Abstract
Abstract. Regime shifts have been reported in many marine ecosystems, and are often expressed as an abrupt change occurring in multiple physical and biological components of the system. In the Gulf of Alaska, a regime shift in the late 1970s was observed, indicated by an abrupt increase in sea surface temperature and major shifts in the catch of many fish species. A thorough understanding of the extent and mechanisms leading to such regime shifts is challenged by data paucity in time and space. We investigate the ability of a suite of ocean biogeochemistry models of varying complexity to simulate regime shifts in the Gulf of Alaska by examining the presence of abrupt changes in time series of physical variables (sea surface temperature and mixed-layer depth), nutrients and biological variables (chlorophyll, primary productivity and plankton biomass) using change-point analysis. Our results show that some ocean biogeochemical models are capable of simulating the late 1970s shift, manifested as an abrupt increase in sea surface temperature followed by an abrupt decrease in nutrients and biological productivity. Models from low to intermediate complexity simulate an abrupt transition in the late 1970s (i.e. a significant shift from one year to the next) while the transition is smoother in higher complexity models. Our study demonstrates that ocean biogeochemical models can successfully simulate regime shifts in the Gulf of Alaska region. These models can therefore be considered useful tools to enhance our understanding of how changes in physical conditions are propagated from lower to upper trophic levels.
Highlights
There is no universal definition of a marine regime shift, they are typically described as an abrupt change in the ecosystem from one state to another, which is detectable in multiple physical and biological components of the system (Lees et al, 2006; Daskalov et al, 2007; deYoung et al, 2008; Andersen et al, 2009; Schwing, 2009)
In the model physical time series, sea surface temperature (SST) exhibits the same signal as the observations: a shift in the intercept www.biogeosciences.net/13/4533/2016/
Using the Gulf of Alaska as a case study, our results demonstrate the usefulness of ocean biogeochemistry model (OBGC) models to infer the chain of events responsible for regime shifts, especially in regions where observations are scarce
Summary
There is no universal definition of a marine regime shift, they are typically described as an abrupt change in the ecosystem from one state to another, which is detectable in multiple physical and biological components of the system (Lees et al, 2006; Daskalov et al, 2007; deYoung et al, 2008; Andersen et al, 2009; Schwing, 2009). Key drivers of marine regime shifts include changes in ecosystem habitat, biotic processes such as dynamics of the foodweb and abiotic processes such as changes in physical and chemical conditions (deYoung et al, 2008) These drivers can be natural or anthropogenic, or a combined influence, which can increase the vulnerability of ecosystems. Excessive fishing is an example of an anthropogenic biotic driver where a decrease in top predators (top-down control) can cause a trophic cascade, resulting in a new bottomup controlled state (Daskalov et al, 2007) Abiotic factors such as climate change or ocean and atmosphere oscillations may initiate bottom-up regime shifts in the food web via changes affecting the abundance of phytoplankton or zooplankton (Cury and Shannon, 2004). Bottom-up driven shifts in biological components of the ecosystem generated by climate shifts manifested in changes in sea surface temperature or mixed-layer depth are considered the most identified (deYoung et al, 2008) and are the focus of this study
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