The role of hydrodynamic regimes and water properties on transports, retentions, and settlements of mollusc larvae at a lagoon and its adjacent open shore in Kai Islands, Indonesia

The study was carried out in the Hoat Lagoon and its adjacent open shore near Ohoililir Village at the West Coast of Kai Kecil Island, Kai islands, Indonesia, at 15 sampling dates between 25 August 2001 and 22 May 2002. The main objective of this study is to increase our understanding of the effects of hydrodynamic regimes and water properties on larval transport, retention, and settlement in a tropical lagoon. The following questions were addressed: How do the currents transport mollusc larvae to the lagoon? How many mollusc larvae are retained in the lagoon? How do water properties and turbulence intensity control the larval settlement? The investigations were guided by two main hypotheses: (1) The greater the larval supply and the longer the retention time of the larvae, the higher the settlement densities in the lagoon are. (2) The higher the turbulence intensity and the higher the seawater density, the smaller the settlement densities are. Most larvae introduced into the lagoon were advected from the Banda Sea (65 to 70 nm away from the study sites) (‘teleplanic larvae) by wind-driven currents during the northwest monsoon or by upwelling during the southeast monsoon. An average of about 38% of the water inflow and about 78 % of the imported larvae were retained in the lagoon after one tidal cycle. Most larvae that were retained in the lagoon settled and utilized the lagoon as a nursery ground. Due to the relatively short turnover time of the total water volume of the lagoon (about 19 hours), it is regarded as a well-flushed lagoon. About 29% of the total water volume is exported during a single outflow, i.e. it took approximately 3 ebb tides to exchange the water mass of the lagoon. This suggests that recruitment occurs the whole year round. Turbulence intensities and seawater densities were significantly negatively correlated with settlement densities.