Explaining the spatial synchrony of fluctuations in North Sea phytoplankton abundance using wavelets

Background/Question/Methods

It is well known that in theory spatially synchronous fluctuations in abundance can result from spatially synchronous fluctuations in the environment: this is known as the Moran effect. The challenge is to identify environmental drivers in real systems and demonstrate the Moran effect in action. Phytoplankton abundance undergoes fluctuations over different timescales at different times, complicating the identification of Moran effects; this seems likely to be a general feature of spatiotemporal ecological data. We introduce wavelet methods to disaggregate synchrony by timescale of fluctuation in order to identify drivers of synchrony. We generate timescale-specific models to determine how much of the spatial synchrony of phytoplankton can be attributed to these drivers. The Continuous Plankton Recorder (CPR) survey at SAHFOS monitors near surface plankton in the sea around the British Isles.   We examine time series of annualised data representing changes in primary productivity (phytoplankton colour index), for 26 areas of the North Sea. We compare with the local fluctuations in relevant environmental variables, and ecological variables including the abundances of important plankton species which predate on phytoplankton.

Results/Conclusions

Both biotic factors (Calanus Finmarchicus abundance, Decapod larvae abundance) and abiotic factors (sea surface temperature and salinity) are significant. Models based on these 4 explanatory variables predict spatial synchrony varying with time and timescale, accounting for 63 percent of the long timescale (fluctuations longer than 4 years) spatial synchrony in phytoplankton colour, and 7 percent of the observed synchrony at short timescales (fluctuations shorter than 4 years). The remaining spatial synchrony must be attributed to other synchronous sources of variance. No single explanatory variable explained more than 3 percent of the observed synchrony at short timescales, although notably salinity explains one spatially synchronised fluctuation in the late 70s. Sea surface temperature explains over 50 percent of the long timescale spatial synchrony, whereas Calanus Finmarchicus explains only 7 percent.