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The allocation of limiting elements among plant organs is an important aspect of the adaptation of plants to their ambient environment. Although eutrophication can extremely alter light and nutrient availability, little is known about nutrient partitioning among organs of submerged macrophytes in response to eutrophication. Here, we analyzed the stoichiometric scaling of carbon (C), nitrogen (N), and phosphorus (P) concentrations among organs (leaf, stem, and root) of 327 individuals of seven common submerged macrophytes (three growth forms), sampled from 26 Yangtze plain lakes whose nutrient levels differed. Scaling exponents of stem nutrients to leaf (or root) nutrients varied among the growth forms. With increasing water total N (WTN) concentration, the scaling exponents of stem C to leaf (or root) C increased from <1 to >1, however, those of stem P to root P showed the opposite trend. These results indicated that, as plant nutrient content increased, plants growing in low WTN concentration accumulated leaf C (or stem P) at a faster rate, whereas those in high WTN concentration showed a faster increase in their stem C (or root P). Additionally, the scaling exponents of stem N to leaf (or root) N and stem P to leaf P were consistently large than 1, but decreased with a greater WTN concentration. This suggested that plants invested more N and P into stem than leaf tissues, with a higher investment of N in stem than root tissues, but eutrophication would decrease the allocation of N and P to stem. Such shifts in plant nutrient allocation strategies from low to high WTN concentration may be attributed to changed light and nutrient availability. In summary, eutrophication would alter nutrient allocation strategies of submerged macrophytes, which may influence their community structures by enhancing the competitive ability of some species in the process of eutrophication.

Freshwater lakes across the world have undergone dramatic changes in biological components and water quality over the past several decades. Previous studies focused on potential drivers mainly on nutrient enrichment in the catchment. However, the relative importance of climate change and the top-down cascade effects of fish stocking on water quality is not fully understood. Here, by compiling 155 lakes data with four periods of field investigation in subtropical lakes in China, we found no significant changes in water total nitrogen and total phosphorus in the past two decades. However, the phytoplankton abundance increased significantly, and the water clarity declined by 44.1%. We further found that carp stocking and climate change are potentially more important than nutrients driving water quality change, which is also evidenced in two lakes (Lake Donghu and Qiandao) with long-term monitoring histories. Specifically, carp stocking can decrease the water clarity directly by stirring up sediment and indirectly by trophic cascade along the food web. For climatic factors, mean annual temperature (MAT) has a positive effect on phytoplankton abundance, while mean annual precipitation has a negative one, with climates overall having little effect on water clarity. In addition, nutrient enrichment and climate change also have strong interactions with carp stocking, which may enhance the top-down effects on water quality. Our findings highlight that either MAT or carp stocking may become an overwhelming driver of water clarity decline, which provides new insights into the conservation strategy for water quality management in the subtropical lakes in China.The cluster of subtropical lakes in the Yangtze River floodplain is one of the largest groups of shallow lakes in East Asia (Wang and Dou 1998). These lakes are among the most threatened ecosystems in China due to long-term intense anthropogenic pressure and water quality degradation. More than 40% of lakes in this region were in a eutrophic state. Previous studies have assessed the long-term changes in water quality across the floodplain lakes indirectly by reconstructing paleolimnological records (Xu et al. 2017;Zhang et al. 2019) and remote sensing archives (Feng et al. 2019a,b;Hou et al. 2020). However, to date, direct long-term data from large-scale spatial investigation is scarce.Water clarity is a fundamental characteristic of lakes and provides general information on water quality. It reflects how

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Ecosystem regime shifts attract a growing concern because irreversible nonlinear, abrupt changes can significantly alter the services that the ecosystem offers to human society. Regime shifts between contrasting alternative states occur when environmental conditions cross a specific threshold, with ecological resilience decreasing as the system approaches the tipping point (Ma et al., 2021). Therefore, measuring and quantifying ecological resilience is critical to improving ecosystem management and conservation. Previous assessments of ecological resilience often focused on identifying thresholds or early warning indicators based on critical slowing down (Contamin & Ellison, 2009;Dakos et al., 2015). However, these methods do not include the specific mechanisms of regime shifts. Positive feedback is the key to understanding the pattern of ecosystem response to environmental changes, with high feedback strength creating