Cage aquaculture and water-quality changes in the LaCloche ...

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Cage aquaculture and water-quality changes in the LaCloche Channel, Lake Huron, Canada: a paleolimnological assessment Saloni Clerk, Daniel T. Selbie, and John P. Smol

Abstract: Lake eutrophication due to cage aquaculture is an area of concern in Ontario; however, without knowledge of pre-impact conditions, it is difficult to determine the extent and magnitude of environmental change. Paleolimnological techniques were used to estimate water-quality conditions prior to, during, and briefly following aquaculture operation in the LaCloche Channel, Lake Huron. Past oxygen and nutrient levels were inferred from assemblages of chironomids and diatoms, respectively, to determine whether recent low-oxygen and nutrient-rich conditions were related to cage aquaculture in operation from 1989 to 1998. Chironomid assemblages exhibited trends consistent with decreased hypolimnetic oxygen levels, with reductions in oxic-type profundal taxa and increased relative abundances of littoral communities. Diatom assemblages reflected a period of nutrient enrichment by increased relative abundances of meso-eutrophic taxa. Improvements in water quality are inferred from assemblages of diatoms in surface sediments, which may correspond to the cessation of fish-farming activities in 1998. In contrast, no sign of deep-water oxygen recovery is recorded by chironomids. These trends are consistent with eutrophication, and suggest that the LaCloche Channel was sensitive to nutrient loading from the cage-aquaculture operation. This study demonstrates the potential of using paleolimnological techniques to track water-quality changes associated with cage farming. Résumé : L’eutrophisation causée par l’aquaculture en cages est un problème préoccupant en Ontario; cependant, sans informations sur les conditions antérieures, il est difficile de déterminer l’étendue et l’importance des changements environnementaux. Nous avons utilisé des techniques paléolimnologiques pour évaluer les conditions de la qualité de l’eau avant, pendant et peu après une opération d’aquaculture dans le chenal LaCloche au lac Huron. Nous avons déduit les concentrations d’oxygène et de nutriments du passé de l’analyse respectivement des chironomidés et des diatomées afin de déterminer si les conditions récentes d’oxygène réduit et de nutriments élevés sont reliées aux activités d’aquaculture en cages qui y ont eu lieu de 1989 à 1998. Les communautés de chironomidés montrent des tendances qui sont compatibles avec une réduction de l’oxygène dans l’hypolimnion, avec une diminution des taxons oxybiontes de la région profonde et une augmentation de l’abondance relative des communautés littorales. Les communautés de diatomées indiquent une période d’enrichissement en nutriments par l’augmentation de l’abondance relative des taxons méso-eutrophes. Nous avons déduit l’existence d’une amélioration de la qualité de l’eau de l’analyse des diatomées des sédiments superficiels, amélioration qui peut correspondre à l’arrêt des activités de pisciculture en 1998. En revanche, nous ne trouvons aucune trace de récupération des conditions d’oxygène dans les eaux profondes à partir de l’examen des chironomidés. Ces tendances sont compatibles avec l’eutrophisation et laissent croire que le chenal LaCloche était sensible à l’apport des nutriments provoqué par les opérations d’aquaculture en cages. Notre étude démontre le potentiel de l’utilisation des techniques paléolimnologiques pour suivre les changements de la qualité de l’eau associés à l’élevage en cages. [Traduit par la Rédaction]

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Introduction Intensive fish farming in lakes is a relatively new pressure on Canadian aquatic ecosystems. Although still regarded as a small industry in Canada, fish farming is rapidly expanding, particularly in Ontario, where cage production of rainbow trout (Oncorhynchus mykiss) has increased dramatically since the mid-1980s (Moccia et al. 1997). The Manitoulin Island – Georgian Bay area (Fig. 1) accounts for over 95%

of cage production of rainbow trout in this province (Moccia et al. 1997) and consequently receives significant nutrient loadings from fish wastes (uneaten fish food and feces). Annual loadings in the North Channel of Lake Huron and Georgian Bay alone are estimated at 15 t of total phosphorus, 90 t of nitrogen, and 500 t of solid waste (Gale 1999b). Untreated wastes from these cage operations are discharged directly into the aquatic environment, and so there is grow-

Received 29 May 2003. Accepted 6 April 2004. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 16 November 2003. J17557 S. Clerk, D.T. Selbie, and J.P. Smol.1 Paleoecological Environmental Assessment and Research Laboratory, Department of Biology, Queen’s University, Kingston, ON K7L 3N6, Canada. 1

Corresponding author (e-mail: [email protected]).

