J. Great Lakes Res. 31(Suppl. 2):23-34 Internat. Assoc. Great Lakes Res., 2005

The US EPA Lake Erie indicators monitoring program 1983 - 2002:trends in phosphorus, silica

and chlorophyll a in the central basin. David C. Rockwell1, Glenn J. Warren1, Paul E. Bertram1, Douglas K. Salisbury2, and Noel M. Burns3 1Great Lakes National Program Office, United States Environmental Protection Agency, 77 West Jackson Blvd, Chicago, Illinois 60604 Telephone: 312-353-1373 Fax: 312-353-2018 e-mail: Rockwell.David@epa.gov, Warren.Glenn@epa.gov, Bertram.paul@epa.gov 2 Computer Sciences Corporation, Federal Sector, 3170 Fairview Park Drive, Falls Church, VA 22042-4506, Telephone: 312-353-3212 Fax:312-353-2018 3 Lakes Consulting, 42 Seabreeze Rd., Devonport. Auckland, New Zealand 1309 Telephone and Fax: 64-7-864-7449 e-mail: lakescon@xtra.co.nz Corresponding Author: David C. Rockwell, E.Mail: Rockwell.David@epa.gov Running head: phosphorus, chlorophyll a, and silica in Lake Erie

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Abstract: During the past 20 years, Lake Erie has exhibited a series of complex chemical changes resulting from changing anthropogenic influences and introductions of exotic species. Since 1990, some apparent trends in nutrient concentrations have been inconsistent with the predictions of models originally used to guide Lake Erie water quality management. We performed time trend analysis on total phosphorus (TP), chlorophyll a Chl a), and dissolved reactive silica (DRS) measurements collected during spring and summer in the central basin of Lake Erie between 1983 and 2001. Three distinct time trends in basin-specific, station-averaged open-water TP concentrations were observed over the 20-y period - 1983-1989 decreases, 1990-1997 increases, and 1997- 2001 decreases. Exceptionally high levels of turbidity and TP were observed in spring 2002, possibly reflecting increasing frequency of winter storm events. Open-water concentrations of TP declined during the 1980’s as annual TP loadings to Lake Erie declined below the 11,000 metric ton target level that had been expected to reduce central basin eutrophication. This was accompanied by a significant increase in available silica and in Chl a concentrations. During the period 1990-2002, when dreissenid mussels were abundant, TP concentration spring increases but summer declines were observed. Unexpected increases in spring central basin concentrations of DRS (exceeding 1 mg/L) were observed. During the same period, Chl a values declined in the spring and rose slightly during summer. Key words: Lake Erie, phosphorus, chlorophyll a, silica, trends, Dreissenidae

Introduction: Lake Erie has a history of anthropogenic influences that have changed the lake ecosystem (Beeton 1965, 1969; International Joint Commission. 1969; Kemp et al. 1976). Accelerated eutrophication was found to be the result of large phosphorus loads from a number of sources, including municipal sewage plants and non-point, agricultural runoff to Lake Erie’s tributaries. In the 1970’s Governmental actions to reverse this trend were taken. Water quality models developed during this period were used to set target levels of phosphorus load input to the lake and of open lake phosphorus concentrations necessary to establish desirable in-lake conditions (Vollenweider 1976, DiToro et al. 1978, and Chapra 1978). A basic assumption of the models was that there exists a stable relationship between phosphorus and chlorophyll a concentrations (and primary productivity), and between the phosphorus load entering the system, and the observed in-lake concentrations. Open-lake concentrations of total phosphorus (TP) in Lake Erie have risen through the 1990’s. However, Chlorophyll a, which serves as a useful indicator of algal biomass (Herdendorf 1984), was at historically low levels in all three basins through the 1990’s (Howell et al. 1996, and Makarewicz et al. 2000). It is unclear whether there has been a change in the basic chlorophyll a - phosphorus relationship in the lake in the years since 1990. Lake Erie, with a hydraulic residence time of between 2.3 and 3.4 years, depending on lake level (Quinn, 1992) should exhibit changes in nutrient concentrations and water quality based on nutrient loading rates. Monitoring programs in the U.S. (Rockwell 2003) and Canada (Charleton

