Observation of chlorophyli-a distribution in the Suruga Bay using the underwater tow device-SCAN FISH

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Objective of our project was to develop appropriate methods to study phytoplankton distribution and spring bloom mechanism in the Suruga Bay as well as in open ocean waters off eastern Japan. We initially used the underwater tow device-SCAN FISH observation system (MACArtney A/S, Denmark) in the open ocean near Japan onboard RV BOSEI-MARU (1777t) of the Tokai University, to record vertical profile of temperature, salinity and chlorophyll-a fluorescence in the upper water column. Later we made necessary operational modification in the system to make it usable from smaller platforms such as fishing boats, in the Suruga Bay.Besides, we analyzed 3-year time series (1999-2001) of satellite derived ocean color and SST date respectively from Orb View2/Sea WiFS and NOAA/AVHRR to monitor spatial and temporal change in sea surface thermal field and its impact on near surface chlorophyll-a concentration in the Suruga Bay. We further developed empirical method to derive vertical profile of chlorop … More hyll a from the near surface pigment concentration for implementation on satellite data. For this purpose in situ water samples were collected during the cruises at surface and at different depths within 100m and chlorophyll a concentration was analyzed using HPLC technique.From the time series data it was noticed that beginning of the spring bloom in the Suruga Bay was delayed by one month during 2000 compared to 1999 and 2001. Our results based on different oceanographic and meteorological data in and around the Suruga Bay indicated that water column stratification started in the late-February in 1999 and 2001. However, during the late-February of 2000 such stratification got disrupted under strong influence of local wind creating deep mixed layer. Such condition did not allow phytoplankton to develop the spring bloom. Our results clearly indicated that vertical stratification is an important parameter in spring bloom phenomenon in the bay. We further used the SCAN FISH, to observe vertical profiles of different oceanographic parameters along with chlorohyll-a fluorescence to understand the mechanism of the bloom phenomenon. Vertical distribution of chlorophyll-a within upper 30m of the water column was noticed to be more strongly influenced by salinity profile than that of water temperature especially in the western sector of the bay. Strong relationship between vertical profile of salinity and chlorophyll-a underlines the importance of density stratification in the spring bloom phenomena in side the bay. This can be attrebuted to influx of plentiful freshwater into the bay from four major rivers along the western margin. Less