Ulva, algae bloom, herbivorous fish, Fundulus
Potential Impact of Herbivorous Fish on Ulva Bloom Biomass
Sponsor: Carol Thornber, Biological Sciences
The addition of anthropogenically produced nitrogen and phosphorous to coastal habitats is a major problem that frequently results in the rapid growth, or bloom, of macroalgae. Within Narragansett Bay, RI, blooms are commonly formed by two species: Ulva compressa and U. rigida. These blooms co-occur with a number of invertebrates that have been previously studied for their impacts on bloom biomass. However, few studies have examined the abundance or potential grazing impacts of herbivorous fish. To address this information gap, we conducted surveys via seining from July 2011 to March 2012. In addition, we assessed gut contents of two suspected herbivores: Fundulus heteroclitus and F. majalis. We found that the most abundant fish species at bloom-impacted sites were Fundulus heteroclitus and F. majalis. Fundulus heteroclitus abundance peaked in September (mean 71.5 individuals per 216 m3 of water) while F. majalis was most abundant in August (mean 108 individuals per 216 m3 of water); no Fundulus were found in January and February. Preliminary results of gut content analysis suggest that F. majalis is carnivorous, as their winter diet consisted of copepods and ostracods. In contrast, F. heteroclitus appears to be herbivorous, as the majority of the gut contents were found to be phytoplankton of the genus Prorocentrum. If F. heteroclitus consumes Ulva as it becomes more abundant during the spring and summer, large populations of these fish could impact Ulva biomass, thereby influencing Ulva bloom severity. In future months, we intend to directly assess F. heteroclitus consumption of Ulva using paired-choice feeding assays to determine the rate of thallus consumption as well as whether they prefer U. compressa or U. rigida, as any feeding preference could influence the species composition of blooms.
Keywords: Ulva, algae bloom, herbivorous fish, Fundulus