Growth of FER studies with time and socioeconomic development
The growth of FER research and practices with time is approximately consistent with the deterioration of water quality and ecological functions in freshwater ecosystems. During the first five years of 21 century, the water qualities of surface freshwater exhibited the worst status. Meanwhile, the freshwaters were also facing increasingly serious issues, such as habitat losses, species extinction, ecological invasion, ecosystem service decline, and so on [16, 30, 36, 44]. That is also when a number of nation-wide FER projects have been launched to alter the situation (e.g., Major National Scientific and Technological Projects on Water Pollution Control and Treatment). The rapid increase in FER studies during the same period (Fig. 1) was mostly the outcome of these projects and efforts. The studies conducted in Suzhou River, Shanghai and Taihu Lake, Jiangsu Province contributed the majority of publications in the East China, which is inevitably ascribed to the severity of water pollution of these two waters. Additionally, the regional GDP and investment into environmental protection had significant correlations with the number of FER studies (Table 1), highlighting the importance of overall economic development and financial investment into the scientific research and environmental protection [12, 23].
Quantitative details of the FER projects
Almost half of the FER studies were conducted in lakes (Fig. 3A), which was likely associated with the bigger pressure from improving the deteriorated water qualities in the large lakes compared to other ecosystem types. And it in some way reflects the fact that it is more difficulty to perform small-scale projects or studies in large running waters. In most studies in which the authors mentioned about the causes of ecosystem degradation, the predominant causes are land reclamation and eutrophication (Fig. 3B), which is consistent with what we mentioned in the introduction.
Most restoration projects are completed at reach scale for rivers and area scale for lakes, the ratio of studies at watershed scale is only 2% (Fig. 3C). It is associated with the fact that, in the past decade, many FER projects were implemented as demonstrations of the national wide and government-driven projects aiming to mitigate water pollution and ecological degradation. The disassembling of watershed-scale planning and over-dispersed investment and management result in the relatively small scales of restoration project. However, there is no doubt that an effective restoration requires coordinated efforts at the watershed, because the sources of impacts to ecosystems are largely generated outside of the freshwaters in the watershed, thus restoration potential may be very limited when entire watersheds are impaired [26, 37]. The tangling of property ownership, expectation for socioeconomic development from the government, interest demand from the public, and the investment mechanism, usually complicates the planning, implementation and evaluation of the restoration projects at the watershed level. Thus, it requires more interactions between different interest groups, more profound pre-restoration planning, and longer-term and bigger-scale monitoring for the watershed-scale FER projects.
Over 60% of the studies assesse the project outcome by comparing to a reference site as the simultaneous control without intervention (Fig. 3D). This is particularly the case if the pre-restoration information about the ecological status is lacking. Actually, for the studies using the status before restoration as the reference, the “status before restoration” in most cases does not necessarily refer the status before degradation, and most project outcome assessments have just suggested whether the restoration actions worked compared with the situation without them.
The most frequently mentioned goals of restoration are improvement of biodiversity and water quality, and ecosystem services (Fig. 4A). The emphasize on biodiversity and water quality is similar with what was found in the study reviewing on the status of river restoration in the United States by Palmer et al. , while the difference is that the goals related to stabilizing channels, improving riparian and in-stream habitat for the US projects account for considerable percentages in the overall projects. Additionally, the methods used in our FER projects are dominated by revegetation (accounting for over 40%, Fig. 4B), which is greatly different from the focus on geomorphic measures for channels and streams in the US projects. It is partly ascribed to the popularity of the Natural Channel Design (NCD) approach for river restoration in the US in 20th century . The broad application of revegetation as an restoration measure has its practical cause. Although spontaneous colonization by plants, following habitat recovery, is normally preferred, sometimes the re-introduction of native pioneer species is necessary in case the habitats are isolated or fragmented, or the seed bank is lacking .
Time scale of the intervention and monitoring are typically shorter than 1 year (Fig. 5). Since most biogeochemical and ecological processes delay the responses of freshwater ecosystems to the interventions, potentially for decades , it is important to recognize that some restoration goals may only be achieved in a long run, which make the short-term monitoring compromising in offering substantial evidences underpinning the solid conclusions about whether the project outcomes are positive. Thus, the expanded duration, at least of post-restoration monitoring is needed to support the evaluation of FER outcomes.
Outcome evaluation of the FER projects
Defining the success of a restoration project has always been an issue for restoration ecologists , and the main categories and key attributes that used to evaluate the outcome of the FER projects have been kept revised and updated during the past years. Besides of morphological and physical characteristics, water quality, biological characteristics and ecological processes, the categories of metrics for evaluating the outcome of FER projects we used in this study also include socioeconomic benefits (Table 1). However, only 3 publications provided direct results concerning economic benefit from the restoration projects. Unlike the most previous studies which mainly used morphological or physical characteristics as indicators, our study shows that biological characteristics and water quality are the most commonly used categories for outcome evaluations (Fig. 6), and at least nearly 80% of the publications provided positive results based on the improvement of the monitored indicators, which can be considered to meet the restoration goals.
Although numerous studies of stream or river restoration indicate that the restoration of channel form and stream bank can lead to rehabilitation of physical habitat, biological recovery is not common even when the in-stream/channel structural variables are completely restored. Simply, habitat may be important ecologically, but it is not sufficient for assessing ecological outcomes, and in the vast majority of cases restoration of habitat does not certainly lead to biological restoration . In fact, as one of the most frequently considered categories in evaluating the restoration effectiveness in our study, water quality usually limits the biological recovery of freshwaters, and thus ecological recovery will not occur until the source of pollutant is removed . Many studies show and interpret the relationships between nutrient levels and ecological structure and function in freshwater ecosystems, and it is the classic theory of eutrophication of shallow lakes underpinning the most of FER practices in China.
In most projects aiming to improve water quality, physical/chemical measures and revegetation are employed, and in most cases, they are proved to be efficient in achieving the goal in our study. However, the pollution mitigations are typically carried out at relatively small scales (e.g., river section or lake region), and the monitoring of water quality and ecological status is almost accordingly conducted within limited spatial and temporal ranges (Fig. 5B), which makes whether the restoration effect can exists in a broader range and a longer term questionable. While, the best management practices at a watershed scale may reduce and delay pollution to the waterbodies and may improve water quality conditions and ultimately restore the ecological processes successfully . But the number of projects focusing on the watershed-scale restoration is still small over the past decades (Fig. 3C), not only in China but across the global [17, 20]. However, there is an increasing number of FER studies starting to include larger-scale and longer-term targets, which is hopeful for more thoroughly understanding restoration ecology and better FER practice.