Food web ecology has generally focused on consumer-driven (“top-down”) versus resource-driven (“bottom-up”) control, yet increasing evidence demonstrates the importance of linkages between the two, particularly in marine ecosystems. Fish and other consumers, which impose top-down control, may also drastically influence the structure of coastal and marine communities by supplying nutrients via the excretion of urine and feces, potentially reinforcing top-down effects. While upwelling brings nitrogen in the form of nitrate to the ocean surface, consumers including fish and invertebrates excrete ammonia, which is more readily taken up by primary producers. Upwelling intensity varies spatially and temporally, so the addition of ammonia from consumers may be important for sustaining kelp forests in low upwelling conditions.
In central Chile, kelp forests of Lessonia trabeculata are patchily distributed, and are influenced by both human fishing pressure and upwelling. Along the central coast, upwelling intensity is localized, varying spatially at the scale of 10s to 100s of kilometers, and kelp forests exist in areas with both weak and strong upwelling. However, the importance of fish-derived nutrients to kelp forests, particularly in areas of low upwelling, is unclear. Aplodactylus punctatus is an herbivorous fish closely associated with L. trabeculata. I conducted an experiment with A. punctatus and L. trabeculata to assess the effect of fish presence on kelp growth and nitrogen enrichment. I measured the growth and decay of kelp blades housed in aquaria with or without fish over 10 days, and conducted stable N isotope analysis of kelp blade and fish tissue before and after the 10-day treatment to assess enrichment of d15N in kelp. I also separately measured the ammonia excretion rate of Aplodactylus punctatus.
Results/Conclusions
Over the 10-day experiment, no change in d15N was observed in kelp blades of either treatment group. Average kelp growth was minimal and exceeded by average decay. However, blade decay was lower in individuals housed with fish than without fish, suggesting that kelp blade decay may be mitigated by fish presence. Further experimentation is warranted to investigate causes of kelp decay in aquaria and assess kelp uptake rate of fish-excreted ammonia under varying degrees of nitrogen limitation.