Sediment, herbivory, and nutrients interact to control algal turf height on a fringing reef
摘要
Algal turf (hereafter turf) on coral reefs is subject to multiple and often interacting anthropogenic stressors, including overfishing, increased sediment, and nutrient influxes. When maintained in a closely cropped state, turf is a key component of healthy reef ecosystems. However, multiple stressors can cause transitions to a longer, sediment-laden state, inhibiting coral recruitment and limiting reef resilience. Therefore, understanding the drivers controlling turf height is important to help predict reef resilience. To evaluate the relative importance of three stressors and their interactions on mediating turf height, we conducted a three-factor fully crossed field experiment manipulating sediment depth, nutrient supply, and access to herbivores and measured turf height four times over 24 days. Our results revealed a significant four-way interaction among our three stressors and time. Herbivores limited turf height in most plots, with average height remaining close to initial levels in plots with access to herbivores compared to an increase in height of ~ 400% in plots without herbivores. However, added nutrients and sediment rather than herbivory maintained short turf in plots subject to these double stressors, contributing to the four-way interaction. In the absence of herbivory, the addition of nutrients increased turf height, but only in the absence of sediment. Thus, the addition of sediment negated the positive effect of nutrients. Further, stressor effects occurred on different time scales, with nutrient effects taking longer to develop than herbivory effects. Our results of complex, context-dependent, and temporally dynamic effects highlight the need for multi-factor experiments to better understand ecosystem responses and inform management. Knowing multiple stressors have significant and complex higher-order interactions is important because local and global stressors are likely to increase in number, frequency and intensity in the Anthropocene.