Originally mesotrophic, the Bay of Quinte ecosystem has experienced eutrophication since the 1940s, which resulted in the decline of once-lush submerged aquatic vegetation (SAV) beds in the upper bay by the mid-1960s. Since 1972, twelve SAV surveys have been conducted along ten index transects, recording:% cover, distance SAV beds extended from shore (extent), maximum depth of colonization (Zc), species composition, and, in later years, wet plant biomass. Offshore secchi depth and ϵpar, (the vertical light extinction rate (m−1) for photosynthetically active radiation), were also recorded either weekly or bi-weekly during the growing season since 1972. During this time, two major changes occurred within the bay: the reduction in point-source phosphorus (P-control) loadings in 1978 and the 1993 invasion by Dreissenid Mussel. SAV response to these changes varied temporally and spatially, with the shallow upper bay showing the greatest response, particularly after Zebra Mussels establishment. In the upper bay, mean secchi depth increased by 8% from 1.2 m prior to P-control (pre-P), to 1.3 m after P-control (post-P) and further increased by 46% to 1.9 m after Dreissena establishment (post-D). Upper bay SAV responded to these invasive species with increases in the means of three variables: Zc from 1.6 to 3.5 m, extent from 114 m to 417 m and wet biomass from 50 g m−2 to 962 g m−2. SAV in the middle and lower bays were in better condition in 1972, with pre-P cover in excess of 50% and Zc of 2.6 and 3.7 m, respectively. SAV cover did increase in the post-D (1994 to 2007) period by approximately 25% and Zc increased to 3.7 and 6.5 m, but the narrow fringing strip of shallower water along the shore in these two deeper bays limited substantial increases in bed extent. Both water clarity and basin morphometry strongly influenced SAV distribution and abundance within the Bay of Quinte.

Introduction

Submerged aquatic vegetation (SAV) is an important component of aquatic ecosystems affecting light, temperature, nutrients and biota (Carpenter and Lodge, 1986). As a buffer between the terrestrial and pelagic environment, SAV reduce the effects of wave action (Keddy, 1982), and retard water flow resulting in increased sedimentation and greater water clarity (Madsen et al., 2001; Horppila and Nurminen, 2003). SAV act as nutrient sinks (Blindow, 1992) and upon senescence, serve as nutrient sources (Best, 1982). SAV provide substrate for epiphytes that are food for zooplankton, which in turn are consumed by planktivorous fish (Keast, 1984). SAV provides spawning and nursery habitat for numerous fish species and have been found to influence fish abundance (Chick and McIvor, 1994), growth rates (Werner et al., 1983) and community structure (Weaver et al., 1997). Randall et al. (1996) estimated that sites with SAV had higher densities of fish and higher species richness than non vegetated sites in three bays in southern Ontario. In addition, dense SAV had a production index (accounts for both biomass and fish size) that was over 2 times higher than found in areas where SAV was sparse or absent.

Project Quinte, a multi-agency research collaboration, was established in 1972 to assess the ecosystem impacts of a planned reduction in phosphorus point-source loadings in the Bay of Quinte. In 1985, this multi-basin bay was designated as one of 43 Areas of Concern (AOC) around the Great Lakes by the International Joint Commission (IJC). Both the Canadian and United States governments committed to restore these AOCs under Annex 2 of the 1987 Great Lakes Water Quality Agreement (GLWQA). A common Remedial Action Plan (RAP) process was outlined in the Annex (IJC, 1987) to define beneficial use impairments (BUI) (Bay of Quinte RAP Coordinating Committee, 1990) and develop remedial action plans (Bay of Quinte RAP Coordinating Committee, 1993). In the Bay of Quinte, some BUI's were caused by excessive algae and loss of fish habitat, which included the loss of extensive submergent aquatic vegetation (SAV) beds, particularly in the upper bay. Ongoing research has continued in the Bay of Quinte to address many beneficial use impairments under the Remedial Action Plan (see http://ww.bqrap.ca for the time sequence of the RAP beginning in 1986). These efforts have created a valuable dataset for the Bay of Quinte in terms of both time series and breadth of ecosystem assessment.

