Species assemblages of zooplanktonic crustaceans in mountain shallow ponds of Chile ( Parque Cañi )

Even though the Chilean lakes are characterized by their oligotrophy, a transition from oligotrophy to mesotrophy, due human intervention, has been reported in some lakes. Nevertheless, there are still some pristine and unpolluted small lakes and ponds in mountain zones, free of human intervention and surrounded by native forests. Nine unpolluted, oligotrophic and pristine water bodies located in Cañi Park, a mountain zone with altitudes between 1000 to 1500 m a.s.l and forests where Nothofagus dombeyi, N. pumilio and Araucaria araucana predominate, were studied. For each sampled lake, zooplankton was collected and environmental parameters were obtained (conductivity, total dissolved solids, and chlorophyll concentration). A null model of species co-occurrence was applied to determine randomness in species associations. All sites revealed low species richness (< 6); the calanoid copepod Boeckella gracilis Daday, 1902 was present in all sites. The results of the null model indicated randomness or absence of regulatory factors in species associations. Only few species occur in practically all localities. Also, a significant inverse association between chlorophyll concentration with percentage of calanoid copepods and a weak direct association between chlorophyll concentration and percentages of cladocerans were found.


MATERIAL AND METHODS
The present study was carried out at the Cañi Park, a privately protected, mountainous land. The local vegetation is characterized by the strong presence of Nothofagus dombeyi (Mirb.) Oerst. and N. pumilio (Poepp. and Endl), which in some areas coexist with N. obliqua (Mirb.) Oerst. In high altitude zones, Araucaria araucana (K. Koch) and N. pumilio predominate (LUEBERT & PLISCOFF 2006). In addition, the local vegetation also includes shrub and herb communities. The shrub communities are characterized by the presence of Chusquea quila  (HOFFMAN, 2005;LUEBERT & PLISCOFF 2006). The park has numerous fishless, shallow ponds that can be accessed via mountain roads located ca 8 km from the main access to the park.
The study site, visited in January, 2008, included nine shallow ponds (Tab. I). January corresponds summer month with highest zooplankton abundances in the southern hemisphere , 2007a conductivity, and total dissolved solids were determined. Chlorophyll a concentration was measured after acetone extraction (WETZEL & LIKENS 1991) and total dissolved solids were measured using an YSI-30 sensor.
Qualitative samples were obtained by performing horizontal hauls in the pelagic and littoral zones using an Apstein net with 20 cm mouth diameter and 100 µm mesh size, applying a methodology similar to that described for southern Table I. Geographical location, chlorophyll "a" concentration, total dissolved solids, conductivity and crustacean species reported for the studied sites.

Site
Geographical location Chlorophyll a (µg/l) TDS (mg/l) Conductivity (mS/cm) Species  (2006). The data were ordered using an absence/presence matrix. A Checkerboard score ("C-score"), a quantitative index of occurrence, measured the extent to which species co-occur less frequently than expected by chance alone (GOTELLI 2000(GOTELLI , 2001. GOTELLI & ENTSMINGER (2004), TIHO & JOHENS (2007) and TONDOH (2006) suggested that the following robust statistical models should be used in a co-occurrence analysis. First, the matrix layout needs to include the species names in rows and the sites in the columns. Second, the following models should be used: 1) Fixed-Fixed: in this model, the row and column sums of the matrix are preserved. Thus, each random community contains the same number of species as the original community (fixed column) and each species occurs with the same frequency as in the original community (fixed row). 2) Fixed-Equiprobable: in this algorithm only the row sums are fixed and the columns are treated as equiprobable. This null model considers all the sites (columns) as equally available for all species, which occur in the same proportions as in the original communities. 3) Fixed-Proportional: this model holds the species occurrence totals the same as in the original community and the probability that a species occurs at a site (column) is proportional to the column total for that sample. A null model analysis was carried out using the software Ecosim version 7.0 (GOTELLI & ENTSMINGER 2004, TIHO & JOHENS 2007, TONDOH 2006, DE LOS RÍOS 2008. Finally a regression analysis using the software Xlstat 5.0 was performed to determine potential associations of chlorophyll "a" concentrations with species number, percentage of calanoid copepods, percentage of cladocerans and percentage of cyclopoid copepods.

RESULTS
The studied sites show low chlorophyll concentrations (Tab. I). The calanoid copepods Boeckella gracilis (Daday, 1902) and C. dubia were found at all studied sites (Tab. I). In addition, the following crustaceans were encountered in many of studied sites: D. pulex, Mesocyclops longisetus (Thiebaud, 1912), Chydorus sphaericus (O.F. Müller, 1785), Diaphanosoma chilense (Daday, 1902), and Hyalella araucana (Grosso & Peralta, 1999) (Tab. I). The overall species number was low, between three species for Negra and Del Risco ponds, and a maximum of six species for Escondida pond (Tab. I). The results of the null model analysis revealed the absence of regulatory factors for all simulations (Tab. II). Many of the studied sites showed a notorious abundance of calanoids copepods and low cladoceran abundance (Tab. III).
The results of the regression analysis did not indicate a significant association between chlorophyll concentration with either species number or with percentage of cyclopoid (Fig. 1). A significant inverse association was observed between chlorophyll concentrations and relative percentages of calanoid, whereas a weak significant direct association was observed between chlorophyll concentrations and percentage of cladocerans (Fig. 1).

DISCUSSION
The results obtained in the present study revealed a low species richness (Tab. I) and high predominance of calanoids (Tab. III) in many of the studied sites. This pattern is in agreement with other similar studies on oligotrophic lakes in Argentinean Patagonia (MODENUTTI et al. 1998), New Zealand (JEPPENSEN et al. 1997, 2000, and Chilean large and deep Patagonian lakes (SOTO & ZÚÑIGA 1991, 2007a. The increase in the relative abundance of daphnids (Cladocera) when chlorophyll concentrations are high, observed for a few studied sites (Tab. III and Fig. 1), agrees with previous descriptions for lakes in Chilean Patagonia (SOTO & ZÚÑIGA 1991, DE LOS RÍOS & SOTO 2007b, DE LOS RÍOS et al. 2007), and New Zealand (JEPPENSEN et al. 1997, 2000. Nevertheless, the present study did not find associations between species number and chlorophyll concentration (Fig. 1), a situation that differs from descriptions for the Chilean Patagonian lakes, for which a direct association between species number and chlorophyll concentration had been reported (SOTO & ZÚÑIGA 1991, DE LOS RÍOS & SOTO 2007b, DE LOS RÍOS et al. 2007   The presence of B. gracilis in all studied sites is a new report, because, since only rarely it has been reported for Chilean lakes (MENU-MARQUE et al. 2000). Nevertheless, in recent studies, this species was reported from high mountain lakes in northern Patagonia (TROCHINE et al. 2006, DE LOS RIOS et al. 2007. In general, the species found in our study are the same as those reported for other similar Argentinean and Chilean small lakes in the Andes mountains of northern Patagonia (38-41°S). Furthermore, the regulatory mechanisms of the local community are similar to those reported for other lakes and ponds of Chilean Patagonia (38-51°S).
Our results indicate that unpolluted freshwater bodies have similar regulatory mechanisms as those reported for the Patagonian lakes of Chile and Argentina. These results are important for management, considering the importance of these water bodies as habitats for aquatic birds (ARAYA & MILLIE 2005), and tourism activities due their unpolluted conditions (STEINHART et al. 1999(STEINHART et al. , 2002.