Do mangroves rather than rivers provide nutrients to coastal environments south of the Amazon River? Evidence from long-term flux measurements

Mangroves play an important role in the biogeochemical cycles of coastal an d marine ecosystems in the tropics. However, few studies have been carried out to clarify the extent to which mangroves represent a source of nutrient s and organic matter for adjacent coastal waters or function, alternatively as a sink. Here we present results of a long-term nutrient-flux study in o ne of the world's largest mangrove areas in North Brazil. These are up to n ow the only published nutrient flux data for Brazilian mangroves, and const itute one of the most comprehensive data sets in this field. Throughout 36 tidal cycles in the course of 1 yr, fluxes of organic carbon (DOC, POC), ni trogen (DON, PON) and dissolved inorganic nutrients (N, Si, P compounds) we re determined in a tidal creek connecting a clearly defined area of mangrov es (2.2 km(2)) with the Caete! Estuary. Additionally, a qualitative whole-e stuary approach was applied (190 km(2) of mangroves). Both methods revealed an out-welling of dissolved organic matter and nutrients from the mangrove s, exceeding considerably that of mangroves in other regions of the world. This net export was (annual average, mmol [m(2) mangrove](-1) d(-1)) DOC ap proximate to 10, DON approximate to 0.7, ammonium approximate to 0.2, silic ate approximate to 6 and phosphate approximate to 0.02. Nitrate fluxes were comparatively low. Silicate and phosphate were exported mainly during the dry season and DON during the rainy season. DOC showed no seasonal trend. E specially ammonium fluxes presented strong asymmetries between day and nigh t. Although only similar to6 % of the fluvial catchment area is covered by mangroves, their nutrient export exceeded that of the hinterland. The annua l export from mangroves on a regional scale (6700 km(2) from 0 degrees S, 5 0 degreesW to 3 degrees S, 42 degrees W) was estimated (mol yr(-1)) as DOC approximate to 30 x 10(9), DON approximate to 2 x 10(9), ammonium approxima te to 0.4 x 10(9), silicate = 15 x 10(9) and phosphate = 0.04 x 10(9). Comp ared with the Amazon River discharge these fluxes are low (1 to 3 % of the Amazon fluxes). The nutrient cycles of coastal marine environments are prob ably mangrove-dominated southeast of the Amazon Estuary, contrary to the ri ver-dominated cycles to the northwest.