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Trends in Water Clarity of the Lower Great Lakes from Remotely Sensed Aquatic Color
J. Great Lakes Res. 33:828–841 Internat. Assoc. Great Lakes Res., 2007
Trends in Water Clarity of the Lower Great Lakes from Remotely Sensed Aquatic Color Caren E. Binding*, John H. Jerome, Robert P. Bukata, and William G. Booty National Water Research Institute Canada Centre for Inland Waters 867 Lakeshore Road, P.O. Box 5050 Burlington, Ontario L7R 4A6 ABSTRACT. Satellite observations of aquatic colour enable environmental monitoring of the Great Lakes at spatial and temporal scales not obtainable through ground-based monitoring. By merging data from the Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), monthly binned images of water-leaving radiance over the Great Lakes have been produced for the periods 1979–1985 and 1998–2006. This time-series can be interpreted in terms of changes in water clarity, showing seasonal and inter-annual variability of bright-water episodes such as phytoplankton blooms, re-suspension of bottom sediments, and whiting events. Variations in Secchi disk depth over Lakes Erie and Ontario are predicted using empirical relationships from coincident measurements of water transparency and remotely-sensed water-leaving radiance. Satellite observations document the extent to which the water clarity of the lower Great Lakes has changed over the last three decades in response to significant events including the invasion of zebra mussels. Results confirm dramatic reductions in Lake Ontario turbidity in the years following mussel colonization, with a doubling of estimated Secchi depths. Evidence confirms a reduction in the frequency/intensity of whiting events in agreement with suggestions of the role of calcium uptake by mussels on lake water clarity. Increased spring-time water clarity in the eastern basin of Lake Erie also corroborates previous observations in the region. Despite historical reports of localised increases in transparency in the western basin immediately following the mussel invasion, image analysis shows a significant increase in turbidity between the two study periods, in agreement with more recent reports of longer term trends in water clarity. Through its capacity to provide regular and readily interpretable synoptic views of regions undergoing significant environmental change, this work illustrates the value of remotely sensing water colour to water clarity monitoring in the lower Great Lakes. INDEX WORDS:
Remote sensing, aquatic colour, water quality, water clarity, Great Lakes.
INTRODUCTION Aquatic resources within the Great Lakes watershed have been impacted over the years by numerous environmental stressors as a consequence of, for example, farming practices, shipping, urbanization, and industrialization. The introduction of non-native invasive species, point-source discharges, nutrient loading and resulting eutrophication and nuisance algal blooms have all led to notable fluctuations in water quality and clarity (Environment Canada 2001). Mandated programs to reduce phosphorus loading to the Great Lakes in the 1970s and 80s resulted in a substantial reversal of the process of eutrophication, with significant reductions in *Corresponding
phytoplankton biomass recorded in the 80s and 90s (Stevens and Neilson 1987, Millard et al. 1996). The introduction of zebra mussels to the Great Lakes ecosystem in 1988 (Herbert et al. 1989) also led to reports of significant localized increases in water transparency in western Lake Erie, Saginaw Bay, and Lake St. Clair in the years immediately following the zebra mussel colonization, attributed in part to the direct removal of particulate matter by filter-feeding (Holland 1993, Leach 1993). However, longer-term studies by Makarewicz et al. (1999) and Barbiero and Tuchman (2004) found no evidence of a persistent and lake-wide increase in water transparency in Lake Erie but instead pointed to decreases in water clarity in the western basin and increased clarity in the eastern basin. Howell et al. (1996) also reported
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Remote Sensing of Great Lakes Water Clarity increased Secchi depths at a monitoring station in the eastern basin of Lake Erie in the years following mussel colonization. Despite several detailed studies, sparse spatial coverage and the discontinuous nature of ground-based monitoring often preclude reliable conclusions regarding long-term lake-wide changes in water quality. Earth observation satellites therefore have a logical place in the monitoring of environmental change within the Great Lakes watershed, providing regular, high resolution coverage of the region that may present more robust evidence of spatial and temporal trends in water quality than intermittent point-sampling alone. The spectral variation of visible radiation leaving a water body, as seen from a remote platform such as an aircraft or satellite sensor, provides information suitable for the detection of color-producing water quality parameters such as phytoplankton pigments, mineral suspended particulates, and dissolved organic matter. The Coastal Zone Color Scanner (CZCS) was a mutli-channel scanning radiometer launched aboard the Nimbus-7 satellite in October 1978 and was the first spacecraft instrument devoted to the measurement of ocean color (Hovis et al. 1980). It remained in operation until June 1986 after which there was a period of 11 years before the next dedicated ocean color platform was launched. The Sea-viewing Wide Fieldof-view Sensor (SeaWiFS), in orbit on the OrbView-2 platform since 1997, was the start of a new generation of satellite sensors delivering daily observations of aquatic color over the world’s water bodies (McClain et al. 2004). Gower (2004) presented a time series of SeaWiFS imagery for the period 1997–2001 and highlighted significant features in Canadian waters worthy of further investigation over longer time-scales. Previously it has been difficult to compare aquatic color observations from different satellites because the instruments, the data processing algorithms and calibration techniques have been different (Antoine et al. 2003). Achieving coherency between satellite missions is critical for assessing long-term changes in aquatic biogeochemistry. As part of NASA’s on-going commitment to the development of a long-term time-series of aquatic color data, the entire CZCS archive has been reprocessed by the Goddard Ocean Biology Processing Group (OBPG) following standard conventions for data formatting, navigation, and calibration, using identical algorithms wherever possible and the same processing code currently used for SeaWiFS. This communication combines these two satellite image archives in order to assess
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ecosystem impacts over the Great Lakes through the analysis of temporal trends in estimated water quality parameters. A time series of water-leaving radiance from both the CZCS and SeaWiFS sensors is presented that can be interpreted in terms of changes in water clarity of the Great Lakes aquatic system. Water-leaving radiance in the green portion of the visible spectrum (~550 nm) may be treated as an indicator of general turbidity; scattering caused by organic and inorganic particles in suspension lead to elevations in the measured water-leaving radiance. While it is not possible to distinguish the different types of scattering materials from a single band reflectance, with prior knowledge of the Great Lakes system it is possible to infer the timing and location of bright-water episodes such as intense phytoplankton blooms, suspended sediments, and whiting events. Results show both spatial and temporal trends in water clarity that may be related to significant events impacting water quality of the Great Lakes over the last three decades. SATELLITE DATA ACQUISITION AND PROCESSING METHODOLOGY The SeaWiFS and CZCS data used in this study were acquired from the NASA Ocean Color Web facility (http://oceancolor.gsfc.nasa.gov/) and processed using the dedicated ocean color processing software SeaDAS v5.0 (Baith et al. 2001). Monthly binned level 3b SeaWiFS images of normalized water-leaving radiance at 555 nm (nLw555) were produced for the Great Lakes region from January 1998 to December 2005. These level 3b products are atmospherically corrected radiances combined into 4.6 km spatial bins, and into one calendar month temporal bins. All of the valid pixels for the given time period and grid square are compiled in the same bin and the weighted mean of all observations is generated. Equivalent L3b monthly binned images were produced from CZCS Band-3 radiance at 550 nm for the period January 1979 to December 1985. Level 2 imagery could be used to provide increased spatial and temporal coverage (1 km, daily), however, in order to smooth daily variability and reduce computation requirements only the monthly binned data were analyzed here. Potentially erroneous pixels corresponding to conditions including land, cloud or ice, sun glint, saturated signals, and atmospheric correction failure were flagged and subsequently removed from further data analysis. SeaWiFS is limited to operate only for sun elevations above 20 degrees which, com-
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Binding et al. TABLE 1. Least square regression coefficients of the form Zs = αnLw β for data sets of in situ Secchi disk depth (Zs) and coincident satellite nLw~550. Data-set Erie SeaWiFS Erie CZCS All Erie All Ontario
FIG. 1. Secchi disk depth (Zs) against coincident SeaWiFS nLw 555 during surveys on Lake Erie from 1998 to 2005 and coincident CZCS nLw550 during surveys on Lake Erie from 1979 to 1985. Least square regression equations of the form Zs = αnLwβ are presented in Table 1. bined with pixels flagged due to winter lake ice formation, results in significant data loss during the winter months. Consequently, the analysis of satellite observations was restricted to the months March through to October each year. ASSESSING CONTINUITY OF SENSOR DATA The approach adopted by the OBPG for the reprocessing of CZCS data is as similar as possible to that used to process SeaWiFS and MODIS data, differing only in the atmospheric correction procedures and the associated assumptions used to determine aerosol contributions. With the absence of suitable near infra-red (NIR) bands on the CZCS, the aerosol correction was performed using the 670 nm band where an iteration scheme was developed to determine the water-leaving radiance and subsequently the aerosol contribution at that wavelength. Band radiances may also show discrepancies because of differences in wavelength characteristics, with the sensor wavebands differing in both nominal wavelength (SeaWiFS is nLw 555 , CZCS is nLw550) and also in bandpass (SeaWiFS is 20 nm, CZCS is 40 nm). In low turbidity waters, optical modeling convolving spectra with both the SeaWiFS and CZCS respective bandpasses estimates that CZCS nLw550 would be between 4 to 6% higher than SeaWiFS nLw555 although this may be higher in more turbid waters.
