Journal of Great Lakes Research 38 (2012) 830–833

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Journal of Great Lakes Research

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Commentary

Response to comment on ‘Temporal trends in phosphorus and lacustrine productivityin Lake Simcoe inferred from lake sediment’

Véronique P. Hiriart-Baer ⁎, Jacqui E. Milne 1, Christopher H. Marvin 2

Aquatic Ecosystem Management Research Division, Water Science and Technology, Directorate, Science and Technology Branch, National Water Research Institute, Environment Canada,867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6

⁎ Corresponding author. Tel.: +1 905 336 6447.E-mail addresses: Veronique.Hiriart-Baer@ec.gc.ca (V

Jacqui.Milne@ec.gc.ca (J.E. Milne), Chris.Marvin@ec.gc.ca1 Tel.: +1 905 336 6432.2 Tel.: +1 905 319 6919.

0380-1330/$ – see front matter. Crown Copyright © 20http://dx.doi.org/10.1016/j.jglr.2012.09.009

a r t i c l e i n f o that a lack of correlation between sediment P and P loadings vio-

Article history:

Received 17 September 2012Accepted 17 September 2012Available online 25 October 2012

Communicated by Gerald matisoff

lates Boyle's first assumption for the application of geochemicalpaleolimnological markers (Boyle, 2001). Ginn et al. (2012) present ev-idence that surface sediment P concentrations are not related to watercolumn P concentrations (Ginn et al., 2012; Fig. 2) and, from this, inferthat sediment P concentrations are not correlated with P loadings. To ex-plore the validity of the argument presented by Ginn et al. (2012) thatwater column P is correlated with P load, we examined the relationshipbetween P loads and mean annual total P concentrations at Lake Simcoestations K-42 and C-6 (Fig. 1). We were unable to detect a relationship

Introduction

We wish to thank Ginn et al. (2012) for their comments regardingour paper “Temporal trends in phosphorus and lacustrine productivityin Lake Simcoe inferred from lake sediment” (Hiriart-Baer et al.,2011). In this paper we demonstrated that organic phosphorus (P) con-centrations in Lake Simcoe sediment cores have increased since ca. 1970coupled with an increase in primary production inferred from carbonstable isotope signatures of organic matter which could be explainedby a change in the P cycle in Lake Simcoe. Ginn et al. (2012) objectedto our interpretation of the results stating that (1) geochemical esti-mates of past P from Lake Simcoe cores report opposite trends tolong-term observational/monitoring data; (2) monitoring data doesnot record increased algal primary production; and (3) geochemicalestimates of past P from dated sediment cores are not tracking changesin lake trophic status. We maintain that (1) water column total P con-centrations are not adequately related to loads and cannot be used toreject sediment P concentrations; (2) independent evidence supportsour data that primary production has generally increased in LakeSimcoe over the last four decades; and (3) organic P is indeed a reliableproxy for paleolimnological reconstructions.

Opposite trends to long-term observational/monitoring data

Ginn et al. (2012) state that the 1980–2010 decline in totalphosphorus (P) water column concentrations is in stark contrast toour trend of increasing sediment phosphorus (P) concentrations and

.P. Hiriart-Baer),(C.H. Marvin).

12 Published by Elsevier B.V. on beh

between mean water column total P and total P loads for either station(K-42 r2=0.003; C-6 r2=0.044). Although first principles would sug-gest that offshore water column P is necessarily correlated to loadings,in Lake Simcoe, total P loadings alone do not appear to explain much ofthe inter-annual total P variability (Fig. 1), a conclusion which has alsobeen suggested by recent modelling efforts (Gudimov et al., 2012). Thisapparent decoupling between loads and offshore water quality is, inpart, due to an increase in the relative importance of non-point sources,which are inherently more difficult to measure, and the fact thatnon-point source discharges often hug shorelines and may notcompletely mix with offshore waters. It is also possible that an un-derestimation bias exists in the total P loading estimates, since sys-tematic sampling protocols, like the one in Lake Simcoe (pre-2000;O'Connor et al., 2011), can ignore some major events which can dis-proportionately contribute to total annual P loads (Gudimov et al.,2011). A lack of a relationship between water column P and load-ings may not invalidate their claim, but it indicates that water col-umn P should not be used to dismiss independent contradictoryevidence.

