Growth and flowering responses of eelgrass to simulated grazing and fecal addition by Brant Geese

Seagrasses provide a wide range of ecosystem services and are prioritized in conservation planning. Management of this dynamic community requires describing seagrass responses to repeated grazing by dependent herbivores. Using two experimental randomized block designs in Humboldt Bay, California, we tested responses of eelgrass (Zostera marina) to separate and combined effects of simulated Brant Goose (Branta bernicla) grazing (clipping to a uniform height) and fecal pellet addition, and then to medium (MED) and severe (SEV) intensities of simulated Brant flock visitation. In Experiment 1, clipping with fecal addition (FC) stimulated more clonal addition of shoots than controls or clipping and fecal addition alone (standardized beta = 0.26 relative to control). The FC treatment also had the largest positive, delayed effect (beta = 0.135) on density of flowering shoots. In Experiment 2, the MED intensity treatment had positive effects on shoot density (beta = 0.149), rates of leaf extension (beta = 0.16), and leaf productivity (beta = 0.20). These MED treatment responses resulted in the highest measures of above-ground (beta = 0.099) and below-ground (beta = 0.32) biomass. Whole shoot (beta(MED) = 0.33, beta(SEV) = -0.36) and landscape (beta(MED) = 0.34, beta(SEV) = -0.23) levels of productivity for the MED treatment reflected the parabolic shape predicted by the compensatory regrowth hypothesis, likely because light and nutrient resources were available to the eelgrass and meristems that were not damaged. Meristem injury in the SEV treatment likely explained why productivity was low, but the high SEV resource levels allowed these plants to recover to control levels in approximately eight to 10 weeks after the last treatment applications. The MED level of shoot and landscape productivity would optimize low intertidal accessibility for Brant to the most nutritiously valuable leaves. Brant grazing and detrital pathways of carbon flow are likely linked with Brant-induced productivity contributing to the detrital pathway when Brant are present and after the birds leave the estuary. The effect of Brant grazing on eelgrass sexual reproduction may increase long-term resilience of estuarine ecosystems.

Automated summary

Seagrasses benefit from moderate grazing and fecal pellet addition, which promotes shoot growth and increases reproductive shoot density. Moderate intensity grazing by Brant Geese also enhances shoot density, leaf extension rates, and productivity of eelgrass.