Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

Summary Published research suggests that the total phosphorus ( TP ) thresholds for the regime shifts between a clear‐water state dominated by submersed macrophytes and a turbid‐water state dominated by phytoplankton in shallow lakes vary with forms of lake basins and climates. However, such hypotheses remain untested by direct field evidence. We therefore tested the hypotheses with empirical data from subtropical lakes on the Y angtze floodplain and also from other lakes in temperate to tropical zones. TP thresholds were found to vary little at moderate depths, but to decrease notably when depth exceeds a level of probably 3–4 m, and increase sharply when depth is below a level of around 1–2 m. TP thresholds were found to be nearly equal in shallow lakes (1–2 m <mean depth <3–4 m; c . 0.1 km 2 <area <at least 350 km 2 ) from temperate to subtropical (probably to tropical) zones, being 80–120 mg m −3 for the forward shift from a clear‐water state to a turbid‐water state and 40–60 mg m −3 for the backward shift. The threshold of turbidity for the forward shift was found to be higher than that for the backward shift, amending the previous hypothesis of the equality of turbidity thresholds for both shifts. Our findings suggest that according to the subequality of TP thresholds, similar target concentrations for in‐lake TP can be set in most shallow lakes world‐wide to mitigate eutrophication.