Bangping Deng, Wei Liu, Jun Li, Tao Yang, Jun Shi, Yiming Yuan, Shengming Cheng, Lei Wu, Chuxu Xiong, Lichen Zhao, Xiping Zheng, Lin Zhu
The phytoplankton community was strongly seasonal and near-monodominant in summer: the diatom Skeletonema costatum contributed 76.3% of total cell density in spring and 98.4% in summer, then collapsed to 0.7% in autumn, when diversity rose as dominance dispersed among larger diatoms; cell density declined by three orders of magnitude from spring to autumn. These shifts tracked the physicochemical environment. Dissolved inorganic nitrogen exceeded the Class I seawater criterion in 53.6% of spring-summer observations, dissolved inorganic phosphorus exceedance peaked in autumn at 89.3%, and summer bottom-water dissolved oxygen fell to 2.10 mg L-1, consistent with regional hypoxia; trace metals remained below Class I sediment criteria. Zooplankton biomass peaked in summer, whereas macrobenthic standing stock declined 14-fold from summer to autumn, with three stations azoic. Canonical correspondence analysis retained dissolved oxygen and temperature across all biological groups, while nutrient variables were retained only for the planktonic communities.
INTRODUCTION: River-influenced continental shelves impose strong seasonal nutrient pressure on phytoplankton assemblages, yet the environmental regulation of phytoplankton assemblages in the outer estuarine zone, and the way their dynamics propagate to higher trophic levels, remains poorly resolved.
METHODS: We conducted three cruises in spring, summer, and autumn 2024 at 28 stations in the exclusive economic zone (EEZ) adjacent to the Yangtze River Estuary, characterizing the phytoplankton community together with water quality, surficial sediment geochemistry, and the accompanying zooplankton and macrobenthos as ecosystem context.
RESULTS: The phytoplankton community was strongly seasonal and near-monodominant in summer: the diatom Skeletonema costatum contributed 76.3% of total cell density in spring and 98.4% in summer, then collapsed to 0.7% in autumn, when diversity rose as dominance dispersed among larger diatoms; cell density declined by three orders of magnitude from spring to autumn. These shifts tracked the physicochemical environment. Dissolved inorganic nitrogen exceeded the Class I seawater criterion in 53.6% of spring-summer observations, dissolved inorganic phosphorus exceedance peaked in autumn at 89.3%, and summer bottom-water dissolved oxygen fell to 2.10 mg L-1, consistent with regional hypoxia; trace metals remained below Class I sediment criteria. Zooplankton biomass peaked in summer, whereas macrobenthic standing stock declined 14-fold from summer to autumn, with three stations azoic. Canonical correspondence analysis retained dissolved oxygen and temperature across all biological groups, while nutrient variables were retained only for the planktonic communities.
DISCUSSION: The phytoplankton community thus emerges as the most environmentally responsive component of this shelf ecosystem, underscoring the value of integrated, multi-season monitoring of net-collected microalgal assemblages to support coastal management.