Mangrove forests belong to the major ecosystems of the biosphere, and about 60-75% of tropical coasts are fringed by this highly productive ecosystem. Objective of this study was an assessment of the role that mangroves in north Brazil play for the nutrient cycles of coastal and marine ecosystems. 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. The nutrient and organic matter exchange between mangrove and adjacent ecosystems was quantified and characterized. Throughout 36 tidal cycles in the course of one year, fluxes of dissolved and particulate organic matter (DOM, POM) and inorganic nutrients (N, P, Si compounds) were determined in a tidal creek connecting a clearly defined area of mangroves with an estuary. The driving forces behind these material fluxes were analyzed. Additionally, material fluxes were estimated on the more extended spatial scale of the whole estuarine mangrove fringe. Aimed at the identification of source and bioavailability of DOM and POM in coastal environments, lignin-derived phenols and stable carbon isotopes were established as tracers for mangrove-derived organic matter and its diagenetic state.

The principle pathway for nutrient and organic matter export from the mangrove was porewater flow from the upper sediment horizon to the tidal creek and subsequently to the estuary. Porewater flow was directly coupled with tides. Aquatic primary production significantly altered the concentration of ammonium, DOM and POM, indicating a labile DOM and POM pool. Products of anaerobic and aerobic leaf litter decomposition in the mangrove sediment provided the bulk of DOM and POM in the tidal creek, as evident from its organic signature. The net export of these degradation products from the mangrove was about 20% of total litter fall. A similar amount of fresh litter, rapidly decomposed in the water column, was directly exported. A highly abundant, grazing macrofauna in the mangrove plays a key role for litter decomposition and export. Strong outwelling of organic matter and inorganic nutrients from the mangrove was determined. About 10 mmol DOC, 0.9 mmol dissolved inorganic and organic nitrogen, 6 mmol silicate and 0.02 mmol phosphate on annual average were exported per square meter mangrove and day. Although mangroves cover only 6% of the fluvial catchment area, their nutrient export exceeded by far that from the hinterland. Organic tracers indicated similar export characteristics of mangroves in adjacent bays or estuaries. DOM and POM were exported from the mangrove to the estuary in similar proportions. POM was rapidly removed from the water column probably by sedimentation. Mangrove-derived DOM, however, was evidently object of long-distance transport, probably because of its recalcitrant and conservative properties. Terrestrial DOM, on the other hand, was quantitatively removed from the water column, indicating a geochemical barrier zone for this type of DOM in the estuary.

It can be concluded that mangroves play a key role for the coastal and marine ecosystems in the studied region in north Brazil, providing inorganic and organic nutrients to these environments. The chemical tracers established in this study to distinguish between marine, mangrove and other terrestrial organic matter sources are a suitable tool to trace the fate and the proportion of mangrove-derived organic matter even in offshore environments.