Water Environment in Little Sand Bay

Yvonne Hsieh       Aug, 2008

In this camping trip, there were three instruments that I was using, i.e. wind sensor, temperature sensor, and dissolved oxygen (DO) sensor. The measurement period was from Jul 15th to Jul 19th.

(1) Wind

Wind forcing is the most important factor governing mass water movement. Variations in wind speed accounted for 93% of the variation in near-surface current speed (George, 1981). Wind induced mixing is a very important mechanism for the deepening of a mixed layer. Wind mixing is mainly caused by two processes, production of turbulent kinetic energy due to shear and injection of turbulent kinetic energy due to breaking of surface waves (Burchard, et al., 1999). The latter effect causes however significant additional mixing close to the surface only (Craig, et al., 1994).

Fig. 1 Locations of the deployments	Fig. 2 wind sensor

The wind sensor was located on the dock close to the inlet of the harbor (see Fig. 1, denoted by a red dot). The height of the wind sensor is 11 ft above the water surface. The wind speed and direction are plotted in Fig. 3. The wind measurement in Devil's Island where is the closest weather station to the Little Sand Bay are as shown in Fig. 4. 

Fig. 3 Wind data in the Little Sand Bay.

Fig. 4 Wind data in the Devil's Island.

(2) Water temperature

Continuous temperature profiling was collected at three locations using Onset HOBO Water Temp Pros at time interval of 1 minute. The two thermal chains inside and outside of the bay are at the spots with the water depth of 2 m (the red dots in the Fig. 1).  Another thermal chain was deployed outside of the bay where the water depth is about 10 m. Fig. 5 shows the colormaps of water temperature inside and outside of the bay where the water depth is 2m. The water inside of the bay was generally warmer than that outside of the bay.

Fig. 5 Colormap of water temperautre.

The water temperature time series for the two 2-m thermal chains inside and outside of the bay are plotted in Fig. 6. In general, the water temperature were correlated well with the air temperature (see Fig. 7). It can be noticed that there were some local stratification events occurring outside of the bay during the study period, e.g. Jul 17th (in the afternnoon) and Jul 19th (during the midnight). The bottom water temperature decreased faster than that in the top layer. One possible reason to cause the rapid decrease in bottom temperature could be the supply of cold water from the offshore. Fig. 8 is the current data measured by ADCP. It indicates that the flow during these two events had similar patterns.

Fig. 6 Time series of water temperature.

Fig. 7 Time series of air temperature.

Fig. 7 ADCP data.

(3) DO

Dissolved oxygen (DO) was attached with the thermal chain inside of the bay at the water depth of 1 m. The time series of DO concentration is displayed in Fig. , which shows  the diurnal variation and peaks occurring in the afternoon when the solar radiation is strongest.  

Fig.  DO concentration.