The James Bay Lowland host one of the largest wetland complexes in the world in part due to the low permeability of marine sediments that suppress groundwater seepage losses. Dewatering of an open-pit diamond mine in the area has depressurized the regional bedrock aquifer. Bioherms, fractured limestone outcroppings formed from ancient coral reefs that protrude to the peatland surface, lack this mantle of low-permeability sediments and provide a direct connection between the peatland (surficial) and the regional (bedrock) aquifers. Well transects and piezometer nests were installed around seven bioherms in the depressurized zone and one in a non-affected zone (control) to monitor the water table drawdown and change in hydraulic gradients around the bioherms. Water tables in the affected bioherms decreased between 2 and 4 m in the first 4 years of dewatering. The drawdown in the bioherms caused a localized water table drawdown in the peat surrounding the bioherms that extended to approximately 30 m from the edge of the bioherm during a dry period. Under wet conditions, drawdown was similar to that at the control site. Hydraulic gradients in the peat (which typically are very small) increased over the field seasons and in a few locations exceeded 1. These gradients represented significant losses to the local, near bioherm, system as at many of the locations surrounding the bioherms vertical seepage losses ranged between 1 and 4 mm/day, which are similar to the seasonal average evaporative water loss of ~ 3 mm/day. The bioherms are acting as efficient drainage nodes; however, their influence is localized to the peat immediately (~ <30 m) surrounding them. Copyright © 2012 John Wiley & Sons, Ltd.