Water Research 188 (2021) 116525 Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres Characterization of sedimentary phosphorus in Lake Erie and on-site quantification of internal phosphorus loading Y.T. Wang a , T.Q. Zhang a , ∗, Y.C. Zhao b , 1 , J.J.H. Ciborowski c , 2 , Y.M. Zhao d , I.P. O’Halloran e , Z.M. Qi f , C.S. Tan a a Harrow Research and Development Center, Agriculture and Agri-Food Canada, Harrow, ON N0R 1G0, Canada b Nanotechnology Engineering Program, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada c Department of Biology, University of Windsor, Windsor, ON N9B 3P4, Canada d Aquatic Research and Monitoring Section, Ontario Ministry of Natural Resources and Forestry, 320 Milo Rd, Wheatley, ON N0P 2P0, Canada e School of Environmental Sciences, Ridgetown Campus, University of Guelph, ON, Canada f McGill University, Department of Bioresource Engineering, Sainte-Anne-de-Bellevue, QC H9 × 3V9, Canada a r t i c l e i n f o Article history: Received 5 May 2020 Revised 8 October 2020 Accepted 14 October 2020 Available online 14 October 2020 Keywords: Surface water quality Lake sediment Phosphorus loads Lake Erie Internal phosphorus loading a b s t r a c t Lake Erie harmful algal blooms and hypoxia are two major environmental problems, and have severe im- pacts on human health, aquatic ecosystems, and the economy. However, little is known about internal loading of phosphorus (P) from sediments, which pose a challenge for assessing the efficacy of current conservation measures on the improvement of lake water quality. A modified Hedley’s extraction proce- dure was employed to analyze representative sediment samples collected from the Lake Erie basin for assessing sedimentary P stock, potential availability for release into lake water, and internal P loading. Inorganic and organic P in the sediments were characterized by sequential extractions in H 2 O, 0.5 M NaHCO 3 , 0.1 M NaOH, and 1.0 M HCl, respectively. In the 0 – 10 cm sediment, total P stock was 172, 191, and 170 metric tons km −2 in the western, central, and eastern basins, respectively. Sedimentary P seems unlikely to contribute to internal P loading in the western basin, while in the eastern basin it can poten- tially contribute to an internal loading of 359 metric tons P yr −1 . In the central basin, 41% of organic P, 15% of non-HCl extractable inorganic P, and 9.7% of residual P in the 0 – 10 cm sediment is potentially available for release into lake water; in the 10 – 20 cm sediment, organic P extracted by NaHCO 3 and NaOH is also partially available. The central basin potentially contributes to internal P loading at a total amount of 10,599 metric tons yr −1 . Internal P loading may not contribute to HABs in the western basin, but it can cause and maintain hypoxia in the central basin and delay the recovery of lake water quality for a lengthy time period in response to external P reduction measures. Crown Copyright © 2020 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) 1 c k e a 2 U a t t 2 t i c h 0 ( . Introduction In the 1960s and 1970s, Lake Erie experienced extensive yanobacterial blooms, episodically reducing dissolved oxygen and illing fish in the lake. The blooms were mainly a consequence of xcessive external phosphorus (P) loading from point sources and gricultural runoff discharge ( Matisoff et al., 2016 ). In response, ∗ Corresponding author. E-mail address: Tiequan.Zhang@Canada.Ca (T.Q. Zhang). 1 Current address: Nanotechnology Engineering Program, University of Waterloo, 00 University Ave W, Waterloo, ON N2L 3G1, Canada. 2 Current address: Department of Biological Sciences, University of Calgary, 2500 niversity Drive NW, Calgary, AB, Canada. y s a H m S p ttps://doi.org/10.1016/j.watres.2020.116525 043-1354/Crown Copyright © 2020 Published by Elsevier Ltd. This is an open access arti http://creativecommons.org/licenses/by-nc-nd/4.0/ ) n external P load reduction program was implemented through he Great Lakes Water Quality Agreement in 1972, and lake condi- ions had improved considerably by the mid-1980s ( Dalo ̆glu et al., 012 ; Zhang et al., 2016 ). In the mid-1990s, however, cyanobac- erial blooms reappeared in the lake and have become increas- ngly extensive and frequent since then, despite continuing suc- ess in achieving the target total P (TP) loading in most of the ears ( Maccoux et al., 2016 ). Harmful algal blooms (HABs) pose ignificant risks to drinking water supplies, quality of human life, nd economic vitality. In 2014, for example, toxins from Lake Erie ABs resulted in a two-day “do not drink” advisory to approxi- ately half a million local residents in Toledo, Ohio ( Wynne and tumpf, 2015 ). To improve Lake Erie water quality and reduce the ublic health risks, the governments of Canada and the United cle under the CC BY-NC-ND license https://doi.org/10.1016/j.watres.2020.116525 http://www.ScienceDirect.com http://www.elsevier.com/locate/watres http://crossmark.crossref.org/dialog/?doi=10.1016/j.watres.2020.116525&domain=pdf http://creativecommons.org/licenses/by-nc-nd/4.0/ mailto:Tiequan.Zhang@Canada.Ca https://doi.org/10.1016/j.watres.2020.116525 http://creativecommons.org/licenses/by-nc-nd/4.0/ Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 S a M d e t B i w t d h c v w p T u a t s P m b i m P o s o 1 P ( r b t P i E i d f g q s s o t o d u p z H P t i o p o a P a m 1 v i 2 2 i t c m s a i m a – fi t N T i 2 p l z c c R t A a 3 3 0 t m t E ( c e k t N ( 0 a < A 2 k 1 c H tates have committed to a further 40% reduction