α 4.518 4.211 4.443 3.505
β –0.991 –0.821 –0.949 –0.923
R2 0.72 0.70 0.71 0.65
n 306 94 400 420
p < 0.05 < 0.05 < 0.05 < 0.05
As a result of these uncertainties it may not be appropriate to make quantitative comparisons between the CZCS and SeaWiFS imagery without first having supporting evidence of continuity in the radiance measurements from both satellite sensors. This can be achieved by comparing the sensor radiances against a known optical property and testing for consistency between the two missions. Whilst this approach will not eliminate sensor biases, it does enable a simple assessment of the continuity of sensor data and identification of any significant discrepancies which may alter trends in time-series analyses. Quality-assessment carried out by the OBPG showed that radiance and chlorophyll retrievals are generally in good agreement with in situ measurements although the data are currently rather limited (Feldman and McClain 2006). Few data sets of optical properties exist dating as far back as the 1970s. However, one consistent measurement made by Environment Canada over the years is water transparency. Over the period of interest 7,780 Secchi disk observations were made within Lakes Erie and Ontario during routine research and surveillance cruises. Comparing these Secchi disk depths (Zs) with coincident satellite radiance measurements from both SeaWiFS and CZCS provide a simple indication of the consistency of the sensor observations. CZCS and SeaWiFS imagery was obtained in the form of L3b daily composites and clear-image radiances coincident with in situ sampling dates were extracted. Lake Erie was chosen for the test, enabling sensor consistency to be assessed across the largest range of measured radiance. CZCS spatial coverage was on average less than 50% of that which is currently available from SeaWiFS over any single year resulting in significantly fewer match-ups (94 compared with 306 for SeaWiFS). Empirical relationships between Secchi disk depths and both CZCS and SeaWiFS waterleaving radiance were formed and showed close agreement (Fig. 1, Table 1). Analysis of variance identified no significant differences in the slopes
Remote Sensing of Great Lakes Water Clarity (F = 2.88, p = 0.16) or intercepts (F = 3.39, p = 0.07) of linear regressions on log transformed data for the two datasets, suggesting it may be reasonable to make quantitative comparisons between radiance data from the two missions. This suggests that any sensor bias may be sufficiently small to be within the inherent variability of the relationship between Secchi depth and water-leaving radiance. VARIATIONS IN WATER-LEAVING RADIANCE Figure 2a and 2b present CZCS nLw550 and SeaWiFS nLw555 for the Great Lakes region averaged over the periods 1979–1985 and 1998–2005, respectively. The large geographical variation in water-leaving radiance is noticeable, with Lake Erie exhibiting the highest radiance and Lake Superior the lowest. Regions of water clarity change may be inferred from these images by simple subtraction of the means (Fig. 2c). Even considering the potential instrument bias discussed above, the differences seen in Figure 2c are suggestive of real environmental change, with large differences (–50% to 100%) between CZCS and SeaWiFS radiances. From Figure 2 it can be seen that Lakes Michigan and Huron appear to have been brighter during the 1979–1985 period than the 1998–2005 period although several shallow near-shore areas such as Saginaw Bay and Lake St. Clair exhibit significantly brighter waters in more recent years. Interestingly, a remote sensing study of the effects of zebra mussels on water clarity of Saginaw Bay during the period 1987–1993 identified distinct increases in water clarity immediately following colonization (Budd et al. 2001). Imagery here suggests this clean-up may have been short-lived, with the bay showing some of the largest increases in water brightness over the Great Lakes. Lake Superior has shown the least change in mean brightness between the two time periods whereas Lake Ontario in its entirety exhibits significantly lower image brightness during the SeaWiFS mission than the CZCS mission two decades prior. Imagery of Lake Erie suggests more complex change with the central and western basins appearing much brighter in recent years whereas the eastern basin waters are less bright than during the CZCS mission years. Temporal variability may be assessed in more detail from time-series plots of monthly lake-averaged water-leaving radiance (Fig. 3), showing seasonal and inter-annual variability in brightness signals that may be related to periods of phytoplankton
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blooms, mineral sediment suspension, and whiting events. Lake Superior, with low levels of waterleaving radiance, and hence low turbidity, exhibits relatively small annual and inter-annual variability, particularly since 1998. Lake Michigan shows substantially greater variability dominated by peaks during August in several years. These bright-water events, predominantly observed in Lake Michigan, but also Lake Ontario and occasionally Lake Erie can be attributed to whiting events, that is the precipitation of highly scattering calcium carbonate under specific temperature and pH conditions (Strong and Eadie 1978, Vanderploeg et al. 1987). These whiting events were first observed in satellite imagery of the Great Lakes in 1973 (Strong and Eadie 1978) and appear to be regular features in imagery during August/September. Lake Huron exhibits an apparent gradual decrease in water brightness over the period of study, although with little change in intra-annual variability. In contrast to all the other lakes, Lake Erie shows a substantial increase in both the magnitude and variability of brightness levels between the CZCS and SeaWiFS missions. The peak in 2002 is driven largely by elevated water-leaving radiance during March/April. Barbiero and Tuchman (2004) also reported a year of unusually low water clarity in 2002 with spring Secchi depths throughout the lake < 2 m. The reason for this peak is unknown, there were no unusual weather events and water levels were not significantly different from other years. Lake Ontario monthly variations show regular spring peaks corresponding to spring phytoplankton blooms in addition to the whiting-driven peaks in August/September. The frequency and intensity of these peaks appears to have decreased during the SeaWiFS mission compared with the CZCS observations. TRENDS IN LAKE ERIE WATER CLARITY The close agreement between the satellite radiance datasets shown in Figure 1 means that the two datasets can be combined to give a simple method of estimating Secchi disk depths in Lake Erie from CZCS and SeaWiFS radiance at ~550 nm with an r.m.s. error of 24.8% of the mean (Zs = 4.443nLw–0.95, R2 = 0.71, n = 400). This relationship was applied to the archived monthly composite images in order to determine the long term trends in water clarity for Lake Erie. Figures 4a and 4b present the average Secchi depth over Lake Erie for the periods 1979–1985 and 1998–2005 respectively.
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FIG. 2. (a) Average CZCS nLw550 for the period 1979–1985, (b) average SeaWiFS nLw555 for the period 1998–2005 and (c) difference between the two images.
