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Proposed Nutrient Criteria for NH’s Estuaries. Philip Trowbridge, P.E. NH Estuaries Project / NH DES November 17, 2008. Chronology. 2005: Orientation to the problem and possibilities for criteria 2006: NOAA assessment methods State of the Estuaries report
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Proposed Nutrient Criteria for NH’s Estuaries Philip Trowbridge, P.E. NH Estuaries Project / NH DES November 17, 2008
Chronology • 2005: Orientation to the problem and possibilities for criteria • 2006: NOAA assessment methods State of the Estuaries report • 2007: Analysis of grab sample data Loading-WQ model from EPA Antidegradation Light attenuation factors from buoy
Chronology (cont.) • 2007: Nitrogen from Lamprey Watershed Normalized N loads compared to other estuaries • 2008: Eelgrass assessment for 303d list Aggregate Kd relationships Kd, turbidity from hyperspectral data Macroalgae from hyperspectral data Historic N loads from watershed
Regulatory Authority • Env-Wq 1703.14 • “Class B waters shall contain no phosphorus or nitrogen in such concentrations that would impair any existing or designated uses, unless naturally occurring.” • Relevant designated uses • Primary Contact Recreation (swimming) • Aquatic Life Use Support
Precedents from Other States • Massachusetts Estuaries Project • Ambient [TN] typically 0.35-0.38 mg N/L for the protection eelgrass • Pensacola Bay • Ambient [TN] = 0.49 mg N/L to maintain current (good) conditions
Methods • Grab samples for water quality • Aggregation by assessment zone • Aggregation by trend station • Grab samples for sediment quality • Aggregation by assessment zone • Datasonde measurements of DO, turbidity • Buoy observations of Kd and clarity factors • Hyperspectral imagery for macroalgae, eelgrass
Conceptual Model (following Bricker et al., 2007) • Nutrient Concentrations • Total Nitrogen • Total Phosphorus • Primary Indicators • Chlorophyll-a • Macroalgae • Secondary Indicators • Benthic Invertebrates and Sediment Quality • Dissolved Oxygen • Eelgrass
Nutrient Concentrations • Median values of TN and TP in different assessment zones • Average concentrations at stations • Limiting nutrient calculations
[TN] = 0.244 mg N/L in offshore GOM waters
Primary Indicators – Chlorophyll-a • Median concentrations in each assessment zone • Average concentrations at stations • Seasonal patterns in different salinity regimes • Relationship to TN • Predicted TN threshold for primary contact recreation designated use
Chl-a threshold for swimming use Proposed TN threshold for swim/boat use (0.67 mg N/L)
Primary Indicators - Macroalgae • Eelgrass and macroalgae mapped using hyperspectral imagery from August 2007 • 1381 acres of eelgrass in Great Bay • 136 acres of macroalgae in Great Bay • Macroalgae covers 6% of areas formerly mapped as eelgrass in 1996 • Median [TN] in Great Bay = 0.42 mg N/L
Secondary Indicators – Benthos • Benthic invertebrate IBI correlated with TN but really controlled by salinity • Total Organic Carbon content >5% indicates organic enrichment • TOC correlated with chlorophyll-a • TN threshold of 0.67 mg N/L is protective of this endpoint.
Secondary Indicators – DO • Minimum DO concentrations at stations • Relationship between DO and Chl-a • Chl-a threshold for DO • Relationship between DO and TN for grab samples • Relationship between DO and TN for datasonde measurements • TN threshold for DO
Proposed Chl-a threshold to meet DO standard Chl-a (yearly 90th %ile) = 6.4 ug/L Chla-a (summer 90th %ile) = 10 ug/L
Apparent threshold (0.57 mg N/L)
TN Threshold for DO • Regression from grab samples • 0.57 mg N/L • Datasondes • No problems for TN=0.29-0.39 mg N/L • Violations for TN=0.45-0.74 mg N/L • Lamprey River DO problems may be natural • Weight of evidence • TN threshold for DO = 0.5 mg N/L
Secondary Indicators - Eelgrass • Average water clarity (Kd) at stations • Conceptual model from Koch (2001) • Threshold for water clarity for eelgrass • Quantification of factors controlling Kd • Relationship between turbidity and nitrogen • Relationship between Kd and nitrogen • Threshold for TN for eelgrass
Conceptual Model (Koch, 2001) Zmax should be >1 m below Zmin for viable eelgrass beds CBP set 0.22 as the minimum value for Iz/Io for eelgrass Zmin = 1 meter for the Great Bay Estuary
Minimum Kd for Eelgrass Survival • Zmin = 1 m • Zmax >=2 m • Iz/Io = 0.22 • Using the equation from Koch (2001), the minimum Kd would be 0.75 m-1 • Use this value as a threshold regardless of cause
Kd threshold (-0.75 m-1) Threshold for TN to maintain water clarity for eelgrass (0.32 mg N/L) Apparent threshold for macroalgae growth (0.42 mg N/L)