Nutrient status of Otsuchi Bay (northeastern Japan) following the 2011 off the Pacific coast of Tohoku Earthquake

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1 J Ocenogr DOI 1.17/s SPECIAL SECTION: ORIGINAL ARTICLE Ocenogrphic oservtions fter the 11 erthquke off the Pcific cost of Tohoku Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost of Tohoku Erthquke Hideki Fukud 1 Ryosuke Ktym 1 Ynhui Yng 1 Hiroyuki Tksu 1 Yuichiro Nishie 1 Atsushi Tsud 1 Toshi Ngt 1 Received: 9 Jnury 15 / Revised: 7 April 15 / Accepted: April 15 The Ocenogrphic Society of Jpn nd Springer Jpn 15 Astrct On 11 Mrch 11, the Snriku cost of Jpn ws struck y mssive tsunmi ssocited with the 11 off the Pcific cost of Tohoku Erthquke. The tsunmi cused lrge disturnces in ottom sediments nd severe destruction of humn-mde structures, nturl lndscpes nd infrstructures such s sewge tretment systems nd quculture fcilities. A serious nd continuing concern is fundmentl ltertion of the nutrient regime in the impcted regions, which my exert profound influence on the processes of ecosystem recovery nd fishery reconstruction. We investigted the nutrient sttus of the wter column of Otsuchi By, smll emyment tht ws hevily dmged y the tsunmi. Our dt collected etween My 11 nd July 14 reveled some prominent fetures in nutrient prmeters tht were potentilly linked to effects of the tsunmi. These fetures include (1) high turidity nd high concentrtions of nitrite nd silicic cid until Septemer 11, (2) the spred of sewter chrcterized y high phosphte concentrtion nd low totl inorgnic nitrogen (mmonium + nitrte + nitrite) to phosphte (TIN/P) rtio during the mixing period etween Novemer 11 nd Jnury 12, nd (3) n increse in the TIN/P rtio during the mixing period of susequent yers to ecome higher thn it ws efore the erthquke. These dt not only provide evidence of the ltertion nd susequent recovery of the nutrient sttus of Otsuchi By fter the erthquke ut lso suggest possile tsunmi-induced ltertion in nutrient stoichiometry which ppered to lst until t lest the eginning of 14. * Hideki Fukud hfukud@ori.u tokyo.c.jp 1 Atmosphere nd Ocen Reserch Institute, University of Tokyo, Kshiwnoh, Kshiw, Chi , Jpn Keywords The 11 off the Pcific cost of Tohoku Erthquke Tsunmi Otsuchi By Nutrients Turidity 1 Introduction On 11 Mrch 11, the estern cost of Jpn ws struck y mssive tsunmi ssocited with the 11 off the Pcific cost of Tohoku Erthquke, which hd moment mgnitude of 9. (Mori et l. 11; Stke et l. 13; Tjim et l. 13). The tsunmi cused serious nd extensive dmge to the costl society nd nturl environment (Kzm nd Nod 12; Suppsri et l. 12). Although the extent nd nture of the dmge differed gretly etween loclities, depending on their societl, environmentl, nd geogrphicl settings, some generl fetures chrcterize the modes of tsunmi-induced disturnces on the Snriku cost of the Iwte nd Miygi prefectures. These fetures include lrge disturnces (mssive resuspension, mixing, nd trnsport) of ottom sediments (references in Goto et l. 14), severe destruction of humn-mde physicl structures (e.g., nks, piers, nd wter gtes) nd nturl costl lndscpes (mudflts, sndy eches, sltmrsh) (Ogswr et l. 12; Okysu et l. 13; Tppin et l. 12; Udo et l. 12), enormous dischrges of lnd-derived deris (soil, wood, houses, crs) to the costl se (Arii et l. 14; Ministry of the Environment of Jpn 14), nd disruption of infrstructures such s sewge tretment systems nd quculture fcilities (Okud nd Ohshi 12; Qi et l. 14). One serious nd ongoing concern rising from the dmge is the ltertion of the nutrient sttus of the impcted mrine environments. Here, the term nutrient sttus refers to the concentrtions, chemicl compositions, nd stoichiometry of mjor nutrients, including nitrogen, phosphorus, nd silic, with considertion of their vriility in time nd spce. It

2 H. Fukud et l. is known tht even slight or moderte chnges in the nutrient sttus of the wter column cn ffect: phytoplnkton productivity, the sis of mrine food wes (Cloern et l. 14; Hecky nd Kilhm 1988; Rlis et l. 1996); the growth of seweeds, including those tht re importnt s quculture products (Coelho et l. ; Hrrison nd Hurd 1); nd the occurrence of hrmful lge, which cn dmge oysters nd other shellfish (Anderson et l. 2; Heisler et l. 8). Therefore, tsunmi-induced chnges to the nutrient sttus of costl mrine environments, if ny, my exert profound nd prolonged influences on the ongoing processes of ecosystem recovery nd fishery reconstruction in the impcted regions. This study ws conducted s prt of the post-tsunmi ecosystem monitoring progrm supported y the Tohoku Ecosystem