Nitrogen and Phosphorus Distributions across the Thermohaline Front in Kii Channel in Winter

Transcription

1 Journal of Oceanography Vol. 49, pp. 407 to Nitrogen and Phosphorus Distributions across the Thermohaline Front in Kii Channel in Winter ICHIRO YUASA, EISUKE HASHIMOTO and HIDEKI UESHIMA Government Industrial Research Institute, Chugoku, Hiro Suehiro 2-2-2, Kure-city , Japan (Received 6 May 1992; in revised form 16 November 1992; accepted 17 December 1992) We have studied nitrogen and phosphorus distributions across the thermohaline front in Kii Channel in winter by using engine-cooling sea water of a ferry boat. On Dec and Jan. 1987, differences of PO 4 -P and DIN across the front are recognized. Especially in the latter case, differences of nutrients concentrations across the front are very obvious. But differences of nutrients across the front on Feb. 1986, Feb. and Mar are not obvious. Inspite of winter, Akashiwo had happened in Osaka Bay, nutrients mostly have already been utilized by phytoplankton in inner part of Osaka Bay. Consequently, differences of nutrients concentrations across the front are nearly zero. 1. Introduction It is well known that a thermohaline front develops on the shelf area of coastal bay in winter. Examples have been observed in Seto Inland Sea and Tokyo Bay among others. Such thermohaline fronts are generated in a transition zone between colder coastal (less brackish) water and warmer oceanic (more brackish) water. Ichie (1951) was the first to point out that a prominent thermohaline front exists in the Kii Channel. Yoshioka (1988) had studied oceanographical structure of the Kii front. Yanagi (1980) showed the existence of a similar front in the Sea of Iyo. Nagashima and Okazaki (1979), Saino (1988) and Yanagi et al. (1989) studied a thermohaline front in Tokyo Bay. These fronts are at transition zones between water masses of a different origin. But distributions of nitrogen and phosphorus across the front are so far not known. Saino (1988) reported that there are differences of nutrient concentrations across the front in Tokyo Bay and the surface coastal water sinks at the front and intrudes into the middle layer of offshore water spreading into a skirt-like shape under the front. But no data has been reported about the thermohaline front of Seto Inland Sea. In order to understand nutrient transport mechanism between Seto Inland Sea and oceanic water, data of nutrient distributions across the front are very important. Therefore, we have studied nitrogen and phosphorus distributions across the thermohaline front in Kii Channel by using engine-cooling sea water of a ferry boat. 2. Methods and Observations The course of the ferry boat is shown in Fig. 1. In Fig. 1 K and O means the typical location of the thermohaline front in Kii Channel and tidal front in Osaka Bay, respectively (Yoshioka, 1988; Yanagi et al., 1988; Yuasa et al., 1992). The course of the ferry boat crosses these fronts. The water in the sea chest was replaced by fresh sea water because of strong suction by the enginecooling pump. So, we sampled 1 liter of engine cooling water at a depth of about 4 m. After sampling, we measured water temperature immediately and salinity was measured by electric-

2 408 I. Yuasa et al. Fig. 1. Observation course of the ferry boat. Dotted lines mean pattern of fronts. K: Kii Channel thermohaline front. O: Osaka Bay tidal front. Stn. A and B: observation station of vertical profile of T, S by Tokushima Prefectural Fisheries Experimental Station. Stn. O-1 ~ 5 and C-1 ~ 4: observation station by Kobe Marine Observatory. Table 1. Date of observations and sampling interval. Date Sampling interval min 8.7 km conductivity meter of Auto-Lab in laboratory. Concentrations of nitrogen and phosphorus in their chemical forms were analysed by chemical analyses in the laboratory as follows: For chemical analyses we brought water samples back to our laboratory stored cold. Water samples for analyses of NH 4 -N, (NO 2 -N) + (NO 3 -N), PO 4 -P and DTP (Dissolved Total Phosphorus) were filtered with Milipore HA filter (0.45 µm)

