A spatial analysis of Didymosphenia geminata (rock snot) in the New York City watershed

Background/Question/Methods

Didymosphenia geminata (didymo) is a nuisance stream alga that has unexpectedly increased in streams and rivers worldwide in recent years. Didymo is native to cool temperate regions of the Northern Hemisphere. During rapid periods of growth (blooms), the diatoms produce long polysaccharide stalks and their mats modify stream ecosystems through extensive benthic growth. Didymo mats have a cotton-like texture and tan color that has led to the common name “rock snot.” Didymo is currently found in seven known locations across New York State and is hypothesized to spread by transportation on recreational equipment (i.e., fishing gear). We investigated 1) the spatial distribution of didymo in Esopus Creek (Catskill Mountains), and 2) the chemical and hydrological causes of blooms through water chemistry analysis and microscopy. We sampled 7 sites weekly in June and July 2011, ranging from the headwaters to just above the inflow of the Ashokan Reservoir. Geographic Information Systems (GIS) were used to analyze spatial patterns of didymo distribution and cell densities within the Esopus Creek and in four tributaries.

Results/Conclusions

Spatially, didymo cell densities increased concurrently with increases in conductivity. Didymo was absent at the furthest upstream site and had the highest densities near a tributary that contributed wastewater effluent from a ski resort. Didymo was absent from several large tributaries throughout the summer. Higher didymo areal densities were found near public fishing access points, as well as in parts of the creek that experienced lower flow velocities. Didymo blooms developed throughout the summer 2011, but blooms were scoured away by a flood in early July. In general, didymo coverage was lower in 2011 than in summer of 2010, likely a result of higher flows. Overall, the extent of the didymo blooms was linked to hydrologic patterns, water chemistry, and human activity. This project will provide valuable information to inform fishermen and other recreational stream-users about how to avoid spread of nuisance didymo cell blooms. We will incorporate data into educational material posted at stream access points along Esopus Creek. Finally, this work illustrates multiple levels of controls on the spread and proliferation of an ecosystem-dominating nuisance species.