Do logging and forest fires influence zooplankton biomass in Canadian Boreal Shield lakes? A. Patoine, B. Pinel-Alloul, E.E. Prepas, and R. Carignan Abstract: Zooplankton biomass was assessed in 20 reference lakes, nine logged-watershed lakes, and nine burned- watershed lakes during three summers following watershed disturbances by logging or wildfires. Biomass of cladocerans, calanoids, cyclopoids, and rotifers was quantified in the 38 lakes for the first year following disturbances. Limnoplankton biomass in four size fractions was quantified during 3 years following disturbances. One year after disturbances, burned-watershed lakes supported 59% more biomass of the rotifer size fraction of limnoplankton (100–200 mm) than reference lakes, while logged-watershed lakes supported 43% less of calanoid biomass. Two years after disturbances, differences in limnoplankton biomass between burned-watershed lakes and reference lakes were more pronounced than during the first year, while logged-watershed lakes supported levels of limnoplankton biomass no different from those of reference lakes. Three years after disturbances, no significant variations could be detected among the three groups of lakes for any of the limnoplankton size fractions. The proportion of watershed area im- pacted by logging activities was on average less than half the proportion impacted by wildfires. Nonetheless, both types of disturbances seemed to have opposite effects on the zooplankton biomass during the first year, and the effects did not extend beyond 2 years. Résumé : La biomasse du zooplancton a été mesurée dans 20 lacs aux bassins versants intacts, neuf lacs aux bassins versants déboisés par la coupe forestière (lacs C) et neuf lacs aux bassins versants brûlés par des feux de forêt naturels (lacs F), et ce, pendant les trois étés qui ont suivi les perturbations de bassins versants. Pendant la première année après perturbations, la biomasse de limnoplancton comprise entre 100 et 200 mm, composée principalement de rotifères, était en moyenne 59% plus élevée dans les lacs F que dans les lacs de référence, tandis que la biomasse des calanoïdes était 43% plus faible dans les lacs C. Pendant la deuxième année, les différences de biomasse de limnoplancton entre lacs F et lacs de référence étaient encore plus élevées comparées à la première année, tandis que les lacs C montraient des biomasses équivalentes à celles des lacs de référence. Pendant la troisième année, aucune différence de biomasse n’était détectable entre les trois groupes de lacs. Les effets des feux de forêt sur la biomasse du zooplancton semblent opposés à ceux des coupes forestières pendant la première année et ne se prolongent pas au-delà de la deuxième année. Patoine et al. 164 Introduction Wildfires and logging each remove about 10 000 km 2 of Canada’s 4.5 × 10 6 km 2 forest each year (Forestry Canada 1991). New strategies in forest management are based on the assumption that emulating natural disturbances such as wildfires in logging practices will sustain boreal forest dy- namics and biodiversity (Hunter 1993). Such strategies, however, were developed without considering aquatic eco- systems, even though water represents 8% of Canada’s sur- face area. Comparing the impacts of wildfires and logging on lake water quality and biota is a necessary step towards developing forest management practices that take into account the global impacts of logging on the forest–lake ecosystem. Stream concentrations of particulate organic matter, phos- phorus, nitrogen, and ions can increase following forest fires (Bayley et al. 1992) and clear-cutting (Likens et al. 1970). Following watershed disturbance, the biomass of stream macroinvertebrate communities has been shown to increase in some cases (Burton and Ulrich 1994) but not in others (Minshall et al. 1997). In comparison with the number of studies on streams, little work has addressed the conse- quences of watershed disturbance on lake aquatic communi- ties. In some cases, forest harvesting was associated with increases in chlorophyll a (Chl a) concentrations and pri- mary production (Rask et al. 1993), moderate increases in cladoceran and copepod density (Rask et al. 1998), and de- creases in catch and biomass per unit effort of brook trout (Salvelinus fontinalis) (Bérubé and Lévesque 1995). In other cases, differences between logged-watershed lakes and refer- ence lakes in Chl a, total phosphorus (TP), total nitrogen (TN), and dissolved organic carbon (DOC) concentrations were undetectable (Lehmann 1994). Presently, confounding factors such as the type of disturbance (logging or fire), the time elapsed since the disturbance, and the extent of natural Can. J. Fish. Aquat. Sci. 57(Suppl. 2): 155–164 (2000) © 2000 NRC Canada 155 Received August 10, 1999. Accepted February 10, 2000. J15314 A. Patoine, 1 B. Pinel-Alloul, and R. Carignan. Groupe de Recherche Interuniversitaire en Limnologie et en Environnement Aquatique (GRIL), Département de sciences biologiques, Université de Montréal, C.P. 6128, succursale Centre-ville, Montréal, QC H3C 3J7, Canada. E.E. Prepas. Sustainable Forestry Management Network, Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada. 1 Author to whom all correspondence should be addressed. patoinea@magellan.umontreal.ca