PARENT SESSION
Thursday, August 10, 1:30-5:00 pm
COS 92 - Nitrogen cycling II
Sultana, Mezzanine Level, Cook Convention Center
Presiders: J Ely and S Baer

Natural and anthropogenic nitrogen uptake in bloom-forming macroalgae.

Thornber, Carol^*,1, Nixon, Scott¹, ¹ University of Rhode Island, Kingston, RI

ABSTRACT- Macroalgal blooms are becoming increasingly common ecological formations in shallow bays and estuaries worldwide. These blooms are spatially and temporally variable and are the result of rapid, seemingly uncontrolled growth of one to several species of macroalgae. They may negatively impact other shallow marine habitats such as seagrass and shellfish beds and are frequently regarded as nuisance species. In Narragansett Bay (RI), several species of green algae (primarily Ulva) and red algae (primarily Gracilaria) form dense aggregations during the summertime. Nitrogen is one of the primary limiting nutrients for macroalgae, and the formation of these blooms is frequently attributed to increased inputs of anthropogenic sources of nitrogen. In this study, we investigated the relative contributions of natural and anthropogenic (from sewage) nitrogen to macroalgal growth via stable nitrogen isotope ratios. We cultured Ulva and Gracilaria in water collected from four locations in Narragansett Bay along a latitudinal gradient of anthropogenic nitrogen inputs and examined the nitrogen isotopic signature as N15. Our northernmost site was at the head of the bay near a major urban center, Providence, while our southernmost site was near the mouth of the bay in a more rural area. We found significant differences in the N15 signature of algae cultured among these different water sources; algae grown in our northern water treatments had enriched signatures of 15N (and thus more anthropogenically derived nitrogen) relative to our southern treatments. These data, when coupled with a previous general model of circulation patterns, will permit the rapid estimation of the relative contribution of the major nitrogen sources to this bay, and their impacts on the population dynamics of local organisms.

Key words: nitrogen, estuarine, algae