BIODIVERSITAS ISSN: 1412-033X Volume 19, Number 3, May 2018 E-ISSN: 2085-4722 Pages: 1044-1053 DOI: 10.13057/biodiv/d190337 Seagrass beds as the buffer zone for fish biodiversity in coastal water of Bontang City, East Kalimantan, Indonesia฀ ADITYA IRAWAN 1,2,♥ , SUPRIHARYONO 1 , JOHANNES HUTABARAT 1 , AMBARIYANTO 1 1 Doctoral Program in Coastal Resources Management, Faculty of Fisheries and Marine Science, Universitas Diponegoro. Tembalang, Semarang 50275, Central Java, Indonesia 2 Faculty of Fisheries and Marine Science, Universitas Mulawarman. Jl. Gunung Tabur, Gunung Kelua, Samarinda 75123, East Kalimantan, Indonesia. Tel./fax.: +62-541-748648, ♥ email: aditya.irawan@fpik.unmul.ac.id฀ Manuscript received: 24 November 2017. Revision accepted: 15 May 2018. Abstract. Irawan A, Supriharyono, Hutabarat J, Ambariyanto. 2018. Seagrass beds as the buffer zone for fish biodiversity in coastal water of Bontang City, East Kalimantan, Indonesia. Biodiversitas 19: 1044-1053. The purpose of this study was to demonstrate the importance of seagrass beds as a buffer zone for fish biodiversity in a coastal ecosystem. The sampling of seagrass and fish were done in seagrass beds associated with mangrove (ST1) and coral reef (ST2). Sampling period was executed from May 2011 to May 2017. Sampling was done on the lowest tide with a six-month interval. Seagrass sampling was carried out using line transect, and fish sampling was performed using set net (belat local name) operated by local fisherman. The results showed that seagrass beds at ST1 composed of C. rotundata and E. acoroides and that at ST2 composed of C. rotundata, E. acoroides, H. pinifolia, H. minor and T. hemprichii. The total number of individual was 42574 divided into 112 fish species. There were 17 species (population: 15.18%) associated explicitly with ST1 and 39 species (population: 34.82%) specifically associated with ST2 while 56 fish species (population: 50.0%) were prevalently spread in both stations. This research proved the existence of seagrass beds as a buffer zone for fish biodiversity.฀ Keyword: Biodiversity, connectivity, fish, seagrass beds INTRODUCTION Seagrass is a macrophyte in coastal water (Billah et al. 2016). Structurally, the construction of seagrass beds provides nursery grounds for fish juvenile (Jones 2014), and coral fishes (Verweij et al. 2008). It also serves as feeding grounds (Coles et al. 1993; Erftemeijer and Allen 1993; Carroll and Peterson 2013) for economically high- value fishes (Hantanirina and Benbow 2013; Blandon and zu Ermgassen 2014), to provide food, food chain and biodiversity, and ecosystem service (Cucio et al. 2016) in supporting the coastal water productivity (Greening and Janicki 2006; Nadiarti et al. 2012; Bahlmann et al. 2015). Seagrass tends to be found close to mangrove forests and coral reefs (Short et al. 2011). This illustrates the importance of interaction in the coastal ecosystem (Hirst et al. 2016). Natural interactions within the coastal ecosystem protect the population's habitat (Unsworth et al. 2007). A combination of two habitats shows that mangroves specifically protects the coast and coral reef that forms a fringing reef for seagrass beds, which serve to control degradation of coral reefs in the long-term (Guannel et al. 2016). By understanding the importance of the role of seagrass beds, we should have been able to manage its sustainability, but in reality, the fact shows that the expanse of seagrass beds have decreased in many parts of the world (Hughes et al. 2009; Jones 2014) and seagrass beds have been globally threatened by anthropogenic factors and climate change (Rasheed and Unsworth 2011; Grech et al. 2012; Unsworth et al. 2012; Duarte et al. 2013; Shafer et al. 2014; Mazarrasa et al. 2015). This same thing happened in the coastal area in Bontang where the expanse of seagrass beds had also been decreased as the result of the damage covering around 321.3 ha area or as much as 45% of total seagrass beds area of 714 ha (DPKP Kota Bontang 2011). Despite the fact that the composition of seagrass beds (consisting of 9 species of total 12 species found in Indonesia) (Tomascik et al. 1997; P3IK/P3O-LIPI 2000; DPKP Bontang 2015) had been known, it is a mangrove forest. Coral reef has become the priority in conservation management, and the critical role of seagrass beds in supporting fishery resources and the productivity in Bontang coastal water tends to be neglected.฀ This study aimed to demonstrate the importance of seagrass beds as the buffer zone for fish species biodiversity in coastal ecosystem especially the association between seagrass beds, mangrove, and coral reef, to initiate an integrated and continuous management system in the coastal ecosystem. This study focused on fish species composition and the distribution of fish length size and its interaction with species characteristic, the density of seagrass stand and the association between seagrass beds and mangrove and coral reef in Bontang coastal water. Mangrove forests and coral reef are creating the importance of interaction between coastal ecosystems. The natural interaction of coastal ecosystem provides protection for population habitat or combination of two habitats which shows that mangrove specifically provides protection to the coast and coral reef that forms a fringing reef for seagrass