that scheme the water-leaving radiance is assumed to be negligible in the near-infrared (IR) part of the spectrum (Gordon and Wang, 1994) . Also, in turbid Case II waters, the current band ratio bio-optical algorithm does not work well, simply because there are often other constituents (e.g., colored dissolved organic matter, or CDOM) whose optical properties may not covary with phytoplankton pigment concentrations Morel and Prieur 1977 , Carder et al. 1991 , Carder et al. 1999 , Muller-Karger et al. 1991 .

On the Seasonal Phytoplankton Concentration and Sea Surface Temperature Cycles of the Gulf of Mexico as Determined by Satellites

Monthly climatologies of near-surface phytoplankton pigment concentration and sea surface temperature (SST) were derived for the Gulf of Mexico from multiyear series of coastal zone color scanner (CZCS) (November 1978 to November 1985) and advanced very high resolution radiometer (AVHRR) (January 1983 to December 1987) images. We complement these series with SST from the comprehensive ocean-atmosphere data set (1946–1987) and Climate Analysis Center (1982–1990), and hydrographic profile data from the NOAA National Oceanographic Data Center (1914–1985). The CZCS ocean color satellite data provide the first climatological time series of phytoplankton concentration for the region. The CZCS images show that seasonal variation in pigment concentration seaward of the shelf is synchronous throughout the gulf, with highest values (>0.18 mg m−3) in December to February and lowest values (∼0.06 mg m−3) in May to July. Variation in SST is also synchronous throughout the gulf, with maxima in July to September and minima in February to March, The amplitude of the SST variation in the western gulf is about twice that observed in the eastern gulf, and SST maxima and minima persist longer in the west. Larger amplitudes in SST variation are also observed toward the margins. While annual cycles of SST and pigment concentrations are out of phase relative to each other, the phases of mixed layer depth change and pigment concentration change are similar. Model simulations suggest that the single most important factor controlling the seasonal cycle in surface pigment concentration is the depth of the mixed layer. The combined use of ocean color and infrared images permits year-round observation of spatial structure of the surface circulation in the gulf and the pattern of dispersal of the Mississippi River plume. Infrared images are most useful between November and mid-May, when strong SST gradients occur. During this time, pigment concentrations are high and can be horizontally homogeneous. In contrast, between late May and October, SST fields are uniform, but the Loop Current and large anticyclonic eddies could be traced with the CZCS. Three anticyclonic eddies were observed in 1979, and at least two were observed in 1980. No eddies were observed during summers of subsequent years in the CZCS time series, but this may be a result of the dramatic decrease in the satellite sampling rate. The series of color images showed that small parcels of Mississippi River water were frequently (2–4 times a year) entrained in the cyclonic edge of the Loop Current, stretched along the Current, and carried to the southeast along the western Florida shelf. However, most of the Mississippi River water flowed to the west, following the Louisiana-Texas coast as far south as the Mexico-United States border. Here, a persistent cyclone may reside, exporting shelf constituents to deeper regions of the gulf.

Hurricanes, submarine groundwater discharge, and Florida's red tides

A Karenia brevis Harmful Algal Bloom affected coastal waters shallower than 50 m off west-central Florida from January 2005 through January 2006, showing a sustained anomaly of ∼1 mg chlorophyll m -3 over an area of up to 67,500 km 2 . Red tides occur in the same area (approximately 26-29°N, 82-83°W) almost every year, but the intense 2005 bloom led to a widespread hypoxic zone (dissolved oxygen <2 mg L -1 ) that caused mortalities of benthic communities, fish, turtles, birds, and marine mammals. Runoff alone provided insufficient nitrogen to support this bloom. We pose the hypothesis that submarine groundwater discharge (SGD) provides the missing nutrients, and indeed can trigger and support the recurrent red tides off west-central Florida. SGD inputs of dissolved inorganic nitrogen (DIN) in Tampa Bay alone are ∼35% of that discharged by all central Florida rivers draining west combined. We propose that the unusual number of hurricanes in 2004 resulted in high runoff, and in higher than normal SGD emerging along the west Florida coast throughout 2005, initiating and fueling the persistent HAB. This mechanism may also explain recurrent red tides in other coastal regions of the Gulf of Mexico.

Gulf of California biogeographic regions based on coastal zone color scanner imagery.

Topographically, the Gulf of California is divided into a series of basins and trenches that deepen to the south. Maximum depth at the mouth is greater than 3000 m. Most of the northern gulf is less than 200 m deep. The gulf has hydrographic features conducive to high primary productivity. Upwelling events have been described on the basis of temperature distributions at the eastern coast during winter and spring and at the western coast during summer. Tidal amplitude may be as high as 9 m in the upper gulf. On the basis of discrete phytoplankton sampling, the gulf was previously divided into four geographic regions. This division took into consideration only the space distribution, taxonomic composition, and abundance of microphytopla nkton. With the availability of the coastal zone color scanner (CZCS) imagery, we were able to in clude the time variability of pigments to make a more detailed biogeographic division of the gulf. With weekly composites of the imagery, we generated time series of pigment concentrations for 33 locations throughout the gulf and for the whole life span of the CZCS. The time series show a clear seasonal variation, with maxima in winter and spring and minima in summer. The effect of upwelling at the eastern coast is clearly evident, with high pigment concentrations. The effect of the summer upwelling off the Baja California coast is not evident in these time series. Time series from locations on the western side of the gulf also show maxima in winter and spring that are due to the eddy circulation that brings upwelled water from the eastern side. Principal- component analysis was applied to define 14 regions. Ballenas Channel, between Angel de la Guarda and Baja California, and the upper gulf always appeared as very distinct regions. Some of these 14 regions relate to the geographic distributions of important faunal groups, including the benthos, or their life cycles. For example, the upper gulf is a place for reproduction and the nursery of many fish species, marine mammals and birds are specially abundant in Ballenas Channel, sardine spawning mostly occurs in the central gulf in spring, and shrimp are abundant off mainland Mexico.