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2001) have documented central basin open lake increasing TP concentrations in the 1990’s during spring and summer. These changes in TP concentration are possibly concordant with estimated phosphorus loads. External total phosphorus loadings showed a decreasing trend from 1982 to 1990 and an increasing trend from 1990 to 1997, with lower loads in 1999 and 2000 (Dolan and McGunagle, this volume). The introduction of Driessena spp mussels in Lake Erie in the late 1980’s has been the subject of many investigations and may be related to the changes in nutrient chemistry and phytoplankton populations (Makarewicz et al. 2000, Klerks et al. 1996, Leach 1993, MacIsaac et al. 1996, Madenjian 1995, Nicholls and Hopkins 1993, Conroy et al., this volume). The rise of Dreissena populations in eastern basin Lake Erie reached a peak population density of 320,000 individuals/m2 maximum by 1991(Howell et al. 1996, but see Patterson et al., this volume). Studies in Saginaw Bay (Lake Huron) documented nutrient changes, effects on benthic algal populations and a shift away from diatom dominance of the phytoplankton community following proliferation of Driessena (Fahnenstiel et al. 1995, Johengen et al. 1995, Gardner et al. 1995, Lowe and Pillsbury 1995). Broad ecological changes have been seen in other lakes where Driessena have become abundant. (Nalepa and Fahnenstiel 1995). Increases in the sediment-water phosphorus flux associated with the presence of zebra mussels help to explain increases in blue-green algae (Bierman, Jr. et al. 2005) in Saginaw Bay. In order to track the changes in nutrient levels expected to result from nutrient reduction actions in Canada and the United States, open water monitoring programs were put into place. On an annual basis, the U.S. Environmental Protection Agency, Great Lakes National Program Office (GLNPO) monitors the three basins of Lake Erie to provide a long-term record of nutrients, chlorophyll a, and plankton communities. This paper presents and compares three trend analyses (spring, summer, and spring+summer combined) in total phosphorus, silica, and chlorophyll a in the central basin during the interval 1983 through 2002. We operationally define two periods of interest with respect to nutrient trends - pre-Dreissena (1983-1989) and Dreissena (1990-2002). Materials and Methods Open Lake Monitoring: The US EPA Great Lakes National Program Office (GLNPO) monitoring program for Lake Erie began in 1983 at a limited number of locations designed to provide data for numerical, nutrient-based eutrophication models (Chapra, 1977; Lesht 1985, DiToro and Connolly, 1980). Water and plankton samples were collected from 1983 to 2002. The exceptions to the regular collections were: spring 1989, the summers of 1992 and 1993 and spring and summer surveys in 1994 and 1995 when no samples were collected. The annual monitoring program began in 1983, with samples being collected at five Central basin stations (Fig. 1A) (Lesht and Rockwell 1985). The number central basin stations were increased to ten in 1985 (Fig. 1B). The resulting network has remained unchanged since that time.