Associted Mrine Sciences (TEAMS) reserch progrm ( Our ojective ws to ssess the nutrient sttus of Otsuchi By susequent to the 11 erthquke. Otsuchi By is semi-enclosed y opening to the western North Pcific. Its length nd width re pproximtely 7 nd 2 4 km, respectively. Three rivers, the Otsuchi, Kozuchi, nd Unosumi, with totl wter dischrge of 3 35 m 3 s 1 (Ano et l. 5), flow into the y. The residentil res of Otsuchi town nd the Unosumi re of Kmishi city, in which out 22, people lived efore the erthquke, re distriuted long the costl zone of the y nd the wtersheds of the rivers. The costl community nd the societl nd industril (especilly fishery-relted) infrstructures of these res were severely dmged y the tsunmi, which reched mximum height in Otsuchi By of 15.1 m (Iwte Prefecture 13). In Otsuchi town, the numer of tsunmi victims reched 16. In the inundted re, pproximtely 6.68 km 2, out 56 uildings collpsed (Yngw nd Ski 14). The mount of rule in Otsuchi town nd the Unosumi re of Kmishi city ws 612 ktons, nd 235 ktons of tsunmi deposits were left on the inundted re (Kmishi city 11; Otsuchi town 14). The sewge tretment plnt of Otsuchi town locted ner the river mouth of the Kozuchi River went out of opertion due to the tsunmi. As n emergency response, primry effluent ws relesed into the Kozuchi River from the end of Mrch 11 until the restortion of norml opertion of the plnt on 31 July 12 (Ministry of Lnd, Infrstructure, Trnsport nd Tourism of Jpn 11; Iwte prefecture 14). Physicl prmeters s well s concentrtions of inorgnic nutrients nd chlorophyll were monitored for more thn 3 yers, from My 11 to July 14. The Ocen Reserch Institute ( predecessor of the Atmosphere nd Ocen Reserch Institute) of the University of Tokyo estlished field sttion t Otsuchi town in 1973 clled the Costl Mrine Reserch Center, which ws reformed s the Interntionl Costl Reserch Center (ICRC) in 3. Since the estlishment of this center, reserchers hve used the fcility to exmine the nutrient nd other ecologicl properties of Otsuchi By. In this study, we took dvntge of this reserch to compre our dt with those collected efore the erthquke (Fukud et l. 7, nd unpulished; Httori 1976, 1977,, 1978, ; Httori et l. 1974). 2 Mterils nd methods 2.1 Study site nd smple collection Wter smples were collected from four sttions (2, 6, 11, nd 16) long trnsect from the center of Otsuchi By to the mouth of the Unosumi River (Fig. 1). Sttions 2, 6, 11, nd 16 were locted ner the center of Otsuchi By (39 36 N, E; wter depth: 46 m), t the point intersection of the trnsect nd the extension line from the Fig. 1 Loctions of smpling sttions in Otsuchi By, Jpn

3 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost joint mouth of the Otsuchi nd Kozuchi rivers (39 29 N, E; wter depth: 39 m), in the vicinity of the re etween the communities of Murohm nd Hkozki (39 11 N, E; wter depth: m), nd t the mouth of the Unosumi River (39 N, E; wter depth: 1 m), respectively. Smples were collected every 2 months from 27 My 11 to 4 Mrch 14. Depth profiles of temperture, slinity nd turidity were determined y conductivity temperture depth (CTD) sensors (ACL218-TPM; Alec Electronics, Jpn: My to July 11; ASTD687; JFE ADvntech, Jpn: Septemer 11 to Mrch 14; ASTD12; JFE Advntech, Jpn: My to July 14). The turidity dt were only collected from 27 My 11 to 28 Novemer 12 ecuse the turidity sensor hd elow-cceptle performnce fter 3 Jnury 13. Secchi depth ws used to estimte the ottom of the euphotic lyer (Secchi depth 2; Holmes 197). The mixed-lyer depth ws determined sed on.125 unit σt criterion from tht of the surfce wter (Levitus 1982). A 12-L Niskin smpler ws used to collect sewter smples t predetermined depths (primrily t 4- to 26-m intervls), with the exception tht only surfce wter smples were collected t Sttions 6, 11, nd 16 on 27 My 11. To determine inorgnic nutrient [nitrte (NO 3 ), nitrite (NO 2 ), mmonium (HN 4 + ), phosphte (PO 4 3 ) nd silicic cid (Si(OH) 4 )] concentrtions, sewter smples were contined in 1-ml crylic resin tues nd stored t C until nlysis. To determine the concentrtion of chlorophyll (herefter Chl. ), susmples of sewter were filtered through GF/F filters (Whtmn Interntionl) nd the filters were stored t C until nlysis. 