1 research outputs found
Assessing the sensitivity of seagrass bed biotopes to pressures associated with marine activities.
This project was commissioned to generate an improved understanding of the sensitivities of
seagrass habitats to pressures associated with human activities in the marine environment -
to provide an evidence base to facilitate and support management advice for Marine
Protected Areas; development of UK marine monitoring and assessment, and conservation
advice to offshore marine industries.
Seagrass bed habitats are identified as a Priority Marine Feature (PMF) under the Marine
(Scotland) Act 2010, they are also included on the OSPAR list of threatened and declining
species and habitats, and are a Habitat of Principle Importance (HPI) under the Natural
Environment and Rural Communities (NERC) Act 2006, in England and Wales.
The purpose of this project was to produce sensitivity assessments with supporting evidence
for the HPI, OSPAR and PMF seagrass/Zostera bed habitat definitions, clearly documenting
the evidence behind the assessments and any differences between assessments.
Nineteen pressures, falling in five categories - biological, hydrological, physical damage,
physical loss, and pollution and other chemical changes - were assessed in this report.
Assessments were based on the three British seagrasses Zostera marina, Z. noltei and
Ruppia maritima. Z. marina var. angustifolia was considered to be a subspecies of Z.
marina but it was specified where studies had considered it as a species in its own rights.
Where possible other components of the community were investigated but the basis of the
assessment focused on seagrass species.
To develop each sensitivity assessment, the resistance and resilience of the key elements
were assessed against the pressure benchmark using the available evidence. The
benchmarks were designed to provide a ‘standard’ level of pressure against which to assess
sensitivity. Overall, seagrass beds were highly sensitive to a number of human activities:
• penetration or disturbance of the substratum below the surface;
• habitat structure changes – removal of substratum;
• physical change to another sediment type;
• physical loss of habitat;
• siltation rate changes including and smothering; and
• changes in suspended solids.
High sensitivity was recorded for pressures which directly impacted the factors that limit
seagrass growth and health such as light availability. Physical pressures that caused
mechanical modification of the sediment, and hence damage to roots and leaves, also
resulted in high sensitivity.
Seagrass beds were assessed as ‘not sensitive’ to microbial pathogens or ‘removal of target
species’. These assessments were based on the benchmarks used. Z. marina is known to
be sensitive to Labyrinthula zosterae but this was not included in the benchmark used.
Similarly, ‘removal of target species’ addresses only the biological effects of removal and not
the physical effects of the process used. For example, seagrass beds are probably not
sensitive to the removal of scallops found within the bed but are highly sensitive to the
effects of dredging for scallops, as assessed under the pressure penetration or disturbance
of the substratum below the surface‘. This is also an example of a synergistic effect
Assessing the sensitivity of seagrass bed biotopes to pressures associated with marine activities
between pressures. Where possible, synergistic effects were highlighted but synergistic and
cumulative effects are outside the scope off this study.
The report found that no distinct differences in sensitivity exist between the HPI, PMF and
OSPAR definitions. Individual biotopes do however have different sensitivities to pressures.
These differences were determined by the species affected, the position of the habitat on the
shore and the sediment type. For instance evidence showed that beds growing in soft and
muddy sand were more vulnerable to physical damage than beds on harder, more compact
substratum. Temporal effects can also influence the sensitivity of seagrass beds. On a
seasonal time frame, physical damage to roots and leaves occurring in the reproductive
season (summer months) will have a greater impact than damage in winter. On a daily
basis, the tidal regime could accentuate or attenuate the effects of pressures depending on
high and low tide. A variety of factors must therefore be taken into account in order to
assess the sensitivity of a particular seagrass habitat at any location.
No clear difference in resilience was established across the three seagrass definitions
assessed in this report. The resilience of seagrass beds and the ability to recover from
human induced pressures is a combination of the environmental conditions of the site,
growth rates of the seagrass, the frequency and the intensity of the disturbance. This
highlights the importance of considering the species affected as well as the ecology of the
seagrass bed, the environmental conditions and the types and nature of activities giving rise
to the pressure and the effects of that pressure. For example, pressures that result in
sediment modification (e.g. pitting or erosion), sediment change or removal, prolong
recovery. Therefore, the resilience of each biotope and habitat definitions is discussed for
each pressure.
Using a clearly documented, evidence based approach to create sensitivity assessments
allows the assessment and any subsequent decision making or management plans to be
readily communicated, transparent and justifiable. The assessments can be replicated and
updated where new evidence becomes available ensuring the longevity of the sensitivity
assessment tool. The evidence review has reduced the uncertainty around assessments
previously undertaken in the MB0102 project (Tillin et al 2010) by assigning a single
sensitivity score to the pressures as opposed to a range.
Finally, as seagrass habitats may also contribute to ecosystem function and the delivery of
ecosystem services, understanding the sensitivity of these biotopes may also support
assessment and management in regard to these.
Whatever objective measures are applied to data to assess sensitivity, the final sensitivity
assessment is indicative. The evidence, the benchmarks, the confidence in the
assessments and the limitations of the process, require a sense-check by experienced
marine ecologists before the outcome is used in management decisions