HABs in the Northwest Pacific: Emergent findings

HABs in the Northwest Pacific: Emergent findings and new directions from SeaWiFS and other data *Yu-Hwan Ahn and **Palanisamy Shanmugam *Ocean Satellite Research Group, Korea Ocean Research and Development Institute, Ansan P.O. Box. 29, Seoul, Korea **Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600 036, India ABSTRACT SeaWiFS RCA-Chl along with sea surface height variations/geostrophic currents, sea surface temperature, wind speed/direction and field observation data, are used to first describe comprehensively the occurrences of various hazardous algal blooms (HABs) and their underlying mechanisms and link to nutrient enrichment during the summer in shelf-slope waters off the Northwest Pacific (NWP). These datasets provide a coherent view of the summertime evolution of HABs and related physical processes in four common dynamic regions: coastal cold/estuary water zones, upwelling zones next to the coast, repeated meanders/eddies, and frontal regimes induced by the Kuroshio and its tributaries. High blooms coincided with the coastal upwelling and cyclonic eddy regimes that followed SST minimum and large negative SSH along with favorable phase of winds. By contrast, relatively low mean RCA were consistent with the fronts and anticyclonic meanders revealing moderate-high SSH fields along with variable winds blown off the NWP coast. These anticyclonic meanders, on some occasions, when nutrient-containing coastal water setoff higher chlorophyll biomass and major currents gained force in August, straddled the continental margin, entraining high chlorophyll water from the coast and from the adjacent cyclonic eddies located nearby into their outer rings that formed a conveyer-belt system of transport to inject coastal blooms into the deep-sea (e.g., East Sea) region of the NWP. The above findings based on satellite data combined with field hydrographic/ bloom observation data evidently illustrated richness of the response of summer HABs to the surface circulation and nutrient enrichment processes in shelf-slope waters off the NWP coast. Keywords: HABs, Ocean color, SeaWiFS, SS, winds, Northwest Pacific

1. INTRODUCTION Continental shelf-slope waters of the Northwest Pacific (NWP) (covering China, Korea, Japan and Russia) in recent decades have undergone a remarkably troubling transformation to more mesotrophic and eutrophic conditions by which Remote Sensing of Inland, Coastal, and Oceanic Waters, edited by Robert J. Frouin, Serge Andrefouet, Hiroshi Kawamura, Mervyn J. Lynch, Delu Pan, Trevor Platt, Proc. of SPIE Vol. 7150, 71500P · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.804767 Proc. of SPIE Vol. 7150 71500P-1 2008 SPIE Digital Library -- Subscriber Archive Copy

there is appearance, persistence and epidemic of summer algal blooms causing fish mortality, shellfish poisoning, physiological impairment, and numerous ecological and health impacts (Table 1). Such blooms of microscopic marine algae producing different forms of hazardous effects on marine organisms and human are therefore broadly termed as “hazardous” algal blooms (HABs). For what is known about HAB occurrences in the NWP region, NOWPAP (2005) supports a widespread belief that summer HABs are increasing in frequency and severity owing to nutrient enrichment of shelf-slope waters supplied either anthropogenically or naturally (Anderson et al., 2002) or owing to climate oscillations and climate change impacts (Edwards et al., 2006). Arising from growing concerns of such an increase in the occurrence of HABs, a number of national, regional and international programs namely the Global Ecology and Oceanography of Harmful Algal Blooms (GEOHAB) (http://ioc.unesco.org/hab/GEOHAB.htm), the Northwest Pacific Action Plan (NOWPAP)

(http://www.nowpap.org/),

the

Korean

Harmful

Algal

Bloom

Research

Group

(KORHAB)

