Spatial-Temporal Variability of Chlorophyll-a Concentration in Cenderawasih Bay and Surrounding Waters

Alianto and Hamuna / JAGI Vol 4 No 2/2020 343 Spatial-Temporal Variability of Chlorophyll-a Concentration in Cenderawasih Bay and Surrounding Waters Alianto , Baigo Hamuna * 1 Department of Fisheries, Faculty of Fisheries and Marine Science, University of Papua, Gunung Salju Street, Manokwari Regency, Indonesia 2 Department of Marine Science and Fisheries, Faculty of Mathematics and Natural Science, Cenderawasih University, Kamp Wolker Street, Jayapura City, Indonesia * Corresponding author e-mail: bhamuna@yahoo.com.sg


Introduction
In marine waters, chlorophyll-a is identical to the presence of phytoplankton which is the primary food source for marine organisms. All phytoplankton in waters has chlorophyll pigments, especially chlorophyll-a (Castro and Huber, 2007). Therefore, chlorophyll-a is an indicator of the abundance of phytoplankton in waters that is very important in the process of photosynthesis (Zhang and Han, 2015). Ecologically, the chlorophyll of phytoplankton plays an important role as a measure of phytoplankton standing-stock and photosynthetic potential of water (Darecki et al., 2005). Measurement of chlorophyll-a concentration is one of the fertility parameters of water expressed in primary productivity (Anderson, 2005;Boyer et al., 2009). Therefore, the abundance of phytoplankton as a major producer is very important in the fisheries sector, especially capture fisheries.
At present, monitoring of chlorophyll-a concentrations in marine waters can be done by utilizing remote sensing technology. Chlorophyll-a remote sensing is one way to determine the state of the sea and the processes that occur therein based on the value of the concentration of water-leaving radiance which is the result of interactions between Open Access sunlight and waters received by satellites. The characteristics of chlorophyll-a that can absorb and reflect certain light spectrums are used to detect the distribution of phytoplankton chlorophyll at sea level from satellites. The spectrum of sunlight reaching the sea surface consists of all visible spectra with wavelengths between 400-700 nm (Van Der Woerd and Pasterkamp, 2008). The maximum chlorophyll-a absorbs in the range of 400-500 nm wavelengths, and the maximum reflects in the range of wavelengths 500-600 nm (O'Reilly et al., 1998).
One of the satellite images that can be used to monitor chlorophyll-a concentrations in ocean waters is the Aqua-MODIS (Moderate Resolution Imaging Spectroradiometer) satellite image. The use of Aqua-MODIS satellite data is very good for monitoring the dynamics of chlorophyll-a concentrations in waters because it has a high temporal resolution so that it can be observed periodically and continuously and its distribution patterns can be analyzed (Hamuna et al., 2015). Various studies that have utilized Aqua-MODIS satellite imagery for monitoring chlorophyll-a concentrations, such as Tarigan and Wiadnyana (2013) who observed chlorophyll-a concentrations in Jakarta Bay. Ha et al. (2014) and Abbas et al. (2019) estimated the concentration of chlorophyll-a in shallow coastal waters. Ali et al. (2016) modeling Aqua MODIS data to estimate chlorophyll-a concentrations in the turbid waters of Lake Erie. Wisha and Khoirunnisa (2018)  This study aims to analyze the variability of chlorophyll-a concentrations in Cenderawasih Bay and its surrounding waters, both spatially and temporally, using Aqua-MODIS satellite data. The results of this study can provide information about the distribution pattern of chlorophyll-a concentration in the waters of Cenderawasih Bay and can be used as a basis for further research in the field of marine and fisheries, considering the waters of the Cenderawasih Bay are very potential areas for fishing ground.

Study Area Description
Cenderawasih Bay is one of the gulf waters located on the border between Papua Province and West Papua Province, Indonesia. Cenderawasih Bay was established as the Cenderawasih Bay National Park, which was definitively determined by Minister of Forestry Decree No. 8009/Kpts-II/2002 on August 29, 2002, with an area of 1,453,500 Ha (Suraji et al., 2015). Administratively, Cenderawasih Bay National Park is located in Teluk Wondama Regency (West Papua Province) and Nabire Regency (Papua Province). Cenderawasih Bay is the largest bay in Indonesia which has a high potential for water resources (BBTNTC, 2009). As a marine national park, most of the biological potential that exists is a diversity of coastal and marine resources.
The waters of the Cenderawasih Bay and the surrounding waters used as the study area are presented in Figure 1. Generally, there are five districts covered by the study area, namely Nabire Regency, Yapen Islands Regency, and Waropen Regency in Papua Province and Teluk Wondama Regency and South Manokwari Regency in West Papua Province.

