Calibration of in situ chlorophyll fluorometers for organic matter

This is a sel -a chi ed e sion o an o iginal a icle. This e sion may di e om he o iginal in pagina ion and ypog aphic de ails. Au ho (s): Ti le: Yea : Ve sion: Copy igh : Righ s: Righ s u l: Please ci e he o iginal e sion: CC BY 4.0 h ps://c ea i ecommons.o g/licenses/by/4.0/ Calib a ion o in si u chlo ophyll luo ome e s o o ganic ma e © The Au ho (s) 2019 Published e sion Kuha, Jonna; Jä inen, Ma ko; Salmi, Pauliina; Ka jalainen, Juha Kuha, J., Jä inen, M., Salmi, P., & Ka jalainen, J. (2020). Calib a ion o in si u chlo ophyll luo ome e s o o ganic ma e . Hyd obiologia, 847(21), 4377-4387. h ps://doi.o g/10.1007/s10750-019-04086-z 2020 RESTORATION OF EUTROPHIC LAKES Calib a ion o in si u chlo ophyll luo ome e s o o ganic ma e Jonna Kuha .Ma ko Ja ¨ inen .Pauliina Salmi .Juha Ka jalainen Recei ed: 1 Feb ua y 2019 / Re ised: 16 Sep embe 2019 / Accep ed: 28 Sep embe 2019 / Published online: 5 No embe 2019 ÓThe Au ho (s) 2019 Abs ac O ganic ma e (OM) o he han li ing phy oplank on is known o a ec luo ome ic in si u assessmen s o chlo ophyll in lakes. Fo his eason, calib a ing luo ome ic measu emen s o OM e o is impo an . In his s udy, chlo ophyll (Chl) luo es- cence was measu ed in si u in mul iple Finnish lakes using wo sondes equipped wi h Chl luo ome e s (ex.470/em.650–700 nm). OM abso bance (A 420 ) was measu ed om wa e samples, and one o he wo sondes was also equipped wi h in si u luo ome e o OM (ex.350/em.430 nm). The sonde wi h Chl and OM luo ome e s was also deployed con inuously on an au oma ed wa e quali y moni o ing s a ion on Lake Konne esi. Fo da a om mul iple lakes, inclusion o wa e colou es ima es in o he calib a ion model imp o ed he p edic abili y o Chl assessmen s ma kedly. When OM abso bance o in si u OM luo escence was used in he calib a ion model, p edic abili y be ween he in si u Chl and labo a o y Chl a assessmen s was also enhanced. Howe e , co ec ion was no supe io o he one done wi h he wa e colou es ima e. Ou esul s demons a ed ha co ec ion wi h wa e colou assessmen s o in si u measu emen s o OM luo escence o e s p ac ical means o o e come he a ia ion due o OM when assessing Chl in humic lakes in si u. Keywo ds Au oma ed moni o ing Chlo ophyll a  Fluo escence O ganic ma e Op ical senso s  Wa e colou In oduc ion Phy oplank on biomass is widely used as an indica o o eu ophica ion in he s a us assessmen o su ace wa e s. Chlo ophyll aconcen a ion is used as a p oxy o phy oplank on biomass, adi ionally quan i ied in labo a o y om wa e samples by e hanol ex ac ion ollowed by spec opho ome ic measu emen a wa eleng hs 665 and 750 nm (Lo enzen, 1967); ISO 10260, 1992). The sho comings o his p o ocol a e also well known: i equi es la ge sample olumes, sample anspo a ion, s o age and handling. The e- o e, spa ial and empo al co e age and ep esen a- i eness o adi ional measu emen s a e gene ally limi ed. Du ing he las wo decades, ield Gues edi o s: Tom Jilbe , Raoul-Ma ie Cou u e, B ian J. Huse & Kale i Salonen / Res o a ion o eu ophic lakes: cu en p ac ices and u u e challenges J. Kuha (&)P. Salmi J. Ka jalainen Depa men o Biological