Extraordinary algal blooms in Chesapeake Bay following a year of tropical storms and record warm temperatures

Background/Question/Methods

There is a growing consensus that harmful algal blooms (HABs) are increasing worldwide in intensity and frequency in response to nutrient pollution.  Phytoplankton populations can respond quickly to changing environmental conditions including nutrient additions with greater abundances and shifting species composition.  The growth and dominance of HAB species can be promoted by long-term increases in nutrient concentrations and episodic nutrient pulses caused by storm events. In Chesapeake Bay and its tributaries, monthly fixed site monitoring of water quality and living resources have identified long-term trends of increased eutrophication.  However due to the ephemeral nature and patchiness of blooms, the ability to detect and study HABs and the potential effects of storm events are limited.  This study utilized higher frequency sampling, including in-situ chlorophyll mapping, to monitor the initiation, development, and transport of algal blooms.  Weekly collections were made from a network of stations and when in-situ chlorophyll measurements were elevated (>15µg l^-1) during 2011 and 2012.  Phytoplankton community analysis was conducted via light microscopy, with water-quality parameters including nutrient concentrations measured using standard methods. 

Results/Conclusions

Multiple seasonal algal blooms developed during the course of the study, including cyanobacteria blooms in the tidal fresh tributaries and large monospecific dinoflagellate blooms in the lower Chesapeake Bay and saline waters of the Rappahannock, York and James Rivers.  The region was affected by two tropical storms in August and September 2011 with associated nutrient runoff, followed by an especially warm winter, and continued warm temperatures into 2012.  The 2011 storms occurred during a well-developed bloom of the HAB forming dinoflagellate Cochlodinium polykrikoides and through flushing, had an immediate dissipative effect.  The dinoflagellate blooms observed in 2012 occurred 5-7 weeks earlier than 2011, lasted 2-3 weeks longer, and were greater in areal coverage and algal abundance than historical records.  The spring bloom was dominated by Heterocapsa triquetra with a maximum development in the mesohaline James River of 1.9 x10⁵ cells ml^-1.  Bloom concentrations as high as 7.6 x10⁴ cells ml^-1 of C. polykrikoides were observed from June to August throughout lower Chesapeake Bay, including the mainstem, western tributaries and eastern shore. This represents an increasingly growing range of this cyst forming species as well as that of the toxic dinoflagellate Alexandrium monilatum,  observed in the Chesapeake Bay since 2007.