Historically, the Bay of Quinte was slightly to moderately oligotrophic prior to the late 1800s with a shift to a eutrophic state by 1890 (Sly, 1986). Water quality deteriorated noticeably after 1930 and by the 1950s, the bay was considered hypereutrophic (Johnson, 1986). In the 1960s, algal blooms occurred in the upper and middle bays and secchi disc depths <0.3 m were recorded (Johnson and Hurley, 1986). SAV beds were described as lush in the 1950s, particularly in the Trenton area, with subsequent declines during the 1960s to half of their estimated extent (Crowder and Bristow, 1986). In the 1970s, macrophytes were restricted to a euphotic zone of 2 m in the upper bay and 4 m in the lower bay. Since 1972, two major changes have occurred: first, a major reduction in point-source phosphorous (P) loadings from sewage treatment plants implemented in the winter of 1977–1978 and, second, the 1993 invasion by Dreissena spp.

Several studies have demonstrated major effects of point-source P and Dreissena invasions on macrophyte density and distribution. In an assessment of the bay's nutrient budgets, Minns et al. (1986) proposed that the collapse of macrophytes in the 1960s was represented by a cusp-catastrophe model wherein macrophytes persisted in abundance until point-source P-loading reached very high levels and excess periphyton growth curtailed their growth. Minns et al. (1986) then inferred from the model that macrophytes might recover if point-source P loadings declined to very low levels and accumulated internal loading dissipated. Higgins and Vander Zanden (2010) showed in their extensive meta-analysis of the effects of Dreissenid Mussels on freshwater ecosystems that littoral primary producers responded positively to the establishment of Dreissena. On average, increases of 182.1% in SAV cover, 39.4% in the maximum depth of SAV colonization, and 170.5% in periphyton biomass occurred. These changes were concurrent with increased water clarity (secchi increases of 50.5% in littoral areas and 38.5% lake-wide) which was primarily attributed to the filtering capacity of Dreissena spp. (Higgins and Vander Zanden, 2010).

Given the potential strong effect of phosphorous and Dreissenia on SAV, the objective of this article was to evaluate the response of SAV in each of the sub-basins of the Bay of Quinte to these two changes. For the upper bay, it was anticipated that SAV response to P reductions may be slow due to internal P loading while increases in water clarity from filtration by Dreissenia may result in a rapid increase in SAV density and extent. It is predicted that SAV response in the middle and lower bays will not be as strong as in the upper bay. This is because the initial SAV condition was not as degraded as that found in the upper bay and basin morphometry limits SAV extent to a relatively small area adjacent to the shoreline. Measures of SAV response that were assessed included percent bottom cover, the distance SAV beds extended offshore, maximum depth of colonization, plant biomass and species composition.

Methodology

Location

The Bay of Quinte is a 254 km2, Z-shaped bay that opens out to the northeastern shore of Lake Ontario (Figure 1). Researchers typically divide the bay into three sections due to differences in basin morphometry and trophic gradients (Johnson and Hurley, 1986). The upper bay is shallowest with a mean depth of 3.5 m, whereas the middle and lower bay mean depths are 5.2 and 24.4 m, respectively. Water generally flows west-to-east in the bay and flushing is dominated by flow from the Trent River near Trenton with annual flushing rates in the upper bay ranging from 9.2 to 14.6 and May to September flushing rates from 1 to 4.6 in the years 1965 to 1981 (Johnson, 1986). Significant water exchange occurs between Lake Ontario and the lower bay through the Upper Gap. During summer, there can be backflow events at depth by Glenora with upwelling into the middle bay (Minns et al., 1986).

Figure 1.

Location of submerged aquatic vegetation transects and Project Quinte water quality monitoring stations from 1972 to 2007. Depths are referenced to IGLD 1985.

Figure 1.

Location of submerged aquatic vegetation transects and Project Quinte water quality monitoring stations from 1972 to 2007. Depths are referenced to IGLD 1985.

Data collection

Reference transects were established in five locations in 1972 as seen in Figure 1: three in the upper bay at Trenton, Belleville and Big Bay, one in the middle bay at Hay Bay outflow and one at Conway in the lower bay (Bristow et al., 1977). A total of 10 transects crossed the bay with sampling starting at 0.5 m water depth and ending 90 m beyond the last macrophyte. Seasonal survey timing varied from mid-July to mid-September with a sampling interval between one to six years. From 1972 to 1988, the survey protocol used a 30 m rope with knots placed at 1 m intervals. Divers then noted SAV absence or presence at each of the 29 knots and the rope was re-positioned further offshore until three successive lengths of rope yielded no macrophytes. During this time, three full surveys of all ten transects were conducted in 1972, 1979 and 1988, whereas partial surveys in 1973, 1974, 1979, 1982 and 1985 were conducted at a subset of these sites. Percent cover was calculated for every 29 m segment based on the % presence of SAV for that segment interval. The depth at the mid-point of each 29 m segment was also recorded along with species composition. The distance that SAV beds extended offshore was estimated by multiplying the number of times the rope was re-positioned until the last SAV was reached, by rope length.

