(課程博士(英文))別紙4Date of submission (month day, year)

July 11れ 2019
Takanobu lnoue

Department
AI'chitecture andStudentlD Number

CivilEn部neerin牙Pplical】t's name

D145505

Shigeru KatoSupervisors

KurikoYokota

MbabaziJames

Abstnct (Doctor)
Iitle ofThesis'重血︑、

Bioavailable phosphorusin ranoa'suspended sedimentinrivers aowing throughContrasting land uses

Approx.80o words
Eutrophication in aquatic ecosystems is intimately linked with the increase in the concentrations ofnitrogen (N), phosphorus (P), a11d carbon (C); though p is the main limiting nutrient for primary
Production in freshwaters.圦lhile considerable information in recent years has pointed to theimportance of considering bioavailability 血 Water quality restoration proorams, established target
P load reduction guidelines are sti11 based on total phosphorus (1P). River water p is usuaⅡy
Separated into particulate p (PP) and dissolved p (DP), and the forms of p that support algal
growth are tenned bioavailable. Dissolved p is mostly available for algal uptake, while pp which
Consists of p associated 、Nith sediment and orσanic material eroded during runoff, contributes aVariable but long・term legacy source of bioavailable pp. Therefore, none of the cotlmlonly
measured p fractions are accurate measures of bioavailable phosphorus (BAP).1n general, pp ismuch more variable than dissolved p, and storm events are extremely important for pp dynalnics
because large portions ofriver pp loads can be transpoded attached to sediment and organic matter
during only a few major high aow events. The fraction of pp that is potentia11y bioavailable(PBAP), may increase if p is boul)d to clay minerals and de即'adable organic matter in suspendedSedin)ents (SS).1t is sti11 Undear how pa11iculate organic carbon (POC) al)d particulate orga11ic
nitrogen (PON), which are important components ofpadiculate organic matter in rivers inauence
the potential availability of pBAP. Furthermore, much is less hlown about the relationshipbetween pBAR and poC 砥ld poN stoichiometry despitethe spike in ss during hiσh aow events
The wide variability in pBAp suggests that both river・aow characteristics a11d la11d use patterns in
the river catcl)1nent may be important in addition to poc and poN concentration. since pp is
UsuaⅡy transported to rivers bound to ss,the qua11tity and type ofsedimentin 川noa'wi11inauence
the concentration of BAp by varying the relative amounts of Dp and pp entering the river.1t is
Poorly 山lderstood how di丘erent land・use types inauence pBAp in river runoff, and few studies
have estimated the alnount pBAp during low aow conditions. Besides, there is uncedainty
regarding the relationship between pBAp a11d particulate orσanic matter constituents U11derdifferent hydr010gical conditions in urban a11d agriculturalrivers.1n this thesis, we investigated theVariation in ss, DP, PP, PBAP, BAP, poc al)d poN in rivers draining through contrastine landUses

The objectives ofthe study were:(i)to estimate pBAp in run0丘fiom agricultural and urban rivers
in orderto assess whetherthe pBAp content wiⅡ be markedly differentin the rivers;(ii) to detennine the BAp propodion in river r山10ff, and to clarify its relationship with Tp in therivers; and iii)to exatnine the relationship between pBAP, poc and poN, and to relate this to theelemental composition ofss from the rivers.

Xve used a new rapid procedure that involves ultrasonic treatment to extract pBAp from ssSamples. A strong statisticalrelationship was observed between pBAp and ss in the river dassed
as agricultural(r2= 0.69),in contrastto the urba110ne where no conelation was observed,implyingthat the particle size a11d composition of ss in both rivers was markedly different. pp was thedominant fraction oflp at high aow and between 6 t079ツ0 ofpp was estimated to be bioavailable.

A strong conelation was observed between BAp a11d Tp in the urban river (r = 0.84), indicatingthat merely using lp as a proxy for BAp is questionable as this overestimates the eutrophication
Potential of some p sources. The BAp and lp relationshゆ in the agricultural river revealed thatelevated pp concentration at high aow may nottranslate into higher BAp in the runoff,implyingthat qua11tification ofBAp in ss is critical for effective watershed management programs. Besides,elevated poc atld poN concentrations in both rivers were associated with low aow. poc a11d

PON concentrations in both rivers decHned at high aow presumably due to the mixing effects of、Nater from different sources in the watershed which could have altered the particulate organic

matter composition. A significa11t conelation existed between poc and ss in both rivers, and poc
to poN ratios of ss from the rivers pointto the change in the particulate organic matter at varylng

ao、N conditions. PBAp exhibited a positive relationshゆ With poc and poN concentration in bothrivers possibly an indication that paniculate organic matter contributes to BAp in the rivers.1ron(Fe) was the dominant element measured in sS 丘'om both rivers, and the pBAp concentration in
the agriculturalriver was positively conelated with the Fe to poc ratio unHke in the urban river.
OveraⅡ,this study shows thatitis importantto quantify BAp in ss since lp includes complex pformsthat may not be available to algae. Furthermore,the origin ofss under changing aows could
be different, with subsurface sediment pathways Hnked t010w aow and contribution from surface
runoff associated with rain events, a11d paniculate organic matter constituents and sedimentelementalcontent could greatly impact BAp in riverrunoff̲ノ