Effect of Chronic Versus Pulse Perturbations on a Marine Ecosystem: Integration of Functional Responses Across Organization Levels
Human impacts accelerate the intensity and frequency of perturbations on ecosystems; approaches that integrate responses across organization levels are, however, lacking, particularly in the ocean. We experimentally simulated the frequency of fertilization (‘chronic’ versus ‘pulse’ events) in orthogonal combinations of two intensities (‘large’ versus ‘moderate’ fertilization) to determine physiological and biological responses by the seagrass Cymodocea nodosa and associated flora (epiphytes and green seaweeds, specifically Caulerpa prolifera), as well as functional changes (community primary and secondary productivity) at the ecosystem level. We predicted that the absence of recovery time from chronic perturbation would more severely affect responses at population and ecosystem levels relative to discrete events (pulses). Nutrient enrichment increased the biomass of C. prolifera irrespective of its frequency, whereas seagrass biomass and shoot density particularly decreased under a chronic scenario. These demographic responses were connected with varying photo-physiological performance of both C. nodosa and C. prolifera. Fertilization, regardless of its intensity and frequency, decreased the maximum photosynthetic rate of C. nodosa, concomitant with increased pigments, particularly under chronic fertilization, and decreased photoprotective (phenols) compounds. In contrast, fertilization boosted the maximum photochemical yield of C. prolifera, in addition to an increase in pigments and photoprotective compounds. Community primary and secondary productivity, however, did not vary under fertilization of varying intensity and frequency. In summary, fertilization precipitated population-level changes in physiological and biological attributes of vegetation. However, fertilization effects did not entirely cascade into ecosystem-level processes, that is, ecosystem productivity, which suggests a functional compensation (that is, increased algal performance to offset losses of seagrass production) during the initial stages of fertilization.
Keywordsnutrients ecosystem perturbation frequency of perturbation marine vegetation temporal variability marine angiosperms green macroalgae Atlantic Ocean
Fernando Tuya was supported by the MINECO ‘Ramón y Cajal’ program. Part of this study was performed through the project ECOSERVEG (BEST initiative, Voluntary Scheme for Biodiversity and Ecosystem Services in Territories of the EU Outermost Regions and Oversees Countries and Territories, Grant No. 07.032700/2012/635752/SUB/B2) and the Spanish MINECO ‘Plan Nacional’ (CGL 2011-23833, ANTROTIDAL). The research staff was partially supported by the Campus Atlántico Tricontinental. We acknowledge Tony Sánchez for his help during fieldwork. Iacopo Bertocci and several anonymous reviewers provided positive criticism on previous drafts.
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