Survey methods were revised when hydroacoustic equipment was obtained. The same reference transects were sampled with hydroacoustics in surveys in 1988, 1994, 2000, 2004 and 2007. A Raytheon DE-719C recording fathometer was used in 1988 and a Lowrance X-16 echosounder was used in 1994 and 2000. Paper echograms produced from these surveys were visually interpreted for percent cover and SAV depth at 3 mm intervals which approximately equals 1 m in actual length. The last macrophyte on the transect was interpreted from the echogram and the SAV bed extent was scaled from the echogram using marks corresponding to GPS coordinates taken in the field.

The last two surveys used a BioSonics DT-X digital echosounder with a 430 kHz single-beam transducer with a pulse length of 0.1 ms and a ping rate of 5 pps. EcoSav V1 and 2 software (BioSonics, 2000) was used to estimate percent cover and SAV depth. Results were mapped using ArcView 3.3 (Environmental Systems Research Institute Inc., 1992) and SAV extent was estimated by measuring from a fixed coordinate that serves as the start point on the transect to the last macrophyte as estimated in EcoSAV. In 1988, 1994 and 2000, divers sampled for species composition and above-ground biomass using 0.25 m2 quadrats at 3 to 6 points along each transect. These points were used to ground-truth the acoustic data while in 2004 and 2007, a combination of rake tosses and underwater video was used for ground-truthing. Leisti et al. (2006) provides detailed information on sampling methodologies for all surveys.

Numerous water quality parameters were measured as part of Project Quinte, including offshore secchi depth, ϵpar (the vertical light extinction rate (m−1) for photosynthetically active radiation), total phosphorous (TP) and Chl a. Transparency is a strong determinant of depth colonization by SAV (Chambers and Kalff, 1985; Hudon et al., 2000). Surveys were conducted on a weekly (1972 to 1982) and bi-weekly (1982 to 2007) basis from mid-May to mid-October at Belleville, Hay Bay and Conway (Figure 1). Measurements were recorded for the Napanee offshore station, but there is a gap from 1983 to 1989 when this site was not sampled. Seasonal means by sub-bay were estimated for this dataset with both the Belleville and Napanee stations included in the upper bay.

Statistical analysis

Datasets were divided into three time stanzas; (1) the pre-phosphorous period (pre-P: 1972 to 1977), (2) a period after phosphorous control was implemented in sewage treatment plants in the upper bay (post-P: 1978 to 1994), and (3) a period after Dreissena was established in all bays (post-D: 1995 to 2007). Additionally, data were separated into upper, middle and lower bays. Kruskal-Wallis tests were conducted to determine differences among time stanzas within each bay. Non-parametric tests were used because transformation of the datasets did not improve normality. Post-hoc comparisons were calculated using the nonparametric multiple comparison (npmc) method in R (Gentleman and Ihaka, 1997), as described in Munsel and Hothorn (2001).

Results

Time series for seasonal mean trophic (TP and Chl a) and transparency (secchi and ϵpar) variables had substantial interannual variability, but improving trends were evident through the three time stanzas (Figure 2). Comparisons among bays showed that lower bay trophic values were consistently and considerably lower and water clarity much higher than the upper and middle bays. Greater similarity existed between the middle and upper bays although the upper bay generally exhibited the highest eutrophic conditions.

Figure 2.

May to October means for (a) total phosphorous, (b) total chlorophyll a (uncorrected for phaeopigments), (c) secchi depth and (d) ϵpar from 1972 to 2007. Upper bay (UB) values have been calculated using data from the Belleville and Napanee stations, middle bay (MB) from the Hay Bay station and lower bay (LB) from the Conway station.

Figure 2.

May to October means for (a) total phosphorous, (b) total chlorophyll a (uncorrected for phaeopigments), (c) secchi depth and (d) ϵpar from 1972 to 2007. Upper bay (UB) values have been calculated using data from the Belleville and Napanee stations, middle bay (MB) from the Hay Bay station and lower bay (LB) from the Conway station.

