Bayesian Network Analysis reveals resilience of the jellyfish Aurelia aurita to an Irish Sea regime shift

Abstract

Robust time-series of direct observations of jellyfish abundance are not available for many ecosystems, leaving it difficult to determine changes in jellyfish abundance, the possible causes (e.g. climate change) or the consequences (e.g. trophic cascades). We sought an indirect ecological route to reconstruct jellyfish abundance in the Irish Sea: since zooplankton are jellyfish prey, historic variability in zooplankton communities may provide proxies for jellyfish abundance. We determined the Bayesian ecological network of jellyfish-zooplankton dependencies using jellyfish- and zooplankton-abundance data obtained using nets during a 2-week cruise to the Irish Sea in 2008. This network revealed that Aurelia aurita abundance was dependent on zooplankton groups Warm Temperate and Temperate Oceanic as defined by previous zooplankton ecology work. We then determined historic zooplankton networks across the Irish Sea from abundance data from Continuous Plankton Recorder surveys conducted between 1970 and 2000. Transposing the 2008 spatial dependencies onto the historic networks revealed that Aurelia abundance was more strongly dependent over time on sea surface temperature than on the zooplankton community. The generalist predatory abilities of Aurelia may have insulated this jellyfish over the 1985 regime shift when zooplankton composition in the Irish Sea changed abruptly, and also help explain its globally widespread distribution.

Figure 1

Herring landings data for the Irish Sea (modified from Ref.). X-axis is time measured in years and y-axis are landings in metric tonnes.

Figure 2

Cruise track in May 2008 showing locations for MIK net (pink squares), Otter trawl (yellow triangles), ring net (dark blue diamonds) and CTD casts (red crosses).

Figure 3

Jellyfish abundance sampling along the cruise route. Coloured circles are different jellyfish species: green Aurelia; red Chrysaora hysoscella; blue Cyanea lamarckii; purple Cyanea capillata and black Rhizostoma pulmo. The size of the circles is proportional to the log jellyfish count, varying between 0 and 9. The cruise track is given by the blue lines; observations the green line; the red line shows the aerial survey flight path, and the yellow line is a zone identified by previous aerial surveys as a jellyfish ‘hotspot’.

Figure 4

Historic Plankton group abundances (from CPR data) and sea surface temperature (from UK Meteorological Office). The three time periods are shown in different colours: Peach 1971–1980; Green 1981–1990, and Blue 1991–2000. The x-axis is the month of the year, and the y-axis shows the relative zooplankton abundance (scaled relative to each groups maximum) for each time period. This relative scaling is to enable clear comparisons of relative abundance between groups for each biogeographical zooplankton group over the three decades.

Figure 5

Jellyfish—zooplankton network apparent in the 2008 cruise data. The occurrence rate is indicated by the width of the edge (the line depicting dependencies between two taxa) - the wider the line, the higher the occurrence rate. Arrows indicate non-mutual dependence between two taxa; for example the Sub Arctic Zooplankton group has a positive dependency (is aggregated) with Warm Temperate Oceanic Zooplankton, but Warm Temperate Oceanic Zooplankton do not have a dependency with Sub Arctic Zooplankton. Where there is a mutual dependency between two groups, such as with Cold Temperate Oceanic Zooplankton and Aurelia aurita, the edge does not have an arrow. Numbers by the lines are the mean interaction strengths of the dependencies, with positive interaction strengths indicating aggregation, negative interaction strengths indicating segregation, and zero indicating different aggregation and segregation behaviours at different densities. Phytoplankton abundances are given by Chlorophyll Concentration.

Figure 6

Historic zooplankton networks for the three time periods. Arrows indicate non-mutual dependence between two taxa; for example Shelf Sea Zooplankton are aggregated with respect to Warm Temperate Zooplankton, but Warm Temperate Zooplankton are not aggregated with respect to Shelf Sea Zooplankton.

Figure 7

The network used to infer historic Aurelia abundances across the three decades (70s: purple edges, 80s: blue edges and 90s: cyan edges). The red edges and nodes were inferred from the cruise data, and are assumed to be constant over the historic study period (1971–2000). Different coloured edges represent different time periods.