Determining Nutrient Thresholds for Didymo Blooms in the Delaware River

Observational surveys and experimental rock transplants were conducted within the Delaware River to document the phenology of Didymosphenia geminata blooms and to evaluate the effects of different nutrient regimes on stalk formation and bloom development. The observational surveys began in October 2012 and continued through May 2013. They showed no D.geminata colonies in the Lower Delaware River prior to April 2013, although substantial blooms of the related stalked diatom, Gomphoneis sp., were documented from November 2012 through January 2013 in the lower-nutrient zone upstream of the Lehigh River confluence. D.geminata was first observed on April 10th, 2013, with a longitudinal pattern of earlier and stronger growth at upstream sites and a progression of development downstream. By late May 2013, however, a reversal of this longitudinal development was observed, with the long-stalked colonies retreating upstream to sites above the Lower Delaware. At no time were the short-stalk colonies of D.geminata, first observed in 2012 below the Lehigh River confluence, documented in 2013. The experimental transplant of D.geminata colonies within the Delaware River initially looked promising when begun in early May 2013. But the experiment coincided with the retreat, or waning, of the D.geminata blooms in the Lower Delaware, and visually distinct D.geminata colonies failed to persist under all experimental treatments, as well as at un-manipulated background rocks above the Lehigh confluence. This study highlights a number of interesting patterns about the growth of dense blooms of stalked diatoms. First, the formation of stalked D.geminata colonies in the Lower Delaware River began in early spring, with no stalked colonies observed through the colder fall and winter months. It is important to note, however, that the timing of D.geminata growth in the Lower Delaware may be different here than in the upstream sections of the Upper and Middle Delaware River, particularly tailwater sections with stable cold-water releases from reservoirs. Second, areas of long-stalk D.geminata were generally associated with soluble reactive phosphorus (SRP; aka, dissolved orthophosphate or dissolved reactive phosphorus [DRP]) concentrations below 10 μg/L. Previous research in New Zealand has indicated a SRP threshold of 2 μg/L for the inducement of stalk production. Our limited observational and experimental work is not yet sufficient to rigorously establish a similar SRP for the Delaware River. Yet it is worth noting the observed patterns in the Delaware during 2013 and highlighting the potential for different phosphorus thresholds for stalk formation in different environmental settings. Finally, extensive mat coverage by other stalked diatoms can occur in the Delaware River. Previous observations have shown Cymbella mexicana blooms in the Upper Delaware River and its tributaries. In the Lower Delaware, heavy coverage by Gomphoneis sp. was observed from November 2012 through January 2013. These observations of additional stalked diatom blooms raise important questions about the likely ecological role and effects of seasonal blooms of D.geminata and other stalked diatoms.