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

fred.short@unh.edu

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