and En i onmen al Science, Uni e si y o Jy a ¨skyla ¨, PO Box 35, 40014 Jy a ¨skyla ¨, Finland e-mail: [email p o ec ed] M. Ja ¨ inen F eshwa e Cen e/Wa e In o ma ion Sys em Uni , Finnish En i onmen Ins i u e (SYKE), La oka anonkaa i 11, 00790 Helsinki, Finland 123 Hyd obiologia (2020) 847:4377–4387 h ps://doi.o g/10.1007/s10750-019-04086-z(0123456789().,- olV)(0123456789().,- olV) spec o luo ome e s o measu e chlo ophyll in si u ha e become inc easingly common wo ldwide (Mein- son e al., 2015). In he ex , we use ‘‘Chl a’’ o e e o chlo ophyll aconcen a ion measu ed in labo a o y, in con as o ‘‘Chl’’ ha e e s o in si u luo escence o chlo ophyll. Due o he ease o measu ing, Chl luo ome e s a e equen ly used o s udy phy oplank on dis ibu ion, ac i i y and popula ion dynamics in si u (P oc o & Roesle , 2010; Zeng & Li, 2015). Fluo ome ic quan i ica ion o Chl is gene ally cos -e ec i e and allows equen obse a ions du ing sudden phenom- ena such as mixing e en s o sho -li ed algal blooms (Jennings e al., 2012; Klug e al., 2012). On he o he hand, in e p e a ion o in si u Chl luo ome e da a is no s aigh o wa d. Values yielded by he luo ome- e s a e a bi a y and need o be calib a ed o, o example, adi ional Chl aconcen a ions in o de o in e p e he measu emen s, and calib a ion is a ec ed by se e al sou ces o a ia ion. In addi ion o phy o- plank on biomass, he in ensi y o Chl luo escence is dependen on he pigmen composi ion and physio- logical eac ions o phy oplank on (Williams & B idges, 1964; Seppa ¨la ¨& Balode, 1998; Richa dson e al., 2010). Physical and chemical condi ions in he wa e also a ec he in si u Chl luo escence. Such a iables include changes in he unde wa e ligh and empe a u e condi ions, back sca e ing and abso p- ion o ligh by o he pa icles han phy oplank on (S ickland, 1968; Ca de e al., 1991; Se a e al., 2009; Downing e al., 2012; Os owska, 2012). Mos p ominen ly, Chl luo escence signal is a ec ed by luo escence o o ganic ma e (OM) ela ed o some- hing else han li ing phy oplank on (P oc o & Roesle , 2010; Twiss, 2011). P ac ices on how o con ol he sou ces o e o due o OM a e unde a igo ous scien i ic discussion. OM is a b oad e m, including DOM (dissol ed o ganic ma e ) and POM (pa icula e o ganic ma e ). In li e a u e, OM co ec ion has been conduc ed by using concep s o soluble luo escence (Ca lson & Shapi o, 1981), yellow subs ances (Twiss, 2011), luo escen o ch omopho ic dissol ed o ganic ma e (CDOM; P oc o & Roesle , 2010) o humic subs ances (Ca l- son & Shapi o, 1981). In his s udy, we use he gene al e m OM o desc ibe o ganic ma e ial o he han phy oplank on in he lake wa e , because we did no ac iona e o cha ac e ize he di e en OM compo- nen s. P e ious s udies ha e shown ha he OM signal is gene ally linked o alloch honous dissol ed humic subs ances in bo eal lakes (Ca lson & Shapi o, 1981; Twiss, 2011). In he ma ine en i onmen , phy oplank- on deg ada ion p oduc s may comp ise a la ge pa o o al OM (Xing e al., 2017). The a ie y o p oposed measu es o con ol he OM signal is b oad, indica ing i s impo ance in Chl luo escence p oblema ics. Ca lson & Shapi o (1981) il e ed a sample wa e o sub ac