Document: KAR-3-48-23

Do bottom-up factors drive variation in herbivore effects? A study of upwelling and herbivory along the central Chilean coast.

NIELSEN, K.J.*

Oregon State University, Corvallis, OR 97331, USA ¹

Abstract:
Variation in upwelling regimes is known to affect community structure in rocky intertidal systems by altering the interactions between major predators and their sessile invertebrate prey. However, very little is known about the link between upwelled nutrient supply and benthic plant-herbivore interactions. I used field experiments to test the hypothesis that macroalgae at sites with stronger and more frequent upwelling events would grow faster and therefore be less impacted by herbivory than algae at other sites. Using satellite images of sea surface temperature and in situ measurements of water temperature, I identified 2 'strong upwelling' and 2 'weak-upwelling' sites along the central coast of Chile. I compared algal growth and herbivore impact among these four sites with three complementary studies. I monitored the growth rates of marked, herbivore-free individuals of two dominant plants, Mazzaella laminarioides and Lessonia nigrescens, in the mid and low zones, respectively. In the low zone I also examined the relationship between the alga Lessonia and the herbivorous snail Scurria scurra which lives obligately on the stipes of Lessonia by measuring the growth rates of marked pairs. In the mid zone I followed the recruitment and growth of macroalgae in cleared experimental plots (with herbivores, without herbivores, and manipulation controls) over one year. Herbivore potential in the mid zone at each site was calculated by seasonal surveys of herbivore abundance. Herbivore effects appeared to be strongest at the weak-upwelling sites but differences in herbivore pressure were also negatively correlated with upwelling strength. Where herbivores were abundant, they had a strong negative effect on macroalgal abundance and diversity. Strong upwelling sites had greater diversity of seaweed species recruiting to bare rock and the highest macroalgal growth rates. The evidence from this study suggests that bottom-up effects associated with upwelling regimes may contribute to variation in plant-herbivore interactions among sites but other sources of variation such as local abiotic conditions, recruitment patterns and historical contingency may also be important.

Keywords: Herbivory, Upwelling, Intertidal, Macroalgae, Bottom-up, Top-down

This abstract is being presented at: 11:00 AM in session:
Oral Session #37: Phytoplankton.