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There is an apparent gap between the prominence of present theoretical frameworks involving ecological thresholds and regime shifts, and the paucity of efforts to conduct simple tests and quantitative inferences on the actual appearance of such phenomena in ecological data. There is a wide range of statistical methods and analytical techniques now available that render these questions tractable, some of them even dating half a century back. Yet, their application has been sparse and confined within a narrow subset of cases of ecological regime shifts. Our objective is to raise awareness on the range of techniques available, and to their principles and limitations, in order to promote a more operational approach to the analysis of ecological thresholds and regime shifts. The observation that managed ecosystems often fail to respond smoothly to changing pressures has generated perplexity and eventually lead researchers to draw parallels between the behaviour of ecological systems and other complex systems with non-linear dynamics, such as the global climate, the human immune system, and the world economy (cf.[1] for a popular account). Initial reports of kelp forest disturbance and recovery [2], freshwater ecosystem shifts engineered by beavers [3], and vegetation shifts affected by fire [4] have lead on to an ever-growing research effort on ECOLOGICAL THRESHOLDS and REGIME SHIFTS (see Glossary), whose underlying theoretical framework [5,6] (Box 1) has been shown to be applicable to a broad range of ecosystems from coral reefs to forests and lakes [7,8]. These concepts are now also making their way into the minds and discussions of policy makers and might soon be translated into legislative frameworks [9].Ecological regime shifts can be defined as abrupt changes on several trophic levels [10], leading to rapid ecosystem reconfiguration between alternative states. These shifts are generally thought to be driven by external perturbations (e.g. climatic fluctuations, overexploitation, eutrophication, and invasive species), but the exact mechanism is often unclear. The subject has become a fast growing scientific discipline, manifested by a 12-fold increase in publications between 1991 and 2006, twice as fast as the growth rate of research effort in ecology as a whole (7.7 % year -1 , ISI Web of Science). Most of the reported cases of ecological regime shifts are inferred from time series of monitoring data, while direct evidence by controlled experiments of the existence of alternative states is difficult to find [11]. Surprisingly, the general techniques available to test for regime shifts and thresholds have only to a limited extent been applied to these data sets. As formal tests of regime shifts have a long history in the context of climate change research (e.g. that formal statistical tests for ecological regime shifts have mostly been restricted to the effects of climate change on marine communities [13]. These observations suggest that there is a need to increase the awareness of ecologists on the availability...

Ocean acidification due to anthropogenic CO 2 emissions is a dominant driver of long-term changes in pH in the open ocean, raising concern for the future of calcifying organisms, many of which are present in coastal habitats. However, changes in pH in coastal ecosystems result from a multitude of drivers, including impacts from watershed processes, nutrient inputs, and changes in ecosystem structure and metabolism. Interaction between ocean acidification due to anthropogenic CO 2 emissions and the dynamic regional to local drivers of coastal ecosystems have resulted in complex regulation of pH in coastal waters. Changes in the watershed can, for example, lead to changes in alkalinity and CO 2 fluxes that, together with metabolic processes and oceanic dynamics, yield high-magnitude decadal changes of up to 0.5 units in coastal pH. Metabolism results in strong diel to seasonal fluctuations in pH, with characteristic ranges of 0.3 pH units, with metabolically intense habitats exceeding this range on a daily basis. The intense variability and multiple, complex controls on pH implies that the concept of ocean acidification due to anthropogenic CO 2 emissions cannot be transposed to coastal ecosystems directly. Furthermore, in coastal ecosystems, the detection of trends towards acidification is not trivial and the attribution of these changes to anthropogenic CO 2 emissions is even more problematic. Coastal ecosystems may show acidification or basification, depending on the balance between the invasion of coastal waters by anthropogenic CO 2 , watershed export of alkalinity, organic matter and CO 2 , and changes in the balance between primary production, respiration and calcification rates in response to changes in nutrient inputs and losses of ecosystem components. Hence, we contend that ocean acidification from anthropogenic CO 2 is largely an open-ocean syndrome and that a concept of anthropogenic impacts on marine pH, which is applicable across the entire ocean, from coastal to open-ocean environments, provides a superior framework to consider the multiple components of the anthropogenic perturbation of marine pH trajectories. The concept of anthropogenic impacts on seawater pH acknowledges that a regional focus is necessary to predict future trajectories in the pH of coastal waters and points at opportunities to manage these trajectories locally to conserve coastal organisms vulnerable to ocean acidification.

Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation.biogeochemistry | climate change

The implicit assumption of many scientific and regulatory frameworks that ecosystems impacted by human pressures may be reverted to their original condition by suppressing the pressure was tested using coastal eutrophication. The response to nutrient abatement of four thoroughly studied coastal ecosystems that received increased nutrient inputs between the 1970s and the 1980s showed that the trajectories of these ecosystems were not directly reversible. All four ecosystems displayed convoluted trajectories that failed to return to the reference status upon nutrient reduction. This failure is proposed to result from the broad changes in environmental conditions, all affecting ecosystem dynamics, that occurred over the 30 years spanning from the onset of eutrophication to the reduction of nutrient levels. Understanding ecosystem response to multiple shifting baselines is essential to set reliable targets for restoration efforts.

Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia.

Science-based, multinational management of the Baltic Sea offers lessons on amelioration of highly disturbed marine ecosystems.

Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.