3 Nitrogen and Phosphorus Distributions across the Thermohaline Front 409 into 250 ml polypropyrene bottles. NH 4 -N was determined by an indophenol blue method (Liddicoat et al., 1975), (NO 2 -N) + (NO 3 -N) by the Cu-Cd column reduction method (Strickland and Parsons, 1968) and PO 4 -P by the molybdenum blue method (Strickland and Parsons, 1965). T-P and DTP were measured by the same procedures as in the case of PO 4 -P after oxidation with potassium persulfate (Menzel and Corwin, 1965). DOP (Dissolved Organic Phosphorus) is taken to be the leftover portion of DTP minus PO 4 -P. PP (Particulate Phosphorus) is determined by subtraction of DTP from T-P. DIN (Dissolved Inorganic Nitrogen) is the sum of (NH 4 -N), (NO 2 - N) and (NO 3 -N). Date of observations and sampling interval are shown in Table 1. In the second series of observation, Tokushima Prefectural Fisheries Experimental Station measured vertical profile of temperature and salinity at Stn. A and B in Fig. 1, and sampled the water of 1 liter at a depth of 0, 5, 10, 20, 30, 50, 75, 100 m. 3. Results and Discussion 3.1 Front structure and distributions of nitrogen and phosphorus on Feb In the first observation on Feb. 1986, we aimed to observe the phenomena of thermohaline front as it was most developed. Distributions of salinity, nitrogen and phosphorus at surface layer are shown in Fig. 2. The abscissa in Fig. 2 is the distance from Osaka Port. In the inner part of Osaka Bay, salinity is low. But from one-third way out of Osaka Bay (the distance from Osaka Port is 20 km), salinity suddenly increased. Near Ishima there is a thermohaline front. Concentrations of nitrogen and phosphorus are very high in Osaka Bay. But outside of the inner part of Osaka Bay (the distance from Osaka Port is from 10 to 20 km), nitrogen and phosphorus decrease suddenly. Therefore, horizontal gradient of nitrogen and phosphorus concentrations are low from southern part of Osaka Bay (the distance from Osaka Port is 40 km) to Kii Strait. Consequently, differences of nitrogen and phosphorus concentrations across the thermohaline front in Kii Channel is not obvious. From the distributions of nitrogen, phosphorus and chlorophyll-a in Osaka Bay by Osaka Prefectural Fisheries Experimental Station on 5, 6 Feb. 1986, concentrations of NH 4 -N, PO 4 -P are zero in most area of Osaka Bay and except for small area where salinity is low. Concentration of T-P is lower than 1 µg-at l 1 except inner part of Osaka Bay. On the other hand, concentration of chlorophyll-a is higher than 10 µg l 1 for the most part of Osaka Bay. From January to March in this year, Heterocapsa triquetra formed red tide in Osaka Bay. This fact corresponds to our finding of observation by ferry boat. If the themohaline front has developed in Kii Channel, differences of nutrients concentrations across the front are not obvious. If there is exchange of coastal water and oceanic water through the thermohaline front, transport of nutrients to oceanic area is zero at least in this period of the observation. 3.2 Front structure and distributions of nitrogen and phosphorus from Nov to Mar Horizontal distributions of T, S and σ t Thermohaline front in Kii Channel develops from autumn to winter by cooling through sea surface in coastal region. The front is the most stable and intense from January to March (Yoshioka, 1988). We observed once per month from Nov to Mar in order to study the variation of nitrogen and phosphorus distributions across the front in response to its forming and development process. Horizontal distributions of temperature, salinity and σ t along the track line of the ferry boat

4 410 I. Yuasa et al. Fig. 2. Horizontal distributions of salinity, nitrogen and phosphorus from Osaka Port to Muroto in Feb observed by ferry boat. are shown in Fig. 3. The abscissa in Fig. 3 is distance from Osaka Port. Yoshioka (1988) studied characteristics of T, S and σ t distributions across the front in detail. In this year, from Nov. to Dec., temperature falls more rapidly in Osaka Bay than in the south Kii Channel because the former is shallower. In Nov a thermohaline front appears near Ishima. It becomes more intense and stable in Dec In Jan. the front is most intense. Differences of concentrations across the front near Ishima are 4.1 C for temperature, 1 for salinity. In Jan. and Feb. there is a high temperature and high salinity water mass in northern part of Kii front. Appearance of this water can be attributed to the meandering of the front.

