Study of Oceanographic Environments of sea of Harima during Red Tides period by means of Multi Platform Remote Sensing

Study of Oceanographic Environments of sea of Harima during Red Tides period by means of Multi Platform Remote Sensing

Seijiro Hayakawa, Takeshi Doihara, Kiichi Hirono
Asia Air Survey Co., Ltd. Tokyo, Japan

Abstract
Red tides due to Chattonella-antiqua were the cause of damages to the young Yellow Tail raising industries in Sea of Harima, Japan, during the summer (from July through August) of 1987.

Recently, Red tides due to C. antiqua, frequently occur all over Sea of Harima. Oceanographic environmental parameters which are responsible for the explosive increase in C. antiqua, however, have not yet been identified.

The difference between sea surface temperature (SST) distribution during two periods, the summer of 1987 and 1985 when Red tides did not occur so much, was studied by means of comparison between NOAA AVHRR data. And weather observation date during those two periods was also compared.

To analyze NOAA AVHRR data acquired in different stage of tides, SST distribution changes due to the ebb and flow was estimated by means of Airborne Multi Spectral Scanner (MSS) data (thermal channel) acquired in time to the tidal cycle.

It is clearly seen on NOAA AVHRR data that the homogeneous water body spread all over Sea of Harima at the period of bloom in 1987. It is noticed that oceanic environments such as upwelling from the bottom due to the strong winds, mixing and spreading of eutrophic water with a plenty of sunlight during the summer of 1987 were different from those of 1985. And those conditions are considered to be necessary to cause Red Tides due to C. antiqua in Sea of Harima.

Intorduction
Sea of Harima is located in the eastern part of the Seto Inland Sea, Japan (Fig. 1). This area is one of the most eutrophic coastal sea in Japan.

Recently, Red tides due to C. antiqua frequently occur all over Sea. of Harima. During the summer (from July through August) of 1987, Red tides due to C. antiqua caused great damages to the young Yellow Tail raising industries in Sea of Harima.

As increase in occurrence of Red tides, the many kinds of study to analyze the relation between oceanographic environments and occurrence of Red tides have been done. FUKASE et al., (1980F) concluded that the upwelling due to the water flow through Kii Channel caused Red tides. And YANAGI (1980) concluded that the upwelling due to the wind caused Red tides. The most important factor of explosive increase in C.antiqua, However, have not yet been cleared. To study Red Tides phenomena, it is necessary to analyze Red tides from an extensive environmental aspect.

Selecting SST as one of parameter representing oceanic environments, the difference between SST distribution during two periods, the summer of 1987 and 1985 when Red tides did not occur so much, was studied by means of comparison between NOAA AVHRR data.

To compare these NOAA AVHRR data acquired in different stage of tides, the extent of mixing area around Naruto Channel due to the ebb and flow had been estimated by Airnorne MSS data collected at times of slacks and maximum tidal streams

Upwelling associated with the wind was mentioned as an important factor for occurrence of Red tides by YANAGI(1981), IOI et al., (1981). The difference between weather condition which may have caused upwelling during the summer of 1987 and /or 1985 was studied.

Data used for analysis and methods

NOAA AVHRR data

Cloud free NOAA AVHRR data obtained during the summer of 1987 (Red tides occurred) and 1985 (not occurred) were used to compare SST distribution and a horizontal extent of mixing area in the surface layer around naruto Channel due to the ebb and flow. The data used for this study are listed on table 1. IN order to compare the difference between oceanic environments during two periods, 23 images of NOAA AVHRR data from 15 JUL to AUG 1985, and 14 from 8 JUL to 30 AUG 1987, were analyzed.
Airborne MSS data

The mixing water body formed around the Naruto Channel due to the ebb and flow affects to SST distribution in Sea of Harima. As each SST distribution estimated from NOAA AVHRR data shows oceanic feature in different tide stage, the maximum extent of mixing area is difficult to know.
Eather observation data

Nutrients and benthic C. antique cells are often supplied into the photic zone form the bottom by upwelling which is one of the major mechanisms supporting the high productivity of coastal waters. YANAGI ( 1981) and IOI et al., (1981) discussed that upwelling especially due to wind blows is one of the most important factor for occurence of Red tides. So, the weather condition around Sea of Harima (hours of sunlight, precipitation, wind speed and wind direction) were compared during the summer (July-August ) of 1987 with those of 1985.

These data had been observed at Tokushima (see Fig. 1)

Comparison of data and its results

The relationship between SST distribution and its change due to the ebb and flow

The maximum extent of mixing area due to the ebb and flow is shown in Table 3. According to ariborne MSS, the front the maximum mixing area is located in 15km north and 22km south around the Naruto Channel.

This result means that mixing area due to the ebb and flow around Naruto Channel doesn't so much affect to the oceanic environment at the center of Sea of Harima.
Difference of SST distribution during two periods, the summer of 1987 (Red tides occurred ) and 1985 ( not occurred)

Comparing SST distribution obtained from NOAA AVHRR data during to periods, the summer of 1987 with 1985, the following were emphasized:
1. Pattern of SST distribution during the summer of 1987 was more homogeneous than that the 1985. During July of 1987, minimum and maximum SST in Sea of Harima were approximately 24. c and 26. c respectively. And at the beginning of August, 26. c and 29. c. At the end of August , 25. c and 28. c. Namely, the range of difference was approximately from 2. c to 3. c. In 1985, minimum and maximum SST were 24. c and 29. c and the range of difference was from 4. c to 5. c.
2. During the summer of 1985, the clockwise moving pattern of warm water body was observed, while it was not in 1987,
3. An extent of mixing area around Naruto Channel during the summer of 1987 was almost the same as 1985.
Difference between weather condition during two periods, the summer of 1987 and 1985.