(http://ioc.unesco.org/hab/HAN29_Final_comp.pdf), have hence recently been implemented to understand the features and mechanisms underlying the population dynamics of HABs, and to improve and develop management and amelioration strategies. In NWP waters which are structured by the major elements of the region’s circulation patterns, notably the Kuroshio and its branch currents (Fig. 1), several of the previous studies have focused on describing certain HABs in the narrow band of coastal waters, contributing diminutive knowledge of the offshore blooms and their underling mechanisms in the NWP region. This may be due to the following limitations: (1) in-situ sampling are confined to point measurements from ship-based surveys that limit the analysis and interpretation about the bloom formation and dynamics, (2) satellite chlorophyll estimates from generic bio-optical (and atmospheric correction) algorithms are either questionable in the vicinity of clouds or invalidated in coastal regions where the waters are generally turbid (high suspended sediment-SS and colored dissolved organic matter-CDOM concentrations) or strongly absorbing aerosols prevail, (3) sea surface temperature estimates from the NOAA-AVHRR are either uncertain under cloudy conditions or preventing clear identification of oceanographic features because of the uniformity of warm waters throughout the bloom initiation areas during the summer months. To overcome these limitations, in this paper, we use the RCA-Chl (from SeaWiFS), sea surface height/geostrophic currents, sea surface temperature, and wind, in conjugation with in-situ observation data, to illustrate the space and time relationships between summer HABs (1998-2006) and their underlying mechanisms off the NWP coast. The analysis in this study also builds on previous work using in-situ hydrographic, nutrients and bloom observations data, and addresses several questions as follows: Why, when and where the summer HABs develop, occur and terminate? Whether there is existence of any new bloom which had not been detected earlier by conventional field and remote sensing techniques? What are the spatial and temporal distributions of summer HABs? Can outbreaks be tied to the transport of offshore waters?, and, if so, can this transport be detected by remote sensing? What mechanisms elucidate the observed patterns? Is there a possible link between the HABs and nutrient enrichment? These are important to be addressed in order for the major monitoring programs to help protect marine ecosystems and to secure and support sustainable development, the economy, and the environment of this region.

Proc. of SPIE Vol. 7150 71500P-2

Table 1. The summer HAB species composition in NWP waters (1998-2006). Star marks indicate the high frequency species and no star marks indicate the relatively low frequency species. The bloom species recorded rarely are not shown here. Abbreviations for four groups of phytoplankton are as follows: Da – Diatoms, Df – Dinoflagellates, Ci – Ciliates,

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2. MATERALS AND METHODS Full resolution (~1 km / pixel at nadir) Level 1A SeaWiFS ocean color radiances for eight narrow spectral channels (first 6 bands with the FWHM 20nm and last 2 bands with the FWHM 40nm) in the visible and near-infrared spectral domains were acquired from the KORDI-HRPT station for the summer periods from June-September 1998-2006, which coincided with the peak bloom seasons in the NWP region. SeaWiFS Level 1A data were processed using SSMM atmospheric correction scheme and red tide index (RI) and red tide index chlorophyll (RCA) algorithms (Ahn and Shanmugam, 2006). These algorithms have been demonstrated to be very effective in detecting and providing an enhanced understanding of HABs in this region (Ahn and Shanmugam, 2006). SST were processed from AVHRR and used to detect SST variability in the bloom region. Similarly, wind fields (speed and direction) from NASA spectrometer SeaWinds onboard QuikSCAT Level 2B data records from June-September1999-2006 were collected and used to determine the direction and velocity of bloom transport along with movements of surface ocean features. Near real-time sea surface height (SSH) variation data generated from a combination of NASA’s Pathfinder TOPEX/POSEIDON (T/P) and Jason-1, and ESA’s (European Space Agency) ERS-2 and ENVISAT satellites were also collected from the DUACS and used for the identification and further understanding of the physical oceanographic features that concentrate and transport HAB species in the surface ocean.