5 Nitrogen and Phosphorus Distributions across the Thermohaline Front 411 (a) Fig. 3. Distributions of temperature, salinity and σ t from Osaka Port to Muroto from Nov to Mar Horizontal distributions of nitrogen and phosphorus Figures 4 and 5 show the monthly variations in nitrogen and phosphorus distributions from Osaka Port to Muroto from Nov to March In the central part of Osaka Bay, nitrogen and phosphorus exibit a sharp change across the Osaka Bay front in which temperature and salinity do so in all observations. This front is the tidal front in Osaka Bay (Yuasa and Ueshima, 1992). In offshore region from the Osaka Bay front, concentrations of NH 4 -N are low, except in Mar Consequently, differences of NH 4 -N concentrations across the Kii front are not obvious. Concentrations of (NO 2 -N) + (NO 3 -N) are high in Osaka Bay. But differences of (NO 2 - N) + (NO 3 -N) concentration across the Kii front are not always obvious. In Dec and Jan they change sharply across the Kii front. But Feb. and Mar differences across the front are not obvious. Outside from the Kii front, (NO 2 -N) + (NO 3 -N) concentrations increase gradually. Concentration of outside area from the front is about 1 ~ 2 µg-at l 1 in Nov. 1986, increasing to 6 ~ 8 µg-at l 1 in Mar This phenomenon means the formation of subtropical mode water in the area outside from the front (Masuzawa, 1969). Therefore, in Mar. 1987, differences of (NO 2 -N) + (NO 3 -N) concentration across the front are not obvious. Horizontal

6 412 I. Yuasa et al. (b) Fig. 3. (continued). distributions of DIN are similar to the pattern of (NO 2 -N) + (NO 3 -N) concentration. Patterns of PO 4 -P distributions are divided into two groups, one from Nov to Jan. 1987, and the other from Feb. to Mar In the former, PO 4 -P concentrations in Kii Channel are not zero. Consequently, PO 4 -P change sharply across the Kii front. Whereas in the latter in Feb. and Mar. PO 4 -P concentrations decrease sharply in the eastern part of Osaka Bay. In Kii Channel PO 4 - P concentrations are very low. Therefore, changes of PO 4 -P across the Kii front is not obvious. Pattern of PP distribution is divided into two groups, too. One is the pattern from Nov to Jan. 1987, and the other from Feb. to Mar In the former case, PP concentrations are low in the central part of Osaka Bay. Whereas in Feb. and Mar. 1987, PP concentrations are high in Osaka Bay but PP concentrations are low in Kii Channel. Consequently, difference of PP concentrations across the Kii front is not obvious. Horizontal distribution of T-P is the same pattern for five observations. On Jan. 1987, difference of T-P concentrations across the front is the largest in series of observations. The variation of concentration in series of observations is

7 Nitrogen and Phosphorus Distributions across the Thermohaline Front 413 (c) Fig. 3. (continued). the smallest for DOP. Differences of nitrogen and phosphorus concentrations across the front are the most intense and stable in Jan among the series of observations. In Jan. 1987, nitrogen and phosphorus decrease sharply on the central part of Osaka Bay but from Tomogashima to Ishima, there are little change of nitrogen and phosphorus, although they change sharply across the Kii front. Nitrogen and phosphorus concentrations near the front and differences across the front in Jan are shown in Table 2. T-P changes from 1.6 to 0.8 µg-at l 1, DOP from 0.5 ~ 0.7 to 0.4 ~ 0.5 µg-at l 1, PO 4 -P from 0.7 to 0.1 µg-at l 1 and DIN from 9 ~ 10 to 2 ~ 3 µg-at l 1 across the front. These changes are the largest difference in the series of observations. But NH 4 -N does not change across the front. In Feb. and Mar. 1987, nitrogen and phosphorus concentrations are very low in Osaka Bay, copying the same pattern as that of Feb There is no recognized changes of nitrogen and phosphorus concentrations across the Kii front except PP in Feb From the data of nutrients and chlorophyll-a distributions of surface layer in Feb observed by Osaka Prefectural Fisheries Experimental Station, red tide, Skeletonema-costatum formed in the eastern part of

8 414 I. Yuasa et al. (a) (b) Fig. 4. Horizontal distributions of phosphorus from Osaka Port to Muroto from Nov to Mar observed by ferry boat.

9 Nitrogen and Phosphorus Distributions across the Thermohaline Front 415 (c) (d) Fig. 4. (continued). Osaka Bay and concentration of chlorophyll-a was very high, 10 ~ 30 µg l 1. Therefore, nutrients decrease sharply in the eastern part of Osaka Bay. At Tomogashima Strait concentrations of nutrients are very low. This feature is similar to our observations by the ferry boat. From the vertical distributions of T, S, nitrogen and phosphorus on both sides of the front (Stn. A and Stn. B in Fig. 1), T, S, nitrogen and phosphorus are well mixed and constant vertically except in Feb on Stn. B. 3.3 Differences of nitrogen and phosphorus concentrations on frontal zone From our observation data, differences of nitrogen and phosphorus concentrations across the Kii front vary depending on both the forming process of the front and the oceanic condition of the inner coastal sea. Differences of nitrogen and phosphorus concentrations across the front are the largest and the most intense on Jan In Table 2, differences of nitrogen and phosphorus concentrations across the Kii front by the authors and by Kondo (1978) are given together with the data of Tokyo Bay front by Saino et al. (1988). From Kondo s data difference of PO 4 -P concentration across the front is not recognized. But DIN concentration changes from 5 to 10 µgat l 1 across the Kii front. Observation of Tokyo Bay was done at the same time with our observation in Kii Channel. Generally, difference of nutrients concentrations across the ther-

