Seagrass meadows commonly reside in shallow sheltered embayments, typical of the locations that are attractive for mooring boats. These boat moorings present a source of small scale long-term repeated physical disturbance to seagrass (Hastings et al., 1995; Macreadie et al., 2015). Seagrasses take root in soft sediments where the heavy chains associated with moorings can uproot rhizomatous tissue and tear shoots (Bourque et al., 2015). Seagrasses need sheltered conditions which reflects the susceptibility of their anchoring within this environment to physical disturbance (Hemminga and Duarte, 2000).

Disturbance occurs as the mooring chain rotates around a central anchor point with tidal and wind induced movements associated with the buoy. This movement is typically exacerbated by the attachment of a boat which acts as a sail. The movement then causes the chain to drag along the substrate and can lead to declines in seagrass (Milazzo et al., 2004). Damage to the seagrass leaves, or sediment resuspension resulting in reduced light levels in the water column can lead to decreased photosynthesis levels (Di Carlo and Kenworthy, 2008). Along with reduced light levels through increased sedimentation in the water column, the direct physical impacts of moorings can cause long-term losses of other resources from the sediment such as organic carbon (Serrano et al., 2016). Physical disturbance can also prevent or limit the recovery of a damaged meadow, for example, Zostera marina seedlings exposed to physical disturbance encountered a threefold higher mortality rate than seedlings protected from disturbance (Valdemarsen et al., 2010).

The life history traits of seagrasses changes between species and across the four families (Unsworth et al., 2015). This leads to key differences in the mechanical strength of seagrass species and how they might be affected by physical stress (de los Santos et al., 2016). Differences in life history traits may ultimately lead to dissimilarities in the ability of different species to remain resilient (resist and recover) to the impacts of disturbance from a standard chain mooring. Due to the global use of boat moorings their damage to seagrass meadows has been quantified in a variety of locations, but the majority of these are within Posidonia spp. meadows, the slowest growing of the seagrasses. Boat moorings placed in Posidonia oceanica meadows have been found to result in a decline in shoot density and meadow cover and to cause rhizomes to become exposed (Francour et al., 1999; Milazzo et al., 2004; Montefalcone et al., 2008). A range of other studies have also documented the impact of moorings on Posidonia spp. (Walker et al., 1989; Hastings et al., 1995; Demers et al., 2013).

Posidonia species’ are generally considered to be long lived and slow growers, and therefore slow to recover from stress. Conversely, Z. marina is a fast growing species, and rapid recovery from small scale disturbance has been reported (Qin et al., 2016). However, very slow long-term recovery from disturbances such as dredging have also been reported (Hilary et al., 2005). Z. marina has a circumglobal distribution throughout the northern hemisphere where it plays a critical role in supporting, amongst others things, fisheries production, carbon storage and sediment stabilization (Jackson et al., 2001; Bertelli and Unsworth, 2014; Mtwana Nordlund et al., 2016). Unfortunately there is extensive and growing evidence of the large scale loss and degradation of Z. marina meadows across its range (Waycott et al., 2009; Jones and Unsworth, 2016; Lefcheck et al., 2017). Understanding how Z. marina responds to physical disturbances such as those resulting from boat moorings remains a key issue for its management.

Given that Z. marina is relatively tolerant to increased sedimentation when compared to Posidonia species’, and coupled with the ability of Z. marina to produce large quantities of seeds and exhibit rapid rhizome growth (Erftemeijer and Lewis, 2006) there exists potential for this species to undergo some level of recovery after physical disturbance. Z. marina may remain unaffected or impacted to a lesser degree (e.g., have reduced cover rather than complete loss) than other species of seagrass. This is the first study we are aware of that attempts to quantify the impact of boat moorings on Z. marina and we argue that given the propensity for moorings to be placed within seagrass habitat and the widespread distribution of this particular species around the globe such information is needed for better informed habitat management.

The present study was conducted in the United Kingdom where understanding threats to the seagrass system at local levels is also of importance. Historical loss of seagrass in the United Kingdom is thought to have been extensive (Wilding et al., 2009) and recent studies have indicated that current populations are in a perilous state (Jones and Unsworth, 2016) and poorly managed (Jackson et al., 2016). To date there exists limited understanding of the key threats to United Kingdom seagrass, specifically the impacts of boat moorings on its abundance.

In this study, we tested the following null hypothesis: Z. marina is not impacted at the small (patch) or meadow scales by the presence of boat moorings. The hypothesis was tested by examining seagrass surrounding boat moorings in seagrass meadows from a range of sites in the United Kingdom using a combination of field based assessments and remote sensing techniques.

This study demonstrates the significant impact swinging chain moorings have on the globally important seagrass species Z. marina. We provide conclusive evidence that across multiple sites, Z. marina is damaged by swinging chain moorings leading to a minimal loss of at least 6 ha of United Kingdom seagrass and the associated fragmentation of at least nine significant meadows. Given the wide scale historical loss of United Kingdom seagrass, its current perilous state and the limited conservation attention it receives (Jackson et al., 2016; Jones and Unsworth, 2016) our findings add to evidence of an already degraded ecologically vital resource. This loss of United Kingdom seagrass from boat moorings is small but significant at a local scale. This is because it fragments existing meadows and ultimately reduces their resilience to other stressors such as eutrophication (Unsworth et al., 2015; Maxwell et al., 2016).

Each swinging chain mooring was found to result in the loss of 122 m² of seagrass. This loss has the capacity to result in a reduction of the extensive ecosystem service value of seagrasses including carbon storage, invertebrate biodiversity, and fish habitat (Frost et al., 1999; Lilley and Unsworth, 2014; Macreadie et al., 2015).

