THE COMPOSITION, DIVERSITY AND COMMUNITY DYNAMICS OF LIMNETIC ZOOPLANKTON IN A TROPICAL CALDERA LAKE (LAKE TAAL, PHILIPPINES)

THE RAFFLES BULLETIN OF ZOOLOGY 2011 59(1): 1 7 Date of Publication: 28 Feb.2011 National University of Singapore THE COMPOSITION, DIVERSITY AND COMMUNITY DYNAMICS OF LIMNETIC ZOOPLANKTON IN A TROPICAL CALDERA LAKE (LAKE TAAL, PHILIPPINES) Rey Donne S. Papa Department of Biological Sciences, Graduate School and Research Center for the Natural Sciences University of Santo Tomas, Manila, Philippines Email: rspapa@mnl.ust.edu.ph; reypaps@yahoo,com (Corresponding Author) Macrina T. Zafaralla Institute of Biological Sciences, University of the Philippines Los Baños, Laguna, Philippines ABSTRACT. Limnetic zooplankton serve as a major food source for pelagic fish. This paper updates the species composition, diversity and community dynamics of limnetic zooplankton in the two basins of Lake Taal, an active caldera lake ecosystem. Samples obtained from January to December 2008 included 15 rotifer, six cladoceran and three copepod species. Eight species are new records for the lake. The zooplankton community is dominated by copepods, which contributed 64% to total abundance and 84% to total biomass. Diversity values for rotifers and cladocerans were low and similar in both lake basins. The composition, diversity and homogenous spatial distribution of zooplankton in Lake Taal is typical for tropical lakes with a high trophic state which may be a response to the prevailing conditions influenced by its location, geological origin and meteorological factors. KEY WORDS. Sardinella tawilis, Pseudodiaptomus brehmi, Tropical Limnology, Volcanic lakes, Eutrophication. INTRODUCTION Lake Taal (formerly known as Lake Bombon) (13 55'- 14 05'N, 120 55'-121 105'E; Altitude: 2.5 masl) is the third largest Philippine lake which covers an area of 268 km 2. It is one of the deepest lakes in the country at 198 m maximum depth and occupies a collapsed caldera formed by volcanic explosions. The lake is fed by water from 37 partly seasonal tributaries while the 8.2 km Pansipit River connects it to Balayan Bay (Mutia, 2001; Papa et al., 2008; Perez et al., 2008; Ramos, 2002). There are two lake basins separated by the largest island in the center known as Taal Volcano Island which is regarded as the worlds lowest active volcano (Zlotnicki et al., 2009). Volcanism has led to the evolution of landlocked endemic species of marine origin such as the freshwater sardine (Sardinella tawilis) and the Taal sea snake (Hydrophis semperi) (Hargrove, 1991). The Philippine government has declared it as a conservation and biodiversity priority (Ong et al., 2002) and a research hotspot for freshwater fauna. Fish surveys by Herre (1927) and Villadolid (1937) have listed 101 species from 32 families making it one of the most diverse freshwater fish populations in the country. Local fisher folk depended on capture fisheries as their primary source of food and livelihood until fish farming of Oreochromis spp. (tilapia) and Chanos chanos (milkfish) in net cages started in 1975 which became a lucrative business thereafter (Aypa et al., 2008). The under-regulated proliferation of aquaculture has unfortunately led to a decrease in water quality (Mamaril Sr., 2001a). Three major zooplankton groups dominate freshwater ecosystems the Copepoda, Cladocera and Rotifera. They vary in contribution to total abundance and biomass depending on trophic status and predation by zooplanktivorous fish (Fernando, 2002). Zooplankton composition, distribution and movement are also influenced by physical and chemical characteristics of the ecosystem (Pinto-Coelho et al., 2005). In lakes, distinct differences between the composition and community dynamics of littoral and limnetic zooplankton may occur which necessitate separate evaluations (Matsumara- Tundisi & Tundisi, 2005). Studies on Philippine freshwater zooplankton have yet to reach the sophistication and level of applied limnology of North American and European institutions (Mamaril Sr., 1986) and appear to be subsumed under fisheries (Mamaril Sr., 2001b). The most comprehensive taxonomic papers by 1