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FIG. 3. Monthly lake-wide average nLw ~550 from CZCS (open circles) and SeaWiFS (filled circles) for each of the Great Lakes. Lake-wide Secchi depths ranged from 2 to 4.5 m and 1.5 to > 6 m for the CZCS and SeaWiFS missions respectively, with the geographic distribution varying considerably between the two periods. Secchi depths of 4 m and greater, while quite widespread in the central and eastern basins during 1979–1985, are confined strictly to the eastern basin during the 1998–2005 period. Figure 4c shows the change in Zs between the two periods
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and confirms the relative increase in water clarity of up to and exceeding 2 m throughout the eastern basin. In contrast, the central and western basins have undergone a period of decreasing water clarity, with average reductions in Zs of 1–2 m. The zebra mussel population is thought to have become established in the western and central basins of Lake Erie in 1988 and completely colonized all three basins by 1989 (Griffiths et al. 1991). Since their introduction, mussels have been held responsible for reports of improved water clarity in the eastern basin; Barbiero and Tuchman (2004) reported evidence of increased springtime water clarity in the eastern basin of Lake Erie, with Secchi depths more than doubling after the mussel invasion, corroborating similar earlier documentation by Howell et al. (1996). An assessment of such seasonal changes in the satellite-derived Zs across the three basins was made for both the CZCS and SeaWiFS missions (Figures 5a and 5b, respectively). During 1979–1985, seasonal Secchi depths for all three basins were similar, exhibiting minima in April and maxima in August/September. By the 1998–2005 period, clear differences in the water clarity of the three basins are evident. Seasonal trends remain similar but there is now up to 4 m difference in Secchi depth between eastern and western basins. T-tests were performed in order to identify significant differences in the monthly-mean Secchi depths between the two observation periods. Those differences found to be statistically significant are shown in Figures 6a and 6b. In agreement with the reports of Barbiero and Tuchman (2004) and Howell et al. (1996), Figures 6a and 6b confirm more than a doubling of the east basin Secchi depths in April and May, with average changes of more than 3 m. Increases in monthly mean Secchi depths of more than 50% are evident throughout the rest of the year in the eastern basin. These results do not change appreciably if data from the 2002 peak are removed, confirming that this single bright event does not introduce excessive bias in the study. Trends in the central and western basins differ markedly to the observed increases in eastern basin water clarity. Reports over the years have documented significant localized increases in water clarity in the western basin (Leach 1993, Holland 1993) and dramatic declines in bio-productivity. However, contrasting evidence reported by Barbiero and Tuchman (2004) in fact suggested statistically significant declines in water clarity in some regions of Lake Erie. Results of image analysis here are in
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FIG. 4. Average Lake Erie Secchi disk depth (meters) for the periods (a) 1979–1985 and (b) 1998–2005 and (c) change in Secchi depth between the two periods. agreement with the findings of Barbiero and Tuchman, suggesting decreased water clarity in the western basin. Spring and fall Secchi depths appear to have decreased by up to 50% in the western basin after the mussel invasion, with no significant change during the summer months. The central basin shows the least temporal change, although still recording significant spring/fall decreases in Zs and increases in summer (July/August) Zs. Several authors have noted dramatic reductions
in phytoplankton biomass in the western basin attributed not only to the zebra mussel invasion but also to significant reductions in phosphate loadings over the years (Nicholls and Hopkins 1993, Makarewicz et al. 1999, Barbiero and Tuchman 2004). Total phosphorus concentrations in both Lakes Erie and Ontario decreased substantially after 1976 following legislation to limit phosphorus loading to the Great Lakes. Nicholls and Hopkins (1993) reported declines in phytoplankton biomass
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FIG. 5. (a) Monthly average CZCS-derived Secchi depth for Lake Erie during 1979–1985 and (b) monthly average SeaWiFS-derived Secchi depth for Lake Erie during 1998–2005. Data averaged for each of the eastern, central, and western basins. in Lake Erie’s western basin of around 5% per year over the period 1970–1985, and related this to decreased phosphorus loading to the western basin of about the same magnitude. Further dramatic declines in total phytoplankton biomass (up to 90%) coincided with the invasion of the lake by zebra mussels (Nicholls and Hopkins 1993). In their longer term study, Barbiero and Tuchman (2004) observed decreases in chlorophyll of 50% in the western basin in the years following the mussel invasion. Despite these reports of lower phytoplankton abundances in the western basin, results from image analysis in the present study suggest that this
has not resulted in significant increases in water clarity. Makarewicz et al. (1999) found that reductions in phytoplankton biomass and chlorophyll did not necessarily translate into increased transparency, stating that transparency was driven more by the re-suspension of inorganic particulate matter in the western basin. Makarewicz et al. observed significant decreases in spring/summer phytoplankton biomass in the western basin of Lake Erie in post-mussel years, with little reduction in biomass in the central and eastern basins during the same period. Despite this, they found significant decreases in spring-time water transparency in the
FIG. 6. Lake Erie monthly mean Secchi depth differences between the periods 1979–1985 and 1998–2005 ± 95% confidence intervals. (a) Difference as a percentage of the 1979–1985 average Zs and (b) absolute difference in meters. Significant differences were confirmed with t-tests at the 0.05 significance level and n-2 degrees of freedom. Those months without bar graphs exhibited no significant differences between the means.
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Binding et al. changes in algal biomass. Barbiero and Tuchman (2004) suggested that seasonal stratification in the eastern basin may limit the direct effects of mussel filtration on surface water clarity during the summer and, therefore, the observed increases in water clarity are more likely due to changes in phosphorus loadings or a reduction in whiting events (discussed in more detail in the following section).
FIG. 7. Secchi disk depth (Zs) against coincident SeaWiFS and CZCS nLw ~550 during surveys on Lake Ontario, along with the power fit for Lake Erie data for comparison. Least square regression equations of the form Zs = α nLwβ are presented in Table 1. western basin and an increase in central and eastern transparency. Others have made similar observations; MacIsaac (1996) reported on reductions in phytoplankton biomass of up to 90% in the Hudson River but only a 7% increase in water transparency following mussel colonization, attributing the discrepancy to particulate re-suspension. Given that there have been substantial reductions in nutrient loadings to the lake, and the known capacity for mussels to remove large quantities of suspended particulates from the water column through filter-feeding, it is not obvious why there has been an increase in turbidity in the western basin. Some speculative reasoning may be differences in the wind regime during the two observation periods leading to increased re-suspension of bottom sediments; data from Lake Erie buoys suggest 20% higher annual average wind speeds during the SeaWiFS observations compared to the CZCS observations (NOAA’s National Data Buoy Center, Station 45005). Long-term effects of lower phytoplankton productivity may also reduce biologically-aggregated material and associated settling, allowing finer disaggregated mineral particles to be resuspended more easily. Other possible explanations could be changes in the timing of ice break-up and variations in the input of sediments from the Sandusky and Maumee rivers. Sources of re-suspended sediments to the deeper eastern basin are far less than the western basin and therefore changes in water clarity here are more likely to be driven by
TRENDS IN LAKE ONTARIO WATER CLARITY Data from Lake Ontario were assessed for regional differences in the relationship between nLw~550 and Zs. While Lake Ontario data points appear to lie in good agreement with those from Lake Erie (Fig. 7, Table 1), ANOVA tests confirm that the two relationships have the same slope (F = 0.02, p = 0.875) but difference intercepts (F = 78.24, p < 0.05). The regression lines are parallel but are offset by 0.25 mW cm–2 µm–1 sr–1. This offset my be caused by differences in the predominant scattering agents in both lakes, with higher scattering mineral particles of Lake Erie producing a higher water-leaving radiance for a given Secchi depth than the algal dominated waters of Lake Ontario. The equation Zs = 3.505nLw–0.92 (R2 = 0.65, n = 420) was therefore used to predict Secchi disk depths within Lake Ontario, with an r.m.s. error in predicted Zs of 23% of the mean. Average Secchi depths over Lake Ontario as estimated from CZCS and SeaWiFS imagery for the periods 1979–1985 and 1998–2005 respectively are presented in Figures 8a and 8b. Secchi depths are largely uniform across the lake at around 3–4 m for the 1979–1985 period and increase noticeably to 6–8 m by the 1998–2005 period. Figure 8c confirms that the entire lake appears to have undergone significant improvements in water clarity, with a lake-wide increase in Secchi depth of between 2 and > 4 m. The north shores show the greatest change in water clarity, in excess of 4 m, with the Niagara River plume region on the southern shore showing the least change. The absence of large regions of bottom sediment re-suspension in Lake Ontario suggests that these temporal changes can be attributed to bio-chemical changes, either a reduction in biological productivity or a reduction in the intensity and/or frequency of whiting events. The cause of the increased water transparency may be speculated upon by looking at the monthly mean Secchi disk depths for the two time periods, where significant seasonality in water clarity is evi-
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FIG. 8. Average Lake Ontario Secchi disk depth (meters) for the periods (a) 1979–1985 and (b) 1998–2005 and (c) change in Secchi depth between the two periods.
dent (Fig. 9). During the 1979–1985 period, seasonality was typified by a small drop in Secchi depth in April, followed by a more significant reduction in August. These episodes may be attributed to spring phytoplankton blooms and whiting events respectively. The SeaWiFS mission data (1998–2005) shows some changes in the seasonal cycle of water clarity in Lake Ontario. There is no clear indication
of a spring bloom influencing water clarity, in fact there is a large increase in Secchi disk depths during the spring and summer, suggesting a lower influence of algal blooms in lake Ontario in recent years. Figure 10 presents the monthly mean significant differences in Secchi depth for Lake Ontario between the CZCS and SeaWiFS periods and con-
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FIG. 9. Monthly average Secchi depth for Lake Ontario during the two observation periods, 1979–1985 (CZCS) and 1998–2005 (SeaWiFS).
firms quantitatively the change undertaken over the decades. The largest monthly change in Secchi depth of 5 m was in April, more than doubling the 1979–1985 period levels of water clarity. Reductions in total phosphorus in Lake Ontario were generally greatest in the March/April period and higher in the near-shore than offshore (Nichols et al. 2001). Associated reductions in algal growth may, therefore, be responsible for the large increases in spring-time water clarity observed here. In agree-
ment, Millard et al. (2003) observed a lake-wide decline in chlorophyll between 1990 and 1996 and attributed it to the combined effects of both phosphorous loading controls and the mussel invasion. Lake Ontario was colonized by mussels shortly after Lake Erie (Griffiths et al. 1991) and potential effects of filtration were noted by Nicholls (2001) and Millard et al. (1996). Nicholls (2001) documented rapid and dramatic reductions in chlorophyll of 65–75% in the 3 years following mussel colonization at southern and eastern stations in Lake Ontario. The later mussel colonization of the north shore was reflected in later improvements in water clarity relative to the south shore. Here, however, the north shore shows greatest change. It may be that with the shallower waters of the northern shores any impact from reduced nutrient loads and/or mussel populations will be more pro nounced. The time-series of Lake Ontario water-leaving radiance in Figure 2 showed the incidence of significant peaks in water-leaving radiance attributed to whiting events that take place regularly in Lake Ontario during August/September. The frequency and intensity of these bright-water episodes is significantly less in recent years than during the 1979–1985 period, suggesting a reduction in whiting events since the establishment of zebra mussels. Estimates of Secchi depths from satellite imagery show that August Secchi depths more than doubled from 2 m to 5 m between the two observation periods (Fig. 10). Barbiero et al. (2006) described a three-fold reduction in August turbidity
FIG. 10. Lake Ontario monthly mean Secchi depth differences between the periods 1979–1985 and 1998–2005 ± 95% confidence intervals. (a) Difference as a percentage of the 1979-1985 average Zs and (b) absolute difference in meters. Significant differences were confirmed with t-tests at the 0.05 significance level and n-2 degrees of freedom. Those months without bar graphs exhibited no significant differences between the means.
