Phosphorus geochemistry of recent sediments in the South Basin of Lake Winnipeg

Abstract

Lake Winnipeg supports the largest commercial fishery on Canadian Prairies. It has been influenced by a variety of environmental forces and anthropogenic activities. To gain a better understanding of recent changes in nutrient status of the lake, it is important to reconstruct its previous history from sedimentary records. Lacustrine sediments are known to be an important sink of many dissolved and suspended substances, including phosphorus, hence, they provide a permanent historical record of changes occurring in the lake. These changes may be induced by natural factors or by anthropogenic activities in the watershed. Phosphorus profiles from dated sediment cores collected in 1999 and 1994 from the South Basin of Lake Winnipeg were investigated to determine phosphorus enrichment in recent sediments. To interpret the nutrient status and depositional conditions responsible for the trends in total phosphorus, three operationally defined forms of phosphorus (P) were determined: non-apatite inorganic P, apatite P, and organic P. Significant increases in sediment phosphorus concentrations were observed in the uppermost 20 cm of the cores and several anomalies were observed at depth. A doubling in total phosphorus relative to aluminum over the last fifty years is largely due to increases in the non-apatite inorganic fraction, suggesting that much of sedimentary phosphorus increase is attributable to changes in the nutrient status of the water column related to anthropogenic inputs. Organic phosphorus exhibits a subtle increase in the upper 20 cm of the gravity cores, likely due to increases in the primary productivity of the lake. Except for the slight increase in deeper sediments, apatite phosphorus, which is thought to be of detrital origin, remained fairly constant over the length of the cores. Anomalous spikes in phosphorus concentrations deeper in the cores, comprised mainly of the non-apatite inorganic phosphorus fraction, likely resulted from natural variation in local oxidizing conditions, possibly induced by changes in water circulation and/or changes in sediment deposition rates due to climatic variation. The results of this investigation contribute to increased understanding of the depositional history of phosphorus in the lake over the last millennium.

Abundance, growth, and life history characteristics of sympatric walleye (Sander vitreus) and sauger (Sander canadensis) in Lake Winnipeg, Manitoba

“…Both the temporal and spatial patterns of increasing growth rate could be the result of changes in nutrient inputs, exploitation, temperature (and hence, growing season), food web structure, or some combination of these factors, although it is not readily apparent why these factors might differentially affect these two species. The south basin of Lake Winnipeg generally receives more nutrientrich inflows than other regions of the lake (Brunskill et al, 1980), and sediment analyses have demonstrated a temporal increase in nutrient inputs in recent decades (Mayer et al, 2006). Higher nutrient inputs could provide a more productive food web for the percid community and may generate faster growth.…”

“…Minimum allowed mesh sizes have declined over time throughout Lake Winnipeg but have remained larger in the north basin where lake whitefish are the primary target and smaller in the channel and south basin where percids are the main target species. In addition to the effects of exploitation, the Lake Winnipeg fish community has experienced considerable environmental change through regulation of both inflowing tributaries and lake water levels (LWCNR, 1975), introductions of exotic species such as carp (Cyprinus carpio), white bass (Morone chrysops), and rainbow smelt (Osmerus mordax) (Stewart and Watkinson, 2004), and eutrophication from increased nutrient inputs (Mayer et al, 2006).…”

An isotopic baseline (δ13C, δ15N) for fishes of Lake Winnipeg: Implications for investigating impacts of eutrophication and invasive species

“…The result has been a trend towards cultural eutrophication as evidenced by extensive algal blooms in recent years (LWSB, 2005). The Red River watershed system is the largest nitrogen and phosphorous source to Lake Winnipeg, contributing more of these nutrients than all other rivers that lead to it combined (Mayer et al, 2006;Patalas and Salki, 1992). Exploitation of Lake Winnipeg for hydroelectric purposes since the 1960s via control of the Nelson River outflow has hindered the lake's ability to discharge nutrients and other anthropogenic compounds during spring freshets, thereby increasing nutrient in-lake retention times and possibly accelerating the eutrophication process .…”

Reproductive allocation in exploited lake whitefish (Coregonus clupeaformis) and walleye (Sander vitreus) populations

“…The period from the late 1980s to the early 2000s was characterized by very different types of change in these two ecosystems. In Lake Winnipeg, productivity was probably increasing in response to increasing nutrient inputs (Mayer et al, 2006), and the walleye population appears to have responded through increased abundance and growth and decreased age at maturity (Johnston et al, in press). In contrast, in eastern Lake Ontario a rapid increase in abundance of invasive dreissenid mussels in the early 1990s was followed by declines in both walleye and lake whitefish abundance (Hoyle et al, 2008).…”