Field and model studies of the fate of particulate carbon in mangrove-fringed Hinchinbrook Channel, Australia

Abstract

A field and model study was undertaken in 1996/1997 of the dynamics of water, fine sediment and particulate carbon in the northern region of the mangrove‐fringed Hinchinbrook Channel, Australia. The currents were primarily tidal and modulated by the wind. Biological detritus acted as a coagulant for the fine cohesive sediment in suspension in the mangrove‐fringed, muddy coastal waters. Plankton and bacteria were the major aggregating agents at neap tides, and mangrove detritus at spring tides. The micro‐aggregates were typically several hundreds of micrometer in diameter and enhanced the settling rate. The fate of fine sediment and particulate carbon was controlled by the dynamics of the coastal boundary layer, a turbid shallow coastal water zone along the mangrove‐fringed coast. A tidally‐modulated, turbidity maximum zone was found in this layer. Wind stirring increased the turbidity by a factor of five. The channel behaves as a sink trapping fine sediment and particulate carbon. However, the sink was ‘leaky’ because the dynamics of the coastal boundary layer generated a net outflow of fine sediment out of the channel along the western coast. The biologically enhanced settling of cohesive sediment limited the offshore extent of the muddy suspension to within a few hundreds of meters from the coast. At spring flood tides, some of this particulate carbon was advected into the mangrove forest where it would remain trapped. On a yearly basis about six times as much particulate carbon was exported out of Hinchinbrook Channel through the coastal boundary layer than was trapped in the fringing mangroves.