Ecology of the Copepods at a Low Salinity Zone of the Ganges Estuary, India

Low salinity (below 5 PSU) zone (LSZ) acts as an ecocline and limits species distribution within an estuary. After Farakka barrage (1975) the Diamond Harbour (DH) of the Ganges estuary turned to a LSZ from meso- to poly-haline zone, which might have affected estuarine inhabitants including plankton (e.g., copepods). The objectives were to study diversity, dominance of the copepods and their associations with environmental variability of the DH. The present study and a review of the previous literature were used for an assessment of the  structural changes of the copepod community over time. Copepod assemblages were sampled seasonally (four times per season) from a sampling site (22.10.59° N; 88.11.22° E) of the DH. Water temperature (°C), salinity and pH were recorded in situ and Total Nitrogen (TN), Nitrate-nitrogen (NO₃-N) and Phosphate (P) levels were measured at the laboratory following the spectrophotometric methods. Seasonal variations of the salinity, pH, NO₃-N and P were not significant but water temperature and TN levels were significant. Copepod richness at DH was the highest in the monsoon (13) followed by the postmonsoon (11) and premonsoon (10). The species richness of the copepods at DH declined from the levels of the 1970s. Presently, Paracalanus parvus is the most abundant copepod at DH followed by B. similis. During the pre-Farakka time; P. parvus and B. similis were rare and species such as Cyclops sp., Bryocamptus sp., Ergasilus sp. which were not found in the present study were abundant. Multivariate analysis suggested significant seasonal variations of the copepod community structure. Salinity, water temperature, pH, P and NO₃-N levels of DH may limit the distribution of a few species. The DH is experiencing a sea level rise at a rate of 5.3 mm/year; therefore, environmental variability may change, which may draw more eurihaline and neritic copepod species towards the DH so a regular monitoring of the LSZ of the estuary is recommended.