3. RESULTS The SeaWiFS-derived RCA-Chl estimates from June-September 1998-2006 display a wealth of information about spacetime distributional patterns of the HABs as well reveal important information concerning the factors affecting the growth and dominance of these blooms organisms in the NWP region (Fig. 2 and Fig. 3). Overall, the heavy algal patches as of these RCA-Chl records discovered from many nearshore and offshore waters showed the mean RCA-Chl typically of 120 mg m-3 (dense blooms can exceed these levels) and TBCA of 3-56 ×103 km2. Of the observed five patches in June, four had shown the development and movement of their centre of distribution from the northern BS to southern BS, the eastern YS to western and southern YS, and the northeastern KSS to southwestern KSS. All of them were persistently low in mean RCA-Chl 1.5-4.3 mg m-3 and covered an area from 4.9 up to 14.3 ×103 km2 (TBCA), in contrast with the most prominent one off the SCS coast characterized by the mean RCA-Chl 10 mg m-3 and TBCA 56 ×103 km2. It should be noted that in July, there was a dramatic change in the spatio-temporal distributions of RCA-Chl patches that had spread several hundred kilometers from the nearshore to offshore or the offshore to nearshore, or moved up or down the coasts of the SCS, ECS, BS, YS, KSS, JS and RS, although some cases were found either persistent or recurrent in a “high occurrence zone” from the Fujian to Zhejiang, the BS to northern YS, and off the YR estuary. Of the observed 18 heavy algal patches, 9 occurrences had the mean RCA-Chl 4.3-13 mg m-3 and TBCA 20-55 ×103 km2, 5 occurrences had the mean RCA-Chl 4.8-11 mg m-3 and TBCA 5-18 ×103 km2, and 2 occurrences had the mean RCA-Chl 1.9-4 mg m-3 and TBCA 1.9-4 ×103 km2. Not surprisingly, similar distributions of the RCA-Chl were observed throughout NWP


waters during August, despite of their abundances being slightly declined as the populations extended over the large spatial scale offshore. For instance, of the 15 records, 5 displayed the mean RCA-Chl 1.5-11 mg m-3 and TBCA 20-53 ×103 km2, 7 displayed the mean RCA-Chl 1.8-10 mg m-3 and TBCA 11-19.5 ×103 km2, and 3 displayed the mean RCAChl 3.5-6 mg m-3 and TBCA 3.2-7 ×103 km2. It seemed that on an average, the July and August blooms themselves accounted for about 66% of the total HAB records during the summer 1998-2006 and some of them are “new” blooms discovered from SeaWiFS data (particularly 17 August 1998, 10 and 11 August 2000, 4 July 2001, 31 July 2002, 6 August 2003, 7 August 2004, and 29 July 2004) off the northeastern Taiwan coast, Jeju and Kyushu (and neighboring region) Islands, which had existed in these waters for many years, but which had not been detected or recognized until the advent of advanced scientific methods to analyze these SeaWiFS data. In September, the RCA-Chl distributions seen predominantly attached to the coasts of the BS-YS and of Korea and Russia moved throughout their oceanic ranges (with the mean RCA-Chl 7-12 mg m-3 and TBCA 6.5-42 ×103 km2), but notably diminished in surface waters of the ECS (with mean RCA-Chl 6-10 mg m-3 and TBCA 11-16 ×103 km2) and completely disappeared in the SCS and China-Taiwan segments. These trends confirm the previous observations that HAB peaks delay from April to June in the SCS, June to August in the ECS and Korean Seas, and July to September in the BS and RS (NOWPAP, 2005).

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SeaWiFS RCA-Chl (August)

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Fig. 2. The spatio-temporal distributions of the SeaWiFS RCA-Chl (relative units) in NWP waters during JuneSeptember 1998-2006 (the date and year is mentioned next to the bloom and month on the top of these figures).


