Top-down modification of bottom-up processes

Whereas most studies have focused on the negative effects of consumers on their prey, today's Marine Ecology lab evaluated the potential positive effects of herbivores on seaweeds. Specifically, we quantified the effects of snails (Littorina littorea) and seaweeds (Fucus vesiculosus) on ammonium concentrations in tide pool microcosms. You see, in addition to eating seaweeds, snails also excrete ammonium as a waste product. And ammonium is the preferred nitrogen source for most seaweeds. As you can see below, seaweeds take up the nitrogen excreted by snails. Where snails alone are present, ammonium rapidly accumulates. Where seaweeds alone are present, ammonium quickly disappears due to uptake. However, when both snails and seaweeds occur in tide pools, ammonium concentrations are identical to those in pools with neither snails nor seaweeds. Why does ammonium decline in -Littorina -Fucus treatments? Probably because microbes in the seawater also use ammonium as a nitrogen source.

Fucus vesiculosus

Nutrients in tide pools