in external TP nd dissolved reactive P (DRP) loads reaching Lake Erie from the aumee River and other priority tributaries ( USEPA, 2016 ). Internal P loading may contribute to the total quantity of P elivered to the lake water column and thus retard the recov- ry of lake water quality ( Penn et al., 1995 ). Evidence suggests hat sediment P regeneration does occur in Lake Erie. For example, urns (1976) observed a dramatic increase in DRP concentration n the hypolimnion waters of the central basin (CB) of Lake Erie hen the waters became anoxic. A recent study on oxygen iso- opic sources of dissolved P in Lake Erie suggested that previously- eposited P may indeed be released from sediments of CB when ypolimnetic hypoxia occurs during the period of summer stratifi- ation ( Elsbery et al., 2009 ). Matisoff et al. (2016) observed large ariations in the diffusive flux of P from bottom sediments in the estern basin (WB) of Lake Erie when using sediment incubation, ore water concentration profile, and bottom chamber methods. hey suggested that internal diffusive recycling of P in the WB is nlikely to trigger HABs by itself. In the CB, internal P loading may mount to 8–20% of the total external input of P to Lake Erie in he sediment incubation and the pore water concentration profile tudies ( Paytan et al., 2017 ). Meanwhile, Matisoff et al. (2016) and aytan et al. (2017) also discussed the limitations of their measure- ent methods, such as ignoring some important processes (e.g. ioturbation by macro-bentric fauna) involved in the internal load- ng of P and great challenges of simulating lake bottom environ- ent during the incubations. In order to better estimate internal loading in the lake, one needs to directly characterize the forms f sedimentary P and assess the on-site potential for releasing dis- olved P into the water column. Forms of P in the sediments of Lake Erie and their nature and rigins were discussed by Williams et al. (1976a and 1976b ) in the 970s. Specifically, sedimentary P was categorized into inorganic (Pi) such as apatite Pi and non-apatite Pi (NAPI), and organic P Po); apatite P is of detrital sedimental rock origin and mainly de- ived from eroding bluffs along the north shore of the CB, while oth NAPI and Po are of anthropogenic origin and can be poten- ially released into overlying water, thereby contributing to HABs. ennuto et al. (2014) further reported that total P concentrations n surficial sediments increased from nearshore-to-offshore in Lake rie. To our knowledge, in the last four decades, no further stud- es have been conducted to update or improve our complete un- erstanding on sedimentary P cycling, as well as to quantify the orms of sedimentary P with respect to their availability for algal rowth in the Lake Erie. Filling this knowledge gap is critical for uantifying sediment P regeneration to the water column given a hift in the prevalence of P inputs from point sources to diffuse ources, the recent resurgence of HABs in the WB and expansion f the hypoxic zone in the CB, and other changing lake condi- ions ( e.g ., colonization by invasive dreissenid mussels and blooms f Cladophora sp. ) associated with nutrient deposition from four ecades ago ( Watson et al., 2016 ). A modified Hedley’s sequential extraction procedure is widely sed to separate P in soils into several fractions associated with lant availability and the chemical nature of P ( Zhang and MacKen- ie, 1997 ). Compared to other sequential extraction schemes, the edley’s one not only measures Pi, but also Po. Considering that o plays an important role in sediment P cycling ( Zhu et al., 2013 ), he Hedley’s procedure was employed in our study to quantify sed- mentary P and assess P availability for release into lake water. To ur knowledge, this study is the first time to apply the Hedley’s rocedure to sedimentary P cycling research of Lake Erie. It not nly provides a research tool for sedimentary P measurements, but lso helps better understand the nature and cycling of sedimentary of Lake Erie. The objectives of this study were to estimate: ( i ) Pi nd Po forms and their respective quantities in surficial lake sedi- 2 ents, ( ii ) spatial distribution of each sediment P form in the 0 – 0 cm sediment of the Lake Erie basin, ( iii ) vertical distribution of arious P forms in the sediment profile, and ( iv ) internal P loading n the lake. . Materials and methods .1. Sediment sampling In 2010, a total of 37 sites were successfully sampled for sed- ment cores across the Lake Erie basin ( Fig. 1 and Table A.1). All hese sampling sites are located in the areas of soft substrates ac- ording to the map of Lake Erie substrate types ( Haltuch and Berk- an, 1999 ), and happened to be offshore. We could not collect any ediment samples in the areas of hard substrates due to the in- bility of the box corer to penetrate into the sediment. As shown n Fig. 3 , many nearshore areas belong to hard substrates, which akes it unsuccessful to collect sediment samples from nearshore reas. Each soft sediment core was then split into 0 – 10 cm, 10 20 cm, and 20 – 30 cm sections for chemical analysis. A modi- ed Hedley’s P fraction procedure was used to separate sedimen- ary P into eight different P fractions, including H 2 O-Pi, H 2 O-Po, aHCO 3 -Pi, NaHCO 3 -Po, NaOH-Pi, NaOH-Po, HCl-Pi, and residue P. he details on sediment sampling, sample preparations and chem- cal analysis can be found in Appendix A. .2. Analysis of variance and P stock and release calculations As