Results

Rainfall data

Precipitation data obtained from PAGASA stations including Pampanga, Bulacan, and Metro Manila stations revealed significant variations between the two seasons. In terms of the wet season, the highest levels of rainfall appeared to be in July, which are in accordance with studies by (Siringan et al., 2008). During the wet season, it generally rains almost every day during July, August, and September, thus insinuating a peak in monthly precipitation levels during July-August (Azanza and Miranda, 2001). July and August had significantly higher rainfall levels than that of October and the difference between the maximum rainfall observed in July 2011 and the minimum rainfall seen in October 2007 is 181.1 millimeters (see Table 1). The mean rainfall for the 4 months during the dry season in this study is 352.93 millimeters. The standard deviation for this set of data is 72.34 which shows that all of the months in the wet season with the exception of October 2007 fall within one standard deviation of the mean. The highest precipitation level for the dry season occurred in January 2001, while the lowest level occurred in March 1996 (see Table 2). The difference between the maximum and minimum precipitation levels is 11.9 millimeters. The mean rainfall for the 4 months in the wet season is 15 millimeters. The standard deviation for this set of data is 4.78 which shows that February 1989, March 1996, and January 1996 all fall within one standard deviation of the mean.

Table 1:Wet season monthly precipitation levels Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA).

Table 2: Dry season monthly precipitation levels from PAGASA.

Algal blooms concentrations using Landsat 4-present

The satellite images acquired from GLOVIS Landsat 4-present collection reveal a stark contrast regarding the presence, concentration, and extent of algae blooms between the wet and the dry seasons both visually as well as numerically from cursor values on ENVI5.0 software. February 1989 has the lowest band 2 value of 33 (see Table 3). Although the image from February 1989 has a low band 2 reflectance, the lighter magenta clouds appear to extend throughout the bay, with the brightest concentrations of algae in the upper nearshore region of the bay where the Pampanga River drains (see Figure 2a). January 1996 has a similar band 2 value of 36, however, the bloom appears to be much more confined to the coastline on the western side, extending all the way to the mouth of the bay (see Figure 2b). March 1996 has the lowest cursor value of 30, and in comparison to the January image from the same year, the magenta clouds in the water are noticeably duller, indicating lower concentrations of algae in the water (Figure 2c). The plume is still relatively confined alongshore, however, it does not extend as far down the western coast as is observed in January. In both images, although the concentration of the algae is most significant in the north-northwest region of the bay where the plume appears at noticeable distances offshore, the reflectance levels of band 2 are still very low in the dry season. In the final image from the dry season, January 2001, the highest cursor value is observed, with a band 2 value of 48, considerably higher than those of the other months. Therefore, it is understandable that out of all 4 images from the dry season, the image from January 2001 has brighter magenta clouds (in contrast to the darker, unaffected areas in the bay) than the other images (see Figure 2d). Again, the highest concentration of algae, evidenced by the magenta clouds, is found in the upper northeast region where rivers clearly drain into the bay. While the clouds do not extend as far down the coast as is seen in January and March 1996, they appear to extend more uniformly throughout the bay, similar to the image from February 1989. In all four images, when observing the plumes originating from the east coast of Manila Bay, they appear to extend further offshore into the bay, adjacent to the northern coastline and originating from rivers (specifically Pasig River) (Siringan et al., 2008); however, for the purpose of this study in order to maintain consistency, cursor values were only taken from the specific coordinates in the upper northwest region of the bay near Pampanga River.

 Table 3: Dry season cursor values based on ENVI band 2 reflectance value.

 Figure 2a: February 1989 Landsat 4-present satellite image (Bands 2 and 5).

Figure 2b: January 1996 Landsat 4-present satellite image (Bands 2 and 5).

  Figure 2c: March 1996 Landsat 4-present satellite image (Bands 2 and 5).

Figure 2d: January 2001 Landsat 4-present satellite image (Bands 2 and 5).

