Genetic diversity, like species diversity, can have substantial effects on primary and secondary productivity, nutrient cycling, and response to disturbance.  However, we know relatively little about the specific mechanisms or traits that contribute to these effects, as well as how diversity among different trophic levels may interact.  Using a combination of field and mesocosm experiments, we examined the ecological effects of seagrass (Zostera marina) genotypic diversity and its interactions with consumer species diversity.  By comparing genotype performance in monoculture and polyculture at the end of these experiments, we delineate the specific mechanisms underlying plant genotypic diversity effects.
Results/Conclusions

Genotypic diversity enhanced seagrass production in response to experimental disturbance in a field experiment, but only at the highest level of disturbance.  Seagrass genotypic diversity also mitigated biomass loss during a natural macroalgal bloom, and this positive effect of diversity on shoot density and above-ground biomass persisted for at least a year, even after densities reached ambient levels.  Seagrass genotypic diversity also increased seagrass biomass in a separate mesocosm experiment, yet these effects depended on consumer species identity. In particular, seagrass biomass was higher in genotypic polyculture than in monoculture only when a specific grazer species (sea hare, Phyllaplysia taylori) was present.  In both experiments complementarity contributed significantly to positive effects of seagrass genotypic diversity, consistent with observations of differential resource use among genotypes.  Dominance effects were also important, though the sign and magnitude of these effects differed between the two experiments (most likely due to variation in experimental duration) and between different response variables.  For instance, positive dominance, in which the genotype that performs well in monoculture has high abundance in polyculture at the expense of other genotypes, was strong for shoot density in the field experiment.  However, this same genotype had relatively low biomass per shoot, resulting in negligible dominance effects on biomass in the same experiment.  Dominance effects on aboveground biomass were actually negative in the mesocosm experiment, consistent with an observed increase in biomass for certain genotypes in polyculture relative to monoculture.  Collectively, our results highlight the importance of plant genotypic diversity for population and community responses, particularly in the face of disturbance.  Further, they suggest that these effects likely result from a complex mix of dominance and complementarity mechanisms that depend on the traits of the specific taxa involved and the response variables of interest.