2.2 Determintion of concentrtions of inorgnic nutrients nd chlorophyll Concentrtions of NO 3, NO 2, HN 4 +, PO 4 3, nd Si(OH) 4 were determined clorimetriclly using n utonlyzer (AACS II or AACS III; Brn+Luee, Germny) ccording to Armstrong et l. (1967). Before the nlysis of Si(OH) 4, frozen smples were stored in the drk t room temperture for ~48 h (Gordon et l. 1993). We used unfiltered sewter smples for the nutrient nlysis ccording to the protocol of WOCE (Gordon et l. 1993). Note tht, lthough inorgnic nutrients in unfiltered smple wters re considered to e mostly solule in ocenic wters (WOCE; Gordon et l. 1993), they cn lso include those ssocited with prticles (e.g., y sorption), especilly in high turidity wters. In this pper, the sum of NO 3, NO 2 nd NH 4 + concentrtion is referred to s totl inorgnic nitrogen (TIN) concentrtion ecuse high-turidity wter ws included in the smples (see Results ). The detection limits, defined s three times the stndrd devition of repeted mesurements of the lnk, were.5,.2,.1,.2, nd.3 μm for NO 3, NO 2, HN 4 +, PO 4 3, nd Si(OH) 4, respectively. Chl. concentrtions were determined y fluorometry (Fluorometer 1-AU; Turner Designs, USA), using the cidifiction method (Holme-Hnsen et l. 1965) fter extrction with N, N-dimethyl formmide (Suzuki nd Ishimru 199). 2.3 Sttisticl nlysis Differences in the vriles mong different periods were tested y Kruskl Wllis one-wy ANOVA on rnks test nd then y the post hoc Dunn s method ecuse the normlity test (Shpiro Wilks) for vriles filed (p <.5). To nlyze the reltionships etween vriles, the reduced mjor xis ws used to define the line of est fit (Quinn nd Keough 2). The sttisticl nlyses were performed using SigmPlot 12.5 (Hulinks, USA). The regression line ws clculted using Excel 13 (Microsoft, USA). 2.4 Nutrient dt collected efore the tsunmi The nutrient sttus of the Otsuchi By efore the erthquke ws exmined using dt reported in the literture. These dt consisted of those collected y Dr. Akihiko Httori etween 1973 nd 1977 (herefter, B1 dt; Httori 1976, 1977,, 1978, ; Httori et l. 1974) nd those otined y the first uthor of the present pper nd his collegues etween 1996 nd 8 (herefter, B2 dt; Fukud et l. 7, nd unpulished). The B1 dt included the nutrient concentrtion dt [NO 3, NO 2, HN 4 +, PO 4 3, nd Si(OH) 4 ] minly collected t the y center ner Sttions 2, 6 nd 11 of this study. These dt were collected Numer of smple B1 B2 A1 A2 A Month Fig. 2 Frequency distriution of the numer of dt efore nd fter 11 off the Pcific cost of Tohoku Erthquke y month of survey. Dt efore My 11 were otined from the literture (see Sect. 2). The dt collected efore the erthquke were seprted into two groups (B nd B ), while those collected fter the erthquke were divided into three groups (A1 My 11 Mrch 12; A2 My 12 Mrch 13; nd A3 My 13 Mrch 14)

4 H. Fukud et l. in different months (Fig. 2). Nutrient concentrtions were determined colorimetriclly [using the cdmium columndizotiztion method for NO 3, the dizotiztion method for NO 2, the phenol-indophenol method for mmonium, the molydte method for PO 4 3 nd the molydte method for Si (OH) 4 ] for smples with or without filtrtion through grss fier filters (GF/C). The B2 dt consisted of those collected minly t the y center ner Sttions 2 nd 11 of this study. These dt were collected through ll sesons, lthough the dt were most intensively collected in My (Fig. 2). Nutrient concentrtions were determined for NO 3, NO 2, HN 4 +, PO 4 3 using n utonlyzer, ccording to the method descried in the present pper. Note tht the B2 dt lck the dt of Si(OH) 4 concentrtion. To fcilitte the comprison of the nutrient sttus etween efore nd fter the erthquke, we seprted the sesonl dt into three periods: strtified period (My Septemer), mixing period (Octoer Jnury) nd Oyshio intrusion period (Ferury April). High turidity following the erthquke lsted until t lest Septemer 11 (Fig. 3d). Between My nd July 11, turidity ws greter thn.25 ppm (kolin turidity) throughout the wter column. At this time, high turidity exceeding 1 ppm ws distriuted in the surfce nd the ner-ottom lyers (Fig. 4, ). In Septemer 11, the high turidity zone ws in the lower prt of the wter column (Fig. 4c). High turidity in the susurfce lyer ws lso oserved in Mrch 12 (Figs. 3d, 4f). Aside from 11, high turidity ner the ottom ws lso oserved etween My nd Novemer 12 (Fig. 3d). 3 Results In this section, the temporl chnges in physicl nd nutrient prmeters t different depths re descried for Sttion 2 (ner the center of the y) s representtive exmple, unless otherwise noted. 