|Title||The response of Lake Michigan benthic algae to an in situ nutrient manipulation|
|Year of Publication||1985|
|Degree||Master of Science|
|Number of Pages||86 pp.|
|University||Bowling Green State University|
|City||Bowling Green, OH|
The idea that Lake Michigan phytoplankton become secondarily growth-limited by silicon, following enrichment with phosphorus, has been well established. Conversely, the possibility that Lake Michigan epilimnetic, benthic algae are growth-limited by silicon is virtually unstudied, and was explored in the present experiment using nutrient-releasing artificial substrates (NRS). Treatments consisted of Si, N+P, Si+N+P, and controls (CONT). Forty-eight NRS were placed in Little Traverse Bay, Lake Michigan, at a depth of 10 m using SCUBA. Four replicate NRS of each treatment were sampled after 7, 14, and 31 days of exposure. Algal biomass as estimated by chlorophyll-a, particulate silica, and total biovolume increased through time (P < 0.0001, for each). Chlorophyll-a was greatest on Si+N+P and N+P substrates compared to Si and CONT substrates (P<0.0001). Particulate silica was greatest on Si+N+P followed by N+P and was lowest on Si and CONT substrates (P<0.0001). Estimates for total biovolume were different among all four treatments, with the means ordered from highest to lowest as Si+N+P, N+P, Si, CONT (P<0.0001). Communities on CONT and Si substrates were composed primarily of diatoms (98%) and changed little, in terms of biomass, throughout the study. The algal biovolume on N+P and Si+N+P substrates increased nearly 9-fold over the controls and consisted primarily of the green alga Stigeoclonium tenue and the blue-green alga Schizotrhrix calcicola. In addition, diatom biovolume was highest on Si+N+P substrates throughout the experiment (P<0.0001), and increased 2-fold on Si+N+P substrates by day 31, consisting chiefly of Achnanthes minutissima (38.9%) and Cocconeis placentula (29.1%). The lack of response to enrichment with Si alone and highest accumulation of chlorophyll-a, particulate silica, total biovolume, and diatom biovolume on Si+N+P substrates suggests that Si may become an important factor to benthic algal growth following enrichment with N+P. Furthermore, a similar shift from a diatom to a green and blue-green dominated algal community has been observed for upper Great Lakes phytoplankton following N+P enrichment, demonstrating the possibility that both communities are structured by comparable nutrient processes. Lastly, the response of algal communities to nutrient addition may be characterized by the reaction of a very few species or guilds of species.