Can. J. Fish. Aquat. Sci. 61: 1691–1701 (2004)

doi: 10.1139/F04-099

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Fig. 1. Map of the North Channel of Lake Huron showing the location of the LaCloche Channel core site (夹). The Ontario Ministry of Natural Resources site (䊏) was operational from 1985 to 1992.

ing concern that cage farming poses a serious eutrophication risk. In the Great Lakes, only one study to date has examined water-quality impacts from cage aquaculture (Hamblin and Gale 2002), despite the importance of this area for largescale aquaculture operations. This reflects the fact that most aquaculture research in Canada has tended to focus on large salmonid cage operations in marine coastal waters rather

than on inland lakes (Chambers et al. 2001). Freshwater cage culture is now receiving increasing attention as an emerging environmental issue. However, there is limited understanding of the environmental impacts of cage aquaculture, as long-term water-quality data are often lacking. The depletion of oxygen in the bottom waters of lakes is of particular concern in cage operations (Beveridge 1984). Increased sedimentation of fish waste materials, as well as © 2004 NRC Canada

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the eventual decomposition of the elevated levels of algae produced, may deplete oxygen in hypolimnetic waters. These anoxic and hypoxic conditions can have severe repercussions for oxygen-dependent organisms, as well as alter processes at the sediment–water interface. For example, deep-water oxygen availability may affect the diversity and abundance of many types of biota, and under anoxic conditions, the release of nutrients (e.g., orthophosphate) can be enhanced, thereby prolonging the effects of eutrophication (i.e., internal loading) (Nürnberg 1995). Enhanced oxygen depletion due to cage farming may be especially problematic in regions within Lake Huron that have been designated “cold water fishery areas”, based on Provincial Water Quality Objectives of ≥6 mg·L–1 dissolved oxygen (DO) (Ontario Ministry of the Environment (OMOE) 1994). Although it is suspected that fish farms are responsible for poor water-quality conditions in some lakes (e.g., algal blooms, low hypolimnetic DO conditions), a lack of historical water-quality data is a major obstacle in determining the actual impacts of the cage farms in question. This is particularly the case in the LaCloche Channel (North Channel of Lake Huron), where deteriorating water-quality conditions were linked to cage-aquaculture activities from 1989 to 1998 (Gale 1999a). With only limited pre-impact data available, perceived and actual impacts from aquaculture activities in the Channel were questionable. Nevertheless, the aquaculture operation was eventually decommissioned. In the absence of monitoring data, paleolimnological techniques may provide powerful methods for inferring historical water-quality conditions to assess potential environmental impacts in aquaculture areas such as the LaCloche Channel. This information can help to answer important management questions by addressing whether the nutrient-rich status of an aquaculture lake is a recent phenomenon, resulting from intensive fish farming, or a natural condition. Chironomids (Diptera: Chironomidae) and diatoms (class Bacillariophyceae) are two commonly used paleoindicators that can provide a continuous record of water-quality changes. For example, fossil chironomids are reliable indicators of past oxygen levels (e.g., Quinlan and Smol 2001a), whereas diatoms are excellent indicators of past surfacewater nutrient levels (e.g., Hall and Smol 1999). In this paleolimnological study we used analyses of fossil chironomid and diatom assemblages archived in the sediments of the LaCloche Channel to track changes in deep-water hypolimnetic anoxia and surface-water nutrient levels, respectively, resulting from aquaculture activities. These data will be used to assess the degree of eutrification and recovery from cage aquaculture in the LaCloche Channel.

