Thresholds of environmental sustainablility
What are ecological thresholds?
Ecological thresholds have been defined as the breaking points of ecosystems. At a threshold there is an abrupt change in an ecosystem quality, property or phenomenon. Thus small changes in an environmental driver can produce large responses in the ecosystem (Groffman et al. 2006). These kind of dramatic shifts have been documented for many systems; from rapid eutrophication of coastal waters to structural changes in fish communities. Trespassing of thresholds may have serious economic consequences (Barbier 2007) (Taylor et al. 2006).
|Table 1: EXAMPLES OF DOCUMENTED SHIFTS IN STATES IN AQUATIC ECOSYSTEMS|
|(Modified from Folke et al. 2002) |
|Ecosystem||Alternative 1||Alternative 2|
|Freshwater systems||Clear water, Benthic vegetation||Turbid water, Blue-green algae|
|Oligotrophic macrophytes and algae||Cattails, and Blue-green algae|
|Game fish abundant||Game fish absent|
|Marine Systems||Hard coral||Fleshy algae|
|Kelp forests||Urchin dominance|
|Seagrass beds||Algae and muddy water|
Despite their intuitive appeal, it is difficult to define thresholds precisely. There are several definitions for the concept. Common features of most definitions are the nonlinearities of the responses in ecological or biological systems to pressures caused by human activities or natural processes. But ecological thresholds do not just refer to sudden jumps in a time series. They imply nonlinear dynamics, with possibilities for alternative stable state, shift of the ecosystem, hysteresis, and points of no return.
In models it is possible to determine the exact threshold, but in nature thresholds shifts or transitions occur within ranges of pressures and not at an exactly specifiable point (Huggett A. 2005). Another important characteristics is hysteresis, which means that the state of a system depends on its history. Even when a change is not irreversible, the return path from an altered state towards the original state can be drastically different from the development that lead to the altered state.
Different scales of coastal thresholds
Thresholds can be detected at different spatial, temporal or functional scales. Analyses of thresholds should recognize the possibility of interacting ecological thresholds at different scales. Events in the coastal areas are on one hand dependent on processes in the water shed, on the other hand on processes of the open sea. Coastal areas are also highly dependent on events in the watershed. Coastal lagoons and enclosed seas often have large drainage areas relative to the sea surface. For example, the drainage area of the Baltic Sea covers 1, 700, 000 km2, or more than four times the entire water area (415, 266 km2). Changes that originate in the land use can be manifested as the trespassing of ecological thresholds in the coastal waters. Hysteris effects mean that cuts in, for example, nutrient inputs do not necessarily induce an immediate response. (Stålnacke 2005).
Regime shifts have been used to explain abrupt changes in ecosystems. The case of the sudden transition of the Sahara from a vegetated wet land to a dry and barren desert some 5500 years ago is one example (Foley et al. 2003). Regime shifts have also been documented for marine ecosystems, and typically involve changes in dominant species, general community composition, and trophic level.
Regime shifts can be characterised as rapid reorganizations of ecosystems from one relatively stable state to another. In the marine environment, regimes may last for several decades and natural shifts have often been linked to changes in climatic conditions. Regime shifts have also been linked to human pressure on systems and have then been seen as a change between a healthy and an unhealthy state of the ecosystem (Rapport 2007).
Climatic changes and human pressures appear to be the main triggering factors causing ecosystem regime shifts. For instance, increased sea surface temperature and possibly change in wind intensity triggered a change in the location of an oceanic bio-geographical boundary along the European continental shelf in the 1970 in western European basin (Beaugrand 2004). This in turn affected different components of North Sea marine ecosystems. The Black Sea provides examples of changes clearly have been caused by human pressures (Daskalov et al. 2007).
Threshold mechanisms in coastal waters
Although there are well documented regime shifts in coastal waters many more have probably occurred, or are likely to occur, if pressures increase (Walker & Meyers 2004) Some of the mechanisms underlying regime shifts are reasonably well known. For instance, the loss of plant communities on the sea floor can be attributed to increasing nutrient concentrations that stimulate the growth of phytoplankton and epiphytic algae, and their expansion in turn shades seagrasses and macroalgae (Krause-Jensen et al. 2008). A threshold of light attenuation of 0.27 m-1, setting a depth for seagrass in the Mediterranean Sea has been detected (Duarte et al. 2007). 
Similarly, an analysis of a large dataset from Danish coastal waters demonstrates that the cover of macroalgal communities in deeper water decreases markedly along a eutrophication gradient (Krause-Jensen 2007). The analysis indicates that algal abundance initially responded slowly to increasing eutrophication, but showed a more marked response at nitrogen concentrations around 35-40 µM, indicating a sec-ond order transition.
In the Ringkøbing Fjord on the west coast of Denmark transitions that indicate thresholds are driven by sluice management that affects the salinity (Håkanson & Bryhn 2007). From 1995 to 1997, a dramatic change took place because of a change in water salinity due to the implementation of a new sluice practice. The ecosystem changed from a nutrient-driven turbid green water to a grazing-controlled clear water.
Implications for management
Regime shift and thresholds can have major implications for ecosystem management(Petersen et al. 2005). . Dramatic changes are always challenging to deal with, but matters are complicated further by the fact that a change that is perceived to be adverse from one perspective may turn out to be beneficial from another. For example the Ringkøbing fjord is now closer to many environmental objectives, even though the improvements were not caused by a reduction of anthropogenic pressures, such as nutrient discharges. However, the southern part of the lagoon is designated as a Ramsar site and as a Special Bird Protection Area. Several of the birds forage on the water vegetation, which has decreased dramatically. Return to the previous turbid green state would be an obligation from the perspective of bird protection, but would not be admissible under the European Union Water Framework Directive. This contradiction between nature conservation and environmental protection may eventually be solved by the gradual increase in macrophyte coverage. Complex interactions with different consequences and management implications have also been documented for coral reefs (Knowlton & Jackson 2008)  and fisheries (Burgess et al. 2003). Thus it is not always a question of avoiding thresholds and regime shifts at any costs, but to learn to live with the possibility and to adapt to new situations. This will require social learning that engages both users of coastal resources, managers and researchers.
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