Philippine Journal of Science 150 (5): 1231-1243, October 2021 ISSN 0031 - 7683 Date Received: 14 Dec 2020 Limnology of Lake Wood: An Ancestral Lake of the Subanen Tribe Marjohn Y. Baludo1*, Rey Donne S. Papa3, and Francis S. Magbanua2 1Institute of Environmental Science and Meteorology; 2Institute of Biology University of the Philippines Diliman, Quezon City 1101 Philippines 3Department of Biological Sciences, College of Science; and Research Center for the Natural and Applied Sciences and the Graduate School University of Santo Tomas, Manila 1015 Philippines Relatively limited information exists on Lake Wood’s limnology and ecology, an ancestral lake of the Subanens. Here, we provide a limnological characterization of the lake – exploring its physical, chemical, and biological features. The data on lake morphometry were gathered using an echosounder; lower depth water samples were collected using a Niskin water sampler for physicochemical analyses while surface water samples were collected at five sampling sites for phosphate (PO4) analysis; and zooplankton samples were collected at several depths through vertical towing in March–April 2019. Lake Wood, located at 320 m above sea level, has an extensive surface area covering 7.38 km2 with a maximum depth of 85 m. Lake water comes from rainfall, small rivers, and groundwater and empties into the Dumanquilas Bay via its only outlet – the Biswangan River. Land use of the lake is predominantly cultivated land. Moreover, based on the trophic state index (TSI), the lake’s current trophic status is eutrophic. During the study periods, thermocline and oxycline formed at 15 m below the surface of the lake. We recorded three cladocerans (Ceriodaphnia cornuta, Diaphanosoma cf. sarsi, and Moina micrura) and one copepod species (Thermocyclops crassus) in the lake, all of which are common limnetic species found in the Philippines. Based on our findings, we recommend more efforts to manage and rehabilitate the lake to control and manage eutrophication’s negative impacts. Finally, our study contributes to understanding lakes in Mindanao that are less represented in Philippine freshwater research. Keywords: eutrophication, Mindanao Island, trophic state index, tropical lake, zooplankton INTRODUCTION interest worldwide (Gunkel 2000) and advances more investigation in tropical lakes (Ramírez et al. 2020; The progress of tropical limnology lags behind temperate Santoso and Toruan 2020). The importance of inland limnology (Talling and Lemoalle 1998; Lewis 2000), as water fisheries in developing countries, particularly in limnology has traditionally been focused on temperate Africa, has increased the number of research programs regions (Fadum and Hall 2020; Santoso and Toruan 2020). in the tropics (Fadum and Hall 2020). The significant Despite the economic, social, and cultural importance differences in the physical, chemical, and biological of lakes, the limited knowledge of tropical limnology processes in tropical lakes compared to temperate lakes propels this field to develop as the primary research (Lewis 1987; Gunkel and Casallas 2002; Crowe et al. *Corresponding Author: [email protected] 2008) call for a more comprehensive study of tropical 1231 Philippine Journal of Science Baludo et al.: Limnology of Lake Wood: Vol. 150 No. 5, October 2021 An Ancestral Lake of the Subanen Tribe lake ecosystems (Gunkel and Casallas 2002). The lack use. Thus, it is imperative to characterize Lake Wood for of large seasonal variation, higher annual irradiance, its ecological and economic importance and conserve and and the smaller Coriolis effect in the tropics combine to protect the endemic species thriving in the lake. create weakly stratified lakes with relatively warm and uniform temperatures plus low physical stability (Lewis 1987). Tropical lakes are more efficient in producing phytoplankton biomass and generally similar to temperate MATERIALS AND METHODS lakes in terms of planktonic and benthic biodiversity (Lewis 2000; Osborne 2012). Furthermore, the increasing Study Site demands on lake water resources in a growing population Lake Wood (7.84° N, 123.17° E) ranks as the country’s living in the vicinity of tropical lakes has resulted in 11th largest lake in terms of area (Davies 1992). Located pressures on tropical lakes (Juma et al. 2014; Zieba et al. in the western part of Mindanao Island, Lake Wood has a 2017; Bogardi et al. 2020). maximum dimension of 4.37 x 2.10 km (Figure 1) and a Lake ecosystems are one of the most frequently studied maximum depth of 85 m. The lake is about 48 km from freshwater ecosystems in the Philippines; however, these Pagadian City and is situated north of Dumanquilas Bay studies focus mainly on Luzon Island (Magbanua et al. (Pratt 1916; Woltereck 1941). The lake receives an average 2017). Limnological studies remain inconsistent and daily rainfall of 8.7 mm within a month, with peak rainfall fragmentary (Papa and Mamaril 2011), especially in the in July (12.42 mm) and a minimum in January (4.24 