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Within the constraints of ice cover and weather, Lake Erie basins have been sampled within similar temperature ranges, with most spring sampling occurring during April where the average water temperatures at the time of sampling ranged from 0.9-6.5o C in the central basin. One exception to April sampling was in 1999, when the CCGS Icebreaker Samuel Risley was used in March and uniform 0.01o C water temperatures were found. Summer surveys take place in August. Water temperature minima and maxima at this time were 20.7o C-26.4o C in the central basin. A survey of the central basin of Lake Erie was typically completed in less than 24 h. In spring, samples were collected at depths of 1 m, mid-water column, 10 m above the bottom, and 1 m above the bottom. Sampling depths in summer, following the onset of stratification in the central basin were 1 m, mid-epilimnion, thermocline, and upper hypolimnion, and 1 m above the bottom. Chemical Analyses: All chemical analyses took place aboard the ship through 1994 using Technicon Autoanalyzers (System II). After 1992, a Lachat flow injection instrument was used for nutrient chemistries, with samples collected, preserved, and shipped to a laboratory in Chicago. Methods for chemical analyses varied only slightly over the reporting period (Table 1). Detailed descriptions of sampling and analytical procedures for GLNPO’s open lake water quality survey of the Great Lakes are available as follows: 1983-1992 (Rockwell et al. 1989), 1993-1999 (Lachat, 1992 and USEPA, 1997) and 2000-2002 (USEPA, 2003). Statistical Methods: Three analyses of long-term trends were undertaken. The first analysis assessed the spring survey data only. The second analysis evaluated trends in the summer survey data only. The third analysis examined interannual trends of the data from both spring and summer surveys combined. The latter analysis assessed seasonally-adjusted data fitted to a polynomial by multiple regression (Burns et al., this volume, and below). Long-term nutrient trend analysis based on spring surveys is GLNPO’s principal method of tracking changes over time (Bertram 1993). This approach rests on the assumption that the spring period is the least biologically active for the lake (the water is still cold), and because mixing is at its maximum the basins are the most homogeneous at this time. Station average concentrations, (means of concentrations from all sample depths for spring and all epilimnion sample depths for summer) collected at individual monitoring stations were used for long-term analysis for spring and summer, respectively. Data from two runs, or passes, through the lake were combined when they were statistically indistinguishable (Lesht and Rockwell 1987). Spring 1989 data were lost due to ship mechanical failure. Analyses of data collected in spring 1992, 1993, 1996 and 1997 were based on the second of two runs because the first run occurred during storm-perturbed conditions. Multiple runs during the spring surveys were stopped in 1998 unless the Central Basin average turbidity exceeded 4 NTU.

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The spring and summer concentration data were regressed against sampling year using a Sen nonparametric trend analysis (Sen, 1968) for the pre-Dreissena (1983-1988) and Dreissena (1990-2002) periods. Median slope, median year, and median parameter concentration measurement were calculated to define the Sen regression line equation: y = median slope*(year) + median measurement Spearman rank correlation coefficients and a two-sided confidence interval (95% upper and lower confidence limits) about the true parameter concentration slope were calculated to evaluate the statistical significance of the Sen regression coefficients. All descriptive, correlation, univariate, and regression statistic calculations were performed using The SAS System for Windows (SAS v8.2). The third long term trend analysis entailed combining spring and summer survey data obtained from the monitoring program. The data were analyzed using ‘LakeWatch’ software (Lakes Consulting 2000), which was especially designed for analysis of monitoring data from lakes and reservoirs (Burns et al. 1999). For these analyses, spring and summer survey data were ‘deseasonalized’. All available data for all years for a parameter were plotted against Julian Day (e.g., Fig. 2). Polynomial regression was then used to produce a least-squares fit through the data. The ‘deseasonalized’ value of an observation was that point’s residual value (i.e., the observed value – value predicted for the day of sampling from the polynomial regression equation). Interannual time trends were then estimated by linear regression of the deseasonalized values against time (e.g., Fig. 3A). As for the spring and summer survey data, spring+summer trends were evaluated for the 1983-1989 pre –Dreissena period, the 1989-1990-2002 Dreissena period, and for the entire 1983-2002 period of record. Results and Discussion Total Phosphorus Seasonal Trends: The epilimnion /isothermal-annualized TP data (Fig. 2) for the pre- Dreissena period summarize the progression of seasonal change in epilimnetic concentration. Total phosphorus concentrations declined from the spring maximum (range 6-26 ug/L; mid April median of 12 ug/L) to reach a minimum near the beginning of August (range 3.5-12 ug/L; mid August median of 6.5 ug/L; Fig. 2). Total phosphorus concentrations probably declined in the epilimnion during summer due to settling of particles into the hypolimnion. The lack of data between the end of April and the beginning of August precludes our knowing whether the reduction is gradual or abrupt. However, Burns et al. (this volume) and Carrick et al. (this volume) suggested that sudden, rapid settling of phytoplankton from the eiplimnion to the hypolimnion results in subthermocline and epibenthic primary production sufficient to raise dissolved oxygen concentrations in deep waters of the central basin. Interannual Trends PreDreissena: The spring and combined spring+summer survey data analyses detected statistically significant decreases in TP during the preDreissena period (Table 2). Most of the interannual decline was evident in the spring (-0.73 ug/L/y, p<0.001; Fig. 3C). Although summer TP concentrations also tended to have declined through the 1980’s (-0.21 ug/L/y), this trend was not statistically significant (p>0.05). The combined spring+summer mean TP