3.1 Wter temperture, slinity, euphotic lyer depth, nd turidity During the oservtion period, the surfce wter temperture egn to increse t the end of Mrch nd then reched the nnul mximum vlue of out 22 C in Septemer (Fig. 3). Susequently, the wter temperture decresed towrd the nnul minimum vlue of round 5 C in Mrch. In the summer period, from My to Septemer, while wter strtifiction ws developed in the upper lyer of the wter column, the slinity of the lower lyer of the wter column ecme greter thn 33.7 (except during 12), indicting intrusion of Tsugru current sline wter into the y (Hnw nd Mitsuder 1987). In this period, the slinity of the surfce wter decresed remrkly ner the center of the y (sttion 2), suggesting high river dischrge. Between Octoer nd Ferury, the mixed-lyer depth deepened except etween Octoer 13 nd Ferury 14 (Tle 1). During this high-mixing period, the entire wter column filled with the highly sline wter, except in utumn of 13. Between Ferury nd April, Otsuchi By ws filled with cold (<7 C) low-slinity (<33.7) wter trnsported y the Oyshio Current (Hnw nd Mitsuder 1987). The euphotic lyer depth vried in the rnge of m (Tle 1). Fig. 3 Temporl vritions in wter temperture, slinity, c density (in σt units), nd d turidity t sttion 2 in Otsuchi By from 27 My 11 to 23 July 14. The turidity dt hd elow-cceptle performnce fter 19 Novemer 11

5 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost Tle 1 Summry of the degree of trnsprency nd the mixed-lyer depth t four smpling sttions in Otsuchi By y dte of the survey Dte of survey Trnsprency (m) Mixed lyer depth (m) Sttion 2 Sttion 6 Sttion 11 Sttion 12 Sttion 2 Sttion 6 Sttion 11 Sttion My (.2) 7.1 (14.2) 6.3 (12.6) 6.1 (Bottom) <5 <5 <5 <5 27 July (19.) 9. (18.) 8. (16.) 3. (6.) <5 <5 <5 <5 Septemer (22.) 12. (24.) 8.5 (.) 9. (Bottom) <5 <5 <5 <5 Novemer (23.) 1. (.) 7.5 (15.) 1. (Bottom) Bottom <5 16 Jnury (24.) 11. (22.) 11.5 (Bottom) 1. (Bottom) 21.9 <5 5.7 <5 22 Mrch (8.) 7. (14.) 5. (1.) 2.5 (5.) <5 <5 <5 <5 22 My (18.) 7. (14.) 5. (1.) 5. (1.) <5 <5 <5 <5 22 July (18.) 9. (18.) 9.5 (19.) 3. (6.) <5 <5 <5 <5 16 Octoer (18.) 9. (18.) 8. (16.) 5.5 (Bottom) 8.2 <5 <5 <5 28 Novemer (23.) 1. (.) 1.5 Bottom) >1 (Bottom) 9.7 <5 <5 <5 3 Jnury (.4) 7.8 (15.6) 6.5 (13.) 3.8 (7.6) 13.5 <5 9.4 <5 12 Mrch (26.) 15. (3.) 14.5 (Bottom) >1 (Bottom) <5 <5 28 My (13.) 6.7 (13.4) 5.5 (11.) 5. (1.) <5 <5 <5 <5 23 July (11.) 4. (8.) 3.6 (7.2) 3. (6.) <5 <5 <5 <5 25 Septemer (.) 8. (16.) 7.5 (15.) 6. (Bottom) <5 <5 <5 <5 19 Novemer (.) 8. (16.) 6.8 (13.6) 4. (8.) <5 <5 <5 <5 Jnury (32.) 16. (32.) 14.5 (Bottom) >1 (Bottom) <5 <5 <5 <5 4 Mrch (13.) 7.5 (15.) 6. (12.) 7.9 (Bottom) <5 <5 <5 <5 28 My (21.) 7.5 (15.) 8. (16.) 5. (1.) <5 <5 <5 <5 23 July 14 c 7. (14.) 2.5 (5.) <5 <5 <5 <5 Numers in prentheses indicte the euphotic lyer depth estimeted from the degree of trnsprency (see Sect. 2) The mixed lyer depth ws sed on.125 unit σt criterion (Levitus 1982) Wter smple ws collected from only 1 m depth t Sttions 6, 11 nd 16 c Wter smple ws collected from Sttions 2 nd Temporl chnges in inorgnic nutrients nd chlorophyll concentrtions fter the erthquke NO 3 ws depleted in the surfce wter in summer, during which strtifiction developed (Fig. 5). NO 3 concentrtion egn to increse with the increse in wter mixing depth nd then reched its nnul mximum in Mrch, when Oyshio wter intruded into Otsuchi By. In Mrch 12, the NO 3 concentrtion reched >1 µm, vlue greter thn those of the similr period in 13 nd 14. However, no remrkle feture of NO 3 concentrtion ws oserved in the high-turidity period etween My nd Septemer 11. Except for 11, NO 2 ws lower thn.3 µm throughout the wter column, except during the high-mixing period etween Novemer nd Jnury (Fig. 5). In 11, however, the NO 2 concentrtion ws greter thn.3 µm in the high-turidity wter tht existed etween My nd Septemer. In this period, the mximum NO 2 concentrtion in the wter column occurred ner the ottom (Fig. 5). The NO 2 concentrtion in the ottom wter in 11 t Sttion 2 decresed from 1.33 µm in My to.38 µm in Septemer, ut then incresed to.83 µm in Novemer. Figure 6 shows the temporl vrition in the nitrite-to-nitrte (NO 2 / NO 3 ) rtio. The NO 2 /NO 3 rtio ws high etween My nd Septemer of 11. Notly, in My 11, the NO 2 / NO 3 rtio exceeded 1 throughout the wter column, wheres in July nd Septemer of 11, the NO 2 /NO 3 rtio ws high (>1) only within the euphotic lyer. Sesonl fluctutions in HN + 4 concentrtion showed different pttern from those of NO 3 nd NO 2 (Fig. 5). HN + 4 concentrtion incresed throughout the wter column in Mrch, nd it remined t reltively higher concentrtion in the ner-ottom lyer thn in the surfce lyer in the strtified period etween My nd Septemer. In 11, however, the HN + 4 concentrtion in the ner-ottom lyer etween My nd Septemer ws lower thn tht in corresponding periods of 12, 13, nd 14. Additionlly, notle susurfce mximum in HN + 4 concentrtion ws oserved t Sttions 2 nd 6 (dt not shown) in Septemer 11 (Fig. 5c).