Materials and methods Study area The LaCloche Channel is located within the North Channel of Lake Huron, Ontario (Fig. 1). The northern shore is bounded by the LaCloche Mountain Range, which is composed primarily of quartzite. Along the south shore, Manitoulin Island borders the Channel and is underlain by dolomitic limestone (Chapman and Putnam 1984). The Eastern Hardwood Forest of North America surrounds the North Channel and is characterized by mixed coniferous and decid-

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uous trees; most of this is second growth because of logging activities during European settlement in the mid-1800s (Sly and Munawar 1988). Historically, this area has been used for a variety of human activities in addition to logging and lumbering, including fur trading, fishing, and mining. Currently, there is little development in the Channel’s catchment, with only one cottage and one small fishing lodge. The LaCloche Channel is characterized by several deep basins (maximum depth 41 m) and a relatively low flushing rate (2.3 times·year–1) (Gale 1999a, 1999b). This area is managed by the OMOE as a cold-water fishery, which requires minimum DO levels of 6 mg·L–1 (OMOE 1994). Historical monitoring data from August 1985 in the forebay of this Channel indicate relatively good oxygen conditions, with hypolimnetic DO levels of 5 mg·L–1 and low nutrient levels of 15 m) (Boyd et al. 2000); however, reductions in nutrient concentrations were found, with autumn 1998 TP levels of 12 µg·L–1 (Gale 1999a). Controversy still exists as to whether the fish farm was directly responsible for these poor water-quality conditions, as long-term water-quality data are required to accurately assess impacts. In this case, however, the aquaculture operation was closed, based on a comparison with monitoring data from a one-time sampling event in 1985. Field and laboratory methods In late August 1999, a 26 cm long sediment core was obtained from the deep-water region of the LaCloche Channel (38 m depth) at the site where fish cages were located © 2004 NRC Canada

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(Fig. 1). The core was collected using a Glew (1989) gravity corer (inner diameter 6.6 cm) and sectioned at the lakeshore immediately after collection using a Glew (1988) extruder. The upper sediments (0–10 cm) were extruded at 0.5-cm intervals, whereas deeper sediments (10 cm to the bottom of the core) were sectioned at 1.0-cm intervals. The sectioned sediment samples were placed in individual Whirl-Pak® bags and stored at 4 °C until analyses were undertaken. To estimate recent dates, sediments were analysed for 137 Cs activity in 18 samples taken over the length of the core (Appleby 2001). Wet samples were freeze-dried and subsampled (~0.35 g) into plastic tubes to a height of ~18 mm. These samples were subsequently analysed for 137Cs activity using gamma-counting techniques. Because of insufficient sediment volume, 210Pb activity was not measured. For chironomid analyses, subsamples of wet sediment (4– 22 g) were deflocculated with 5% KOH for ~4 days at room temperature, sieved through a pair of nested Nitex® mesh sieves (200 and 95 µm), and dehydrated with three successive treatments of ethanol (75%, 95%, and 99%). Individual chironomid head capsules were hand-sorted using a Leica® MZ-12 dissecting microscope (8–120× magnification) and mounted on glass slides using Euparal® (refractive index 1.485). If a minimum of 40 head capsules (Quinlan and Smol 2001b) were not found, sediments were resampled from freeze-dried material that was used for 137Cs analysis. Taxonomic identification of chironomids was undertaken using a Leitz Dialux 20 microscope at 125–630× magnification. Head-capsule identifications were based primarily on the structure of the mentum. Specimens with a complete or almost complete mentum were counted as one head capsule, whereas those with approximately half a mentum were counted as a half of a head capsule. Identifications were made according to Walker (1988, 2000) and Wiederholm (1983). The majority of specimens were identified to the generic level, but several taxa had to be combined into higher taxonomic categories because they were indistinguishable. These included Cricotopus/Orthocladius and Corynoneura/ Thienemanniella. Also, a large number of taxa encountered belonged to the tribe Tanytarsina, which is a broad taxonomic group (Walker 1988). Most species from this group are littoral; however, some taxa inhabit the profundal zone, such as Micropsectra. Micropsectra can be differentiated on the basis of a short apical spur on the antennal pedestal, and a bifid premandible. Premandibles were often missing from our specimens and so we differentiated Micropsectra as Micropsectra type, primarily based on the presence of a short apical spur. Diatom slides were prepared for light microscopy following methodology similar to that of Battarbee et al. (2001). A homogenized subsample of ~0.2 g of wet sediment from each core interval analysed was treated with 10% HCl solution to remove carbonate material. Samples were left undisturbed for 24 h to settle and subsequently the supernatant liquid was aspirated. The sediment samples were then washed with deionized water and allowed to settle for 24 h before being aspirated. This rinsing procedure was repeated four times. The organic component of the samples was digested by treatment with an equal mixture by weight of concentrated sulphuric acid (H2SO4) and nitric acid (HNO3), heated in a water bath (~80 °C) for ~5 h, and left to settle


