Shifts between synchronous and asynchronous population dynamics in a pelagic jellyfish-zooplankton-phytoplankton food web
The factors driving jellyfish population dynamics are poorly understood, especially in tropical systems. Recent studies have suggested that bottom-up processes dominate plankton-jellyfish trophic interactions and may determine the occurrence of jellyfish blooms. However, it remains unclear whether jellyfish exert top-down influences as an important pelagic predator or if bottom-up factors influence jellyfish populations. This uncertainty is due partly to the complex and open systems in which jellyfish occur. Working within the natural simplified microcosms of ‘marine lakes’—isolated bodies of seawater entirely surrounded by land—can provide a tractable system to study trophic dynamics in marine systems. Within marine lakes in Palau, the abundance of Mastigias papua medusae, which form the top of a planktivorous trophic chain, fluctuates dramatically interannually. We assessed the population sizes of medusae, microzooplankton and phytoplankton, as well as environmental characteristics of the lake, each month across a 13 year time series within one lake. M. papua population sizes were assessed by counting all medusae caught in over a dozen replicated vertical net hauls spread across the lake.Plankton population sizes were estimated by identifying and enumerating specimens from triplicate vertical hauls.
M. papua population densities and environmental data from the entire dataset were analyzed, as well as zoo- and phytoplankton composition and abundances enumerated for a four year subset of the time series. Analyses of these population fluctuations suggest a strong bottom-up correlation between zoo- and phytoplankton (rmax=0.99, ravg=0.48), but weak coupling with M. papua (rmax=0.92, ravg= -0.18), which is not expected for jellyfishes. Additionally, this trophic structure between phyto- and zooplankton does not persist through time, rapidly moving between synchronous (r=0.81, R2=0.70) and asynchronous (r=0.40, R2=0.14) periods. The lack of a tight coupling between M. papua and plankton should have implications for the degree of impact on plankton community dynamics in jellyfish blooms if the rest of the time series shows a similar trend. Jellyfish-affected ecosystems may react in more complex ways than previous studies have indicated.