Nutrient Criteria Development for New Hampshires Estuaries

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Nutrient Criteria Development for New
Hampshires Estuaries

• P. Trowbridge, P.E.
• December 7, 2007

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Topics to Cover

• Guiding questions and nitrogen loading rates for
  Great Bay compared to other estuaries
• Estuarine nutrient criteria in other states
• Deadline for establishing nutrient criteria for
  NHs estuaries
• Develop group consensus on how to proceed in
  order to meet the deadline

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Guiding Questions (from Jim Hagy, EPA)

• Q. Has the system degraded from a prior state?
  Why?
• Q. Is the estuary degraded relative to other
  estuaries?
• Q. Are there environmental measures or indicators
  associated with nutrient over enrichment?
• Q. Are nutrient loads significantly above natural
  levels?

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Eelgrass Cover (ac) 1949-81 3,222 2005
2,291 Percent Change -29
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Water Quality in GBE relative to Similar New
England Estuaries
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Environmental Indicators of Nutrient Enrichment

• Eelgrass distribution and biomass
• Nitrogen concentrations in water
• Water clarity
• Watershed nitrogen loading
• Watershed sediment loading

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Nitrogen Loading Rates in Great Bay Compared to
Other Systems
Normalized by Surface Area

• Hauxwell et al. 2003 Eelgrass disappears at gt60
  kg/ha/yr
• Latimer et al. 2007 At 160 mg/m3, less
  than 5 of eelgrass remains
• Nixon et al. 2001 Compiled loadings of
  eelgrass and macroalgae systems

• Great Bay loading rate is 182 kg/ha/yr
• Great Bay loading rate is 280 mg/m3 (normalized
  by RT)
• Great Bay loads were at high end of
  eelgrass-dominated systems

Normalized by Volume Residence Time
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Nitrogen Loading Rates in Great Bay Compared to
Other Systems

• Steward Green 2007 watershed loads to maintain
  eelgrass 2.4-3.2 kg/ha/yr

• Great Bay watershed loading rate 3.8 kg/ha/yr

Normalized by watershed area
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Watershed Nitrogen Yields for Estuaries Similar
to the GBE
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Relationship of Water Quality to Watershed
Nitrogen Yields
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Guiding Questions

• Q. Has the system degraded from a prior state?
  Why? YES, eelgrass loss.
• Q. Is the estuary degraded relative to other
  estuaries? YES, compared to Casco et al.
• Q. Are there environmental measures or indicators
  associated with nutrient over enrichment? YES,
  eelgrass, TN, N loads.
• Q. Are nutrient loads significantly above natural
  levels? YES, compared to Casco et al. and when
  normalized by estuarine area or volume.

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Numeric Criteria Status for States
ALL Estuaries
Some Estuaries
21 of 27
Existing nutrient criteria are all based on
response variables paired with watershed loading
Slide courtesy of Jacques Oliver, EPA
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Rationale for 12/31/08 Deadline for a
Recommendation

• Process began three years ago. Competing
  priorities for NHEP staff in 2009.
• Municipalities need clear direction for WWTF
  upgrades and NPDES permits.
• Losing eelgrass biomass at 100 tons/yr.
• Implementation will be slow.
• 2009 SOE conference will be a good opportunity to
  disseminate the results.
• NHEP Management Plan will be updated in 2010 Add
  nitrogen reduction action plans.

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Options for the Next Year (see handout)

• Option 1 Develop a long-term trend of nitrogen
  and sediment loads to the estuary and compare to
  historic eelgrass distribution
• Option 2 Develop different nutrient criteria for
  different segments of the estuary
• Option 3 Designate the Great Bay Estuary as a
  Tier I waterbody for nitrogen and sediment

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Options (cont.)

• Option 4 Reference concentration approach within
  Great Bay
• Option 5 Reference approach for other estuaries
  in the ecoregion