for 24 h. Eight deionized water rinses were conducted, as outlined above, until neutral pH was attained. An aliquot of homogenized slurry was then pipetted in a series of sequential deionized-water dilutions onto cover slips. Addition of a small amount of 10% hydrochloric acid (HCl) aided in the dispersion of diatom microfossils on the cover slip. Cover slip samples were evaporated and mounted on glass slides using Naphrax® mounting medium (refractive index 1.74). Diatom valves were identified and enumerated along parallel transects from the centre of each cover slip, using a Leica DMRB microscope equipped with differential interference contrast optics (numerical aperture 1.30; magnification 1000×). A minimum of 300 valves were counted for each sample. Diatom taxonomy and nomenclature followed that of Krammer and Lange-Bertalot (1986–1991) and Cumming et al. (1995). Two species of Stephanodiscus (S. minutulus and S. parvus) were counted as one taxon because of difficulties in differentiating them under light microscopy. Numerical analysis Biostratigraphic zones for both chironomid and diatom assemblages were determined through cluster analysis using constrained incremental sum of squares (CONISS) (Grimm 1991). To compare the major patterns of variation between chironomids and diatoms, the data were summarized by detrended correspondence analysis (DCA) using untransformed data (Birks 1998). In the DCA detrending was by segments, and nonlinear rescaling was implemented (ter Braak and Šmilauer 1998). Finally, the relationship between profundal and littoral chironomid species and samples was evaluated using a principal components analysis (PCA) ordination. The PCA was based on a covariance matrix with relative abundance data using the CANOCO software (ter Braak and Šmilauer 1998).

Results and discussion Chronology The 137Cs profile for the LaCloche Channel core was distinct, and displayed the characteristic rise, peak, and decline in activity (Appleby 2001), which reflects historical nuclear testing (Fig. 2). A well-resolved 137Cs peak is observed around 13.5 cm in the core and provides a marker for the 1963 horizon of maximum atmospheric weapons testing prior to a worldwide nuclear test ban treaty. The presence of this distinct peak suggests that cesium mobility through the sediments was not likely, as this would be represented by a more gradual change in cesium level through the upper intervals. For these reasons we are confident that the 137Cs profile is indicating 1963, and that our core covers the period of aquaculture activity, as well as pre-impact conditions. A linear interpolation of sedimentation rates between 1963 and 1999 is not realistic, as sedimentation rates would not likely have remained constant over this time period because of eutrophication. We acknowledge that our geochronology is approximate. Paleolimnological analyses Chironomids and diatoms were well preserved and consisted of three distinct communities based on CONISS (Figs. 3, 4). Stratigraphic zones are not, however, identical between indi© 2004 NRC Canada

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Cs activity in sediments from the LaCloche Channel site.

cators, as diatoms respond positively to nutrient inputs (Hall and Smol 1999), whereas chironomids are mainly benthic and respond to changes in oxygen and food availability resulting from nutrient inputs (Little et al. 2000). Nevertheless, DCA analysis indicates that major shifts in the composition of chironomid and diatom assemblages occurred at similar depths (~9.0 cm for chironomids and ~8.0 cm for diatoms) after 1963 (~13.5 cm) as dated by 137 Cs analysis (data are available from the authors). These changes occur in Zone 2 of both chironomid and diatom profiles (Figs. 3 and 4). A total of 34 chironomid and 78 diatom taxa were identified from the core. There is a clear shift in the composition of the fossil communities from the bottom to the top of the core, with profundal chironomids comprising relatively more of the assemblages in older sediments than in recent sediments. Significant shifts in the diatom community are also evident, most notably in nutrient-sensitive planktonic diatoms, which dominate assemblages throughout the core. Changes in both chironomid and diatom communities are consistent with eutrophication and are discussed in more detail below. Biostratigraphic Zone 1: pre-impact conditions Chironomid assemblages deposited during the earliest period of the core indicate well-oxygenated bottom waters, with limited periods of hypoxia (Fig. 3). These conditions are inferred from stable abundances of profundal chironomids characteristic of oxic lakes, such as Heterotrissocladius, Micropsectra type, and Sergentia. Heterotrissocladius and

