Algae fight on the side of light. Inverts, not so much...

by jebyrnes in I'm a chordata, urochordata!

So, given my recent transition, it gives me great pleasure to talk about a paper that kind of sums up both halves of my split personality these days. Inverts and algae - who will win! The recent Miller and Etter paper in Ecology I think is not only a great piece of experimental ecology - careful, painstaking, and thorough - but it's also just a nice piece of natural history looking at the New England subtidal.

Oh, and maybe I worked on this project as an undergrad tech, and, well, maybe one of the authors is my current office-mate.

When I head out on a dive, I tend to do two things. The first is to hover above the surface, and take in the grandeur of the rocky benches below - ah, me, the beautiful algal bed, swaying two and fro, teeming with life. I run my hands through the fronds, examining their texture, peering at the life within.

The second is to head right for the nearest vertical surface, even if it's a tiny boulder just poking up above the substrate. I stick my nose close to it, and peer at all of the encrusting invertebrates duking it out for space. If there are no rock walls, I peer into the cracks and crevices to see where squirts, sponges, and bryozoans have managed to eek out a bit of life, safe from their algal foes.

A sampling of some invertebrates from a New Zealand Fjord. Photo credit Franz Smith, from Treasures of the Sea

But why are the foes? Why do vast walls of ascidians and the like tend to only occur on walls, and why is this a DMZ as far as many algae are concerned?

Of course, one of you will shout, flow is better on walls - haven't Witman, Sebens, and others taught us anything? Well, maybe. But someone else may say that it's light - I mean, obviously, those walls shade algae, so only the inverts can survive. And then someone else pipes up - No! Predators! It's hard out there for a crab - and it may not have access to those big walls. Or maybe it's sedimentation - more on the horizontal than the vertical - or maybe...

Yeah, you get the point.

So Miller set out to see just what's going on. What are the local factors that mediate algal-invertebrate competition? In Maine and Massachusetts, he set up a detailed series of manipulations of each of these factors within cages on horizontal substrates, attempting to add in varying levels of light limitation and predator restrictions. For each manipulation, there was a series of controls that also ruled out differing hydrodynamic regimes that were artefacts of the initial cage design. Then he measured changes in each and every variable, and sampled the heck out of the cages over time (including in the winter...in Maine! It had just finished snowing, for chrissakes!)

Basically, we dove, chipped out a bunch of divots in the substrate, z-sparred in a series of blank granite tiles, and slapped an dizzying array of caged, each slightly different than the others, over a series of weeks. This is the experiment that taught me to stop worrying, and love the z-spar.

(Note, to test the light idea, we also tried bolting mirrors to rock walls. Then a storm came, and lo, the mirrors, they acted like airplane wings...and were found days later scatter all about the seafloor.)

The take-home is that it's pretty much all light. Cages with clear or no shades were dominated by algae. Cages with shades were dominated by what you'd expect to find on walls, with the exception of a few species which (maybe) were excluded due to sedimentation. That, and inverts are often negatively phototactic, preferentially setting where it is darker.

Bray-Curtis dissimilarities (how I dislike them...mostly) showing the clustering of different community types. Note that the shaded treatments and the actual wall samples overlap, while the unshaded treatment cluster way off to the other side for the most part

Predation by urchins may play a role - insofar as it can exclude algae, but that can only really be seen in looking at past work in the system and comparing it to the results here. That, and, heck, urchins can climb walls. So, really, their distribution matters only insofar as the periodic pulses that can wipe out algae.

A conceptual model of subtidal coexistence from this paper. Light plays the key role in mediating the competitive relationship between these two groups, although sedimentation and heavy grazing may play a role at different points in time

What I find most interesting here is that it really explores the major types of niche differentiation that occur due to landscape heterogeneity, and finds just which ones can mediate coexistence on a regular basis. Sure, sedimentation and herbivory/predation/flow will play their role, but the irregular landscape really matters by setting up light gradients. These types of gradients are all over the natural world, but they can easily and flippantly be ascribed to the wrong reason. Heck, one can even look at things like just where MPAs are placed and see all sorts of correlations with pre-existing human behavior and political concessions rather than straight-up biology. It takes a careful eye and an exacting design to really teases apart these correlated differences to find the mechanism behind the pattern - even in the icy waters of Maine.

With your tech shivering due to inadequate drysuit underwear.

Miller, R.J., Etter, R.J. (2008). SHADING FACILITATES SESSILE INVERTEBRATE DOMINANCE IN THE ROCKY SUBTIDAL GULF OF MAINE. Ecology, 89(2), 452. DOI: 10.1890/06-1099.1... Read more »