A time series of % SAV cover-by-depth interval was calculated for each bay (Figure 3). SAV cover in the upper bay was generally sparser than the middle or lower bays throughout the time series, but especially in the pre-P period. SAV cover increased substantially in the upper bay after the invasion by Dreissena. Moderate-to-high SAV densities were recorded in the middle and lower bays from the beginning to the end of the time series. All bays experienced fluctuations in the maximum depth of SAV in the post-P period, with a substantial increase in Zc occurring in the lower bay in 1988. Maximum SAV depths were consistent in the post-D period for the upper and middle bays, with depths of 4 and 3 m respectively.

Figure 3.

Submerged aquatic vegetation cover by depth interval for (a) upper, (b) middle and (c) lower bay areas in the survey years between 1972 and 2007. Mean cover was calculated from individual cover values along each of the reference transects. Upper bay cover was based on data from the north and south transects at Trenton, Belleville and Big Bay; middle bay cover from east and west transects at Hay Bay and lower bay cover from north and south transects at Conway.

Figure 3.

Submerged aquatic vegetation cover by depth interval for (a) upper, (b) middle and (c) lower bay areas in the survey years between 1972 and 2007. Mean cover was calculated from individual cover values along each of the reference transects. Upper bay cover was based on data from the north and south transects at Trenton, Belleville and Big Bay; middle bay cover from east and west transects at Hay Bay and lower bay cover from north and south transects at Conway.

Time stanza means of trophic variables (Table 1) indicated that secchi depths increased whereas ϵpar decreased through time for all three bays. Kruskal-Wallis tests in conjunction with post-hoc comparisons show both secchi and ϵpar differed significantly among all time stanzas in the upper bay. The largest increase in water clarity in the upper bay occurred between the post-P and post-D periods, where secchi depths increased by 46% and ϵpar decreased by 31%. Between the pre-P and post-P period, secchi depths in the upper bay increased by 8% and ϵpar decreased by 18%. The middle and lower bays also experienced the largest increase in clarity between the post-P and post-D time stanzas with a significant difference occurring only with the post-D period.

Table 1.

Time stanza means by bay for transparency (secchi, ϵpar) and submerged aquatic vegetation variables. Significance was tested prior to averaging with Mann-Whitney tests for SAV biomass and Kruskal-Wallis tests for remaining variables. The differences in superscript letters indicate significant differences (α = 0.05) were found in the post-hoc comparisons. ϵpar is the vertical light extinction rate (m−1) for photosynthetically active radiation, N is the number of samples, Zc is the maximum depth of colonization.

Submerged Aquatic Vegetation Variables
SecchiSecchiϵparϵpar% Cover% CoverExtentExtentZc meanWet BiomassWet Biomass
meanNmeanNmeanNmean (m)N(m)Zc Nmean (g m-2)N
Upper Bay             
 Pre-P 1.2A 324 1.82A 323 17A 71 197AB 10 2.3A 10   
 Post-P 1.3B 397 1.50B 374 42B 301 114A 28 1.6A 27 50 33 
 Post-D 1.9C 338 1.04C 338 51C 2227 417B 18 3.5B 18 962 18 
 Significance (P) <0.0001  <0.0001  <0.0001  0.01  <0.0001  <0.0001  
Middle Bay             
 Pre-P 1.4A 162 1.40A 160 50AB 14 102AB 2.6A   
 Post-P 1.5A 237 1.24B 224 49A 119 115A 10 2.2A 10 226 14 
 Post-D 2.3B 169 0.81C 169 61B 394 276B 3.7B 903 
 Significance (P) <0.0001  <0.0001  0.01  0.03   0.07  
Lower Bay             
 Pre-P 2.9A 162 0.63A 162 56AB 17 123 3.7A   
 Post-P 3.0A 236 0.55B 230 57A 53 91 4.3A 429 
 Post-D 5.5B 167 0.37C 166 72B 170 93 6.5B 1363 
 Significance (P) <0.0001  <0.0001   0.31  0.02  0.01  
Submerged Aquatic Vegetation Variables
SecchiSecchiϵparϵpar% Cover% CoverExtentExtentZc meanWet BiomassWet Biomass
meanNmeanNmeanNmean (m)N(m)Zc Nmean (g m-2)N
Upper Bay             
 Pre-P 1.2A 324 1.82A 323 17A 71 197AB 10 2.3A 10   
 Post-P 1.3B 397 1.50B 374 42B 301 114A 28 1.6A 27 50 33 
 Post-D 1.9C 338 1.04C 338 51C 2227 417B 18 3.5B 18 962 18 
 Significance (P) <0.0001  <0.0001  <0.0001  0.01  <0.0001  <0.0001  
Middle Bay             
 Pre-P 1.4A 162 1.40A 160 50AB 14 102AB 2.6A   
 Post-P 1.5A 237 1.24B 224 49A 119 115A 10 2.2A 10 226 14 
 Post-D 2.3B 169 0.81C 169 61B 394 276B 3.7B 903 
 Significance (P) <0.0001  <0.0001  0.01  0.03   0.07  
Lower Bay             
 Pre-P 2.9A 162 0.63A 162 56AB 17 123 3.7A   
 Post-P 3.0A 236 0.55B 230 57A 53 91 4.3A 429 
 Post-D 5.5B 167 0.37C 166 72B 170 93 6.5B 1363 
 Significance (P) <0.0001  <0.0001   0.31  0.02  0.01  

