Primary production and environmental variables from 1971 to 2021 in the North Basin of Lake Biwa

Abstract: Global climate change and anthropogenic oligotrophication are expected to reshape the dynamics of primary production (PP) in aquatic ecosystems however, few studies have explored their long-term effects. In theory, the PP of phytoplankton in Lake Biwa may decline over decades due to warming, heightened stratification, and anthropogenic oligotrophication. Furthermore, the PP of large phytoplankton, which are inedible to zooplankton, along with biomass-specific productivity (PBc), could decrease. In this study, data from 1976–2021 and active fluorometry measurements taken in 2020 and 2021 were evaluated. Quantitatively, the temporal dynamics of mean seasonal PP during 1971–2021 were assessed according to the carbon fixation rate to investigate relationships among environmental factors. Qualitatively, phytoplankton biomass, PP, and PBc were measured in two size fractions [edible (S) or inedible (L) for zooplankton] in 2020 and 2021, and the L:S balance for these three measures was compared between 1992 (low-temperature/high-nutrient conditions) and 2020–2021 (high-temperature/low-nutrient conditions) to assess seasonal dynamics. The results indicated that climate change and anthropogenic oligotrophication over the past 50 years have diminished Lake Biwa's PP since the 1990s, impacting the phenology of PP dynamics. However, the L:S balance in PP and PBc has exhibited minimal change since 1992. These findings suggest that, although climate change and oligotrophication may reduce overall PP, they do not markedly alter the inedible/edible phytoplankton balance in terms of PP and PBc. Instead, as total PP declines, the production of small edible phytoplankton decreases proportionally, potentially affecting trophic transfer efficiency and material cycling in Lake Biwa.摘要：全球气候变化与人为贫营养化预计将重塑水生生态系统的初级生产力（Primary Production, PP）动态，但目前鲜有研究探讨其长期影响。理论上，受气候变暖、水体分层加剧以及人为贫营养化影响，琵琶湖（Lake Biwa）的浮游植物（Phytoplankton）初级生产力可能在数十年间出现下降。此外，对浮游动物（Zooplankton）不可食用的大型浮游植物，及其生物量比生产力（Biomass-specific Productivity, PBc）也可能出现降低。本研究评估了1976年至2021年的观测数据，以及2020年与2021年的主动荧光法（active fluorometry）测量结果。在定量层面，本研究以碳固定速率为依据，分析了1971年至2021年的平均季节性初级生产力时间动态，以探究其与环境因子间的关联；在定性层面，研究于2020年与2021年针对两个粒径分级的浮游植物生物量、初级生产力及生物量比生产力进行测定，即分为可被浮游动物食用的小型组分（S组）与不可食用的大型组分（L组），并对比了1992年（低温/高营养盐条件）与2020-2021年（高温/低营养盐条件）下上述三项指标的L:S平衡情况，以分析其季节动态。研究结果显示，过去50年间的气候变化与人为贫营养化自20世纪90年代起已导致琵琶湖的初级生产力出现衰减，并对初级生产力动态的物候特征产生了影响；然而，自1992年以来，初级生产力与生物量比生产力的L:S平衡并未出现显著变化。上述发现表明：尽管气候变化与贫营养化可能会降低整体初级生产力，但并未显著改变初级生产力与生物量比生产力维度下不可食用/可食用浮游植物的平衡比例。相反，随着总初级生产力下降，小型可食用浮游植物的产量出现比例性缩减，这可能会对琵琶湖的营养传递效率与物质循环产生影响。