Seagrass production around artificial reefs is resistant to human stressors

Breeze blocks placed in one of the ocean’s most endangered habitats provide an unexpected lift for fish.

Seagrass meadows are found across the world, reaching from the tropics up into the lower reaches of the Arctic circle. They are incredibly valuable habitats, providing a nursery for young fish as well as sucking vast quantities of carbon dioxide from the atmosphere.

However, with an area of seagrass the size of a football pitch being lost every 30 minutes, it’s more important than ever to find out how to turn things around. A new study in the Caribbean has shown that artificial reefs can help to bolster their growth in the tropics, even as threats such as fishing and nutrient pollution continue.

Dr Jacob Allgeier, a co-author of the paper, says, ‘By attracting fish, whose faeces provide concentrated nutrients for the seagrass, the artificial reefs increase the primary production of the entire ecosystem.’

‘We are now investigating how this cascades up the food web. The new energy has to go somewhere, so we are quantifying how it affects invertebrates and fish with our evidence suggesting that it is fuelling increases in both.’

The findings of the study were published in Proceedings of the Royal Society B.

 

Artificial reefs and seagrass

One of the biggest issues affecting seagrass is nutrient pollution, often from the release of human sewage. While the influx of nutrients can initially boost the growth of the meadows, it also promotes the growth of algae which reduces the amount of sunlight getting to the seagrass and harms it in the long run.

Alongside fishing which causes levels of the fish faeces that fertilise the meadows to drop, it was thought that the combination of these two issues might work in unexpected ways to hinder the growth of seagrass.

But the current study has revealed some surprising results. It has found that the productivity of seagrass in both disturbed and undisturbed meadows was increased by the presence of an artificial reef, while algae didn’t actually seem to pose an issue, even in areas where nutrient pollution was high.

Mona Andskog, the PhD student who led the research, explains, ‘Artificial reefs built in seagrass create a positive feedback loop. They attract fish that use the reefs for shelter which, in turn, supply new nutrients from their faeces that fertilise the seagrass around the reef.’

‘This increased primary production can increase invertebrate production by providing more food and shelter for invertebrates, which in turn provide more food for fishes.’

Experiments in Haiti, at some of the most fished sites included in the study, also showed that the artificial reefs were providing additional benefits to the fish. Large numbers of small fish were found at the site because of the difficulty in using nets around the reef, meaning that the overall biomass of fish was at times larger than in unfished areas measured elsewhere in the study.

While artificial reefs present a promising option for tropical seagrasses, they’re likely to have a much more limited impact on temperate meadows. These waters already tend to have higher nutrient levels, meaning that any contribution the reef would made to overall growth would be small.

The scientists now hope to explore how the placing of artificial reefs can affect seagrass ecosystems, as well as expanding their research to the Dominican Republic.

‘We will be testing how different configurations of artificial reef clusters can affect the production and fish community composition,’ Jacob says. ‘This includes the number of artificial reefs in each cluster, as well as their arrangement.’

‘As with this research, we hope to simultaneously use the reefs to test fundamental questions about production in these highly impacted ecosystems as well as optimising the positive feedback that is initiated by the artificial reefs.’