## 3 Results and Discussion  
### Moored ADCP  
Water temperature in the observed area varied between 24-27 \({}^{\circ}\)C during the day and night time, average salinity 33 psu, and average pH 8.03. The water temperature was relatively constant during day and night throughout the observation time [39]. The observation was carried out during the wet season. The results of the MVBS time series, the vertical distribution of MVBS and the monthly average water temperature in the survey area are shown in Figure 3.  
The time series MVBS data from the surface to a depth of 11.5 m were filtered because the values were below \(-\)100 dB. A high level of MVBS mainly occurred at the near bottom of the observed depth, indicated as the SSL. Near the surface and middle columns layer, the MVBS value was much lower when compared with the near-bottom area. Overall, the MVBS value in the SSL from the time series data ranged between \(-\)100 dB to \(-\)60 dB. The highest MVBS value was between \(-\)95 dB to \(-\)70 dB at 12 m to 16 m of depth.  
The extracted data ranging from 11.5 to 16 m were indicated as an SSL. The zooplankton behavior based on the MVBS pattern was moved up and down as shown in the MVBS echogram and classified as the diurnal cycle. The echogram indicated that zooplankton migrated upward during the night time in 14-16 m of depth. The more detailed look in Figure 4 with the black and white strips below the echogram represents night (black) and day time (white), allowing the identification and analysis of oscillatory MVBS value from moored ADCP time series data.  
All MVBS data selected for a daily pattern of zooplankton vertical migration were in depths between 11.5 and 16 m. Based on time, there was an increase and decrease in the value of MVBS in the morning and night time. The yellow to red color variation indicates zooplankton vertical migration. The red color represents the larger MVBS value produced by a high density of zooplankton compared to the density that is represented by a blue color. MVBS in the center of the SSL gradually moved down in the daylight and went up at night (Figure 5). On 5, 16, 22, 23 April the MVBS value was slightly lower than other days, ranged from -95 to -85 dB, and it appears that the weak acoustic signal is marked in blue color. This was probably due to zooplankton moving away from the observed area, indicated by MVBS values during that time [40; 41; 42].  
Generally, the zooplankton migrated to the top of the SSL at midnight, as shown in Figure 5. The zooplankton leaving the deeper layer started two hours before sunset, ascended to a shallow depth at night time and settled on the top layer, and moved again to the deeper layer starting at sunrise. The MVBS showed changes around 05:30 and 18:00. In the early morning, the strong MVBS at the 12 m of depth propagated downward to a deeper depth of 15.5 m, while in the twilight time the strong MVBS propagated upward. This pattern, eventually, was general daily vertical migration [43] but in some cases, zooplankton was not moving to the surface area of the water column, due to sunlight penetration to the water column. There was research for light intensity as one of the factors controlling  
Figure 4: Extracted MVBS of moored ADCP data at 11.5-16 m. The black bar below the echogram indicated the night time while the white bar indicated the daylight time.  
Figure 3: Time series (26 days) of filtered MVBS, the distribution of MVBS, and monthly average temperature. Yellow to red color in red square dashed line in 12–16 m of depth indicates the zooplankton movement during one cycle of the sun.  
vertical of migration [44; 45]. Another study was found that detritus near the seabed might be a possible food source for zooplankton [46].