10 416 I. Yuasa et al. (a) (b) Fig. 5. Horizontal distributions of nitrogen from Osaka Port to Muroto from Nov to Mar observed by ferry boat.

11 Nitrogen and Phosphorus Distributions across the Thermohaline Front 417 (c) Fig. 5. (continued). Table 2. Deviation of nitrogen and phosphorus concentrations on the thermohaline front of Kii Channel and Tokyo Bay. Kii Channel Outside of the front Yuasa et al. T-P DOP 0.5 ~ ~ ~ 0.2 (January 1987) PO 4 -P NH 4 -N 0.1< 0.1< 0 DIN 9 ~ 10 2 ~ 3 +7 Kondo PO 4 -P 0.4 ~ < 0 ~ +0.1 (Feb ) DIN C Tokyo Bay Open Sea C Saino et al. PO 4 -P NH 4 -N (January 1987) DIN Chl.a Unit: nitrogen, phosphorus: µg-at l 1. chlorophyll-a: µg l 1.

12 418 I. Yuasa et al. mohaline front of Tokyo Bay is larger than that of Kii Channel. The special difference of NH 4 - N concentration across the front is 8 µg-at l 1 in Tokyo Bay, whereas null in Kii Channel. The cause of these features is the difference of the distance from origin of nitrogen and phosphorus (river mouth and landcoast) to the position of thermohaline front. In Tokyo Bay, the position of the front is 40 km from origin, whereas in Kii Channel, about 100 km. The position of the thermohaline front in Tokyo Bay is comparable to that in the inner part of Osaka Bay in the case of the Seto Inland Sea. Therefore, in the Seto Inland Sea, the time necessary to reach the front from land is longer than that in Tokyo Bay. Nutrients, for example NH 4 -N, PO 4 -P, are largely utilized by phytoplankton before reaching the front area. On the thermohaline front in Tokyo Bay, there is convergence of flow on surface layer. Consequently, surface coastal water sinks at the front and intrudes into the middle layer (Yanagi et al., 1989). The thermohaline front plays a significant role in material transport from coastal area to oceanic area. By the data of Kobe Marine Observatory (1987) observed in Kii Channel on Feb similar distributions of nitrogen and phosphorus to those of Tokyo Bay are recognized as shown in Fig. 6. This figure shows the differences of nitrogen and phosphorus Fig. 6. Vertical sections of temperature, nitrogen, phosphorus and chlorophyll-a from Osaka to Kii Channel on 4, 5 Feb observed by Kobe Marine Observatory. Observation stations are shown in Fig. 1.

