Use of remote sensing in monitoring and forecasting of harmful algal blooms Richard P. Stumpf*a and Michelle C. Tomlinsona NOAA Ocean Service, 1305 East West Highway, Silver Spring MD 20910
a
ABSTRACT Harmful algal blooms (HABs) have impacts on coastal economies, public health, and various endangered species. HABs are caused by a variety of organisms, most commonly dinoflagellates, diatoms, and cyanobacteria. In the late 1970’s, optical remote sensing was found to have a potential for detecting the presence of blooms of Karenia brevis on the US Florida coast. Due to the nearly annual frequency of these blooms and the ability to note them with ocean color imagery, K. brevis blooms have strongly influenced the field of HAB remote sensing. However, with the variability between phytoplankton blooms, heir environment and their relatively narrow range of pigment types, particularly between toxic and non-toxic dinoflagellates and diatoms, techniques beyond optical detection are required for detecting and monitoring HABs. While satellite chlorophyll has some value, ecological or environmental characteristics are required to use chlorophyll. For example, identification of new blooms can be an effective means of identifying HABs that are quie intense, also blooms occurring after specific rainfall or wind events can be indicated as HABs. Several HAB species do not bloom in the traditional sense, in that they do not dominate the biomass. In these cases, remote sensing of SST or chlorophyll can be coupled with linkages to seasonal succession, changes in circulation or currents, and wind-induced transport—including upwelling and downwelling, to indicate the potential for a HAB to occur. An effective monitoring and forecasting system for HABs will require the coupling of remote sensing with an environmental and ecological understanding of the organism. Keywords: harmful algal blooms, ocean color, chlorophyll, sea surface temperature, forecasting, modeling
BACKGROUND Harmful algal blooms (HABs) have a severe impact on coastal economies. The cost of these blooms has been estimated at more than $50 million per year in the United States (Anderson et al., 2000)1. Aquaculture in Norway and Korea has such value as to warrant significant investments in reducing HAB impacts. The impacts on shellfisheries, aquaculture, and public health have pointed to a need to forecast the blooms. In the case of wild and commercial fisheries, early detection can lead to targeting specific beaches, thereby avoiding coast-wide closures and also harvesting the stock or protecting it prior to impact by the HABs. Recreational shellfisheries, such as razor clams along the Washington (USA) coast can be managed to assure that the short (several day) openings are not ruined by late closures. HABs in the Gulf of Mexico impact humans by respiratory irritation and dead fish. Knowing the likely impact of the HABs will reduce the risk to both public health and tourism. Remote sensing can be a part of this by detecting blooms and detecting conditions that favor the blooms. HABs are generally characterized as toxic, although algal blooms can be harmful through excess biomass leading to hypoxia and anoxia. The toxic forms cover a range of algae, including dinoflagellates, diatoms, cyanobacteria, prymesiophytes, and pelagophytes (Landsberg, 2003; Stumpf and Tomlinson, 2005) 2, 3. To complicate this, a few species, including the diatoms in the Pseudo-nitzschia genus, are not always toxic. Similarly some species are toxic only to fish and not apparently accumulative in mollusks, such as Prorocentrum spp. (Lansberg, 2002) 2, which can produce intense blooms in estuaries (Stumpf and Tyler, 1988) 4.
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[email protected]; phone 1-301-713-3028; ccma.nos.noaa.gov/rsd/
Edited by Frouin, Robert J.; Babin, Marcel; Sathyendranath, Shubha. Proceedings of the SPIE, Volume 5885, pp. 148-151 (2005).
