Catchment scale human interventions in the Pamba Basin (Kerala, India) and their impact on estuarine ecosystem in the southern Vembanad Estuary

Abstract

Anthropogenic inputs nowadays are the major source of nutrients leading to cultural eutrophication in the coastal aquatic systems. While river inputs of anthropogenic nutrients into coastal seas is considered a global problem, the database is biased towards temperate regions, developed countries and major river systems. Little is known on the amount and composition of nutrient fluxes from densely populated catchments in tropical regions and, in particular from small- and medium-sized rivers. The South Indian Pamba River is a prime example in this respect because of its manifold human interventions such as the Sabarimala pilgrimage, the largest pilgrim centre in the world, agricultural practices and sewage disposal. The goals of this study were (i) to define cause-effect relationships by assessing the effect of various human interventions such as the pilgrims, agriculture and sewage disposal in combination with the seasonal variations in hydrology on the nutrients and organic matter composition (ii) to quantify land use specific nutrient inputs (iii) to assess the respective impacts in the Pamba River and (iv) to understand the ecological consequences in the Vembanad estuary. The Pamba River and the Vembanad estuary were sampled from March 2010 to February 2012. 9 sampling campaigns were carried out to cover the south west monsoon (SWM, May/June 2010), north east monsoon (NEM, October to December 2011) and pre monsoon (PM, March/April 2010 and January/February 2012). A socioeconomic survey on agricultural practices was conducted to collect information on the type, time and quantity of fertilizer application during November 2011. Dissolved organic nitrogen (DON) was the dominant form of N and accounted for 99% of the total dissolved nitrogen (TDN) pool in the Pamba River. Land use specific cause-effect relationships were identified for the Pamba catchment. The global maximum DON concentration (29,302 µM) and yield (745 kg ha-1 yr-1) in the Pamba River were due to the pilgrim activities, high population density, agricultural and livestock farming as well as the lack of infrastructure for sanitation facilities. Domestic sewage inputs (26 kg ha-1 yr-1) introduced into the Pamba catchment was an order of magnitude higher than the other human impacted world rivers. Dissolved inorganic nitrogen (DIN) was dominated by ammonium (NH4 ) due to the effluents from pilgrim activities in the upstream temple region, while nitrate (NO3-) was the major N species in the downstream agriculture segments. DIN yields in the Pamba River were lower in agriculture segments with systematic fertilizer management, i.e. in the paddy and rubber dominated regions, than in the settlement with mixed tree crop (SMT) regions where farmers tend to apply more fertilizer than required by the plants because of a lack of knowledge. Annual NPK fertilizer inputs in the catchment were 95 kg ha-1 yr-1, of which 13% of total N as DIN and 1% of phosphate-P reached the river mouth. The average yield for the Pamba catchment amounted to 3.5 kg ha-1 yr-1 of DIN and 0.2 kg ha-1 yr-1 of phosphate-P (PO43--P). Seasonal variations in hydrology exhibited marked changes in the dissolved nutrients and organic matter loading in the Pamba River, i.e. the loads were high during the SWM and NEM compared to the PM. Suspended organic matter were mainly composed of planktonic as indicated by high chlorophyll a values (0.7 to 34.2 µg L-1) and terrestrial sources in the Pamba River. High nutrient concentrations were observed in the Vembanad estuarine region receiving direct inputs from the upstream catchments, thereby exhibiting signs of eutrophication. Phytoplankton abundance was high in the upper estuary and ranged between 1,513 to 3,552 cells L-1 and < 900 cells L-1 in the mid and lower estuary. Uptake by the phytoplankton and water hyacinth (Eichhornia sp.) in the Vembanad estuary were the major sink of the nutrients in the Vembanad estuary. From this study it is evident that the lack of technological investments not only affects the drinking water and river biodiversity, but also has a huge impact on the riverine nutrient concentrations. These results suggest that installation of adequate sanitation measures for waste disposal and treatment together with improved agricultural and animal management practices could strongly reduce the anthropogenic nutrient inputs into the Pamba River and in turn leads to less impact in the receiving Vembanad estuary. This study also underscores the need for more regional scale quantitative studies from densely-populated tropical river catchments in order to improve global nutrient budgets and the assessment of the consequences of anthropogenic nutrient inputs into coastal aquatic systems.