Aguilera, M., Arias, R. M., & Manzur, T. (2019). Mapping microhabitat thermal patterns in artificial breakwaters : Alteration of intertidal biodiversity by higher rock temperature. Ecology and Evolution, (June), 1–13. https://doi.org/10.1002/ece3.5776
Featured Image: Sami Hurmerinta, flickr.com
A temperature oasis under threat
The coasts are a temperature refuge for humans, plants, and animals alike: during the summer, we flock to them for cool water and the ocean breeze, and during the winter, we are drawn towards the warmer coastal temperatures. However, like many urban environments, temperatures of coastlines may be getting warmer and more variable. When people live along and use coastlines, we build artificial structures. Seawalls protect areas from seawater that may encroach and cause erosion. Jetties and groins are structures that project from the land towards the water in order to break up currents and waves. Boat ramps provide opportunities for recreation and commercial activities. Riprap, or piles of artificial rock, provide a lining along channels or coastlines to prevent erosion. Together, these structures aid in disaster prevention and/or mitigation, allow for economic growth, and provide opportunities for people to enjoy the coast through tourism and recreation. However, these structures are not without consequence: they may impact coastal species, especially algae and invertebrates that settle on hard surfaces.
Even though both natural and artificial structures provide solid substrate for coastal marine organisms, these substrates are not made equally. Scientists Aguilera et. al. are based in Chile, where human populations and development are concentrated along the coast. They conducted a study with two main goals: 1) to compare temperature patterns within natural boulder fields versus artificial riprap and 2) to discover how these development-influenced temperature patterns impact how many species are present and where these species establish homes.
Natural substrates have many little nooks, crannies, and pools for algae and invertebrates to cool off or hide from predators, waves, and wind. Aguilera et. al. predicted that artificial substrate would be warmer overall and have less variation in temperature over their surfaces because they are smoother with fewer hiding spots. Additionally, the scientists predicted that these warmer, less variable temperature patterns would cause fewer species to establish themselves on riprap compared to those established on natural boulders.
Aguilera et. al. selected paired testing sites: one natural boulder field paired with one artificial riprap site. They tested 5 total pairs, some temperate and some subtropical, located in areas with a variety of uses for humans. They measured temperatures at selected areas on the tops and sides of rocks, as well as the unions between them. Infrared thermal images allowed the scientists to measure differences in temperature even within just a few centimeters. Those centimeters are a big deal for some smaller organisms!
In addition to measuring rock temperature, scientists measured air and water temperature, rock slope, geographic orientation, and rock smoothness vs roughness.
Artificial structures are less hospitable for coastal organisms
Artificial structures were less rough and sloped than natural substrates. As a result of these differences, artificial structures also had warmer temperatures. A smoother surface means there are fewer nooks and crannies to take refuge from heat. These warm temperatures were associated with a lower number of species, and less variation in the types of species present on riprap. This pattern remained consistent whether species moved around (ie, snails) or stayed put (ie, mussels, barnacles, and algae).
The best of both worlds? Providing natural homes while building necessary infrastructure
Artificial structures on the coast don’t look great for biodiversity. But, they are sometimes necessary in order to keep people safe and economies running!
Fortunately, there are ways that we can build necessary infrastructure while better protecting natural biodiversity. Coastal green infrastructure is growing in feasibility and popularity. We can get creative with our riprap by making grooves and pits in the rocks to provide microhabitats, and installing artificial tilt boxes to create ledges. We could transplant mussels to create a rougher and more differentiated surface, and create pools in between rocks to retain more water and provide cool areas for organisms.
These ideas are a few of many that should be incorporated into urban coastal planning. Aguilera and his team suggested that future studies test these methods to incorporate when, where, and how they could be the most effective and feasible.