High discharge intensified low net ecosystem productivity, hypoxia, and acidification at three outlets of the Pearl River Estuary, China

Clarifying the influence of hydrological variations on ecological function is a topic of considerable interest in watershed ecological flow assessment and water resource management. Net ecosystem productivity (NEP) is a useful composite indicator of ecosystem function, reflecting material cycling and energy flow. However, the effects of hydrological variations on NEP, especially the influence mechanism, remain unclear due to the complex environmental characteristics in estuaries. We analysed the spatial–temporal variability of the aquatic environment and NEP through in-situ monitoring and field sampling from December 2018 to January 2020 at three outlets (Humen, Jiaomen, and Hongqimen) with different hydrological conditions in the Pearl River Estuary (PRE), China, and explored the influence mechanism of hydrological variation on NEP. The 155 groups of effective metabolism values were estimated using Odum's open-water method. The daily ecosystem respiration (ER) was higher than the gross primary production (GPP); therefore, water bodies were dominated by net heterotrophy at the three outlets. The daily NEP (–4.34 ± 1.40 mg O₂ L–¹d–¹), O₂ concentration (5.2 ± 1.02 mg L–¹), and pH (7.53±0.24) were lowest at Humen, which also had the largest discharge and tide volume, deepest water depth, and widest channel. Seasonally, the NEP in the summer (–3.30 ± 1.39 mg O₂ L–¹d–¹) and autumn (–3.19 ± 1.60 mg O₂ L–¹d–¹) was lower than those in the spring (–1.56 ± 1.92 mg O₂ L–¹d–¹) and winter (–2.17 ± 1.50 mg O₂ L–¹d–¹). The inhibitory effect of increased discharge on the metabolic rate exceeded the stimulation provided by seasonal factors, such as increased temperature and solar radiation. The scour and dilution effect caused by discharge increase reduced chlorophyll a concentration; meanwhile, the increase in turbidity resulted in a decrease in the photosynthetic rate and GPP. ER was stimulated by heterotrophic microorganisms and high total suspended solids, resulting in a decrease in O₂ and endogenous organics, thus causing the low NEP, hypoxia, and acidification phenomenon. Our results suggest that lengthening the discharge pulse period in summer and autumn will further decrease NEP and increase the area of hypoxia and acidification at the three outlets in the PRE.