We suggested that an increase in the relative proportion of solublereactive phosphorus (SRP) over the last four decades may have led toan increase in primary production and organic P burial in LakeSimcoe. As a counter-argument, Ginn et al. (2012) show SRP and TPconcentrations at five stations in Lake Simcoe from 1980 to 2008(Ginn et al. 2012; Table 1). Based on these data which show a de-crease in the proportion of SRP to TP between 1980 and 1999 andthereafter a steady ratio, it would appear that the SRP hypothesis pro-posed by Hiriart-Baer et al. (2011) does not hold; however; a fewpoints need to be made. First, SRP:TP ratios in streams would be amore convincing argument that SRP inputs have not changed inLake Simcoe. To our knowledge, long-term trends in the proportionof SRP:TP in Lake Simcoe tributaries have not been reported exceptfor the period between 1982 and 1992 (Peat and Walters, 1994) but

alf of International Association for Great Lakes Research. All rights reserved.

5

10

15

20

25

1980 1987 1994 2001 2008

5

10

15

20

25

30

40 80 120 160

Total P loads (T/yr)

a b

c d

10

30M

ean

Tot

al P

(µg

/L-1

)

Fig. 1. Mean water column total P concentrations at station K-42 and C-6 over time (a) andtheir relationship with total P loads to Lake Simcoe (b). Note: Panels (a) and (c) werereproduced from Fig. 1 in Ginn et al. (2012).

831V.P. Hiriart-Baer et al. / Journal of Great Lakes Research 38 (2012) 830–833

increases have been recorded in some Lake Erie tributaries, particu-larly in agricultural watersheds, which make up 47% of the totalland use in the Lake Simcoe watershed (Ohio EPA, 2010; Palmer etal., 2011). Similarly increases in SRP:TP ratio from 0.12±0.11 to0.37±0.16 in the 1970s and early 2000s, respectively, have beenrecorded in TwelveMile creek, anOntariowatershedwith 44% agricultur-al land use (data from the Provincial Water Quality Monitoring Network;Niagara Peninsula Conservation Authority, 2000). Second, offshore SRPconcentrations (and P ratios) do not necessarily reflect shoreline inputs

Fig. 2. Spatially integrated conductivity values in Kempenfelt Bay during the May 2009 LaKempenfelt Bay (K-38, K-39 and K42 from west to east).

(i.e. runoff and tributaries) which dominate P loads (Palmer et al.,2011). In 2009 and 2010 we conducted spatial surveys of surfacewater quality parameters for Lake Simcoe using a flow-through systemequipped with various sensors (e.g. conductivity, chlorophyll a, coloureddissolved organic matter). Looking at conductivity, a common indicatorof watershed inputs (e.g. Higgins et al., 2012), we see that the offshoremonitoring stations can fail to detect areas influenced by shoreline inputsof material (Fig. 2). Finally, given the large increase in aquatic macro-phytes abundance since the early 1980s (Ginn, 2011) as well as exces-sive growth of inshore filamentous algae (Winter et al., 2007) andassociated epiphytes, it is likely that water column SRP concentrationsare under biological control and that offshore, and arguably nearshore,measurements of SRPmaynot accurately reflect the amount of bioavail-able P in the system (Higgins et al., 2012).

Monitoring data does not record increased algal primaryproduction

Ginn et al. (2012) state that euphotic chlorophyll a (chl a) concen-trations and phytoplankton biovolume have decreased or remainedstable since 1980 (Ginn et al. 2012; Fig. 1) contradicting our carbonstable isotope data which suggest increased primary production hasincreased since the 1970s. It is important to note that chl a concentra-tions and phytoplankton biovolumes are not the equivalent of prima-ry production; while often used interchangeably, the former reflectstanding stocks while the latter are a rate process. Carbon stable iso-topes in sediments are used to reconstruct past primary production inlakes. This approach is not novel to our study and has been used exten-sively to infer changes in paleoproductivity (e.g. Hodell and Schelske,1998; Routh et al., 2009; Vreca andMuri, 2006) once the data are prop-erly corrected for the Suess effect (Verburg, 2007). Althoughwe are notentirely certain whether Ginn et al. (2012) are challenging this method

ke Simcoe cruise. Stars indicate the position of the long-term monitoring stations in

201011

Increasing P loads

Mussel invasion

Climate change

Increased urbanization

20001

Increased macrophyte biomass

Increased primary production

Increased organic P burial

Beak Consultants Ltd 1992, Winter et al. 1992, Winter et al. 2007, LSRCA/OMOE 2009

Evans et al. 2011

Stainsby et al. 2011, Conservation Ontario

Ginn 2011

Ginn 2011

Hawryshyn et al. 2011; Hiriart-Baer et al. 2011

Hiriart-Baer et al. 2011

Cau

seE

ffect

Declining P loadsHigh P loads Decreasing P loads to 1980 levels

19901970 1980

Increased macrophyte biomass

Increased primary production

Increased organic P burial

Fig. 3. Partial compilation of known stressors in Lake Simcoe and some of the ecosystem effects that have been observed over the last four decades.