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Micropsectra are well-known indicators of high oxygen levels, whereas Sergentia thrives in mesotrophic lakes with moderate oxygen depletion (Quinlan et al. 1998). Sporadically high abundances of Chironomus, a taxon highly tolerant of hypoxia (Sæther 1979), indicate that some episodes of lowoxygen conditions may have occurred during this period, possibly as a result of anthropogenic activities in the catchment (e.g., logging). Overall though, the profundal community is dominated by oxic-type taxa, which indicates that hypoxia must have been limited in the Channel prior to the commencement of aquaculture activities. The composition of the diatom assemblages occurring in Zone 1 indicates oligotrophic to mesotrophic nutrient levels, which is consistent with well-oxygenated conditions in the deeper waters inferred from the chironomids (Fig. 4). For example, high abundances of diatoms characteristic of low to moderate nutrient levels such as Tabellaria flocculosa str. IIIP, Cyclotella pseudostelligera, Cyclotella bodanica v. aff. lemanica, and Cyclotella michiganiana (Reavie and Smol 2001) are recorded during this period. Moderately rich nutrient conditions during this time are also supported by abundances of Aulacoseira ambigua, Asterionella formosa, and Fragilaria crotonensis, and probably indicate some nutrient enrichment from local human activities in the catchment. Biostratigraphic Zone 2: inferred aquaculture period Peak abundances of oxic-type profundal chironomids are observed in the upper portion of Zone 2, including Micropsectra type and Sergentia (Fig. 3). Shortly thereafter, Micropsectra type, a taxon that requires high levels of oxygen (Sæther 1979), declines, followed by the mesotrophic indicator Sergentia, indicating progressively lower oxygen availability in the bottom waters. The commencement of fish-farming activities in 1989 may be responsible for lowered oxygen availability in this period. Decreases in oxictype taxa have been associated with lower oxygen and higher nutrient conditions in other eutrophication studies as well (e.g., Walker et al. 1993; Little et al. 2000; Francis 2001). Interestingly, declines in these oxic-type indicators are not mirrored by increases in anoxic-type taxa, such as Chironomus. For anoxic-type taxa to dominate assemblages, low oxygen and high nutrient concentrations (which affect food availability) are required (Quinlan and Smol 2001a). Based on the declines observed in the oxic-type indicators Micropsectra type and Sergentia, it appears that nutrient loadings from the fish farm were high enough to deplete oxygen levels, but not high enough to sustain profundal populations of anoxic-type taxa (e.g., Chironomus), which have high food requirements. In their training set, composed of oligotrophic to mesotrophic lakes, Quinlan and Smol (2001a) also found that anoxic-type taxa did not dominate chironomid assemblages in lakes with extremely low end-of-summer oxygen concentrations. Another notable feature of Zone 2 is the appearance of Polypedilum around 9 cm (Fig. 3). This taxon favours organic muds (Sayer et al. 1999) and has been found to dominate extant chironomid assemblages in sediments near cage farms in Lake Letowskie, Poland (Dobrowolski 1987). Although no sedimentation data are available from the LaCloche Channel site, it is likely that loadings were significant to the benthic environment, as an estimated 164 kg of © 2004 NRC Canada

Fig. 3. Relative abundances of the main chironomid taxa in sediments from the LaCloche Channel. Profundal and littoral taxa were classified according to the closest available training set to our study area (Quinlan and Smol 2001a). Stratigraphically constrained cluster analysis (Grimm 1991) was used to delineate three major zones in the chironomid stratigraphy. 137Cs analysis indicates that the 13.5-cm depth was deposited ~1963. The ratio between the relative abundances of littoral and profundal chironomids is shown as %L:%P.

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Fig. 4. Relative abundances of the 12 most abundant diatom taxa (>5% in at least two samples) in the core from the LaCloche Channel. cm depth was deposited ~1963. Diatom zones were constructed using stratigraphically constrained cluster analysis (Grimm 1991).

