Mussels power mudflats

The intertidal mudflats in the Wadden Sea are important for many species of birds and fish. But what creates a healthy mudflat? University of Groningen ecologist Britas Klemens Eriksson and colleagues have discovered that mussel banks play a vital role. The results were published last week in the journal Estuarine, Coastal and Shelf Science.

Harvesting and changing conditions in recent decades have caused a strong decline in the cockle and mussel banks in the Wadden Sea. This has sparked attempts to restore them. Eriksson has spent years working on projects trying to understand how mussels work and what their function is. ‘In all these years, we noticed that diatom algae seemed to thrive around the mussel banks. They form thick brownish biofilms. We started to wonder if the mussels were responsible for this.’

Diatoms, small algae encased in silica cell walls, are ‘primary producers’: they use photosynthesis to harvest sunlight and create the energy used by grazers and other organisms further up the food chain. Their presence around mussel banks was obvious, but the big question was whether there was a causal relationship between the two.

Storm

‘We decided to try to find out. Of course, you can’t move mussel banks around, so we used long-time observations and a small-scale experiment.’ In the experiment, small plots were seeded with different densities of mussels and protected from predation by birds and crabs. They also created artificial banks without mussels but with the same effect on hydrodynamics.

The experiment survived one summer before a storm blew away the stakes that marked the enclosures and held up the bird and crab repellers. But the data was convincing. ‘The more mussels in a bank, the more diatom biomass.’ Merely manipulating the hydrodynamics had no effect on the diatom density. ‘The mussels excrete a lot of waste that is rich in nutrients. Diatoms use this for food. The banks also protect the biofilms, because they break the waves and create a calmer environment.’

Eriksson shows satellite images of the Wadden Sea on which mussel banks are just visible. ‘Look, you can see a kind of plume around the banks.’ The slight discoloration shows the effect of the mussel banks on their surroundings. Biofilms can extend to over 200 meters from the main banks. ‘We are still investigating, but it could well be that about half the intertidal mudflats are affected by the mussel banks.’ The mussel banks therefore create a rich environment. ‘If you take away the mussels or cockles, all this extra productivity disappears. The energy flow through the ecosystem starts with the diatoms.’

Political reasons

So biological processes in the Wadden Sea appear to control biomass production on the mudflats. ‘This is contrary to the prevailing opinion that the Wadden Sea is limited by nutrient input from the North Sea.’ In this theory improved water quality caused a reduction in the area’s productivity, because fewer phosphates in the rivers meant less eutrophication of the North Sea. ‘A sloppy assumption made more for political reasons than based on scientific proof’, says Eriksson.

Indeed, back in the 1980s it was believed that harvesting mussels or cockles was not a problem: they would grow right back. In the 1990s, biologists saw that this wasn’t the case. Now it is clear why the mussel banks don’t grow back: they are the engine which drives the ecosystem. ‘And this is based on our data from intertidal mudflats. We are still evaluating the importance of the subtidal areas that are permanently underwater.’ But it is already clear that restoring the mussel banks, as is now being attempted at several locations in the Wadden Sea, is vital.

Reference: Friederike G. Engela, Javier Alegria, Rosyta Andriana, Serena Donadi, Joao B. Gusmao, c, Maria A. van Leeuwe, Birte Matthiessen, Britas Klemens Eriksson: Mussel beds are biological power stations on intertidal flats. Estuarine, Coastal and Shelf Science, 15 May 2017 DOI 10.1016/j.ecss.2017.04.003