Options for Developing Numeric Nutrient Criteria for New Hampshire’s Estuaries
Option 1: Develop a long-term trend of nitrogen and sediment loads to the estuary and compare
to trends in eelgrass distribution
• Use watershed export models to predict nitrogen and sediment loads in 1962, 1974, 1998,
2001 and 2005 using historic land use datasets for Rockingham and Strafford counties.
• Compile eelgrass distribution from GBERC (1949-1981), 1996, and 2005.
• Develop temporal relationships for nitrogen loading, sediment loading, and eelgrass
distribution.
• Set eelgrass distribution goal at some percentage of the GBERC distribution (Tampa Bay
used 95%).
• Use the temporal relationships to select the nitrogen and sediment loading rates
corresponding to the eelgrass distribution goal.
Pro
Follows precedent from Indian River
Lagoon and Tampa Bay.
Will result in criteria for nitrogen and
sediment loads from the watershed.
Easy to understand.
Con
Historic record of eelgrass and land use is
incomplete.
Oldest eelgrass cover from GBERC spans
32 years (1949-1981).
Criteria would be set by interpolation
(maybe extrapolation).
Does not determine actual cause of
eelgrass decline.
Option 2: Develop different nutrient criteria for different segments of the estuary
• Split the Great Bay Estuary into at least three sub-estuaries: Great Bay, Little Bay, and
the Piscataqua River.
• Quantify “impairments” (e.g., eelgrass loss, macroalgae cover, water clarity) in each subestuary.
• Relate nitrogen or sediment loads to impairments, if possible. Alternatively, only define
criteria for response variables.
Pro
Does not mix the different areas which
behave differently (Great Bay is intertidaldominated, Piscataqua River is sub-tidal
dominated).
Would allow for different criteria in
different areas.
Possible to establish criteria for response
variables only, not loads.
Con
No uniform data on macroalgae.
Eelgrass presence in Great Bay is
misleading because shallow water allows
it to persist when it should not.
Not able to calculate separate nitrogen and
sediment loads to sub-estuaries.
Research may not find a gradient between
sub-estuaries.
Option 3: Designate the Great Bay Estuary as a Tier I waterbody for nitrogen and sediment
• Administrative process to “hold the loads” at current levels.
• Need to carefully define the reference loads for nitrogen and sediment
• Need to prove that 90% of assimilative capacity has been used up.
• Temporary measure while research for more specific criteria continues.
Pro
Administrative process which would
result in no increase in loads from current
levels.
Con
Would be the first time process was used
in NH. There might be legal hurdles.
Without a standard, a weight-of-evidence
approach would be needed to justify
limits.
Temporary limits until a better standard is
developed.
Option 4: Reference concentration approach within Great Bay
• Select a reference station within the estuary where eelgrass loss is minimal.
• Calculate 75th percentile TN concentration at the reference station and the 25th percentile
concentration at all other stations.
• Set numeric criteria for [TN] between 25th percentile of whole estuary and 75th percentile
of reference site (e.g., using the Coastal Marine Laboratory as a reference condition, the
criteria would be 0.3 mg N/L).
• Use water quality models (simple or complex) to determine nitrogen loads which would
result in TN concentrations at the criteria.
Pro
Follows EPA guidance.
Would result in criteria for nitrogen
concentrations in estuarine waters.
Could also be used to set criteria for water
clarity and suspended solids.
Would result in at least 10% reduction of
nitrogen loads from current levels.
Con
No other state has set criteria for nitrogen
concentrations.
Hard to find a reference location besides
the mouth of the harbor.
May not be appropriate to have one
number for the whole estuary.
To determine watershed loads, would
need a water quality model of the estuary.
Option 5: Reference approach for other estuaries in the ecoregion
• Compile water quality and eelgrass loss data for four reference estuaries in the Gulf of
Maine (Casco Bay, Blue Hill Bay, Englishman’s Bay, Passamaquoddy Bay) and one
impaired estuary (Narragansett Bay).
• Compare nitrogen loading rates by watershed area.
• Select the highest acceptable watershed nitrogen yield from the reference estuaries as the
goal for Great Bay. The target will most likely will be the yield for Casco Bay (2.7
kg/ha/yr), which matches the TMDLs for Indian River Lagoon (2.4-3.2 kg/ha/yr).
Pro
Allows comparison of Great Bay to
pristine estuaries in same ecoregion
Would result in criteria for nitrogen yield
from watershed.
Easy to understand.
Would result in at least 20% reduction of
nitrogen loads from current levels.
Con
Would need to gather data from reference
systems in Maine with help from MEDEP.
Based on watershed yield, ignoring
differences in estuary hydrography.
Does not determine actual cause of
eelgrass decline.
Water clarity criteria would be needed for
deep water areas (e.g., Piscataqua River).