the risk of hypoxia in narragansett bay a synthesis of available data
Post on 21-Dec-2015
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The Risk of Hypoxia in Narragansett Bay
A Synthesis of Available Data
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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)
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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)
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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
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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
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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
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• 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
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Where the July and August Insomniac surveys fit into the structure of a hypoxic event at the Bullock’s
Reach buoy
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Large regions of the bay show a coherent pattern with DO measured at the Bullock’s Reach Buoy
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• Deep stations are generally well correlated within a given region of the bay
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• 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
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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
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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
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Additional Material
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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
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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
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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)
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Large regions of the bay show a coherent pattern with DO measured at the Bullock’s Reach Buoy
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• Deep stations in different regions of the bay are generally very well correlated
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GSO dock does not show on this slide
Roger Williams is a future location