Data Acquisition
Data in this study used chlorophyll-a images from the Aqua-MODIS (Moderate Resolution Imaging Spectroradiometer) Level 3 monthly composite from January to December 2019. The chlorophyll-a image downloaded is spatial resolution 4km x 4km in the Hierarchical Data Format (HDF) from the NASA website (https://oceancolor.gsfc.nasa.gov/l3/).

Data Processing and Analysis
Image data processing is a way to manipulate image data into the desired output. The chlorophyll-a concentration of Aqua-MODIS Level 3 monthly composites from January to December 2019 obtained is a chlorophyll-a image that has been processed according to the following OC3M algorithm (O'Reilly et al., 2000): Rrs (443) Rrs (550) > Rrs (490) Rrs (550)  ) where, Ca is the concentration of chlorophyll-a (mg.m -3 ), R is the reflectance ratio, and Rrs is the remote sensing reflectance.
The extracted chlorophyll-a image can be displayed in the SeaDAS version 7.2 (SeaWiFS Data Analysis System) software. The stages of image processing carried out begins with cropping according to the research location. The chlorophyll-a image that has been cut according to the location of the study is then carried out the Export Mask Pixels process to obtain the concentration of chlorophyll-a. The chlorophyll-a concentration of each pixel based on the latitude and longitude of the process is obtained in the ASCII (American Standard Code for Information  Interchange) format. Furthermore, monthly chlorophyll-a concentration data is tabulated using Microsoft Excel to determine the maximum, minimum, and average monthly chlorophyll-a concentration and then displayed in a time-series graph.
Although the Aqua-MODIS satellite image data used is Level 3, there are still blank pixels (no value) due to cloud cover so they do not have chlorophyll-a concentration values. Therefore, interpolation is needed to get the value of chlorophyll-a concentration in these pixels (Mursyidin et al., 2015). In this study, the interpolation process was carried out using Surfer 11 software. The kriging method was used to interpolate the concentration of chlorophyll-a and to produce a spatial distribution map of chlorophyll-a concentration. Maps of the average distribution of monthly chlorophyll-a concentrations obtained are stored in JPEG format so that they are easily observed visually.
Analysis of chlorophyll-a concentration used in this study is a descriptive analysis that includes temporal and spatial analysis. The temporal analysis was carried out based on the time-series of chlorophyll-a concentrations to determine the temporal fluctuation of chlorophyll-a concentrations in 2019. Spatial analysis was carried out to determine the spatial distribution of chlorophyll-a concentrations in the study area. Spatial analysis was carried out based on the color degradation of the chlorophyll-a concentration on the map.

Temporal Variability of Chlorophyll-a Concentration
Chlorophyll-a concentrations in Cenderawasih Bay and surrounding waters are shown in Figure 2. There are variations in chlorophyll concentration in the study area, both monthly and seasonal chlorophyll-a concentrations. Monthly average chlorophyll-a concentration ranges from 0.1988 -0.3415 mg.m -3 . The highest average chlorophyll-a concentration was in March and the lowest in January. The maximum chlorophyll-a concentration in April and the minimum in August, which is around 9.1089 mg.m -3 and 0.0975 mg.m -3 , respectively. Generally, the concentration of chlorophyll-a in Cenderawasih Bay and its surrounding waters is dominated by a low concentration, which ranges 0.1482 -0.3158 mg.m -3 . The average chlorophyll-a concentration in the Transition I (West-East) and East Seasons ranged from 0.2391 -0.3415 mg.m -3 and decreased in the Transition II Season (East-West) to the West Season with a range of 0.1988 -0.2822 mg.m -3 .
Chlorophyll-a concentrations in the waters of the Madura Strait using Aqua-MODIS data ranged from 2.001 -3.00 mg.m -3 (Trijayanto and Sukojo, 2015). Based on the season, the concentration of chlorophyll-a obtained in this study was relatively the same as the concentration of chlorophyll-a in several marine waters in Indonesia.
Chlorophyll-a concentrations in the Seram Sea and the Banda Sea are lower in the West Season and Transition I Season, ranging from 0.15 -0.30 mg.m -3 , while the concentration of chlorophyll-a in the East and East-West Transition Season ranges between 0.30 -0.60 mg.m -3 (Manery, 2014). The research results of Putra et al. (2017) showed a higher concentration of chlorophyll-a in August (East Season) compared to the chlorophyll-a concentration in Transition Season II (September -November). Likewise, the results research of Mahabror and Zaky (2016) that the concentration of chlorophyll-a in the southern waters of Aru decreased dramatically in the West Season compared to the previous season.
Based on the trophic status criteria of marine waters referring to Hakanson and Bryann (2008), the Cenderawasih Bay and surrounding waters are dominantly classified as oligotrophic and mesotrophic categories (chlorophyll-a concentration < 6 mg.m -3 ). Besides, the concentration of chlorophyll-a can also be used as an indicator of the fishing ground area. According to Arifin (2009) that the concentration of chlorophyll-a range between 0.26 -0.29 mg.m -3 is a good water condition for fishing activities. Specifically, the results of research by Manery (2014) showed that the concentration of chlorophyll-a in areas of high potential, medium potential, low potential, and very low potential for skipjack tuna ranged from 0.40-0.60 mg.m -3 , 0.20 -0.40 mg.m -3 , 0.10 -0.20 mg.m -3 , and 0.00 -0.10 mg.m -3 , respectively. Based on the dominant chlorophyll-a concentration, the Cenderawasih Bay and its surroundings are in the low to the high potential category for skipjack tuna fishing.

