Vertical distribution of the endemic Pseudodiaptomus brehmi (Kiefer, 1938) and the introduced Arctodiaptomus dorsalis (Marsh, 1907) (Copepoda: Calanoida) during the stratified and mixed states of Lake Taal, Philippines

This study provides the first side-by-side assessment of the vertical distributions of the endemic Pseudodiaptomus brehmi and the introduced Arctodiaptomus dorsalis in Lake Taal, Philippines, during mixed and stratified lake states to evaluate species-specific responses to seasonal changes in physicochemical conditions and water-column structure. Physicochemical parameters differed markedly between lake states, reflecting pronounced seasonal heterogeneity. Arctodiaptomus dorsalis (17 ind. L−1) outnumbered P. brehmi (9 ind. L−1), with the highest densities recorded during mixed conditions. Depth of Average Individual (DAI) analyses revealed overlapping distributions during mixing, whereas stratified periods showed vertical segregation, with P. brehmi occupying deeper or more variable layers and A. dorsalis aggregating consistently below the metalimnion. Canonical Correspondence Analysis explained 98% of the variation in species–environment relationships and showed that A. dorsalis copepodites were positively related to temperature, pH and chlorophyll-a, whereas all stages of P. brehmi were negatively associated with these variables. These contrasting environmental associations suggest that P. brehmi exhibits greater sensitivity to seasonal environmental conditions, while A. dorsalis maintains a comparatively stable distribution within the productive strata. Our findings indicate that seasonal changes in water-column structure may influence niche partitioning that can contribute to the coexistence of endemic and introduced calanoid copepods in Lake Taal.