Masters Thesis

Experimental effects of ammonium on eelgrass (Zostera marina L.) shoot density in Humboldt Bay, California

Humboldt Bay contains two populations of eelgrass; Zostera marina L., a sparse population in North Bay and a significantly denser population in South Bay. No study has attempted to determine the cause of this density difference despite the importance of shoot density for protecting eelgrass community members. My objectives were to describe some of the ambient conditions in the water column and sediment of Humboldt Bay relevant to eelgrass, to experimentally determine if the variation in shoot density is caused by either ammonium limitation or toxicity, and to determine if ambient ammonium levels are affecting other eelgrass metrics. Ambient conditions were determined through monthly water samples, sediment cores, and data loggers. The effect of ammonium on density and other eelgrass metrics was examined through the in situ application of ammonium/phosphate fertilizer. The water column in Humboldt Bay contained insufficient ammonium to saturate eelgrass uptake and in the summer was nitrate limited as well. However, the sediment contained sufficient ammonium and the experiment demonstrated that the density difference between bays was not due to ammonium limitation. The application of fertilizer to the sediment of North Bay caused a significant decrease in eelgrass density and below ground biomass. The high levels of phosphate in North Bay treatment plots indicated phosphate not ammonium, toxicity. North Bay eelgrass may be vulnerable to an increase in nutrient loading. The abundance of Phyllaplysia taylori L. was a significant predictor of the change in density indicating that light levels may also be impacting densities between the bays.

Items in ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.