mm) Visayas and Mindanao Islands (Magbanua et al. 2017) (Figure 2; Climate Hazards Center 2021). It is situated at 6 3 and in small lakes in the country (Brillo 2015). Most of 320 m above sea level, with a volume of 356 x 10 m . the research on Philippine lakes focused on aquaculture A mountainous terrain surrounds the lake, particularly in and fisheries, pollution, and eutrophication. Only a few the lake’s northwestern portion where Mt. Imbing (900 have been dedicated to freshwater lake ecology and m) sits. Both the northeast and southwest shores comprise biodiversity studies (Zafaralla 2001; Aquino et al. 2008; rugged landscapes and are flanked to the western and Papa and Mamaril 2011; Bannister et al. 2019; Mendoza eastern lakeshore areas with relatively gentle and flat et al. 2019; Navarrete et al. 2019). There is still a paucity plains. Southeast of the lake is of low elevation where the of information available on the status of the Philippine Biswangan River feeds water into the Dumanquilas Bay. lakes, except for Laguna de Bay, the country’s largest lake (Guerrero 2001). Lake Morphology A reconnaissance survey was conducted from 31 Oct–02 Lake Wood has a surface area of 7.38 km2, with a Nov 2018 to generate the bathymetric map of Lake Wood maximum depth of 85 m. The lake, Danao in the local using a Lowrance™ echosounder, model LCX-27c, a dual- term, is part of the ancestral domain of the Subanens. It frequency (50/200 kHz) recordable echo sounder equipped harbors an endemic migratory fish, Rasbora philippina, with a global positioning system (GPS). The echo-sounding which is locally known as porang. In the literature, the equipment was used to record lake depth measurements. The lake was first reported to the American administration GPS equipment simultaneously determined and recorded by Captain Smith, 14th Cavalry, United States Army, each depth’s location. Due to some limitations, only a total who explored it in 1904 during their expedition from of about 428 transverse depth soundings at known locations Misamis to Dumanquilas Bay. The lake was known as were collected to cover the entire lake. Measured depths were Lake Leonard Wood, with an outline similar to figure then interpolated to create contour plots using the “kriging” eight (Pratt 1916). As part of the Wallacea expedition, gridding method (Oliver and Webster 1990) in Surfer 16 Woltereck noted that the lake is similar to Lake Danao in (Golden Software, LLC., Colorado, USA). Leyte (Woltereck 1941). The maximum and mean depths were established based To date, only one published work has been done on Lake on the generated bathymetric map. The lake’s volume was Wood (Superales et al. 2013). While Superales et al. calculated by multiplying its mean depth with its surface (2013) determined the water quality, trophic status, and area (Lakewatch Florida 2001). This computation yielded fish fauna of Lake Wood, its basic limnology remains an estimated volume of the lake that determines the many unknown. Thus, this paper presents additional information lake ecosystem processes. on the lake morphometry, land use/land cover of the watershed, and zooplankton composition. Update on the trophic status classification was also done using multi- Land Use and Catchment parameter indices different from Superales et al. (2013). The data used to map the watershed and its land use/land Lake Wood serves as a habitat for fish and wildlife, a cover were derived from Lehner and Grill (2013) and the source of food and water for domestic and agricultural Philippine Geographic Information System (PhilGIS) 1232 Philippine Journal of Science Baludo et al.: Limnology of Lake Wood: Vol. 150 No. 5, October 2021 An Ancestral Lake of the Subanen Tribe Figure 1. Map showing the location of the study area and sampling site. The left panel shows the entire Philippines, and the right bottom panel presents the boundary of Zamboanga del Sur with the relative location of Lake Wood. The right topmost panel displays the study site (X) with the sampling points where water samples were collected for PO4 analysis. Map generated using QGIS Software Version 2.18. Sampling and Analyses Zooplankton sampling and measurement of environmental variables, except for PO4, were carried out on 7–8, 11–12, 14–15, 20–21 Mar 2019 and 5–6, 9–10, 15–16, 19–20 Apr 2019 at five sampling depths (1, 15, 30, 50, and 70 m) to determine whether water stratification occurs. Water samples for dissolved oxygen (DO), conductivity, and water temperature were collected using a Niskin water sampler and were measured three times in situ using YSI EcoSense® DO200A and YSI EcoSense® EC300A meters during noon and midnight. Physicochemical data were tested for differences in depth and sampling period using two-way ANOVA. Figure 2. Monthly rainfall data in Lake Wood from 1981– 2018. Data from Climate Hazards Group InfraRed A 1-L water sample was also collected at the same depths Precipitation with Station (CHIRPS) satellite data to determine the chlorophyll-a concentration (Chl-a).