Cease or alter maintenance activities on subtidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
Definition: ‘Ceasing or altering maintenance activities’ includes actions taken to avoid or reduce the disturbance, damage or removal of native organisms from structures, with the aim of enhancing their biodiversity.
Subtidal rocky habitats experience intermittent disturbance from storms, sedimentation, pollution and other human activities, which lead to fluctuations in biodiversity (e.g. Balata et al. 2007). These pressures are often more pronounced and frequent on artificial structures, especially those built in urban areas with high human activity and poor water quality, and/or in areas of high wave energy (Airoldi & Bulleri 2011; Moschella et al. 2005). Artificial structures are also often subject to disturbance from maintenance activities carried out to ensure they remain fit-for-purpose, safe, and aesthetically acceptable. Maintenance can include repairing or reinforcing points of weakness such as eroded cracks or holes, moving or replacing dislodged components, or cleaning regimes using physical or chemical methods. Such activities can further disturb, damage or remove biodiversity from structure surfaces (Harasti et al. 2010; Mamo et al. 2020), reduce the availability of microhabitats for organisms to shelter in (Moreira et al. 2007), and leave bare space available to opportunistic non-native or other nuisance species (Viola et al. 2017).
Although some maintenance is likely to be essential, there may be opportunities to cease or alter activities that disturb, damage or remove native organisms from subtidal artificial structures, to maintain or enhance their biodiversity. Altering activities could include using lower-impact methods, reducing the frequency or adjusting the timing of maintenance to avoid disturbance, damage, removal or the creation of bare space on surfaces when non-native or problematic species are more likely to occupy it (Airoldi & Bulleri 2011; Viola et al. 2017). It could also include allowing natural weathering of structure surfaces to occur, creating texture and microhabitat spaces (Moreira et al. 2007).
Studies of the effects of real or simulated maintenance activities to illustrate their impact compared with no or altered maintenance, where it is not clear that ceasing/altering maintenance would be a feasible conservation action, are not included but are informative (e.g. Airoldi & Bulleri 2011; Mamo et al. 2020; Viola et al. 2017). Studies that investigate cleaning activities to control or remove non-native or nuisance species are similarly not included where these actions are indiscriminate, simultaneously removing all biodiversity (e.g. Novak et al. 2017).
Airoldi L. & Bulleri F. (2011) Anthropogenic disturbance can determine the magnitude of opportunistic species responses on marine urban infrastructures. PLoS ONE, 6, e22985.
Balata D., Piazzi L. & Benedetti-Cecchi L. (2007) Sediment disturbance and loss of beta diversity on subtidal rocky reefs. Ecology, 88, 2455–2461.
Harasti D., Glasby T.M. & Martin-Smith K.M. (2010) Striking a balance between retaining populations of protected seahorses and maintaining swimming nets. Aquatic Conservation: Marine and Freshwater Ecosystems, 20, 159–166.
Mamo L.T., Porter A.G., Tagliafico A., Coleman M.A., Smith S.D.A., Figueira W.F. & Kelaher B.P. (2020) Upgrades of coastal protective infrastructure affect benthic communities. Journal of Applied Ecology, 58, 295–303.
Moreira J., Chapman M.G. & Underwood A.J. (2007) Maintenance of chitons on seawalls using crevices on sandstone blocks as habitat in Sydney Harbour, Australia. Journal of Experimental Marine Biology and Ecology, 347, 134–143.
Moschella P.S., Abbiati M., Åberg P., Airoldi L., Anderson J.M., Bacchiocchi F., Bulleri F., Dinesen G.E., Frost M., Gacia E., Granhag L., Jonsson P.R., Satta M.P., Sundelöf A., Thompson R.C. & Hawkins S.J. (2005) Low-crested coastal defence structures as artificial habitats for marine life: using ecological criteria in design. Coastal Engineering, 52, 1053–1071.
Novak L., López-Legentil S., Sieradzki E. & Shenkar N. (2017) Rapid establishment of the non-indigenous ascidian Styela plicata and its associated bacteria in marinas and fishing harbors along the Mediterranean coast of Israel. Mediterranean Marine Science, 18, 324–331.
Viola S.M., Page H.M., Zaleski S.F., Miller R.J., Doheny B., Dugan J.E., Schroeder D.M. & Schroeter S.C. (2017) Anthropogenic disturbance facilitates a non-native species on offshore oil platforms. Journal of Applied Ecology, 55, 1583–1593.
Supporting evidence from individual studies
A replicated, randomized, controlled study in 2007–2008 on a subtidal swimming-enclosure net in Sydney Harbour estuary, Australia (Harasti et al. 2010) found that enclosure-net panels cleaned only along the top section supported a higher abundance of seahorses Hippocampus abdominalis and Hippocampus whitei than panels cleaned only along the bottom or from top-to-bottom. Over four months, enclosure-net panels cleaned only along the top supported more seahorses (20% of original abundance) than panels cleaned along the bottom (5%) or from top-to-bottom (3%). Maintenance activities were altered on a polypropylene swimming-enclosure net (length: 150 m; height: 3–4 m from sea surface to seabed; mesh size: 100 mm) in November 2007. Net panels (4-m sections) were either cleaned along the top only (surface to 1 m depth), the bottom only (seabed to 1 m above), or from top-to-bottom (surface to seabed). There were four panels of each treatment, randomly arranged along the net. Seahorses were removed during cleaning then replaced in the same position, while all other organisms were scraped from the net. Some panels were left uncleaned but this treatment was not considered a feasible conservation action since the weight of the net could cause it to break or sink. Eleven seahorses on each panel were tagged and monitored over four months.Study and other actions tested
A replicated, paired sites, controlled study in 2008 on eight subtidal pontoons in the Delaware Inland Bays, USA (Marenghi et al. 2010) found that reducing the frequency of cleaning activity did not increase the survival or growth of transplanted oysters Crassostrea virginica on floats attached to the pontoons, nor did it alter the non-mobile invertebrate, mobile invertebrate and fish community composition or increase their species diversity, richness or abundance on and around floats, but it did increase the macroalgal abundance. Data for all comparisons were reported as statistical model results. Over four months, transplanted oyster survival and growth was similar on floats cleaned every four or two weeks. The same was true for the overall community composition, and the species diversity, richness and abundance of non-mobile invertebrates and of mobile invertebrates and fishes on and around oyster floats. The abundance of macroalgae was higher on floats cleaned every four than every two weeks. Maintenance activities were altered on floats holding transplanted oysters attached to pontoons during June–September 2008. Hatchery-reared oysters (61 mm average length) were transplanted into wire baskets (25 mm mesh size) submerged 0.2 m beneath plastic floats (1.0 × 0.7 × 0.3 m) and attached to pontoons. One float with oysters (6 l) and one without were attached to each of eight pontoons in June 2008. Floats were cleaned with a freshwater hose every four weeks on four pontoons and every two weeks on four. Oyster survival and growth was monitored, non-mobile invertebrates on oyster shells were counted, and mobile invertebrates and fishes on and around floats were netted (3 mm mesh size) and counted over four months.Study and other actions tested