J. Great Lakes Res. 31:253–266 Internat. Assoc. Great Lakes Res., 2005 Spatial Variability in Water Quality and Surface Sediment Diatom Assemblages in a Complex Lake Basin: Lake of the Woods, Ontario, Canada Sergi Pla 1 , Andrew M. Paterson 2,* , John P. Smol 1 , Bev J. Clark 2 , and Ron Ingram 2 1 Paleoecological Environmental Assessment and Research Laboratory (PEARL) Department of Biology Queen’s University Kingston, Ontario K7L 3N6 2 Ontario Ministry of the Environment Dorset Environmental Science Centre 1026 Bellwood Acres Road Dorset, Ontario P0A 1E0 ABSTRACT. Lake of the Woods (LOW) is an international waterbody spanning the Canadian provinces of Ontario and Manitoba, and the U.S. state of Minnesota. In recent years, there has been a perception that water quality has deteriorated in northern regions of the lake, with an increase in the fre- quency and intensity of toxin-producing cyanobacterial blooms. However, given the lack of long-term data these trends are difficult to verify. As a first step, we examine spatial and seasonal patterns in water quality in this highly complex lake on the Canadian Shield. Further, we examine surface sediment diatom assemblages across multiple sites to determine if they track within-lake differences in environmental con- ditions. Our results show that there are significant spatial patterns in water quality in LOW. Principal Component Analysis divides the lake into three geographic zones based primarily on algal nutrients (i.e., total phosphorus, TP), with the highest concentrations at sites proximal to Rainy River. This variation is closely tracked by sedimentary diatom assemblages, with [TP] explaining 43% of the variation in diatom assemblages across sites. The close correlation between water quality and the surface sediment diatom record indicate that paleoecological models could be used to provide data on the relative importance of natural and anthropogenic sources of nutrients to the lake. INDEX WORDS: Water quality, diatoms, total phosphorus, spatial patterns, Lake of the Woods. INTRODUCTION Lake of the Woods (LOW) is a waterbody of in- ternational importance, spanning the Canadian provinces of Ontario and Manitoba, and the state of Minnesota, USA. In addition, Shoal Lake, which is hydrologically linked to LOW, is a drinking water reservoir for the city of Winnipeg, Manitoba. Al- though algal blooms have been reported in the southern portion of the lake since the early 1800s (McElroy and Riggs 1943), anecdotal evidence sug- gests that water quality has deteriorated in the northern bays in recent years. Furthermore, algal identifications in the late-1990s found large popula- * Corresponding author. E-mail: andrew.paterson@ene.gov.on.ca 253 tions of toxin-producing cyanobacteria at several locations in the lake (H. Kling and C. Herbert, un- published data). In the absence of historical data, however, increases in bloom frequency and inten- sity are difficult to verify. Furthermore, data de- scribing the relative importance of natural (e.g., geological) and anthropogenic sources of algal nu- trients are scarce, and thus the relative sources of nutrients to the lake are not fully understood. Due to the size and hydrological complexity of LOW (Fig. 1), the water quality is highly variable and difficult to monitor. Consequently, few studies have quantified seasonal or spatial patterns in water chemistry or biological assemblages across the lake. Recent measurements of nutrient concentra- tions and water transparency indicate that there is a