mentioned above, we could not collect hard sediment sam- les. Moreover, hard substrates are unlikely to sequester P to any arge extent. Therefore, we assumed that P stock is low or nearly ero in hard substrates, which were then excluded from our cal- ulations of spatial distributions of sedimentary P, N, and C con- entrations, P stock, and internal P loading in the current study. egarding the details on data analysis, P stock and release poten- ial calculations, and internal P loading determinations, please see ppendix A, where the rationales for estimating internal P loading re also discussed. . Results and discussion .1. Distribution of P among sequentially extracted P fractions in the –10 cm sediment In surface sediments (0–10 cm), TP concentrations ranged be- ween 498 and 1180 mg kg −1 with a mean ± SD of 889 ± 152 g kg −1 ( Fig. 2 ; n = 37), which were reasonably consistent with hose previously reported in surficial sediment of nearshore Lake rie (450-750 mg TP kg −1 ) ( Pennuto et al., 2014 ). Williams et al. 1976a ) reported a larger variation (188-2863 mg kg −1 ) in TP con- entration in 48 surficial sediment samples from this lake; how- ver, 90% of those samples fell within the range of 463-1387 mg g −1 , similar to the range found in the present study. Among he eight P fractions extracted (i.e., H 2 O-Pi, H 2 O-Po, NaHCO 3 -Pi, aHCO 3 -Po, NaOH-Pi, NaOH-Po, HCl, and residual P), the least Pi 0.8-20.9 mg kg −1 ; 0.1-2.3% of TP) and Po (0.7-5.0 mg kg −1 ; 0.1- .5% of TP) were those extracted by deionized water. These data re consistent with previous reports that H 2 O-Pi accounted for 3% of TP in the sediments collected from 44 European, North merican, and Chinese lakes ( Zhu et al., 2013 ; Kopáček et al., 005 ). Sedimentary NaHCO 3 -Pi and -Po ranged between 7-159 mg g −1 and between 0-27 mg kg −1 , respectively, accounting for 0.9- 4.3% and 0-2.6%, respectively, of TP in the sediments. The NaOH-Pi fraction accounted for 0.8-20.4% of TP, with con- entrations ranging between 6-241 mg kg −1 . Compared with the O and NaHCO extractants, the 0.1 M NaOH solution extracted 2 3 Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 Fig. 1. Map of Lake Erie showing locations of the 37 sampling stations (from west to east) in the west, central, and east basins. m T a e w s r r s s T s P w a a m l 3 s s f e ( S t i h f i d t o o t c e c w t i c s i d L a o m t b a l o p e d t P t ( o w s t t t I b t i c a ( ore Po (6-129 mg kg −1 ), thereby accounting for 0.8-13.0% of P. The HCl-Pi, ranging between 367-807 mg kg −1 , was the most bundant P form in Lake Erie sediments, accounting for, on av- rage, 57% of TP in sediment. The second most dominant P form as residual P, accounting for 7.9-23.2% of TP in sediment. De- pite the dominance of HCl-Pi in sediment TP, no significant cor- elation was found between TP and HCl-Pi. In comparison, the emaining P fractions were all significantly correlated with TP in ediment ( r = 0.34-0.84, P ≤ 0.05, data not shown), even though ome of the fractions only accounted for very small percentages of P. This indicates that the variations in the quantity of TP among ites are mainly caused by non-HCl-Pi fractions (i.e. H 2 O-Pi, H 2 O- o, NaHCO 3 -Pi, NaHCO 3 -Po, NaOH-Pi, NaOH-Po, and residual P), hich is understandable considering that HCl-Pi is of natural origin nd is inert, leading to its relatively uniform distribution in space nd time ( Williams et al., 1976b ). The HCl-Pi would not contribute uch to availability of sediment P to phytoplankton due to its very ow solubility ( Burns, 1976 ). .2. Spatial distribution of various P fractions in the 0–10 cm ediment Spatial distribution of sedimentary P (0–10 cm) varied with re- pect to P fractions, and none of the fractions were distributed uni- ormly across the lake ( Fig. 3 ). On average, H 2 O-Pi was the low- st in the CB (mean ±SD of 3.2 ±1.2 mg kg −1 ), followed by the EB 7.2 ±5.7 mg kg −1 ), with the highest in the WB (8.3 ±5.7 mg kg −1 ). edimentary H 2 O-Pi was mainly concentrated in the western por- ion of WB, and the area surrounding Ottawa County, Ohio (includ- ng the Sandusky basin and the southeast portion of the WB). The igh H 2 O-Pi concentrations observed in these locations reflect the act that these areas receive about 80% of annual external P load- ng to the lake ( Scavia et al., 2014 ). However, external P loading ata fail to explain high H 2 O-Pi concentrations in the sediment of he eastern basin (EB), because this basin only receives about 10% f P loading to the lake ( Scavia et al., 2014 ). To the east-southeast f Long Point, high P concentrations were not only observed for he H 2 O-Pi fraction, but also for all the remaining P fractions ex- ept for HCl-Pi. The deepest region of the lake ( i.e ., EB) experi- 3 nces the longest period of annual stratification and has high P oncentrations, probably because the hypolimnial water is always ell-oxygenated and the thermocline prevents water exchange be- ween the epilimnion and hypolimnion, locking the P in the sed- ment ( Williams et al., 1976b ; Mortimer, 1987 ). In addition, surfi- ial water flow from west to east dominates in the lake, and some uspended particles containing P eventually reach EB through sed- ment suspension-resettling cycles and gradually accumulate in the eeper areas. Therefore, concentrated P in EB east-southeast of ong Point could be due to the long-term accumulation of P in the rea. Patterson et al. (2005) observed that the northeast nearshore f Long Point had the highest Dreissena