Abstract
Jellyfish (pelagic Cnidarians and Ctenophores) form erratic and seemingly unpredictable blooms with often large, transient effects on ecosystem structure. To rapidly capitalize on favorable conditions, jellyfish can employ different life histories, which are either a life cycle with one annual sexual reproduction event and an overwintering benthic stage (metagenic life cycle), or continuous reproduction and a holoplanktonic life cycle. However, the links between life history, blooms, and environmental variability are unclear. Here, we examine how environmental variability can drive the bloom dynamics of typical jellyfish in coastal enclosed or semi-enclosed temperate ecosystems. With a simple community model, we reproduce typical seasonalities of the two strategies and trophic cascades triggered by abundant jellyfish, demonstrating how erratic blooms can be generated by irregular changes in the environment. Consistent with literature observations, we predict that metagenic jellyfish dominate early in the season, compared to holoplanktonic organisms, and are favored by increased seasonality. Our results reveal possible mechanisms driving coastal patterns of jellyfish blooms, and factors that are important for the outcome of competition between jellyfish with different life cycles. Such knowledge is important for our understanding of jellyfish blooms, which have large consequences for human activities and well-being, and may improve our ability to predict and manage local ecosystems.
Highlights
Jellyfish outbreaks are widely recognized for their variable and unpredictable dynamics, with individuals suddenly appearing in large numbers, only to later be seemingly absent from the ecosystem (Boero et al, 2008)
The model is run for long multi-annual time periods, and reproduces the main features of a typical temperate seasonal cycle, with a phytoplankton bloom in the early spring followed by increased zooplankton biomass (Figure 2)
The main findings of the study can be summed up as: (1) A simple model can produce seasonal patterns in biomasses of the main plankton groups that are typical of temperate environments, including trophic cascades triggered by high jellyfish biomasses
Summary
Jellyfish outbreaks are widely recognized for their variable and unpredictable dynamics, with individuals suddenly appearing in large numbers, only to later be seemingly absent from the ecosystem (Boero et al, 2008). Most bloom-forming jellyfish are large and have zooplanktivorous diets (Hamner and Dawson, 2009), and may effectively control grazer populations when abundant (Feigenbaum and Kelly, 1984; Olesen, 1995; Schneider and Behrends, 1998; Møller and Riisgård, 2007c; Tiselius and Møller, 2017) This in turn releases primary producers from predation control, eliciting phytoplankton blooms (Møller and Riisgård, 2007b) and favoring jellyfish in the competition with fish (Riisgård et al, 2012; Robinson et al, 2014), Environmental Variability and Jellyfish Blooms in part through the effect of reduced water clarity and light on fish visual feeding efficiency (Aksnes, 2007; Haraldsson et al, 2012; Schnedler-Meyer et al, 2016). This stimulates nutrient release from the bottom and further algal growth, and since many jellyfish are tolerant to hypoxia, they have a competitive advantage over other, more sensitive organism groups in such conditions (Decker et al, 2004; Purcell, 2012)
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