Swinging boat moorings: Spatial heterogeneous damage to eelgrass beds in a tidal ecosystem

Highlights

• Swing boat moorings locally impact Zostera marina habitats.

• SCUBA-diving sampling revealed larger effects than expected by aerial survey.

• Coupling observations and models gives clues to critical phases of the chain movements.

• Repeated physical disturbance induces phenotypical plasticity in Z. marina plant structure.

Abstract

Seagrass meadows are currently known to be subjected to huge physical disturbances including boat moorings in shallow bays. We aimed to identify the impact of permanent swing mooring on the fast-growing seagrass Zostera marina in a mega-tidal area. Coupling the hydrodynamic MARS3D model to simulate mooring chain movements and in situ measurements of plant traits, we analyzed the structural responses of the eelgrass bed to scraping disturbance in the western English Channel (France). A comparison of the results with a reference site without any permanent swing boat mooring showed a significant impact on eelgrass structure (shoot density, leaf size, leaf dry weight), depending on the direction and distance from the mooring. Zostera marina was absent close to the mooring fixation point in three out of the four directions we evaluated. Beyond 5 m, the canopy height remained lower than in the reference site, most likely due to regular disturbances by mooring chains. Conversely, shoot density beyond 5 m was higher than in the reference site. This adaptive response counter-balanced the decrease in canopy height at these distances. The fluctuations of the structure of the eelgrass cover (number of shoots, leaf length) at a small spatial scale was clearly in accordance with the scraping intensity simulated by the MARS3D model. The tidal currents coupled to tidal amplitude variability imply a small-scale heterogeneous effect of permanent mooring on the benthic compartment, previously undetected by an aerial survey. The present results highlight the interest of coupling approaches so as to understand how physical pressure influences fast-growing species traits. The resulting important modifications could imply a more functional impact such as biodiversity loss and carbon sequestration, which is beyond the scope of the present paper.

Introduction

Despite their high ecological, economical and conservation value (Costanza et al., 1997; Jackson et al., 2013), seagrass meadows are known to be largely impacted by a wide range of direct and indirect human-induced disturbances (Orth et al., 2006 and reference therein) leading to accelerated seagrass losses across the world over the last decades (Short and Wyllie-Echeverria, 1996; Waycott et al., 2009). Among the major causes, repeated physical disturbances such as those caused by swinging boat moorings are one of the major reasons of local decline (Orth et al., 2006). Damage occurs when the heavy mooring chains rotate with the tidal current and wind movements associated with the buoy. This movement causes the chain to scrape up the surrounding substrate, uprooting seagrasses and resulting in clear damage on the seagrass cover around the fixing point of the mooring (Walker et al., 1989; Hastings et al., 1995; Demers et al., 2013; Unsworth et al., 2017; Sagerman et al., 2019). This always produces long-lasting impacts due to direct regular physical disturbance such as sediment erosion, and partial or total destruction of the seagrass cover (see Jackson et al., 2013). This has been clearly demonstrated on both slow-growing tropical species such as Posidonia sp. and Amphibolis sp. (Walker et al., 1989; Hastings et al., 1995; Demers et al., 2013) and temperate eelgrass such as Zostera marina (Collins et al., 2010; Unsworth et al., 2017). However, a recent systematic review on the effects of mooring infrastructure on benthic vegetation concluded that (1) the effects could vary according to the study and (2) there were too few studies to identify the reasons (Sagerman et al., 2019).

