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Marine Regime shifts is defined as rapid reorganizations of ecosystems from one relatively stable state to another. The idea of a regime shift was first used in investigations of freshwater and terrestrial systems. It is now been used for marine and estuarine systems to describe rapid shifts between two alternate stable environmental states. The definition has broadened to include shifts in biological systems as a response to physical drivers. Therefore a regime shift could constitute as a change in species abundance, community composition and trophic organization occurring at regional or greater spatial scales in response to either an external physical or anthropogenic driver or due to internal processes. The restructuring of the ecosystem due to the physical and/or biological processes has to establish a new quasiequilbrium state.

Causes of Regime Shifts in the Ocean
Regime shifts began to be looked at in the marine environment after direct observations and retrieval of historical records showed examples of multi-decadal variability in stock abundances of fish. An interest began to build in the correlation among the abundance of various fish-stocks in individual ecosystems.

Regime shifts have been found due to changes of external physical variables on the water and internal biological variables. Water temperature changes and wind pattern shifts have been correlated to regime shifts. Wind pattern changes would alter migration patterns of fish leading to them being in a more or less favorable environments. Wind shifts could increase upwelling in regions leading to more favorable nutrient conditions for one species over another. Temperature variations are similar in altering the environment the fish are in. Overfishing of one species is an example of how regime shifts can be caused anthropogenically. Removing a fish species from a certain region at high rates could alter trophic levels allowing another species to flourish.

Patterns of Marine Regime Shift
Regime shifts have been described in three qualitatively different patterns. These patterns were found by relating a biotic response (e.g., abundance of a particular species) to an abiotic control variable (e.g., ocean variability). This would be a relation of an ecosystem response or state to an external forcing or condition. The three patterns found have been classified as smooth, abrupt, and discontinuous.

A smooth regime shift is represented by a quasi-linear relationship between the response and control variable. This means that the response variable follows the same path as the control variable. An abrupt regime shift is represented as a non-linear relationship between the response and control variable. This means that small changes in an ocean variable like temperature would lead to abrupt shifts in population abundances within that ecosystem. A discontinuous regime is one where the trajectory of the response variable is different when the control variable increases or decreases. Within this shift the ecosystem cannot go back to a state of equilibrium with a small change in the altered variable.

Detection of Marine Regime Shifts
Regime shifts are detected through a meta-analysis. A researcher must analyze data sets covering substantial periods of time in order to determine if an event is a potential regime shift. This is to ensure that the data being used can be compared before, during, and after the potential regime shift being looked at. The Intergovernmental Panel on Climate Change (IPCC) guidelines for assessment reports demand that time series analyzing biological changes must be at least 20 years long, extend several years prior to and after the proposed regime shift, and end in 1990 or later. Since regime shifts can affect the whole ecosystem, it is suggested that observations include physical and biological parameters that cover more than one trophic level in order to identify the control mechanisms within the ecosystem.

Examples of Marine Regime Shifts
A Regime Shift was seen in the North Atlantic Ocean from the 1920’s through the 1960’s. During the 20’s and 30’s there was a dramatic warming of the northern North Atlantic due to increased southerly winds, which pumped warm air into the northern North Atlantic and Arctic. A result of this warming was a dramatic change in the marine ecosystems in the North Atlantic due to increased water temperatures. The most profound change in the ecosystem was the response of the Atlantic cod off West Greenland. The Atlantic cod increased in numbers and spread gradually northward from near the southern tip of Greenland to Upernavik, a distance of over 1200 km. This increase in cod abundance led to the development of the cod fishing industry off the coast of Greenland. Catches rose to a peak in the early 1960s before declining rapidly. The decline came during a period of decreasing air and ocean temperatures. Zooplankton samples were also collected in June-July every year from 1950 to 1985 on transects across three of West Greenland’s offshore banks. Zooplankton abundances were greater in the warmer periods of the 1950s and 60s compared to any other period. Cod could utilize the zooplankton as nutrients during their development allowing them to grow in larger quantities. Other species in the warm period that prefer warm waters like herring, coalfish, and redfish reproduced more successfully in areas north of their normal range. Cold-water species such as capelin no longer migrated south along the West Greenland resulting in a decrease in their abundance in southwestern Greenland but an increase in their abundance in areas northward. Overall West Greenland saw a shift in fish abundances (e.g. increases in cod and decreases in capelin) due to an environmental variable (e.g. warming of the waters due to increased southerly winds) between 1920 and 1960.

The fishing industry itself can cause a regime shift. Over-fishing of one fish species can lead to another fish species flourishing in that ecosystem. This has been seen in the California Current with the schooling fish, primarily sardines and anchovies. During the 1930’s and 1940’s, sardines were a major commercial fish. After 1951 due to over fishing the sardine stock crashed and could not support any fishery. Anchovy stocks began to rise since they now could utilize more of the nutrients in the water once taken up by the sardine stocks. The anchovy fishery then crashed in 1990 due to overfishing and again we have seen an increase in sardine stocks. This would be an example of a multi-decade regime. Regime shifts can have economic implications. An example of this would be the salmon populations in the northern pacific. Due to a rise of 0.5˚C in mean sea surface temperature in 1976 in this area, salmon populations migrated north. The Gulf of Alaska and the Bering Sea fishermen reaped the benefits financially from the sudden increase in salmon stocks while fishermen in Washington and Oregon suffered.