OM backg ound om Chl luo escence in No h Ame ican lakes and Leppa ¨e al. (1995) ollowed he same p o ocol in Finnish lakes. Twiss (2011) and P oc o & Roesle (2010) used mul iple wa eband senso s o measu e luo escen OM in lakes and o es ablish a calib a ion model be ween Chl and OM, whe eas Xing e al. (2017) used a single wa eband senso o luo escen OM o co ec in si u Chl luo escence alues in deep e ical p o iles om ma ine en i onmen . In si u Chl luo ome e s a e becoming inc easingly common in he moni o ing o inland wa e s, bu in e na ional s anda ds o ield ope a ion a e s ill unde de elop- men (e.g. La igne e al., 2012; C emella e al., 2018). In he nea u u e, inc ease in wa e colou may become mo e impo an due o b owning o bo eal lakes (Mon ei h e al., 2007; E ans e al., 2012; K i zbe g & Eks o ¨m, 2012). Co ec ions o Chl o OM luo escence would hus become inc easingly ele an in moni o ing. In bo eal lakes, he connec ion be ween dissol ed OM and wa e colou is ypically s ong and a he uni o m due o he p esence o humic subs ances (Ca lson & Shapi o, 1981; Hessen & T an ik, 1998; Keski alo & Elo an a, 1999). Resul s by Leppa ¨e al. (1995) indica ed good co ela ion be ween wa e colou and backg ound luo escence in ou een lakes in Eas e n Finland. Fo his eason, and he simplici y o wa e colou measu emen , he main aim o his s udy was o sc u inize wa e colou as a p oxy o OM in co ec ion in humic lakes. In his s udy, we es ed an in si u OM luo escence senso o co ec Chl esul s. Majo luo escence peaks o humic subs ances in wa e consis o exci a ion be ween 330 and 390 nm and emission be ween 420 and 500 nm wa eleng hs (Ma ilainen e al., 2011), and he OM senso deployed in his s udy co e ed he same wa eleng h ange. We hypo hesized ha co ec ing luo ome ic Chl da a wi h (1) a e age o median wa e colou , (2) wa e colou a he measu emen dep h assessed ia abso bance (A 420 ), o (3) in si u luo ome ic 123 4378 Hyd obiologia (2020) 847:4377–4387 measu emen s o OM imp o es, and equally well, he p edic abili y be ween luo ome ic Chl and adi- ional labo a o y Chl aassessmen . We aimed also o ob ain in o ma ion o he seasonal OM a ia ion wi hin one lake and o de e mine whe he con inuous moni o ing o Chl could bene i om he OM calib a ion. Ma e ials and me hods S udy lakes This s udy was conduc ed on lakes loca ed in Cen al and Sou he n Finland (N-Eu ope) a ying in size, nu ien concen a ion and wa e colou (Table 1). To unde s and lake-speci ic e ec s o di e en calib a- ion models, Lakes Konne esi and Jy a ¨sja ¨ i in Cen al Finland and Lakes Vesija ¨ i and Vanaja esi in Sou he n Finland we e sc u inized sepa a ely. O he lakes, Jy a ¨sja ¨ i and Vanaja esi ep esen small and la ge humic lakes, espec i ely. Based on hei ypical Chl aand o al phospho us concen a ions (Table 1), hese lakes a e oligo-meso ophic. Lakes Konne esi and Vesija ¨ i ep esen la ge clea wa e lakes, he la e being oligo ophic and he o me a mo e eu ophic lake. In si u Chl measu emen s and Chl a labo a o y analysis In si u measu emen s on he selec ed lakes we e done wi h wo sepa a e YSI6600 mul ipa ame e sondes ( ype V2-4; YSI Inc., Yellow Sp ings, OH, USA; he ea e Sondes 1 