1 Nippon Suisan Gakkaishi 56(11), (1990) Mean Seasonal Changes of Dissolved Inorganic Nutrients in the Ofunato Estuary Yasuhiro Hayakawa* (Received February 19, 1990) Mean seasonal changes of dissolved inorganic nutrients (silicates, phosphates, and nitrogen compounds) in the Ofunato estuary were represented on the basis of monthly measurements from April 1979 to March 1989, and were compared with those in coastal waters. Estuarine nutrients increased in all layers in winter and spring, and decreased in the middle layer in summer and autumn, during which, however, high concentrations were observed in the surface and the bottom layer. A rapid depletion of nutrients occurred from February to March in the estuary in relation to phytoplankton blooms, and from April to May in coastal waters. Estuarine nutrients were greatly influenced by fluxes of nutrients from the river into the surface and from bottom sediments into the bottom, as well as fluxes from coastal waters in which the seasonal changes were mainly attributed to those in the oceanographic structure. The high overall levels of primary productivity in estuarine waters are attributed to the great availability of key nutrients. They are dissolved, generally inorganic compounds of silicon, nitrogen and phosphorus. In the Ofunato estuary, these nutrients are carried from coastal waters, rivers and sewages adjacent to this estuary, and then modified through biological processes in estuarine waters. Seasonal changes in fluxes from coastal and riverine waters have great influences on those in nutrients supporting primary production in estuarine waters. Thus, understanding seasonal changes in nutrients is essential to the preservation and utilization of estuarine waters. However, few studies showed the mean seasonal changes in nutrients which were described for a whole year on the basis of the data from long-term monitoring. Monthly measurements ( ) of coastal waters off Todogasaki (39 32'N, 'E) in the north of the Ofunato estuary, showed that remarkable seasonal changes in nitrate and chlorophyll-a were repeated every year; with high nitrate in winter and high chlorophyll-a in spring, both being low in summer.') The riverine fluxes of nutrients into the Ofunato estuary were characterized by an intermittent increase according to rain falls.2) Nutrients in this estuary have been monitored monthly for more than 10years, and it has been observed that the seasonal changes were repeated similarly every year, though great fluctuations were observed in the surface layers in summer due to the yearly change in the river flow. The present study aims to represent mean seasonal changes, i.e. 10-year average patterns of seasonal changes in nutrients in the Ofunato estuary ( ), in comparison with those in coastal waters ( ) close to this estuary. Such results will be useful not only in estimating the effect of coastal waters on estuarine waters, but also in assessing the optimum scale of aquaculture to be operated in this estuasy. Materials and Methods Study Area and Sampling The Ofunato estuary is located on the northeastern coast (Sanrihu) of Japan facing east to the Western North Pacific Ocean, where the Tugaru Warm Current and the Oyashio waters have a great influence upon the coastal environments. It is typical of the semi-enclosed waters, with its surface area of 7.7km2, the maximum depth of 38m, and a sill depth of 15m at the mouth (Fig. 1). The sill is a man-made heap, forming the lower part of the breakwater constructed for protection from the tunami. The Sakari River and the Sakari Sewage, the major two sources of terrestrial nutrients, pour into the head of the estuary with mean discharges of 3.3 and 0.2 (m3/ sec), respectively.2) Water sampling at the middle (OF06, bottom depth of 25m) and just at the outside (OF15, 37m) of the estuary, was carried out monthly from April 1979 to March Additional