density and dry tissue ass in all of Lake Erie. Therefore, high H 2 O-Pi and -Po concen- rations in the northeast nearshore of Long Point in EB may have een due to abundant dreissenid mussels in the area which are ble to remove suspended particles from the water column, and re- ease dissolved nutrients into the water column, or deposited them n the nearshore lake bottom ( Hecky et al., 2004 ). Comparing the resent study with that of Patterson et al. (2005) shows that ar- as of high H 2 O-Pi generally correspond to locations where mean ensity and dry tissue mass of Dreissena are high. Such an explana- ion for high H 2 O-Pi observed in the northeast nearshore of Long oint is also supported by the measurements of nitrogen (N) iso- ope ( δ15 N) signatures in suspended particulate matter in Lake Erie Upsdell, 2005 ). Particulate N observed in the northeast nearshore f Long Point did not come from any other part of the lake, but as most likely produced through nutrient recycling of N by dreis- enid mussels. We speculate that the same mechanism also applies o the observed H 2 O-Pi in this study. Both NaHCO 3 -Pi and NaOH-Pi exhibited similar spatial distribu- ion patterns, with high concentrations occurring in the WB and he middle part of the CB, particularly along the southern shore. n our view, such P distribution patterns mainly reflect the com- ined effects of external P loading and the lake circulation pat- erns ( USDOI, 1968 ; USEPA, 1984 ). Maumee River water, contribut- ng 39% of TP loading to the Lake Erie, and the associated parti- les primarily flow from west to east along the southern shore, nd eventually enter the CB through the Middle/South Channels Michalak et al., 2013 ; Scavia et al., 2014 ). The Sandusky River, ac- Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 Fig. 2. Inorganic (Pi) and organic phosphorus (Po) fractions in the 0 - 10 cm soft sediments of Lake Erie. The samples are numbered according to longitude from west to east ( Fig. 1 ). c i t t e H w a S a t n ( t s i K r o E e s 1 t fi l H N c i w i d t a P ( b ( M l T o i N i r t r p ( I r t h a r ounting for 11% of TP loading to the lake, contributes the major- ty of sediment P in the Sandusky basin and enters the lake at he southwestern corner of the CB ( Scavia et al., 2014 ). The De- roit River water (carrying almost 23% of TP loading to Lake Erie) nters the lake at the northwestern corner ( Michalak et al., 2013 ). owever, this river contributes 90% of the water inflow to the lake, hich suggests that the P in the Detroit River may not contribute s much to sedimentary P concentration as do the Maumee and andusky rivers. In the CB, both the prevailing southwest winds nd the earth’s rotation generate currents that cause a geostrophic ransport of water toward the south shore, probably causing sig- ificant P deposition into the sediment along the southern shore USEPA, 1984 ). Compared with other P fractions, HCl-Pi exhibited a unique spa- ial distribution pattern: high concentrations along the northern hore of both the northeastern CB and the western EB (includ- ng the embayment north of Long Point) ( Fig. 3 ). According to emp et al. (1976) and Williams et al. (1976b) , HCl-Pi is mostly de- ived from the sediment through shoreline erosion, 83% of which ccurs from the bluffs along the north shore of the CB between rieau and Long Point. It was reported that HCl-Pi (apatite P) av- raged 545 mg kg −1 in 14 bluff samples collected from the north hore of Lake Erie, accounting for about 95% of TP ( Williams et al., 976b ). Therefore, surface transport of water and suspended par- icles toward the southeast, subsurface transport of re-suspended ne materials, and upwelling adjacent to the north shore may col- ectively create the observed distribution pattern of sedimentary Cl-Pi ( USEPA, 1984 ; USDOI, 1968 ).. Overall, various Po fractions (e.g., H 2 O-Po, NaHCO 3 -Po, and aOH-Po) and residual P, also predominated in Po, were all con- 4 entrated in the middle part of the CB, east-southeast of Long Point n the EB, and southern part of the WB, except for NaHCO 3 -Po, hich was concentrated in the middle part of the CB. The similar- ties reflect one or more common mechanisms controlling spatial istribution of Po. Significant linear correlations were observed be- ween organic C concentration and H 2 O-Po, NaHCO 3 -Po, NaOH-Po, nd residual P ( P < 0.0 0 01, data not shown). Furthermore, these fractions were also linearly related with total N in sediments P < 0.0 0 01, data not shown). In fact, linear relationships have een widely reported between organic C and Po in lake sediments Williams et al., 1976a ; Bostan et al., 20 0 0 ). Similarly, Kemp and udrochova (1972) reported that 90% of total N in sediments col- ected from a Lake Ontario sediment core was in an organic form. hese results suggest that organic N and P are integral components f the organic matter of the lake sediments. Thus, it is not surpris- ng to find similar spatial distribution patterns for organic C, total , and Po and residual P ( Fig. 3 and Fig. A.1). The measured C:N ratio values ranged between 5.7 and 14.5 n our study, with a median value of 9.8 ( Fig. 4 ). Generally, C:N atios are directly related to the sources of P and organic mat- er in lake systems ( Kemp et al., 1977 ; Thomas et al., 1991 ). The eported range of C:N ratio is about 5-8 for freshwater phyto- lankton ( Kendall et al., 2001 ), 14-20 for aquatic macrophytes Kemp et al., 1977 ), 20-546 for vascular land plants ( Meyers and shiwatari, 