Change detection in shallow coral reef environments using Landsat 7 ETM+ data

Abstract This paper aims to clarify the potential of the new Landsat 7/Enhanced Thematic Mapper Plus (ETM+) sensors for change detection in coral reef environments. We processed images of two reef sites in Florida and Hawaii acquired over short time intervals (2 weeks and 3 months). During these periods, reefs were not affected by major disturbances (phase shift, strategy shift, bleaching, and hurricanes). This stability allowed us to assess the bias in change detection analysis. Two methods for change detection analysis were applied. The first one estimates the atmospheric conditions (Rayleigh and aerosol radiances, ozone and diffuse transmittances) using an ETM+/SeaWiFS multisensor approach. The second method is an empirical correction based on pseudoinvariant features that compensates for different atmospheric conditions as well as for any sensor (noise) or environmental (water column, sea surface state) conditions. The atmospheric correction alone did not provide an accurate match in images across time due to significant whitecaps and possible sun glint and its products required an empirical adjustment. Therefore, for the images in this study there was not substantial benefit in performing an atmospheric correction compared to an empirical correction alone. Both methods resulted in a minimum uncertainty of 4, 3, and 3 digital counts, respectively, in ETM+ Bands 1–3. Finally, we completed the study of real images by the analysis of ETM+ reflectance spectra for a large variety of coral reef objects. We concluded that the assessment of the rates of change in three ubiquitous classes ‘sand,’ ‘background’ (including rubble, pavement, and heavily grazed dead coral structure), and ‘foreground’ (including living corals and macroalgae) emerges as the most reproducible and feasible application for the ETM+ sensor.

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.

How precise are SeaWiFS ocean color estimates? Implications of digitization-noise errors

Various subtle but important digitization round-off and noise errors are found in SeaWiFS imagery. These errors often cause large relative errors at a pixel and cause pixelization or “speckling” across the image, which is particularly obvious in the SeaWiFS chlorophyll standard product. Using simulations and current SeaWiFS algorithms, we show the effect of digitization-noise errors on the SeaWiFS ocean-color data products. It is found that the SeaWiFS mission goals, namely to estimate water-leaving radiances to within ±5% and chlorophyll-a concentrations to within ±35% for Case I waters, cannot always be met. For maritime aerosol conditions the errors in the estimated chlorophyll concentrations over oligotrophic waters can easily reach ±60% and the absolute values over adjacent pixels can vary 2–3 fold. Several schemes can be used to reduce this type of error, among which the spatial smoothing of the atmospheric-correction bands is recommended. This is because the errors in those bands are propagated and exaggerated to the visible bands and to the chlorophyll product through the atmospheric-correction process. The smoothing scheme, however, will cause more pixels to be discarded over cloud edges. Ultimately, a significant increase in both digitization bits and sensitivity is essential to achieve the stated mission goals. Such improved specifications are available on MODIS.

The influence of Loop Current perturbations on the formation and evolution of Tortugas eddies in the southern Straits of Florida

Large cyclonic eddies on the northern edge of the Florida Current are the dominant mesoscale features within the southern Straits of Florida. The most prominent of these features is a quasi-stationary eddy that forms near the Dry Tortugas. Our observations, compiled from 3 years of advanced very high resolution radiometer measurements in the Straits of Florida and Gulf of Mexico, demonstrate a strong relationship between the generation of anticyclonic rings from the Gulf of Mexico Loop Current and the evolution of Tortugas eddies within the southern Straits of Florida. In six cases, Tortugas eddies evolve from cyclonic frontal eddies which form along the boundary of the Loop Current. The eddies remain stationary near the Dry Tortugas until they are impacted by an approaching Loop Current frontal eddy. The length of time an eddy spends near the Dry Tortugas is increased when the Loop Current sheds an anticyclonic ring. The involvement of a Loop Current frontal eddy in the ring-shedding process results in a delay in its, and hence the Tortugas eddys, downstream propagation. Results suggest that the lifetime of a Tortugas eddy can be as long as 140 days when a ring-shedding event occurs, or as short as 50 days in the absence of any ring-shedding events. Upon entering the Straits of Florida, the Tortugas eddies are deformed by the narrowing topography and shrink to approximately 55% of their original size as they propagated downstream. The shrinking of these eddies is accompanied by an accelerated translation from 5 km/d in the western Straits of Florida to 16 km/d in the east.

Surface-ocean color and deep-ocean carbon flux: how close a connection?

Abstract Seven years of simultaneous, quasi-continuous data collected by the Nimbus-7 Coastal Zone Color Scanner and by a deep-ocean sediment trap in the Sargasso Sea allow the derivation of empirical relationships between remotely sensed ocean color and the sinking of particulate carbon into the deep sea. In agreement with earlier observations, the results indicate a 1.5-month lag between surface-ocean events observed by the satellite and arrival of a record of those events, carried by sinking particles, at a depth of 3200 m. In addition, the results suggest that the sea-surface area most influential on particle-flux characteristics recorded by the sediment trap in the Sargasso Sea lies to the northeast of the traps mooring site. The results point towards possible ways of quantifying the role of marine biota in the regulation of atmospheric carbon dioxide through use of satellite observations.