Jackson Estuarine Laboratory
University of New Hampshire
Frederick T. Short
Research Professor
Department of Natural Resources
and the Environment
Eelgrass & Nitrogen:
The GOOD, the BAD and
the ALGAE
2011, 2018
Piscataqua River. NH 2004
7 months of growth from a single seedling
Nitrogen is Critical to Eelgrass
Fenchel et al. 1973
DIN DON
The nitrogen cycle in an eelgrass meadow: Dissolved Inorganic Nitrogen (DIN) and Dissolved Organic Nitrogen (DON) are the primary forms that contribute
to Total Nitrogen in the water. The natural N cycle in the sediment and the water column includes uptake of Nitrogen (ammonium and nitrate + nitrite) by
the plants through the roots and leaves. In a pristine, non-eutrophied system as depicted here, the primary source of new N is N2 fixation in the water
column and the sediments.
Water
from Hubbard Brook Research Foundation 2003
N
Eelgrass
from Hubbard Brook Research Foundation 2003
N
Seaweed
1996
Mapped Eelgrass from 1984 - 2016
Great Bay, New Hampshire
The Adams Point eelgrass bed in August 2009:Loss of biomass and plant density
Jackson Estuarine Lab
The Adams Point eelgrass bed in August 2013:Loss of biomass and plant density
2009
Site OCC
0
10 0
200
P-Value <0.005
R2 = 0.83
Site T1
0
100
200
P-Value <0.001
R2 = 0.96
Site T3
0
100
200
P-Value <0.005
R2 = 0.98
Site R2
0
100
200
P-Value <0.005
R2 = 0.96
-
0
100
Site OCC
0
10 0
200
P-Value <0.005R2 = 0.83
Site T1
0
100
200
P-Value <0.001
R2 = 0.96
Site T3
0
100
200
P-Value <0.005
R2 = 0.98
Site R2
0
100
200
P-Value <0.005
R2 = 0.96
-E
elg
rass
Bio
mas
s (g
m)
-2
0
100
200
2000 2001 2002 2003 2004 2005 2006 2007 2008
Site DP
P-Value = 0.142
R2 = 0.95
Y = -21.63x + 43395
Y = -27.79x + 55761
Y = -47.45x + 95230
Y = -25.34x + 50855
Y = -29.88x + 59958
Years
Piscataqua River
Eelgrass Decline
Beem and Short, 2009, NH Port Authority
Eelgrass Biomass Losses (2005-2009)
65%
100%
20%
Little Bay & Piscataqua River
Great Bay
Portsmouth and Little Harbors
Loss
Loss
Loss
1996
2004
2013
Eelgrass Decline in Great Bay, NH
List of potential eelgrass stressors:
Possible Stressors Causing Reduced Light• Phytoplankton • Suspended Sediments • Seaweeds • Epiphytes • CDOM
Other Possible Stressors• Bioturbation• Water Temperature • Salinity • Wasting Disease
Stressor Impacts
INCREASING LEVEL CAUSESTRESSOR
Phytoplankton HIGH Nitrogen
Seaweeds HIGH Nitrogen
Suspended Sediments HIGH Loss of Eelgrass
Nitrogen Effect
Short et al. 1995
Nitrogen addition to mesocom tanks
Ambient Enriched NN
From Short et al. 1990. CZM
Mean (SE)
Area (acres) = -52.6*yr + 107464
R² = 0.734
0
1,000
2,000
3,000
1990 1995 2000 2005 2010 2015 2020
Year
Eelgrass Distribution in Great Bay (1992-2015)
FT Short / UNH 2016
Eelgrass Distribution
in Great Bay (1992-2015)
49% Loss
1996
Converting Eelgrass Cover to Biomass
y=2.2962x-42.027R²=0.72498
0
50
100
150
200
250
300
0 20 40 60 80 100
EelgrassBiomass(g/m
2)
PercentCover
AbovegroundBiomassvs.EelgrassCover
Data: USEPA and UNH Seagrass Lab
Eelgrass Distribution and Biomass
in Great Bay (1992-2015)
Biomass (ton) = -54.8*yr + 110520 R² = 0.705
Area (acres) = -52.6*yr + 107464 R² = 0.734
0
1000
2000
3000
1990 1995 2000 2005 2010 2015 2020
Year FT Short / UNH 2016
49% Loss
85% Loss
Seaweed
Nitrogen Enrichment > Algae
phytoplankton seaweed
seaweed
seaweed
SeagrassNet Monitoring
SeaweedIncrease inGreat Bay
0.0
2.0
4.0
6.0
8.0
10.0
12.0
2007 2008 2009 2010 2011 2012 2013 2014
Norm
alizedQuad
ratCount
GreatBay2007-2014 Eelgrass
Seaweed
0.0
2.0
4.0
6.0
8.0
10.0
12.0
2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
Normalized
Qua
dratCou
nt
GreatBayTrends2007-2014
Eelgrass
SeaweedEelgrass and seaweed (macroalgae) in the July SeagrassNet quadrat photos from Great Bay NH9.2 for 2007-2014, showing eelgrass percent cover and seaweed presence, measured as normalized occurrence in quadrats along Transects A, B, and C combined.
Great Bay, New Hampshire
Dense healthy eelgrass Great Bay, 1999
Seaweed and sparse eelgrass in Great Bay, 2009
Nitrogen Causes Eelgrass Loss in Great Bay2013
Plankton clouds the waterand shades eelgrass
Nuisance seaweed overgrows eelgrass
Epiphytes
Epiphytes on eelgrass in Great Bay, New Hampshire, 2015
Epiphytes on eelgrass in Great Bay, New Hampshire
August 2008
y = 24.25x - 47540
R² = 0.883
0
200
400
600
800
1000
1200
1400
1985 1990 1995 2000 2005 2010 2015
N L
oad
ing
to
GB
E
(to
ns/
yr)
Total N Loading
Total N Loading
Increase of TN loading since 1988Total N discharge to GBE 2013 SoE.xlsx&
Short 1992, GBE Estuarine Profile
Suspended Sediment
• River SS inputs have not changed.
• SS increasing in Great Bay.
• What can be done?- Bring back the eelgrass
to filter the water
Clear water
Loss of Nitrogen Filtration by Eelgrassin Great Bay
R²=0.63795
-
1,000
2,000
3,000
4,000
5,000
6,000
1995 2000 2005 2010 2015
NitrogenFiltra
on(kgN/day)
<-- 2618 from Sewage
2016
Estuaries with Seagrass Decline
Waquoit Bay• High N Loads Eelgrass Lost
Boston Harbor• Reduced N Load Eelgrass Recovering
Chesapeake Bay• High N Loads Eelgrass Lost
Tampa Bay• Reduced N Load Eelgrass Recovering
Thank You