Do logging and forest fires influence zooplankton biomass in Canadian Boreal Shield lakes?

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. Resume : La biomasse du zooplancton a ete mesuree dans 20 lacs aux bassins versants intacts, neuf lacs aux bassins versants deboises par la coupe forestiere (lacs C) et neuf lacs aux bassins versants brules par des feux de foret naturels (lacs F), et ce, pendant les trois etes qui ont suivi les perturbations de bassins versants. Pendant la premiere annee apres perturbations, la biomasse de limnoplancton comprise entre 100 et 200 mm, composee principalement de rotiferes, etait en moyenne 59% plus elevee dans les lacs F que dans les lacs de reference, tandis que la biomasse des calanoides etait 43% plus faible dans les lacs C. Pendant la deuxieme annee, les differences de biomasse de limnoplancton entre lacs F et lacs de reference etaient encore plus elevees comparees a la premiere annee, tandis que les lacs C montraient des biomasses equivalentes a celles des lacs de reference. Pendant la troisieme annee, aucune difference de biomasse n'etait detectable entre les trois groupes de lacs. Les effets des feux de foret sur la biomasse du zooplancton semblent opposes a ceux des coupes forestieres pendant la premiere annee et ne se prolongent pas au-dela de la deuxieme annee.