Spatial Distribution of Chlorophyll-a Concentration
Spatially, the distribution of chlorophyll-a concentrations in Cenderawasih Bay and surrounding waters differs between coastal waters and offshore waters, where coastal waters tend to have higher chlorophyll-a concentrations than offshore waters (Figure 3). The distribution of chlorophyll-a concentrations is almost evenly distributed in offshore waters. While the concentration of chlorophyll-a in coastal waters tends to vary each month, especially in coastal waters around the border between Nabire Regency and Waropen Regency because many large rivers flow around the coastal waters. High concentrations of chlorophyll-a in these waters occur in April, May, July, and August. The maximum and minimum chlorophyll-a concentrations from the interpolation results are presented in Table 1. There are differences in the chlorophyll-a concentration between the data obtained from the Aqua-MODIS satellite imagery and the interpolation results. However, the interpolation results show that the root mean square error (RMSE) is relatively small, ranging from 0.47101 to 1.80772. This shows that the results of the interpolation of chlorophyll-a concentration are quite accurate.
Various research results show that the distribution of chlorophyll-a is relatively the same as the results of this study (Tarigan and Wiadnyana, 2013;Syahdan et al., 2014;Samad et al., 2016;Hamuna and Dimara, 2017). The high distribution of chlorophyll-a concentrations in coastal waters is caused by the presence of a large supply of nutrients through run-off from the mainland, while the low concentration of chlorophyll-a in offshore waters due to the absence of direct nutrient supply from the mainland (Hartuti et al., 2004). The abundance of phytoplankton chlorophyll in a network is highly dependent on the availability of ammonia, nitrates, and silicates (Alianto et al., 2018). The ecological impact of the entry of organic waste from land to estuary causes the waters to become more fertile and an increase in nutrients which is a very important substance in the life of organisms such as phytoplankton (Wang et al., 2015;Wisha and Maslukah, 2017).
The distribution of chlorophyll-a concentrations is also highly correlated with oceanographic conditions (Mann and Lazier, 2006). The abundance and distribution of phytoplankton in the sea as an indicator of chlorophyll-a concentration are not only influenced by nutrients but also the physical conditions of waters such as light penetration, temperature, salinity, and surface currents (Lo et al., 2004;Djumanto et al., 2009), so that abundance is very volatile according to season and location of water (Arinardi et al., 1997). High and low sea surface temperature will affect the intensity of upwelling and chlorophyll-a concentration, especially in offshore waters (Putra et al., 2017).

Conclusion
In this study, information has been produced about the variability of chlorophyll-a concentrations in Cenderawasih Bay and surrounding waters, both spatial and temporal variability. The average value of chlorophyll-a concentration increases from March to June and then decreases in July or August. The average value of chlorophyll-a concentration ranged from 0.1988 -0.3415 mg.m -3 . The highest average chlorophyll-a concentration was in March and the lowest in January. The concentration of chlorophyll-a in Cenderawasih Bay and its surrounding waters is dominated by a low concentration, which ranges 0.1482 -0.3158 mg.m -3 . Generally, the variability of chlorophyll-a concentrations in the study area is influenced by seasons. The average chlorophyll-a concentration is high in the Transition I (West-east) and East seasons and will decrease in Transition II (East-West) until the West season. Spatially, chlorophyll-a concentrations in coastal areas are higher than in offshore waters. High chlorophyll-a concentrations are found around the border between Nabire Regency and Waropen Regency, especially in April, May, July, and December. The amount of run-off flow that supplies nutrients from the mainland greatly affects the high concentration of chlorophyll-a in the coastal area.