13 Nitrogen and Phosphorus Distributions across the Thermohaline Front 419 across the front. These observations indicate that the front is very important element for transport of materials as there are differences of nitrogen and phosphorus concentration. From Figs. 4 and 5 concentrations of (NO 2 -N) + (NO 3 -N) and PO 4 -P in the area outside of the front, have increased gradually from Nov to Mar Consequently, on Mar. 1987, differences of (NO 2 -N) + (NO 3 -N) and PO 4 -P concentrations across the front are not obvious. This phenomenon is possibly due to the formation of subtropical mode water (Masuzawa, 1969). This means that differences of (NO 2 -N) + (NO 3 -N) and PO 4 -P concentrations across the front are dependant on seasonal variation of the behaviour of oceanic water outside of the front. 3.4 Relation between nitrogen (or phosphorus) and salinity As Tokyo Bay data is obtained only once, we can not study the monthly variation of nitrogen and phosphorus distributions across the front. But from our observations, nitrogen and phosphorus distributions vary monthly according to development process of the front and biological activity in Osaka Bay. Therefore, we will discuss the relation between each element of nitrogen (or phosphorus) distributions and physical processes or biological processes. Relation between nitrogen (or phosphorus) and salinity on Feb are shown in Fig. 7. A linear regression does not seem applicable between nutrients (NH 4 -N, PO 4 -P) and salinity. Both NH 4 -N and PO 4 -P are nearly zero in Osaka Bay and Kii Channel. Especially PO 4 -P becomes lower as salinity does so. In Fig. 8, relation between phosphorus and salinity from Nov to Mar is shown. T-P can be linearly regressed to salinity in all months. But the relation between PO 4 -P and salinity is different on each month. On Jan. 1987, PO 4 -P is linearly related to salinity. But on Feb. and Mar there are no linear relation between PO 4 -P and salinity. The concentration of PO 4 -P is considerably lower than that estimated only by physical mixing process. DOP is more or less high in Osaka Bay, but monthly variation is small and stable. On Nov., Dec. and Jan., each phosphorus concentration is inversely proportional to salinity as a whole especially on Jan., both T-P and PO 4 -P are inversely proportional to salinity. PO 4 -P ratio to T-P is maximum on Jan in all observations. Concentration of phosphorus is determined by physical mixing process in all areas. On the other hand characteristics of phosphorus (or nitrogen) concentrations on Feb. and Mar are different from that on Jan From Jan. to Feb. 1987, PP has become higher than PO 4 -P, making the pattern of broken lines curved downward. This tendency is recognized in nitrogen concentration in Fig. 9, as well. Namely, there are no linear relation between nitrogen concentration and salinity. Especially PO 4 -P concentration is very low where salinity is smaller than 32. This is probably because of large uptake by phytoplankton in Osaka Bay. Background of these variations is the magnitude of bio-chemical activity in coastal sea, especially Osaka Bay. The area of red tide (Akashiwo) corresponds to chlorophyll-a distribution, Akashiwo was observed in Osaka Bay widely on Feb. 1986, Consequently, nutrients concentrations were very low in the Akashiwo area and in off-shore area from the Akashiwo area nutrients were nearly zero. On these months if thermohaline front have developed in Kii Channel, differences of nutrients across the front are nearly zero. From observation of Akashiwo in Osaka Bay by Osaka Prefectural Fisheries Experimental Station, occurrence of Akashiwo in Osaka Bay from Dec. to Mar. for last 10 years is shown in Table 3. Unexpectedly, a tendency similar to our observations of 1986 and 1987 FY is recognized in many years. Usually Akashiwos of Heterosigma, Heterocapsa triqetra and Skeletonema costatum occur in the inner part of Osaka Bay in Jan. or Feb. every year. In these periods, surface nutrients concentrations were very low in Osaka Bay (Osaka Prefectural

14 420 I. Yuasa et al. (a) (b) Fig. 7. Relations between nitrogen (or phosphorus) and salinity on Feb

15 Nitrogen and Phosphorus Distributions across the Thermohaline Front 421 (a) (b) (c) (d) Fig. 8. Relations between phosphorus and salinity from Nov to Mar

16 422 I. Yuasa et al. (a) (b) (c) Fig. 9. Relations between nitrogen and salinity from Nov to Mar

17 Nitrogen and Phosphorus Distributions across the Thermohaline Front 423 Table 3. Red tide in Osaka Bay in winter from 1979 to Date Station Area (km 2 ) Plankton ~ 23 Sakai ~ Izumiotsu Prorocentrum micans 2. 5 ~ 7 off Nishinomiya Heterocapsa triquetra 3. 5 ~ 6 Wadamisaki ~ Izumiotsu Heterosigma ~ 18 Wadamisaki ~ Ozaki Chaetoceros borealis ~ 26 inner part 320 Skeretonema costatum, Heterosigma 3. 5 Wadamisaki ~ Sennan 420 Heterosigma 3.17 ~ 24 eastern part 380 Skeretonema costatum eastern part 460 Skeretonema costatum ~ 2.10 inner part 900 Skeretonema costatum 3. 4 off Nishinomiya 100 Heterocapsa triquetra inner part 170 Skeretonema costatum ~ 8 off Kobe 45 Prorocentrum minimum 2.22 ~ 23 eastern part 470 Skeretonema costatum, Heterocapsa inner part 170 Heterosigma ~ 3.22 inner part 200 Heterocapsa inner part 280 Skeretonema costatum inner part 360 Heterosigma 3.15 ~ 5. 8 Kobe ~ Kishiwada 630 Skeretonema costatum inner part 470 Skeretonema costatum 2. 4 inner part 150 Heterocapsa 3. 5 ~ 6 inner part 460 Skeretonema costatum ~ 3.18 Wadamisaki ~ Hamadera Heterocapsa triquetra inner part Skeretonema costatum 3. 8 inner part 220 Skeretonema costatum Fisheries Experimental Station, 1988, 1989). We can understand that nutrient distributions in Dec and Jan are controlled by the physical process and there is a large difference of nutrient concentration across the Kii front. A front has a very important role in transporting nutrients from coastal area to oceanic sea. But in the case of Feb. and Mar nutrients have been utilized by phytoplankton in low salinity area and the role of thermohaline front of Kii channel for nutrient transport is comparatively small. Generally, nutrient concentration in winter is determined by the physical mixing process, but the bio-chemical process in coastal sea and seasonal variation of the behaviour of oceanic water outside of the front are important factors for transport through thermohaline front. Existence of Kii Front does not always mean that nutrient concentration is different from water masses separated by the front. History of water mass, for example duration time, experience of physical, chemical and biological processes in coastal sea and seasonal variation of the behaviour of oceanic water outside of the front are very important factors. Thus, the tidal front in Osaka Bay plays the first important role for distributions and transport of nutrients even in winter. In our observation data (Figs. 3, 4 and 5), there are temperature and salinity gaps in the central part of Osaka Bay.

