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Part 1
Biodiversity
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1
UNDERSTANDING HARMFUL ALGAL BLOOM (HAB)
OCCURRENCES IN MANILA BAY, PHILIPPINES
ROSA CELIA POQUITA-DU and PETER ALAN TODD
Harmful algal blooms (HAB) are a yearly event in the waters of Manila Bay, Philippines. The very first major bloom was caused by a toxic dinoflagellate species, Pyrodinium bahamense var. compressum, which is known to cause Paralytic Shellfish Poisoning (PSP). This chapter reviews HAB episodes in the bay recorded from the 1980s to 1990s, and compares them with the more recent events in the 21st century. Pyrodinium blooms occurred for almost a decade from 1988 until 1998, before being succeeded by a non-toxic species, Noctiluca scintillans, that was first recorded in the year 2001. No PSP cases were recorded between then and the year 2010. More recent bloom episodes, however, were caused by another toxic dinoflagellate species, Alexandrium minutum. The numbers of recorded PSP cases were higher in the 1980s and 1990s bloom events compared to the more recent toxic blooms. The decrease in incidences of PSP do not necessarily mean that the ecological health of the bay has improved, but instead could be due to changes in the physico-chemical properties of the water favouring one phytoplankton species over another. Even though sea surface temperatures and rainfall are known to be associated to HABs, the present study found no significant correlations among the variables tested. Due to the complexity of the environmental issues identified in Manila Bay, it is essential to conduct more comprehensive studies on the biological, physical, and chemical components of the bay to facilitate a clearer understanding of HAB episodes.
1. Introduction
Almost every coastal country in the world is affected by Harmful Algal Blooms (HABs), commonly known as red tides (Anderson, 2007), and they represent one of the major marine environmental problems in Southeast Asia (SEA). The phenomenon is caused by the proliferation of phytoplankton species which results from a combination of physical, chemical and biological factors including natural mechanisms of species dispersal, pollution-related nutrient enrichment of water bodies, climatic shifts, and transport of algal species via ship ballast water (Anderson, 1998 & 2007; Smayda, 1989; Hallegraeff, 1993). Coastal waters in the SEA region are increasingly contaminated by pollutants from various sources, sometimes reaching levels that cause the rejection of shellfish for human consumption and export (Chua et al., 1989). The Philippines is one of the countries greatly affected by HABs, with almost yearly recurrences at several coastal areas. Since 1999, the country has recorded the highest number of Paralytic Shellfish Poisoning (PSP) incidences caused by Pyrodinium bahamense var. compressum blooms in the region (Azanza and Taylor, 2001). PSP results from the ingestion of a potent endotoxin called saxitoxin (STX), which is produced by some species of dinoflagellates. These toxins are accumulated in the tissues of bivalve organisms (e.g. mussel, oysters) and are harmful when consumed by humans. Saxitoxin is classified as a neurotoxin which is capable of causing paralysis to the victim within 24 hours after ingestion (Popkiss et al., 1979).
The Philippines has experienced a considerable number of red tide outbreaks in the coastal waters in all three major islands: Luzon, Visayas and Mindanao. This phenomenon has been continuously monitored over the last three decades starting from the 1980s to the present. Up to the year 2001, the country has experienced a total of 42 toxic algal blooms which resulted in a total of 2,107 PSP cases with 117 fatalities (Furio & Gonzales, 2004). Reported economic loss in the Philippines during the 1983 bloom (the very first HAB occurrence in the country) was approximately USD 5 million. The first bloom that occurred in Manila Bay in 1988 resulted in losses of USD 300,000 per day during the height of the episode (Corrales and Maclean, 1995; Azanza and Taylor, 2001).
HABs have brought tremendous negative impacts, not just to the marine ecosystem, but also to the economy and human health; therefore, understanding bloom dynamics can potentially improve existing mitigation measures and further minimize economic losses. This chapter (1) examines the recorded toxic and non-toxic algal blooms in Manila Bay from 1988 to 2011, and discusses the mechanisms of each causative species; (2) evaluates the roles of two environmental factors, i.e. Sea Surface Temperature (SST) and rainfall, in initiating bloom episodes in the bay and; (3) recommends steps towards improving existing management strategies.