Remote Sensing of Great Lakes Water Clarity values in Lake Ontario and a near doubling of Secchi depths following the zebra mussel colonization. Measured total suspended solids suggested most of this decrease in turbidity was due to a reduction in non-algal particulates. There have been recent suggestions that the long suspected role of zebra mussels in cleaning up Great Lakes water clarity may not be through direct filtering, which may be impeded by deep water and summer stratification, but through the enhanced uptake of calcium by the mussel populations, lowering the calcium carbonate saturation of the water column and resulting in far fewer whiting events (Barbiero and Tuchman 2004, Barbiero et al. 2006). Figure 9 illustrates that during the 1998–2005 period average Secchi depths decline to a minimum in September, 1 month later than the CZCS period average, suggesting the whiting events occur later now than in the 1970s and 80s. Calcium solubility decreases with increasing temperature and with the mussel uptake reducing calcium concentrations in the lake, calcium saturation and hence precipitation would be initiated at higher lake temperatures (i.e., later in the year). DISCUSSION AND CONCLUSIONS The Great Lakes have been subject to numerous socio-environmental influences, management actions, and accidental events over the decades which have created ecosystem changes which would have benefited high resolution monitoring available only through remote sensing techniques. Accurate determinations of long-term change from ground-based monitoring are made difficult by predominantly small, discontinuous, and geographically localized datasets of environmental parameters. Results presented here provide a means of examining largescale and long-term environmental changes over the Great Lakes. The advantages over ground-based monitoring are clearly in substantial increases in both spatial and temporal coverage; even with data loss due to cloud, lake coverage far exceeds that of typical monitoring programs. With the ongoing free availability of SeaWiFS and MODIS imagery and proposed future sensors, remote sensing of aquatic color is a cost-effective tool for observing lakewide water quality, offering important support in monitoring and mitigation of adverse ecosystem impacts. A simple method of estimating Secchi depths from satellite measured aquatic color is presented and used in the retrospective analysis of water clar-
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ity conditions of Lakes Ontario and Erie. Analysis of imagery over two periods, 1979–1985 and 1998–2005, identified: • a near doubling of spring time Secchi depths in the eastern basin of Lake Erie (increasing by up to 2 m) • a decrease in Secchi depths of up to 50% in the western basin of Lake Erie (decreasing by 1–2 m) • a more than doubling of Secchi depths over Lake Ontario (increasing by 2 – > 4 m) • evidence for calcium carbonate precipitation occurring later in the year and an overall reduction in the frequency/intensity of events These results contradict early reports of significant localised water clarity improvements in the western basin of Lake Erie following the introduction of zebra mussels (Holland 1993, Leach 1993). However, it is important to note that many studies of the impact of mussels on water clarity dealt with a short period of time, usually colonization ± 3 years or less (Leach 1993, Millard et al. 1996, Nicholls 2001). Results here deal with longer-term impacts and with the absence of imagery for the 1986–1997 period give no indication of the immediate impact of the mussel invasion. The water clarity trends identified here are, however, in agreement with long-term ground-based studies of water quality change reported by Barbiero and Tuchman (2004) and Makarewicz et al. (1999) and may be linked to the combined effects of a decrease in productivity associated with lower nutrient loadings to the lakes, particulate removal through mussel filter-feeding, and a decrease in the intensity and frequency of whiting events brought about by calcium uptake by mussel populations. Given the varying environmental conditions and inherent subjectivity with which Secchi disk measurements are made, the relationships determined in this study are surprisingly robust, with average errors of less than 25%. These errors are consistent with NASA’s goal of retrieving water quality information (primarily chlorophyll) from ocean color sensors with an accuracy of 35% (McClain et al. 2004). Nevertheless, the authors do not propose anything more than regional application of these relationships; image analysis was not extended to the entire Great Lakes region and would require further validation before being applied outside the current area of interest. The relationships may break down
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in waters strongly affected by dissolved organic matter where a positive correlation between Secchi depth and water-leaving radiance might be expected because of the dominance of absorption over scattering in such waters. The observations here are based predominantly on waters where particulate backscattering determines the level of water-leaving radiance, although some of the variance in the data will no doubt be due to varying quantities of absorbing materials present. ACKNOWLEDGMENTS This work was carried out with the financial support of the Canadian Space Agency under the Government Related Initiatives Project “Wealth of Water.” REFERENCES Antoine, D., Morel, A., Gentili, B., Gordon, H.R., Banzon, V.F., Evans, R.H., Brown, J.W., Walsh, S., Baringer, W., and Li, A. 2003. In search of long-term trends in ocean color. EOS 84(32): 301, 308–309. Baith, K., Lindsay, R., Fu, G., and McClain, C.R. 2001. SeaDAS, a data analysis system for ocean-color satellite sensors. EOS Trans. AGU, 82. Barbiero, R.P., and Tuchman, M.L. 2004. Long-term dreissenid impacts on water clarity in Lake Erie. J. Great Lakes Res. 30:557–565. ——— , Tuchman, M.L., and Millard, E.S. 2006. Postdreissenid increases in transparency during summer stratification in the offshore waters of Lake Ontario: Is a reduction in whiting events the cause? J. Great Lakes Res. 32:131–141. Budd, J.W., Drummer, T.D., Nalepa, T.F., and Fahnenstiel, G.L. 2001. Remote sensing of biotic effects: Zebra mussels (Dreissena polymorpha) influence on water clarity in Saginaw Bay, Lake Huron. Limnol. Oceanogr. 46:213–223. Environment Canada. 2001. Threats to sources of drinking water and aquatic ecosystem health in Canada. National Water Research Institute, Burlington, Ontario. NWRI Scientific Assessment Report Series No. 1. Feldman, G.C., and McClain, C.R. 2006. Implementation of CZCS Processing within the OBPG,Ocean Color Web, Eds. Kuring, N., Bailey, S.W., Thomas, D., Franz, B.F., Meister, G., Werdell, P.J., Eplee, R.E., MacDonald, M., Rubens, M. NASA Goddard Space Flight Center. http://oceancolor.gsfc.nasa.gov/CZCS/ czcs_processing/ (23 October 2006). Gower, G.F.R. 2004. SeaWiFS global composite images show significant features of Canadian waters for 1997–2001. Can. J. Remote Sensing, 30:26–35. Griffiths, R.W., Schloesser, D.W., Leach, J.H., and
Kovalak, W.P. 1991. Distribution and dispersal of the zebra mussel (Dreissena polymorpha) in the Great lakes region. Can. J. Fish. Aquat. Sci. 48:1381–1388. Herbert, P.D.N., Muncaster, B.W., and Mackie, G.L. 1989. Ecological and genetic studies on Dreissena polymorpha (Pallas): a new mollusc in the Great Lakes. Can. J. Fish. Aquat. Sci. 46:1587–1591. Holland, R.E. 1993. Changes in planktonic diatoms and water transparency in Hatchery Bay, Bass Island area, western Lake Erie since the establishment of the zebra mussel. J. Great Lakes Res. 19:617–624. Hovis, W.A., Clark, D.K., Anderson, F., Austin, R.W., Wilson, W.H., Baker, E.T., Ball, D. Gordon, H.R., Mueller, J.L., El-Sayed, S., Sturm, B., Wrigley, R.C., and Yentsch, C.S. 1980. NIMBUS-7 Coastal Zone Color Scanner: System description and initial imagery. Science 210:60–63. Howell, E.T., Marvin, C.H., Bilyea, R.W., Kauss, P.B., and Sommers, K. 1996. Changes in environmental conditions during Dreissena colonization of a monitoring station in eastern Lake Erie. J. Great Lakes Res. 22:744–756. Leach, J.H. 1993. Impacts of the zebra mussel (Dreissena polymorpha) on water quality and fish spawing reefs in western Lake Erie. In Zebra mussels. Biology, impacts and control, T.F. Nalepa and D.W. Schloesser, eds., pp. 381–397. Boca Raton, FL: Lewis Publishers. MacIsaac, H.J. 1996. Potential abiotic and biotic impacts of zebra mussels on the inland waters of North America. Amer. Zool. 36:287–299. Makarewicz, J.C., Lewis, T.W., and Bertram, P. 1999. Phytoplankton composition and biomass in the offshore waters of Lake Erie: pre- and post-Dreissena introduction (1983–1993). J. Great Lakes Res. 25:135–148. McClain, C.R., Feldman, G.C., and Hooker, S.B. 2004. An overview of the SeaWiFS project and strategies for producing a climate research quality global ocean bio-optical time series. Deep Sea Res. II 51:5–42 Millard, E.S., Myles, D.D., Johannsson, O.E., and Ralph, K.M. 1996. Phytoplankton photosynthesis at two index stations in Lake Ontario 1987–1992: assessment of the long-term response to phosphorus control. Can. J. Fish. Aquat. Sci. 53:1092–1111. ——— , Johannsson, O.E., Neilson, M.A., and ElShaarawi, A.H. 2003. Long-term seasonal and spatial trends in nutrients, chlorophyll a and light attentuation in Lake Ontario. In State of Lake Ontario (SOLO)—Past, Present and Future, M. Munawar, ed., pp. 97–132. Ecovision World Monograph Series, Aquatic Ecosystem Health and Management Society. Nicholls, K.H. 2001. CUSUM phytoplankton and chlorophyll functions illustrate the apparent onset of Dreissenid mussel impacts in Lake Ontario. J. Great Lakes Res. 27:393–401. ——— , and Hopkins, G.J. 1993. Recent changes in Lake
Remote Sensing of Great Lakes Water Clarity Erie (north shore) phytoplankton: cumulative impacts of phosphorous loading reductions and the zebra mussel introduction. J. Great Lakes Res. 19:637–647. ——— , Hopkins, G.J., Standke, S.J., and Nakamoto, L. 2001. Trends in total phosphorous in Canadian nearshore waters of the Laurentian Great Lakes: 1976–1999. J. Great Lakes Res. 27:402–422. Stevens, R.J.J., and Neilson, M.A. 1987. Response of Lake Ontario to reduction in phosphorus load, 1967–82. Can. J. Fish. Aquat. Sci. 44:2059–2069. Strong, A.E., and Eadie, B.J. 1978. Satellite observations
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of calcium carbonate precipitation in the Great Lakes. Limnol. Oceanogr. 23:877–887. Vanderploeg, H.A., Eadie, B.J., Liebig, J.R., Tarapchak, S.J., and Gower, R.M. 1987. Contribution of calcite to the particle-size spectrum of Lake Michigan seston and its interaction with the plankton. Can. J. Fish. Aquat. Sci. 44:1898–1914. Submitted: 10 January 2007 Accepted: 7 August 2007 Editorial handing: David J. Schwab
