Bottom-up and top-down processes affect eelgrass communities in British Columbia

Background/Question/Methods

Due to human actions, most ecosystems today are subjected to disturbances that affect their natural trophic interactions. Two common anthropogenic perturbations to coastal marine ecosystems are the removal of predators (resulting from overharvesting) and eutrophication. In many ecosystems, these factors frequently occur together, although little is known about the net effects of simultaneous predator removal and eutrophication on community structure. Eelgrass mesograzer communities provide ideal study systems to ask these questions due to the fast responses of species to environmental manipulations. Many mesograzers live on eelgrass and feed on epiphytes, which are highly responsive to nutrient addition. They are preyed on by small fish and larger crustaceans, which can be easily excluded with mesh cages. In a seven week field experiment, we excluded predators and added nutrients in a factorial design (n=10) in a large eelgrass (Zostera marina) meadow at Tsawwassen, British Columbia. Artificial Seegrass Units (ASUs) were used to collect mesograzers. We tested the hypotheses that predator removal and nutrient addition both increase grazer abundance and decrease richness, and that these factors combined increase grazer abundance and decrease richness above the effect of a single perturbation alone. We measured the responses of final abundance and community composition of snails and amphipods, two important grazers in this system.

Results/Conclusions

Both predator exclusion and nutrient addition had strong effects on the eelgrass mesograzer community, and responses varied significantly among taxa. We found 636 (+/- 300) individual mesograzers per ASU, comprising six amphipod and two snail species. Predator exclusion decreased total grazer abundance and richness, but increased the abundance of some species. Changes in community composition resulting from predator manipulations were driven by variable responses of invertebrate taxa. Nutrient addition increased total grazer abundance, but had no significant effect on richness. There were no significant interactive effects of predator exclusion and nutrient addition on the mesograzer community. We conclude that both bottom-up and top-down processes affect British Columbia’s eelgrass communities. This study demonstrates that eelgrass ecosystems in British Columbia are likely vulnerable to human perturbations like overfishing and eutrophication. Modifications of bottom-up and top-down processes on the herbivore community likely influence the overall productivity and functioning of this ecosystem, which may ultimately affect fisheries that depend on healthy eelgrass systems.