Herlambang Aulia Rachman

Although researchers have investigated the impact of Indian Ocean Dipole (IOD) phases on human lives, only a few have examined such impacts on fisheries. In this study, we analyzed the influence of negative (positive) IOD phases on chlorophyll a (Chl-a) concentrations as an indicator of phytoplankton biomass and small pelagic fish production in the eastern Indian Ocean (EIO) off Java. We also conducted field surveys in the EIO off Palabuhanratu Bay at the peak (October) and the end (December) of the 2019 positive IOD phase. Our findings show that the Chl-a concentration had a strong and robust association with the 2016 (2019) negative (positive) IOD phases. The negative (positive) anomalous Chl-a concentration in the EIO off Java associated with the negative (positive) IOD phase induced strong downwelling (upwelling), leading to the preponderant decrease (increase) in small pelagic fish production in the EIO off Java.

The Banda Sea region is one of the locations in Indonesian waters that have high Coastal Upwelling intensity which related to the monsoon pattern. The calculation of Upwelling Index (UI) based on Wind data show that the peak coastal upwelling is from June to September. Analysis of SST trends was carried out in the July-September period based on NOAA OISST data from 1982-2017. The results show that there are differences in the value of SST trends that occur in the Coastal and Oceanic regions. In general, the SST trend in the Banda Sea waters has a positive value (warming) in both the coastal and ocean areas. While the UI trend in coastal and oceanic regions in the Banda Sea tends to decline from the 1982-2017 period. This condition shows that external factors quite influence oceanographic conditions in Banda Sea waters. It is also thought to have caused a decrease in the intensity of the upwelling trend found in the Banda Sea. The results show that the SST trend in the Banda Sea tends to increase during the peak upwelling season (June-September). Meanwhile, the Upwelling intensity trend shows a decreasing pattern which is also confirmed by decreasing trend in chlorophyll concentration.

Teluk Benoa merupakan daerah estuari semi tertutup yang terdapat di wilayah selatan Bali. Kawasan ini merupakan daerah yang menjadi muara bagi beberapa sungai besar yang terdapat di Bali. Sungai merupakan salah satu sumber aliran material seperti angkutan sedimen yang dialirkan menuju ke laut. Proses transpor sedimen di Teluk Benoa dapat didekati dengan pemodelan numerik FVCOM (Finite Volume Coastal Ocean Model) untuk mengetahui distribusi sedimen yang terdapat di daerah teluk. Hasil model numerik menunjukkan bahwa proses pergerakan sedimen akan mengikuti bentuk pola arusnya. Pola pergerakan arus di Teluk Benoa akan mengikuti pola pasang surutnya dimana saat akan pasang pola arus akan mengarah ke dalam teluk dan sebaliknya pada saat surut. Kecepatan arus rata-rata pada bagian mulut mencapai 0.8 m/s pada saat menuju pasang dan 0.9 m/s pada saat menuju surut. Besar kecepatan arus akan mengalami perlambatan pada saat kondisi pasang tertinggi dan surut terendah yakni hanya mencapai rata-rata sekitar 0.3 m/s. Konsentrasi sedimen tertinggi pada saat pasang atau surut terjadi pada bagian dekat muara sungai yang mencapai 100 mg/L dan mulut teluk yang mencapai 150 mg/L. Tingginya konsentrasi sedimen di mulut teluk diakibatkan adanya erosi pada bagian dasar perairan akibat kecepatan arus yang cukup tinggi. Hasil validasi elevasi pasang surut pada model didapatkan root mean square error (RMSE) sebesar 9.18 cm dengan koefisien determinasi (R²) mencapai 0.97. Untuk perbandingan hasil model sedimen dengan data observasi berupa Total Suspended Solid memiliki tingkat korelasi sebesar 0.77. STUDY OF SEDIMENT TRANSPORT AT BENOA BAY USING NUMERICAL MODELLING Benoa Bay is a semi-enclosed estuary area located in the south of Bali. This area is became estuary for several rivers in Bali. The river is a source of material such as sediment transport stream that flowed into the sea. Sediment transport processes in Benoa Bay can be approximated by numerical modeling FVCOM (Finite Volume Coastal Ocean Model) to determine the distribution of sediments found in the bay area. Research carried out by the whole domain Benoa Bay area in January 2016. The results of numerical models show that the process of sediment movement will follow the current pattern shape. The pattern of the movement of currents in the Benoa Bay will follow the pattern of tide where the current will flow lead into the bay at high tide and vice versa. Average flow velocity at the mouth reaches 0.8 m/s at the time to the tide and 0.9 m / s at the time towards low tide. Current speed will slow down when the condition of the highest and lowest tide that is only reached an average of about 0.3 m/s. The highest sediment concentrations at high tide or low tide occurs in the section near the mouth of the river that reached 100 mg/L and the mouth of the bay, which reached 150 mg/L. The high concentration of sediment at the mouth of the bay due to the erosion of the bottom waters due to the flow velocity is high. The results of the validation by tidal elevation models obtained root mean square error (RMSE) of 9:18 cm with a coefficient of determination (R²) reached 0.97. For the comparison of model results with observational data in the form of sediment Total Suspended Solid has a level of correlation of 0.77. Keywords: Benoa Bay, Sediment, Numerical modelling