Fig. 3 shows the weather condition around Sea of Harima in July and August of 1985 and 1987. From these data, the following were emphasized:

In the summer of 1987, at previous period of the bloom (11-16) July), the range of the wind speed which blew from south-south-east at Tokushima was from 5 to 7 m sec-1 . It is considered that strong winds were blowing over the Sea of Harima. When the young Yellow Tail raising industries were damaged exceedingly (from the end of July to the beginning of August ) , wind speed was weaker and hours of sunlight was ling every day.
In the summer of 1985 (From 1 of July to 5 of August),, the winds was weaker than that of the same period of 1987.

Discussion
According to weather observation data, strong winds were blowing over the Sea of Harima at previous period of the bloom of 1987. Consequently it was possible that this wind might have caused the upwelling of bottom water which contained nutrients and benthic C. antiqua cells abundantly and the mixing near the surface. After that, wind speed was weak and hours of sunlight was long. So, the mixed eutrophic water with benthic C. antiqua cells might be spread in the surface layer with a plenty of sunligh. On the other hand according to NOA AVHRR data, the pattern of SST distribution obtained during the summer of 1987 was more homigeneous, and the difference of minimum and maximum temperature was approximately 2 to 3' c while it was 4 to 5 c in 1985. It is considered that these sea condition that cannot deny the above-metioned inference, can be responsible for the rapid growth to C. antiqua.

During the summer of 1985, the winds might not be so strong enough to cause upwelling. And on account of the existence of the clockwise moving pattern clearly seen in 1985, spreading of homigeneous water which was mixed with eutrophic bottom water can not be admitted. It is considered that these weather and sea condition were not suitable for the explosive increase in C. antique.

Conclusion
Using Multi-Platform remote sensing and weather observation data, oceanographic environments of Sea of Harima in the summer of 1987 (Red tides occurred) were compared with these of 1985 ( not occurred). As a results of this study, it is noticed that oceanic environments such as upwelling from the bottom due to the strong winds, mixing and spreading of eutrophic water with a plenty of sunlight during the summer of 1987 were different from those of 1985. And those conditions are considered to be necessary to cause Red Tides in Sea of Harima.

In the case of this study, there was no other data useful in regard to ocean environments at the previous period of C. antiqua blooming, but NOAA AVHRR data and weather observation data. Therefore, simultaneously obtained data by shipboard, such as, nutrients, salinity, vertical temperature distribution, with remote sensing were expected to indentify the relationship parameters in the future.

Acknowledgement
The authors wish to express their gratitude to Dr. Minoru Fujimoto of Nansei Regional Fisheries Research Laboratory. This report represents part of the results obtained through the project titled "The study of development of techniques for Red tides" funded by the Fisheries Agency of Japan.

References

FUKASE et al., ( 1980) : Red tides due to C. antiqua in Sea of Harima in 1978. Papers in J. Oceanogr. Soc. Japan, APR., 1980, 283 - 284.
YANAGI ( 1980) : Ocean environments related to the occurrency of Red tides in Sea of Harima I. Papers in J. Oceanogr. Soc. Japan, OCT, 1980, 61-62
YANAGI ( 1981) Ocean environments related to the occurrence of Red tides in Sea of Harima, II. Papers in J. Oceanogr. Soc. Japan, APR, 1981, 82-83
IOI et al., ( 1981) : Red tides in Sea of Harima. Papers in J. Oceanogr. Soc. Japan, OCT, 1981 254-255

Table 1NOAA AVHRR data used in this study

1985		1987
JUL. 15 23 24 25 26 27 28 29 30 31	AUG. 1 2 3 13 14 17 18 19 20 21 22 26 29	JUL. 8 23 24 25 26	AUG. 3 11 15 16 17 19 28 29 30

Table 2 Airborne MSS data used in this study

Date	Observation time	Tidal Stage	Tidal Stream
AUG. 31'88	8:20 - 9:30 11:40 - 12:25 14:50 - 15:42 17:45 - 18:33	Between Neap and VSpring tide	Maximum northern slackMaximum southernSlack
OCT.1 OCT. 2 OCT.3	15:55 - 16:55 16:55 - 17:15 09:30 - 10:17 13:07 - 13:55	NeapTide	Maximum southern Slack SlackMaximum northern
OCT.8 OCT.9 OCT.27 NOV. 23	16:00 - 16:55 14:15 - 15:03 13:10 - 14:00 09:00 - 10:00	SpringTide	Maximum northern SlackMaximum southernslack

Table 3 Distance of thermal front location of mixing area from Naruto Channel

Tidal srteam \ Tidal stage	Maxaimum nortern	Slack	Maximum Southern	slack
Neap tide	* N 132 (Km)	N 15 _	N 14 _	Indistinct S 20 - 21
Between neap and Spring tide	N7.5 N 5 - 7	N 8.5 _	S 17.5 S12 - 14	S 22 S18 - 21
Spring tide	N 12.5 -	N 15 S 7 - 8	S 14 - 11 _	S 5 - 11 _

* N: From Naruto Channel to Sea of Harima
S: From Naruto Channel To Kii Channel
Upper: Airborne MSS data
Lower: NOAA AVHRR data