Our study found that seagrass cover (%) and canopy height increases with increasing distance away from the mooring. Seagrass was mostly present within the area defined as containing a scar, although it was very low in density, with canopy height reduced. The average distance from a mooring that was scarred was 5.4 m. This radial scar of 5.4 m is lower than those recorded for Posidonia australis (e.g., 9 m) (Demers et al., 2013). This smaller scarred area may reflect the greater capacity of Z. marina to recover relative to the more climax species P. australis, alternatively it could also be the result of differences in tidal regimes.

Although the vast majority of chain moorings caused a loss of seagrass (79%), there were some that did not. The reasons for this are not clear, but may be a result of differences in chain thickness and length, duration the mooring had been in place, tidal movement, the amount of use of the mooring, or the size of the boat using the mooring. Improved understanding of these factors may help lead to solutions that can reduce the impact of swinging chain moorings on Z. marina.

Although seagrass was recorded within the scarred areas the cover was always less than 5%. This low cover is significant as although it offers little in the way of ecosystem service provision such as fish habitat (McCloskey and Unsworth, 2015) it suggests that recovery might be possible if a mooring is removed or replaced with an ecologically sensitive design (Demers et al., 2013).

The present study did not determine whether low cover was the result of new propagules or seeds, or whether it remained from rhizomes present in the sediment. Such recovery potential requires further consideration. The capacity of the seagrass to recover will depend upon the multiple attributes of the resilience of the system (Unsworth et al., 2015). Some of the sites examined in the present study are considered ecologically largely healthy (e.g., good water quality) except for the mooring and other boat damage (Jones and Unsworth, 2016), meaning that if the moorings were replaced with environmentally sensitive gear there is considerable potential for seagrass recovery.

The results presented here add to the argument regarding the effect moorings have on seagrass in general, supporting (Walker et al., 1989; Hastings et al., 1995; Montefalcone et al., 2008; Demers et al., 2013; Serrano et al., 2016) who report negative impacts of moorings for a variety of seagrass species (see Appendix 1).

Although we determined scar area to have a mean diameter of 5.4 ± 3.5 m our study documents how seagrass cover at a distance of between 16 and 20m from the mooring anchor point was lower than that at the control sites. This could indicate the impacted area is larger than just the scarred area, beyond scar damage would be due to the moorings causing seagrass thinning through lower levels of disturbance at the extreme ends of the mooring chain and at distances away from the chain.

Thinning has the potential to impact the ecosystem services provided by seagrass. Studies in Australia have illustrated how long-term carbon stores can be lost as a result of mooring damage (Macreadie et al., 2015; Serrano et al., 2016) and research in Canada has demonstrated invertebrate diversity and abundance is lower in areas with moorings and high levels of recreational boat use (Leatherbarrow, 2006).

Our study found a negative correlation between seagrass cover and algal cover, although this correlation was weak, it suggests that algae may colonize and grow in areas where seagrass has been removed or has faced cover declines through thinning. The presence of macro-algae can limit the recovery potential of seagrass and cause further declines through competition for light and space (Orth et al., 2006). Other ecosystem stressors such as eutrophication can exacerbate this competition by supporting increased algal cover (Short et al., 2006). The influence of light reduction due to shading of the seagrass by the presence of a boat on the moorings may also contribute to such stress.

Seagrass meadows examined in the present study were not found to be impacted at the whole meadow scale by moorings. All impacts were found to be within an area of approximately 20 m radius from the mooring. At the two sites examined for such impacts, seagrass was variable and did not provide a clear consistent pattern of change with respect to distance from the mooring. Seagrass cover at Studland Bay was much more consistent than at Durgan, this may be due to areas of Durgan being anchored on throughout the bed, leading to exacerbated habitat fragmentation (Francour et al., 1999; Collins et al., 2010). The distance from the moored area was also found not to impact the algal cover (%), this again could be due to the quality of the seagrass meadow as a whole not being compromised by moorings.

Satellite imagery and aerial photographs are effective means of assessing seagrass loss (Hastings et al., 1995; Ward et al., 2003). By using a simplistic method of assessing seagrass extent the present study was broadly able to calculate the area of loss of seagrass surrounding a boat mooring (subject to clear imagery). Areas of seagrass scarring determined using remote imagery were not significantly different to those determined in the field (based on seagrass < 5% cover), although some site specific differences were recorded (underestimations with remote methods). We suggest that the depth of the meadow may have influenced this, as has been found in previous studies (Ferwerda et al., 2007). Although the use of such imagery is a quick and cost effective way to monitor the loss of seagrass and has allowed us to propose that at least 6 ha of seagrass in the United Kingdom has been lost due to boat moorings, caution should be taken as this figure is likely an underestimations of seagrass loss.

We find strong evidence that swinging chain moorings result in a significant loss of Z. marina. This loss and impact is isolated to the surrounding area to the mooring and does not translate to a meadow scale. However, an abundance of moorings, as is seen throughout the United Kingdom and globally at sites containing seagrass, can lead to a significant loss of habitat and ultimately ecosystem functioning. In spite of the higher growth and recovery potential of Z. marina relative to species of Posidonia spp. impacts are still of the same magnitude, although a little smaller in area. Although this study mostly documents the impact of these installations, we did record a limited few instances of no impact, the reason for this requires further study and may help mitigate further loss of seagrass from singing chain moorings. The findings of the present study confirm the need for more widespread use of seagrass friendly mooring systems to reduce seagrass loss and to support recovery (Cullen-Unsworth and Unsworth, 2016).

BW and RU contributed to study design, data collection, analysis and write up; LC-U and BJ contributed to write up and analysis.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.