Trends in Water Clarity of the Lower Great Lakes from Remotely Sensed Aquatic Color Caren E. Binding*, John H. Jerome, Robert P. Bukata, and William G. Booty National Water Research Institute Canada Centre for Inland Waters 867 Lakeshore Road, P.O. Box 5050 Burlington, Ontario L7R 4A6 ABSTRACT. Satellite observations of aquatic colour enable environmental monitoring of the Great Lakes at spatial and temporal scales not obtainable through ground-based monitoring. By merging data from the Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), monthly binned images of water-leaving radiance over the Great Lakes have been produced for the periods 1979–1985 and 1998–2006. This time-series can be interpreted in terms of changes in water clarity, showing seasonal and inter-annual variability of bright-water episodes such as phytoplankton blooms, re-suspension of bottom sediments, and whiting events. Variations in Secchi disk depth over Lakes Erie and Ontario are predicted using empirical relationships from coincident measurements of water transparency and remotely-sensed water-leaving radiance. Satellite observations document the extent to which the water clarity of the lower Great Lakes has changed over the last three decades in response to significant events including the invasion of zebra mussels. Results confirm dramatic reductions in Lake Ontario turbidity in the years following mussel colonization, with a doubling of estimated Secchi depths. Evidence confirms a reduction in the frequency/intensity of whiting events in agreement with suggestions of the role of calcium uptake by mussels on lake water clarity. Increased spring-time water clarity in the eastern basin of Lake Erie also corroborates previous observations in the region. Despite historical reports of localised increases in transparency in the western basin immediately following the mussel invasion, image analysis shows a significant increase in turbidity between the two study periods, in agreement with more recent reports of longer term trends in water clarity. Through its capacity to provide regular and readily interpretable synoptic views of regions undergoing significant environmental change, this work illustrates the value of remotely sensing water colour to water clarity monitoring in the lower Great Lakes. INDEX WORDS:
Remote sensing, aquatic colour, water quality, water clarity, Great Lakes.
INTRODUCTION Aquatic resources within the Great Lakes watershed have been impacted over the years by numerous environmental stressors as a consequence of, for example, farming practices, shipping, urbanization, and industrialization. The introduction of non-native invasive species, point-source discharges, nutrient loading and resulting eutrophication and nuisance algal blooms have all led to notable fluctuations in water quality and clarity (Environment Canada 2001). Mandated programs to reduce phosphorus loading to the Great Lakes in the 1970s and 80s resulted in a substantial reversal of the process of eutrophication, with significant reductions in *Corresponding
phytoplankton biomass recorded in the 80s and 90s (Stevens and Neilson 1987, Millard et al. 1996). The introduction of zebra mussels to the Great Lakes ecosystem in 1988 (Herbert et al. 1989) also led to reports of significant localized increases in water transparency in western Lake Erie, Saginaw Bay, and Lake St. Clair in the years immediately following the zebra mussel colonization, attributed in part to the direct removal of particulate matter by filter-feeding (Holland 1993, Leach 1993). However, longer-term studies by Makarewicz et al. (1999) and Barbiero and Tuchman (2004) found no evidence of a persistent and lake-wide increase in water transparency in Lake Erie but instead pointed to decreases in water clarity in the western basin and increased clarity in the eastern basin. Howell et al. (1996) also reported
author. E-mail: [email protected]
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Remote Sensing of Great Lakes Water Clarity increased Secchi depths at a monitoring station in the eastern basin of Lake Erie in the years following mussel colonization. Despite several detailed studies, sparse spatial coverage and the discontinuous nature of ground-based monitoring often preclude reliable conclusions regarding long-term lake-wide changes in water quality. Earth observation satellites therefore have a logical place in the monitoring of environmental change within the Great Lakes watershed, providing regular, high resolution coverage of the region that may present more robust evidence of spatial and temporal trends in water quality than intermittent point-sampling alone. The spectral variation of visible radiation leaving a water body, as seen from a remote platform such as an aircraft or satellite sensor, provides information suitable for the detection of color-producing water quality parameters such as phytoplankton pigments, mineral suspended particulates, and dissolved organic matter. The Coastal Zone Color Scanner (CZCS) was a mutli-channel scanning radiometer launched aboard the Nimbus-7 satellite in October 1978 and was the first spacecraft instrument devoted to the measurement of ocean color (Hovis et al. 1980). It remained in operation until June 1986 after which there was a period of 11 years before the next dedicated ocean color platform was launched. The Sea-viewing Wide Fieldof-view Sensor (SeaWiFS), in orbit on the OrbView-2 platform since 1997, was the start of a new generation of satellite sensors delivering daily observations of aquatic color over the world’s water bodies (McClain et al. 2004). Gower (2004) presented a time series of SeaWiFS imagery for the period 1997–2001 and highlighted significant features in Canadian waters worthy of further investigation over longer time-scales. Previously it has been difficult to compare aquatic color observations from different satellites because the instruments, the data processing algorithms and calibration techniques have been different (Antoine et al. 2003). Achieving coherency between satellite missions is critical for assessing long-term changes in aquatic biogeochemistry. As part of NASA’s on-going commitment to the development of a long-term time-series of aquatic color data, the entire CZCS archive has been reprocessed by the Goddard Ocean Biology Processing Group (OBPG) following standard conventions for data formatting, navigation, and calibration, using identical algorithms wherever possible and the same processing code currently used for SeaWiFS. This communication combines these two satellite image archives in order to assess
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ecosystem impacts over the Great Lakes through the analysis of temporal trends in estimated water quality parameters. A time series of water-leaving radiance from both the CZCS and SeaWiFS sensors is presented that can be interpreted in terms of changes in water clarity of the Great Lakes aquatic system. Water-leaving radiance in the green portion of the visible spectrum (~550 nm) may be treated as an indicator of general turbidity; scattering caused by organic and inorganic particles in suspension lead to elevations in the measured water-leaving radiance. While it is not possible to distinguish the different types of scattering materials from a single band reflectance, with prior knowledge of the Great Lakes system it is possible to infer the timing and location of bright-water episodes such as intense phytoplankton blooms, suspended sediments, and whiting events. Results show both spatial and temporal trends in water clarity that may be related to significant events impacting water quality of the Great Lakes over the last three decades. SATELLITE DATA ACQUISITION AND PROCESSING METHODOLOGY The SeaWiFS and CZCS data used in this study were acquired from the NASA Ocean Color Web facility (http://oceancolor.gsfc.nasa.gov/) and processed using the dedicated ocean color processing software SeaDAS v5.0 (Baith et al. 2001). Monthly binned level 3b SeaWiFS images of normalized water-leaving radiance at 555 nm (nLw555) were produced for the Great Lakes region from January 1998 to December 2005. These level 3b products are atmospherically corrected radiances combined into 4.6 km spatial bins, and into one calendar month temporal bins. All of the valid pixels for the given time period and grid square are compiled in the same bin and the weighted mean of all observations is generated. Equivalent L3b monthly binned images were produced from CZCS Band-3 radiance at 550 nm for the period January 1979 to December 1985. Level 2 imagery could be used to provide increased spatial and temporal coverage (1 km, daily), however, in order to smooth daily variability and reduce computation requirements only the monthly binned data were analyzed here. Potentially erroneous pixels corresponding to conditions including land, cloud or ice, sun glint, saturated signals, and atmospheric correction failure were flagged and subsequently removed from further data analysis. SeaWiFS is limited to operate only for sun elevations above 20 degrees which, com-
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Binding et al. TABLE 1. Least square regression coefficients of the form Zs = αnLw β for data sets of in situ Secchi disk depth (Zs) and coincident satellite nLw~550. Data-set Erie SeaWiFS Erie CZCS All Erie All Ontario
FIG. 1. Secchi disk depth (Zs) against coincident SeaWiFS nLw 555 during surveys on Lake Erie from 1998 to 2005 and coincident CZCS nLw550 during surveys on Lake Erie from 1979 to 1985. Least square regression equations of the form Zs = αnLwβ are presented in Table 1. bined with pixels flagged due to winter lake ice formation, results in significant data loss during the winter months. Consequently, the analysis of satellite observations was restricted to the months March through to October each year. ASSESSING CONTINUITY OF SENSOR DATA The approach adopted by the OBPG for the reprocessing of CZCS data is as similar as possible to that used to process SeaWiFS and MODIS data, differing only in the atmospheric correction procedures and the associated assumptions used to determine aerosol contributions. With the absence of suitable near infra-red (NIR) bands on the CZCS, the aerosol correction was performed using the 670 nm band where an iteration scheme was developed to determine the water-leaving radiance and subsequently the aerosol contribution at that wavelength. Band radiances may also show discrepancies because of differences in wavelength characteristics, with the sensor wavebands differing in both nominal wavelength (SeaWiFS is nLw 555 , CZCS is nLw550) and also in bandpass (SeaWiFS is 20 nm, CZCS is 40 nm). In low turbidity waters, optical modeling convolving spectra with both the SeaWiFS and CZCS respective bandpasses estimates that CZCS nLw550 would be between 4 to 6% higher than SeaWiFS nLw555 although this may be higher in more turbid waters.
