The Risk of Hypoxia in Narragansett Bay A Synthesis of Available Data
Dec 21, 2015
The Risk of Hypoxia in Narragansett Bay
A Synthesis of Available Data
Objectives:
• To assess whether hypoxia poses a risk to aquatic life in Upper Narragansett Bay
• Obtain an estimate of the spatial extent of this risk
• Outline some approaches to integration of existing dissolved oxygen datasets with the EPA saltwater DO criteria (2000)
Available Data
• “Insomniac” surveys – Prell, Deacutis, Murray (1999-2003)
• Nu-shuttle cruises – Berman, Oviatt, Melrose (1998-2004)
• Monitoring Buoys – Bergondo, Kester (1999-2004)
• Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape Cod to Cape Hatteras
– EPA (2000)
Data Types and LimitationsSpatial Temporal
Horizontal Vertical
Insomniac Surveys
Extensive75+ stations throughout the upper bay
ExtensiveRoughly 1.5 m depth intervals
Limited 2-3 surveys per summer
Nu-Shuttle Cruises
LimitedOnly deep channels sampled
Extensive Limited~ 1 survey per month
Monitoring Buoys
Limited5-10 locations in NB 1999-2003
LimitedSurface and bottom instruments
Extensive15 minute sampling interval
What do the EPA Criteria Tell Us
• EPA criteria address both growth and mortality for different life stages:– Larvae – growth and mortality effects are
incorporated into a larval recruitment model– Juveniles and Adults are considered together
• Both persistent and cyclic/episodic hypoxia are accounted for in the criteria
Experimental Mortality Data
• Plot from the EPA criteria document including protective limits
• Shows DO concentrations associated with 50% mortality (LC50) for species that commonly inhabit estuaries between Cape Cod and Cape Hatteras
2.9 mg/L - Limit for Protection against Larval Mortality
2.3 mg/L Limit for Protection against Juvenile Mortality
Winterflounder
Tautog
• Average bottom DO in most of the Upper Bay is in a range known to cause harm to aquatic life
• DO divisions based upon larval protection limits
Where the July and August Insomniac surveys fit into the structure of a hypoxic event at the Bullock’s
Reach buoy
Large regions of the bay show a coherent pattern with DO measured at the Bullock’s Reach Buoy
• Deep stations are generally well correlated within a given region of the bay
• Cross section of the shipping channel from the Providence River to the south end of Prudence Island
• Transects show DO (color contours) and density (σt)
• In this case – the Nu-shuttle cruise occurred 4 days after the Insomniac survey
Conclusions
• Aquatic life in Upper Narragansett Bay is at risk from hypoxia - both mortality and growth effects
• The spatial extent of this risk is large – including almost all areas northward from the south tip of Prudence Island
• Integration of the existing data can provide information about patterns of hypoxia in Upper Narragansett Bay and guide future monitoring efforts
Acknowledgements
Many thanks for the free exchange of data and ideas:
Deanna Bergondo Mark BermanDana Kester Chris MelroseChris Deacutis Heather StoffelWarren Prell Heidi TraversDavid Murray Robin WeberDon Pryor Catherine WalkerCandace Oviatt Peter August
Additional Material
Comparison of Insomniac Summer Averages and
Summer Averages from Monitoring Buoys
• The Insomniacs data and Buoy data are completely independent, yet they give a very similar summer average at the Bullock’s Reach and North Prudence Buoy sites
Bullock’s
DO (mg/L)
N. Prudence
DO (mg/L)
Avg. from Nearby stations
3.36 3.67
1999 - 4.51
2001 3.28 3.70
2002 3.45 4.20
2003 3.20 3.00
Avg. from Buoy Data
3.31 3.85
Larval Time-to-death Curves from the EPA criteria
• These time-to-death curves are based upon studies done on larvae
• Mortality after 24 hours does not increase significantly at a given DO concentration
Absolute Minimum DO measured at each station in any “Insomniac” Survey.
Note that areas in red are below 1.2 mg/L (from the EPA VPDO criteria Implementation Guidance document)
Large regions of the bay show a coherent pattern with DO measured at the Bullock’s Reach Buoy
• Deep stations in different regions of the bay are generally very well correlated
GSO dock does not show on this slide
Roger Williams is a future location