Tracking recent recovery from eutrophication in a high ... - Springer Link

Journal of Paleolimnology 28: 377–381, 2002.  2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Tracking recent recovery from eutrophication in a high arctic lake (Meretta Lake, Cornwallis Island, Nunavut, Canada) using fossil diatom assemblages 1, 2 1 Neal Michelutti *, Marianne S.V. Douglas and John P. Smol 1

Paleoecological Environmental Assessment and Research Laboratory ( PEARL), Department of Biology, Queen’ s University, Kingston, Ontario K7 L 3 N6, Canada; 2 Paleoenvironmental Assessment Laboratory ( PAL), Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5 S 3 B1, Canada; * Author for correspondence ( e-mail: michelut@ biology.queensu.ca) Received 11 February 2002; accepted in revised form 30 April 2002

Key words: Cornwallis Island, Diatoms, Eutrophication, High Arctic, Paleolimnology, Recovery, Resolute Bay, Sewage

Abstract Meretta Lake (Resolute Bay, Cornwallis Island, Nunavut, Canada) is a high arctic lake that received raw sewage for almost 50 years from the Canadian Department of Transport Base. The lake was sampled from 1968–72 during the International Biological Programme, as part of the Char Lake Project. As the number of users at the Transport Base declined throughout the 1990s, so too did the lake’s nutrient levels, and Meretta Lake is now classified as oligotrophic. A previous diatom-based paleolimnological study revealed marked species assemblage shifts coincident with sewage inputs beginning in the late 1940s; however, because the core was taken at a time when nutrient levels were still relatively high (i.e., 1993), the diatom record did not yet track any signs of recovery. In this present study, we examined fossil diatom assemblages from a sediment core taken in 2001. Our results indicate a shift to the pre-impact diatom assemblages in the most recent sediments, indicating that the paleolimnological record is tracking the decreased nutrient inputs to this high arctic lake, and confirms that no significant lags exist in these largely ice-covered lakes.

Introduction Meretta Lake, located on Cornwallis Island in the Canadian High Arctic, had received raw sewage for nearly 50 years from the Canadian Department of Transport Base (i.e., the ‘North Base’) through a series of watercourses and utilidors or discharge pipes (Schindler et al. 1974). The first detailed studies of high arctic lake eutrophication were done on Meretta Lake during 1968–72 as part of the Char Lake Project (CLP), which was one component of the International Biological Programme (IBP) (Rigler 1972, 1974; Schindler et al. 1974). However, as Rigler (1974) noted in the final IBP report, some conclusions were still tentative, as the original conditions of the lake were unknown. This prompted Douglas and Smol (2000) to perform a diatom-based paleolimnological

study to reconstruct pre-impact conditions from a core collected in 1993. They recorded marked species assemblage shifts coincident with eutrophication from the North Base. Sewage discharges into Meretta Lake began in 1949 but declined substantially, especially since the early 1990s, as the number of users at the North Base decreased. By December 1998, no further effluent was being released into the lake (Douglas and Smol 2000). Water chemistry measurements recorded annually from 1992–99 indicated that total phosphorus (TP) concentrations and other trophic state variables were nearing ‘natural’ values for high arctic lakes by the late-1990s (Douglas and Smol 2000). Despite the declining nutrient concentrations, the diatom record for the 1993 core analysed by Douglas and Smol (2000) showed no signs of recovery, as the TP

378 concentration in 1993 (18.3 mg / L) was still relatively high by arctic standards (most high arctic lakes have TP concentrations ,10 mg / L and sometime much lower). Also, slow sedimentation rates (|1 cm / 18 years) likely obscured any signs of recovery in the recent sediments. In order to determine if species shifts tracked changes in Meretta Lake’s nutrient levels, Michelutti et al. (2002) examined periphytic (moss and rock) diatom assemblages collected annually from 1992– 99. They showed that, as TP levels decreased, there was a shift from eutrophic to more oligotrophic assemblages. The question remained, however, whether sediments deposited in the 1990s would contain fossil diatom assemblages that tracked these nutrient declines. In this present study, we used highresolution sampling techniques on a recently-recovered sediment core (2001) to determine if the diatom assemblages indicative of a post-1990 recovery trend could be retrieved from the surface sediments.

