My first impression of tidal flats in the Oosterschelde (Eastern Scheldt), a major river in South Holland, was the ease with which you could walk across them. In San Francisco Bay, where the tidal flats are muddy (rather than sandy), each step sank deep into the mud. The crossing to the oyster restoration site for monthly monitoring was always an exhausting adventure. Tuesday's visit to the tidal flats of the Oosterschelde brought back fond memories of tromping through the mud in California.
How can oyster reefs help restore the Oosterschelde?
Anyways, the barrier reduced the tide range in the river (i.e. the tide does not go as high or as low as it used to). Since intertidal habitats depend on being dry and wet for a certain amount of time, this smaller tide range has lead to rapid shrinking and loss of tidal flat and marsh habitat. Many pilot projects are in the ground already to test ways to slow/stop this degradation, including artificial oyster reefs and tidal flat sand replenishments. The EMERGO project will look at how well these (and other) actions address the unraveling of intertidal habitats.
Let's visit some artificial and natural oyster reefs!
In 2009, small test oyster reefs (2 of 12x4m) were constructed at the mudflats of Viane. The artificial reefs begin as metal cages filled with dead oyster shells. This is similar to the San Francisco Bay project, where bags of oyster shells were used. New oysters like to colonize on the dead oyster shells. Over time, the live oysters stick together and make the structure sturdier, removing the need for the metal casing, which disintegrates away. A year later, this project was deemed successful because the reefs were stable, some oysters had colonized, and sedimentation was occurring behind them. Larger reefs (200x10m, 2 at Viane and another at De Val) were constructed in 2010. We visited these three larger reefs, as well as some vibrant natural reefs nearby. See bottom of post for a map marking the artificial reef locations. Source
Our walk across the mudflat at Viane was a long one - it took a couple hours and ~6 km to visit the natural and artificial reefs at this site. We first visited a natural oyster reef.
Of all three reefs, the westernmost reef at Viane showed the most growth. Clumps of happy oysters pointed their shells up towards the sky. The second reef at Viane was less promising, which may be because it was built higher on the mudflat. Oysters don't like to be dry for long periods of time, so they usually don't grow past a certain elevation "ceiling," relative to the tide. Another reason may be that a bar of sand has been moving slowly across the area, filling in the gaps in the reef where oysters could otherwise grow.
You can find many interesting things on the mudflat, a few of which I've shared above (worms) in the photos below...