The Biogeochemistry of the Caeté Mangrove-Shelf System

Abstract

Mangrove biogeochemistry addresses the interrelation between primary production and nutrient cycles, material fluxes and secondary production. It requires data on source identification, quantification and turnover of organic matter and inorganic nutrients in sediments, porewater and in the water column. This information can be derived from organic biomarkers, and it is one of the important tasks in the field of biogeochemistry to develop such proxies. Lignin phenols, for example, are suitable source indicators to distinguish between organic matter derived from mangroves, other terrigenous sources and aqueous organisms. Pentacyclic triterpenols can be applied as chemotaxonomic tracers to identify organic matter from leaves of different mangrove species. After an initial rapid degradation, these markers leave a typical chemical imprint in mangrove sediments. It is important to consider the preservation potential of these markers ranging from weeks to thousands or even millions of years. The concentrations of dissolved inorganic nutrients (silicate, phosphate, ammonium and nitrite) indicate sources of water masses in tidal creeks on different time scales and also reflect autotrophic and microbial activity. Inorganic and organic nutrient flux calculations on daily, seasonal, annual and centurial time scales reveal the significance of the mangrove ecosystem for adjacent ecosystems, in particular the coastal shelf. Dissolved organic carbon (DOC) from mangroves contributes a major fraction of organic carbon which is exported to the coastal ocean while suspended matter and riverine DOC is rapidly removed from the water column within the estuary. Stable carbon isotopes show that mangroves represent the main source of terrigenous DOC in the open ocean off northern Brazil. Tidal range and porewater concentrations are driving forces behind coastal outwelling of nutrients and organic matter from mangroves. Molecular organic investigations can reveal the influence of microbial and photochemical decay. Sunlight efficiently destroyed aromatic molecules during transport offshore, removing about one-third of mangrove-derived DOC. The remainder was refractory and may thus be distributed over the oceans. On a global scale, it is estimated that mangroves account for >10% of the terrestrially-derived, refractory DOC transported to the ocean, while they cover only <0.1% of the continents’ surface. These numbers also emphasize the significance of mangroves for global biogeochemical cycles.