Site description Meretta Lake (728 41.759 N, 948 59.589 W) is located in the hamlet of Resolute Bay (Qausuittuq) on Cornwallis Island, Nunavut, Canada. Physical, chemical, and biological data gathered during the IBP (1968– 72), as well as descriptions of the catchment and of sewage inputs, have been summarized elsewhere (e.g., Masemann (1971), Cruickshank (1971), Rigler (1972, 1974, 1978), Schindler et al. (1974)). Photographs of the utilidors (i.e., the sewage disposal system) and of the lake are included in Douglas and Smol (2000). Briefly, Meretta Lake is ice-covered for approximately 10 months of the year, and in cooler years it may remain partially covered all summer by a central float of snow and ice. The lake has two basins, but similar to the work during the IBP, and that of Douglas and Smol (2000), our sampling was performed in the northern basin (area 5 20 ha; Z max 5 9 m; Z mean 5 3.25 m).

Methods On July 20, 2001, a 21 cm sediment core was recovered from the northern basin of Meretta Lake using a Glew (1991) gravity corer (i.d. 5 4.0 cm). The core was sectioned on-site into 0.5 cm intervals using a Glew (1988) extruder. Diatom preparation

followed standard techniques (Wilson et al. 1996) and counting procedures and taxonomic references followed Douglas and Smol (1993). The focus of this paper is a detailed analysis on the uppermost 5 cm of sediment.

Results and discussion Our diatom data show a subtle but clear response to the recent declines in nutrient levels measured in Meretta Lake over the last decade (Figure 1). In the most recent sediments, we recorded a shift in the fossil diatom assemblages to a composition that is returning to the pre-impact assemblage (Douglas and Smol 2000). To emphasize that a recovery has indeed occurred, and to further support Douglas and Smol’s (2000) finding that Meretta Lake’s pre-impact diatom assemblage was relatively stable and overwhelmingly dominated by Fragilaria pinnata (and its varieties), we identified and enumerated fossil diatoms from the 9.5–10, 14.5–15, and 19.5–20 cm depths of our 2001 sediment core. Our pre-impact data are in accordance with those of Douglas and Smol (2000), as diatom composition at all of these depths was dominated by the Fragilaria pinnata complex (relative abundances .60%), with lesser amounts of F. construens v. cf. pumila (relative abundances ,10%), and Achnanthes, Amphora, Cymbella, Navicula, and Nitzschia taxa (each with relative abundances ,5%). However, in this present study, we focus only on the upper 5 cm of our sediment core, as our intent was to determine whether the recent sediments (i.e., post-1990s) contained diatom assemblages indicative of Meretta Lake’s return to oligotrophic conditions. The pre-1940s assemblage, dominated by small, benthic Fragilaria sensu lata diatoms (largely F. pinnata and its varieties), changed markedly after eutrophication from the North Base in the late 1940s, with a large relative increase in F. construens v. cf. pumila, along with smaller increases in Nitzschia and Navicula species, with compensatory declines in the original Fragilaria assemblage (Douglas and Smol 2000). From the base of our core at 5 cm depth to between 1.5–2 cm depth, the trends in our fossil diatom assemblages appear identical to the Douglas and Smol (2000) core. The assemblage at the bottom (5 cm depth) of our core is dominated by the F. pinnata complex, which begins to decrease as F. construens v. cf. pumila increases until it peaks between the 1–2 cm depth (Figure 1). The similarities

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Figure 1. Fossil diatom assemblages from the Meretta Lake sediment core collected in July 2001. The dotted line shows the point of coincidence with the diatom record from the core collected by Douglas and Smol (2000) in 1993. The diatom assemblages above the dotted line reflect the reduced nutrient inputs into Meretta Lake during the 1990s, and represent the beginning of a return to pre-impact assemblages. The diatom assemblages identified and enumerated from the 9.5–10, 14.5–15, and 19.5–20 cm depths of this sediment core (not shown in figure) corresponded with the pre-impact assemblages of Douglas and Smol (2000), and were dominated by the Fragilaria pinnata complex (relative abundances .60%), with lesser amounts of F. construens v. cf. pumila (relative abundances ,10%), and Achnanthes, Amphora, Cymbella, Navicula, and Nitzschia taxa (relative abundances ,5%). The most common taxa (i.e., present in greater than 0.5% relative abundance at any one interval) comprising the genera groupings of Achnanthes, Amphora, Cymbella, Navicula, and Nitzschia were ¨ A. subatomoides (Hustedt) Lange-Bertalot, A. ziegleri Achnanthes curtissima Carter, A. grana Hohn & Hellermann, A. minutissima Kutzing, ¨ Lange-Bertalot, Amphora inariensis Krammer, A. libyca Ehrenberg, A. pediculus Kutzing (Grunow), Cymbella reichardtii Krammer, C. ¨ N. jaernfeltii Hustedt, N. silesiaca Bleisch, Navicula absoluta Hustedt, N. cf. minusculus, N. cf. schoendfeldii, N. crytpocephala Kutzing, ¨ ¨ Nitzschia bryophila (Hustedt) Hustedt, N. dissipata (Kutzing) Grunow, N. perminuta (Grunow) modica Hustedt, N. phylletpa Kutzing, Peragallo, N. pura Hustedt.

