The tidal current influences Segara Anakan Cilacap estuary, in which the highest tide occurs during the full moon. During the high tide, the tidal current flows into the Segara Anakan lagoon through west and east Plawangan and mix at the current meeting place in the eastern of Motean, the most extensive mangrove area in Segara Anakan. This condition allows accumulation of litter (organic matter) that may trigger the decomposition process and change water quality. The water quality changes influence plankton community structure. This study purposes were to observe plankton community structure, water quality, and to analyze the correlation between plankton community structure (richness, abundance, and distribution) and water quality (temperature, water transparency, pH, salinity, dissolved O₂, and free CO₂) at the mixing water during full moon in Segara Anakan Cilacap. This research used survey method and purposive sampling. Sampling was conducted for three periods of full moon phases between October and December 2017 at six selected stations. Data of plankton structure community and water quality was analyzed descriptively whereas the relationship of both was analyzed using Principal Component Analysis. The result showed that plankton richness was relatively similar (8-9 species) at all stations, mainly Pleurosigma angulatum as a cosmopolitan species. The highest plankton abundance was found at Station 3 (2 653 individuals/l) mainly P. angulatum (1 606 individual/l) as dominant species.  Water quality of the mixing water showed that dissolved O₂ was ineligible to water quality standards threshold, mainly at Station 3 (2.23 mg/l), while free CO₂ concentration was within an ideal concentration (8.8 mg/l) as station experiencing litter decomposition process. The relationship between plankton community structure and mixing water quality was that the increase of phytoplankton abundance (mainly P. angulatum) towards Station 3 was triggered by rising free CO₂ due to litter decomposition process.

Novita Deffi Fahmiaty

Moh. Husein Sastranegara

Dwi Nugroho Wibowo