1993 ), and generally greater than 16 for glacial mate- ials ( Kemp et al., 1977 ). In our study, the CB had lower C:N ra- ios, particularly in the middle of the basin ( Fig. 4 ). It seems that igh Po and residual P in the CB were primarily derived from utochthonous phytoplankton-derived organic matter. It is widely ecognized that the WB is richer in phytoplankton than the CB Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 Fig. 3. Spatial distribution of concentrations (mg kg −1 ) of various P fractions in 0–10 cm soft sediments of Lake Erie. Grey areas represent hard substrates. Fig. 4. Spatial distribution of sediment C:N ratio values in 0–10 cm soft sediments of Lake Erie. Grey areas represent hard substrates. a i r p m t t v E p B h m t t t t C nd EB due to shallow waters and its major tributaries supply- ng nutrients ( Allinger and Reavie, 2013 ). Thus, high organic and esidual P along with low C:N ratios in the CB suggests that sus- 5 ended particles in the lake are rich in autochthonous organic atter and associated P can be transported from west to east in he lake, which is consistent with a net eastward movement of wa- er current in the lake (USEPA. 1984 ; Zhou et al., 2015 ). One pre- ious study did show that suspended particulate matter in Lake rie had C:N ratios of 4.7-6.9 and was primarily derived from lankton ( Upsdell, 2005 ). In addition, Shinohara et al. (2012) and ostan et al. (20 0 0) reported that Po accounted for significantly igher proportions of TP in suspended particles than in lake sedi- ent. Our results agree with Kemp et al. (1977) who reported that he biggest cluster of high C:N ratios of Lake Erie was located in he WB, particularly in the area close to the mouth of the De- roit River. Kemp et al. (1977) also found that these high C:N ra- ios in the WB were similar to those measured for the Lake St. lair, which has a shallow water depth and a large standing crop Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 Table 1 Significant differences in P concentrations (mg P kg −1 ) of sediments between different depths (i.e. 0–10 cm, 10–20 cm, and 20–30 cm); the results are based on 23 sediment core samples (n = 5, 12, and 6 for the western, central, and eastern basins, respectively), which all contained 0–10 cm, 10–20 cm, and 20–30 cm layers. P fractions Sediment depth (cm) Mean concentration (mg kg −1 ) West basin Central basin Central basin H 2 O-Pi 0 - 10 6.33 a 3.05 a 8.99 a 10 - 20 3.97 a A 1.07 b A 5.61 b A 20 - 30 3.12 A 1.3 A 5.65 A H 2 O-Po 0 - 10 1.95 a 3.11 a 1.85 a 10 - 20 1.83 a A 2.12 b A 1.32 a A 20 - 30 1.55 A 2.23 A 1.74 A NaHCO 3 -Pi 0 - 10 74.9 a 87.2 a 65.1 a 10 - 20 81.2 a A 56.4 b A 71.1 a A 20 - 30 80.5 A 59.5 A 80.8 A NaHCO 3 -Po 0 - 10 7.16 a 14.1 a 5.65 a 10 - 20 5.13 a A 9.31 b A 7.53 a A 20 - 30 4.06 A 6.95 B 7.07 A NaOH-Pi 0 - 10 102 a 132 a 68.3 a 10 - 20 120 a A 106 a A 88.8 a A 20 - 30 122 A 107 A 105 A NaOH-Po 0 - 10 65.5 a 107 a 51.6 a 10 - 20 69.2 a A 88.2 b A 46.4 a A 20 - 30 48.6 A 64.8 B 52.2 A HCl-Pi 0 - 10 414 a 431 a 565 a 10 - 20 437 a A 418 a A 586 a A 20 - 30 427 A 428 A 536 A Residual P 0 - 10 171 a 202 a 151 a 10 - 20 154 a A 183 b A 139 a A 20 - 30 144 A 177 A 144 A For any individual P fraction, means followed by the same lowercase letter are not significantly different at p < 0.05 between 0–10 cm and 10–20 cm depths, followed by the same capital letter are not significantly different at p < 0.05 between 10–20 cm and 20–30 cm depths. o t i E a t t t i w P p m d ( T t d s P 3 a i t f C s f f a c i P u i t t 2 H e 2 a p P d f i P p o p m a t c ( s p t t t P t t f macrophytes. Therefore, they suggested that the suspended ma- erials from Lake St. Clair are the most significant source of sed- ment in the WB. On average, the C:N ratios were higher in the B than those in the CB ( Fig. 4 ). It seems that the shallows cre- ted by the Pennsylvania ridge, which separates the two basins in- errupts the continuing pattern of decreasing C:N ratio from west o east. This could be the case if sediment composed of plank- on sources mostly settles in the CB, which is possible consider- ng that transport velocities decrease when the flow enters the ide CB. The ridge may become a barrier for plankton-sources in the CB sediments to be transported to the EB. The lower rimary productivity in the EB and high water transparency pro- ote the nearshore growth of macrophytes ( Cladophora sp. ), which erive major sources of the sediment P from the Grand River Valipour et al., 2016 ) and are less related to the WB and CB. he relatively steep slope and sediment resuspension cycling could ransport suspended particles to settle down in the areas at the eeper end of the basin ( De March, 1978 ), causing higher organic edimentary P in these areas. The areas with high Po and residual in the CB and EB overall are deep water. .3. Vertical distribution of P in sediment cores Vertical distribution of sedimentary P varied among P fractions nd basins ( Table 1 ). In the WB, we found no significant difference n the concentration of each P form among the vertical layers. In he EB, the only significant difference in P concentration occurred or H 2 O-Pi between 0 - 10 cm and 10 - 20 cm sections. In the B, in contrast, significant differences in P concentrations were ob- erved between 0 -10 cm and 10 - 20 cm for all P fractions except or NaOH-Pi and HCl-Pi, and between 10 - 20 cm and 20 - 30 cm or NaHCO 3 -Po and NaOH-Po fractions. Our data suggest that P deposited in the CB was more algal vailable than that