Abstract Estuaries of large Siberian rivers, Yenisei and Ob, play a pivotal role in transformation and sedimentation of the suspended matter from the riverine runoff. The mesozooplankton community is one of the key players in estuarine functioning. Most of the studies in the Yenisei region were performed at the end of the productive season (August–September) and concentrated on the structure of zooplankton community. The main goal of this study was to characterize structure and grazing of the zooplankton community in the Yenisei estuarine and plume-affected shelf region in early summer soon after the discharge maximum. More specifically, we assessed zooplankton biomass and analyzed the distribution patterns of the dominant species in July; assessed herbivorous feeding rates; and grazing pressure of these species during the period of high phytoplankton production. Zooplankton were collected and feeding rates were studied along a quasi-latitudinal transect in the Yenisei Estuary and the adjacent shelf in July 2016. Freshwater zone (FWZ), frontal zone of the Yenisei plume (YFZ) and shelf zone outside the plume (MSZ) were distinguished according to salinity and temperature. Highest biomass (12–101 mg DW m−3) was recorded in the YFZ. Several specific aspects of distribution and grazing were distinguished. Cladocera were highly abundant in the FWZ and grazed up to 14% of phytoplankton there. Species which dominated zooplankton in the YFZ avoided the upper freshened layer, and maximum abundance of both marine and brackish species coincided with the gradient layer. The brackish Limnocalanus macrurus penetrated far into the north where this species was supposedly exported earlier with the maximal runoff. Low ingestion (0.1–0.9% body carbon) and demographic structure of L. macrurus suggest termination of seasonal development of the population. Herbivorous feeding of the marine copepods exceeded their metabolic demands in the estuarine zone and was insufficient in the marine shelf zone. The highest daily grazing pressure in the YFZ (up to 21% phytoplankton biomass) was exerted by Calanus glacialis. Analysis of our results together with the published data implies that timing of ice retreat is among the main factors controlling seasonal development of the dominant zooplankton populations.

中文翻译：

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初夏叶尼塞河口及邻近陆架优势中型浮游动物的分布与放牧（2016年7月）

摘要 西伯利亚大河叶尼塞河和鄂毕河河口对河流径流悬浮物的转化和沉积起着关键作用。中型浮游生物群落是河口功能的关键参与者之一。叶尼塞地区的大部分研究是在生产季节结束时（8 月至 9 月）进行的，并且集中在浮游动物群落的结构上。本研究的主要目标是描述叶尼塞河口和受羽流影响的陆架区浮游动物群落的结构和放牧特征，该地区在初夏排放量达到最大值后不久。更具体地说，我们评估了浮游动物生物量并分析了 7 月份优势物种的分布模式；评估草食性摄食率；这些物种在浮游植物高产时期的放牧压力。2016 年 7 月，沿叶尼塞河口和邻近陆架的准纬度横断面收集了浮游动物并研究了摄食率。 淡水区 (FWZ)、叶尼塞羽流的锋面区 (YFZ) 和羽流外的陆架区 (MSZ)根据盐度和温度区分。在 YFZ 中记录了最高的生物量（12-101 mg DW m-3）。对分布和放牧的几个具体方面进行了区分。枝角类在 FWZ 中非常丰富，占浮游植物的 14%。长江流域浮游动物占主导地位的物种避开了上层新鲜层，海洋和微咸物种的最大丰度与梯度层重合。微咸的 Limnocalanus macrurus 深入北部，据推测该物种较早以最大径流出口。L. macrurus 的低摄入量（0.1-0.9% 身体碳）和人口结构表明种群季节性发展的终止。海洋桡足类的食草性摄食超过了河口区的代谢需求，而在海洋陆架区则不足。YFZ 中最高的每日放牧压力（高达 21% 的浮游植物生物量）由 Calanus glacialis 施加。对我们的结果和已发表数据的分析表明，冰退的时间是控制主要浮游动物种群季节性发展的主要因素之一。9% 身体碳）和 L. macrurus 的人口结构表明种群季节性发展的终止。海洋桡足类的食草性摄食超过了河口区的代谢需求，而在海洋陆架区则不足。YFZ 中最高的每日放牧压力（高达 21% 的浮游植物生物量）由 Calanus glacialis 施加。对我们的结果和已发表数据的分析表明，冰退的时间是控制主要浮游动物种群季节性发展的主要因素之一。9% 的身体碳）和 L. macrurus 的人口结构表明种群季节性发展的终止。海洋桡足类的食草性摄食超过了河口区的代谢需求，而在海洋陆架区则不足。YFZ 中最高的每日放牧压力（高达 21% 的浮游植物生物量）由 Calanus glacialis 施加。对我们的结果和已发表数据的分析表明，冰退的时间是控制主要浮游动物种群季节性发展的主要因素之一。YFZ 中最高的每日放牧压力（高达 21% 的浮游植物生物量）由 Calanus glacialis 施加。对我们的结果和已发表数据的分析表明，冰退的时间是控制主要浮游动物种群季节性发展的主要因素之一。YFZ 中最高的每日放牧压力（高达 21% 的浮游植物生物量）由 Calanus glacialis 施加。对我们的结果和已发表数据的分析表明，冰退的时间是控制主要浮游动物种群季节性发展的主要因素之一。