2 Fig. 1. Map of the Ofunato estuary and sampling stations in estuarine waters (OF06 and OF15) and coastal waters (TB01, TB02, OS01, and OS02). Note that the estuary is semi-enclosed with a sill at the mouth. sampling for plant pigment was made monthly at OF06 (Jan to Dec. 1989) and at OF15 (Jan to Dec. 1989). Stations OF06 and OF15 are referred to as estuarine stations. Hydrographic measurements in coastal waters at the stations; TB01 (55m), TB02 (135m), OSO1 (55m), and OS02 (165m), which are parts of the coastal observational stations (the Tsubakijima line and Osaki line), have monthly been carried out by Iwate Prefectural Fisheries Experimental Station for a few decades. Nutrients were monitored monthly from 1984 to These stations are referred to as coastal stations. Sample Analysis Nutrients and chlorophyll-a (Chi. a) were measured in every 5m layer at the estuarine stations. Water temperature (T), salinity (Sal), and dissolved oxygen (DO) were measured also in the 1 and 3m layers in addition to every 5m layer. Water samples were analyzed after filtraration (Millipore, 0.45ƒÊm) for dissolved reactive silicate (DRSi), dissolved inorganic phosphate (DIP), nitrite (NO2-N), and nitrate (NO2-N) by the method of Strichland and Parsons,3) and ammonium (NH4-N) by the indophenol method.4) Dissolved inorganic nitrogen (DIN) was the sum of these three nitrogen compounds. Another sample of about 5l filltered through a glass filter (Whatman, GF/C) was analyzed for Chi. a by the method of Lorenzen.5) T, Sal, and DO were measured by a thermometer (TOHO DENTAN,
3 Fig. 2. Mean seasonal changes in water temperature ( Ž) and salinity ( ñ) at TB01 and TB02 (Tsubakijima line) in coastal waters ( ). Low temperatures less than 6( Ž) and high temperatures over 20( Ž) are dark and light shaded, respectively. High salinities over 34.0( ñ) are dark shaded. ET5 and ET5D), a salinometer (AUTOLAB, Model 601, MKIII), and the Winhier method,3) respectively. Measurements of T, Sal, DRSi, DIP, and nitrite plus nitrate ((NO2+NO3)-N) in the coastal stations were made by the staff of Iwate Prefectural Fisheries Experimental Station. Nutrients were maesured by an auto-analyser (Technicon). Data Analysis Monthly data of T, Sal, DO, and nutrients in each layer in the estuarine staions were averaged monthly for 10 years (Apr to Mar. 1989), while Chl. a at OF06 for 5 years (Jan to Dec. 1989) and at OF15 for 2 years (Jan to Dec. 1989). There was a lack of measurements in several layers (especially, 15m at OF06, 15, 25, 30, and 35m at OF15), but the calculated value was taken as the mean value. Mean values of T and Sal in the coastal stations were also calculated for the same periods ('79-'89), while nutrients for 5 years (Apr to Mar. 1987). Isopleths based on these means were drawn to describe mean seasonal changes in nutrients and other aquatic environmental factors. The mean seasonal change averaged for all layers and mean vertical profile averaged annually were also represented for DO, Chl. a, and nutrients. Overall mean and standard deviation of nutrients in the upper layer (less than 50m depth) were given from all data at each station. The layer deeper than 50m is referred to as the lower layer. Results and Discussions Temperature and Salinity Mean seasonal changes in T and Sal at the coastal and estuarine stations are shown in Figs. 2, 3, and 4. Relatively low temperatures of 5-6 Ž were found from March to April, and high temperatures of Ž in the surface early in September. The thermal stratification was well developed in the upper layer from June to September. In the lower layer at the coastal stations, less saline waters below ca. 33.7%. (Oyashio) were encountered from March to May, while more saline waters over 34.0%. (Tugaru Warm Current) from August to January. In the upper layer at the coastal stations, lower salinities were found from March to October, while higher salinities from November to January. Extremely low salinities below 33.0% were found on the surface shallower than 5m at the estuarine stations from March to November. During 1979 and 1989, two hydrographically extreme events were observed off Sanriku. First, the highest temperature of Ž was found in the upper layer in September of 1979, and high temperatures above 20 Ž, accompanied with high
