Articles sourced and retained for the systematic map
The search for relevant articles using the selected search strings and search engines returned a total of 2297 articles. The full set of 2297 articles was composed of 691 articles for nuclear (689 peer reviewed articles, 2 grey literature reports), 939 for oil & gas (913 peer reviewed articles, 29 grey literature reports) and 667 for offshore wind farms (667 peer reviewed articles, 0 grey literature reports) (Fig. 1).
Article selection then proceeded based on the inclusion criteria. A total of 1960 were discarded either because they were duplicates or on review of their titles were judged to be spurious. The total available articles considered in the second filter (abstract review) were 337 (nuclear 85, oil & gas 200, offshore wind 52). The Fleiss kappa test was carried before proceeding to filter 2 at this stage and returned a result of 0.53. This result falls within the range of moderate agreement between reviewers. The team discussed the non-agreement to clarify any remaining inconsistencies in understanding and clarification of criteria being used in the selection process. The team then continued with the selection of articles using filter 2 which resulted in a further 199 articles being discarded across all the energy systems. These were split as follows: 40 articles (38 peer reviewed, 2 grey literature), excluded for nuclear; 132 (104 peer reviewed, 28 grey literature), articles for oil & gas and 27 (27 peer reviewed) articles for offshore wind.
The remaining 138 articles were then read in full resulting in the further exclusion of 88 articles. These articles were excluded for reasons ranging from “no quantifiable data” to “not accessible” and are listed in Additional file 3 by energy type.
Fifty articles were included in the final systematic map composed of 11 articles for nuclear, 22 for oil & gas and 17 for offshore wind. All articles identified for full article review were available and accessible from on-line journals or hard copies from library archives. The full list of articles included in the systematic map is listed in Additional file 4.
Many of the articles had more than one study as presented in Fig. 1 and seen in the systematic map database (Additional file 5). The total number of studies from which data was extracted totalled 208 studies. These were split: 22 for nuclear, 145 for oil & gas, and 41 for offshore wind.
The final set of articles used in the systematic review spanned the period 1977–2013 (Fig. 2; column B in Additional file 5). The nuclear and oil & gas articles included studies from the 1970s and 1990s respectively which is unsurprising giving the presence of nuclear energy from the 1950s and oil & gas from the late 1800s. Articles focusing on offshore wind energy were more recent, as the first offshore wind farm was only constructed in the 1990s. It is evident from the consistent and increasing presence of these publications over time that investigating the environmental impacts of energy systems is of continuing importance.
Location of articles
The geographical locations of the studies in the articles are clustered around the main production spots of the three energy systems around the world and associated large marine ecosystems (Fig. 3; column I of Additional file 5). Oil & gas studies covered the greatest geographical spread ranging from the north Beaufort Sea down to the South Brazil shelf and across to the Laptev Sea, reflecting the proliferation of this fuel around the world. Studies focusing on nuclear energy were concentrated around a latitudinal zone spanning N22° to N53°, stretching from the NE US Continental Shelf to the Celtic-Biscay Shelf and onto the South and East China Sea. Offshore wind studies were clustered in and closely linked to waters of the Irish Sea, North Sea and Baltic Sea, reflecting Europe’s lead in the construction and operation of offshore wind farms.
Subject details
The subjects of the reviewed studies for nuclear and oil & gas appeared to reflect a tradition of benthic impacts, with fewer studies considering the pelagic system (Fig. 4; column E of Additional file 5), possibly because benthic systems may be expected to endure longer lasting effects compared to the more dynamic pelagic communities. Conversely, studies exploring the impacts of offshore wind farms had a more specific focus on key and highly mobile species, including marine mammals, birds and fish possibly because wind arrays have a tradition of being considered to cause potential harm to migration routes [23]. Reviewed studies thus seemed to suggest that emphasis is typically placed on evaluating impacts on specific ecosystem components in each case, and that gaps may remain regarding reviews of potential impacts on different ecosystem components for each energy type.
Additionally, by comparing subject type with the lifecycle stage of each energy system (i.e. construction, operation or decommissioning), it was evident that most studies have been focused on the operation phase (Fig. 5; column AC of Additional file 5). Within these studies on operation, 63% of nuclear, 52% of oil & gas and 12% of offshore wind studies focused on benthic impacts.
Outcome variable
The outcome variables used in the search terms included thirty different possibilities, but only ten of these were identified in the studies reviewed. The most studied outcome across all energy systems in absolute terms was population change and ecosystem function/process, followed by abundance, biodiversity and community richness (Fig. 6; column Z of Additional file 5).
However, if these studies are analysed by energy type and subject, then in relative terms ecosystem function/process was the most common variable investigated particularly in the benthic community, while abundance and population were frequently studied across the majority of subjects (Fig. 7).
Quality of study and design
Ninety-nine percent of all studies across all energy systems reviewed were designed to collect primary data, and so received the highest score for this attribute. Secondary data made up the remaining 1% (Fig. 8a; column Q of Additional file 5). The use of fully structured comparator design (i.e. BACI design), which are deemed to provide a more robust basis for impact studies, were restricted to 28 of the studies reviewed (15% of the total) specifically in oil & gas and offshore wind farm studies (Fig. 8b; column R of Additional file 5). Forty percent of offshore wind studies were found to follow a BACI design. The next most popular comparator type in absolute terms was the spatial comparator (control-impact, CI), making up 48% of all studies reviewed for oil & gas. Nuclear studies were found to have the highest proportion of studies (45%) where no spatial or temporal comparator was employed to measure impacts, as studies often reported on data collected over a particular time period during exposure (e.g. operation). This could suggest that impacts are measured through potential changes to the ecosystem during operation, but no real controls are employed.
Replication was the third category considered important in ranking the strength of individual studies (Fig. 8c; column W of Additional file 5). Fifty percent of the studies retained for the systematic map included either temporal or spatial replication, which scored the highest in this category. Over half of all offshore oil & gas and offshore wind focused studies matched this attributes.
Impact on ecosystem services
Ecosystem service impacts were not explicitly mentioned or considered in any of the studies reviewed. It was the objective of this systematic map to introduce this extra dimension to allow the studies to be compared in terms of ecosystem services across energy systems.
The results show that the species and habitats evaluated in the studies and impacted by offshore energy systems, provide a range of ecosystem services across the provisioning, regulating, supporting and cultural categories. Regulating services were the most commonly associated with the benthic species studied followed by supporting, provisioning and cultural. Across the other species types, cultural services were particularly dominant in the studies of impacts of marine mammals and birds from offshore wind farms. Supporting and provisioning services were also associated with the demersal and pelagic species investigated (Fig. 9; column AB of Additional file 5).
Translating changes into ecosystem services highlights the opportunity for species other than those traditionally studied to be researched based on the ecosystem services they provide. With the marine environment becoming ever more intensively used, and demands for ecosystem based management, a holistic investigation of a range of species that provide a suite of ecosystem services is required to inform planning and operational decisions.