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Cs analysis indicates that the 13.5-

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solid waste can be generated per tonne of fish produced (Cho and Bureau 1998). Thus, the appearance of this taxon may be related to increased loadings from aquaculture activities in the LaCloche Channel. Changes in the diatom assemblages in Zone 2 provide additional evidence of eutrophication from aquaculture activities (Fig. 4). More productive conditions ~9.0 cm are inferred from increases in several mesotrophic diatom species (e.g., Fragilaria crotonensis, F. nanana, and A. ambigua), coupled with concurrent decreases in relative abundance of the dominant oligotrophic indicator, Cyclotella comensis (Reavie and Smol 2001). The increase in C. comensis observed earlier in Zone 2 is likely related to increased atmospheric deposition of inorganic nitrogen (NO3) from industrialization around the Great Lakes system (Pappas and Stoermer 1995). The rise in this taxon, which may be favoured under low-silica and high-nitrate conditions (e.g., during summer blooms), has also been observed in other, typically oligotrophic, parts of the Laurentian Great Lakes (Stoermer and Kreis 1980; Stoermer et al. 1996). The stratigraphical plot of the DCA sample scores indicates a substantial compositional change in both chironomid and diatom assemblages in Zone 2 (data are available from the authors). Differences in the timing of these changes were minimal between chironomids (9.0 cm) and diatoms (8.0 cm) and, based on our inferred chronology, are approximately coincident with the aquaculture operation. Biostratigraphic Zone 3: expansion and removal of cage farm Profundal chironomid assemblages from the most recent sediments in Zone 3 are characterized by the disappearance of Sergentia. This provides strong evidence of reduced oxygen availability in deep-water habitats and contrasts sharply with pre-impact assemblages that had stable abundances of this taxon. The disappearance of Sergentia has been associated with the development of anoxic conditions in past eutrophication studies (Little et al. 2000; Francis 2001). Other oxic-type profundal taxa are present in similar (e.g., Heterotrissocladius) or only slightly reduced (e.g., Micropsectra type) abundances compared with background levels. This suggests that low-oxygen conditions were restricted in the Channel, perhaps to the deepest basin, where Sergentia thrived. Lower oxygen conditions during this period are also supported by the appearance of Cryptochironomus and Parakiefferiella sp. B., taxa which, according to Quinlan and Smol (2001a), have lower oxygen optima than either Sergentia or Micropsectra type. Low-oxygen conditions inferred from chironomids during this period are supported by oxygen profiles recorded during the late 1990s, which showed anoxic conditions in the hypolimnion during the late summers (Gale 1999b). Zone 3 is also marked by increased abundances of several littoral taxa. Many of the littoral taxa that dominate recent sediments are associated with macrophytes (e.g., Corynoneura/ Thienemanniella, Cricotopus/Orthocladius, Dicrotendipes; Brodersen et al. 2001) (Fig. 3), suggesting that macrophyte biomass increased in the Channel as a result of nutrient enrichment from the fish farm. This is supported independently by the documentation of some of the macrophytes that these