and 2) ha we e equipped wi h Chl luo ome e s (ex.470/em.650–700 nm) wi hou em- pe a u e co ec ion. Sonde 1 was used o measu e Chl luo escence on six o he s udy lakes including bo h humic and clea wa e lakes (Table 1). Samples o he labo a o y analysis o Chl awe e aken a he dep h o 1 m and se e al o he dep hs om he lakes measu ed wi h Sonde 1 (see he link in Da a a ailabili y o alues). Samples we e aken wi h a 4.4 l Limnos- ube sample and s o ed in cool (6°C) and da k un il he analysis wi hin 24 h a e he collec ion. Chl a(SFS- ISO 10260:1992) was measu ed in he labo a o y a e il a ion o 0.5–1.0 l o sample wa e h ough GF/C il e s. Chl awas measu ed using he cold e hanol ex ac ion me hod and wa e colou spec opho ome - ically. Wa eleng hs 665 and 750 nm we e measu ed Table 1 Loca ions and basic limnological cha ac e is ics o he s udy si es Lake Coo dina es (*WGS84) A ea (ha) Dep h (m) To -P, (lgl -1 ) Chl a, (lgl -1 ) Wa e colou , (mg P l -1 ) Sonde La Lon Max Mean Al aja ¨ i 62°18050.00400 25°43011.65500 210 16.5 3.8 29 16.5 80 1 Jy a ¨sja ¨ i 62°14011.67100 25°4602.86800 314 25.0 7.0 25 10.8 70 1 Konne esi 62°37057.36600 26°36016.50400 19,028 57.1 10.6 6 4.2 25 1 Ruokoja ¨ i 62°15032.90200 27°18041.2800 464 8.2 1.3 30 10.6 110 1 Vanaja esi 61°9019.71300 24°13044.98200 10,261 23.9 7.7 24 16.0 50 1 Vesija ¨ i 61°301.66200 25°3504.92600 6,471 40.0 6.1 27 9.6 10 1 Alasenja ¨ i 61°0046.56400 25°44035.2900 275 15.2 6.1 14 4.5 10 2 Alinen Rau ja ¨ i 61°11043.09800 25°601.24100 50 12.0 No da a 23 23.0 100 2 A kiomaanja ¨ i 61°3014.65900 25°44026.69200 208 20.2 5.1 17 5.3 25 2 Jou ja ¨ i 60°58036.40600 25°4206.56700 40 5.0 3.3 25 24.0 20 2 Me asja ¨ i 61°103.69500 25°4104.86800 24 2.6 1.5 31 19.3 50 2 Pa ¨a ¨ja ¨ i 61°3048.7600 25°7056.99800 1,352 85.0 14.8 12 5.5 70 2 Ruuhija ¨ i 61°1038.01100 26°0028.05400 573 18.7 5.6 18 10.8 30 2 Tyo ¨ ja ¨ i 60°59044.51500 25°2802.63500 56 7.0 1.5 23 No da a 50 2 Limnological da a om he da abase o Finnish En i onmen al Ins i u e (SYKE) o Dolman e al. (2015) Sonde e e s o ei he o he wo mul ipa ame e sondes (Sonde 1 and Sonde 2) ha was deployed on he lake To -P o al phospho us concen a ion, Chl a chlo ophyll aconcen a ion 123 Hyd obiologia (2020) 847:4377–4387 4379 wi h he Shimadzu UV-1800 spec opho ome e (Shi- madzu Co., Kyo o, Japan) in 5-cm qua z cu e es. Sonde 2 was used o measu e Chl luo escence in eigh lakes, also including humic and clea wa e lakes (Table 1). Ve ical p o iles we e eco ded, as o Sonde 1, and in eg a ed (0–2 m) samples o Chl acollec ed wi h a Limnos sample . Chl asamples we e il e ed using Wha man GF/F il e s and s o ed in a eeze (-20°C) un il measu emen wi hin 1–3 weeks a e sampling using a ho e hanol ex ac- ion me hod and spec opho ome e (SFS-ISO 10260, 1992; Dolman e al., 2015). The me hods o Chl aex ac ion we e di e en o he wo da ase s, because he ex ac ions we e ca ied ou in wo di e en ins i u es (Lammi Biological S a ion o Uni e si y o Helsinki and Uni e si y o Jy a ¨skyla ¨). Howe e , bo h me hods a e gene al and well-es ab- lished p ocedu es in labo a o y analysis o Chl a. Wa e colou o he s udy lakes (colou ypical ) In his s udy, we i s es ed he epilimne