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concentrations declined at an average rate of 0.52ug/L/y (p<0.05; Table 2; Fig. 3A) between 1983 and 1989. Interannual Trends, Dreissena Period: Over the 13 y of the Dreissena period, spring TP concentrations increased at an estimated annual rate of 0.57 ug/L/y (p<0.001, Table 2; Fig. 3C). In contrast, the reverse trend was observed during summer sample surveys. The concentration of summer TP declined by an average of 0.14 ug/L/y (p<0.001). However, the summer declines did not compensate for the annual spring increases in that when TP trends were estimated on a spring+summer basis between 1990 and 2002, concentrations have risen at an annual rate of 0.22 ug/L/y (Table 2; Fig. 3B). These increases in TP concentrations during the Dreissena period occurred even though most of the external loads from 1990 through 2002 were below the target level of 11,000 tonnes, and the external loads are not increasing in the long term (Dolan and McGunagle, this volume). In fact, the linear 20-y trend estimate for all seasonally-adjusted TP data indicates that concentrations in 2002 were equivalent to values in 1983 (net annual change of 0.012 ug/L/y, p>0.05). The Dreissena-period rise in central basin TP concentrations does not match predictions of the numerical models of TP dynamics. Model simulations of TP concentrations and loadings by DePinto et al. (2002) suggest that the TP concentrations of the central of Lake Erie are only partially explained by external total phosphorus loads. They hypothesize that unless the loading estimates are markedly inaccurate (but see Dolan and McGunagle, this volume), the rising TP concentrations most likely reflect a decrease in the sediment deposition rate of TP. Closer examination of the Dreissena period revealed two subperiods of change within the overall increase (Fig. 3C). Concentration of TP increased significantly during the first subperiod (1989-1997; 0.40 ug/L/y). External total phosphorus loadings to Lake Erie were 16,825 tonnes in 1997 due to extensive flooding, producing the highest central basin loads over the last decade (Table 3). However, TP declined significantly during the following 5 years (1997- 2001; -0.89 ug/L/y). There was little spring or summer precipitation during the years 1999 through 2001, which resulted in relatively low external loadings of TP from tributaries (Dolan and McGunagle, this volume), and this appears to correlate with the negative trend in central basin TP concentrations over the most recent time interval. Nevertheless, some TP concentrations measured in 2002 were the highest observed during the entire period of record, and the station-averaged mean for the basin was equivalent to mean concentrations observed from 1983-1985 (Table 3). The range of 2002 values is in line with an extrapolation of trends seen over the 1989-1997 period (i.e., an annual increase of 0.4 ug/L/y extended from 1989 to 2002; Fig. 3C). Dissolved Reactive Silica (DRS) Interannual Trends PreDreissena Period: Spring central basin DRS concentrations increased at a rate of 7.1 ug/L/y (p<0.001, Table 2 and Fig. 4). Summer concentrations exhibited an even higher annual rate of increase (25.7 ug/L/y; p<0.01, Table 2). The estimated annual rise determined for combined spring+summer surveys was 4.6 ug/L/y (p<0.05). This increase in DRS accompanied the institution of effective phosphorus loading controls that led to the decline of TP loads to below

