Evaluating trophic cascades as drivers of regime shifts in different ocean ecosystems

In ecosystems that are strongly structured by predation, reducing top predator abundance can alter several lower trophic levels-a process known as a trophic cascade. A persistent trophic cascade also fits the definition of a regime shift. Such 'trophic cascade regime shifts' have been reported in a few pelagic marine systems-notably the Black Sea, Baltic Sea and eastern Scotian Shelf-raising the question of how common this phenomenon is in the marine environment. We provide a general methodology for distinguishing top-down and bottom-up effects and apply this methodology to time series from these three ecosystems. We found evidence for top-down forcing in the Black Sea due primarily to gelatinous zooplankton. Changes in the Baltic Sea are primarily bottom-up, strongly structured by salinity, but top-down forcing related to changes in cod abundance also shapes the ecosystem. Changes in the eastern Scotian Shelf that were originally attributed to declines in groundfish are better explained by changes in stratification. Our review suggests that trophic cascade regime shifts are rare in open ocean ecosystems and that their likelihood increases as the residence time of water in the system increases. Our work challenges the assumption that negative correlation between consecutive trophic levels implies top-down forcing.