Didymosphenia geminata (Lyngbye) M.Schmidt (herein referred to as didymo) is a benthic freshwater diatom capable of producing nuisance blooms that impact the recreational, ecological, and aesthetic value of freshwater ecosystems. Recently, didymo has expanded its range and ecological tolerances leading to massive blooms in areas where it was previously undocumented or had existed in low abundance (Spaulding and Elwell, 2007; Blanco and Ector, 2009; Whitton et al., 2009). Since 2007, these blooms have been increasingly observed in Pennsylvania (PA), yet currently no inventory of its distribution or assessment of watershed vulnerability exists for the state (PFBC, 2011). The intent of this study was to conduct a comprehensive assessment of didymo throughout PA and provide relevant technical information that could help inform management efforts. This study addresses three important research questions regarding didymo in PA streams along with their associated results and implications.
Research Question 1) What is the current and historic distribution of didymo in Pennsylvania?
We combined literature and historical collections with comprehensive field surveys to assess the current distribution of didymo. In total, 1752 historical records were compiled and supplemented with 40 samples collected in streams throughout PA. Of these, 96 records detected didymo from six watersheds within or intersecting the political boundary of PA, including the Delaware River and East Branch Dyberry Creek in northeastern; Pine Creek and Trout Run in northcentral; Gunpowder Falls in southern; and Youghiogheny River in southwestern PA. We also conducted a more intensive survey within the Pine Creek Watershed, a region where didymo blooms have been recently discovered (June 2013). Currently, didymo can be found on the substrate along a 24-mile reach from ranging from West Branch Pine Creek at Crippen Run, downstream to the Tiadaghton access in the Pine Creek gorge. Didymo coverage was greatest near the mouth of West Branch Pine Creek, with cell densities decreasing rapidly progressing downstream. All tributaries to Pine Creek other than the West Branch, including Ninemile Run, Babb Creek, Cedar Run, Slate Run, and Little Pine Creek, were not colonized by didymo.
Research Question 2) What is the potential habitat suitability for nuisance didymo blooms in Pennsylvania?
We assessed water chemistry, stream morphology, and other data to examine the potential habitat suitability of nuisance didymo blooms throughout PA. We incorporated targeted water quality sampling in concert with compilation of existing data sources to construct a database of Soluble Reactive Phosphorus (SRP; >18,000 samples), which is a nutrient known to limit didymo abundance. SRP concentrations were negatively correlated to the distribution of didymo across PA, with didymo restricted to watersheds with mean SRP concentrations <10 μg/L. This pattern was complex, however, and potentially linked to water management practices (regulated versus unregulated streams). The link between SRP and didymo was apparent in the Pine Creek Watershed. Didymo abundance was greatest in the upstream reaches of Pine Creek where SRP was lowest, then appeared to decline downstream as SRP increased.
We created an in-depth didymo habitat suitability index using 12 physicochemical parameters known to influence didymo blooms. Results suggest that, at least in PA, suitable didymo habitat is marginal at best, largely due to lack of flow regulation. Episodic nuisance didymo blooms may arise in unregulated streams when environmental conditions permit, but enduring blooms are most likely restricted to habitats downstream of bottom-release dams in this region. We then coupled high frequency sampling with continuous data collection at West Branch Pine Creek to elucidate relationships between physicochemical parameters and didymo coverage. Streamflow, temperature, and pH appeared to exert control over didymo coverage throughout the study period, with streamflow appearing to exert the most control at this location with consistently low SRP concentrations. Antecedent periods of stable flow were required for accumulation of didymo stalk material at West Branch Pine Creek.
Research Question 3) How does didymo presence affect the nutritional content of biofilms?
We collected periphyton from 44 sites across PA in the summer and fall of 2015. Fatty acid analysis was conducted on the periphyton to assess the nutritional relevance of biofilms colonized by didymo. To date, the fatty acid results are provisional, due to an unforeseen bias in the analytical chemistry methodology. The presence of didymo in the biofilm and season of collections (summer or fall) best explained variation in physiologically important fatty acid profiles (sensu essential fatty acids). Individual fatty acids also varied in with respect to season and didymo presence. When grouped according to fatty acid function group (e.g., saturated versus unsaturated), the effect of season was strongest, which is unsurprising given seasonal differences in photoperiod, temperature, nutrients, and carbon loading. Significant differences in saturated and mono-unsaturated fatty acids also suggest differences in nutritional quality between didymo and didymo-free biofilms.