Oyster reefs have plenty to contend with. Over-harvesting has seen an 85 per cent decline in the number of reefs over the past 100 years. But it looks like sea level rise is one challenge oysters may be able to keep abreast of, a new study, out today in journal Nature Climate Change, suggests.

Once, oysters were so abundant in the Chesapeake Bay that boats risked capsizing on the huge expanses of the creatures. But these days, the Chesapeake oyster population is less than one per cent of what it was due to over-harvesting and declining water quality.

Oysters attach to each other’s shells to form reefs of living and dead creatures, sometimes hundreds of metres wide. Oyster reefs are useful multi-taskers, filtering and cleaning water, providing habitat for fish and crustaceans, stabilising the shoreline, maintaining a healthy water pH and providing income for local communities.

With numbers already under pressure, scientists have been concerned that sea level rise (SLR) poses a further threat to oyster communities. But past assessments of how quickly oyster reefs can grow upwards have significantly underestimated their speed, a group of scientists from the University of North Carolina and Northeastern University has found.

The study suggests reefs should be able to keep up with any sea level rise caused by climate change.

Safe haven or vulnerable to sea level rise?

The scientists looked at oyster reefs situated in intertidal areas – areas of shore that are underwater when the tide is in but become exposed when the tide goes out.

Oysters in intertidal reefs grow faster because the oysters stack on top of each other in clusters. This means attempts to restore reefs in intertidal areas could have a high chance of success – if they are able to keep their heads above water.

As the climate changes, previous estimates have suggested that reefs will be threatened by sea level rise. Older studies suggested that reefs only grow at around 0.5 centimetres a year – not fast enough for them to thrive in rising oceans.

But according to the new study, these estimates are far too modest. Between 2010 and 2011, the team looked at five intertidal reefs constructed in a scientific reserve between 1997 and 2000.

The scientists measured reef growth in two ways. First, they cored reefs by driving an aluminium tube into the reef with a jackhammer. Removing these cores provided cylindrical samples of the whole reef and allowed them to calculate the reefs’ vertical growth rates.

Next, the scientists measured reef growth using laser scanners that collected about a million samples of reef elevation, and created a three-dimensional model of the reefs.

The team found reefs grow much faster than previously thought. Lead author Dr Antonio Rodriguez at the University of North Carolina tells Carbon Brief:

“The main conclusion is that intertidal reefs have the potential to grow vertically by 11 centimetres per year, which is rapid enough to keep up with any future rate of sea-level rise.”

So why does the new study conclude that oysters can grow faster?

Dr Rodriguez says that the underwater parts of oyster reefs can grow rapidly. But the crest of the reef – the part that peeks out from the water – grows more slowly, and can’t grow faster than sea levels rise.

This helps to explain why other studies underestimated potential reef growth by as much as an order of magnitude. He says:

“Once the top of the reef reaches that elevation where the oysters are exposed 50 per cent of the time, the reef can only grow as rapidly as sea-level is rising.”

Implications for conservation 

The scientists’ findings have important implications for oyster-loving policymakers and conservationists. Rodriguez says:

“The message is that oyster-reef restoration has a high likelihood of success in intertidal areas.  If intertidal reefs are restored close to marsh shorelines one could end up with a reef that will help protect the shoreline from erosion, filter water, provide fish habitat, and be able to keep up with sea-level rise. No rock sill can do those things.”

Not the only problem

Sea level rise is only one of a number of natural and human-caused threats oyster reefs face. The paper lists “harvesting, degrading water quality, increasing rates of SLR, warming, disease, ocean acidification, and parasitism”, all of which are making oyster reef habitats and the services they provide to the wider oceans unsustainable. It adds:

“Loss of these reefs in estuaries that otherwise lack alternative hard substrates [solid grounding for other species, like oyster reefs,] is a global problem.”

Rodriguez says the other effects of climate change could make oyster reefs grow more slowly. He tells Carbon Brief:

“We report the first measures of reef growth in the paper (whole reef growth).  The next steps should be to look at how additional stressors may impact that growth.  Top of the list should be human impacts and disease.”