In marine investigations, the maximum chlorophyll-a (Chla) concentration is often reported to occur at a specific depth below the ocean surface, a phenomenon known as subsurface Chla maxima (SCM). However, SCM has long been overlooked in artificial reservoirs, which may lead to a serious underestimation of the primary productivity level and trophic status of reservoirs. To better understand the temporal and spatial variability of SCM and the mechanisms leading to SCM development, this study conducted a detailed survey in a large subtropical reservoir (Xinanjiang Reservoir, XAJR) from September 2020 to August 2021. The seasonal thermal stratification, in situ variables (WT, pH, DO and Chla), nutrient concentrations (DSi, NO₃⁻, DIP and DCO₂), Chla maxima depth and magnitude of the riverine region (S1), transition region (S2) and the central part of the XAJR (S3 and S4) were all thoroughly investigated. Thermal stratification and SCM in XAJR exhibited significant seasonal and spatial heterogeneity. Phytoplankton biomass in the epilimnion was limited by dissolved CO₂ from June to October in the warm seasons, while it was primarily limited by phosphorus in the other seasons, according to the nutrient limitation analysis. Along the water column, dissolved CO₂ limitation occurred mainly above the SCM layer, and the water column below the SCM layer gradually transitioned from dissolved CO₂ limitation to phosphorus limitation. Furthermore, as the thermal stratification developed, the upstream water mass moves along the middle of the water column as density flow toward the reservoir, providing nutrients for the development of the SCM. This research contributes to a better understanding of the temporal and spatial variation of SCM and nutrient supply in deep and large stratified reservoirs.

在海洋调查中，最大叶绿素-a (Chla) 浓度经常被报道发生在海洋表面以下的特定深度，这种现象被称为次表层 Chla maxima (SCM)。然而，长期以来，人工水库中的SCM一直被忽视，这可能导致对水库初级生产力水平和营养状况的严重低估。为更好地了解 SCM 的时空变异性和 SCM 发展机制，本研究于 2020 年 9 月至 2021 年 8 月对一个大型副热带水库（新安江水库，XAJR）进行了详细调查。季节热分层、原位变量（WT、pH、DO 和 Chla）、营养浓度（DSi、NO ₃ ^-、DIP 和 DCO ₂)、河流区域 (S1)、过渡区域 (S2) 和 XAJR 中部 (S3 和 S4) 的最大叶绿体深度和幅度都进行了深入调查。XAJR中的热分层和SCM表现出显着的季节和空间异质性。根据营养限制分析，浮游植物生物量在温暖季节的 6 月至 10 月受到溶解 CO ₂的限制，而在其他季节则主要受到磷的限制。沿水柱，溶解CO ₂限制主要发生在SCM层上方，SCM层下方水柱逐渐由溶解CO _2过渡限制磷限制。此外，随着热分层的发展，上游水团沿水柱中部移动，随着密度流向储层，为SCM的发育提供养分。该研究有助于更好地了解深层和大型分层水库中SCM和养分供应的时空变化。