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the target level in the early 1980’s. An increase in available dissolved silica may be expected (Stoermer et al. 1996) when phosphorus levels decrease sufficiently to reduce diatom productivity and silica uptake, and hence reduction of the soluble silica fraction, i.e. a reversal of the eutrophication process suggested by Schelske and Stoermer (1986). Interannual Trends Dreissena Period: Springtime concentrations of DRS continued to increase significantly (16.6 ug/L/y, p<0.001; Table 2, Fig. 4) through the Dreissena period. However, the increase was not maintained through the summer. The summer time trend was slightly negative but not significantly different from zero (-8.2 ug/L/y, p>0.05; Table 2). The combined spring+summer trend analysis was slightly positive but also not significantly different from zero (p>0.05; Table 2). Station-averaged DRS appeared to have reached an asymptote by about 2001 (Fig. 4), and DRS concentrations exceeding 1,000 ug/L have been common throughout the Dreissena period. The increase in the soluble silica fraction during this period appears to represent a different mechanism than the eutrophication process suggested by Schelske and Stoermer (1986). Increases in DRS are not expected to accompany increases in TP concentrations that would generally increase algal biomass. Stimulated algal growth generally results in decreasing DRS (Conley et al. 1993). Chlorophyll a Interannual Trends PreDreissena Period: Although TP spring and spring+summer concentrations declined during the period 1983-1989, Chl a rose during this interval. The estimated spring Chl a concentration rise was not significantly different from zero (0.18 ug/L/y; p>0.05; Table 2; Fig. 5), but it was significant during the summer (0.40 ug/L/y, p<0.01; Table 2; Fig. 5). The summer rise was sufficient to indicate substantial interannual increase for the combined spring+summer Chl a analysis (0.36 ug/L/y, p<0.001; Table 2), a time of year during which the trend in TP concentration was significantly decreasing (-0.52 ug/L/y, p<0.05; Table 2). These Chl a increases were unexpected given the decrease in total TP loadings and decrease in open lake TP concentrations observed. Interannual Trends Dreissena Period: In the Dreissena period, spring Chl a concentrations were highly variable among stations, especially between 1991-1993 and 1996 (Fig. 5). There was a slight but significant decreasing trend (-0.09 ug/L/y; p<0.05;Table 2) in spring survey values. However, summer survey Chl a concentrations rose at a rate of 0.09 ug/L/y (p<0.01). There was no net change in spring+summer combined Chl a concentration. The 20-y central basin Chl a trend for 1983-2002 was a marginally significant (p<0.05) decline of 0.07 ug/L/y; p<0.05). Conclusions Pre-Dreissena TP concentrations decreased fairly consistently among years, especially in spring, during the 1983-1989 pre-Dreissena period of record. Spring TP concentrations were below the 10 ug/L target levels set by the Great Lakes Water Quality Agreement in both spring and summer surveys in 1988 and in 1990. During the Dreissena period, TP concentrations exhibited significant annual increases in both spring and summer seasons. Spring TP concentrations returned to levels >10 ug/L in 8 of the 10 years reported during the Dreissena period and in 3 of these years, spring

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TP concentrations exceeded 15 ug/L. No spring surveys had exceeded 15 ug/L during the pre-Dreissena period. External phosphorus loading is one of several factors controlling the total phosphorus concentrations in the Driessena period. While it will continue to be a driving factor of total phosphorus concentrations in the Lake, it was not a statistically significant predictor (p>0.05) during the Dreissena period, and other factors must be influencing central basin TP concentrations. Simulations of Lake Erie TP dynamics suggest that a reduction in the settling rate of total phosphorus inputs improves model fits to the central basin concentrations observed in recent years (DePinto et al. 2002). Phosphorus can be present in solution, bound to organic or inorganic particles, or contained within living tissues. Changes to any of the compartments can potentially contribute to reduced TP settling rates. Dreissenid filtering and processing activities are frequently cited as a potential source of altered particle and nutrient dynamics (Vanderploeg et al. 2002, Hecky et al. 2004), converting biogenic phosphorus into excreted nutrients. Conroy et al. (2005) argued that the replacement of zebra mussels by quagga mussels in Lake Erie may have increased nutrient flux and TP remineralization rates. However, effects are more pronounced for nitrogen than for TP flux. Furthermore, driessenid nutrient excretion effects are most important in the unstratified nearshore and the western basin, and are a function of local densities (Conroy et al. 2005). Dreissenid numbers and biomass in the western and central basins have been declining rather than rising since the late 1990’s (Patterson et al., this volume), so if the open-water nutrient effects observed in our time trend analyses stem from dressenid effects, the effects are likely lagged by several years. Central basin offshore waters have become more turbid through the 1990s (Barbiero and Tuchman 2004, Burns et al., this volume). Barbiero and Tuchman (2004) attributed this to greater contributions of sediment loads from tributaries (mainly the Maumee R.) rather than to phytoplankton biomass. Our data have detected a gradual decrease in Chl a spring concentrations, but a gradual rise in Chl a summer concentrations through the dreissenid period. Munawar and Munawar (1999) documented that phytoplankton particle size has declined. Because settling rate is a function of particle size, this, too, could translate into a reduction in TP settling rate. Finally, particle settling rates are also determined by water viscosity, which is a function of temperature. Spring TP concentrations in spring 2002 were the highest observed since the 1970s. This was also the coldest month of May ever recorded in the lower Great Lakes. Delayed spring warming of water may slow the settling velocity of particles entering the lake (and their associated TP and development of Chl a) during spring run-off, contributing to reduced rates of TP loss from the water column and lower Chl a in the spring survey. Further research will be necessary to evaluate the relative importance of various factors to particle flux and their ultimate role in the apparent changes to Lake Erie`s central basin TP dynamics. Dissolved reactive silica concentrations significantly increased in the central basin during both the pre- Dreissena and Dreissena periods. Pre-Dreissena period DRS concentrations (<100 ug/L) were less than in the Dreissena period, when annual maximum DRS concentrations frequently exceeded 1,000 ug/L. Since colonization by Dreissena, the relationships among TP concentrations, dissolved

