Repeated observations of abrupt and persistent recruitment collapses in gadoids – a potential scenario in relation to climate change?
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Management, surveillance and studies of ecosystems and their constituent species are generally based on the assumption of simple dose-response relationships. Gradual environmental changes or perturbations are expected to cause corresponding changes in the abundance of affected species. However, a unique time series (since 1919) of 0-group gadoid abundance data from the Norwegian Skagerrak coast shows repeated incidents of abrupt and persistent recruitment collapses in gadoid fishes. It is proposed that the recruitment collapses are caused by gradually increasing nutrient loads, which result in abrupt changes in the planktonic community and thereby reduced food variability for the 0-group gadoids. The results suggest that: 1. There are alternative stable states with different environmental optima in marine ecosystems. 2. The change from one stable state to another appears as a catastrophic shift. 3. Different stable states may persist under overlapping environmental conditions, suggesting that marine ecosystems are highly resilient. 4. The eutrophication-induced shifts are linked to increasing turnover rates in the algal community with increasing primary productivity. This favours small, fast-growing algal species, which are then grazed by smaller herbivorous zooplankton. The fact that marine ecosystems may not respond in a gradual dose-response manner, but shift abruptly between alternative stable states seriously challenges our present approach of managing ecosystems and their constituent species, and the way we monitor potential impact of environmental changes. This is of particular concern at the prospect of global warming. Keywords: Ecology, recruitment, eutrophication, regime shifts, climate change