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 and marine ecosystems in the tropics. However, few studies have been carried out to clarify the extent to which mangroves represent a source of nutrients 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 one of the world's largest mangrove areas in North Brazil. These are up to now the only published nutrient flux data for Brazilian mangroves, and constitute 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), nitrogen (DON, PON) and dissolved inorganic nutrients (N, Si, P compounds) were determined in a tidal creek connecting a clearly defined area of mangroves (2.2 km2) with the Caeté Estuary. Additionally, a qualitative whole-estuary approach was applied (190 km2 of mangroves). Both methods revealed an outwelling of dissolved organic matter and nutrients from the mangroves, exceeding considerably that of mangroves in other regions of the world. This net export was (annual average, mmol [m2 mangrove]–1 d–1) DOC ≈ 10, DON ≈ 0.7, ammonium ≈ 0.2, silicate ≈ 6 and phosphate ≈ 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. Especially ammonium fluxes presented strong asymmetries between day and night. Although only ~6% of the fluvial catchment area is covered by mangroves, their nutrient export exceeded that of the hinterland. The annual export from mangroves on a regional scale (6700 km2 from 0° S, 50° W to 3° S, 42° W) was estimated (mol yr–1) as DOC ≈ 30 ×109, DON ≈ 2 ×109, ammonium ≈ 0.4 ×109, silicate ≈ 15 ×109 and phosphate ≈ 0.04 ×109. Compared with the Amazon River discharge these fluxes are low (1 to 3% of the Amazon fluxes). The nutrient cycles of coastal marine environments are probably mangrove-dominated southeast of the Amazon Estuary, contrary to the river-dominated cycles to the northwest.