A new test of the Light:Nutrient hypothesis

To review: I love ecological stoichiometry (ES). I find it a fascinating subject and a useful framework for understanding ecological phenomena. However, ES is still relatively new, with a lot of the empirical work restricted to plankton (esp. Daphnia and algae). So it is always interesting to see theories developed predominantly in the pelagic system examined in other habitats.

One of the more interesting ideas out there is the light:nutrient hypothesis of Urabe and Sterner (1996; the ideas have been expanded in several later papers). Essentially, Urabe and Sterner noted that as you increase the amount of light, you decrease the nutrient content of algae. That's because as the light increases, but the availability of nutrients doesn't, then the algae is able to produce a larger and larger amount of carbon. As the algae become more and more nutrient-poor, the herbivores that graze on these algae get less and less nutrients from their food.

So what happens is a unimodal curve relating growth to the amount of algae present. That's depicted in Figure 1 from Urabe and Sterner (on the left). Part A of this figure shows the growth of algae and the nutrient content of the algae along a gradient of increasing light. As the light goes up, there's more and more algae with less and less phosphorus (and important nutrient). Then in Part B, focus on the line labeled "G, herbivore growth rate". See what it does along that same light gradient? First it increases with light, then it slowly decreases with light.

This kind of response was documented by Urabe and Sterner (1996) and several papers since then, but almost entirely in zooplankton (specifically, Daphnia spp.). However, there are lots of questions about this, primarily revolving around: How often does this occur in natural ecosystems?

This effect depends on a lot of things: The nutrient demands of the algae, the nutrient demands of the consumer, having an appropriate level of nutrients and light, etc. There really isn't much investigation of this effect outside of lakes (most of those studies focused on zooplankton).

A paper just published in Ecology by Hill et al. (2010; full cite below) seeks to investigate whether this occurs for snails growing on periphyton in streams. The authors looked at snail growth in two streams for several years, and primarily looked for evidence of that hypothesized unimodal relationship between light and growth. They did this by measuring the nutrient content of algae, the amount of light, estimating algal productivity, and the amount of growth in snails from month to month.

What's pretty interesting is that they didn't find this effect at all (see figure to the right).

Still, there's good theoretical reason for this effect to be possible, so why wasn't it observed? Well, the authors speculate that the extremely high density of herbivores present cause algal density to be held very low (i.e., competition between snails is so great that food is always in short supply). Recall from the Sterner and Urabe figure above that the mechanism driving this effect is the amount of algal biomass: There's just a lot of food. However, if the food is kept so low that algae can never achieve high biomass production, then food will probably remain limiting despite becoming less nutritious.

The authors did confirm that the algal part of the light nutrient hypothesis was occurring: As light increased (due to changing canopy density), the nutrient content of the algae decreased (less nitrogen and phosphorus). That just didn't translate into less growth in the herbivores.

There were some other things happening in this study that I'd like to know more about. For example, the amount of light was varying with the season, so it was also varying with a lot of other factors, like temperature. Without having thought about it too much, I'm not sure how those other changing conditions might affect different aspects of this system. I'm also not sure I completely buy the explanation that exploitive competition was driving this relationship, but I'll have to think about that some more.

The authors also point to some evidence suggesting that this effect might be widespread for benthic herbivores, although I find that evidence to be weak at this point.

Summary:

The authors of this paper tested the light:nutrient hypothesis in a snail-periphyton system and do not see the predicted relationship between light and herbivore growth rates. The expected unimodal response may be suppressed by heavy competition between grazers.