6 H. Fukud et l. Fig. 4 Sptil distriution of turidity (in logrithmic scle) on 22 My, 27 July, c Septemer, nd d Novemer 11, nd e 16 Jnury nd f 22 Mrch 12 PO 3 4 in the upper prt of the wter column ws lmost depleted during the strtified period, nd then incresed throughout the wter column in the mixing period (Fig. 5). Although this generl pttern ws common etween the 3 yers, PO 3 4 concentrtions etween Septemer 11 nd Mrch 12 were notly higher thn those in the corresponding period fter 2 yers. High- PO 3 4 wter (>.5 µm) first ppered in the ner-ottom lyer t Sttion 2 in Septemer 11 (Fig. 7 e). Susequently, PO 4 3 concentrtion incresed in the photic lyer in Novemer nd then spred throughout the entire y during the highmixing period etween Jnury nd Mrch, except tht the concentrtion ws low in the euphotic zone in Mrch when spring phytoplnkton loom ws likely initited, s indicted y high Chl. concentrtion t the depth of 1 m (Fig. 5f). From Novemer 11, the concentrtion of TIN lso egn to increse from the lower prt of the wter column nd then reched its pek in Mrch 12 (Fig. 7j). The TIN/P rtio displyed lrge vriility in time nd spce (Figs. 6, 7k o). The men TIN/P in the y ws notly low in Novemer 11 [5.8 ±.6 (n = )] nd Jnury 12 [6.9 ±.3 (n = )]. In Mrch 12, TIN/P in the photic lyer ecme greter thn 1. The TIN/P rtio in the mixing period in the next winter ws close to the Redfield rtio (16) [mens of Novemer 12 nd Jnury 13 were 15.2 ± 1.5 (n = ) nd 13.6 ± 1.8 (n = ), respectively], which ws followed y the higher TIN/P rtio in Jnury 14 (23.8 ± 5.: n = ) (Fig. 6). The pttern of sesonl fluctution in Si(OH) 4 concentrtion ws generlly chrcterized y high nd low concentrtions during the mixing nd strtified period, respectively (Fig. 5). In Septemer 11, however, higher Si(OH) 4 concentrtion compred with those in the corresponding period of 12 nd 13 ws oserved. The distriution of Si(OH) 4 in Septemer 11 ws complex nd differed from the distriutions of the other nutrients (Fig. 8), eing chrcterized y the presence of the susurfce mximum round the ottom of the euphotic lyer t Sttions 2, 6, nd 11 (Fig. 8). The Si(OH) 4 concentrtion in Mrch 12 tended to e higher thn those in Mrch 13 nd 14, trend similr to tht of NO 3 nd PO 4 3 concentrtions (Fig. 5). The Si(OH) 4 -to-tin rtio (Si/TIN) in Mrch 12 ws reltively high (15.9 ± 3.5: n = ), compred to the corresponding rtio during the mixing period

7 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost Fig. 5 Temporl vrition in concentrtions of NO 3, NO 2, c NH 4 +, d PO 4 3, e Si(OH) 4, nd f Chl. t Sttion 2 in Otsuchi By from 27 My 11 to 23 July 14 in the winters of 12 nd 13 [7.8 ± 1.9 (n = ) nd 12. ± 3.4 (n = ), respectively] (Fig. 6c). Chl. concentrtion in the euphotic lyer vried in the rnge of µg L 1 during the investigtion period, with the highest vlue t the depth of 5 or 1 m in Mrch 12. We identified no notle feture or trend tht might e relted to the tsunmi impct (Fig. 5f). 3.3 The nutrient concentrtions efore nd fter the erthquke Men concentrtions of inorgnic nutrients clculted for strtified, mixing nd Oyshio intrusion periods were compred with the B1 dt ( ), the B2 dt (1996 8), nd the dt collected during three successive yers following the erthquke (A1 dt: My 11 Mrch 12; A2 dt: My 12 Mrch 13; A3 dt: My 13 Mrch 14) (Fig. 9). This comprison reveled severl notle fetures: (1) during the strtified period of the yer immeditely fter the erthquke (A1), medin NO 2 concentrtions were more thn twofold nd significntly (p <.5) higher thn the men vlues found for the corresponding period in B1 nd B2, (2) irrespective of the period, medin Si(OH) 4 concentrtions in A1 were significntly (p <.5) higher thn the corresponding vlues found in B1, nd (3) the medin PO 4 3 concentrtion in the mixing period of A1 ws more thn twofold nd significntly (p <.5) higher thn those in the corresponding period in B1 nd B2 s well s in A2 nd A3. TIN did not disply mrked difference in concentrtion etween efore nd fter the erthquke, except tht the medin TIN concentrtion determined during the mixing period of A2 ws higher thn those in B1 nd B TIN/P rtio efore nd fter the erthquke The temporl chnges in the phosphorus to nitrogen nutrient stoichiometry were exmined y plotting the concentrtions of TIN (y-vrile) nd PO 4 3 (x-vrile) in twodimensionl spce for the individul dtsets collected efore (B1, B2) nd fter (A1, A2, A3) the erthquke (Fig. 1). In ech of these grphs, reference line is given