Even if the loss of seagrass habitats seems to be relatively restricted to a few meters surrounding the mooring, at least 18% of the seagrass lost between 1941 and 1992 at a regional scale are thought to be the consequence of boat mooring in Western Australia (Hastings et al., 1995). The over-dispersion of mooring areas along the coastline still largely constitutes a substantial source of repeated destruction of seagrass beds (Walker et al., 1989; Unsworth et al., 2017; Sagerman et al., 2019). In addition, seagrass loss must also be considered by including the critical alteration of the functional role of seagrass at the local and regional scales. It is well known that the loss of surface area as well as increased fragmentation could modify the functions and ecosystem services supported by the seagrass bed. This could therefore affect the associated biodiversity (Backhurst and Cole, 2000), and more generally ecosystem functioning. The decline of the dominant Atlantic species Zostera marina L. (1753), which is widespread across Europe, could notably affect the carbon storage capacity (Martin et al., 2005; Ouisse et al., 2011 for example) and reduce resuspension and turbidity (Widdows et al., 2008; Reidenbach and Thomas, 2018 for example). Habitat loss may also result in loss of refuge for juveniles, thereby directly or indirectly diminishing the role of this area as a nursery and its feeding functions for many organisms (Boström et al., 2014), including epifauna and flora (Reed and Hovel, 2006; Gustafsson and Boström, 2009), infauna (Silberberger et al., 2016), benthic macrofauna (Fredriksen et al., 2010), fish and birds (Plummer et al., 2013). It is also well known that these highly productive areas provide important carbon sources that directly or indirectly sustain coastal food webs (Jaschinski et al., 2008; Jephson et al., 2008; Baeta et al., 2009; Ouisse et al., 2012), related biodiversity and commercial resources (Wennhage and Pihl, 2002; Cole and Moksnes, 2016).

Initiatives proposed by international conventions on biological diversity (Ramsar Convention, Bern Convention) and then European directives (Habitat Directive EU, Ospar Convention) have led to eelgrass habitats being specifically targeted for conservation and restoration (Wynne et al., 1995). More recently, policy and management aimed at protecting Z. marina habitats in France was tiered from an international scale down to a regional scale. Although seagrass beds have been recorded all around the western coasts of France, they consist of more or less isolated population units growing best when located in shallow subtidal (10–15 m maximum) or intertidal water areas, on muddy-sandy seabeds (and sometimes even on gravel) (Borum et al., 2004). Unfortunately, despite the European directives, these sheltered bays and estuaries represent excellent sites for the expansion of new marina moorings.

While there is a wealth of information on the impacts of anchoring and boat mooring on growth and reproductive success processes, especially for slow-growing seagrasses (Francour et al., 1999; Hastings et al., 1995; Montefalcone et al., 2006; Demers et al., 2013), studies of boat mooring damage on faster-growing seagrasses such as Z. marina are relatively scarce. Aerial satellite imaging and in situ video imaging evidenced that boat mooring led to complete destruction of the Z. marina cover up to 5 m around the weighted mooring point in mega-tidal ecosystems (Unsworth et al., 2017). Nevertheless, size range of vessels, depth and maintenance on boat mooring can be modify the impact on seagrass bed (Glasby and West, 2018). In addition, tidal amplitude and current direction variability over each tidal cycle could influence the distance and the direction of scraping by the mooring chain. This repeated circular non-regular movement of the mooring chain could thus entail non-homogeneous disturbances on Z. marina according to the direction and the distance from the fixing point of the mooring. In addition, Z. marina has a relatively huge capacity to recover from repeated environmental disturbances such as clam harvesting (Qin et al., 2016) by adapting its energy allocation according to environmental conditions. This phenotypical plasticity (number of leaves, canopy height, biomass) could thus result in more complex responses of seagrasses to human-induced disturbances than just the scarred area.

The present study aims to address how irregular movements of mooring chains induced by the tidal cycle affect the structure of Z. marina bed responses, in order to understand a key issue for the management of boat mooring areas along European coasts. We tested the hypothesis that mooring impact in mega-tidal system would (1) depend on distance and direction from the fixing point of each mooring and (2) vary over the neap and spring tides cycle. To test these hypothesis we first mimicked the theoretical scraping movement of the mooring chain to highlight the critical scraping phases of the tidal cycle based on the results of the hydrodynamic model. Second, shoot density, seagrass canopy height and leaf dry weight were analyzed to detect the potential variations of the impact of the chain according to distance and direction to the mooring.