DataSheet_1_Top-Down and Bottom-Up Control in the Galápagos Upwelling System.pdf

Increased standing macroalgal biomass in upwelling zones is generally assumed to be the result of higher nutrient flux due to upwelled waters. However, other factors can strongly impact macroalgal communities. For example, herbivory and temperature, via their effects on primary producers and the metabolic demands of consumers, can also influence macroalgal biomass and productivity, respectively. We assessed the effects of nutrient availability, temperature, and herbivory on macroalgal biomass on a subtidal nearshore rocky reef in the Galápagos Islands. We manipulated nutrient availability and herbivory in field experiments performed in two seasons: the first during a cool, upwelling season, and the second during a warm, non-upwelling season. Excluding macro-herbivores had a clear effect on standing macroalgal biomass, independent of season or nutrient availability. However, we found different interactive effects of nutrients and macro-herbivores between the two seasons. During the cool season, macroalgal biomass was significantly higher in herbivore exclusions than in open areas under ambient nutrient conditions. However, when nutrients were added, macroalgal biomass was not significantly different across all herbivore treatments, which suggests reduced top-down control of herbivores (hence a greater standing algal biomass) in open areas. In the warm season, macroalgal biomass was significantly higher in herbivore exclusions compared to open treatments, both with and without nutrient addition. Furthermore, biomass reached 2X in herbivore exclusions with nutrient additions, which hints nutrient limitation only during warm, low-upwelling conditions. Overall, our results support the hypothesis that macro-herbivores reduce macroalgal biomass in this system and suggest that nutrient availability, but not temperature, modulate herbivory.

人们通常认为，上升流区大型海藻（macroalgal）现存量生物量的提升，源于上升流海水所携带的更高营养盐通量。然而，其他多种因素亦可对大型海藻群落产生显著影响。例如，植食作用与温度，分别通过作用于初级生产者以及消费者的代谢需求，对大型海藻的生物量与生产力产生调控效应。
本研究针对加拉帕戈斯群岛的一处潮下近岸岩礁，评估了营养盐有效性、温度与植食作用对大型海藻生物量的影响。研究在两个季节开展野外实验，对营养盐有效性与植食作用进行操控：第一季为凉爽的上升流季，第二季为温暖的非上升流季。
排除大型植食动物对大型海藻现存量生物量具有明确影响，且该效应不受季节或营养盐有效性的调控。但研究发现，两个季节中营养盐与大型植食动物的交互效应存在显著差异。在凉爽季节，原位营养盐条件下，植食动物排除组的大型海藻生物量显著高于开放区域。然而，当添加营养盐后，所有植食动物处理组间的大型海藻生物量并无显著差异，这表明开放区域内植食动物的下行控制作用被削弱（因此现生藻类生物量更高）。
在温暖季节，无论是否添加营养盐，植食动物排除组的大型海藻生物量均显著高于开放处理组。此外，添加营养盐的植食动物排除组中，生物量达到开放处理组的2倍，这暗示仅在温暖、低上升流的环境下存在营养盐限制。
总体而言，本研究结果支持"大型植食动物会降低该系统内大型海藻生物量"这一假说，并表明是营养盐有效性（而非温度）调控了植食作用的效应。