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reactive silica and Chl a appear to be no longer consistent with the Schelske and Stoermer et al. (1986) hypothesis of phosphorus enrichment, silica utilization and biogeochemical silica depletion in the Great Lakes. During the 1990’s major increases in DRS occurred despite increasing TP concentrations, suggesting that other factors than nutrients are limiting diatom growth. Such factors may include grazing by Dreissena delaying the growth of algae, lowering spring Chl a concentrations, limiting micronutrients, ultraviolet light penetration, and others (Matisoff and Ciborowski, this volume). Chlorophyll a trends are opposite from what would be expected from the changes in TP concentrations when significant trends were observed in these two parameters. In the pre-Dreissena period, Chl a was increasing in both spring and summer seasons whereas TP declines were detected by all trend analyses. In the Dreissena period, when phosphorus concentrations were increasing in the spring and DRS was abundant, Chl a concentrations decreased, but rose slightly during summers when phosphorus concentrations were decreasing. This outcome may represent insufficient time for uptake of the available nutrients or colder spring water temperatures slowing growth, but perhaps this accounts for the rising concentrations of Chl a observed in summer survey. Predation by Dreissena during the spring prestratification period could potentially delay algal population increases and change the diatom species composition during the Dreissena period (R. Barbiero, Computer Science Corporation, pers. com.). These influences could change species composition of the community and the Chl a concentrations per cell. The variations seen in Chl a may respond differently to the particulate non-point source P loads and may not have as direct an effect on the lake as the more soluble municipal loads which were reduced by advanced phosphorus removal processes at municipal sewage plants (Murray Charlton pers. communication). The effect of the Dreissena on the chemistry of the lake is not constant due to changing populations of Dreissena (Barbiero and Tuchman 2004, Patterson et al., this volume). Better knowledge of the interannual patterns of change in Dreissena populations through the 1990’s would help us interpret the extent to which observed trends in water quality parameters have been influenced by variations in Dreissena species composition, distribution, and abundance rather than by contemporaneous changes in environmental conditions. Acknowledgements We thank Murray N. Charlton, Environment Canada and Jan J. H. Ciborowski, University of Windsor who provided insightful review comments and valuable background information. This document has not been subjected to the US Environmental Protection Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by U.S. EPA.

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Table 1 Measurement methods, method working range, and years data are available Measurement

Working Range Instrument Method ( No. )

Method

Years Data Available

Silica 0.01-2.00 mg-Si/L Technicon 0.01-2.00 mg-Si/L Lachat (10-114-27-1-B)

Molybdate

1983-1992 1993, 1996-2002

Total Phosphorus

0.002-0.050 mg-P/L Technicon 0.001-0.050 mg-P/L Lachat(10-115-01-1-F)

H2SO4 and Persulfate digestion, Molybdate, Ascorbic Acid Reduction

1983-1992 1993, 1996-2002

Chlorophyll a

0.0 –11ug Chl a /L

Acetone extraction, fluorometric

1983-1993 1996-2002

Table 2. Annual changes in water quality parameters estimated from trend analysis for total phosphorus (TP), dissolved reactive silica (DRS), and chlorophyll a (Chl a) in the central basin of Lake Erie during pre-dreissenid (1983-89) and dreissenid (1989-2002) periods. Statistically significant coefficients are listed in bold face.