Papa and Zafaralla: Zooplankton of Lake Taal Table 1. Zooplankton species lists from previous limnological surveys in Lake Taal. Brehm, 1939 Mamaril, 2001 Schiemer et al. (eds.), 2008 Ceriodaphnia rigaudi Brachionus angularis Bosmina fatalis Diaphanosoma sarsi Brachionus calycifl orus Ceriodaphnia cornuta Latinopsis australis Brachionus diversicornis Diaphanosoma spp Moina dubia Brachionus falcatus Moina micrura Simocephalus vetula Brachionus forfi cula Microcyclops varicans Brachionus havanaensis Thermocyclops crassus Kiefer, 1939 Brachionus plicatilis Tropodiaptomus vicinus Thermocyclops hyalinus Brachionus quadridentatus Brachionus urceolaris Woltereck et al., 1941 Hexarthra fennica Brachionus angularis Keratella procurva Brachionus calycifl orus Keratella tropica Brachionus calycifl orus dorcas / dorcas spinosa Keratella valga valga Brachionus plicatilis Lecane luna Brachionus forficula Lecane patella Diurella dixon-nuttalli Lecane ungulata Keratella valga valga Testudinella patina Pedalia fennica Trichocerca dixon-nuttali Diaphanosoma sarsi Tripleuchlanis plicata Ceriodaphnia rigaudi Biapertura pseudoverrucosa Moina dubia Bosmina fatalis Simocephalus vetula Bosmina longirostris Thermocyclops hyalinus Ceriodaphnia cornuta Chydorus barroisi Chydorus ventricosus Hauer, 1941 Diaphanosoma sarsi Pedalia intermedia Dunhevedia crassa Pedalia fennica Ilyocryptus spinifer Latinopsis australis Ueno, 1966 Moinodaphnia macleayi Brachionus calycifl orus Simocephalus vetulus Pompholyx complanata Ectocyclops pharelatus Ceriodaphnia cornuta Eucyclops serrulatus Diaphanosoma sarsi Mesocyclops leuckarti Latinopsis australis Microcyclops varicans Moina dubia parva Thermocyclops crassus Mesocyclops cf. thermocyclopoides Tropodiaptomus vicinus Mamaril Sr. and Fernando (1978) and Mamaril Sr. (1986) listed 125 species belonging to Rotifera (61 spp.), Cladocera (49 spp.) and Copepoda (9 spp.). Studies on Lake Taal zooplankton were pioneered by the Wallacea Expedition of the 1930s (Table 1) (Brehm, 1939; Hauer, 1941; Kiefer, 1939b; Woltereck et al., 1941). Follow up studies by Ueno (1966) and Mamaril Sr. (2001b) have led to the enumeration of 37 species in Lake Taal including both littoral and limnetic forms. We want to update the species composition of limnetic zooplankton in Lake Taal and provide additional insights into its community structure. Specifically, we want to compare the present zooplankton composition with previous studies considering recent developments in zooplankton taxonomy and systematics. We would also like update aspects of zooplankton ecology that have not been presented in previous studies by comparing composition, biomass and diversity in the two basins of Lake Taal based on monthly surveys in the year 2008. MATERIALS AND METHODS Sampling. Zooplankton were collected monthly from January to December 2008 in six sampling sites representing the north and south basins of the lake (Fig. 1). Collection was done by making two replicate vertical tows from 40 m depth using an 80µm mesh size conical plankton net (diameter = 30 cm) (Amarasinghe et al., 2008; Vijverberg et al., 2008). Data Analysis. Identification was done to species level based on descriptions, taxonomic keys and illustrations in Mamaril Sr. & Fernando (1978), Mamaril Sr. (1986), Korovchinsky (1992), Dussart & Defaye (2001), Fernando (2002) and Petersen (2009). Sorting, identification and counting were done under dissecting and compound microscopes. Density was determined by counting three 1 ml Sedgewick Rafter sub samples per replicate. Biomass was computed based on the methods of Amarasinghe et al. (2008) and Wetzel & Likens (1991). Biomass was not determined for rare 2

