The observation of complex trajectories of coastal ecosystems in response to eutrophication and subsequent oligotrophication suggests that the expectation that eutrophication can be reverted back to historical reference values by reducing nutrient inputs alone may be unsupported, as oligotrophication does not seem to be occurring to the expected extent. Ad hoc explanations are typically offered to account for failure of coastal ecosystems to return to reference status upon reducing nutrient inputs (e.g., alternative nutrient sources, internal loading, shifts in limiting nutrients, colimitation effects of nutrients and light, decreased filter-feeders activity, e.g., Colijn and Cadée 2003; Paerl et al. 2004; Philippart and Cadée 2000). A decline in chlorophyll a concentration in moderately eutrophic coastal ecosystems may be triggered by factors other than oligotrophication, particularly through changes in food web structure (e.g., Heck and Valentine 2007; Cloern 2001). These observations are comparable to the much longer experience in reverting lake eutrophication. Lake ecosystems have been reported to follow convoluted trajectories following nutrient reduction, with internal loading, changes in food webs, the impacts of climate change, and 10–15-year time lags from nutrient reduction to reduced algal biomass put forward as the causes for the complex lake trajectories observed (Jeppesen et al. 2005).
We submit that the expectation that ecosystems will return to the original conditions tracking reversed trajectories following reduced nutrient inputs may be fundamentally flawed, as it ignores the consequences of shifting baselines, deriving from concurrent changes in multiple pressures, along the time elapsed between pressures and restoration (about 30 years, Table 1). For instance, historical overfishing has altered coastal food webs, removing ecological buffers (Duarte 1995) and rendering coastal ecosystem more prone to eutrophication (e.g., Heck and Valentine 2007; Jackson et al. 2001). Environmental changes induced by human activity, superimposed upon natural trends and fluctuations, have led to major, extreme, and unprecedented changes in the environment since coastal eutrophication emerged in the 1970s that could potentially affect the trophic status of coastal waters (Table 1). These changes are of such magnitude and simultaneously affect so many fundamental factors affecting the functioning of coastal ecosystems that they would have forced, through the combination of their separate and synergetic effects, coastal ecosystems to drift away from their “reference” status even in the absence of direct anthropogenic nutrient inputs.
Failure of coastal ecosystems to return to past ecological conditions upon significant reduction in nutrient inputs is disturbing both to scientists and managers, as current models and management frameworks (e.g., EU Water Framework Directive, EC 2000) assume a direct, continuous response of coastal ecosystems to altered nutrient inputs. These frameworks are derived from the widespread observation of water quality and ecosystem deterioration upon increasing nutrient inputs (e.g., Boesch 2002; Nixon 1995) and implicitly assume that opposite trajectories would be followed upon nutrient reduction, returning the ecosystems to acceptable deviation from preexisting reference conditions (e.g., EU Water Framework Directive, EC 2000). Lack of recovery of coastal ecosystems following reduced nutrient inputs is likely to create frustration, potentially leading to inaction when targets of ecosystem restoration efforts fail to be met. The scientific community is to bear much of the burden for the frustration of managers and society in general when confronted with failure to restore eutrophied ecosystems, as the role of nutrient inputs in controlling phytoplankton biomass has been generally oversimplified in the dialog between scientists and managers, possibly driven by the benign intention to deliver a message clear enough as to prompt restoration efforts. Thorough, sophisticated accounts of the interplay between nutrient inputs and other factors, intrinsic and extrinsic to the ecosystems, in controlling phytoplankton biomass abound in the scientific literature (e.g., Cloern 2001; Jackson et al. 2001; Sharp 2001; Wilkerson et al. 2006; Howarth and Marino 2006) but have not been effectively communicated to managers and policy makers.
Emphasis in returning ecosystems to a particular past state, an unlikely outcome in a world of shifting baselines, should be replaced by targets ensuring the maintenance of key ecosystem functions and, thereby, the constant supply of valuable ecosystem services to society. For instance, the examination of the four cases presented may be interpreted as evidence that nutrient reduction was ineffective. Yet, reduced nutrient inputs have halted further eutrophication, reducing associated damage and vulnerabilities, and improved some indicators of ecosystem status in many ecosystems (e.g., oxygen levels, macrophyte cover; Carstensen et al. 2006; Kemp et al. 2005). Moreover, the literature also contains examples of ecosystems that did improve following reduced nutrient inputs, such as Tampa Bay (Greening and Janicki 2006) or the Potomac River (Kemp et al. 2005). Hence, efforts to reduce nutrient inputs to eutrophied coastal ecosystems have indeed delivered important benefits by either leading to an improved status of coastal ecosystems or preventing damages and risks associated to further eutrophication.