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Fig. 3. The mean RCA-Chl concentrations (relative units) and TBCA (×103 km2) inferred from the SeaWiFS images during June-September 1998-2006. The pixels flagged as clouds were not used in either the mean RCA-Chl or the TBCA calculations (determined by the RCA-Chl >1 mg m-3). Fig. 4 allows us to more closely examine the relationship between the physical parameters (SSH and SST) and SeaWiFS RCA-Chl in shelf-slope waters of the NWP. Note that a tight inverse correlation exits between the RCA-Chl and SSH and SST, suggesting that high Chl are closely associated with cold surface waters consistent with the nutrient-rich upwelling and vertical mixing/cold-eddies. The relatively low Chl seem to be coupled with slightly warmer waters (>24oC) consistent with the fronts and shoaling thermocline/mesoscale filaments/anticyclonic features induced by Kuroshio and its branch currents. This suggests that dense blooms occur in the upwelling/cold-eddy regions, less dense blooms occur in the warmer water regions, and more frequent, moderate-dense blooms occur in the large part of the coastal and offshore regions where the intermediate/combination of these conditions prevail under favorable environmental and meteorological settings. These findings are consistent with previous studies clearly demonstrating a strong influence of the physical mechanisms on the development and distribution of phytoplankton blooms through processes that govern temperature, nutrient fluxes and dynamics (Wilson and Adamec, 2001; Lehahn et al, 2007). 20

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4. DISCUSSION AND CONCLUSION The findings in various studies suggest that the occurrence and severity of HABs are closely associated with various physical, chemical, biological and geological processes, and also the conditions that favor high levels of primary production (Haddad and Carder, 1979; Walsh et al., 2006). In shelf-slope waters of the NWP, nutrient concentrations are generally in excess of those needed for algal growth resulting mainly from the runoff from land (and atmospheric deposition), extent of intrusion of deep waters, convergence and mixing. Of these, the river (e.g., Yangtze)/runoff waters themselves input the concentrations of DSi (SiO32− = 53.39%), DIN (NH4−, NO2− and NO3− = 51.43%), and DIP (PO43− = 47.33%), with silicate being most influenced by the river plumes during summer. Equally, upwellings are considered important to reintroduce essential nutrients to the surface layer near the shelf break (in association with Kuroshio and its branch currents) that locally enhances algal biomass concentrations. Other physical processes such as the cyclonic eddies and frontal regimes similarly result in an increased nutrient flux from subsurface waters to the surface layer that stimulates abnormally high abundances of algal matters in the ECS and along the east coast of Korea and in other regions, heightening the regenerated production rates. On some occasions, large scale meanders propagating adjacent to the shelf also lead to the localized areas of upwelling and downwelling (likely more nutrient-rich) off the NWP coast (e.g., eastern coast of Korea), which strongly influence ocean biota and promote the shelf-slope exchange of water containing HABs. In contrast, some semi-enclosed bays (in the KSS) are cut off from these large-scale circulation patterns that promote mixing, favoring the C. polykrikoides and other blooms to span for more than 6-8 weeks, because of long turnover times for these waters, lack of freshwater inflow and restricted exchange with the open sea which make it difficult to disperse these blooms once they become established (Ahn and Shanmugam, 2006). References 1.

Y.H. Ahn and P. Shanmugam, “Detecting red tides from satellite ocean color observations in optically complex Northeast-Asia coastal waters”, Remote Sensing of Environment, 103, 419-437, 2006.

2.

K.D. Haddad and K.L. Carder, “Oceanic intrusion: One possible initiation mechanism of red tide blooms on the west coast of Florida”, In Toxic dinoflagellate blooms (pp. 269-274): Proc. 2nd Int. Conference, Elsevier, 1979..

3.

NOWPAP, “Integrated report on harmful algal blooms (HABs) for the NOWPAP region”, NOWPAP-CEARAC Report, 5-5 Ushijimashin-machi, Toyama City, Toyama 930-0856, Japan, 2005.

4.

D.M. Anderson, P.M. Glibert and J.M. Burkholder, “Harmful algal blooms and eutrophication: Nutrients sources, composition, and consequences”, Estuaries, 25, 704-726, 2002.

5.

M. Edwards, D.G. Johns, S.C. Leterme, E. Svendsen and A.J. Richardson, “Regional climate change and harmful algal blooms in the northeast Atlantic”, Limnology and Oceanography, 51, 820-829, 2006.

6.

J.J. Walsh et al., “Red tides in the Gulf of Mexico: Where, when and why?”, Journal of Geophysical Research, 111, doi: 10.1029/2004JC002813, 2006.
