Natural and Anthropogenic Influences on Nutrient Export from Tropical
Mountainous Rivers into the Arabian Sea
Abstract
The land-sea connection of nutrients via rivers is an essential
component in the global nutrient biogeochemistry. Rivers alone transport
approximately 80% of the dissolved nutrients from land to ocean and
play a vital role in nourishing riverine, coastal and oceanic
ecosystems. During the past decades, global scale investigations have
estimated the riverine nutrient flux to the oceans along with the
significant drivers. However, most of the previous estimations are based
on nutrient discharge by a few major rivers. Despite being large in
number and high yields, the tropical coastal/mountainous rivers have
received less attention. To fill the gap, we have investigated 70
west-flowing coastal rivers, draining the Western Ghats (WG), for their
Dissolved Inorganic Nutrient (DIN) transport characteristics and
influencing factors. Previous studies in the selected estuaries along
the WG coast are combined to update the land-sea fluxes estimation to
understand the spatial pattern of DIN over the WG rivers-the Arabian sea
coast continuum. Altogether, the DIN fluxes from entire WG (including
non-sampled rivers) region to the estuaries are 664, 241 and 6277 Tg
yr-1 for Nitrate (N), Phosphate (P) and Silicate (DSi)
respectively. The natural factors, such as discharge, cropland, and
forest cover each explain 30–85% of the spatial variation in DIN
levels at the basin scales. DIN concentrations of rivers reduces to
~ 90% after reaching the respective estuaries. Based on
Redfield ratio of C:N:P (106:16:1), annual export of 66.4 Tg of
dissolved inorganic N from the WG rivers would support 439.9 Tg of new
carbon production in the Arabian Sea coast. The humid tropical climate
and high population density (> 300 people per
km2 among the bio-diversity hotspots) of the WG region
favour the high DIN export, thus, making the Arabian sea coast highly
productive among the global coastal regions. Keywords: Dissolved
Inorganic Nutrients; Rivers-Coast Continuum; Influencing Factors; Small
Mountainous Rivers; Western Ghats.