A Stroll to the Oosterschelde Oyster Reefs

[Nerding, not kayaking]

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.

The EMERGO project team: Bram, Lodewijk, Tom, and Oliver

Some background (skip if you're just here for the pretty pictures)...
This was the first field visit for the EMERGO project, a project by TU Delft and NIOZ (the Netherlands Institute of Sea Research) that I'll be working on during my time in the Netherlands. The overarching goal is to understand the morphological and ecological responses of tidal flats to different restoration measures. The project was sparked, in part, by the ongoing loss of intertidal habitats (tidal flats, oyster reefs, marshes, sea grasses) in the Oosterschelde. Between 1976 and 1986, a storm surge barrier was constructed across the mouth of the river. This barrier was the largest component of the 50 year Delta Works flood management project to protect the Netherlands from flooding after devastating floods in 1953. Stay tuned for a future post about this barrier - an epic bike trip is in the works that will involve biking across many of the storm surge barriers.

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 go on a fieldtrip!
This week, two professors (Bram, my advisor from TU Delft and Tom, a prof from NIOZ), two new PhD students (Lodewijk and Oliver), and I visited a few artificial oyster reefs and a massive tidal flat nourishment project.

Lugworm, or "wadpier" in Dutch. The worms live under the sand and eat+poop piles of sand, which cover the entire mudflat.

Worm poop piles everywhere

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. The map below marks the artificial reef locations. Source

Artificial reef at de Val. The reef begins at the bottom of the levee and extends out into the mudflat.

At de Val. As is the case in most of the Dutch estuaries, a large levee runs along the back of the intertidal habitats.

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. 

A natural bed of oysters near the Viane artificial reefs.

Happy beds of natural oysters

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. 

The EMERGO team checking out one of the artificial oyster reefs at Viane.

Oyster reef at Viane, the most successful of the three artificial reefs

Of course, you can find many other interesting things on the mudflat, a few of which I've shared above (worms) and in the photos below...

Mud waves, caused by waves over shallow water covering the mudflat.

Some areas were covered in seaweed

... and shells