R/EH-9

Impact/Outcome:

This section describes how this project has advanced scientific knowledge and/or made a difference in the lives of coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

RECAP: In this completed study, researchers explored what conditions promote the growth, persistence, and demise of the cyanobacteria Microcystis (blue-green algae) in the upper Sassafras River. Their results indicate that elevation in pH, phosphorus release from sediments, inhibition of coupled nitrification-denitrification, and nitrogen fixation are factors in the persistence of cyanobacterial blooms.

RELEVANCE: Despite efforts to reduce algae-fueling phosphorus (P) and nitrogen (N) in the Chesapeake Bay watershed, blooms of blue-green algae, or cyanobacteria, occur in a number of the Bay’s tidal freshwater tributaries. Although most studies occur after a bloom is evident, this study examined the factors that trigger blooms with the hope of targeting ways to avoid them.

RESPONSE: The principal investigators on this completed project are Diane Stoecker and Jeffrey C. Cornwell of the University of Maryland Center for Environmental Science Horn Point Laboratory. Their specific objectives included examining the role of “internal” sources of nutrients in maintaining algae blooms (e.g., sediment nutrient regeneration and pH-mediated phosphate release.) They also sought to examine the role of nutrient depletion and increasing salinity on the breakdown of the bloom. 

RESULTS: The researchers’ results indicate that elevation in pH, phosphorus release from sediments, inhibition of coupled nitrification-denitrification, and nitrogen fixation are factors in the persistence of cyanobacterial blooms. Nutrient loading, particularly phosphorus loading, combined with shallow sediments and warm summers, can create an environment conducive to cyanobacterial growth. But the cyanobacterial assemblage itself appears to “engineer” the biogeochemistry of the sediment-water interface and water column to perpetuate blooms and make their reoccurrence more likely.

The researchers’ initial manuscript was accepted for publication in the online European journal “Biogeosciences.” It extends the observation of pH-driven sediment phosphorus releases to an understanding of pH effects on the nitrogen cycle. The scientists submitted a second journal article to "Aquatic Microbial Ecology.” It concludes that 1) nitrogen fixation, largely at night in the dark, was sufficient to supply an important part of the nitrogen needed to sustain the cyanobacterial bloom, 2) that elevated pH decreased photosynthetic rates, but still limited growth of competing species, 3) and in combination with sediment nutrient releases, the bloom controlled the nutrient cycling in the water column.