Season & TP (ug/L) DRS (ug/L) Chl a (ug/L)

(Analysis) Estimate 1983-89 90-2002 1983-89 90-2002 1983-89 90-2002

Spring

(Sen)

Reg. Coeff.

95% CI -0.73*** -0.99 - -0.40

0.57*** 0.39 – 0.74

7.1*** 5.1 – 10.5

16.6*** 4.9 – 41.8

0.18 -0.12 – 0.43

-0.09* -0.22 – -0.01

Summer

(Sen)

Reg.Coeff.

95% CI

-0.21 -0.47 - 0.05

-0.14*** -0.22 0 -0.07

25.7** 10.4 – 42.8

-8.2 -10.4 – -5.9

0.40** 0.12 - 0.75

0.09** 0.05 – 0.15

Spr+Sum (Polynomial)

Reg. Coeff.

95% CI -0.52* -0.93– -0.11

0.22* 0.04 –- 0.39

4.6* 0.7 – 9.9

5.4 -2.7–- 13.4

0.36*** 0.16 – 0.56

-0.08 -0.20 – 0.04

*p<0.05 **p<0.01 ***p<0.001

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Table 3. Central basin total phosphorus (TP) concentrations and total phosphorus basin loads. Loading estimates taken from Dolan and McGunagle (this volume).

YearTP Conc. (g/L)

TP load (Tonnes/y)

1982 34511983 15.0 22041984 15.42 35731985 14.58 25361986 10.1 29221987 11.72 22871988 7.35 14351989 6.08 23121990 7.42 22091991 9.33 23891992 11.37 16651993 11.23 25671994 18831995 22481996 9.86 27101997 12.92 32201998 8.98 29731999 11.39 11982000 7.19 19512001 6.68 10692002 14.62 1559

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Figure Captions

Figure 1. Lake Erie surveillance stations 1983-1985 (upper map) and 1986-2003 (lower map).

Figure 2. Total phosphorus concentrations observed during spring and summer surveys, 1983-1989. The curve fitted through the data is the polynomial best fit.

Figure 3. A. Line plots of time trends of TP concentration in the central basin of Lake Erie, 1983-1989. Upper curve represents raw data; lower curve represents residual values (observed value minus value predicted for the observation date by regression equations estimated as in Fig 2). Straight lines drawn through the upper and lower curve in each panel represent linear regression estimates of the raw values and residual values vs. time, respectively. B. Time trends of TP concentration in the central basin of Lake Erie, 1990-2002. Remaining explanation as for panel A. C. Time trends of TP concentrations during spring in the central basin of Lake Erie. Straight lines drawn through station average points represent Sen Regression trends for 1983-1989 (left curve; filled circles) and 1990-2002 (right curve; filled squares).

Figure 4. Time trends of dissolved reactive silica concentrations during spring in the central basin of Lake Erie (note Logarithmic scale, extending from 0.001 to 10.0). Lines represent Sen regression trends drawn through station averaged points for 1983-1989 (left curve; filled circles) and 1990-2002 (right curve; filled squares).

Figure 5. Time trends of Chlorophyll a concentrations during spring in the central basin of Lake Erie. Points represent station means. Lines represent Sen regression trends drawn through station average points for 1983-1989 (left curve; filled circles) and 1990-2002 (right curve; filled squares).

.

17

Figure 1.

Figure 2

18

Figure 3.

19

Figure 4.

Central Lake Erie Chlorophyll aSpring Station Means

Slope = - 0.1Not Significant P<0.09

0

1

2

3

4

5

6

7

8

9

10

1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Chlo

roph

yll a

(ug/

L)

WQ Objective1983-19881990-2002Linear (1990-2002)

Figure 5

20