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 algae presses
Sites and Critters

Red macroalgae artwork… and some science

Algae presses are a cool and creative way to document macroalgal specimens you encounter in the field (and they make great gifts too!). These specimens came from a dive site near Centennial Park in Elliott Bay, Seattle. They represent some of the more common and dominant species I see growing on riprap. I made the presses by placing them on thick paper between two pieces of plywood held together by bolts that I then tightened as the algae dried.

These are presses of red algae species I commonly find on riprap in Seattle.  Tiny pieces of these algae are mixed into adjacent sediments.

These are presses of red algae species I commonly find on riprap in Seattle. Tiny pieces of these algae are mixed into adjacent sediments.

Riprap, the rocky material that makes up jetties, breakwaters, and seawalls, supports an abundance and wide diversity of red macroalgae.  One of the questions I’ve been most interested in testing is whether the red macroalgae growing on riprap get incorporated into neighboring soft sediments.  I recently collected sediment samples along transects extending perpendicularly from riprap installations (see earlier post), and I’m happy to say that I have a finding to report!  After weeks of sorting through the sediment samples, I have found that the amount of red macroalgae that is mixed into soft sediment decreases as you move away from riprap.

The next step is to identify whether these differences in algal content might influence the community of organisms that live in soft sediment.  In ecology, food webs that are altered by the influx of resources from adjacent habitats are said to be “subsidized” or influenced by “spatial subsidies.”  (I’ll write a post soon to give more background on the spatial subsidies literature.)

To test for spatial subsidies, I’ll be conducting field experiments in which I enrich soft sediment plots with fixed volumes of shredded red macroalgae. At the end of 8 and 16 weeks, I’ll be collecting core samples from these enrichment plots and testing for differences in community structure. More to come on that.  In the meantime, I’m excited to say riprap-originating algae do in fact make it to neighboring soft sediments… what is their affect there?  The answer to that is coming soon!