832 V.P. Hiriart-Baer et al. / Journal of Great Lakes Research 38 (2012) 830–833

as a whole or simply rejecting our data because they do not align withmonitoring data, we argue that our findings are consistentwith a recentpaleolimnological study that showed an increase in sedimentary chl afor all basins in Lake Simcoe since ca. 1950s to late 1990s/early 2000s(Hawryshyn et al., 2010, 2012; Fig. 3). According to these authors, theanalysis of sedimentary chl a by visible reflectance spectroscopy is con-sidered a “proxy for past primary production”. Furthermore, the studyreports an increase in pennate diatom assemblages in the mid to late20th century, an assemblage “commonly associated with elevated nu-trient levels” and a change in nutrient cycling following the invasionof dreissenid mussels is suggested as a possible mechanism for theobserved shifts in diatom communities (Hawryshyn, 2010). We willnot enter the debate of whether climate change, as suggested byHawryshyn et al. (2012), and/or changes in the P cycle, as suggestedby Hiriart-Baer et al. (2011), are responsible for increases in primaryproduction. The point being that both studies provide evidence that pri-mary production has increased in Lake Simcoe over at least the last fourdecades, albeit not monotonically. If the sedimentary chl a measure-ments and the diatoms shifts reported by Hawryshyn et al. (2012) arecorrect, then we respectfully suggest that our data showing increasedprimary production in Lake Simcoe over the last four decades are con-sistent with this pattern.

Geochemical estimate of past P do not track lake trophicstatus changes

Ginn et al. (2012) argue that the sedimentary P record is not areliable indicator for tracking changes in lake trophic status becausesediment P has a strong upward migration influenced by redox chem-istry and water column dissolved oxygen concentrations. While weagree that sedimentary P does have mobility, and that P flux fromsediments can occur depending on the location and time of year,our interpretation of the sedimentary P data was based predominant-ly on changes in the organic-P content of the sediment. A recent studyby Ostrofsky (2012) investigated the mobility of different P forms infive lakes with differing characteristics. This study showed that themain fractions of P that migrate upward in sediments are reductant-soluble bicarbonate/dithionite extracted P and NaOH extracted P,while apatite-P and organic-P show no significant movement. The lattertwo P forms can consequently be considered more reliable proxies forpaleolimnological reconstructions. We are not implying that organic-Pis refractory; it is indeed a labile form of P but most of the diagenesisoccurs shortly after sedimentation (Hupfer et al., 1995). There remainsan appreciable amount of P that is permanently stored in the sediment,which is likely a result of limited oxygen diffusion in the sediment andthe exhaustion of other electron acceptors required tomineralize organ-ic matter (Ostrofsky, 2012 and references therein). Since our interpreta-tions of the sedimentary P profiles were based largely on changes inorganic-P content and the concomitant changes in the stable isotopedata, we maintain that our interpretation of the organic-P profiles and

our hypothesis of changes in the P cycle in Lake Simcoe are well-founded.

Conclusion

Lake Simcoe has undergone many ecosystem changes over the lastfour decades in response to multiple stressors on the system (Palmeret al., 2011). Stressors such as P loadings, increased urbanization(Ginn, 2011; Palmer et al., 2011), climate change (Stainsby et al.,2011) and the establishment of dreissenid mussels (Evans et al.,2011) have undoubtedly impacted Lake Simcoe, individually and incombination, and are likely causally related to some of the ecosystemresponses we have reported (Fig. 3). Ginn et al. (2012) suggest that,despite contemporary P loads (average 69.7±7.9 T/yr; 2000–2006;LSRCA/OMOE, 2009; Winter et al., 2007) being on par with thosemeasured in the 1980s (average 76.3±17.0 T/yr; 1980–1989; BeakConsultants Limited, 1992), these additional stressors have had noother effect on Lake Simcoe then to ameliorate water quality condi-tions. The paleolimnological markers suggest otherwise, and wemaintain that our interpretation of the sediment record is soundbased on: (1) a lack of correlation between water column total Pand P loads, implying over-looked P inputs from non-point sources;(2) an independent study on paleoproductivity that also indicatesgeneral increases in primary production in Lake Simcoe from 1950sto late 1990s/early 2000s; and (3) the use of organic-P (a relativelystable P form) in the sediment profile to infer changes in P cyclingin Lake Simcoe.

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