Spatial subsidies from riprap… say what?

As promised, I wanted to provide some background information about spatial subsidies and explain one way in which I think they may occur in urban marine ecosystems…

Since the late 1990s, there has been growing recognition among ecologists that the structure of ecological communities may be heavily influenced by the movement of resources from neighboring habitats. This movement of nutrients or energy has been identified by the term spatial subsidies, and has been the subject of considerable discussion in recent ecological literature. Spatial subsidies have been shown to alter the abundance of recipient species in a variety of different ecosystems.  The effects of subsidies appear to be particularly apparent among consumers of lower trophic levels (things that eat plants, for instance).  They may also impact other trophic levels, either directly or through indirect interactions.

photograph of red macroalgae on riprap

A photoquadrat of red macroalgae growing on riprap.

The introduction of artificial rocky material, such as riprap, to urban marine environments may alter neighboring soft sediment communities by providing them with new spatial subsidies.  Riprap is most commonly introduced to soft sediment environments that are not already protected by naturally occurring rocky material.  The biological community on riprap is substantially different from that in neighboring soft sediments, and has the potential to introduce a considerable amount of biomass into adjacent habitats in the form of detritus or debris.

My initial findings suggest that several species of red macroalgae may provide a flux of detrital material into soft sediment habitats. In addition, the sediment close to riprap installations is coarser than that farther away and contains shell hash from barnacles and jingle shells, which are found in high density on riprap.

Several studies have considered how soft sediment communities are altered by the presence and proximity of rocky structures, but with mixed results (see Davis et al. 1982, Ambrose and Anderson 1990, Posey and Ambrose Jr. 1994, Barros et al. 2001, Fabi et al. 2002, Jaramillo et al. 2002, Martin et al. 2005, Bertasi et al. 2007).  While some studies have found differences in the soft sediment community at varying distances from rocky substrates, the mechanism proposed to explain these differences has primarily been physical in nature. Martin et al. (2005) and Bertasi et al. (2007) attributed differences in soft sediment species richness and composition near rocky material to hydrodynamic patterns that trap coarser materials that are transported there by waves.  Fabi et al (2002) considered both physical factors and increased predation as potential reasons for differences in infaunal community structure adjacent to artificial reefs. Barros et al. (2001) provided one of the only suggestions that addition of reef-originating material into soft sediments nearby could be altering infaunal assemblages.

In my next post, I’ll tell you more about the experiments I’ve set up to test whether soft sediment communities are impacted by spatial subsidies from riprap. Thanks for reading and please don’t hesitate to contact me if you have any questions!


Photograph of ocean and riprap at dusk
Big Ideas, Ecosystem Services, Recent Research

Climate change and the proliferation of shoreline armoring

Louise Firth and colleagues recently published this article in Environmental Science Processes & Impacts: Climate change and adaptational impacts in coastal systems: the case of sea defences. It provides a brief exploration of shoreline armoring in the face of climate change. The general idea is this: as sea levels rise, coastal cities and developments are requiring increases in coastal defense structures (breakwaters, riprap, etc). These structures carry negative and potentially positive impacts for marine ecosystems. Why not construct them with these impacts in mind?

Photo of coastline with riprap and seawall

(c) Nigel Chadwick

“There is no doubt,” Firth and colleagues state in their paper, “that [armoring structures] modify the natural environment and can have deleterious impacts…” They cite research that has demonstrated how armoring structures act as stepping stones for species undergoing range expansions and how they have facilitated biological invasions. However, they may have potentially beneficial impacts as well, by supporting species of conservation importance and increasing habitat heterogeneity, as Firth et al. (2013) note.

So what does this mean for the construction of coastal defense structures? If the objective is to enhance intertidal biodiversity, Firth et al. (2013) provide these guidelines:

  • ”Build structure lower in the intertidal zone.”  Areas that are submerged for longer tend to support a greater number of species. Would this alter habitat that would otherwise be unaltered? That’s a discussion for another day I suppose.
  • Avoid smooth rocky material“, as these types of surfaces tend to be to be colonized by fewer species.  Specifically, they suggest a mixture of hard and soft rock to create greater surface roughness.
  • Create rock pools,” which should provide refuges for some species at low tide and support greater diversity.
  • Create pits” and crevices.  These provide hiding places and habitat heterogeneity.
  • Deploy precast habitat enhancement units.” Firth et al. (2013) note that a variety of such units are currently being tested around the world at the moment.  More on this soon in future posts!