A massive invasion of the Neotropical calanoid copepod Arctodiaptomus dorsalis (Marsh, 1907) has been taking place in the Philippines since at least 1993. This copepod’s fecundity is known to depend on food concentration, increasing temperature appears to have detrimental effects on its hatching success and survivorship, but parameters that are truly predictive of the successful integration and propagation of this species in particular bodies of water have not yet been identified. We conducted a spatial analysis of morphometric features and reproductive capacity, the latter proxied by clutch size (CS), of different populations of A. dorsalis inhabiting 11 tropical lakes in the Philippines. Correlations between either morphometry or CS and selected water quality parameters were investigated based on collections and measurements made from December 2018 to April 2019. Means of morphometric measurements differed significantly (P < 0.001) among the populations of A. dorsalis in all lakes. Total length (TL) of adult males, TL of non-ovigerous adult females, and CS of ovigerous females all showed significant (P < 0.001) inverse correlations with lake depth, accounting for about 30% of the observed variability among lakes (r = –0.55, –0.55, and –0.58, respectively). Shallow lakes, which are more homogenous in terms of nutrient distribution throughout the water column, thus provide a more suitable habitat than deep lakes for A. dorsalis and provides a partial explanation of the widespread occurrence and abundance of the species in the shallower, more eutrophic lakes in the Philippines.

Food web studies rely heavily on the use of traditional gut content analysis or the fairly popular stable isotope analysis rather than fatty acid analysis (FAA) despite its straightforward process, and ability to identify and characterize more diverse trophic pathways. In this study, we employed fatty acid (FA) biomarkers as a preliminary attempt to trace and characterize trophic pathways in food webs of four tropical lakes of volcanic origin in Luzon Island, Philippines - three clustered maar lakes (Yambo, Pandin and Calibato) and a large caldera lake (Taal). These lakes have a long history of human disturbance but limited ecological data. Knowledge of basal food sources and existing trophic dynamics of organisms are generally non-existent. Particulate organic matter (POM), zooplankton, and fish species were collected in August 2019 from the study lakes as representative of three trophic guilds. Non-metric multidimensional scaling (NMDS) and principal component analysis (PCA) were conducted to analyse FA profiles and characterize trophic relationships between representative organisms. For the POM, within lakes comparison of taxon-specific FA profiles showed a significant difference between the surface and near bottom depths, with the former dominated by photoautorophs and the latter by chemotrophs, suggesting the ability of FAA to effectively delineate between micro-organisms. Between lakes comparison also showed significant difference between the caldera and maar lakes, with the latter containing higher composition of bacterial FA, reminiscent of the considerably smaller lakes’ response to the impact of unmitigated organic loadings from anthropogenic activities. Taken together with the primary consumers’ FA profiles, analysis confirmed the ability of FAA to discriminate between FA profile sources. PCA explained >70% of the variance in the FA compositions for three trophic guilds in the two deepest lakes, which delineated both zooplankton and fish species food selectivity in each lake, alluding to FAA’s capacity to characterize dietary reliance of various species in an environment with numerous food sources. Although certain limitations were encountered, such as the specificity of the sampling depths for POM, and the small sample size of the representative species of the third trophic level, this study demonstrated the effectiveness of FAA as a powerful ecological tool for disentangling intricate lake food webs comprising various food sources. Overall, this study provided baseline information on basal food sources and trophic pathways of representative organisms from four tropical lakes. Taken together, FAA studies have wide application in understanding food webs, including anthropogenically-threatened lake ecosystems.