Macrophyte species strongly affects changes in C, N, and P stocks in shallow lakes after a regime shift from macrophyte to phytoplankton dominance

Shallow lakes are important stocks of carbon (C), nitrogen (N), and phosphorus (P), yet little is known about the influence of alternative primary producer dominance on C stocks or the impact of different macrophyte species on the role of shallow lakes as elemental stocks. We used Yangtze shallow lakes dominated by a monsoon climate as a research site to test the hypothesis that changes in elemental stocks in the water column and sediment after a shift to a phytoplankton-dominated state depend on the macrophyte species originally present. We used a dual approach, combining multi-year monitoring and multi-lake comparisons of lakes that were, at least once, dominated either by fast-decomposing Potamogeton crispus or slow-decomposing P. maackianus. Elemental concentrations generally decreased in the water column and increased in sediment after a shift from P. maackianus presence to absence. Only a minor reallocation of elemental stocks was found in lakes where P. crispus disappeared. This difference is likely caused by a combination of the different biomass and decomposition rates between species, further illustrated by the amount of dead plant material in the sediment after loss of plants. After P. maackianus loss, plant material was found in the sediment in high amounts for up to 6 years, whereas after P. crispus loss the coarse material was absent in <1 year. Suspended and dissolved concentrations (i.e., the mobile pool) of C increased 1.5–1.9-fold and P increased 2.0–4.3-fold after the shift, whereas N tended to decrease or stay unchanged. Higher mobile pools of C and P after macrophytes loss implies a more vulnerable watershed, supporting higher phytoplankton biomass in the lakes and causing serious downstream eutrophication problems