Mesozooplankton distribution in relation to the salinity gradient in a tropical hypersaline lake

Fluctuations in salinity are known to influence coastal marine communities. In this study, we present the effect of salinity on the distribution characteristics of mesozooplankton in Pulicat lake, the second largest lagoon situated along the south east coast of India. During different seasons of the study period, salinity varied from hyposaline (12.57) to hypersaline (139.74) in the lake. Salinity values were moderate in southern (33.75 ± 0.7) and northern inlets (29.38 ± 4.33), which were connected with the sea; while in the central region of the lake salinity was observed to be higher (52.32 ± 4.57). Mesozooplankton biovolume and abundance were high in the southern and northern inlets whereas a severe reduction in diversity and abundance was observed in the central region of the lake manifesting that increase in salinity is negatively impacting the mesozooplankton distribution. Copepod (Acartia tonsa, Acrocalanus gibber, Paracalanus parvus, Oithona brevicornis and Oithona similis), crustacean and molluscan larvae dominated the mesozooplankton abundance. A. tonsa predominance (79%) over the lake and across different seasons indicates its tolerance to a wide range of salinity fluctuations i.e., brackish to hypersaline environment of the Pulicat. The distribution of euryhaline species A. tonsa in all the sampling locations throughout the study period indicated their prevalence in the lake. Multilinear analysis between biotic and abiotic components through redundancy analysis (RDA) also suggested that salinity is the major constraint factor for the distribution of mesozooplankton. In conclusion, this study showed that the increased salinity levels have profound effects on mesozooplankton community and distribution.