Frequent regime shifts in trophic states in shallow lakes on the Boreal Plain: Alternative "unstable" states?

Abstract

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.

Does the responses of Vallisneria natans (Lour.) Hara to high nitrogen loading differ between the summer high-growth season and the low-growth season?

“…) suggested to entail high risk of macrophyte loss in Yangtze shallow lakes (Wang et al, 2014) and the level (0.018 mg L −1 ) suggested as a threshold for a dominance shift from macrophytes to phytoplankton in Canadian lakes (Bayley et al, 2007). The relatively minor effect of nitrogen may be due to a weak development of periphyton compared with other studies showing a strong positive effect of nitrogen on periphyton growth (Özkan et al, 2010) and an associated decline in macrophyte growth (Olsen et al, 2015;Zhao et al, 2016).…”

Local environment overrides regional climate influence on regime shift in a north temperate lake

“…At a local scale, the processes determining lake biological regimes include both top-down and bottomup interactions, and thus, it is crucial to examine the relative roles of different factors at different levels (Bayley et al 2007;Brönmark et al 2010;Hargeby et al 2007;Scheffer and Jeppesen 2007;Xu et al 2012). In terrestrial grassland ecosystems, local food web interactions have been shown to override temperature effects (Suttle et al 2007), and similarly, it has been experimentally demonstrated that changes in the local food web composition may alter the response to climate change (Hansson et al 2013).…”

Towards a more precisely defined macrophyte-dominated regime: the recent history of a shallow lake in Eastern Poland

“…In lakes where submerged plants cover almost the entire bottom surface, however, as is the case for Lake Kleszczów, it may be several times higher (Bayley et al, 2007). The threshold value is influenced not only by the vegetation cover, its composition, and seasonal dynamics of biomass, but also by the structure of ichthyofauna and contribution of planktivorous and piscivorous fish to the strength of top-down interactions.…”

“…The year-round persistence of the submerged vegetation must have been of high importance for the lake with retention of nutrients in the plant biomass, even in winter, restricting the development of phytoplankton (Scheffer, 1998). Moreover, the continuous presence of macrophytes reduces the risk of e.g., cold conditions delaying the growth of submerged macrophytes in early spring, which could favour algal development (Blindow et al, 1993;Bayley et al, 2007;Sayer et al, 2010b;Hilt et al, 2013). Year-round vegetation provides rich resources of epiphytic fauna as food for fish, reducing their pressure on zooplankton (Diehl & Kornijów, 1998;Kornijów et al, 2005Kornijów et al, , 2016.…”

“…The lake water showed relatively low electrical conductivity, suggesting low concentration of dissolved mineral substances. The concentrations of nutrients, nitrogen and phosphorus, and chlorophyll a were generally low, indicating mesotrophic conditions typical of lakes dominated by macrophytes (Zimmer et al, 2003;Bayley et al, 2007).…”