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!


This Week In The Lab

Soft sediments – they’re alive!!!

On Aug 3, I told you about the dives we’d done to collect soft sediments.  Tube of macrofauna in ethanolSince then I’ve spent many an hour in the lab processing the samples we collected.  Processing involves (1) quantifying the volume of riprap-originating algal material in each sample, (2) quantifying sediment grain size, (3) identifying each type of shell hash, and (4) identifying and counting all macrofauna.

Macrofauna are organisms that live in soft sediment.  Until just a few years ago, I had no idea how many critters actually live embedded in sand and mud. From above, the soft sediment landscape appears barren, almost devoid of life.  But it’s actually alive in a way I never imagined – at a tiny scale.  After many hours of picking through sand and mud in my sediment cores, I am again amazed at the density and diversity of macrofaunal life forms.  They come in all shapes and sizes, colors and textures, life histories and strategies.  Worms, clams, snails, amphipods, ostracods, sea cucumbers, and more!  It’s overwhelming.

For now, I’m simply pulling specimens out of sediment samples and storing them in ethanol (as in the photo), but I’ll be identifying them over the coming months and look forward to sharing.  Their stories are bazaar, amazing, surreal – from vicious hunters with exploding mouth parts to little arthropods that spend their entire lives moving about the seafloor in the organic equivalent of a hamster ball. I’ll highlight my favorites as they arise.  Stay tuned!