Exploring Marine Restoration Opportunities in the Cleddau Estuary
Desk Based Research into the Opportunities, Constraints, and Environmental Nutrient Thresholds for Seagrass, Saltmarsh, and Native Oysters
Purpose & Back Ground
Milford Haven, along with the estuary of the Cleddau rivers in West Wales, is a protected area within the Pembrokeshire Marine Special Area of Conservation and is home to seagrass, saltmarsh and native oysters. However, as none of these protected and designated features are in good environmental status there is a desire to support recovery and restoration.
This review and data analysis considers opportunities, constraints, and environmental nutrient thresholds for seagrass (Zostera marina & Zostera noltii), saltmarsh, and native oysters (Ostrea edulis). It focusses on identifying what is currently ‘known’ and ‘unknown’ through desk-based research and stakeholder interviews to support future restoration efforts. Recommended future research is also suggested to address important ‘unknowns’.
WWF Cymru commissioned the Pembrokeshire Coastal Forum(PCF) who worked in Partnership with Project Seagrass to deliver this research between November 2023 and July 2024.
Aims
Through desk-based research this work wanted to bring together the information on the challenges and the opportunity for restoration of marine and coastal habitats within the Milford Haven Waterway and the Cleddau Estuary with the aims of creating better understanding of where restoration projects could be pursued with higher confidence of a successful outcome for nature.
Background
Milford Haven Waterway or the Cleddau Estuary (Daugleddau in Welsh), is a large estuary in Western Wales and its catchment area comprises a great proportion of Pembrokeshire. It is a ria estuary (drowned river valley). The outer reaches of the estuary is home to heavy industry(oil, gas refinery, gas fired power station) with the upper reaches relatively undeveloped shores.
The catchment area is defined by intensive agricultural activity, predominantly focused on dairy farming. However, sheep rearing and early potato cultivation also hold significance locally. Dairy farming, in particular, is a key driver of the local economy within the river catchment.
The estuary is known to have elevated nutrients, being identified as ‘hyper nutrified’ in the upper estuary by NRW and research showing nitrogen level on average 3 times higher in the upper reaches than near the mouth of the estuary. These elevated nutrients contribute to high levels of smothering opportunistic macro algae regularly in some area, particularly small inlets and pills.
There are existing seagrass and native oyster restoration projects within the estuary.
Key Findings
- There is a variety of constraints to each of the habitats including coastal squeeze, disease, abundance of suitable substrate, and excess nutrients.
- Each of the habitats has varying level of sensitivity or impact from excess nutrients.
- Seagrass being the most sensitive to impacts of excess nutrients particularly subtidal seagrass(Zostera marina) and with the most understood nutrient threshold for intertidal seagrass(Zostera noltii) which this work hypothesises an extreme threshold of .75mg/l of total nitrogen.
- Excess nutrients affects saltmarsh resilience through imbalance of above ground biomass and below ground biomass which in turn more suspectable to affects of wave action, alongside nutrient tolerant species being more dominant and less diversity of plants with the marsh.
- Native Oyster not overly sensitive to nutrients and research suggests higher importance of existence of mature oysters in sufficient numbers without levels of Bonamia high enough to enable larvae production alongside suitable substrate for settlement.
- None of the constraints identified would prevent restoration efforts form being pursued, though varying nutrient levels within the waterway should be considered with nutrient sensitive species and more affected areas.
- There are also potential benefits of more seascape / landscape restoration efforts over species specific projects as there are mutual benefits or interactions between seagrass, saltmarsh and native oysters.
- It is also important to highlight that nutrient mitigation in terrestrial and coastal settings should also be considered when planning marine restoration as current levels of nutrient inputs have negative impacts.