[Update of EMR feature article : Tanner JE, Andrew D. Irving, Milena Fernandes, Doug Fotheringham, Alicia McArdle and Sue Murray-Jones (2014) Seagrass rehabilitation off metropolitan Adelaide: a case study of loss, action, failure and success.Ecological Restoration & Management 15: 3, 168-179. https://onlinelibrary.wiley.com/doi/10.1111/emr.12133]
Key words: Amphibolis, Posidonia, Recruitment facilitation, Seagrass loss
Introduction: Over the last half century or so, over 6,000 hectares of seagrass has been lost off the Adelaide coast due to anthropogenic nutrient and sediment inputs. This loss has led to coastal erosion, decreased habitat, loss of carbon storage, and decreased fish abundance. Recent improvements to wastewater treatment and stormwater runoff have led to some natural recovery, but changes in sand movement resulting from the loss now prevent recolonization of many areas. Our September 2014 feature article in EMR described how SARDI have been working with other state government agencies and universities to develop a cost-effective technique to restore these areas. Typical seagrass restoration costs on the order of AUD$1 million per hectare, but by facilitating natural recruitment of Amphibolis, yet over the last 17 years we have developed a technique that only costs a few tens of thousands of dollars. As described in the feature, this technique uses hessian sand bags (Fig. 1) to provide a stable recruitment substrate while seedlings become established, and has resulted in the re-establishment of small trial patches of seagrass restoration (10-100 m2) which are now over 10 years old (Fig. 2) Importantly, these sites have been colonized by Posidonia and Zostera seagrasses, and provide habitat for faunal assemblages that are similar to those of nearby natural meadows, suggesting potential for small plots to act as ‘starters’ for ecosystem recovery.
Further work undertaken: Since our original article in EMR, we have continued monitoring the 1 hectare trial patches and expanded our focus to include additional species in the restoration, especially Posidonia. We have also started assessing how bags degrade over time under different storage conditions, as operationalizing this technique will require bags to be stored potentially for a month or more between filling and deployment. Importantly, the SA Government has now allocated funds for a proof of application, which will involve the deployment of hessian bags over approx. 10 hectares in late autumn 2020.
Further results to date: Two 1 hectare trials were deployed in June 2014, with 1,000 bags in each (Fig. 3). After 9 months, these bags had an average 6.2 Amphibolis seedlings each, which was typical for bags deployed outside the winter recruitment season in previous years. After a further 12 months, this increased to 9.2 seedlings per bag, within the range of densities previously seen for small-scale winter deployments (7-23 seedlings per bag). A further 12 months later, densities had decreased to 3.1 seedlings per bag. In 2017, a third 1 hectare trial was established with 2,500 bags, although these bags only had 1.2 seedlings each after 9 months. Unfortunately loss of nearly all marker stakes on all three plots due to suspected disturbance by fishing gear meant that further monitoring was not possible. It should be noted that for the successful small-scale deployments, stem densities between 2 and 5 years were very low, and it was only after 5-7 years that success was evident.
Planting Posidonia seedlings into the bags showed good success over the first 3-4 years, with seedlings becoming established and developing into what appeared to be adult plants with multiple shoots, which did not allow individual seedlings to be identified (Fig. 4). However, leaf densities declined substantially in the 12 months following the February 2016 survey, and recovery has been slow in the 2 years since. Trials with different fill types (different sand/clay mixes, different amounts of organic matter added) indicated that this did not influence establishment success or growth, and neither did planting density. Small and large seeds, however, tended to fare poorly compared to those of intermediate size (10-13 mm). These results have been supported by short-term tank experiments, which also showed that there is only a short window for collecting fruits (those collected on 28 Dec formed an average 3.3 120 mm long leaves each after 2 ½ months, while those collected 6 days earlier or 3 days later formed < 2 leaves which were no more than 80 mm long). After collection, fruits that did not release their seedling within 2-3 days performed poorly, and seedlings were best planted within 10 days of release. Whilst earlier Posidonia field experiments were undertaken by divers planting seedlings, which is time consuming and expensive, in 2017 seedlings were planted either onshore or on the boat, and then glued into the bags prior deployment. This was as successful as planting underwater after 1 and 2 years, with an average 20% seedling survival, and leaf lengths of 20-25 cm, across all treatments.
Bags filled with moist sand rapidly dried out in storage, and did not deteriorate any quicker than those filled with dry sand, although it should be noted that in this experiment all bags had good air circulation around them, which would not be the case if they were stored in bulk. Bags left outdoors exposed to the elements deteriorated quicker than those stored indoors, and pallet wrapping led to them rapidly becoming mouldy.
Lessons learned and future directions: While the hessian bag method has resulted in the successful establishment of small patches of seagrasses that have persisted for around a decade, and which are now functioning like natural patches due to colonization by other marine plants and animals, the development of the technique has not been straightforward. Refining the technique has required the development of a good understanding of the timing of recruitment, and the willingness to put conventional wisdom to the test. This work has also required funders to take a long-term view, and to be willing to accept the fact that success cannot be established within a conventional 3-year funding cycle. In this case, it was only 5-7 years and 2 funding cycles after deployment that we saw our small-scale trials being successful. Now that we have established the technique at a small-scale, we are experiencing a new set of challenges with scaling up. The 1 hectare plots have not been as successful as we had hoped. In part, this may be due to low bag density – our small-scale plots were equivalent of approx. 10,000 bags per hectare, not the 1,000-2,500 that we have used. Consequently, our next trial with involve a range of bag densities, from 1,000 to 10,000 bags per hectare. In our previous article, we had indicated that we were looking at developing novel coatings to improve the life of the hessian bags, however, this proved cost prohibitive and reduced the ability of seedlings to attach to the bags. Instead, we have now commenced a new collaboration with textile scientists to look at alternative natural fibres that might last longer than hessian but still be cheap, effective and biodegradable.
Stakeholders and Funding bodies: SA Department for Environment & Water, SA Water, Adelaide & Mount Lofty Ranges Natural Resource Management Board, Australian Research Council, South Australian Research & Development Institute, Flinders University
Contact information: A/Prof Jason Tanner, Principal Scientist – Environmental Assessment & Rehabilitation, SARDI Aquatic Sciences, PO Box 120, Henley Beach, SA. 5022. Tel: +61 8 8429 0119. Email: firstname.lastname@example.org