The apparent lack of a common trajectory of coastal ecosystems represents a major challenge for scientists but also for managers and policy makers, who must consider the possible occurrence of shifting baselines and regime shifts in the ecosystems to evaluate possible outcomes of restoration efforts. Managers, and the public at large, must be educated to accommodate the possibility that ecosystem responses may display hysteresis, due to time lags in the responses and nonlinearities in the system, in the expected outcomes of managerial interventions (Kemp et al. 2005), so that expectations include observed ecosystem responses. It is, however, critical that scientific understanding of ecosystem trajectories progresses to identify what ecosystem traits determine the likelihood of individual coastal ecosystem to show the different trajectories proposed here (Fig. 1) in response to reduced nutrient inputs, to better inform managerial decisions. More generally, the scientific underpinnings of ecosystem restoration efforts must progress to reach the capacity to forecast the trajectories of ecosystems subject to multiple simultaneous pressures and changes, thereby considering the dynamic nature of reference conditions. This requires a dynamic approach to ecosystem responses, considering not only direct responses to pressures but also random drifts, shifting baselines, and nonlinear effects. Ecological thresholds must be identified, also providing boundaries for management strategies, and methods to identify these without the need to cross them are urgently needed (Strange 2007).
The impact of shifting baselines on the capacity to return ecosystems to an idealized past reference status is not exclusive of coastal ecosystems but affects all domains of environmental restoration. An example is the failure to restore the ozone layer to levels before chlorofluorocarbon (CFC) release after the implementation of the Montreal Protocol in 1987, which has been argued to derive from the combined effects of climate warming and the release of new chemicals impacting on ozone levels (Weatherhead and Andersen 2006), illustrating the need to consider shifting baselines. As for nutrient reduction efforts, the decreased release of CFCs in the Montreal Protocol was not ineffective, as it prevented further deterioration of the ozone layer. The widespread aim of preserving ecosystems unchanged in protected areas, such as national parks, represents an effort to anchor them on the shores of Neverland against the action of shifting baselines and clearly irreversible events, such as species extinctions and invasions, which will likely be met with failure. Fisheries science has also faced a general failure to predict stock recovery following reduced fishing pressure, which has led to pleas for a more ecosystem-based approach to fisheries science and management (Botsford et al. 1997), which should consider community and food web interactions and the dynamic nature of marine ecosystems, including their responses to climate change. Similarly, coastal ecosystems must be managed from an ecosystems perspective that extends beyond the role of nutrient inputs to consider the concurrent dynamics of the drivers of change and their interactions.
Society, and its actors most directly involved in ecosystem stewardship, scientists, managers and policy makers, must reconsider the targets of their efforts to focus on maintaining a healthy environment, abandoning scenarios revolving around idealized Neverlands, to confront shifting baselines. Whereas we can reasonably predict the statistical trend that ecosystems will follow upon changes in nutrient inputs, we must acknowledge that our capacity to predict the trajectories of individual ecosystems is still primitive as is challenged by elements of idiosyncrasy in the individual responses that cannot be derived from the statistical responses of multiple coastal ecosystems. Confronted with the need to act to prevent further ecosystem deterioration, we must accept the limits to governability of coastal ecosystems and the need to consider some degree of what Jentoft, addressing similar failures to manage fisheries, has referred to as “a technology of foolishness,” which emphasizes institutional experimentation and learning by doing (Jentoft 2007).
There is a major pending revolution in concepts, paradigms, and approaches in the way we manage nature that should no longer strive at delivering a planet to future generations identical to that we experienced at one point of our lives but one that maintains functional integrity and services of ecosystems conducive to a sustainable future. Ecology and restoration science must, as the character Wendy in J.M. Barrie’s play, grow to face change.