In the upper bay, SAV percent cover was significantly different over the three time stanzas, more than doubling between the pre- and post-P period, then a smaller 23% increase between the post-P and -D period when mean cover reached 51%. SAV cover in the middle and lower bays were already at 50 and 56% respectively in the pre-P period, did not change significantly in the post-P period and increased by 25% in the post-D period.

Mean bed extent more than tripled in the upper bay and doubled in the middle bay between the post-P and -D periods. Bed extent decreased in the upper bay between the pre- and post-P periods whereas the middle bay extent increased slightly, but both of these changes were not significant. In the lower bay, small changes in bed extent over the three time stanzas were not significant.

The mean maximum depth of SAV colonization (Zc) occurred during the post-D period, and was significantly different than the other two time stanzas in all three bays. The greatest increase in mean Zc for all three bays occurred between the post-P and -D time stanzas, with the largest increase of 123% occurring in the upper bay. Substantial increases in the mean wet biomass were recorded for all three bays between the post-P and post-D period, although the middle bay difference was not significant due to high variability. The greatest increase occurred in the upper bay where biomass increased from 50 to 962 g m−2.

The relative frequency of occurrence for the five most dominant species in the Bay of Quinte changed considerably over the time period (Figure 4). Not all species were detected in each of the surveys; Ceratophyllum demersum was absent in 1979 in the upper bay and was absent through most surveys in the middle and lower bays. Myriophyllum spicatum was not detected in the middle bay in 1982, and in the lower bay, Zosterella dubia was absent in 1972 and 1982 and Elodea canadensis absent in 1982.

Figure 4.

Time series of the frequency of occurrence for the five most abundant species in the (a) upper, (b) middle and (c) lower bay areas between 1972 and 2007. Time series were based on vegetated point data from north transects at Trenton, Belleville and Big Bay for the upper bay, Hay Bay east transect for the middle bay and the Conway north transect for the lower bay.

Figure 4.

Time series of the frequency of occurrence for the five most abundant species in the (a) upper, (b) middle and (c) lower bay areas between 1972 and 2007. Time series were based on vegetated point data from north transects at Trenton, Belleville and Big Bay for the upper bay, Hay Bay east transect for the middle bay and the Conway north transect for the lower bay.

Typically, the relative frequency for all species in all three bays for the entire time period was less than 30%. The exception in the upper bay was the dominance of M. spicatum in the pre-P period, E. canadensis in 1979 and Z. dubia in 1982. In the middle bay, both Z. dubia and V. americana had a relative frequency of 40 in 2004, while in 2007, Z. dubia dominated at 65% while V. americana decreased. In 1972 in the lower bay, E. canadensis dominated, while in 1979, 1982 and 1994, V. americana recorded relative frequencies of over forty percent.

Discussion

SAV responses to perturbations differed considerably among the Bay of Quinte sub-basins due principally to trophic gradients and morphometric differences. Point-source P loadings were highest in the shallow upper bay, likely stemming from the high human population density in this area. Major tributaries are also located in the upper bay region and serve to dilute P concentrations downstream when water levels and flushing rates are high. The upwelling that occurs in the middle bay provides further dilution, because Lake Ontario waters are more oligotrophic than the Bay of Quinte (Johannsson et al., 1998). Higher P loading, less dilution, shallower water and higher temperatures all combined to make the upper bay more eutrophic than the middle or lower bays throughout the study period. At the onset of the study, the lower bay was considered mesotrophic and did not experience the hypereutrophic conditions of the upper bay before P-control was implemented. Nicholls (2012) provided a detailed analysis of the TP and Chl a time series for the Bay of Quinte.