species. Species diversity for rotifers and cladocerans was computed using the Shannon-Wiener Diversity Index (H ) and compared across sampling months using One-way ANOVA. Copepod diversity was not determined due to the low number of species. Biomass values in the six sampling sites which represent the north and south basins of the lake were log (x+ s / 2) (where s is the lowest biomass value) transformed and compared across sampling months using One-way ANOVA. Statistical analyses were performed using PAST Software Version 1.81. RESULTS There were 15 rotifer, six cladoceran and three copepod species in Lake Taal. Brachionus was represented by seven species followed by Diaphanosoma (3) and Filinia (2). All other genera were represented by one species each. No new species or Philippine records were found, although a comparison with previous studies revealed that six rotifers, two cladocerans and one copepod are new records for the lake (Table 2). Ten rotifer and one copepod species were Fig. 1. Map of Lake Taal with the six sampling sites (NB North Basin, SB South Basin). The insert shows the location of Lake Taal and the other lakes mentioned in the text (P Lake Paoay, Lb Lake Laguna de Bay, N Lake Naujan and Ln Lake Lanao). Table 2. Limnetic zooplankton species of Lake Taal collected in the year 2008 (*denotes new record while # denotes rare species). Phylum Rotifera Class Monogononta Family Brachionidae Anuraeopsis navicula (Rousselet, 1910)* # Lecane bulla (Gosse, 1851)* # Keratella tropica (Apstein, 1907) Brachionus forfi cula Wierzejski, 1891 Brachionus havanaensis Rousselet, 1911 Brachionus falcatus Zacharias, 1898 # Brachionus quadridentatus Hermann, 1783 # Brachionus angularis Gosse, 1851 # Brachionus diversicornis (Daday, 1883) # Brachionus calycifl orus (Pallas, 1766) # Family Synchaetidae Polyarthra vulgaris Carlin, 1943* Family Hexarthridae Hexarthra intermedia (Wiszniewski, 1929) Family Filinidae Filinia opoliensis Zacharias, 1898* # Filinia longiseta (Ehrenberg, 1834)* # Family Trichocercidae Trichocerca capucina (Wierzejski and Zacharias, 1893)* # Phylum Arthropoda Class Branchiopoda Order Cladocera Family Moinidae Moina micrura Kurz, 1874 Family Bosminidae Bosmina fatalis Burkhardt, 1924 Family Sididae Diaphanosoma excisum Sars, 1886* Diaphanosoma sarsi Richard, 1895 Diaphanosoma tropicum Korovchinsky, 1998 Family Daphniidae Ceriodaphnia cornuta Sars, 1885 Class Maxillopoda Order Calanoida Family Diaptomidae Tropodiaptomus vicinus (Kiefer, 1933) Family Pseudodiaptomidae Pseudodiaptomus brehmi Kiefer, 1939* # Order Cyclopoida Family Cyclopidae Thermocyclops crassus (Fischer, 1853) considered rare since they had low densities and only occurred for a few months. Fig. 2. Average abundance (Ind. / l) and number of species of Copepoda, Cladocera and Rotifera from the upper 40 m in Lake Taal in the year 2008. The Copepoda were grouped into nauplii, Calanoida and Cyclopoida. The three major zooplankton taxa were present throughout the year. There were between 15 and 20 species co-existing each month. The copepods comprised 64% of the average zooplankton abundance and 84% of total biomass which included copepodites and nauplii (Fig. 2). The months of January and March had the highest copepod abundances. January also had the highest zooplankton abundance which may be attributed to the copepod nauplii. Cyclopoid copepods outnumbered calanoid copepods for ten months. The rotifers were most abundant in June. Eleven of the13 common zooplankton species had similar over-all biomass in both basins; however, some species were noticeably more abundant 3

Papa and Zafaralla: Zooplankton of Lake Taal Table 3. One-way ANOVA test statistic (F) and probability (P) for zooplankton biomass. Significance was accepted if P < 0.02; degrees of freedom (d.f.) = 5. Taxa F P C. cornuta 1.383 NS B. fatalis 0.647 NS M. micrura 3.055 NS D. excisum 2.218 NS D. tropicum 0.153 NS D. sarsi 0.102 NS T. vicinus Male 1.822 NS T. vicinus Female 1.025 NS T. crassus Male 0.679 NS T. crassus Female 0.608 NS K. tropica 2.913 P < 0.02 H. intermedia 1.175 NS B. havanaensis 0.300 NS B. forfi cula 0.258 NS P. vulgaris 6.232 P < 0.02 in the north basin for the first three months (cladocerans and copepods) or for most of the year (rotifers) (Fig. 3). Keratella tropica and Polyarthra vulgaris had higher biomass in the north basin for the entire year based on the results of Oneway ANOVA (Table 3). Other rotifers including Hexarthra intermedia and Brachionus havanaensis had higher biomass in the north basin for six and two months, respectively but did not show significant