4 Fig. 3. Mean seasonal changes in water temperature ( Ž) and salinity ( ñ) at OS01 and OS02 (Osaki line) in coastal waters ( ). Other explanations are as in Fig. 2. Fig. 4. Mean seasonal changes in water temperature ( Ž) and salinity ( ñ) of OF06 and OF15 in estuarine waters ( ). Low salinities less than 33.0( ñ) are light shaded. Other explanations are as in Fig. 2. salinities %, were observed both at the coastal and estuarine stations at least till the end that year. It was reported that this event was attributed to the large scale warm core eddy derived from the Kuroshio Current.6-7) Second, extremely cold (below 2 Ž at the coastal staions and below 4 Ž at the estuarine stations) and less saline (below 33.0%) waters were observed in the upper layer during March and April of 1984, which was due to the Oyashio Water overlain by ice-melt water from the Okhotsk Sea.8) Mean values including these extremes, however, described a major characteristic of mean seasonal changes in T and Sal in the study area. Most studies of the hydrographic, variations in coastal waters off Sanriku showed remarkable
5 Fig. 5. Mean seasonal changes in dissolved reactive silicate (ƒêm) at the coastal stations ( ). Relatively high concentrations over 10(ƒÊM) and low concentrations less than 5(ƒÊM) are dark and light shaded, respectively. Mean seasonal changes averaged in all layers (TB01 and OS01) and in the upper layer shallower than 50m (TB02 and OS02) are shown together with the mean vertical profile averaged annually. seasonal changes which come from predominant influences of the Tugaru Warm Current in summer and autumn as well as of the Oyashio waters in winter and spring, and from the occasional influence of the warm core eddy (the Kuroshio water).9,10) Studies of seasonal and short-term variations in the Tugaru Warm Current showed that the southward component of the current velocity near coasts was maximum (ca. 60cm/sec) during Sep.-Nov., and minimum (ca. 20cm/sec) in March,11) and the outflow pattern of the current in autumn varied in a day cycle associated with the changes of volume transport from the Tugaru strait.12) Moreover, correlations between temperature variations at the northern station off Sanriku (39 15'N, 'E) and at the southern station off Johban (36 19'N, 'E) are found with no lag time in the upper layer (less than 50m depth) and with some delay of up to a month in the deeper layer (over 75m) at the southern station.13) Mean seasonal changes in T and Sal at the coastal stations showed a greater and earlier influence of the Oyashio water and the Tugaru Warm Current upon coastal waters, especially in the lower layer, at OS01 and OS02 (Fig. 3) than at TB01 and TB02 (Fig. 2). A great but modified effect of coastal waters upon the lower layers in the estuary were shown (Fig. 4); low T and Sal in early spring, and high Sal in autumn and winter at OF06 and OF15. Silicate At the coastal stations (Fig. 5), relatively high concentrations over 10(ƒÊM) were found in the upper layer from January to April, while those existed in deeper layers than 100m at OS02 throughout the year. Relatively low concentrations of less than 5(ƒÊM) were found in the upper layer from May to October. A rapid decrease was observed during April and May. Thus, this resulted in the remarkable seasonal changes averaged in the upper layer and a gradual increase with depth in mean vertical profile, as are also shown in Fig. 5. At the estuarine stations (Fig. 6), the same seasonal changes as at the coastal stations were recognized, except for high concentrations in the surface observed intermittently from March to November and earlier decrease of DRSi in the
6 Fig. 6. Mean seasonal changes in dissolved reactive silicate (ƒêm) at the estuarine stations ( ). Mean seasonal changes averaged in all layers are shown together with the mean vertical profile averaged annually. Other explanations are as in Fig. 5.