between our diatom stratigraphy and that of Douglas and Smol (2000) end at that point (i.e., between 1.5–2 cm depth, or above the dotted line in Figure 1), as F. construens v. cf. pumila and, to a lesser extent, Nitzschia and Navicula species begin to decrease near the surface of our core. This most recent shift likely represents the fossil diatom assemblages that were deposited in the post-1990s sediment. Similar to the sediment core assemblages, the periphytic (moss and rock) diatoms, collected annually from 1992–1999, also showed increases in the F. pinnata complex concomitant with declining nutrient concentrations (Michelutti et al. 2002). However, while the sediment core assemblages were dominated

by F. construens v. cf. pumila during periods of higher nutrient concentrations, the periphyton was characterized by high relative abundances (greater than 70%) of Nitzschia taxa (i.e., N. perminuta, N. inconspicua, N. liebertruthii, N. frustulum). In general, nearly all of the taxa identified in the periphyton were also observed in the sediment core samples, indicating that the sediments are reliable integrators of the various habitats in the lake. The differences in diatom composition between the periphyton and sediment core samples suggest that input from other habitats that were not sampled by Michelutti et al. (2002) were also important. The fact that changes in Meretta Lake’s nutrient levels were tracked by some-

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Figure 2. Light micrographs of the common small, benthic Fragilaria sensu lata diatoms recovered from the Meretta Lake sediment core collected in July 2001. Note that all Fragilaria pinnata varieties (a–k) were grouped into a Fragilaria pinnata complex. (a–e) Fragilaria pinnata ‘coarse’. (f–i) Fragilaria pinnata ‘fine’. (j–k) Fragilaria pinnata f. subrotunda Mayer. (l–n) Fragilaria construens (Ehrenberg) Hustedt. (o–p) Fragilaria construens v. venter (Ehrenberg) Grunow. (q–t) Fragilaria construens v. cf. pumila.

what different diatom assemblages in the periphyton and sediment core samples indicates that, even in low-diversity high arctic lakes, there are redundancies in the diatom taxa. Throughout the High Arctic, F. pinnata and its varieties tend to be dominant (often overwhelmingly) in alkaline, oligotrophic sites which maintain extensive ice-cover for much of the year (e.g., Smol (1988), Blake et al. (1992), Douglas et al. (1994), Laing and Smol (2000), Smith (2002)). The dominance of F. pinnata is likely related to the fact that most other taxa are unable to establish large populations under the severe limnological conditions that exist at such high latitudes. As noted by Douglas and Smol (2000), it is likely that the ice and snow conditions present at Meretta Lake overrode some of the effects of the chronic phosphorus-loading, thus preventing the truly

dramatic species shifts recorded in similar eutrophication studies in more temperate regions. The relatively muted diatom response to eutrophication in Meretta Lake involved increases in F. construens v. cf. pumila, and Nitzschia and Navicula species; taxa that attain greater relative abundances with higher nutrient concentrations (Douglas and Smol 2000; Michelutti et al. 2002). The increase in the F. pinnata complex and the concomitant decreases of F. construens v. cf. pumila and Nitzschia and Navicula species near the surface of our core reflect the declining nutrient concentrations, and the return to preimpact assemblages. This study shows that paleolimnological techniques can be used to track very recent environmental changes related to nutrient concentrations, even in high arctic lakes that have relatively slow sedimentation rates.

381 Acknowledgements This work was funded by Natural Sciences and Engineering Research Council (NSERC) grants awarded to JPS and MSVD, as well as an Ontario Graduate Scholarship (OGS) and Northern Studies Training Programme (NSTP) grant awarded to NM. We are especially grateful to the Polar Continental Shelf Project (PCSP) for logistical and financial support. We also thank the hamlet of Resolute Bay for permission to undertake this work. Comments by A.P. Wolfe improved the quality of the manuscript. This is PCSP contribution 04901.

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