in the EB and WB ( Table 1 ). There was a de- reasing trend in H O-Pi concentration with depth from 0 to 20 cm 2 6 n the EB, but the percentage ( i.e ., 38%) of potential available H 2 O- i in total H 2 O-Pi was less than that ( i.e ., 67%) in the CB. In partic- lar, the CB was characterized as having higher Po concentrations n sediments ascribed to a large load of organic debris from phy- oplankton. Generally, Po concentration declines with depth below he sediment surface due to mineralization of Po ( Shinohara et al., 012 ; Ahlgren et al., 2011 ; Ahlgren et al., 2005 ; Joshi et al., 2015 ). alf-life times of monoester-P, diester-P, and pyrophosphates were stimated to be about 23, 21, and 13 y, respectively ( Ahlgren et al., 005 ). Summer hypoxia occurs annually in the bottom of the CB, nd oxygen depletion rates have recently increased due to anthro- ogenic factors ( Zhou et al., 2013 ), which would further promote release in the CB ( Hupfer et al., 2007 ). Oscillating redox con- itions caused by the recurrent presence and absence of oxygen avour the growth of some microorganisms in the surficial sed- ments ( Hupfer et al., 2007 ). These microorganisms can take up in excess and store it intracellularly in the form of polyphos- hate under oxic conditions, and potentially release it into the verlying water under anoxic stress ( Hupfer et al., 2007 ). Polyphos- hate was estimated to account for up to 10% of TP in the upper- ost sediment layer ( Hupfer et al., 2007 ). Overall, higher avail- bility of sediment P with the CB is consistent with the observa- ion that the hypolimnetic waters of the CB showed a dramatic in- rease in the concentration of DRP when the waters became anoxic Burns, 1976 ). As expected, H 2 O-Pi was the most available P fraction in the ediment: 67 and 38% of this form in the 0 - 10 cm stratum was otentially available in the CB and EB, respectively, compared to hat in the 10-20 cm depth ( Table 1 ). However, the H 2 O-Pi frac- ion is unlikely an important contributor to internal P loading due o its low concentration. In comparison, only 36% of the NaHCO 3 - i fraction in the CB is potentially algal available, but it can con- ribute 29.2 mg P kg −1 sediment to internal P loading - about 13 imes more than that with H 2 O-Pi. Across the three basins of the Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 l t i t s t fi c E P c t N t P c s s o p t t ( t 2 o a 6 a g t o a t r 3 t o fi N a 1 t l W s i i 9 t t 1 t A P t o 1 s l t o n t i l a 1 p P L o e P b i t f t K e u s h w a i w i l e p o P l i w t c 7 E m s b l h s 2 i P p e i w f t a a t p o a 2 a e t ake, no significant differences in H 2 O-Pi and NaHCO 3 -Pi concen- ration were found between 10 - 20 cm and 20 - 30 cm depths, ndicating that both P fractions reach stable or equilibrium condi- ions at the 10 - 20 cm depth and below, and will be permanently equestered in the deep sediment. Regarding HCl-Pi, it is expected o observe largely identical P concentrations in the sediment pro- le considering its natural origin, inert nature, and few temporal hanges in the processes governing apatite sedimentation in Lake rie ( Williams et al., 1976b ). Our study confirms that the biogeochemical cycling of organic plays an important role in P release into the overlying water ontributing to the eutrophic status of lakes ( Zhu et al., 2013 ). In he 0 - 10 cm sediment stratum of CB, 34% of H 2 O-Po, 34% of aHCO 3 -Po, and 18% of NaOH-Po can be potentially released to he overlying water. Furthermore, small proportions of NaHCO 3 - o and NaOH-Po ( i.e ., 25% and 27%, respectively) in the 10 - 20 m depth stratum are also potentially available. Nevertheless, it eems that both NaHCO 3 -Po and NaOH-Po can become a long-term ource for P release. This may be related to the concentrations f various Po species in each P fraction. Labile monoester, diester, hytate-like P, and some unknown Po are present in each P frac- ion ( Zhu et al., 2013 ). However, compared with the H 2 O-Po frac- ion, the latter two Po fractions contain much more phytate-like P approximately 5-10 times more) and unknown Po species (16-20 imes), which are relatively refractory in the sediment ( Zhu et al., 013 ). According to Eq. (A.5), about 51% of NaHCO 3 -Po and 40% f NaOH-Po in the 0 - 10 cm sediment stratum can be potentially lgal available. Similarly, Ahlgren et al. (2005) reported that 50- 0% of Po contained in lake sediments was degraded and eventu- lly released to the water column. Residual P in agricultural soils is enerally believed to be recalcitrant and unavailable for plant up- ake ( Hedley et al., 1982 ). However, our results indicate that 9.7% f residual P in the 0 - 10 cm layer of sediment is potentially avail- ble for transformation and subsequent release to the bottom wa- er. Søndergaard et al. (1996) also found a similar proportion of the esidual P in sediment could eventually become algal available. .4. Sedimentary P stocks and release and internal P loading Based on the results of the present study, a conceptual scheme o explain sedimentary P stocks and release to the water column f Lake Erie was developed ( Fig. 5 ). Briefly, the TP stock in the sur- cial lake sediment (0–10 cm), consisting of: ( i ) Po ( i.e ., H 2 O-Po, aHCO 3 -Po, and NaOH-Po, ( ii ) non-HCl-Pi (i.e. H 2 O-Pi, NaHCO 3 -Pi, nd NaOH-Pi), ( iii ) residual P, and ( iv ) HCl-Pi, were 197, 219, and 95 metric tons km −2 in WB, CB, and EB, respectively. The poten- ial availability of sedimentary P in the 0 -10 cm sediment for re- ease