A trophic flow model of the Segara Anakan lagoon, Cilacap, Indonesia

Abstract

Segara Anakan is a large lagoon located along the southern coast of Java, Cilacap district, Indonesia, fringed by the only large estuarine-mangrove forest left in Java. This thesis evaluates the changes in the Segara Anakan lagoon (SAL) ecosystem by using a trophic modelling approach based on data derived from the identification and evaluation of habitat and resource abundance changes in the SAL over the last 25 years. Complementary datasets i.e. satellite images, field data and secondary data were used to determine these changes. The SAL ecosystem was mapped from 1978 through 2004 using satellite images and a GIS package to determine coastal habitat changes. The main changes involved the conversion of a large part of the estuary to new land (2966.8 ha) and mangrove area (3497.2 ha), and the subsequent conversion of these areas, and of older sections of the mangrove forest, to rice agriculture, semi-intensive fishponds, new settlements and other land uses (11315.6 ha). The major changes have occurred in the western area of the SAL (Ujung Alang, Ujung Gagak, Klaces and Penikel), both in extent and spatial distribution of some vegetation classes, with large area increases in rice fields, dry land agriculture, aquaculture and settlement area, and decreases in mangrove cover, mud flat and lagoon area. In the eastern area (Kembang Kuning, Sapuregel and Donan), changes are less extensive, but some of the trends verified in the western area can also be identified such as decreases in mangrove area and increases in aquaculture, settlement and industrial areas. Trophic flow models of the SAL system were developed in order to evaluate the changes in the ecosystem between the decades of 1980 and 2000. The models were structured by defining 21 functional trophically linked groups of the ecosystem including commercially important fish groups, shrimps and bivalves. The mass-balanced models for the two periods show how the different groups of the system were affected by the habitat transformations. Estimates of biomass per trophic level, mixed groups impact, ecotrophic efficiency, ascendancy and several other ecosystem attributes derived from the trophic modeling analysis show that trophic functioning as well as average catches of the SAL ecosystem differed between the two periods. Model biomass and mean annual production of commercially important fish and shrimp groups were smaller in the 2000s as were the total catches. Primary production required to sustain catches in SAL ecosystem was 250.19 t km-2 y-1 in the 1980s and 391.74 t km-2 y-1 in the 2000s. Developing a trophic flow model for the sustainable management of the SAL lagoon�s living resources requires an integrated understanding of ecosystem functions and their changes related to human activities and environmental change. These results indicate (i) high temporal and spatial changes of coastal habitat and its resources and (ii) changes in trophic functioning as well as average catches in the SAL ecosystem. The observed changes are both due to human activities (such as mangrove conversion, resources exploitation, and pollution) and environmental impact in the study area (mainly through the filling of the lagoon with riverine sediments derived from the hinterland). Management measures of the lagoon�s resources applied for many years suffer from the lack of an integrated ecosystem understanding, and hence, had little success. The results of this study are expected to contribute to a more sustainable management and conservation of the SAL ecosystem.