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Proposed Nutrient Criteria for NH’s Estuaries

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

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  1. Proposed Nutrient Criteria for NH’s Estuaries Philip Trowbridge, P.E. NH Estuaries Project / NH DES November 17, 2008

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Nutrient Concentrations • Median values of TN and TP in different assessment zones • Average concentrations at stations • Limiting nutrient calculations

  9. [TN] = 0.244 mg N/L in offshore GOM waters

  10. 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

  11. Chl-a threshold for swimming use Proposed TN threshold for swim/boat use (0.67 mg N/L)

  12. 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

  13. 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.

  14. 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

  15. 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

  16. Apparent threshold (0.57 mg N/L)

  17. Median TN = 0.29 mg N/L

  18. Median TN = 0.39 mg N/L

  19. Median TN = 0.45 mg N/L

  20. Median TN = 0.51 mg N/L

  21. Median TN = 0.57 mg N/L

  22. Median TN = 0.74 mg N/L

  23. 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

  24. 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

  25. 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

  26. Predicted Presence/Absence of Eelgrass Based on Measured Kd

  27. 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

  28. Kd Regression Equation from Buoy

  29. Predicted Kd from Hyperspectral Imagery

  30. Cumulative Attenuation by Factors

  31. Approx. 20 times more POC than predicted from chlorophyll-a

  32. 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)

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