α 4.518 4.211 4.443 3.505
β –0.991 –0.821 –0.949 –0.923
R2 0.72 0.70 0.71 0.65
n 306 94 400 420
p < 0.05 < 0.05 < 0.05 < 0.05
As a result of these uncertainties it may not be appropriate to make quantitative comparisons between the CZCS and SeaWiFS imagery without first having supporting evidence of continuity in the radiance measurements from both satellite sensors. This can be achieved by comparing the sensor radiances against a known optical property and testing for consistency between the two missions. Whilst this approach will not eliminate sensor biases, it does enable a simple assessment of the continuity of sensor data and identification of any significant discrepancies which may alter trends in time-series analyses. Quality-assessment carried out by the OBPG showed that radiance and chlorophyll retrievals are generally in good agreement with in situ measurements although the data are currently rather limited (Feldman and McClain 2006). Few data sets of optical properties exist dating as far back as the 1970s. However, one consistent measurement made by Environment Canada over the years is water transparency. Over the period of interest 7,780 Secchi disk observations were made within Lakes Erie and Ontario during routine research and surveillance cruises. Comparing these Secchi disk depths (Zs) with coincident satellite radiance measurements from both SeaWiFS and CZCS provide a simple indication of the consistency of the sensor observations. CZCS and SeaWiFS imagery was obtained in the form of L3b daily composites and clear-image radiances coincident with in situ sampling dates were extracted. Lake Erie was chosen for the test, enabling sensor consistency to be assessed across the largest range of measured radiance. CZCS spatial coverage was on average less than 50% of that which is currently available from SeaWiFS over any single year resulting in significantly fewer match-ups (94 compared with 306 for SeaWiFS). Empirical relationships between Secchi disk depths and both CZCS and SeaWiFS waterleaving radiance were formed and showed close agreement (Fig. 1, Table 1). Analysis of variance identified no significant differences in the slopes
Remote Sensing of Great Lakes Water Clarity (F = 2.88, p = 0.16) or intercepts (F = 3.39, p = 0.07) of linear regressions on log transformed data for the two datasets, suggesting it may be reasonable to make quantitative comparisons between radiance data from the two missions. This suggests that any sensor bias may be sufficiently small to be within the inherent variability of the relationship between Secchi depth and water-leaving radiance. VARIATIONS IN WATER-LEAVING RADIANCE Figure 2a and 2b present CZCS nLw550 and SeaWiFS nLw555 for the Great Lakes region averaged over the periods 1979–1985 and 1998–2005, respectively. The large geographical variation in water-leaving radiance is noticeable, with Lake Erie exhibiting the highest radiance and Lake Superior the lowest. Regions of water clarity change may be inferred from these images by simple subtraction of the means (Fig. 2c). Even considering the potential instrument bias discussed above, the differences seen in Figure 2c are suggestive of real environmental change, with large differences (–50% to 100%) between CZCS and SeaWiFS radiances. From Figure 2 it can be seen that Lakes Michigan and Huron appear to have been brighter during the 1979–1985 period than the 1998–2005 period although several shallow near-shore areas such as Saginaw Bay and Lake St. Clair exhibit significantly brighter waters in more recent years. Interestingly, a remote sensing study of the effects of zebra mussels on water clarity of Saginaw Bay during the period 1987–1993 identified distinct increases in water clarity immediately following colonization (Budd et al. 2001). Imagery here suggests this clean-up may have been short-lived, with the bay showing some of the largest increases in water brightness over the Great Lakes. Lake Superior has shown the least change in mean brightness between the two time periods whereas Lake Ontario in its entirety exhibits significantly lower image brightness during the SeaWiFS mission than the CZCS mission two decades prior. Imagery of Lake Erie suggests more complex change with the central and western basins appearing much brighter in recent years whereas the eastern basin waters are less bright than during the CZCS mission years. Temporal variability may be assessed in more detail from time-series plots of monthly lake-averaged water-leaving radiance (Fig. 3), showing seasonal and inter-annual variability in brightness signals that may be related to periods of phytoplankton
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blooms, mineral sediment suspension, and whiting events. Lake Superior, with low levels of waterleaving radiance, and hence low turbidity, exhibits relatively small annual and inter-annual variability, particularly since 1998. Lake Michigan shows substantially greater variability dominated by peaks during August in several years. These bright-water events, predominantly observed in Lake Michigan, but also Lake Ontario and occasionally Lake Erie can be attributed to whiting events, that is the precipitation of highly scattering calcium carbonate under specific temperature and pH conditions (Strong and Eadie 1978, Vanderploeg et al. 1987). These whiting events were first observed in satellite imagery of the Great Lakes in 1973 (Strong and Eadie 1978) and appear to be regular features in imagery during August/September. Lake Huron exhibits an apparent gradual decrease in water brightness over the period of study, although with little change in intra-annual variability. In contrast to all the other lakes, Lake Erie shows a substantial increase in both the magnitude and variability of brightness levels between the CZCS and SeaWiFS missions. The peak in 2002 is driven largely by elevated water-leaving radiance during March/April. Barbiero and Tuchman (2004) also reported a year of unusually low water clarity in 2002 with spring Secchi depths throughout the lake < 2 m. The reason for this peak is unknown, there were no unusual weather events and water levels were not significantly different from other years. Lake Ontario monthly variations show regular spring peaks corresponding to spring phytoplankton blooms in addition to the whiting-driven peaks in August/September. The frequency and intensity of these peaks appears to have decreased during the SeaWiFS mission compared with the CZCS observations. TRENDS IN LAKE ERIE WATER CLARITY The close agreement between the satellite radiance datasets shown in Figure 1 means that the two datasets can be combined to give a simple method of estimating Secchi disk depths in Lake Erie from CZCS and SeaWiFS radiance at ~550 nm with an r.m.s. error of 24.8% of the mean (Zs = 4.443nLw–0.95, R2 = 0.71, n = 400). This relationship was applied to the archived monthly composite images in order to determine the long term trends in water clarity for Lake Erie. Figures 4a and 4b present the average Secchi depth over Lake Erie for the periods 1979–1985 and 1998–2005 respectively.
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FIG. 2. (a) Average CZCS nLw550 for the period 1979–1985, (b) average SeaWiFS nLw555 for the period 1998–2005 and (c) difference between the two images.
