Ecological considerations for an urban marine environment

In this post Filippo Ferrario discusses his recent paper ‘The overlooked role of biotic factors in controlling the ecological performance of artificial marine habitats

Summer is going to be over soon and many of us have spent the vacation enjoying sea beaches, practising aquatic sports or looking for a pristine marine landscape, where to hike or a wonderful sunset to cherish.

Did you realize how much the coasts have been modified by adding hard infrastructures to the landscape because of coastal urbanization? Nearly 60 % of the world’s population lives within 100 km of the coast and that percentage is increasing. Coastal urbanization means building commercial ports and industrial facilities, coastal roads, houses and leisure-oriented infrastructures (e.g. marinas and wharves, just to cite some). To cope with augmenting energetic demand we are building more offshore installations such as traditional oil-and-gas and newer renewable energy plants. On top of this, the increasing storminess and sea level rise due to climate change are exacerbating coastal hazards thus calling for more coastal defences; too often these are hard artificial structures such as breakwaters, seawalls, groynes and dykes.

What is maybe harder to realize – or less evident – is that every time we build something in the ocean it will be sooner or later colonized by a variety of marine benthic organisms (i.e. those that lives in close relationship with the sea bottom) such as algae, mussels and oysters, tube-worms, barnacles, sponges and corals. Artificial structures thus become novel habitats for biological communities; nonetheless these man-made habitats are not surrogates to natural reefs as they are often colonized by communities different to those typical in natural habitats. Artificial structures can be seen as a new tile in the natural landscape mosaic. But how well will the new tile fit into the existing artwork, ultimately depends on how much the new community will resemble those naturally occurring on the rocky reefs in the area.

Low structural complexity biological community colonizing a breakwater (Photo credit F. Ferrario).
Low structural complexity biological community colonizing a breakwater (Photo credit F. Ferrario).

In the past, ecologists have investigated this question mainly looking at the degree of similarity between benthic communities growing on artificial and natural substrata. It turns out that the diversity of biological community on these structures is often lower – either as number of species found, abundance of organism or genetic diversity – compared to natural rocky reefs. We also now understand that is not granted at all that artificial habitats will end up resembling the natural ones at some point – there are cases where a century of deployment has not been enough. In addition, artificial structures can help invasive species to spread along wide coastal areas. These species, introduced by human activities, often cause ecological as well as economic damage, such as the loss of native species or threatening the productivity of fish farms.

More recently, researchers are trying to understand which factors make artificial habitats different from natural ones. We’re thus realizing that certain features of artificial structures, such as chemical composition of the material, texture, structural complexity or inclination of surfaces, play a role in influencing the composition of the colonizing communities.

One question, however, is rarely asked: Are the interactions between the different species inhabiting artificial habitats an important factor in making these more similar to the natural ones? And if so, how important?

Well, this is what we tried to figure out in our study. In particular we tried to understand why, in the Mediterranean Sea, algal vegetation typified by brown macroalgae of the genus Cystoseira is notably scarce on breakwaters built for coastal defence – one of the most common artificial habitats. Cystoseira algae are an important element in the rocky reefs of the Mediterranean Sea where they form dense canopies functionally equivalent to “forests”. Cystoseira canopies offer ecologically valuable services: they play a key role in primary production and nutrient cycling, offer shelter to juvenile fishes, and provide optimal light and sedimentation conditions for other benthic species living underneath the canopy.

Cystoseira algae form dense canopies on the rocky reefs of the Mediterranean Sea (Photo credit F. Ferrario).
Cystoseira algae form dense canopies on the rocky reefs of the Mediterranean sea (Photo credit F. Ferrario).

To answer our question, we designed a series of caging experiments where the juveniles of Cystoseira species were outplanted on both artificial habitats (i.e. breakwaters) and natural rocky reefs, some of them protected by a cage and others not. We found out that Cystoseira juveniles were able to thrive and survive on artificial habitats only when protected by cages, while we did not observed any effect of caging in the natural habitat. This result clearly showed that the principal cause of the different performance of Cystoseira on artificial and natural habitats had to be identified in a biological interactions occurring between animal species and the juvenile algae (e.g. grazing). Moreover, we were able to assess that this biological disturbance was not caused by small mesograzers (e.g. amphipods) as in other systems but rather by a pool of different macrogazers (e.g. fish and crabs).

A caging experiment allowed identification of biological interactions on artificial and natural habitats. Experimental set-up (Photo credit F. Ferrario).
A caging experiment allowed identification of biological interactions on artificial and natural habitats. Experimental set-up (Photo credit F. Ferrario).

While enhancing ecosystem services in urban nature in the terrestrial environment is nowadays an integral part of nearly all cities (e.g. oxygen from trees, pollination zones, green roofs and walls), the same approach is unfortunately severely lacking in urban marine environments which remain largely unmanaged.

As a famous song from the seventies tells us, “the ocean is a desert with its life underground and a perfect disguise above”. This disguise has often erroneously brought people to ignore how things work in the marine realm. On the contrary, the more we will be able to understand colonization processes on artificial habitats the more we will be able to manage these marine urban environments to conserve important marine species, maintain coastal ecosystem functions and preserve the integrity of the marine landscape.

Our study reveals that it is time to integrate ecological considerations (such as the interactions between marine species) in the planning, construction, and maintenance of urban marine environments thus reducing the ecological footprint of structures like breakwaters, seawalls and piers. Just focusing on structural or environmental factors would be similar to relying on a good harvest based only on soil fertility and irrigation without taking any precautions against pests, and birds feeding on recently cast seeds.

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