ic wa e colou o he s udy lakes as a pa ame e (he ea e colou ypical ) o co ec Chl measu emen s. Fo mos o he s udy lakes, hese colou alues, also used o lake ypology classi ica ion, we e a ailable om he da abase o Finnish En i onmen Ins i u e (SYKE, h ps://wwwp2.ympa is o. i/sc ip s/ki jaudu.asp, equi es ee egis a ion and use o websi e ansla o ). Wa e colou was analysed ei he wi h a Hellige/AVM Neo- compa a o o spec opho ome ically (wa eleng h 410 nm) acco ding o SFS-EN-ISO 7887:2011. The wa e colou o he me hods a e highly compa able. Fo ou o he s udy lakes, colou ypical alue was aken om Dolman e al. (2015), and we e de e mined using spec opho ome ic analysis om 0.45 lm p e- il e ed samples (SFS-EN-ISO 7887, 2011, 410 nm). On si e wa e colou measu emen s (colou Lab ) F om he lakes measu ed wi h Sonde 1, samples o he labo a o y analysis o wa e colou (colou Lab ) we e aken a he same dep hs as samples o labo a o y analysis o Chl a(see he link in Da a a ailabili y o alues). Samples we e aken as subsamples om he 4.4-l Limnos- ube sample and s o ed in cool and da k condi ions un il he analysis wi hin 24 h a e he collec ion. Wa e colou was measu ed in he labo a- o y a e il a ion o 0.5–1.0 l o sample wa e h ough GF/C il e s. Abso bance a 420 nm wa e- leng h was measu ed wi h he Shimadzu UV-1800 spec opho ome e using 1-cm qua z cu e es and con e ed o mg P l -1 (SFS-EN-ISO 7887, 2011). In si u luo escence o o ganic ma e Sonde 1 was equipped wi h Cyclops-7 o ganic ma e (OM) luo ome e (ex.350/em.430 nm; Tu ne Designs Inc., Sunny ale, CA, USA). The OM luo- ome e was calib a ed o wa e empe a u e as in Wa as e al. (2011), o which he empe a u e coe icien (q)o -0.009 was used o ans o m da a o a e e ence empe a u e o 20°C (RFU 20 ). Con inuous wa e quali y moni o ing on Lake Konne esi To es he pe o mance o di e en calib a ion me hods and o s udy seasonal a ia ion in Chl and OM, Sonde 1 was con inuously also deployed in Lake Konne esi du ing he open wa e season in 2013 (June–Oc obe ). One p o ile in e e y h ee hou s a 0.5-m s ep was eco ded om he 42-m-deep wa e column. Daily a e ages o Chl and OM om 1.5 o 2.0 m dep h we e aken in o analysis. Su ace (0– m) Chl ada a om 2013 we e collec ed by he egional en i onmen al agency KES-ELY du ing he open wa e pe iod (June–Oc obe , n= 5, Da abase o Finnish En i onmen Ins i u e) and supplemen ed by ou Chl asamplings (n= 4) om 1 m and 2 m dep hs wi h he Limnos sample . Reg ession models (Table 2a, b) de eloped o he mul iple lakes da ase we e es ed o calib a e he con inuous Chl da a. Compa ison o he calib a ion me hods We compa ed Chl luo ome e da a and co esponding Chl a esul s om labo a o y analyses and es ablished calib a ion equa ions o he Chl luo ome e by i ing a linea eg ession model. Then we added wa e colou (colou Typical ) in o he calib a ion equa ion. Fo Sonde 1, we also used colou Lab aken a he ime o p o ile measu emen s and included o in si u OM luo ome e da a in o he calib a ion models. Su ace (a e age o 1–2 m) Chl and OM luo ome e esul s om Lake Konne esi (Cen al Finland) we e co ec ed wi h each o he es ablished calib a ion equa ions, excluding 123 4380 Hyd obiologia (2020) 