Manila Bay
Manila bay is a semi-closed body of water situated in the western part of Luzon between 14.23° and 14.87°N and 120.53° and 121.03°E (Figure 1.1). Its coastline extends to approximately 190 km coastline with a surface area of 1,700 km2, estimated volume of 2.89 × 1010 m3 and an average depth of 17 m (Jacinto, et al., 1998; Velasquez, et al., 2002). The climate of the bay is characterized by two pronounced seasons: (1) dry from November to May, and (2) wet from June to September. Generally, August is the rainiest and February the driest. Three major wind regimes are observed in the bay: (1) Northeast monsoon from October to January, (2) south-easterly from February to May, and (3) south-westerly trade winds from June to September. The circulation pattern of the surface water is influenced by the interplay of the surface wind stress, tidal forces, and freshwater discharges (De las Alas and Sodusta, 1985). It receives drainage directly from almost 17,000 km2 of watershed composed of 26 catchment areas. A total of 131 rivers and creeks drain into the Bay and high organic loading from these rivers led to the eutrophication of the waters (Bajarias and Relox, 1996). It is bounded by coastal cities and municipalities of Bulacan and Pampanga in the North, Cavite and National Capital region or NCR (including Manila, Pasay, Paranaque, Las Pinas and Navotas) in the East, Bataan in the West and Northwest. Also, within the watershed are the non-coastal cities and municipalities of the NCR (Quezon City, Caloocan, Makati, Pasig, Marikina, Mandaluyong, Muntinlupa, Valenzuela, Malabon, San Juan, Pateros and Taguig), provinces of Nueva Ecija and Tarlac in Region 3, and Rizal and Laguna in Region 4. There are several residential areas near the Bay region that stretch from the low lying coastal zone to upland areas. A large number of migrants have moved to the urban areas around Manila Bay, which has contributed to poverty. Many live in shanty houses in waterways (by the rivers and coastal zones), under bridges and abandoned railways (Ashby, 2008).
Fig. 1.1. Map of Manila Bay showing the neighboring provinces.
Manila Bay plays an important role to the economy of the country due to its numerous contributions (Jacinto et al., 2006). The bay supports several industries from shipping ports to major fishing grounds and numerous aquaculture farms (Prudente et al., 1994). Not only does it have high economic value, but it also had been declared as a critical habitat in Metro Manila as it supports numerous endemic wildlife species, migratory birds, as well as mudflats and mangrove resources (Su et al., 2009). The bay used to be one of the major sources of fishes, shellfishes and other aquaculture products for exports, but the volume of desirable species has diminished considerably over time. Massive economic and biological overfishing in the bay occurred as early as 1948 (Muñoz, 1991). It is considered to be eutrophic due to the accumulated large amounts of nutrients, mostly coming from domestic discharges, particularly near Pasig River watershed (Jacinto et al., 1998), and some from indiscriminate discharge of effluents from industries around the area (Jacinto, et al., 2006). Results from the study conducted by Chang et al. (2009) showed high concentrations of nitrogen species, particularly ammonium, in Pasig River which indicated the river to be the main source of nutrient input to the bay. It receives all outflow from Manila’s sewerage system and only about a quarter of the entire population of Manila (approximately 1.14 million) is served by the communal sewerage system (Gunnerson and Cuellar, 1988).
2. Methods
Most of the data were obtained from government reports prepared by Bureau of Fisheries and Aquatic Resources (BFAR) — Marine Biotoxins Monitoring Unit (MBMU) and the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAG-ASA) for the records of HAB occurrences in Manila Bay, and the Average Monthly Rainfall respectively. Other data sources included the International Research Institute (IRI) Climate Data Library1 for the Average Monthly SST data and also published quantitative studies on HAB occurrences in Manila Bay.
Acquisition of Secondary Data
(i) Record of PSP cases in Manila Bay
Data of HABs in Manila Bay were obtained from BFAR Central Office in Quezon City, Philippines. The monitoring unit mainly focused on harmful algal blooms associated with shellfish poisoning. Acquired data include: (a) number of illnesses & deaths associated with PSP cases in Manila Bay from 1988 when it first occurred in the area until the recent date (mid 2012); (b) causative phytoplankton species for each bloom; (c) implicated shellfish and levels of PSP toxins; (d) specific locations where the bloom started and; (e) dates/duration of shellfish ban. The sampling protocol applied by MBMU in detecting PSP events employs measurements of two key parameters which include: (a) toxicity level in shellfish meat samples and; (b) presence of PSP causing organisms. Collection of samples is on a regular basis wherein the frequency of sampling depends on the site. Field sites which are representative of the whole area of concern and specifically chosen based on the logistic reports of red tide occurrence and availability of shellfish resources are already established. One of the primary sampling stations in the Philippines is Manila Bay, which covers Cavite, Navotas (Metro Manila), Bulacan, and Bataan. Sa...
