Modelling the effects of variable salinity on the temporal distribution of plankton in shallow coastal lagoons

In shallow lagoons, there is a much greater ecological diversity than fully marine water bodies and communities comprise of representatives from both freshwaters and coastal sea. In the present study, an attempt has been made, through mathematical modelling, to study the effect of changing salinity on freshwater and marine plankton. The model proposed here is a five-compartment (Nutrient (N), Freshwater phytoplankton (P), Marine phytoplankton (Q), Zooplankton (Z) and Detritus (D)) ecological model. The unique feature of the model is its ability to simulate plankton dynamics, as a response to salinity variations, with freshwater and marine species of phytoplankton represented separately. Numerical experiments and sensitivity analysis show that changes in salinity levels, nutrients and grazing are the controlling factors. For the case of intermediate salinity (17 psu approx.), the nutrient availability and palatability by zooplankton play a significant role. It is in this case, the two species are found to coexist. As a case study, the model is applied to Chilika Lagoon (19°28′N–19°54′N and 85°06′E–85°36′E), the largest brackish water lagoon on the east coast of India which has distinct and variable characteristics. The model is able to reproduce most of the significant characteristics of the phytoplankton distribution in Chilika, viz., bimodal oscillations, major plankton peaks and increased productivity after the salinity level was increased due to the opening of a new mouth connecting Bay of Bengal.