taxa are associated with in the Channel (Morton and Venn 2000). Another littoral taxon that increases markedly in the surface sediments is the Cladotanytarsus mancus group. Although this taxon grouping is not associated with macrophytes, previous studies have shown increased abundances during eutrophication (e.g., Quinlan et al. 2002). Our results agree with previous studies that have also shown littoral taxa to dominate assemblages during periods of prolonged anoxia associated with eutrophication (Walker et al. 1993; Little et al. 2000). The relative increase of the littoral community in the recent sediments is recorded in the proportion of littoral to profundal taxa, which shows that the littoral community makes up a greater proportion of the total assemblage in sediments deposited from about 7 cm to the surface compared with background conditions (Fig. 3). PCA of species and samples from the LaCloche Channel is consistent with these findings, as many of the profundal taxa are positioned near the older, pre-impact sediments, whereas recent sediments are dominated by littoral taxa (Fig. 5). These results suggest that nutrient loading from the fish farm likely resulted in a more eutrophic chironomid community in which littoral taxa benefited, likely through increased habitat availability (e.g., macrophytes), whereas profundal oxic-type taxa were restricted by low hypolimnetic oxygen levels. Diatoms in most of Zone 3 (e.g., 6.5–1.0 cm) provide strong evidence of continued nutrient enrichment in the Channel (Fig. 4). Greater nutrient levels during this period are inferred from the marked shift from the more oligotrophic indicator Cyclotella comensis around 6.5 cm to the eutrophic indicator Stephanodiscus minutulus/parvus and to a lesser extent Asterionella formosa (Reavie and Smol 2001). Fragilaria crotonensis, a taxon that often characterizes ongoing anthropogenic nutrient enrichment (Hall and Smol 1999), increases around 5 cm, providing further support for an interpretation of more productive conditions during this period. An unprecedented peak in relative abundance of Diatoma tenuis (~5 cm) is also observed in Zone 3. This taxon is associated with high nutrient environments and can bloom under specific nutrient-enriched conditions in spring (Kreis et al. 1985; Krammer and Lange-Bertalot 1991). The expansion of the fish farm in the 1990s may have been responsible for elevating nutrients in the Channel during this period. Elevated nutrient levels inferred from diatoms are corroborated by water-chemistry data collected during the 1990s (Gale 1999a). Most recent surface sediments (~1 cm to the surface) likely reflect a period of reduced aquaculture operations, as the fish farm was scaled back in 1997 and operations ceased in 1998. These sediments are marked by decreases in nutrient-rich taxa such as Fragilaria nanana, F. crotonensis, A. ambigua, A. formosa, and S. minutulus/parvus, coupled with a rebound in abundance of the low-nutrient indicator C. comensis. These reductions indicate improvements in surface water quality. In contrast, the absence of Sergentia from chironomid assemblages in the surface sediments indicates no corresponding improvements in deep-water oxygen levels. Monitoring data collected in the autumn of 1998, following removal of the fish farm, supports improvements in surface water quality inferred from diatoms, as measured © 2004 NRC Canada

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Fig. 5. Principal components analysis (PCA) based on relative abundance data for chironomid taxa greater than 2.0% in at least two intervals and samples in the LaCloche Channel (䊏, samples from deeper than 5.25 cm; ⵧ, intervals ≤5.25 cm). HTRTRS, Heterotrissocladius; MICROP, Micropsectra type; SERGEN, Sergentia; POLYPE, Polypedilum; PROCLD, Procladius; PARAKB, Parakiefferiella sp. B; CHIRON, Chironomus; PENTAN, tribe Pentaneurini; CRYPCH, Cryptochironomus; CRICOR, Cricotopus/ Orthocladius; PARACH, Parachironomus; CORYTH, Corynoneura/Thienemanniella; MICROT, Microtendipes; CLDYM, Cladotanytarsus mancus group; TANYSL, tribe Tanytarsina; PARATT, Paratanytarsus type; CORYNC, Corynocera oliveri type; STMPLLL, Stempellinella; PSEDCH, Pseudochironomus; DICROT, Dicrotendipes; GLYPTO, Glyptotendipes; MICRTT, Microtendipes type; CRYPTE, Cryptotendipes; PGASTL, Pagastiella; PARATE, Paratendipes; CLADOP, Cladopelma; PSECTM, Psectrocladius (Monopsectrocladius); SYNORT, Synorthocladius; NANOCL, Nanocladius; OLIVDA, Oliveridia/Hydrobaenus; CLDTYA, Cladotanytarsus group A; TANYPL, Tanytarsus pallidcornis group; DOITRX, Doithrix/Pseudorthocladius.

average TP levels from several stations in the Channel were significantly lower (12 µg·L–1) than during operations (40 µg·L–1). The lack of recovery in hypolimnetic oxygen levels, inferred from chironomids, is also corroborated by DO profiles taken in 1998 and 1999 that indicate the persistence of anoxic conditions (Gale 1999a; Boyd et al. 2000). Sensitivity of the LaCloche Channel to aquaculture activities Inferences of hypolimnetic anoxia from chironomids and epilemnetic nutrients from diatoms indicate significant changes in bottom-water oxygen conditions and open-water nutrient levels, which are consistent with eutrophication in the LaCloche Channel. The most marked changes in both chironomids and diatoms after ~1963 occurred almost concurrently (i.e., 1 cm apart) and show a clear shift to more productive conditions from background sediments. Furthermore, our inferences of lower oxygen conditions and higher