As for the wet season, the images have significantly brighter magenta clouds, and thus much higher band 2 reflectance values than those of the dry season (see Figure 3a-d). The first image from the wet season, July 2001, has the highest cursor value at the designated coordinates from which it was obtained, with the band 2 value totaling 106 (see Table 4). While there is certainly a general cloudy appearance throughout the bay, especially when compared to images from the dry season, the most extreme concentrations of the algal bloom are found almost entirely alongshore in the upper region of the bay (see Figure 3a). While the plume does extend some distance offshore, the nearshore concentrations are much higher. The image also shows that the highest plume concentrations in the nearshore upper region appear at the mouths of large rivers, specifically the Pampanga River, and other rivers/streams that drain directly into the bay. The image from August 2003 shows a fairly different distribution of algae, although it has a fairly similar cursor value to that of the prior month with high reflectance value of band 2  at 102 (see Figure 3b). The bright magenta clouds in the image, although not quite as bright as those in July 2001, extend much further offshore. While the bay similarly has an overall cloudy appearance in this image, the algal bloom here appears in very high concentrations both nearshore as well as at distances far offshore in contrast to the prior image where the highest algal concentration is primarily confined alongshore and nearshore in the east-west direction. Moreover, the algal bloom extends to the mouth of the bay in the August 2003 image in noticeably higher concentrations than that of July 2001. The third image for the wet season from October 2007 shows an algal distribution that differs from the two aforementioned images (see Figure 3c). This image contains the lowest band 2 value of 97. As evidenced in the prior images, the magenta clouds are similarly observed throughout the entire bay in this image. While the previous two images both reveal fairly consistent and high algae concentrations along the majority of the northern coastline (although they differ in their offshore extent), in the image from October 2007, the brightest clouds and thus highest algae levels are found in circular distributions directly at the mouths of three rivers that drain into the bay. The plume does not extend east to west along the coast as is observed in the previous images, nor does it extend nearly as far offshore as in the image from August 2003. The final image from July 2011 has a similar band 2 value to that of August 2003, at 101. The distribution pattern of the algal bloom in this image can most similarly be compared with that of August 2003 (see Figure 3d). The plume in the July 2011 image, however, does not show the algae in quite as high concentrations as in August 2003, which is deduced from two different observations. First, the image, overall, has lower band 2 reflectance values, and second, the highest concentration of algae is found in a more restricted region, specifically at the mouth of the Pampanga River. While the plume does extend alongshore in the upper region of the bay, the magenta clouds decrease in brightness as one goes from west to east along the northern coastline and are much more scattered than those observed in the August 2003 image. The plumes do, however, extend considerable distances offshore in concentrations that appear relatively similar to or only slightly less than those nearshore. 

 Table 4: Wet season cursor values based on ENVI band 2 reflectance value.

Figure 3a: July 2001 Landsat 4-present satellite image (Bands 2 and 5). 

Figure 3b: August 2003 Landsat 4-present satellite image (Bands 2 and 5).

Figure 3c: October 2007 Landsat 4-present satellite image (Bands 2 and 5).

Figure 3d: July 2011 Landsat 4-present satellite image (Bands 2 and 5).

After obtaining the cursor value for each month of the dry and the wet seasons, which indicates levels of greenness, or algae, in the water based on reflectance, as well as precipitation data for each month, the relationship between the two variables was assessed in a scatter plot. As previously stated, although correlation cannot be deduced due to possible confounding variables and factors, there does appear to be a strong relationship between the two variables with an r value of .84 for the dry season and a r value of .79 for the wet season (see Figure 4a,b). Essentially, as precipitation levels increase, band 2 reflectance values simultaneously increase. Thus, there appears to be a strong relationship between the two variables.

Figure 4a: Scatter plot showing relationship between monthly precipitation levels and cursor values in the dry season.

 Figure 4b: Scatter plot showing relationship between monthly precipitation levels and cursor values in the wet season.










Sea Surface Temperature and Ocean Currents

January 1996 was the only month that provided clear enough results to observe surface current patterns. From this image, it is apparent that there are scattered regions of warmer and cooler sea surface temperatures (see Figure 5). The brighter clouds throughout the water column represent warmer temperatures than the darker regions. It also appears that the currents tend to be heading in a northeast direction. Brighter clouds, and thus warmer temperatures seem to be most consistent nearshore; however, there are warm water patches dispersed throughout.

Figure 5: January 1996 Landsat 4-present satellite image (band 6 - thermal imagery).