7 Fig. 8. Mean seasonal changes in dissolved inorganic phosphate (ƒêm) at the estuarine stations ( ). Other explanations are as in Fig. 7. surface during February and March than that at the coastal stations. Mean concentrations of DRSi from the Sakari River and the Sakari Sewage were estimated ca. 192 and 225(ƒÊM), respectively (unpublished data by Hayakawa). High silicates in the surface may be caused by riverine runoffs. Near the bottom (especially at OF06), high concentrations were observed late in summer in contrast with low ones in the overlying layers. High DRSi at the bottom may be due to the decomposition of particulate matters accumulated through biological processes. Especially at OF06, accumulated matters may be derived partly from the chemical flocculation near the head of the estuary where the Sakari river pours. The mean seasonal changes in all layers were less remarkable at OF15, and very large at OF06 (high in summer and autumn), though both were different from those at the coastal stations. Mean veitical profiles of DRSi in the setuary showed high concentrations both at the surface and at the bottom. Phosphate At the coastal stations (Fig. 7), relatively high concentrations over 0.5(ƒÊM) were found in the upper layer from January to April, while those existed in deeper layers at OS02 throughout the year. Relatively low concentrations less than 0.2(ƒÊM) were found in the upper layer from May to October. At the estuarine stations (Fig. 8), seasonal changes were characterized by very high concentrations in the the bottom (over 3ƒÊM at OF06) late in summer. High DIP was correlated with low DO (Fig. 12). Depletion of DIP earlier in spring and intermittent high concentrations in the surface were not necessarily observed. Relatively low DIP from the river (0.7ƒÊM) seemed to be related to little occurrence of high DIP at the surface in spite of high DIP from the sewage (33.5ƒÊM), since the discharge from the river amounted to more than 70% of the total discharges into this estuary.2) Mean vertical profile indicated high concentrations in the bottom.
8 Fig. 9. Mean seasonal changes in nitrite and nitrate nitrogen (ƒêm) at the coastal stations ( ). Relatively high concentrations over 5(ƒÊM) and low concentrations less than 1(ƒÊM) are dark and light shaded, respectively. Fig. 10. Mean seasonal changes in nitrite and nitrate nitrogen (ƒêm) at the estuarine stations ( ). Other explanations are as in Fig. 9.
9 Fig. 11. Mean seasonal changes in dissolved inorganic nitrogen (ƒêm) at the estuarine stations ( ). Relatively high concentrations over 10(ƒÊM) and low concentrations less than 2(ƒÊM) are dark and light shaded, respectively. Nitrogen At the coastal stations (Fig. 9), relatively high concentrations of (NO2+NO3)-N over 5(ƒÊM) were found in the upper layer from December to April, while those existed in deeper layers at OS02 throughout the year. Relatively low concentrations less than 1(ƒÊM) were found in the upper layer from June to October. At the estuarine stations (Fig. 10), a rapid decrease of (NO2+NO3)-N were observed earlier in spring than that at the coastal satations. Intermittent high concentrations at the surface were frequently observed from April to November. However, concentrations in the surface were not so lage that mean seasonal changes of (NO2+NO3)-N averaged in all layers at OF06 showed less remarkable seasonal changes than those of DRSi. This might be related to less fluxes of nitrogen from the Sakari River than those of DRSi.2) Mean seasonal changes in DIN at the estuarine stations are shown in Fig. 11. High values over 10(ƒÊM) were found in the surface, and those over 20(ƒÊM) were observed in the bottom layers at OF06, while low ones below 2(ƒÊM) prevailed in the layers of 5-15m in summer. Since differences between DIN and (NO2+NO3)-N corresponded to NH4-N, very high values near the bottom at OF06 late in summer were due to ammonium nitrogen. Overall Mean of Nutrients