Elizabeth Brunner, St. Mary's College of Maryland

Class Year:

2009

Mentor:

Judy O'Neil, Ph.D.

Abstract:

Summer nitrogen limitation in the Sassafras River, a freshwater Chesapeake Bay tributary, selects for N₂-fixing cyanobacteria that could provide a "new" nitrogen source to this already nutrient enriched system. Despite the prevalence of these diazotrophic (N₂-fixing) blooms, little attention has been focused on the role of N₂-fixation in nutrient enriched, eutrophic estuaries like the Chesapeake Bay, nor has the role of cyanobacterial N-fixation been considered in N budgets. We analyzed weekly water samples from four stations for total suspended solids (TSS), volatile suspended solids (VSS), chlorophyll a, dissolved nutrients, and nitrogenase activity via the acetylene reduction assay. We performed two nutrient addition bioassay trials, monitoring the same parameters with additions of phosphate (5, 10 μM) and nitrate (10, 20 μM). Despite a cool, relatively wet summer without large cyanobacterial blooms, we found N₂-fixation to be present across the sample sites, with hot spots at Budd's Landing, a neighborhood marina, and Georgetown, an urban marina. Traditional water quality parameters such as suspended material and chlorophyll a were found to be poor indicators of N₂-fixation. In our bioassays, we found results consistent with N-limitation. Nitrogen additions stimulated phosphate uptake without increasing levels of N₂-fixation. Phosphate additions increased N₂-fixation without increasing chlorophyll a.

Location:

Horn Point Laboratory