847:4377–4387 colou Lab , o illus a e he easibili y o he calib a ion o au oma ed wa e quali y moni o ing. Calcula ions o modelling e iciency (ME = 1 -P(y o -y p ) 2 /P(y o -y m ) 2 ), whe e y o ep esen s obse ed alues, y p p edic ed alues, and y m he mean o obse ed alues, and mean absolu e pe cen age e o (MAE(%) = 100[P(|y o -y p |/y o |)]/ n,), whe e nis he numbe o pai s, we e conduc ed acco ding o Maye & Bu le (1993) using Mic oso Excel (2010, Mic oso Co., Redmond, WA, USA). Linea eg essions and s a is ical es s we e conduc ed wi h SPSS so wa e ( e sion 22.0, IBM Co., Chicago, IL, USA). Resul s wi h P 0.05 we e epo ed signi ican . Resul s In he lakes measu ed wi h Sonde 1 (see Table 1), manually sampled Chl aconcen a ion a ied be ween 0.5 and 17.4 lgl -1 , and in he lakes measu ed wi h Sonde 2 be ween 3.6 and 43.7 lgl -1 (Fig. 1). A e Table 2 Uni a ia e and mul i a ia e linea eg ession models o he calib a ion o he Sonde 1 (a) and Sonde 2 (b) luo ome e s using he labo a o y measu ed chlo ophyll a a. Model equa ion o Sonde 1 nR 2 P (1) Chl a= 1.717(Chl) -1.944 71 0.537 0.001 (2) Chl a= 3.024(Chl) -0.162(colou ypical )-1.877 71 0.914 0.001 (3) Chl a= 2.843(Chl) -0.133(colou Lab )-1.067 71 0.816 0.001 (4) Chl a= 2.288(Chl) -5.109(OM) ?3.613 71 0.879 0.001 b. Model equa ion o Sonde 2 nR 2 P (1) Chl a= 0.811(Chl) -4.902 71 0.537 0.001 (2) Chl a= 1.076(Chl) -0.165(colou ypical )-2.938 71 0.914 0.001 (Chl a) concen a ions (lgl -1 ) and wa e colou (colou ypical , colou Lab ,mgP l -1 ) and in si u luo escen o ganic ma e OM (RFU). nnumbe o obse a ions (samplings on lakes) used o cons uc he models Fig. 1 Linea eg ession (b oken line) be ween he obse ed Chl aconcen a ions and es ima ed Chl aconcen a ions based on he calib a ion wi h measu ed Chl ain he s udy lakes (model 1; see Table 2a, b). Solid line = 1:1 line. Do s—Sonde 1 da a; open ma ke s—Sonde 2 da a; MAE mean absolu e pe cen age e o (%), ME modelling e iciency (%) 123 Hyd obiologia (2020) 847:4377–4387 4381 calib a ing he luo ome e eadings only wi h he Chl aconcen a ions (model 1, Table 2a, b), he eg ession coe icien s, modelling e iciency (ME) and mean absolu e pe cen age e o s (MAE) we e a he weak (R 2 = 0.51 and 0.74, MAE = 89% and 38%, ME = 0.53 and 0.86, o Sondes 1 and 2, espec i ely, Fig. 1). The espec i e p edic abili ies o model 2, includ- ing bo h Chl aand wa e colou (colou ypical )as co ec ing a iables, we e no ably be e o bo h sondes (R 2 = 0.81 and 0.92, MAE = 66% and 24%, ME = 0.82 and 0.95, o Sondes 1 and 2, espec i ely, Fig. 2). Fo Sonde 1, we also cons uc ed models 3 and 4, including Chl aand colou Lab o OM luo escence, espec i ely. Model 3 yielded sligh ly be e p e- dic abili y (R 2 = 0.85, MAE = 64%, ME = 0.86) han he wa e colou (colou ypical , Fig. 3). Fo model 4, in oducing OM luo escence as a calib a ion ac o was ela i ely e ec i e (R 2 = 0.77, MAE = 65%, ME = 0.78), bu no supe io o models 2 o 3. Chl a alues yielded by di e en models did no di e signi ican ly om each o he (Rela ed Samples Wil- coxon Signed Rank Tes , P[0.2 o each combina- ion pai o models). The ela ionship be ween colou Lab and OM luo- escence (RFU 20 ) was loga i hmic (Fig. 