Overall mean and standard deviation of nutrients at each station were shown in Table 1. Those of common logarithms of nutrient concentrations are also shown, since they sometimes fit well to log-normal distributions. Very slight differences in DIP and (NO2+NO3)-N between stations were found, while higher DRSi at the estuarine stations than at the coastal stations. Ammonium nitrogen was high at OF06. The atomic ratio of Si/P in the upper layer at the coastal stations were ca with correlation coefficients of Little correlation between nutrients at the estuarine stations was found, because of large
10 Table 1. Overall mean and standard deviation of nutrient concentrations (ƒêm) in the Ofunato estuary and coastal waters
11 Fig. 12. Mean seasonal changes in dissolved oxygen (ml-o2/l) at the estuarine stations ( ). Relatively high DO over 8(ml-O2/L) and low DO less than 4(ml-O2/L) are light and dark shaded, respectively. variability. Estuarine waters seemed to be characterized by higher silicates, ammonium nitrogen and their large variability. Dissolved Oxygen High DO in the surface in spring and low values near bottom late in summer were marked features in estuarine waters (Fig. 12). At OF06, the deficiency of DO was such that chemical oxygen uptake by sulfides as well as biological one might possibley contribute to DO depletion near the bottom.14) The fact that a high concentration of nutrients near the bottom at OF06 was coincident with DO deficiency suggested liberation of nutrients from settling and settled sediments. DO depletion and nutrients liberation may be understood by the similar process as reported in Osaka Bay15) and in Lake Biwa,16) since reduced advective transport may permit organic materials to accumulate in the estuary, and reduced diffusion by stratification during summer may retain nutrients liberated near the bottom. The sill at the mouth of the estuary may make a great contribution to the reduction of advection and diffusion, since there was little DO depletion at OF15 just outside of the sill. Chlorophyll-a. Mean seasonal changes in Chl. a at the estuarine stations are shown in Fig. 13. There was observed patchy distribution of Chl. a with high concentrations over 2(ƒÊg/L) in several layers from February to April and in the surface from May to September. Low Chl. a below 0.5(ƒÊg/L) was found in the deeper layers during summer and autumn, and in most layers in January. High Chl. a at the estuarine stations occurred about a month earlier in comparison with spring blooms in coastal waters off Sanriku (April to May),1) which was coincident with earlier decrease in nutrients at the estuarine stations. It was also reported that spring blooms (Feb.-Apr.) in Funka Bay17) was related to a decrease of nutrients, especially of silicates.18,19) The estuarine rapid
12 Fig. 13. Mean seasonal changes in chlorophyll-a (ƒêg/l) at OF06 ( ) and OF15 ( ) in estuarine waters. Relatively high concentrations over 2(ƒÊg/L) and low concentrations less 0.5(ƒÊg/L) are dark and light shaded, respectively. decrease of nutrients in spring may be due to diatom blooms followed by little supply of terrestrial nutrients. Intermittent high Chl. a in the surface during summer and autumn may be attributed to occasional blooms with the large supply of nutrients from the river and the sewage. It was likely that two major supplies of nutrients from coastal waters during winter and from terrestrial sources during summer contributed greatly to the high overall levels of the primimary productivity in this estuary. Factors Affecting Seasonal Changes in Nutrients There was a considerable influence of the Oyashio water on concentrations of nitrate off Sanriku during March and May in ) At the coastal stations, relatively high concentrations of nutrient persisted at least until May in 1984, while a slight influence on the estuarine stations was observed. The warm core eddy (Kuroshio) had only a slight influence over the nutrients