Mangroves density in the porong estuary

Abstract

The Porong Estuary is an estuarine ecosystem influenced by the Porong River’s interaction with the Java Sea. This estuary plays a significant ecological role, acting as source that provides nutrients and organic materials that are then transported through the river and mixed in with the tidal currents. Some aquatic biota, for example crabs (Scylia serrata), oysters (Crassostrea cucullata), shrimps, and fishes utilize this estuary as a shelter, a nursery ground, and a source of food. Furthermore, because it is surrounded by numerous fish and shrimp ponds, this estuary is also relied upon by thousands of people. In response to the Sidoarjo mudflow disaster that occurred on May 29, 2006, the Porong River has been used as a channel diverting the mudflow to the Java Sea since 2007. This channeling activity will potentially increase the sedimentation rate and decrease the water quality of the Porong Estuary. Therefore, to investigate the potential impact of mudflow channelling activity and provide time series data, the Institute for Marine Research and Observation (IMRO) has been monitoring the ecological condition of the Porong Estuary regularly since 2009; this includes estimating the mangroves density. The estimation of mangroves density was carried out based on remote sensing data. In 2003, 2007, and 2009, the data used was Landsat ETM+. Meanwhile, in 2010, the estimation was done based on WorldView-2 imagery. By comparing the results gathered between 2003 to 2007, before being used as a channel to divert the mudflow, the surface area of mangrove forests in the Porong Estuary decreased significantly, i.e., 666.709-435.365 ha. This generally occurred to mangroves with high and moderate density. On the other hand, from 2007 to 2009 the mangrove area started to increase again, going from 435.4 to 550.1 ha. By 2010, its area continued to increase reaching 654.2 ha. In general, from 2007 to 2010, mangroves with moderate and high density increased significantly from 33.964 to 256.81 ha and from 26.37 to 344.27 ha, respectively. These results show that channelling activity of the Sidoarjo mudflow to the Porong River has provided more substrate to the Porong Estuary area. Moreover, the coastal dynamics around the Porong Estuary give mangroves an opportunity to grow, especially in areas where sediment transport is dominant.