Synecology of seagrass ecosystem due to increased anthropogenic pressure: case study in barranglompo and bonebatang islands of spermonde archipelago, South Sulawesi

Abstract

Anthropogenic pressures along coastal areas particularly in small islands have increased in accordance with increasing of human population. A study has been conducted in two small islands within Spermonde Archipelago, South Sulawesi i.e. Barranglompo and Bonebatang Islands to elaborate impacts of increased anthropogenic pressure to the assemblages of seagrass beds and their associated organisms. Combined field sampling and laboratory analysis were applied to provide data and information on the impacts of anthropogenic activities occurred in small islands to the seagrass ecosystem. In Barranglompo Island, there were two activities with high intensity i.e. ship/boat transportation and domestic sewage disposal. Analysis of carbon contents of seagrass Enhalus acoroides showed that this species contributed to carbon stocks as much as 0.49-1.05 ton/ha in Barranglompo Island, while in Bonebatang Island, the values ranged between 0.08-0.34 ton/ha. Meanwhile, nitrogen concentrations in Barranglompo Island were significantly higher than those in Bonebatang Island. This was supported by lower C:N ratios and higher N:P ratios in Barranglompo Island and significantly different nitrate contents of water column between both Islands (p  0.01). Potential anthropogenic activities affecting nutrient composition in Barranglompo Island were domestic sewage disposal and drainage of liquid household sewage. Based on the water quality parameter measured, It was concluded that anthropogenic activities in Barranglompo and Bonebatang Islands affected turbidity and Total Suspended Solid (TSS) only. Seagrass communities in Barranglompo and Bonebatang Islands were composed of eight and seven species, respectively, that grow in mixed community. Thalassia hemprichii was species with the highest important value index in both islands. Most of macroalgae found in both islands live as epilithic (litophytic) and epipelic (rhizophytic). Three positive and two negative associations in Barranglompo and one positive and five negative associations in Bonebatang were formed among pairs of macrophytes. Analysis of macrophyte structure in both islands classified Barranglompo in moderate ecological status, whereas Bonebatang had high ecological status. This status indicated that in Barranglompo was detected a change in macrophyte composition due to anthropogenic pressure. The highest similarity index was found among stations in the same island indicating that different islands have different macrophyte assemblages. Among all sea urchin species found in both islands, Tripneustes gratilla and Diadema setosum were two dominant species having the highest density. Index of Preponderance revealed that T. hemprichii was the largest seagrass diet within almost all sea urchin guts. Electivity index indicated that sea urchins prefer several seagrass species especially T. hemprichii.