differences over-all. The cladocerans and copepods had higher biomass in the north basin during the first three months of the year which became more similar with the biomass values in the south basin as the months progressed. Tropodiaptomus vicinus had the highest recorded biomass in a single month (January). Except for Brachionus havanaensis, the other common species were present in almost all months. Rotifer diversity (15 spp.) ranged from 0.77 to 1.58 while cladoceran diversity (6 spp.) was between 0.78 and 1.62 (Fig. 4). Lowest and highest diversity was observed in February and April, respectively. No differences in H was found between the north and south basins for both rotifers (F = 2.865; 1 d.f.; P > 0.02) and cladocerans (F = 1.543; 1 d.f.; P > 0.02). DISCUSSION Limnetic and littoral zooplankton have to be treated separately when assessing species richness due to differences in ecology and sampling methods used for these two areas (Matsumara- Tundisi & Tundisi, 2005). The 37 species previously listed for Lake Taal included both littoral and limnetic forms (Mamaril Sr., 2001b). We focused on the community structure and dynamics of Lake Taal s limnetic zooplankton because of their importance in the diet of pelagic fish species such as S. tawilis (Papa et al., 2008). A comparison with previous zooplankton surveys may help better understand the current set of data and update the taxonomy and systematics employed by previous authors Fig. 3. Mean monthly biomass (μg / l) of common zooplankton species from the north and south basins of Lake Taal for the year 2008. 4

(Tables 1 & 2). Thermocyclops crassus was the only cyclopoid copepod species we encountered. It was the only copepod listed in Kiefer (1939b) and Woltereck (1941) while Mamaril Sr. (2001b) found it together with four other cyclopoids. Amarasinghe et al. (2008) then reported T. crassus together with Microcyclops varicans in the lake. In the year 2001, a calanoid copepod, Tropodiaptomus vicinus was included in a list of Lake Taal zooplankton (Mamaril Sr., 2001b) where it was stated that T. vicinus was included based on Zafaralla (1992). The FISHSTRAT zooplankton data also included T. vicinus (Schiemer et al., 2008). In this study, we found T. vicinus together with another calanoid species, Pseudodiaptomus brehmi. This rare demersal species has only been reported from marine habitats after it was first described by Kiefer using specimens from Lake Naujan (Mindoro Is.) (Kiefer, 1939a) which is less than 100 km south of Lake Taal. This is the first record of P. brehmi in Lake Taal and its second recorded occurrence in a freshwater habitat. The cladocerans Bosmina fatalis, Ceriodaphnia cornuta, Moina micrura and Diaphanosoma sarsi and D. tropicum have been recorded in past surveys while D. excisum is a new record for the lake. Bosmina and Moina were described as rare in limnetic areas of Lake Taal compared to nearby Lake Laguna de Bay (Mamaril Sr., 2001b). Nine of the 15 rotifer species we found have been earlier reported by Woltereck et al. (1941), Hauer (1941), Ueno (1966) and Mamaril Sr. (2001b). The remaining six species - A. navicula, L. bulla, P. vulgaris, F. opoliensis, F. longiseta and T. capucina are new records for the lake. We also found Brachionus diversicornis which Mamaril Sr. (2001b) previously listed as a new record for Lake Taal and the Philippines. A comparison with rotifer species richness in other Philippine lakes showed that Lake Taal had less rotifer species than Lake Laguna de Bay (19 spp.) (Mamaril Sr., 2001b) but have more than Lake Paoay (11 spp.) (Aquino et al., 2008) and Lake Lanao (seven spp.) (Lewis Jr., 1979). Other Southeast Asian lakes have similar numbers of rotifer species (Fernando, 1980). There have been no routine monthly zooplankton surveys in Lake Taal covering an entire year prior to this study. Fig. 4. Monthly variations in Shannon-Wiener Diversity (H ) Index values of rotifers and cladocerans in the north and south basins of Lake Taal. The FISHSTRAT survey from August 1998 to July 2000 only included six zooplankton collections with analysis later limited to microcrustaceans (Vijverberg