Sedimentologic, Chemical, and Isotopic Constraints on the Anthropogenic Influence on Chilika Lake, India

Chilika Lake, the largest Asian lagoon on the east coast of India, has a surface area of 1160 km² or about 900 km², respectively for the wet, monsoon vs. dry winter-spring season. The average depth is only about 1.2 m. It is separated from the Bay of Bengal by a 100 m to 1.5 km wide sand bar of about 30 km length, separating the outer channel that connects the lagoon naturally to the sea. Long-shore development of this sand bar as of the Late Holocene increasingly isolated the lagoon from the sea, until final closure in 1992. Given the population increase in the catchment and according changes in land use policies, agricultural practices, and water resource management, Chilika Lake has been subjected to increasing anthropogenic influence. As a consequence the unique biodiversity and also primary production within the lagoon decreased, while eutrophication and siltation increased. As a counter-initiative it was decided to artificially open the lagoon to the sea by dredging.

To help trace and quantify the anthropologic effects on Chilika Lake, a combined sedimentologic, chemical, and isotopic study of the lagoon and its sediments was is in progress. First results from a campaign during the monsoon season suggest that the large gradients in salinity, sediment and nutrient input, as well as primary productivity within the lagoon are controlled by variable fluxes of water, sediment, and nutrients from the three separate catchments to the lagoon. Trends in changes of salinity, H- and O-isotope compositions of waters, but also of concentrations and C- and/or N-isotope compositions of the dissolved inorganic carbon (DIC), particulate organic matter (POM), and aquatic plants indicate that mixing in the lagoon occurs between new freshwater inputs and evaporated water within the basin itself. Except for the outer channel, mixing with seawater is limited. In contrast, the C-isotope composition of the organic matter in the sediments either suggests a higher overall proportion of “marine” or of estuarine-derived POM during the past. The latter may be important during the dry season, coupling salinity increase to the changes in DIC and POM carbon isotope compositions. The salinity-DIC-H-, O-, C-isotope compositions of water are compatible with evaporation as the main driver for salinity increase, rather than admixtures with seawater.