Biogeochemistry of a tropical river affected by human activities in its catchment: Brantas River estuary and coastal waters of Madura Strait, Java, Indonesia T.C. Jennerjahn a, ) , V. Ittekkot a , S. Klo¨pper a , Seno Adi b , Sutopo Purwo Nugroho b , Nana Sudiana b , Anyuta Yusmal b , Prihartanto b , B. Gaye-Haake c a Center for Tropical Marine Ecology, Fahrenheitstrasse 6, D-28359 Bremen, Germany b Directorate of Technology for Land Resources and Region Management, Agency for the Assessment and Application of Technology, Jl. M.H. Thamrin 8, 10340 Jakarta, Indonesia c Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany Received 5 November 2003; accepted 23 February 2004 Abstract On a global scale tropical SE Asia is critical in terms of fluvial nutrient and sediment input into the ocean as well as in terms of human modifications of the coastal zone altering these inputs. In order to obtain information on the biogeochemistry of the Brantas River estuary and adjacent coastal waters of the Madura Strait in eastern Java, Indonesia, a densely-populated area which is strongly affected by human activities in the river catchment, we investigated water, suspended matter and sediments from estuarine and coastal waters and plants and soils from the catchment collected during the dry season. Water samples were analyzed for dissolved nutrients and phytoplankton abundance and composition. Suspended matter, sediment, plant and soil samples were analyzed for carbon, nitrogen, amino acids and stable carbon and nitrogen isotopes. Nutrient concentrations were high in the river and then decreased rapidly seaward. Runoff from agricultural soils may be a major nutrient source. Phytoplankton was dominated by diatoms and most abundant in the estuarine mixing zone. A fairly high phytoplankton abundance despite nitrate-depletion and an N/P ratio !4 in coastal waters indicates additional nitrogen sources. Biogeochemical characteristics and stable isotopes indicate the autochthonous origin of suspended organic matter (OM) during the dry season. Major part of the fluvial input appears to be trapped in the inner estuary. Similar information obtained from the analysis of sediments suggests seasonal differences in the quantity and origin of suspended sediments and OM transported by the river with high inputs into coastal waters during the rainy season, the period of peak river discharge. The amino acid composition in combination with stable isotopes indicates agricultural soils to be a major OM source during that time. A high proportion of amino acid-bound nitrogen in suspended matter and sediments and the presumed sources of dissolved nitrogen other than nitrate to sustain primary productivity in the coastal zone suggest that organic nitrogen may play a more important role for coastal food webs and the nitrogen cycle in tropical regions than previously thought. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: biogeochemistry; rivers; stable isotopes; amino acids; nutrients; Indonesia 1. Introduction Rivers are connecting land and ocean and nourish the continents as well as the coastal ocean and the people living in the respective areas. Natural and anthropogenic environmental changes of the past decades led and will lead to substantial changes of these ecosystems and the biogeochemical cycles of the coastal ocean. Investiga- tions of the past decade have shown that small and mid- sized rivers play a vital role in this respect as they are carrying a sediment load one to two orders of magnitude larger than that of the major river systems (Milliman and Syvitski, 1992). However, particularly in the second half of the 20th century, the diversion and damming of ) Corresponding author. E-mail address: tim.jennerjahn@zmt-bremen.de (T.C. Jennerjahn). Estuarine, Coastal and Shelf Science 60 (2004) 503e514 www.elsevier.com/locate/ECSS 0272-7714/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2004.02.008