Riverine inputs into Manila Bay, Philippines potentially drive changes in chlorophyll-a and total suspended matter concentrations measured from MODIS

Abstract

Water quality along the northern coast of Manila Bay is deteriorating due to anthropogenic influence, and the use of remote sensing is an effective tool for environmental monitoring. This study estimated the chlorophyll-a (chl-a) and total suspended matter (TSM) concentrations in Manila Bay from 2002 to 2016 and evaluated the possible environmental factors that contributed to the spatiotemporal changes in these two parameters. MODIS images were processed through the Case 2 Regional Coast Color model to determine monthly chl-a and TSM concentrations. Manila Bay was divided into six zones based on spectral characteristics. Each zone was then compared fo r the environmental variables precipitation, runoff, sea surface temperature, and wind speed downloaded from the ECMWF Reanalysis v5 global dataset. Zones 1-3 are located in the northern half of the bay and showed higher chl-a (3.2±0.9 to 8.3±2.2 μg/L) and TSM (2.0±0.7 to 11.0±2.5 g/m 3 ) than Zones 4-6 (chl-a: 0.9±0.4 to 1.9±0.8 μg/L, TSM: 0.7±0.2 to 1.3±0.5 g/m 3 ). The highest chl-a and TSM are in Zone 1, located at the mouth of Pampanga River, which is the largest watershed in Manila Bay. It is also an area with extensive mariculture activity. Within Zone 3 is the mouth of Pasig River, a localized area with anomalously high chl-a and TSM due to the high amount of organic load from urbanization. Pearson correlation of the environmental variables in each zone shows that precipitation (0.15-0.68) and runoff (0.38-0.79) are more correlated with water quality than sea surface temperature and wind speed. Paired t-test of chl-a and TSM also show a significant difference between the wet (June to November) and dry (December to May) seasons. Results suggest that water quality is largely influenced by precipitation and runoff. This means that effective river basin management could be the key to improving water quality in Manila Bay.