Farm Findings: Seaweed farms create ‘Hanging Gardens’ similar to natural seaweed habitats.

If you’ve stared into a tide-pool or dipped your head beneath the surface just off the coast of the U.K., you may be familiar with the swaying masses of green, red and brown that characterise the shallows. Shallow-reef, or “rocky shore,” habitats are the most accessible parts of our oceans, and play host to a huge range of species, both plant and animal. Seaweeds of all colours, limpets, crabs, gobies, blennies – these creatures are some of the first ocean residents we get to know.

A rope colonized by seaweed and shellfish at one of Câr-y-Môr's sea farms. Recorded on SubCam 2.

Being so recognizable and widespread, shallow-reef areas are of special interest to ocean conservationists. Simultaneously, seaweed farming has become increasingly popular for cultivating sustainable economic growth in coastal communities. As such, understanding how the ropes and buoys of the farms may reproduce the benefits of shallow-reef’s is intriguing to conservationists and farmers alike. 

At Car-Y-Mor on the southwest coast of Wales, we sought to understand how the “header lines” of a seaweed farm were taken over by local species. Header lines are ropes link buoys, under which the “cultivation lines” are suspended to grow crop seaweed (Fig. 1).

What we found?

1. The header lines of Car-Y-Mor's farm act similarly to naturally occurring shallow reefs in the area, where red seaweeds dominate in Autumn and Winter and brown seaweeds proliferate in the Spring and Summer.

   
   

2. The longer the lines were left out at sea, the more they behaved like naturally occurring reefs.

   
   

Figure 1. (Left) Seasonal line coverage by seaweed species on the header lines at (a) Porthlysgi (2yr old line), (b) Carn ar Wig (2yr old line), and (c) St. Justinian (newest line installed in August 2022). The coverage is represented by various colours, each corresponding to a different species. (Right) A seaweed farm's set-up of a header-line and cultivation-line suspended from buoys.

The many surfaces created by the ropes, buoys and anchors of the seaweed farm in the water column allowed organisms to naturally settle and grow, creating reef-like microhabitats that mirror those of local shallow reefs. Despite the harvesting of seaweeds, these surfaces remained relatively undisturbed and year on year the microhabitats become more similar to those present naturally, indicating a net gain in the abundance of marine species.

Our findings suggest that seaweed farms may act as “hanging gardens” where wild seaweeds local to the area flourish alongside cultivated ones and follow their natural, seasonal patterns. By mimicking natural systems, these hanging gardens could offer shelter, breeding grounds, and feeding opportunities to local species.

It is important to remember, however, that these header-lines are still “micro”-habitats. They cover less area and have a smaller variety of species than totally natural reefs. Furthermore, they are temporary, with a lifespan of 5-8 years. Nonetheless, their ability to mimic local environments is expected to produce a net positive impact on biodiversity while they are at-sea, and there are other potential benefits that need more research.

For example, when seaweed falls from the header lines, it may settle to the seafloor and begin to grow, increasing the chances of creating a new habitat that is more permanent than the header-line itself.

Our findings were very promising and suggest that seaweed farming and the protection of wild seaweed habitats go hand in hand. We’re excited to continue our research on this topic, and look forward to sharing more in the future.

References

1. Bunker, F.StP. D. 2021. Pembrokeshire Marine SAC intertidal rocky shore monitoring. Analysing changes and trends 2005 to 2019. NRW Evidence Report No: 417, xvi + 131pp, Natural Resources Wales.

2. Bekkby, T., Torstensen, R.R.G., Grünfeld, L.A.H., Gundersen, H., Fredriksen, S., Rinde, E., Christie, H., Walday, M., Andersen, G.S., Brkljacic, M.S., Neves, L., Hancke, K., 2023. ‘Hanging gardens’—comparing fauna communities in kelp farms and wild kelp forests. Front. Mar. Sci. 10. https://doi.org/10.3389/fmars.2023.1066101

3. Hawkins, S. J., Evans, A. J., Firth, L. B., Genner, M. J., Herbert, R. J. H., Adams, L. C., Moore, P. J., Mieszkowska, N., Thompson, R. C., Burrows, M. T., & Fenberg, P. B. (2016). Impacts and effects of ocean warming on intertidal rocky habitats. In Explaining ocean warming: causes, scale, effects and consequences (pp. 147-176). https://doi.org/10.2305/IUCN.CH.2016.08.en