The upper bay is considerably different from the middle and lower bays in terms of morphometry. Most of the upper bay is shallow, with 87% of the area <5 m deep. In addition, much of the upper bay slopes gently into expansive but only slightly deeper areas. In contrast, the area <5 m deep in the middle and lower bays is 39 and 22% respectively. SAV habitat in these two bays is generally found along the narrow strip of shallow water adjacent to the shore and is limited to these areas by steep gradients that lead rapidly to substantially deeper waters where SAV cannot establish and survive.

Although secchi depth and ϵpar improved consistently through the three time stanzas for all bays, the greatest percentage increase for both variables occurred after Dreissena establishment. Many studies have noted the importance of water clarity on SAV distribution (Chambers and Kalff, 1985; Hudon et al., 2000; Lammens et al., 2004), and several found increases in density and distribution of SAV after Dreissena invasions (Skubinna et al., 1995; Knapton and Petrie, 1999; Zhu et al., 2006; Higgins and Vander Zanden, 2010). In the Bay of Quinte, SAV showed limited response to reduced P loading in the period 1978 to 1994. As Minns et al. (1986) had predicted, SAV recovery following reductions in P loadings was delayed because of P reflux from the sediment pool. SAV response after the invasion of Dreissenid Mussels was dramatic, particularly in the upper bay where extent and Zc increased by more than 125%.

One exception to the higher post-D response of SAV parameters was a substantial increase in SAV cover in the upper bay between the pre-P and post-P period. Much of this difference can be attributed to the survey protocol which allowed SAV gaps of up to 90 m before determining the last SAV on the transect. In the upper bay, both mean SAV extent and Zc were greater during the pre-P period than the post-P period, but cover was significantly lower. Percent cover was lower because the Trenton north transect in the pre-P period had many gaps before the end of the transect was reached. In the post-P period, the number of gaps and location of the last SAV had been reduced, thus substantially increasing cover and decreasing both extent and Zc. The sparse SAV found furthest offshore in the pre-P period may have been from the seed bed of the earlier, extensive SAV beds. In the post-D period, the considerable increase in extent indicates that SAV are regaining previously occupied territory. By 2007, the Trenton transect had doubled in extent from the 2004 survey, but this also introduced areas with sparser SAV so the cover average did not increase substantially.

In 1972 in the middle and lower bays, SAV was in better condition in terms of density, Zc and extent than compared with the upper bay. The greatest increase in density and Zc for these two bays occurred after the invasion by Dreissena, but increases in extent were limited due to the steep slope in some areas, particularly in the lower bay. Steep slopes presented challenges in assessing both extent and Zc in the lower bay because SAV in this area had already advanced to the edge of the slope in 1972, so assessment of potential expansion beyond the edge had to occur within a very small area along the steep slope. In the middle bay in 1972, SAV still had room for expansion before reaching the drop-off in bathymetry in that area. It achieved that extent on the east transect in the survey following Dreissenid Mussel establishment and the concurrent increase in water clarity. Steeper slopes can influence sediment stability (Hakanson, 1982) and the deposition of nutrient-rich material. Although slopes <3% had little effect on the distribution of fine-grained sediments, Rowan et al. (1986) found that these fine sediments were generally absent on slopes >7.5%. Duarte and Kalff (1986), noted that there was relatively low SAV biomass on slopes >2.2% which they attributed to slope effects on substrate type. In a 1990 study, Duarte and Kalff estimated that a slope of 15% formed the upper threshold for SAV presence.

Of the five most dominant species in the Bay of Quinte, all but V. americana were partially to highly tolerant to degraded water quality (Albert and Minc, 2004; Croft and Chow-Fraser, 2007). With improving water quality in the upper bay, V. americana increased in relative frequency from two in 1972 to a maximum of 24 in 2007. Additionally, less tolerant species such as Potamogeton richardsonii, Potamogeton zosteriformis and Najas flexilis all had higher occurrences in the post-D period. M. spicatum decreased substantially in the upper bay after 1974; declines in this invasive have been reported elsewhere (Carpenter, 1980; Trebitz et al., 1993) and are attributed to multiple factors. No clear trends in species composition were evident in either the middle or lower bays.

