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Abstract: 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 (17psu 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. [ABSTRACT FROM AUTHOR]

Abstract: The quality of coastal waters can be markedly influenced by the growth and distribution of phytoplankton. In this study, a non-linear mathematical model is used to explain the seasonal variability of plankton in shallow coastal lagoons. A local stability analysis for dynamical systems is undertaken in order to estimate the range of values of model parameters. Numerical experiments and sensitivity tests with different parameters show that of the thirteen parameters in the model, the main parameter affecting the productivity is the growth rate of the phytoplankton, which depends on light and nutrients. The time of maximum planktonic growth, in particular, is mostly dependent upon the parameter values within the growth rate term. As a case study, the model is then applied to Chilika Lagoon (19∘28′N–19∘54′N and 85∘06′E–85∘36′E), the largest brackish water lagoon with estuarine character on the east coast of India. Interest in detailed analysis of the ecology of the lagoon and the various factors affecting it is due to the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors — eutrophication, weed proliferation, siltation, industrial pollution and depletion of bioresources. The model is tested for different sectors of the lagoon which have distinct characteristics in terms of depth, light penetration, nutrients and hence planktonic growth. A fine-tuning of the model parameters, with the help of the stability analysis discussed for a general model, was performed in order to validate the model results with the data available for the lagoon. Even in its simplest form, the model is able to reproduce most of the significant characteristics of the planktonic distribution in the lagoon — the bimodal oscillations, and major plankton peaks. [Copyright &y& Elsevier]

Chilika Lagoon (19°28′-19°54′N, 85°06′-85°36′E), on the Orissa coast, India, is one of the world's unique ecospheres. It is the largest brackish water lagoon with estuarine character. Because of its rich biodiversity and socioeconomic importance, it was designated as a "Ramsar site"--a wetland of international importance--in 1981. Interest in detailed analysis of the circulation, biotic, and abiotic factors affecting the lagoon and its limnology is a result of the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors--eutrophication, weed proliferation, siltation, industrial pollution, and depletion of bioresources. This paper describes the development of a two-dimensional depth-averaged hydrodynamic model for Chilika Lagoon and the resulting simulation of currents and salinity corresponding to (i) the Southwest and Northeast monsoon seasons and (ii) pre- and post-mouth-opening conditions. Numerical experiments are performed to understand the different factors--wind, tides and freshwater influx--and their independent and collective roles. Comparison of results obtained with one or both of the openings is made to quantify and validate the changing salinity pattern, which has improved the productivity of the lagoon. Our study shows the salinity levels to be much lower during the southwest monsoon compared to the northeast monsoon, which is validated by observations. The decrease in salinity is attributed to more freshwater influx during the southwest monsoon. There is a significant increase (14%-66%, depending on the sector) in salinity after opening of the new mouth, the maximum change being observed in the channel that connects the lagoon to the sea. The constriction in the lagoon that blocks the tidal effects entering the lagoon is found to be responsible for maintaining the main body of the lagoon with low salinity. The dynamic model will be an important input for our on going work in seasonal study of the ecology of Chilika Lagoon. [ABSTRACT FROM AUTHOR]