18 424 I. Yuasa et al. Acknowledgements We would like to thank Prof. N. Hayakawa of the Nagaoka Technology and Science University for his discussion and encouragement. We are deeply grateful to Mr. Jho of Tokushima Prefectural Fisheries Experimental Station for sampling of sea water and the Japan Car Ferry Company for the use of their ferry boats to make observation. This work was financed by Quality enhancing technology for stagnant waters of the Seto Inland Sea, of the Environment Agency of Japan. References Ichie, H. (1951): On the hydrography of the Kii Suido. Bull. Kobe Mar. Obs., 164, (in Japanese with English abstract). Kobe Marine Observatory (1987): The results of Oceanographic observations of Osaka Bay, Kii Channel and South of Japan in Feb. and Mar Oceanogr. Prompt Rep. Kobe Mar. Obs., 107, Kondo, M. (1978): Distribution and seasonal change of the nutrient salts in the Seto Inland Sea. Umi to Sora, 54, (in Japanese). Kuroda, K., N. Baba, H. Takahashi and S. Bessho (1977): Distributional properties of chlorophyll contents in Kii Channel and its adjacent areas. Umi to Sora, 53(1), 1 14 (in Japanese with English abstract). Liddicoat, M. L., M. I. Tibbits and E. I. Butler (1975): The determination of ammonia in seawater. Limnol. Oceanogr., 14, Masuzawa, J. (1969): Subtropical mode water. Deep-Sea Res., 16, Menzel, D. W. and N. Corwin (1965): The measurement of total phosphorus in sea-water based on liberation of organically bond fractions by persulfate oxidation. Limnol. Oceanogr., 10, Nagashima, H. and M. Okazaki (1979): Observations of temperature, salinity and current velocity at the mouth of Tokyo Bay in winter. Bull. Coast. Oceanogr., 16, (in Japanese). Osaka Prefectural Fisheries Experimental Station (1988): Yearly Data Report, FY1986. Osaka Prefectural Fisheries Experimental Station (1989): Yearly Data Report, FY1987. Saino, T. (1988): Circulation of nutrients in Tokyo Bay. Bull. Coast. Oceanogr., 25, (in Japanese). Strickland, J. D. H. and T. R. Parsons (1965): A manual of sea-water analysis (2nd ed., revised). Fish. Res. Board Can. Bull., 125, Strickland, J. D. H. and T. R. Parsons (1968): A practical handbook of sea-water analysis. Fish. Res. Board Can. Bull., 167, Yanagi, T. (1980): A coastal front in the sea of Iyo. J. Oceanogr. Soc. Japan, 35, Yanagi, T. and S. Takahashi (1988): A tidal front influenced by river discharge. Dyn. Atmos. Oceans, 12, Yanagi, T., A. Isobe, T. Saino and T. Ishimaru (1989): Thermohaline front at the mouth of Tokyo Bay in winter. Cont. Shelf Res., 9, Yoshioka, H. (1988): The coastal front in the Kii Channel in winter. Umi to Sora, 64, Yuasa, I. and H. Ueshima (1992): A tidal front in winter influenced by river discharge. J. Oceanogr., 48,