DISCUSSION HABs in the Gulf of Mexico, and some other parts of the world, are referred to as “red tides” as they sometimes turn the water mahogany brown. In the late 1970’s two analyses occurred that brought HABs to the view of the remote sensing community. The first involved ocean color. The airborne Ocean Color Scanner (OCS) designed for Coastal Zone Color Scanner (CZCS) development, was deployed in Florida and had an overflight of a red tide on October 14, 1977 (Mueller, 1979) 5. An area of discolored water was identified as corresponding to the location of high concentrations of Karenia brevis taken in a sample the previous day. In 1978, the CZCS was launched. In November 1978 it collected an image that showed high chlorophyll associated with a large Karenia brevis bloom (Steidinger and Haddad, 1981) 6. This image became the “poster bloom” for remote sensing of harmful algal blooms, although there is a low probability that all the high chlorophyll in the image is, in fact, Karenia. Optics was not the sole tool for HAB detection, however. Steidinger and Haddad6 also showed that thermal fronts were correlated with the location of blooms offshore in 1979-1980 and their transport onshore. The fronts identified patterns that were not resolvable with CZCS imagery at that time. These two cases, optical detection and identification of related patterns capture the two end points of remote sensing for HABs. Some blooms may be observable with ocean color techniques, others can be found only by their associations with other factors. In fact, spectral analysis is the most problematic solution for HABs. From an optical perspective, dinoflagellates and diatoms are quite similar and in some cases, indistinguishable from satellite, even under monospecific conditions with no other optical materials present (case I waters) (Schofield et al., 1999) 7. In coastal areas with high levels of particulates and dissolved pigments, optical techniques and chlorophyll are unreliable. “High” chlorophyll is the norm in coastal areas, especially those influenced by upwelling or rivers. Blue-green algae (cyanobacteria), which can produce scum, offer a better potential target for optical detection. However, these may be found by the scum layers, not necessarily by the bio-optical models of the characteristics of algae in the water (Stumpf and Tomlinson, 2005; Kahru et al., 2000) 4,8. Even Steidinger and Haddad’s6 work on frontal impacts on HAB initiation, a division exists between the HAB-oriented community and the remote sensing community. The remote sensing community has generally concentrated on techniques based on optical detection. The HAB community has concentrated on oceanographic and ecological associations. The transfer of knowledge from the ecological side back to remote sensing offers the greatest promise, but it necessitates different perspectives on the choice and implementation of remote sensing techniques. With Karenia brevis being considered the most promising for optical techniques, previous work has shown that satellite chlorophyll can be correlated with the cell concentrations in pure blooms of Karenia (Tester et al., 1998) 9. Measures of chlorophyll provide a poor indicator of the presence of Karenia (Stumpf et al., 2003) 10. A more reliable indicator is based on ecological relationships. Karenia is typically a “classic” bloom-forming organism in that it typically will develop to concentrations that dominate the biomass. These blooms are most common along the southwest Florida (Gulf) coast, occurring virtually every year between late summer and winter (and occasionally in late summer on the Florida panhandle. The blooms appear rapidly, so identifying “new blooms” is a good indicator of Karenia (Stumpf et al., 2003; Tomlinson et al., 2004) 10, 11. New blooms are defined as the chlorophyll anomaly against the previous two months, lagged by 15 days. While some new blooms during the HAB season are diatoms (e.g. Rhizosolenia spp.) or Trichodesmium--leading to some false positives--this method has rarely missed the start of a Karenia event. The misses have occurred when a Karenia bloom developed unusually within a diatom bloom or when a late season (early spring) bloom occurred in the midst of a normal spring bloom so that it was not detectable (against a background of persistent high chlorophyll). New blooms that start after upwelling conditions are more likely to be Karenia than other algae (Stumpf et al., 2003) 10. In some areas, such as Japan, the onset of new blooms observed qualitatively after rainfall events also tend to indicate the onset of a HAB (Ishizaka, 2003) 12. Oceanographic characteristics and dynamics that influence HABs can be found from a combination of SST, chlorophyll, and ancillary data such as winds. Keafer and Anderson (1993) 13 found that occurrences of Alexandrium tamarense could be correlated with sea surface temperature patterns (SST). This finding was significant in that this organism rarely blooms at sufficiently high concentrations to alter either the chlorophyll biomass or the optical characteristics of the water.
In upwelling areas, blooms of Dinophysis spp. often develop along fronts. These blooms are too weak to inpact the color or influence chlorophyll concentrations. The onset of the HAB is indicated by development of an upwelling front in the appropriate season followed by upwelling relaxation owing to a wind shift (Sordo et al., 2001) 14. The relaxation may transport the HAB into the estuaries where they can impact shellfisheries. Fortunately for fisheries, but unfortunately for forecasting, these blooms do not occur every year, so some additional method of characterization is needed. Temperature can also identify transport. Tester et al. (1991) 15 found that a filament from the Gulf Stream, together with shoreward directed wind transport explained the appearance of Karenia brevis on the North Carolina coast. Raines et al. (2001) 16 showed that upwelling conditions off Ireland allowed SST to be used to separate chlorophyll patterns into regions where Karenia mikimotoi dominated as opposed to other algae. Other observations have been of associations of HABs with fronts, especially off large rivers (e.g. the Pearl River estuary in China) (Tang et al., 2003; Yin et al., 1999) 17, 18 . The future of remote sensing of HABs will advance most effectively with a coupling of ecological linkages and physical forcing with the appropriate imagery types. A continued assumption that high chlorophyll indicates HABs, or that spectral analysis will extract most HABs, will lead to slow progress in the field. Detection of HABs is an important component of forecasting the likelihood of HABs in an area. However, the ultimate need and objective of coastal managers is a forecast and detection should not be used as an alternative goal. Examining characteristics of the blooms in context of unique oceanographic conditions, ecological associations, and detectable characteristics, as well as coupling viable models of transport and circulation, will lead to robust methods of providing advanced warning of harmful algal blooms.
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