Shallow lakes are known to exhibit alternative states in their biotic structure. Lakes dominated by submersed aquatic vegetation (SAV) are ''clear,'' while lakes dominated by algae (high concentrations of chlorophyll a [Chl a]) are ''turbid.'' The roles of total phosphorus (TP) and SAV in defining these alternative states were examined for up to 6 yr in 24 naturally eutrophic shallow lakes (12.4-670.4 mg TP L 21 ) with variable SAV cover (0-100%) on the western Boreal Plain in Alberta, Canada. Clear lakes had ,18 mg Chl a L 21 . Sixty-seven percent of lakes were clear in any given year, but individual lakes did not remain clear over consecutive years. While 29% did not switch, 71% of lakes were unstable, with 57% switching states once and 14% switching more than once. To increase the temporal and spatial scale of analysis, we used Landsat Thematic Mapper satellite imagery (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) to classify clarity for up to 20 yr in 82 naturally eutrophic shallow lakes. Approximately 80% of lakes were unstable, with 7% switching once and more than 73% switching 2-9 times. Only 20% of lakes were stable and clear. Switches in lake clarity were related to TP but were also dependent on the abundance of SAV. For lakes with high SAV, the TP threshold for the transition from clear to turbid was 275 mg TP L 21 , while for lakes with low SAV, the TP threshold was 50 mg TP L 21 . Given the harsh winter conditions, including lakes with ice depths of up to 0.7 m and anoxia leading to winterkill of aquatic communities, these systems are strongly abiotically regulated and lack mechanisms that maintain a lake in a stable state.

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). Wetlands in the southern Boreal Plains were regionally variable and dynamic. Drought in 2006 caused significant increases in TP, TN, chlorophyll a (chla), conductivity, silica and significant decreases in maximum depth and light penetration. Increased agricultural activity within a 1.6 km buffer surrounding wetlands enhanced nutrients but not chla concentrations or submersed aquatic vegetation (SAV) coverage. Wetlands with higher amounts of agriculture in the buffer zone tended to be shallower with decreased secchi depths. Due to shallow depths, SAV thrived even under hypereutrophic conditions with high agricultural encroachment in the buffer zone. Our study suggests that SAV has a significant role in maintaining clear water states in Boreal Transition Zone wetlands and shallow lakes through its ability to suppress concentrations of chla and TP.

Summary We examined how lake and landscape factors affected the persistence of alternative regimes in 23 predominantly fishless, shallow lake ecosystems over seven years in the Boreal Plains, Alberta, Canada. The alternative regimes were dominance by SAV (submerged aquatic vegetation) or by phytoplankton. The relationship between alternative regime persistence and a suite of lake and landscape variables were determined using regression tree analysis. We also compared the environmental variables and biological communities among alternative regimes using Kruskal–Wallis nonparametric tests and post hoc tests. Persistence of the SAV‐rich regime was best predicted by water level stability. Switches from a SAV‐rich regime to a phytoplankton‐rich regime have been attributed to drought, which concentrates total phosphorus (TP) and other solutes and increases turbidity, favouring phytoplankton dominance. Lakes with stable water levels are inferred as less sensitive to drought, and presumably experience less evapoconcentration during drought, which helps maintain the SAV‐rich regime. Lakes with more water level drawdown may also delay or inhibit SAV growth through physical disturbance. In contrast, the persistence of the phytoplankton‐rich regime was highest in lakes with higher TP and a lower proportion of surrounding wetlands. We postulate that lakes with less wetland in the buffer are more responsive to variations in precipitation because these systems have a smaller water storage capacity and lack peatlands that otherwise attenuate the effects of drought via shallow groundwater inputs. The relative persistence of lakes in the study area was related to lake and landscape factors that affect the relative sensitivity of the lakes to drought conditions. The biological community in these predominantly fishless lakes appears to play a weak role in maintaining the alternative regimes compared to fish‐bearing lake systems.

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