The absence or presence of SAV in shallower lakes can have a major impact on whether the lake experiences shifts to a turbid or clear-water regime (Genkai-Kato and Carpenter, 2005; Scheffer and van Nes, 2007). This is due to SAV ability to stablilize sediment resuspension and prevent P-recycling from the sediments, although many other factors, such as climate, lake size and bathymetry also play a role.

Additional factors, including depth, exposure, temperature and water levels can impact the distribution and abundance of SAV. Depth affects several factors, but the principal impact is light attenuation with increasing water depth through scattering and absorption (Wetzel, 1983). Wave exposure has direct effects through removal of plant biomass, displacement of seedlings and transport of propagules. Indirect effects of exposure include the erosion, transport and deposition of sediments and their associated nutrients and organic matter (Keddy, 1982; Cyr, 1998) and increased turbidity from resuspension of the sediments in shallow lakes (Madsen et al., 2001; Horppila and Nurminen, 2003). Water temperature affects SAV physiological responses such as seed germination, development, photosynthesis rates, oxygen consumption, dormancy and turion formation (Pip, 1989, Rooney and Kalff, 2000). High water levels during the early growing season reduce light available to plants; low water levels can damage SAV through ice scour or freezing in the winter, and by wave action or dessication in summer. SAV may also be affected by herbivory (Lodge, 1991) by invertebrates, fish, mammals and waterfowl. The impact of one or a combination of these factors on SAV will vary depending on their magnitude, timing, duration and the physiology and life history of the SAV species.

A major rationale for listing loss of fish habitat as a beneficial use impairment (BUI) under the Remedial Action Plan in the Bay of Quinte was the substantial reduction in the offshore SAV in the upper bay in the 1960s. An interim target was developed for the delisting criteria for this IBU which requires both an areal and density increase in SAV so that 30% of the bay has a SAV cover of >50%; conditions that were believed to have existed in the early 1900s. This target was based on the anticipated increase in water clarity through reduction of algal densities due to decreased phosphorous point source loadings (Johnson, 1986). The final determination of whether the target has been met will be based on findings from an SAV model to be developed for Bay of Quinte using data from 1972 to 2004.

An interim assessment of the BUI target can be expressed as the percentage occupied by SAV of the total across bay width of the 3 transects in the upper bay (Figure 5). Extent in the upper bay had increased substantially in the post-D period, primarily due to changes at the Trenton site. Littoral slope influenced the magnitude of these changes because the low sloping site allowed rapid expansion in bed extent as SAV moved into slightly deeper waters. This interim assessment of the delisting target indicated that the extent criterion had been met by 2007.

Figure 5.

Time series of submerged aquatic vegetation (SAV) bed extent for the three upper bay locations between 1972 and 2007, expressed as a percentage of the total distance across the bay that was occupied by SAV at Trenton, Belleville and Big Bay.

Figure 5.

Time series of submerged aquatic vegetation (SAV) bed extent for the three upper bay locations between 1972 and 2007, expressed as a percentage of the total distance across the bay that was occupied by SAV at Trenton, Belleville and Big Bay.

Conclusions

As reported elsewhere, the SAV in the Bay of Quinte experienced a greater response in bed extent and Zc to the invasion by Zebra Mussel rather than the reduction in P source loadings. In addition to increases in water clarity, basin morphometry and the sheltered nature of the Bay of Quinte played a key role in the magnitude of the response. Although the lowest SAV density in the post-D period was in the upper bay, the gently sloping sites allowed for substantial expansion of SAV beds when waters became sufficiently clear. This 35-year dataset will be used to estimate if the SAV component of the delisting criteria has been met; an important step towards designating the Bay of Quinte as an Area in Recovery. This assessment has documented changes in the bay showing that excessive eutrophication and invasive species had profound effects on the SAV community. Maintaining current, low P loading levels and preventing invasions of plants or animals that could be detrimental to SAV will be essential if a healthy SAV community is to be sustained in the future.

Acknowledgements

Funding for this project was provided by the Great Lakes Action Plan. The authors gratefully acknowledge all the people who contributed to this 35 year dataset, particularly Adele Crowder, Jeff Warren and GLLFAS field crews. Thanks also to Erin Gertzen who ran the post-hoc comparisons in R. Both Erin and Nick Lapointe provided most helpful comments which substantially improved this manuscript.

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