Physiochemical controls of the growth of the invasive freshwater diatom, Didymosphenia geminata, in Rondout Creek, New York

Background/Question/Methods

The freshwater diatom, Didymosphenia geminata (didymo), is native to oligotrophic waters in temperate and alpine regions of the northern hemisphere. Didymo is considered a nuisance alga when stream conditions support the development of thick mats that cover a significant portion of the stream bed. These thick didymo mats are composed of a diatom cell sitting atop an extracellular polysaccharide stalk that likely assists in nutrient uptake and facilitates didymo blooms in oligotrophic streams. In summer of 2011, we deployed nutrient diffusing substrata (NDS) in Rondout Creek (Catskills, NY) to determine if didymo cell growth increases in response to nutrient enrichments that favor redox gradients. Didymo mats were detected in Rondout Creek beginning in March 2011. NDS release nutrients locally through a porous glass frit allowing algal and microbial colonization. We used 7 NDS treatments: individual treatments of acetate (labile dissolved organic carbon; DOC), iron (Fe), sulfate (S), and phosphate (P), and combination treatments of acetate + sulfate (DOC+S), iron + sulfate (Fe+S), and acetate + sulfate + iron (DOC+S+Fe). After 14 day incubations, we measured didymo cell densities, chlorophyll a, b, and c concentrations, gross primary production (GPP), and community respiration (CR) on NDS biofilms. 

Results/Conclusions

The DOC, DOC+S, and DOC+S+Fe treatments had the highest mean didymo cell densities, but lowest net ecosystem metabolism.  The P treatment increased chlorophyll a, however, chlorophyll a was not correlated with didymo cell densities.  Finally, chlorophyll b:c ratios were all below 1 except for the P treatment.  Overall, these data suggest that high community respiration on treatments with labile DOC likely established low intra-mat oxygen concentrations and high redox potential, and these conditions were where didymo thrived.  In contrast, when P was added to NDS we observed low didymo cell densities, high chlorophyll a, and high chlorophyll b:c ratios. These results suggest that other algae grew quickly and outcompeted didymo.  In general, water chemistry may be a key determinant of didymo mat proliferation and interspecific competition between didymo and other periphyton, thereby driving spatial and temporal variation in didymo bloom size and range expansion.