8 H. Fukud et l. 4 Discussion Fig. 6 Temporl vritions in NO 2 /NO 3 rtio, TIN/P rtio, nd c Si/TIN rtio t Sttion 2 in Otsuchi By from 27 My 11 to 23 July 14 to indicte the Redfield rtio (N:P = 16). In B1, the plots scttered rodly, displying lrge nd nonsystemtic devition from the Redfield rtio. The correltion etween TIN nd PO 4 3 ws insignificnt (p >.5). In contrst, in B2, there ws positive correltion etween TIN nd PO 4 3 (R =.851, n = 7, p <.1), with the slope of the regression eing 13. (Tle 2). During the yer immeditely fter the erthquke (A1), the correltion etween TIN nd PO 4 3 ws weker thn tht in B2 nd there ws notle devition towrd low TIN/P rtios, especilly during the mixing period. Reflecting this devition, the slope of the regression tht relted TIN to PO 4 3 ws gentle (1.) (Tle 2). During susequent yers (A2 nd A3), the devition in TIN/P rtio from the Redfield rtio ws moderted, s indicted y the incresed slopes of the TIN to PO 4 3 regression, which were 15.9 nd.1 for A2 nd A3, respectively (Tle 2). During the mixing period, there ws notle devition towrd high TIN/P rtios: 18.3 ± 6.8 (n = ) for A3 (Fig. 6). The steep slope of the regression tht relted TIN to PO 4 3 for A3 ws resulted from this devition (Fig. 1). Bsed on the physicl dimensions of the y (length 7 km; width 2 4 km) nd moderte residul current (c. 3 5 cm s 1 ; Otoe et l. 9), it is inferred tht the wter residence time within the y is on the order of week, or less thn month. Despite the corse resolution (2-month intervl) of our smpling reltive to the wter residence time, we identified some notle fetures in the nutrient sttus of the y following the 11 erthquke nd tsunmi event. These fetures include (1) high turidity, (2) high NO 2 (up to 1.33 μm) nd Si(OH) 4 (up to 32.5 μm) until Septemer 11, (3) the spred of high-phosphte wter nd notly low TIN/P rtio (men 6) during the mixing period etween Novemer 11 nd Jnury 12, nd (4) n increse in TIN/P, which pproched or exceeded the Redfield rtio (N/P = 16), during the mixing periods of the susequent yers. The identifiction of these fetures is sed on the evlution of the time-series nutrient dt collected over 3.5-yer period following the erthquke event, nd the comprison of the nutrient dt collected fter the erthquke with those collected efore the erthquke, the ltter eing discontinuous nd lrgely different from the former in nture, especilly in regrd to sptiotemporl resolutions. Given these inherent limittions in our pproch, ny inference on possile connection etween the erthquke nd the nutrient sttus of Otsuchi By should e mde with extreme cution. Any nomlous fetures found in the yer (or yers) following the erthquke might not e truly ssocited with the erthquke nd tsunmi, ut would rther e reflection of nturl vriility cused y other forces. Keeping these limittions in mind, the detils re discussed in the following susections. 4.1 High turidity until Septemer 11 In My nd July 11, high-turidity wter ws distriuted in the surfce lyer nd lower prt of the wter column (Figs. 3, 4). In the following Septemer, the depth of the high-turidity zone extended down to the deeper prt of the y. Nishi et l. (13) estimted the distriution pttern of the mximum velocity of the mssive tsunmi nd the ottom sheer stress in Otsuchi By from numericl simultion, nd then evluted the tendency for erosion using the Shields numer (ottom sheer stress divided y the sumerged density nd the dimeter of sediment) s n indictor. They pointed out tht the sefloor of the inner prt of Otsuchi By could e esily eroded y the tsunmi current nd tht resuspended sediments would tend to e trnsported y it. Chnges in the chemicl composition of the sediment surfce in Otsuchi By fter the erthquke re

9 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost Fig. 7 Sptil distriution of e PO 4 3, e j TIN, nd k o TIN/P rtio long the trnsect from Sttion 2 to Sttion 16 on, f, k 27 July 11,, g, i Septemer 11, c, h, m Novemer 11, d, i, n 16 Jnury 12, nd e, j, o 22 Mrch 12. White solid lines show the ottom of the euphotic lyer in greement with the ehvior of the sediment under the tsunmi current (Niki et l. 15). Other dt lso suggest tht mssive mounts of sediments were introduced into Otsuchi By due to the tsunmi. Okysu et l. (13) estimted tht out one-third of the sediments from the sndr locted t the river mouth of the Unosumi River were deposited on the inundted re long the river. Although it is not completely cler wht mechnism cused the highturidity condition to lst more thn 6 months, the fct tht high turidity ws found not only ner the ottom ut lso in the surfce lyer nd in the inner prt of the y suggests tht riverine input of tsunmi deposits on lnd contriuted to the mintennce of the high-turidity condition for prolonged period. In ddition, tmospheric deposition of dust might hve lso contriuted to the increse in turidity, lthough this ssertion requires further investigtion. 4.2 Unusul nutrient sttus during the high turidity period During the high-turidity period, the concentrtion of NO 2 ws remrkly high (up to 1.33 μm) reltive to

