Of Geese And Grass: Investigating Impacts Of Brant Grazing On Eeelgrass

Grazing pressure is important in structuring plant communities, particularly in aquatic environments. Eelgrass (Zostera marina), a keystone species in estuaries across North America, is experiencing widespread population declines. As a keystone species, eelgrass provides numerous important functions, including: 1) water filtration, 2) sediment stabilization, 3) providing refuge and nursery habitat for numerous species, and 4) carbon sequestration. Benefits which could be lost if eelgrass beds continue to diminish. California has experienced some of the greatest eelgrass declines, and Morro Bay, CA experienced a 96% decrease in eelgrass from 2007 to 2017. Most studies to date have examined bottom-up stressors, such as sedimentation and nutrient load, but little research has been done on top-down effects. Brant geese (Branta bernicla) are specialist grazers of eelgrass, feeding voraciously on it in the winter and spring months to build energy stores for their flight to summer breeding grounds in Alaska. Simulated brant grazing studies conducted in Humboldt, CA indicate brant might play an important role in maintaining the health and productivity of eelgrass communities through selective grazing and overcompensation – where plants purportedly grow more in response to herbivory. While there has been some experimental evidence of overcompensation, the idea is not well supported overall. Research was conducted on eelgrass beds in Morro Bay over the 2018-19 brant season. We hypothesized that: 1) brant grazing would decrease growth and overall condition of eelgrass; and 2) brant would selectively graze younger, nitrogen-rich blades. Four study sites were established, each with four open plots that allowed for natural brant grazing (treatment), and four enclosed plots that excluded brant and prevent grazing (control). Polyvinyl chloride (PVC) pipe was used to create cage-like structures around small plots of eelgrass, thus excluding brant, and exclosure effectiveness was confirmed with rigorous observations. Data on eelgrass growth and brant activity was gathered regularly at each site. We predicted that: 1) eelgrass open to brant grazing would show decreases in blade length, stipe density, and other growth and condition proxies before and after brant season when compared with eelgrass protected from brant grazing; and 2) eelgrass grazed by brant would have more epiphytes because brant selectively graze younger blades and leave older blades that accumulate more epiphytes. There were no significant differences in growth or condition of eelgrass between grazed (treatment) and ungrazed (control) plots. Brant activity was detected at each study site and brant exclosures were effective, with no evidence of brant grazing found in control plots. There was no difference in epiphyte load between grazed and ungrazed plots. The findings of this study are contrary the only previous studies examining the relationship between brant grazing and eelgrass growth. Our study suggests brant do not have a significant effect on eelgrass in Morro Bay.