Coastal upwelling fronts as a boundary for planktivorous fish distributions Marine Ecology Progress Series

Fronts have long been considered as bio-aggregators across the food web, serving as important foraging grounds for multiple trophic levels. However, the effect of fronts on intermediate trophic levels is not well understood. We hypothesized that for animals whose metabolic rates are strongly temperature dependent, physiological tolerance will have a more significant impact on their distributions than other biotic factors. We examined this hypothesis through assessment of the spatial variability of planktivorous fish and their dominant zooplankton prey associated with the seasonal and latitudinal variability of the upwelling fronts in the Northern California Current System. Acoustically observed fish biomass dominated by planktivorous species was higher offshore of the upwelling front than inshore. In contrast, zooplankton scattering layers dominated by euphausiids were generally associated with the 200 m isobath, regardless of the position of the front. Fish distributions were consistently found offshore of the upwelling front, aggregating them in the regions of warmer temperature. This suggests that the upwelling front acts as a shoreward boundary for planktivorous fish. With the offshore movement of the upwelling front away from the 200 m isobath as the upwelling season progressed, overlap between planktivorous fish and their zooplankton prey would be decreased. The boundary effect of coastal upwelling fronts on the distributions of mid-trophic level organisms indicates their important role in predator-prey interactions and energy transfer through food webs via a radically different mechanism than previously assumed.长期以来，海洋锋（front）被视为整个食物网中的生物聚集区，为多个营养级的生物提供了重要的觅食场所。然而，学界对海洋锋作用于中间营养级生物的效应尚缺乏充分认知。我们提出假说：对于代谢速率强烈依赖温度的动物而言，生理耐受性对其分布的影响要显著强于其他生物因子。我们以北加利福尼亚洋流系统中上升流锋（upwelling front）的季节与纬度变化为研究背景，通过评估浮游食性鱼类（planktivorous fish）及其优势浮游动物猎物的空间变异性，对该假说开展了检验。声学观测显示，以浮游食性鱼类为优势类群的鱼类生物量，在上升流锋的外海区域高于近岸区域。与之相反，以磷虾类（euphausiids）为优势类群的浮游动物散射层通常与200米等深线相关联，不受海洋锋位置的影响。浮游食性鱼类的分布始终集中在上升流锋的外海区域，聚集于水温较高的海域。这表明上升流锋可作为浮游食性鱼类的向岸边界。随着上升流季节推进，上升流锋向外海移动并远离200米等深线，浮游食性鱼类与其浮游动物猎物之间的栖息重叠度将随之降低。沿岸上升流锋对中营养级生物分布的边界效应，表明其通过一种与此前假设截然不同的机制，在食物网的捕食者-猎物相互作用及能量传递中发挥着关键作用。