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FIG. 3. Monthly lake-wide average nLw ~550 from CZCS (open circles) and SeaWiFS (filled circles) for each of the Great Lakes. Lake-wide Secchi depths ranged from 2 to 4.5 m and 1.5 to > 6 m for the CZCS and SeaWiFS missions respectively, with the geographic distribution varying considerably between the two periods. Secchi depths of 4 m and greater, while quite widespread in the central and eastern basins during 1979–1985, are confined strictly to the eastern basin during the 1998–2005 period. Figure 4c shows the change in Zs between the two periods
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and confirms the relative increase in water clarity of up to and exceeding 2 m throughout the eastern basin. In contrast, the central and western basins have undergone a period of decreasing water clarity, with average reductions in Zs of 1–2 m. The zebra mussel population is thought to have become established in the western and central basins of Lake Erie in 1988 and completely colonized all three basins by 1989 (Griffiths et al. 1991). Since their introduction, mussels have been held responsible for reports of improved water clarity in the eastern basin; Barbiero and Tuchman (2004) reported evidence of increased springtime water clarity in the eastern basin of Lake Erie, with Secchi depths more than doubling after the mussel invasion, corroborating similar earlier documentation by Howell et al. (1996). An assessment of such seasonal changes in the satellite-derived Zs across the three basins was made for both the CZCS and SeaWiFS missions (Figures 5a and 5b, respectively). During 1979–1985, seasonal Secchi depths for all three basins were similar, exhibiting minima in April and maxima in August/September. By the 1998–2005 period, clear differences in the water clarity of the three basins are evident. Seasonal trends remain similar but there is now up to 4 m difference in Secchi depth between eastern and western basins. T-tests were performed in order to identify significant differences in the monthly-mean Secchi depths between the two observation periods. Those differences found to be statistically significant are shown in Figures 6a and 6b. In agreement with the reports of Barbiero and Tuchman (2004) and Howell et al. (1996), Figures 6a and 6b confirm more than a doubling of the east basin Secchi depths in April and May, with average changes of more than 3 m. Increases in monthly mean Secchi depths of more than 50% are evident throughout the rest of the year in the eastern basin. These results do not change appreciably if data from the 2002 peak are removed, confirming that this single bright event does not introduce excessive bias in the study. Trends in the central and western basins differ markedly to the observed increases in eastern basin water clarity. Reports over the years have documented significant localized increases in water clarity in the western basin (Leach 1993, Holland 1993) and dramatic declines in bio-productivity. However, contrasting evidence reported by Barbiero and Tuchman (2004) in fact suggested statistically significant declines in water clarity in some regions of Lake Erie. Results of image analysis here are in
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FIG. 4. Average Lake Erie Secchi disk depth (meters) for the periods (a) 1979–1985 and (b) 1998–2005 and (c) change in Secchi depth between the two periods. agreement with the findings of Barbiero and Tuchman, suggesting decreased water clarity in the western basin. Spring and fall Secchi depths appear to have decreased by up to 50% in the western basin after the mussel invasion, with no significant change during the summer months. The central basin shows the least temporal change, although still recording significant spring/fall decreases in Zs and increases in summer (July/August) Zs. Several authors have noted dramatic reductions
in phytoplankton biomass in the western basin attributed not only to the zebra mussel invasion but also to significant reductions in phosphate loadings over the years (Nicholls and Hopkins 1993, Makarewicz et al. 1999, Barbiero and Tuchman 2004). Total phosphorus concentrations in both Lakes Erie and Ontario decreased substantially after 1976 following legislation to limit phosphorus loading to the Great Lakes. Nicholls and Hopkins (1993) reported declines in phytoplankton biomass
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FIG. 5. (a) Monthly average CZCS-derived Secchi depth for Lake Erie during 1979–1985 and (b) monthly average SeaWiFS-derived Secchi depth for Lake Erie during 1998–2005. Data averaged for each of the eastern, central, and western basins. in Lake Erie’s western basin of around 5% per year over the period 1970–1985, and related this to decreased phosphorus loading to the western basin of about the same magnitude. Further dramatic declines in total phytoplankton biomass (up to 90%) coincided with the invasion of the lake by zebra mussels (Nicholls and Hopkins 1993). In their longer term study, Barbiero and Tuchman (2004) observed decreases in chlorophyll of 50% in the western basin in the years following the mussel invasion. Despite these reports of lower phytoplankton abundances in the western basin, results from image analysis in the present study suggest that this
has not resulted in significant increases in water clarity. Makarewicz et al. (1999) found that reductions in phytoplankton biomass and chlorophyll did not necessarily translate into increased transparency, stating that transparency was driven more by the re-suspension of inorganic particulate matter in the western basin. Makarewicz et al. observed significant decreases in spring/summer phytoplankton biomass in the western basin of Lake Erie in post-mussel years, with little reduction in biomass in the central and eastern basins during the same period. Despite this, they found significant decreases in spring-time water transparency in the
FIG. 6. Lake Erie monthly mean Secchi depth differences between the periods 1979–1985 and 1998–2005 ± 95% confidence intervals. (a) Difference as a percentage of the 1979–1985 average Zs and (b) absolute difference in meters. Significant differences were confirmed with t-tests at the 0.05 significance level and n-2 degrees of freedom. Those months without bar graphs exhibited no significant differences between the means.
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Binding et al. changes in algal biomass. Barbiero and Tuchman (2004) suggested that seasonal stratification in the eastern basin may limit the direct effects of mussel filtration on surface water clarity during the summer and, therefore, the observed increases in water clarity are more likely due to changes in phosphorus loadings or a reduction in whiting events (discussed in more detail in the following section).
FIG. 7. Secchi disk depth (Zs) against coincident SeaWiFS and CZCS nLw ~550 during surveys on Lake Ontario, along with the power fit for Lake Erie data for comparison. Least square regression equations of the form Zs = α nLwβ are presented in Table 1. western basin and an increase in central and eastern transparency. Others have made similar observations; MacIsaac (1996) reported on reductions in phytoplankton biomass of up to 90% in the Hudson River but only a 7% increase in water transparency following mussel colonization, attributing the discrepancy to particulate re-suspension. Given that there have been substantial reductions in nutrient loadings to the lake, and the known capacity for mussels to remove large quantities of suspended particulates from the water column through filter-feeding, it is not obvious why there has been an increase in turbidity in the western basin. Some speculative reasoning may be differences in the wind regime during the two observation periods leading to increased re-suspension of bottom sediments; data from Lake Erie buoys suggest 20% higher annual average wind speeds during the SeaWiFS observations compared to the CZCS observations (NOAA’s National Data Buoy Center, Station 45005). Long-term effects of lower phytoplankton productivity may also reduce biologically-aggregated material and associated settling, allowing finer disaggregated mineral particles to be resuspended more easily. Other possible explanations could be changes in the timing of ice break-up and variations in the input of sediments from the Sandusky and Maumee rivers. Sources of re-suspended sediments to the deeper eastern basin are far less than the western basin and therefore changes in water clarity here are more likely to be driven by
TRENDS IN LAKE ONTARIO WATER CLARITY Data from Lake Ontario were assessed for regional differences in the relationship between nLw~550 and Zs. While Lake Ontario data points appear to lie in good agreement with those from Lake Erie (Fig. 7, Table 1), ANOVA tests confirm that the two relationships have the same slope (F = 0.02, p = 0.875) but difference intercepts (F = 78.24, p < 0.05). The regression lines are parallel but are offset by 0.25 mW cm–2 µm–1 sr–1. This offset my be caused by differences in the predominant scattering agents in both lakes, with higher scattering mineral particles of Lake Erie producing a higher water-leaving radiance for a given Secchi depth than the algal dominated waters of Lake Ontario. The equation Zs = 3.505nLw–0.92 (R2 = 0.65, n = 420) was therefore used to predict Secchi disk depths within Lake Ontario, with an r.m.s. error in predicted Zs of 23% of the mean. Average Secchi depths over Lake Ontario as estimated from CZCS and SeaWiFS imagery for the periods 1979–1985 and 1998–2005 respectively are presented in Figures 8a and 8b. Secchi depths are largely uniform across the lake at around 3–4 m for the 1979–1985 period and increase noticeably to 6–8 m by the 1998–2005 period. Figure 8c confirms that the entire lake appears to have undergone significant improvements in water clarity, with a lake-wide increase in Secchi depth of between 2 and > 4 m. The north shores show the greatest change in water clarity, in excess of 4 m, with the Niagara River plume region on the southern shore showing the least change. The absence of large regions of bottom sediment re-suspension in Lake Ontario suggests that these temporal changes can be attributed to bio-chemical changes, either a reduction in biological productivity or a reduction in the intensity and/or frequency of whiting events. The cause of the increased water transparency may be speculated upon by looking at the monthly mean Secchi disk depths for the two time periods, where significant seasonality in water clarity is evi-
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FIG. 8. Average Lake Ontario Secchi disk depth (meters) for the periods (a) 1979–1985 and (b) 1998–2005 and (c) change in Secchi depth between the two periods.
dent (Fig. 9). During the 1979–1985 period, seasonality was typified by a small drop in Secchi depth in April, followed by a more significant reduction in August. These episodes may be attributed to spring phytoplankton blooms and whiting events respectively. The SeaWiFS mission data (1998–2005) shows some changes in the seasonal cycle of water clarity in Lake Ontario. There is no clear indication
of a spring bloom influencing water clarity, in fact there is a large increase in Secchi disk depths during the spring and summer, suggesting a lower influence of algal blooms in lake Ontario in recent years. Figure 10 presents the monthly mean significant differences in Secchi depth for Lake Ontario between the CZCS and SeaWiFS periods and con-
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FIG. 9. Monthly average Secchi depth for Lake Ontario during the two observation periods, 1979–1985 (CZCS) and 1998–2005 (SeaWiFS).