10 H. Fukud et l. Fig. 8 Sptil distriution of Si(OH) 4 long the trnsect from Sttion 2 to Sttion 16 on Septemer 11. White solid lines show the ottom of the euphotic lyer those in the corresponding periods of 12 nd 13. Such high NO 2 concentrtion hs rrely een oserved in Otsuchi By efore the erthquke (Fukud et l. 7, nd unpulished; Httori 1976, 1977,, 1978, ; Httori et l. 1974). An unusul feture in the high NO 2 concentrtion ws high NO 2 /NO 3 rtio (2 6) throughout the wter column in July nd Septemer 11. Generlly, in oxic wter columns of costl mrine environments, NO 2 concentrtion is low (<.5 μm; Kmykowski nd Zentr 1991). This is ecuse in oxic sewter columns, NO 2 is primrily produced s n intermedite product of nitrifiction (oxidtion of NH 4 + to NO 3 ), during which the ccumultion of NO 2 is generlly known to e smll (i.e., the oxidtion of NH 4 + to NO 2, rther thn the oxidtion of NO 2 to () 1.5 My-Septemer Octoer-Jnury Ferury-April (c) My-Septemer Octoer-Jnury Ferury-April (µm) NO TIN (µm) 1 5., c c, c , c, c 51,, c 53, , , 53 c B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 () My-Septemer Octoer-Jnury Ferury-April (d) My-Septemer Octoer-Jnury Ferury-April Si(OH) 4 (µm) (µm) PO c c B1 A1 A2 A3 B1 A1 A2 A3 B1 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 B1 B2 A1 A2 A3 Fig. 9 Box plots of concentrtions of NO 2, Si(OH) 4, c TIN, nd d PO 4 3 otined from two different periods efore the erthquke (B nd B ), nd three different periods following the erthquke (A1 My 11 Mrch 12; A2 My 12 Mrch 13; nd A3 My 13 Mrch 14). Dt were grouped ccording to: strtified period: My Septemer; high mixing period: Novemer Jnury; Oyshio intrusion period: Ferury April. The ottom nd top of the ox indicte 25th nd 75th percentiles, respectively, nd the line inside the ox indictes the medin. Error rs 1th nd 9th percentiles. Closed circle outlier. The letters elow the oxes indicte group ctegorized y difference in men vlues sed on Kruskl Wllis one-wy ANOVA on rnks with post hoc Dunn s method with significnce level of p <.5. The numers elow the oxes indicte the numer of dt points

11 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost Fig. 1 Reltionship etween TIN nd PO 4 3 for the dt collected efore (B nd B ) nd fter (A1 My 11 Mrch 12; A2 My 12 Mrch 13; nd A3 My 13 Mrch 14) the erthquke for strtified period (closed circles), high mixing period (open circles), nd Oyshio intrusion period (closed inverted tringles). The liner regression line nd the Redfield rtio re indicted y the solid nd dshed lines, respectively TIN (µm) : B : B2 16:1 16: My 11-Mr. 12: A Y = 13. X -.67 R 2 =.725, n = 7 My 12-Mr. 13: A2 16:1 16: TIN (µm) Y = 15.9 X -.65 Y = 1. X R 2 R 2 =.96, n = 12 =.588, n = My 13-Mr. 14: A3 16:1 PO 4 3- (µm) TIN (µm) 1 5 Y =.1 X -.88 R 2 =.612, n = PO 4 3- (µm) Tle 2 Summry of regression nlysis etween totl inorgnic nitrogen (sum of nitrte, nitrite nd mmonium: TIN) nd phosphte Period Slope (95 % CI) Intercept (95 % CI) R 2 n p References Before the Tsunmi ( ).67 ( 1.2 to.32) <.1 Fukud et l. (7) Fukud et l., unpulished. After the Tsunmi My 11 Mrch ( ) 1.9 ( 1.93 to.24) <.1 This study My 12 Mrch ( ).65 (.97 to.33) <.1 This study My 13 Mrch 14.1 ( ).88 ( 1.42 to.35) <.1 This study (PO 3 4 ) in Otsuchi By. The regression lines were estimted y reduced mjor xis method (see Sect. 2) NO 3, usully represents rte limiting step of nitrifiction; Loms nd Lipschultz 6; Wrd 2). However, under hypoxic conditions, the nitrifiction my result in the ccumultion of NO 2, s oserved in wstewter tretment plnts (Jinlong nd Ning 4; Ruiz et l. 3). Also, under noxic conditions, the prominent ccumultion of NO 2 my occur s consequence of relese during dissimiltory reduction of NO 3 (Knowles 1982;

12 H. Fukud et l. Philips et l. 2). Our dt showing the mximum NO 2 concentrtion in the ner-ottom lyer in My nd July might e reflection of the NO 2 relese from hypoxic or noxic sediments to the overlying wter. However, this scenrio would not completely explin the reltively high NO 2 concentrtion nd high NO 2 /NO 3 rtio even in the surfce mixed lyer. The trnsport of NO 2 relesed from the sediment to the surfce lyer would e strongly impeded y the presence of pycnocline etween My nd Septemer. This rises the possiility tht lrge quntity of nitrite ws dischrged into the y vi inflowing surfce nd ground wters. Frozen fish products, which were stored in refrigerted wrehouses, were relesed to the surrounding re y the tsunmi, nd the rotting mterils covered the lndside of the y (Ksi nd Koyshi 12; Tru et l. 12). It is likely tht mssive putrefction of fish nd other orgnic mteril following the tsunmi deposit resulted in the cretion of reduced environments round the y, which ws promoted y microil ctivity enhnced due to wrmer tempertures in My nd July. In support of this hypothesis, Sugihr et l. (in preprtion) detected high Mn concentrtion in prt of shell of Mytilus glloprovincilis, which ccumulted fter the erthquke, collected from intertidl zone of Otsuchi By in Septemer 11, suggesting tht reduced environment in which Mn could exist in dissolved form ppered round the y. During the lter prt of the high-turidity period, the concentrtion of Si(OH) 4 incresed throughout the wter column until My 12. The rnge of the concentrtion of Si(OH) 4 in the high-turidity period in 11 ws significntly higher thn those oserved in the strtified periods of 12 nd 13 nd tht reported previously in (B1). In contrst to the concentrtion of Si(OH) 4 in the high-turidity period, the rnges of the concentrtions of TIN nd PO 4 3 did not differ lrgely from those efore the erthquke. This decoupling mong the distriutions of Si(OH) 4, TIN, nd PO 4 3, nd the distriution of turidity in the y suggests tht high mount of Si(OH) 4 ws introduced in ssocition with the riverine input of sediments. 