4): OM luo escence i s inc eased apidly along colou Lab , bu in lakes wi h colou Lab [50 mg P l -1 i le elled o (Fig. 4). Sc u iniza ion o he pe o mance o each model o a sub-se o lakes co obo a ed he gene al o e iew and demons a ed ha , p io o any OM co ec ion, he a ia ion in he luo ome e eadings calib a ed only wi h he Chl a a ied no ably om he Chl ameasu e- men s e en inside one lake (Figs. 5,6). In humic Lakes Jy a ¨sja ¨ i and Vanaja esi, he p edic abili y be ween he wo was mode a e, bu mean absolu e e o was high (R 2 = 0.88, MAE = 97%, ME = 0.83, Fig. 5). In clea wa e lakes Konne esi and Vesija ¨ i, he p edic abili y wi hou any OM co ec ion was weake bu mean absolu e e o sligh ly lowe (R 2 = 0.56, MAE = 86%, ME = 0.71, Fig. 6). How- e e , when any o he OM co ec ion models (models 2–4 in Table 2a, b) was applied, he p edic abili ies inc eased in all ou lakes. In Jy a ¨sja ¨ i and Vana- ja esi, he eg ession coe icien s, mean absolu e e o s and he modelling e iciencies o models 2–4 a ied only li le (R 2 = 0.93–0.97, MAE = 25–36%%, ME = 0.93–0.97, Fig. 5). Simila ly, clea wa e lakes Konne esi and Vesija ¨ i bene i ed om each o he calib a ion me hods a he equally (R 2 = 0.74–0.76, MAE = 74–79%, ME = 0.85–0.87, Fig. 6). In he oligo ophic, humic Lake Konne esi, Chl a a ied be ween 1.8 and 6.2 lgl -1 in June–Oc obe 2013, colou ypical was 25 mg P l -1 and OM Fig. 2 Linea eg ession (b oken line) be ween he obse ed Chl aconcen a ions and es ima ed Chl aconcen a ions based on he calib a ion wi h measu ed Chl aand wa e colou es ima e (colou ypical , Table 1). Reg ession model 2 was used he e (see Table 2a, b). Solid line = 1:1 line. Do s— Sonde 1 da a; open ma ke s—Sonde 2 da a; MAE mean absolu e pe cen age e o (%), ME modelling e iciency (%) 123 4382 Hyd obiologia (2020) 847:4377–4387 luo escence (RFU 20 ) a ied be ween 0.95 and 58 (Fig. 7). When models 1, 2 and 4 (Table 2a) we e used o co ec he au oma ed luo ome e moni o ing da a, only li le di e ences be ween he modelled Chl a esul s we e ound (Fig. 7). Howe e , he dec ease o wa e colou in au umn 2013, moni o ed wi h in si u OM, p oduced a pe iod wi h highe Chl luo escence in Sep embe han he models using co ec ion wi h only Chl ao Chl aand colou ypical (Fig. 7). Discussion Ou s udy demons a ed he use o wa e colou as an es ima e o OM in con olling he OM luo escence in Chl in si u luo ome e da a. The applicabili y o a ypical colou alue o he lake in calib a ion was alida ed by compa ing i wi h he calib a ions done using simul aneously measu ed wa e colou and in si u OM luo escence. As we expec ed, inclusion o any o hese pa ame e s in o he calib a ion model imp o ed he p edic abili y be ween he in si u Chl luo escence and Chl a. A non-linea ela ionship be ween OM and wa e colou is well documen ed (Wa as e al., 2011; Coble e al., 2014). High concen a ions o dissol ed o ganic ma e ial in he wa e column a ec he OM luo es- cence measu emen s by supp essing he alues a highe dissol ed OM concen a ions (Wa as e al., 2011). We also obse ed his in ou s udy lakes (Fig. 4), and i may explain why he in si u OM calib a ion was no a supe io me hod o e he mo e simplis ic wa e colou