firms quantitatively the change undertaken over the decades. The largest monthly change in Secchi depth of 5 m was in April, more than doubling the 1979–1985 period levels of water clarity. Reductions in total phosphorus in Lake Ontario were generally greatest in the March/April period and higher in the near-shore than offshore (Nichols et al. 2001). Associated reductions in algal growth may, therefore, be responsible for the large increases in spring-time water clarity observed here. In agree-
ment, Millard et al. (2003) observed a lake-wide decline in chlorophyll between 1990 and 1996 and attributed it to the combined effects of both phosphorous loading controls and the mussel invasion. Lake Ontario was colonized by mussels shortly after Lake Erie (Griffiths et al. 1991) and potential effects of filtration were noted by Nicholls (2001) and Millard et al. (1996). Nicholls (2001) documented rapid and dramatic reductions in chlorophyll of 65–75% in the 3 years following mussel colonization at southern and eastern stations in Lake Ontario. The later mussel colonization of the north shore was reflected in later improvements in water clarity relative to the south shore. Here, however, the north shore shows greatest change. It may be that with the shallower waters of the northern shores any impact from reduced nutrient loads and/or mussel populations will be more pro nounced. The time-series of Lake Ontario water-leaving radiance in Figure 2 showed the incidence of significant peaks in water-leaving radiance attributed to whiting events that take place regularly in Lake Ontario during August/September. The frequency and intensity of these bright-water episodes is significantly less in recent years than during the 1979–1985 period, suggesting a reduction in whiting events since the establishment of zebra mussels. Estimates of Secchi depths from satellite imagery show that August Secchi depths more than doubled from 2 m to 5 m between the two observation periods (Fig. 10). Barbiero et al. (2006) described a three-fold reduction in August turbidity
FIG. 10. Lake Ontario monthly mean Secchi depth differences between the periods 1979–1985 and 1998–2005 ± 95% confidence intervals. (a) Difference as a percentage of the 1979-1985 average Zs and (b) absolute difference in meters. Significant differences were confirmed with t-tests at the 0.05 significance level and n-2 degrees of freedom. Those months without bar graphs exhibited no significant differences between the means.
Remote Sensing of Great Lakes Water Clarity values in Lake Ontario and a near doubling of Secchi depths following the zebra mussel colonization. Measured total suspended solids suggested most of this decrease in turbidity was due to a reduction in non-algal particulates. There have been recent suggestions that the long suspected role of zebra mussels in cleaning up Great Lakes water clarity may not be through direct filtering, which may be impeded by deep water and summer stratification, but through the enhanced uptake of calcium by the mussel populations, lowering the calcium carbonate saturation of the water column and resulting in far fewer whiting events (Barbiero and Tuchman 2004, Barbiero et al. 2006). Figure 9 illustrates that during the 1998–2005 period average Secchi depths decline to a minimum in September, 1 month later than the CZCS period average, suggesting the whiting events occur later now than in the 1970s and 80s. Calcium solubility decreases with increasing temperature and with the mussel uptake reducing calcium concentrations in the lake, calcium saturation and hence precipitation would be initiated at higher lake temperatures (i.e., later in the year). DISCUSSION AND CONCLUSIONS The Great Lakes have been subject to numerous socio-environmental influences, management actions, and accidental events over the decades which have created ecosystem changes which would have benefited high resolution monitoring available only through remote sensing techniques. Accurate determinations of long-term change from ground-based monitoring are made difficult by predominantly small, discontinuous, and geographically localized datasets of environmental parameters. Results presented here provide a means of examining largescale and long-term environmental changes over the Great Lakes. The advantages over ground-based monitoring are clearly in substantial increases in both spatial and temporal coverage; even with data loss due to cloud, lake coverage far exceeds that of typical monitoring programs. With the ongoing free availability of SeaWiFS and MODIS imagery and proposed future sensors, remote sensing of aquatic color is a cost-effective tool for observing lakewide water quality, offering important support in monitoring and mitigation of adverse ecosystem impacts. A simple method of estimating Secchi depths from satellite measured aquatic color is presented and used in the retrospective analysis of water clar-
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ity conditions of Lakes Ontario and Erie. Analysis of imagery over two periods, 1979–1985 and 1998–2005, identified: • a near doubling of spring time Secchi depths in the eastern basin of Lake Erie (increasing by up to 2 m) • a decrease in Secchi depths of up to 50% in the western basin of Lake Erie (decreasing by 1–2 m) • a more than doubling of Secchi depths over Lake Ontario (increasing by 2 – > 4 m) • evidence for calcium carbonate precipitation occurring later in the year and an overall reduction in the frequency/intensity of events These results contradict early reports of significant localised water clarity improvements in the western basin of Lake Erie following the introduction of zebra mussels (Holland 1993, Leach 1993). However, it is important to note that many studies of the impact of mussels on water clarity dealt with a short period of time, usually colonization ± 3 years or less (Leach 1993, Millard et al. 1996, Nicholls 2001). Results here deal with longer-term impacts and with the absence of imagery for the 1986–1997 period give no indication of the immediate impact of the mussel invasion. The water clarity trends identified here are, however, in agreement with long-term ground-based studies of water quality change reported by Barbiero and Tuchman (2004) and Makarewicz et al. (1999) and may be linked to the combined effects of a decrease in productivity associated with lower nutrient loadings to the lakes, particulate removal through mussel filter-feeding, and a decrease in the intensity and frequency of whiting events brought about by calcium uptake by mussel populations. Given the varying environmental conditions and inherent subjectivity with which Secchi disk measurements are made, the relationships determined in this study are surprisingly robust, with average errors of less than 25%. These errors are consistent with NASA’s goal of retrieving water quality information (primarily chlorophyll) from ocean color sensors with an accuracy of 35% (McClain et al. 2004). Nevertheless, the authors do not propose anything more than regional application of these relationships; image analysis was not extended to the entire Great Lakes region and would require further validation before being applied outside the current area of interest. The relationships may break down
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in waters strongly affected by dissolved organic matter where a positive correlation between Secchi depth and water-leaving radiance might be expected because of the dominance of absorption over scattering in such waters. The observations here are based predominantly on waters where particulate backscattering determines the level of water-leaving radiance, although some of the variance in the data will no doubt be due to varying quantities of absorbing materials present. ACKNOWLEDGMENTS This work was carried out with the financial support of the Canadian Space Agency under the Government Related Initiatives Project “Wealth of Water.” REFERENCES Antoine, D., Morel, A., Gentili, B., Gordon, H.R., Banzon, V.F., Evans, R.H., Brown, J.W., Walsh, S., Baringer, W., and Li, A. 2003. In search of long-term trends in ocean color. EOS 84(32): 301, 308–309. Baith, K., Lindsay, R., Fu, G., and McClain, C.R. 2001. SeaDAS, a data analysis system for ocean-color satellite sensors. EOS Trans. AGU, 82. Barbiero, R.P., and Tuchman, M.L. 2004. Long-term dreissenid impacts on water clarity in Lake Erie. J. Great Lakes Res. 30:557–565. ——— , Tuchman, M.L., and Millard, E.S. 2006. Postdreissenid increases in transparency during summer stratification in the offshore waters of Lake Ontario: Is a reduction in whiting events the cause? J. Great Lakes Res. 32:131–141. Budd, J.W., Drummer, T.D., Nalepa, T.F., and Fahnenstiel, G.L. 2001. Remote sensing of biotic effects: Zebra mussels (Dreissena polymorpha) influence on water clarity in Saginaw Bay, Lake Huron. Limnol. Oceanogr. 46:213–223. Environment Canada. 2001. Threats to sources of drinking water and aquatic ecosystem health in Canada. National Water Research Institute, Burlington, Ontario. NWRI Scientific Assessment Report Series No. 1. Feldman, G.C., and McClain, C.R. 2006. Implementation of CZCS Processing within the OBPG,Ocean Color Web, Eds. Kuring, N., Bailey, S.W., Thomas, D., Franz, B.F., Meister, G., Werdell, P.J., Eplee, R.E., MacDonald, M., Rubens, M. NASA Goddard Space Flight Center. http://oceancolor.gsfc.nasa.gov/CZCS/ czcs_processing/ (23 October 2006). Gower, G.F.R. 2004. SeaWiFS global composite images show significant features of Canadian waters for 1997–2001. Can. J. Remote Sensing, 30:26–35. Griffiths, R.W., Schloesser, D.W., Leach, J.H., and
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of calcium carbonate precipitation in the Great Lakes. Limnol. Oceanogr. 23:877–887. Vanderploeg, H.A., Eadie, B.J., Liebig, J.R., Tarapchak, S.J., and Gower, R.M. 1987. Contribution of calcite to the particle-size spectrum of Lake Michigan seston and its interaction with the plankton. Can. J. Fish. Aquat. Sci. 44:1898–1914. Submitted: 10 January 2007 Accepted: 7 August 2007 Editorial handing: David J. Schwab