Manipulation of food quality and quantity by black brant geese

High abundance and survival of eelgrass (Zostera marina), a foraging resource in Humboldt Bay, California, is imperative to the success of the black brant goose (Branta bernicula nigricans) population. The grazing optimization model predicts an increase in net primary production of leaf biomass with moderate levels of grazing, exceeding that of ungrazed plants. I used a modified replicated block design to test this hypothesis. Experimental effects of clipping (simulated grazing) and fecal application on leaf growth rate, shoot density, shoot biomass, leaf nutrient content, rhizome biomass, and reproductive potential of eelgrass were examined during the spring and summer for two years. Moderately grazed eelgrass experienced higher leaf growth rate in year two and higher shoot densities in both years. Young leaves had higher nitrogen content than old leaves. In year one, grazed and fertilized eelgrass had higher number of inflorescences. Shoot biomass was greater in the grazed and fertilized treatments in the last period of 2004 and in periods one, three and four in 2005. Rhizome biomass was greater in the grazed treatments during period four of 2004 and in the moderately grazed treatment in 2005. Eelgrass has a central role in the success of many species living in the estuarine environment. Further understanding the interactions among species in this delicate ecosystem will help agencies manage for a healthy seagrass habitat and promote high species diversity in bays worldwide.