nutrient levels from recent sediments are in agreement with monitoring data. These results suggest significant nutrient loadings to the Channel, most likely because of cageaquaculture activities. Our inference that the LaCloche Channel was sensitive to cage-farming activities is supported by site-specific factors that identify the Channel as being more susceptible to eutrophication problems. For example, the deep basins that characterize the Channel could provide isolated depositional zones for fish wastes (Gale 1999b). In these areas, oxygen depletion would be enhanced because of restricted mixing between surface and hypolimnetic waters. Leaching of nutrients from solid wastes would be greater in these deeper basins as well, because of the longer time it takes for this organic matter to settle to the bottom (Tlusty et al. 2000). The capacity of the site to assimilate wastes is also thought to be low, owing to restricted exchange rates between the cages and the surrounding water (Hamblin and Gale 2002) © 2004 NRC Canada

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and the relatively low flushing rate (2.3 times·year–1) (Gale 1999a, 1999b). These factors would contribute to the Channel being especially sensitive to localized impacts from nutrient loadings. The maintenance of productive and oxygen-depleted conditions may have been related to the duration and intensification of operations in the Channel. With continued operation and expansion of the fish farm, not only would overall nutrient loadings be greater, but the likelihood of phosphorus release from the sediments under reduced oxygen levels (i.e., internal loading) also increases. Monitoring data gathered in 1997 support this hypothesis, as nutrient levels 1 m from the sediments were extremely elevated (up to 98 µg·L–1 TP) (Gale 1999a). In past studies, higher nutrient-release rates have also been recorded from sediments at cage sites than at control sites, especially in lakes with longer histories of aquaculture activities (e.g., Kelly 1992, 1993; Temporetti and Pedrozo 2000). The results of our study imply that recent removal of the cage operation led to improvements in surface water quality but limited, if any, recovery of deep-water oxygen levels by the time of core retrieval in 1999. This indicates that benthic communities may take longer to recover from past aquaculture loadings, compared with planktonic communities. Other eutrophication studies have also described a similar pattern of lake recovery (e.g., Walker et al. 1993; Little et al. 2000). Doughty and McPhail (1995) observed severe impacts on sediments of a vacated cage site in terms of elevated oligochaete abundances, even 3 years after cessation of fishfarming activities, indicating that it may take several years before benthic organisms recover. The rate of oxygen uptake by sediments may be controlled by past organic loadings (Granéli 1978), so a lag can be expected between declines in sediment deposition rates from fish wastes and sediment uptake of oxygen. In the case of the LaCloche Channel, improvements in oxygen conditions may be realized only after further reductions in TP levels. For example, modelling in the LaCloche Channel (Gale 1999a) predicts anoxic conditions in the entire hypolimnion at a TP level of 13 µg·L–1, whereas only 5% of the hypolimnion is expected to be below 6 mg·L–1 DO under historic TP levels (5 µg·L–1) (Gale 1999a). In conclusion, knowledge of the environmental impacts of cage aquaculture is critical to the protection and management of aquatic resources, especially considering that the Ontario cage-aquaculture industry is predicted to double production within the next 10 years (Gale 1999b). Our paleolimnological data suggest that chironomid and diatom microfossils can effectively track changes in water quality (e.g., hypolimnetic oxygen levels, nutrient levels) associated with cage farming. Moreover, these data may provide information on natural lake conditions that are unavailable from historical data, but necessary for selecting sites, determining actual water-quality impacts from aquaculture activities, and setting realistic mitigation goals (Smol 2002).

Acknowledgements The authors thank Dr. P. Werner and P. Gale (OMOE, Sudbury) for help in collecting the core. Thanks are also extended to Dr. K. Rühland, Dr. R. Quinlan, J. Sweetman, and


Dr. P. Werner for their comments on an earlier draft of the manuscript. Dr. O. Heiri and an anonymous reviewer provided valuable comments the manuscript. The authors also acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) postgraduate scholarship and Ontario Graduate Scholarship programs. This project was funded by a NSERC strategic grant to J.P.S.

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