calib a ion. P oc o & Roesle (2010) ou lined simila ly ha OM may lead o an unde es ima ion o Chl aby abso bing exci a ion o emission wa eleng hs o , on he o he hand, OM may cause seemingly in ensi ied Chl emission by con- ibu ing o he signal de ec ed by Chl luo ome e s. In Fig. 3 Linea eg ession (b oken line) be ween he obse ed Chl aconcen a ions and es ima ed Chl aconcen a ions based on he calib a ion wi h measu ed Chl aand abso bance-based wa e colou (A 420 , le panel, model 3 in Table 2b) o in si u OM luo escence ( igh panel, model 4 in Table 2b). Solid line = 1:1 line; MAE mean absolu e pe cen age e o (%), ME modelling e iciency (%) Fig. 4 Rela ionship be ween wa e colou and OM luo es- cence (RFU 20 ) in he s udy lakes 123 Hyd obiologia (2020) 847:4377–4387 4383 ou da a, he la e was ypical o mos o he s udied lakes, excep o he eu ophic and mos clea wa e lakes (colou B10 mg P l -1 ). Ou esul s ag ee wi h p e ious s udies s a ing ha wi hou OM calib a ion, he in e p e a ion o in si u Chl esul s is gene ally misleading. P oc o & Roesle (2010) collec ed wa e samples om a lake, pond, s eam and a bog in Maine (USA) wi h a ying OM le els and measu ed phy oplank on and OM luo es- cence wi h he mul iple wa eband luo ome e s in he labo a o y. They obse ed a linea ly inc easing OM signal along he OM concen a ion in he dilu ion se ies de e mined luo ome ically and spec opho o- me ically. Based on he linea cong uence be ween he OM and algae mul iexci e signals, hey s a ed ha he quan i y o OM is o cen al impo ance. Goldman e al. (2013) also showed signi ican o e es ima ion o Chl concen a ion wi h inc eased OM concen a ions in an es ua y. Leppa ¨e al. (1995) ound a high backg ound luo escence in humic lakes o Sou h- Eas e n Finland, caused by wa e colou , which lead o ecommenda ions o he Chl luo ome y calib a ion agains he OM backg ound. In ou s udy, he imp o emen due o calib a ion was no ha clea in he con inuously measu ed luo ome e da a o Lake Konne esi. Howe e , in he end o July 2013, OM luo escence dec eased and he OM calib a ed Chl luo escence (model 4) inc eased leading o a be e cong uence wi h he measu ed Chl a, which migh indica e ha he con inuous on-line Chl measu emen s could bene i om he OM cali- b a ion (Fig. 7). Likely, he signi icance o in si u OM co ec ion is emphasized, compa ed o labo a o y analyses, when Chl is measu ed wi h a high equency in eal ime. In Lake Konne esi, he au oma ed sys em eco ded a dec ease o OM luo escence owa ds he end o he summe , which is a known phenomenon in bo eal lakes and associa ed wi h pho ochemical Fig. 5 Pe o mance o he calib a ion equa ions in humic Lakes Jy a ¨sja ¨ i and Vanaja esi (see Table 1 o lake cha ac e is ics). B oken line ep esen s linea eg ession be ween he obse ed Chl aconcen a ions and es ima ed Chl aconcen a ions based on he calib a ion wi h measu ed Chl a(model 1, see Table 2a, b o he models) o measu ed Chl aand wa e colou es ima e (model 2), abso bance- based wa e colou (A 420 , model 3) o in si u OM luo escence (model 4). Solid line = 1:1 line; MAE mean absolu e pe cen age e o (%), ME modelling e iciency (%) 123 4384 Hyd obiologia (2020) 847:4377–4387