Abstract
Jellyfish outbreaks are increasingly viewed as a deterministic response to escalating levels of environmental degradation and climate extremes. However, a comprehensive understanding of the influence of deterministic drivers and stochastic environmental variations favouring population renewal processes has remained elusive. This study quantifies the deterministic and stochastic components of environmental change that lead to outbreaks of the jellyfish Pelagia noctiluca in the Mediterranen Sea. Using data of jellyfish abundance collected at 241 sites along the Catalan coast from 2007 to 2010 we: (1) tested hypotheses about the influence of time-varying and spatial predictors of jellyfish outbreaks; (2) evaluated the relative importance of stochastic vs. deterministic forcing of outbreaks through the environmental bootstrap method; and (3) quantified return times of extreme events. Outbreaks were common in May and June and less likely in other summer months, which resulted in a negative relationship between outbreaks and SST. Cross- and along-shore advection by geostrophic flow were important concentrating forces of jellyfish, but most outbreaks occurred in the proximity of two canyons in the northern part of the study area. This result supported the recent hypothesis that canyons can funnel P. noctiluca blooms towards shore during upwelling. This can be a general, yet unappreciated mechanism leading to outbreaks of holoplanktonic jellyfish species. The environmental bootstrap indicated that stochastic environmental fluctuations have negligible effects on return times of outbreaks. Our analysis emphasized the importance of deterministic processes leading to jellyfish outbreaks compared to the stochastic component of environmental variation. A better understanding of how environmental drivers affect demographic and population processes in jellyfish species will increase the ability to anticipate jellyfish outbreaks in the future.
Highlights
Extreme events such as droughts, storms and floods are becoming more frequent with climate change
We focus on Pelagia noctiluca (Forsskål), a common holoplanktonic scyphozoan with a wide geographic distribution that extends from the warm and temperate waters of the world’s oceans up to the North Sea [25, 26]
Jellyfish outbreaks may be a deterministic response to escalating levels of environmental degradation [16, 23]
Summary
Extreme events such as droughts, storms and floods are becoming more frequent with climate change These environmental changes may provoke extreme ecological responses in populations and species assemblages that may result in severe impacts to natural ecosystems [1,2,3,4]. Outbreaks occur when the alignment in space and time of certain environmental drivers (abiotic and biotic), result in favourable conditions for population renewal processes [7]. Defining these conditions quantitatively and understanding when and where they will occur are key tasks to forecast population outbreaks. This endeavour is challenging, because episodic events may be inherently unpredictable [8, 9]
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