to the water column varies with basin and P fractions. In the B, all sediment P fractions remain stable or in an equilibrium tatus, while in the EB, only 2.4% of non-HCl-Pi can be released nto the overlying water over time, potentially contributing to the nternal P loading of 359 metric tons yr −1 . In the CB, 41, 15, and .7% of Po, non-HCl-Pi, and residual P, respectively, would be po- entially released into the overlying water over time, contributing o internal P loading of a total of 8232 metric tons yr −1 in the 0– 0 cm sediment according to Eq. (A.4) and Eq. (A.14). In the CB, he TP stock was 201 metric tons km −2 in the 10–20 cm sediment. mong all the measured P fractions, only NaHCO 3 -Po and NaOH- o are potentially available for release into the overlying water in his deeper sediment layer, contributing to the internal P loading f 2367 metric tons yr −1 according to Eq. (A.15). In the CB, the 0– 0 cm sediment overall contributes 78% of the P released from the ediment, while the remaining 22% comes from the older sediment ayer ( i.e. , 10-20 cm). These results are consistent with a report hat for two Swiss glacial lakes, the top sediment layer, deposited ver the last 10 years, accounted for 60-80% of the areal hypolim- 7 etic mineralization rate in the sediments, while sediments older han 10 years contributed only 20-43% to sediment-based mineral- zation rate ( Matzinger et al., 2010 ). According to Eq. (A.16), Lake Erie may receive the internal P oading of up to 10,958 metric tons yr −1 across the three basins, pproximately equal to the whole lake target external P load of 1,0 0 0 metric tons yr −1 . Similarly, Nürnberg and LaZerte (2016) re- orted that internal P load was about 0.75–1.3 times the external load in Lake Winnipeg, Manitoba, between 1999 and 2012. In ake Simcoe, Ontario, internal P load was estimated to be 45-89% f the external load ( Nürnberg et al., 2013 ). Our internal P load stimate for the CB is higher than that (2400 metric tons yr −1 ) aytan et al. (2017) measured in the CB using sediment core incu- ation experiments. One of the major reasons for this discrepancy s that the core incubations accounted only for diffusive P flux in he internal P load estimate ignoring bioturbation by macro-bentric auna ( Paytan et al., 2017 ). In fact, P flux from the sediments due o Hexagenia sp. may be greater than diffusive P flux ( Chaffin and ane, 2010 ; Paytan et al., 2017 ). In comparison, our approach to stimate internal P loading does not distinguish specific individ- al processes (e.g. diffusive P flux and bioturbation) through which edimentary P can be released into the overlying water; instead, it ighlights a common consequence of all the P release processes, hich would lead to decreased P concentrations of the sediment s long as sedimentary P release actually occurs. Another reason s that Paytan et al. (2017) performed the core incubations at 7 °C, hich is below the bottom water temperature of 9–18 °C observed n the CB between June and October, a key period of P release into ake water due to summer hypoxia ( Schertzer et al., 1987 ). In gen- ral, the P release rate increased with increasing incubation tem- erature; when the temperature was below 7 °C, P release was low r even below zero ( Holdren and Armstrong, 1980 ). In addition, aytan et al. (2017) pointed out that the concentration of the over- ying water increased over time during the core incubations, reduc- ng the concentration gradient and then the P flux. In our study, e did not measure a detectable P release from the sediment in he WB. In comparison, Matisoff et al. (2016) reported that the WB ontributes to the internal P loads of 378–808 metric tons yr −1 , 3- % of the 11,0 0 0 metric tons yr −1 target load for P delivery to Lake rie from external sources. However, they pointed out that they ight have overestimated the annual P fluxes because their mea- urements were conducted during the warmer time of the year. Our internal P load estimate shows that internal loading must e an important source of P in Lake Erie. Since 1980s, external P oading has remained below the P reduction target in most years; owever, TP concentration in the CB water increased with time de- pite irregular temporal pattern of external P loading ( Scavia et al., 014 ). In our view, internal P loads may contribute to the increases n lake water TP concentration in the CB. Large amount of internal loads may have different impacts on HABs in the WB and hy- oxia in the CB. Compared with the majority of external P load ntering the WB, internal P loading occurs in the CB. After the P s moved up to the upper water column due to fall overturn, it ould then move from west to east through the water flow. There- ore, internal P loads in the CB may not have much chance of en- ering the WB and contributing to HABs. It is reported that HABs re more likely triggered by relatively short-term loads of immedi- tely available P in the WB ( Scavia et al., 2014 ). Thus, a good con- rol of external P loads delivered to the WB would be a key to the revention, control and mitigation of HABs. However, the recovery f western basin water quality may be delayed due to legacy P, gricultural practice changes, climate change, etc. ( Sharpley et al., 013 ; Michalak et al., 2013 ; Scavia et al., 2014 ). Both hypoxic area nd days are positively related to annual external P loads deliv- red to the WB and CB ( Rucinski et al., 2014 ). In order to combat he growing threat of hypoxia, the Annex 4 committee of the 2012 Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 Fig. 5. Conceptual scheme of sedimentary P stock and release to the water in the 0–20 cm soft sediment of Lake Erie. Total P stock (TPS) consists of P1, organic P, including H 2 O-Po, NaHCO 3 -Po, and NaOH-Po; P2, non-HCl extractable inorganic P, including H 2 O-Pi, NaHCO 3 -Pi, and NaOH-Pi; P3, residual P; and P4, HCl extractable inorganic P. Each P fraction is represented by a circle, the size of which represents the relative magnitude of P stock; a solid line perimeter of a circle means that the P fraction is not potentially available for release to the overlying water, while a dashed line circle means that part of the P fraction is potentially available for P release. Numerical percentage represents the proportion of P potentially available for release into the overlying water in total P contained in a specific P fraction (i.e. organic P, non-HCl extractable inorganic P, or residual P) of the 0–10 cm sediment. In the CB, annual internal P loads (i.e. 10,599 metric tons yr −1) consists of 8,232 metric tons yr −1 derived from organic P, non-HCl extractable inorganic P, and residual P in the 0–10 cm sediment and 2367 metric tons yr −1 derived from organic P in the 10–20 cm sediment. The TPS is 201 metric tons km −2 in the CB. G t t ( t e i t a l l H i E a w a c d i a d s d w l P 4 D c i A s I i C A m reat Lakes Water Quality Agreement set updated P load reduc- ion targets of 40% entering the WB and CB to achieve 60 0 0 metric ons P loads yr −1 delivered into the CB. According to our estimate 10,599 metric tons P yr −1 ) of internal P loads for the CB, it may ake many years to see the desired responses of lake water body ven if the updated reduction target can be met by implement- ng appropriate P reduction measures. In the CB, HABs are also of- en detected by satellites in the offshore waters, but currently they re not severe environmental concerns due to short durations and imited sizes ( Chaffin et al., 2019 ). One may ask a question why arge amount of internal P loads in the CB can not cause large ABs in the CB and EB, which can be explained by the follow- ng two reasons. One is the P dilution effect, as that the CB and B combine to account for over 95% of water volume of the lake nd hold 18 times as much water as the WB. The other is that the ater in the CB and EB is too cold for developing HABs in most reas ( Schertzer et al., 1987 ; Ralston et al., 2014 ). Many climate hange scenarios indicate that Lake Erie area will become warmer ue to climate change, which may further increase internal P load- ng ( Gibbons and Bridgeman, 2020 ). Moreover, lake water temper- tures are expected to increase more rapidly than air temperature ue to decreased ice cover ( Gibbons and Bridgeman, 2020 ), which uggests that lake water will become more favorable rapidly for eveloping HABs in the CB and EB. Therefore, HABs may become orsening in the CB and even EB if external P loads entering the ake is not well controlled and continue to deposit large amount of in the sediment. . Conclusions • The Hedley’s sequential extraction procedure was successfully used to separate sedimentary P into H 2 O-Pi, H 2 O-Po, NaHCO 3 - Pi, NaHCO -Po, NaOH-Pi, NaOH-Po, HCl-Pi, and residual P. 3 8 • Our study was the first time to completely determine the po- tential availability of each P form for release into overlying wa- ter in the WB, CB, and EB, and based on the results to calculate internal P loads of each basin from the sediment. • The TP stock is 197, 219, and 195 metric tons km −2 in the 0– 10 cm sediment of the WB, CB, and EB, respectively. Internal P loads from the 0–10 cm sediment could total 8,232 and 359 metric tons yr −1 in the CB and EB, respectively, while the 0– 10 cm sediment unlikely release detectable P into the overlying water in the WB. In the 10–20 cm sediment, the TP stock of the CB is 201 metric tons km −2 and likely contributes to internal P loads of 2,367 metric tons yr −1 mainly deriving from NaHCO 3 - Po and NaOH-Po. • Internal P loads from the sediment unlikely contributes to HABs in the WB, but it may cause and maintain hypoxia in the CB and delay the recovery of water body quality for a lengthy time period in response to external P reduction measures. eclaration of Competing Interest The authors declare that they have no known competing finan- ial interests or personal relationships that could have appeared to nfluence the work reported in this paper. cknowledgements This research was carried out as part of the Erie Comprehen- ive Collaborative Survey program convened for the 2009 Lake Erie ntensive Year through the Lake Erie Millennium Network. Fund- ng was provided by the Ontario Ministry of the Environment for anadian-based sampling and by the US Environmental Protection gency for US-based sampling (National Coastal Condition Assess- ent program for shoreline samples; Great Lakes National Pro- Y.T. Wang, T.Q. Zhang, Y.C. Zhao et al. Water Research 188 (2021) 116525 g C L o a H v S f R A A A B B C C D D E G H H H H H J K K K K K M M M M M M N N P P P P R R S S S S S T U U U U V W W W W Z ram Office for offshore samples). We are grateful to Environment anada for arranging ship time for offshore sampling (CCGS SHARK imnos and CCGS SHARK Shark ) and for diver-assisted collections f sediments and benthos from nearshore areas. Sediment sample nalyses for P fractions were conducted by Mr. D. Lawrence from RDC, Agriculture and Agri-Food Canada. 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