Riprap community structure in Germany


So many of the world’s coastal cities are located at the mouths of rivers. These estuarine habitats were once vast, shallow landscapes of soft sediment, but they have become speckled with hard artificial structures over the course of urbanization. Do these artificial structures provide opportunities for a new suite of species to establish and thrive? How does these species compare to those that live in the natural soft sediment habitats that remain?

These were the questions posed by Markus Wetzel and colleagues in a recent paper in the journal Marine Environmental Research. The team collected samples from riprap and soft sediment habitats at seven different sites in the Weser estuary in northern Germany. All of this work was conducted in the subtidal (below the lowest low tide), at 2-3m depth.

Nymphon sp. (a sea spider, or pycnogonid). (c) Steve Trewhella
Nymphon sp. (a sea spider, or pycnogonid). (c) Steve Trewhella

Subtidal riprap in the Weser estuary supported a diverse community of organisms, including mussels, barnacles, polychaete worms, crabs, sea stars, urchins, and even sea spiders (see photo). Riprap was also inhabited by large populations of the invasive barnacle, Amphibalanus improvisus, as well as well as invasive crabs, clams, and polychaete worms. While several endangered or threatened species were also found on riprap, the abundance of these species was not significantly different between riprap and soft sediment habitats. Overall, they found riprap communities to be more diverse than soft sediments, with 35 species found in hard substrate samples and only 12 species in sediment samples.

Wetzel and colleagues raise an interesting question in their discussion: “Do we elevate the ecological value of estuaries by adding artificial structures?” These structures do appear to increase diversity and increase opportunities for species that rely on hard substrates to establish and survive. Many of the species that live on artificial structures are suspension feeders (filter feeding from the water column), which, Wetzel et al. point out, are known to recycle nutrients and play an important ecological role. Algae that grows on artificial structures could also be subsidizing surrounding soft sediment communities (though this remains untested) and shell material from from these structures could be changing the physical characteristics of soft sediment environments in their vicinity.

The question is certainly an interesting one, but the jury is still out. To find an answer, not only is much more data needed, but we’ll also need to clarify how we want to quantify “ecological value”. Like all things, adding artificial structures to estuaries has its trade offs. It seems we would be wise to understand the full extent and character of these trade offs , as well as our criteria and objectives for evaluating them, before adding more hard substrate to our coastlines.