at the estuarine stations late in autumn of Studies of nutrients in Fuknka Bay20-22) revealed that seasonal changes in nutrients were due to those in the oceanographic structure. It was associated with both the Tugaru Warm Current and the Oyashio water, with the uptake by the phytoplankton blooms, especially of diatoms (greater than 10ƒÊm),23) and with the regeneration by their decomposition. Off Sanriku, it was also discussed that an increase in nitrate from autumn to early spring was caused by local vertical mixing followed by intrusion of the Oyashio water, and a rapid decrease in spring was related to the phytoplankton bloom followed by intrusion of the Tugaru Warm Current with less nutrients.1) But it still remains to be proven whether the nutrients-rich waters at the coastal stations in winter had their origin only in the local vertical mixing or not. The present study supported that above-stated factors (Oyashio, Tugaru Warm Current and phytoplankton blooms) had a predominant effect on mean seasonal changes in nutrients at the coast-
13 al stations, and showed that those at the estuarine stations were influenced greatly by fluxes of nutrients from the river into the surface layer and from settling and settled sediments into the bottom layer, in addition to fluxes from the coastal waters into the estuary. Acknowledgements The author is very grateful to Dr. T. Shimomura, the former professor of School of Fisheries Sciences, Kitasato University, for his making a beginning of this study, and also to the former students; T. Ono, C. Murakami, S. Murakami (1979), A. Ohno, J. Wada, O. Kagaya, S. Komachi, T. Kojiya (1980), T. Takeuchi, K. Totsuka, F. Nakamura (1981), S. Ishikawa, K. Kumagai, T. Takahashi (1982), S. Katada, S. Hasegawa, K. Mori (1983), M. Okamoto, K. Tekawa (1984), A. Adachi, S. Abe (1985), S. Hasegawa, N. Hamada (1986), A. Satoh, J. Masatani (1987), K. Ikeda, S. Chiba, H. Naitoh (1988), N. Itoh, N. Nakabayashi, and H. Hayashi (1989) for their helps in sampling and measuring nutrients at the estuarine stations. He also wishes to thank especially Mr. M Shimizu, Iwate Prefectural Government, for valuable comments on nutrients at the coastal stations. This study was partly supported by a Grant-in- Aid from the Ministry of Education, Science and Culture. References 1) M. Shimizu: Bull. Tohoku Reg. Fish. Res. Lab., 49, (1987). 2) Y. Hayakawa: Nippon Suisan Gakkaishi, 52, (1986). 3) J. D. H. Strickland and T. R. Parsons: Fish. Res. Bd. Canada, 167, (1968). 4) Y. Nimura: Nippon Suisan Gakkaishi, 39, (1973). 5) C. J. Lorenzen: Limnol. Oceanogr., 12, (1967). 6) K. Mizuno and M. Akiyama: Sora to Umi, 2, (1980). 7) S. Muto: Bull. Tohoku Reg. Fish, Res. Lab., 44, (1982). 8) K. Okuda: Bull. Tohoku Reg. Fish. Res. Lab., 48, (1986). 9) K. Mizuno: Bull. Tohoku Reg. Fish. Res. Lab., 46, (1984). 10) K. Hanawa and H. Mitsudera: J. Oceanogr. Soc. Japan, 42, (1987). 11) Y. Ueno and M. Yamazaki: Bull. Tohoku Reg. Fish. Res. Lab., 49, (1987). 12) I. Yasuda, K. Okuda, M. Hirai, Y. Ogawa, H. Kudoh, S. Fukushima, and K. Mizuno: Bull. Tohoku Reg. Fish. Res. Lab., 50, (1988). 13) M. Hirai: Bull. Tohoku Reg. Fish. Res. Lab., 44, (1982). 14) A. Kawai and H. Maeda: Nippon Suisan Gakkaishi, 50, (1984). 15) H. Joh, S. Yamochi, T. Abe, and A. Kawai: Nippon Suisan Gakkaishi, 50, (1984). 16) H. Maeda, M. Kumagai, Y. Oonishi, H. Kitada, and A. Kawai: Nippon Suisan Gakkaishi, 53, (1987). 17) K. Nakata: Bull. Japan. Soc. Fish. Oceanogr., 41, (1982). 18) S. Tsunogai: Bull. Fac. Fish. Hokkaido Univ., 30, (1979). 19) S. Tsunogai and Y. Watanabe: J. Oceanogr. Soc. Japan, 39, (1983). 20) M. Yanada, Y. Maita and S. Fukase: Bull. Fac. Fish. Hokkaido Univ., 27, (1976). 21) Y. Maita: Bull. Japan. Soc. Fish. Oceanogr., 34, (1979). 22) K. Ohtani and K. Kido: Bull. Fac. Fish. Hokkaido Univ., 31, (1980). 23) Y. Maita and T. Odate: J. Oceanogr. Soc. Japan, 44, (1988). Nippon Suisan Gakkaishi : Formerly Bull. Japan. Soc. Scl. Fish.