Unsworth, Richard et al., "Options for Long-Term Seagrass Monitoring at Porthdinllaen, Wales"

Unsworth, Richard, Chiara Bertelli, and Jim Bull. Options for Long-Term Seagrass Monitoring at Porthdinllaen, Wales, 2014. https://doi.org/10.13140/RG.2.1.3830.7286.

Executive Summary
The seagrass commonly referred to as ‘Eelgrass’ (Zostera marina L.) is one of only two ‘true’ seagrass species found in the UK and is mostly restricted to a maximum of about 7 m of water depth (below chart datum). This is the result of its high light requirements as a photosynthetic organism. Estimates from across the range of Zostera marina suggest it requires between 12 and 37% of surface irradiance to survive in the long-term. These high light requirements and the delicate nature of these plants make seagrass habitat susceptible to impacts. In Porthdinllaen there exists over 20 hectares of seagrass that has been defined as being in an unfavourable condition by Natural Resources Wales. The primary anthropogenic impact of concern upon seagrass at Porthdinllaen is the damage caused by both permanent and annual moorings placed in the seagrass. There are also significant issues related to anchor damage and vehicle use that is thought to be damaging the seagrass. For these reasons the relevant authorities group of the Pen Llŷn a’r Sarnau SAC are undertaking a process of co-management with stakeholders to bring about a reversal of this degradation.

A significant component of this is the creation of a long-term monitoring programme of the seagrass that will help understand the effect of any future management actions, act as an early warning of any future concerns and contribute to reporting on condition of the SAC feature. This report reviews the options available for developing such a seagrass monitoring and assessment programme at Porthdinllaen. It considers previous work conducted at Porthdinllaen and examines options for monitoring seagrass by considering existing programmes in other parts of the UK and best practice seagrass monitoring globally. Options are reviewed by examining both seagrass sampling methods and metrics and the relative merits of different ones.

Here we propose that a monitoring programme at Porthdinllaen is split into six separate components, the first five we propose as being essential whilst the sixth is suggested. These are proposed as they collect scientifically robust data, are well proven and can be collected without too much specialist equipment. They also have the capacity to involve volunteers. By creating both an intertidal and a sub-tidal programme volunteers with and without SCUBA expertise can take part in
the programme. Such programmes need to continue to build on the good collaborative work with Seasearch and develop further links with University student groups (e.g. Marine Biology Society at Swansea University).

  1. An assessment of sub-tidal seagrass extent that determines seagrass presence or absence and depth repeatedly throughout the bay using GPS. Study can utilise freedivers, light weight dropdown video or a light weight hand grab.
  2. Spatial assessment of the intertidal seagrass extent and the presence of impacts. Study would utilise observers walking with a handheld GPS. Seagrass shoot density estimates and associated data would be collected.
  3. Detailed subtidal SCUBA based surveys that assess seagrass status within randomly assigned quadrats radiating out from pre-determined seagrass sampling points spread in a stratified fashion throughout the whole seagrass meadow.
  4. Detailed intertidal walking surveys that assess seagrass status within randomly assigned quadrats radiating out from pre-determined seagrass sampling points spread in a stratified fashion throughout the whole seagrass meadow.
  5. Assessment of environmental conditions using loggers (temperature and light) deployed over the long-term.
  6. Assessment of fish trophic structure as an indicator of the ecosystem state.

All data collection would need to be managed by a trained and experienced scientist and involve detailed write up. This includes annual re-evaluation of sampling efficiency. By creating a long-term data set that uses metrics comparable to existing monitoring programmes in the UK data should over time be examined relative to other UK data to consider longer-term climate driven change. We also suggest that key data and significant findings are made rapidly available to stakeholders and the general public in order to increase the impact of the programme.