Low-latitude waters of the Indian Ocean are warming faster than other major oceans. Most models predict a zooplankton decline due to lower productivity, enhanced metabolism and phytoplankton size shifts that reduce trophic transfer efficiency. In May−June 2019, we investigated mesozooplankton biomass and grazing along the historic 110° E transect line from the International Indian Ocean Expedition (IIOE) of the 1960s. Twenty sampling stations from 39.5 to 11.5° S spanned latitudinal variability from temperate to tropical waters and a pronounced 14°C gradient in mean euphotic zone temperature. Although mesozooplankton size structure was similar along the transect, with smaller (<2 mm) size classes dominant, total biomass increased 3-fold (400 to 1500 mg dry weight m−2) from high to low latitude. More dramatically, gut-fluorescence estimates of grazing (total ingestion or % euphotic zone chl a consumed d−1) were 14and 20-fold higher, respectively, in the low-latitude warmer waters. Biomass-normalized grazing rates varied more than 6-fold over the transect, showing a strong temperature relationship (r2 = 0.85) that exceeded the temperature effects on gut turnover and metabolic rates. Herbivory contributed more to satisfying zooplankton energetic requirements in low-chl a tropical waters than chl a-rich waters at higher latitude. Our unexpected results are inconsistent with trophic amplification of warming effects on phytoplankton to zooplankton, but might be explained by enhanced coupling efficiency via mixotrophy. Additional implications for selective herbivory and topdown grazing control underscore the need for rigorous field studies to understand relationships and validate assumptions about climate change effects on the food webs of tropical oceans. OPEN ACCESS

中文翻译：

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东印度洋 110°E 横断面中浮游动物生物量和温度增强的放牧

印度洋的低纬度水域比其他主要海洋变暖得更快。大多数模型预测浮游动物的减少是由于生产力降低、新陈代谢增强和浮游植物大小变化降低了营养转移效率。2019 年 5 月至 6 月，我们调查了 1960 年代国际印度洋探险队 (IIOE) 沿历史性东经 110° 横断面的中型浮游动物生物量和放牧情况。从 39.5 到 11.5° S 的 20 个采样站跨越了从温带到热带水域的纬度变化和平均透光区温度明显的 14°C 梯度。尽管沿横断面的中型浮游动物大小结构相似，较小（<2 毫米）大小类别占主导地位，但总生物量从高纬度到低纬度增加了 3 倍（400 至 1500 毫克干重 m-2）。更戏剧性的是，在低纬度较温暖的水域中，放牧的肠道荧光估计值（总摄入量或 % euphotic zone chl a 消耗 d-1）分别高出 14 和 20 倍。生物量标准化放牧率在横断面上变化超过 6 倍，显示出强烈的温度关系 (r2 = 0.85)，超过了温度对肠道周转率和代谢率的影响。与高纬度富含叶绿素的水域相比，食草动物对满足低叶绿素 a 热带水域的浮游动物能量需求的贡献更大。我们意想不到的结果与浮游植物对浮游动物的变暖效应的营养放大不一致，但可能通过混合营养增强耦合效率来解释。选择性食草和自上而下放牧控制的其他影响强调需要进行严格的实地研究，以了解关系并验证气候变化对热带海洋食物网影响的假设。开放获取