4.3 Possiility of high PO 4 3 loding from the sediment during the mixing period in 11 During the mixing period etween Octoer 11 nd Jnury 12, PO 4 3 concentrtion ws high reltive to those found in the corresponding period of the susequent yers (12 13, 13 14) nd those reported for the period efore the erthquke ( , ). The increse in PO 4 3 concentrtion egn from the ner-ottom lyer in the center of the y in Septemer, nd wter with low TIN/P rtio, which ws much lower thn the Redfield rtio, then spred to whole of the wter column in the mixing period. This could e explined y the spred of high PO 4 3 nd low DIN/P wter formed ner the ottom to the whole y due to upwelling induced y estwrd wind t the inner prt of Otsuchi By which my cuse three-dimensionl mixing to trnsport sediment-contct wter to the surfce (Furuy et l. 1993; Otoe et l. 9; Shikm 199). High PO 4 3 nd low TIN/P wter ws formed ner the ottom, presumly ecuse of the relese of PO 4 3 from noxic sediment (Howrth 1988) ccompnied y depletion of TIN due to denitrifiction (Burgin nd Hmilton 7; Howrth 1988). This might e relted to the tsunmi-induced loding of sediments nd deris to the ottom of the y. 4.4 The nutrient sttus following 12 nd 13 The low TIN/P rtio in the mixing period of 11 ws moderted y intrusion of Oyhsio wter in Mrch 12, following which the TIN/P rtio in the mixing periods of 12 nd 13 ecme higher thn those in , pproching the Redfield N/P rtio of 16. Generlly, in the offshore region of the Snriku region, the TIN/P rtio in susurfce wter is typiclly round (Fukud et l., unpulished dt; Ksi et l. 1997; Whitney 11). Therefore, Otsuchi By wters during the mixing nd Oyshio intrusion periods in nd were generlly more enriched in N nd depleted in P reltive to the source sewter tht intrudes into the y in winter. In contrst, efore the erthquke (B2; ), the TIN/P rtio ws close to the vlue of the source wter during the mixing nd Oyshio intrusion period. Possile mechnisms cn e hypothesized to explin the increse in TIN/P following the second yer fter the erthquke: (1) n increse in TIN/P rtio of orgnic nd inorgnic mterils dischrged into the y from lnd; (2) decrese in denitrifiction in the sediment; (3) incresed nitrogen fixtion; nd (4) ltertion in nitrogen to phosphorus stoichiometry of iotic communities in nd/or round the y. Future studies should exmine these possiilities in greter detil vi the continution of monitoring nd thorough evlution of nutrient cycling mechnisms in the y. 4.5 Concluding remrks During our oservtion period, temporl chnges in nutrient sttus following the erthquke were detected. Representtive physicl disturnces cused y the tsunmi on the costl ecosystem include physicl erosion nd reconstruction of the sefloor. In Otsuchi By, distinctive ccumultions of NO 2, Si(OH) 4, nd PO 4 3 occurred or egn efore the diminishment of high turidity. Such physicl erosion nd reconstruction of the sefloor my hve een one of the key processes controlling the nutrient sttus following

13 Nutrient sttus of Otsuchi By (northestern Jpn) following the 11 off the Pcific cost the tsunmi, lthough other fctors (e.g., nutrient dischrge from distured lnds surrounding the y) my hve lso contriuted to the ltertion in the nutrient sttus. The role of the reconstructed sefloor in the nutrient dynmics remins uncler. In ddition to the possile ltertion in the nutrient sttus during the susequent yer of the erthquke (11 12), our results lso suggested tht the chnge in the nutrient sttus continued until t lest the eginning of 14. In the investigted region, lrge-scle pulic works to restore infrstructure, reconstruction of fisheries including the reuilding of quculture fcilities, nd other relted chnges in humn ctivity will continue for t lest severl yers. Continution of the monitoring of the nutrient sttus is necessry to provide scientific sis for the sustinle use of the ecosystem services provided y Otsuchi By. Acknowledgments We thnk T. Ae nd M. Ae of Otsuchi town (owners of the fishery vessel Myoho-Mru) for their ssistnce with the smpling in Otsuchi By during the difficult times of My nd Septemer 11 following the erthquke. We lso owe our grtitude to the stff of the Interntionl Costl Reserch Center, Atmosphere nd Ocen Reserch Institute, University of Tokyo for technicl nd logisticl support. K. Niki nd other stff of the Iwte Fisheries Technology Center provided helpful comments on the condition of the sediment fter the erthquke. This study ws supported y the Reserch Fund for Reconstruction Aid for the Est Jpn Gret Erthquke Disster, Jpnese Society of Fisheries Ocenogrphy PICES/ ICES; the Mitsui & Co., Ltd. Environment Fund; nd the reserch progrm Tohoku Ecosystem-Associted Mrine Sciences of the Ministry of Eduction, Culture, Sports, Science nd Technology. References Ano A, Otoe H, Tkgi M (5) River wter flowing into Otsuchi By. 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