Sediment and nutrient distribution and resuspension in Lake Winnipeg

Severe algal blooms in Lake Winnipeg since the late 1990s have been attributed to increased watershed nutrient loading, much of which is associated with suspended particles. Within-lake transport and fate of this nutrient fraction and the importance of internal loading via resuspension, however, are unknown. We measured radioisotopes (⁷Be, ²¹⁰Pb, ¹³⁷Cs), metal and nutrient contents of suspended solids in major tributaries and lake-water, in sediment traps and in bottom sediments to estimate sediment resuspension and mass accumulation rates using two models. Sedimentation rates calculated from ¹³⁷Cs and ²¹⁰Pb dated cores and sediment traps indicated that most (95-99%) suspended material is derived from bottom sediment; mixing models using⁷Be/²¹⁰Pb and ¹³⁷Cs yielded similarly high estimates (82 and 84%, respectively). ¹³⁷Cs profiles in cores indicated that up to ~7cm remains actively resuspended for times up to 23years before incorporation into deeper sediments. Total and bioavailable phosphorus (TP, BAP) in this top sediment layer were generally lower in the North than the South Basin, likely reflecting inputs from the Assiniboine and Red Rivers at the southern end of Lake Winnipeg, with an average of ~30% TP as BAP. Estimates of average sediment-associated internal TP loading for the South Basin (0.264g/m²/y) were ~2× those for the North Basin (0.146g/m²/y). Together, this internal loading is comparable to the magnitude of the external loading. Our results indicate that surficial sediments in Lake Winnipeg will remain a significant and active source of internal nutrient loading for several decades, a process which may delay the response of the lake to external nutrient management.