et al., 2008). A more frequent (e.g. biweekly) sampling regime is usually recommended for more in-depth zooplankton ecology studies (Gliwicz, 1986). Unfortunately, financial and logistical constraints did not permit the conduct of biweekly surveys. We observed similar composition in both lake basins for the dominant zooplankton species (Fig. 3). Due to its tropical location, meteorological factors affect the physical conditions of the lake throughout the year. The watershed has a pronounced dry (November to March) and wet season (July to September) which can also be classified based on monsoon seasons: Northeast (December to March), Southeast (March to April), and Southwest (May to September) (Guerrero III, 2002). Vijverberg et al (2008) added an Intermediate Season (October to November) characterized by low rainfall and low temperatures. These conditions affect lake mixing which influences zooplankton distribution. Tropical zooplankton are more prone to wind dispersal due to their smaller size and slower swimming speeds compared to temperate species (Blukacz et al., 2009). It was possible that this influenced spatial homogeneity in distribution either after lake mixing (cladocerans and copepods) or for the entire year (rotifers). The varying response of these two groups may be attributed to differences in tolerance for higher trophic states and levels of predation. Rotifers had higher biomass in the north basin due to their tolerance for more eutrophic environments; excessive feeding of cage-cultured fish increase nutrient levels in the north basin. This also serves as the origin of excess nutrients that are transported by wind and currents to other locations in the lake. We observed low diversity and a lack of seasonality in occurrence (Fig. 4), which was the general observation for zooplankton of tropical lakes (Fernando, 2002). The highest H values were 1.58 (rotifers) and 1.62 (cladocerans) with H max values for 15 rotifer and 6 cladoceran species at 2.7 and 1.8 respectively. Cladocerans and rotifers had low diversity due to the dominance of C. cornuta and B. fatalis among cladocerans and K. tropica, B. forfi cula and H. intermedia among rotifers. Low diversity may be related to lake trophic status. Lake Taal is now classified as meso- to eutrophic (Perez et al., 2008). Eutrophic lakes are known to support fewer zooplankton species with higher abundances (Matsumara-Tundisi & Tundisi, 2005) where cyclopoid copepods and cladocerans are dominant over calanoid copepods (Pinto-Coelho et al., 2005). Our data shows that the copepods contributed the highest abundance and biomass of the three major groups almost every month similar to results from Aquino & Nielsen (1983), Perez et al. (2008) and Zafaralla (1992). The cyclopoids and cladocerans were also more abundant compared to the calanoid copepods. S. tawilis prefer calanoid copepods over other zooplankton which may have also influenced zooplankton composition and abundance. Lowest calanoid densities were recorded during months when S. tawilis were previously found to consume high quantities of calanoid copepods (Papa et al., 2008). Eutrophication in Lake Taal may be attributed to natural and man-made nutrient inputs from volcanic sediments, increased mixing layer due to thermal vents, wash-offs from the watershed and feeding of caged fish (Amarasinghe et 5

Papa and Zafaralla: Zooplankton of Lake Taal al., 2008; Vista et al., 2006; White & San Diego-McGlone, 2008). Our data shows that based on analysis of monthly samples collected for an entire year, the limnetic zooplankton of Lake Taal is typical of tropical eutrophic lakes. This is exemplified by high copepod abundance, the dominance of cyclopoid copepods and cladocerans over calanoid copepods, the higher biomass of rotifers in the north basin as well as having low species diversity while being present all throughout the year. ACKNOWLEDGEMENTS We would like to thank Reiner Eckmann and Augustus C. Mamaril Sr. for reviewing the manuscript, T. Chad Walter for verifying our identification of P. brehmi and comments on the manuscript. 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