Synchronised Phenology and Climate Change – Part Two: A Codding Mess

As we discussed in our last blog, the decoupling of phenological relationships can have devastating consequences for ecosystems. The internal dynamics within ecosystems and between trophic levels can be hindered and links between species effectively broken, having dramatic effects on the constitution of food webs and the internal stability and resilience of ecosystems as a whole (Edwards and Richardson 2004).

This is exactly what Beaugrand et al. (2003) believe is happening in the North Sea, where concerns are emerging regarding mass declines in the biomass and recruitment (when juvenile organisms survive and are added to a population) of Atlantic cod (Gadus Morhua) since the mid-1980s. Whilst admittedly linked to severe overfishing in the area, Beaugrand et al. also argue that fluctuations in prey species (plankton) of juvenile cod are severely reducing the survival of young. Beaugrand ­et al. suggest that the survival of larval cod relies heavily on three biological parameters of the prey plankton; their mean size, seasonal timing and abundance – all of which are undergoing heavy influence by climatic factors.

Atlantic Cod (Gadus Morhua)

In fact, the argument is such that the phenology of plankton species, which determines timescales of peak abundance, are being dramatically effected by changing sea surface temperature. Arguably, this reduces the number of prey for juvenile cod, resulting in reduced growth and consequently survival (Beaugrand et al. 2003).

A similar study, conducted by Edwards and Richardson (2004), expands upon this idea, and investigates the effects that sea surface warming of 0.9^oC over the past 60 years is having on different pelagic taxa that together facilitate the upwards transfer of energy to juvenile cod through the food web. Specifically, they look at diatoms and dinoflagellates (primary producers), copepods (secondary producers) and non-copepod holozooplankton and meroplankton (secondary and tertiary producers).

Edwards and Richardson (2004), by calculating the change in timing of seasonal cycles for each biota, observed what they describe as “substantial temporal modifications” in peak abundance over the past 30 years (a similar time-frame as that referred to by Beaugrand et al. 2003). Specifically, they found that the peak in meroplankton moved forward by an average of 27 days, dinoflagellates by an average of 23, copepods by 10 days and non-copepod zoo plankton by 10 days over the study period. Diatoms, however, had no significant advancement in their spring  bloom and only a relatively minor 5 days in their autumn bloom.

Here a few examples of the output from Edwards and and Richardson's (2004) study can be seen. a details the seasonal cycle of the dinoflagellate Ceratium fusus and the diatom Cylindrotheca closterium, and there is a clear pattern of change for the former and a clear lack of change for the latter. b shows how the interannual variability for seasonal peaks for the two species across the study period (note, diatoms have a spring and autumn peak, so show two peaks in the data). c shows the change in timing of seasonal peaks over the entire 45-year study period for the 66 taxa, against seasonal peak from 1958. Clear change is shown for certain taxa, whilst others, largely those belonging to diatoms, show little change.

Edwards and Richardson (2004) explain these advancements as a result of phenological change due to temperature increase; supposedly, temperature plays a key role in influencing plankton physiological, affecting a range of variables including reproduction, mortality, respiration, and embryonic development. It is for this reason that there has been significant advancement (of a greater magnitude, Edwards and Richardson argue, than studies have shown in terrestrial ecosystems)  in seasonal cycle in response to climate warming for large parts of the ecosystem. Conversely, diatoms have remained arguably stationary, leading Edwards andRichardson to believe that they are more dependent on the photoperiod or intensity to influence life cycles.

The issue arises from the variability in response that we see. Although a large proportion of the pelagic ecosystem IS responding, the intensity of this response is highly variable, and is leading to severe asynchrony between successive trophic levels. This creates inefficiency in the transfer of marine production to higher trophic levels, as peaks in species are no longer able to take full advantage of each other and transfer the most energy possible up the trophic level. Less production means less prey, and less prey can have dramatic effects on larger members of the ecosystem such as the Atlantic Cod (Edwards and Richardson 2004). This is the issue at hand; our fishy friend is currently being held hostage from the bottom up, in what Beaugrand et al. (2003) refer to as bottom-up control.

An understandably morose Atlantic Cod

This provides an excellent example of the dangers of phenological change, and really helps portray the intricacies of the problem. Note how all members in the study conducted by Edwards and Richardson experienced differing levels of advancement, making the problem of desynchronization not just a problem between one or two species, but between all trophic levels. This allowed change right at the very bottom of the trophic scale to affect a species that exists right at the very top, and everything in-between. This is dramatically destabilising for ecosystems, and if allowed to persist, could create great uncertainty in the future of ecosystems, and possibly lead to collapse.

In our quest for understanding, we will expand upon this more in our next blog. The issue here is huge, and there is a lot of ground to cover.