The cultural eutrophication of Lac la Biche, Alberta, Canada: a paleoecological study

Abstract

Abstract:A multiproxy paleoecological investigation of Lac la Biche, a large boreal lake in northeastern Alberta, Canada, revealed that the lake was eutrophic before European settlement but has undergone additional cultural eutrophication in the past 30 to 50 years. Annual fluxes to sediments of phosphorus, nitrogen, carbon, and inorganic sediments have increased with time. A declining N-P ratio has increasingly favored nitrogen-fixing cyanobacteria. Increased deposition of microbial pigments and diatom frustules and a recent shift in diatom species also indicate increasing eutrophication. Biogenic silica increased with time, but there is no evidence of a near-surface decline that would indicate silica limitation. Stable isotopes suggest that an increasing proportion of carbon deposited in sediments is of in-lake origin, indicating increased productivity. In the basin nearest the town of Lac La Biche, an increase in d 15 N followed the construction of the sewage treatment plant, but more recently, decreased d 15 N in both basins suggests that nitrogen fixation has become a more important source of nitrogen. Despite documented damage to the fishery of the lake, zooplankton fossils do not show evidence of a strong trophic cascade. The study illustrates the power of a multiproxy approach in obtaining reliable paleolimnological conclusions.Résumé : Une étude paléoécologique avec plusieurs variables de substitution au lac La Biche, un grand lac boréal du nord-est de l'Alberta, Canada, montre que le lac était eutrophe avant l'arrivée des européens, mais qu'il a subi une eutrophisation culturelle supplémentaire au cours des 30 à 50 dernières années. Les apports annuels aux sédiments de phosphore, d'azote, de carbone et de sédiments inorganiques se sont accrus au cours des années. Un rapport N-P en dé-clin a de plus en plus favorisé les cyanobactéries fixatrices d'azote. Les dépôts croissants de pigments microbiens et de frustules de diatomées, de même qu'un changement récent dans les espèces de diatomées, indiquent une intensification de l'eutrophisation. Il y a une augmentation dans le temps de la silice biogène, mais aucune indication d'un déclin près de la surface qui pourrait signaler une limitation de la silice. Les analyses d'isotopes stables laissent croire qu'une proportion croissante du carbone déposé dans les sédiments provient du lac lui-même, ce qui indique une productivité accrue. Dans le bassin versant le plus près de la ville de Lac-La-Biche, une augmentation de d 15 N a suivi la construction d'une usine de traitement des eaux usées; plus récemment, une réduction de d 15 N dans les deux bassins fait penser que la fixation d'azote est devenue une source plus importante d'azote. Malgré les données sur les dommages causés aux pêches dans le lac, les fossiles du zooplancton n'appuient pas l'existence d'une importante cascade trophique. Notre étude est un exemple du potentiel des méthodes à nombreuses variables de substitution pour l'obtention de conclusions paléolimnologiques fiables.[Traduit par la Rédaction]

Blooming algae: a Canadian perspective on the rise of toxic cyanobacteria

“…Many small lakes in the region are naturally eutrophic, but several have experienced rapid cultural eutrophication since the mid-20th century, and there are indications of increasing cyanobacterial dominance based largely on the paleolimnological record, as historical records are lacking (Schindler et al 2008).…”

Contrasting responses of dimictic and polymictic lakes to environmental change: a spatial and temporal study

“…More locally, the few century-scale paleolimnological records from Alberta, which have all come from polymictic lakes, have shown enhanced eutrophication trends since *CE 1960, but these were all sites where urbanization has provided a substantial proportion of the nutrient signal (Blais et al 2000;Schindler et al 2008). In a neighboring study region (Saskatchewan), a series of paleolimnological studies did report a significant interaction between waterquality indicators and agriculture in a series of polymictic lakes (Hall et al 1999;Dixit et al 2000;Quinlan et al 2002), but climate was a much stronger predictor of chironomid community changes (i.e.…”

“…Given that nutrient-rich soils and low-lying topographies are characteristic of this region, many authors have concluded that the trophic status of these lakes is strongly influenced by the natural landscape (Blakney 1998;Blais et al 2000;McEachern andCharette 2003/2004;Devito et al 2005;Donahue 2006;Schindler et al 2008). Several studies in Alberta and in a neighboring nutrient-rich region (Saskatchewan), however, have shown that lakes were responsive to agriculture and urbanization (Hall et al 1999;Dixit et al 2000;Blais et al 2000;Quinlan et al 2002;Schindler et al 2008). Thus, we set out to evaluate whether agricultural development has had a detectable effect on the water quality of lakes with naturally high levels of nutrient loading from the catchment.…”

Fisheries restoration potential for a large lake ecosystem: using ecosystem models to examine dynamic relationships between walleye, cormorant, and perch

“…At Lac la Biche, changes to ecosystem conditions through the 20th century were numerous. Fish populations were heavily harvested, walleye were extirpated, major fish kills occurred in 1946 and 1965, the watershed was cleared for agriculture, treated sewage discharge into the lake began, and settlement around the lake increased (Champagne 1992, Schindler et al 2008. Any one of these activities could have altered important ecosystem parameters, thereby eroding system resilience (Folke et al 2004) and changing the system's potential for ASSs.…”