Lake regime shift from submerged macrophyte to phytoplankton affected phosphorus speciation in sediment and eutrophic state in Caohai Lake, Guizhou, China

To understand the effect of regime shift in Caohai Lake in Yunnan-Guizhou Plateau, SW China from submerged macrophyte dominance to phytoplankton dominance on the specification and distribution of phosphorus and on ecological and environmental states, changes in phosphorus specification in the sediments and water were studied. The form, composition, and distribution of phosphorus in sediment were sampled in July 2020 (before regime shift) and July 2021 (after regime shift) were analyzed. Results reveal that phosphorus content in sediment was lower than that those of Erhai Lake and Dianchi Lake, Yunnan, SW China, on the same plateau, and was lower than those of Taihu Lake, Chaohu Lake, and Poyang Lake in the middle-lower Changjiang (Yangtze) River Plain. Organic phosphorus (Or-P) was the main form (up to 60%), followed by inactive phosphorus (Ina-P), and the active phosphorus (Act-P), the least, which is opposite to those of Taihu Lake and Poyang Lake in the middle-lower Changjiang River Plain in the eastern China. Or-P content was high, indicating a high potential risk of phosphorous release. After the regime shift, the total phosphorus in sediment decreased from 0.87 +/- 0.13 to 0.70 +/- 0.13 g/kg. The proportion of Or-P and Act-P decreased from 68.23% to 65.32% and from 5.35% to 4.69%, respectively. In contrast, the proportion of Ina-P increased from 26.42% to 29.99%. The Moran's I index revealed that the heterogeneity of the spatial distributions of the total phosphorus (S-TP) and Act-P in the sediments before regime shift was significant (P<0.1). However, the heterogeneity of the spatial distributions of S-TP and the various forms of phosphorus after regime shift was not significant (P>0.05). The regime shift aggravated the eutrophication of the lake, the trophic level index (TLI) increased from 48.42 to 54.49 (P<0.01), and the previously mesotrophic lake became a mildly eutrophic lake. The results of this study revealed the impact of regime shift in the lake from submerged macrophyte dominance to phytoplankton dominance on the composition and spatial distribution of phosphorus in sediments and provided a basis for the restoration of eutrophicated and aquatic ecosystem degraded lakes.