Response of eelgrass
Zostera marina to CO
2
enrichment: possible impacts of climate change
and potential for remediation of coastal habitats
Sherry L. Palacios
1,
*, Richard C. Zimmerman
2
1
Ocean Sciences Department, University of California Santa Cruz, 1156 High St., Santa Cruz, California 95064, USA
2
Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave., Norfolk,
Virginia 23520, USA
ABSTRACT: Projected increases in dissolved aqueous con-
centrations of carbon dioxide [CO
2
(aq)] may have significant
impacts on photosynthesis of CO
2
-limited organisms such as
seagrasses. Short-term CO
2
(aq) enrichment increases photo-
synthetic rates and reduces light requirements for growth and
survival of individual eelgrass
Zostera marina L. shoots grow-
ing in the laboratory under artificial light regimes for at least
45 d. This study examined the effects of long-term CO
2
(aq)
enrichment on the performance of eelgrass growing under
natural light-replete (33% surface irradiance) and light-limited
(5% surface irradiance) conditions for a period of 1 yr. Eelgrass
shoots were grown at 4 CO
2
(aq) concentrations in outdoor
flow-through seawater aquaria bubbled with industrial flue
gas containing approximately 11% CO
2
. Enrichment with
CO
2
(aq) did not alter biomass-specific growth rates, leaf size,
or leaf sugar content of above-ground shoots in either light
treatment. CO
2
(aq) enrichment, however, led to significantly
higher reproductive output, below-ground biomass and vege-
tative proliferation of new shoots in light-replete treatments.
This suggests that increasing the CO
2
content of the atmo-
sphere and ocean surface will increase the area-specific pro-
ductivity of seagrass meadows. CO
2
(aq) enrichment did not
affect the performance of shoots grown under light limitation,
suggesting that the transition from carbon- to light-limited
growth followed Liebigs Law. This study also demonstrated
that direct injection of industrial flue gas could significantly
increase eelgrass productivity; this might prove useful for
restoration efforts in degraded environments. The broader
effects of CO
2
(aq) enrichment on the function of natural
seagrass meadows, however, require further study before
deliberate CO
2
injection could be considered as an engineer-
ing solution to the problem of seagrass habitat degradation.
KEY WORDS: Eelgrass ·
Zostera marina · Carbon dioxide ·
Climate change · Productivity
Resale or republication not permitted without
written consent of the publisher
Rising CO
2
concentrations derived from combustion
of fossil fuel can increase the productivity and flower-
ing rates of seagrass
Zostera marina.
Photo: S. L. Palacios
O
PEN
PEN

A
CCESS
CCESS Mar Ecol Prog Ser 344: 113, 2007
berth 1996, ONeill & Oppenheimer 2002), levels not
reached since the Cretaceous (Retallack 2001). These
CO
2
increases may have dramatic impacts on global
climate (Keeling 1997), global carbon cycles (Tren-
berth 1996), ocean circulation (Manabe & Stouffer
1994, Sarmiento et al. 1998), biotic diversity (e.g. Kley-
pas et al. 1999, Ehleringer et al. 2001), and marine
ecosystem function (Denman 1996).
Climate change and rising atmospheric CO
2
are pre-
dicted to increase the fecundity (Koch & Mooney 1996,
DeLucia et al. 1999) and water use efficiency of terres-
trial plants (Retallack 2001), alter biomass partitioning
between their source and sink tissues (Chu et al. 1992),
and decrease the nutritive value of plant material by
diluting essential elements (N, Fe, etc.) with carbon
(ONeill & Norby 1996). Additionally, rising atmo-
spheric CO
2
concentration is predicted to favor the sur-
vival of C
3
over C
4
species, thereby altering plant com-
munity assemblages and their associated herbivore
populations (Ehleringer et al. 2001). In contrast, down-
regulation of productivity after prolonged exposure to
elevated [CO
2
] in some terrestrial species indicates
that some changes due to CO
2
enrichment may be
short-lived (Arp 1991, Woodward 2002).
The ocean environment is also expected to undergo
significant changes in response to rising CO
2
concen-
trations. The greenhouse effect is predicted to increase
ocean temperatures by 1 to 3°C, melt polar ice, freshen
surface waters at high latitudes and raise sea level
by 0.5 m in the next 50 to 100 yr (Trenberth 1996).
These temperature changes will affect heat sensitive
organisms directly and alter ocean currents (Manabe
& Stouffer 1994, Sarmiento et al. 1998). Elevated atmo-
spheric CO
2
will also increase the dissolved aqueous
CO
2
concentration [CO
2
(aq)] in seawater (Zeebe &
Wolf-Gladrow 2001).
The direct response of marine ecosystems to long
term CO
2
enrichment is less clear. The resulting drop
in seawater pH may cause widespread decline of car-
bonate accreting systems such as coral reefs (Kleypas
et al. 1999). Marine photosynthesis is generally not
CO
2
limited, because most marine algae derive 80 to
90% of their dissolved inorganic carbon (DIC) require-
ments from dehydration of the abundant HCO
3
(Beer
1996), which represents about 88% of the total DIC
content of seawater (Zeebe & Wolf-Gladrow 2001).
This efficient utilization of HCO
3
for photosynthesis
contributes to the low minimum light requirement for
algal growth, which is on the order of 1% of surface
irradiance (Luning & Dring 1975). In contrast, seagrass
light requirements are in excess of 11% of surface irra-
diance (Dennison & Alberte 1985, Duarte 1991), due
primarily to carbon limitation of photosynthesis (Zim-
merman et al. 1995, 1996, Beer & Koch 1996, Beer &
Rehnberg 1997, Zimmerman et al. 1997, Invers et al.
2001). Although seagrasses are capable of dehydrating
HCO
3
, many appear to rely on CO
2
(aq) for at least
50% of the carbon used for photosynthesis in nature
(Durako 1993, Beer & Koch 1996, Beer & Rehnberg
1997). Short-term enrichment of
Zostera marina L.
(eelgrass) with CO
2
(aq) in the laboratory under artifi-
cial illumination increased leaf photosynthesis and
shoot productivity 3-fold, while simultaneously decreas-
ing daily light requirements (Zimmerman et al. 1997).
Terrestrial studies have demonstrated that long-term
effects of changes in important variables, such as CO
2
availability, can be difficult to predict from short-term
exposure (Arp 1991, Woodward 2002). Consequently,
objectives of this study were to determine (1) if pro-
longed CO
2
(aq) enrichment permanently enhances the
productivity of eelgrass shoots growing under natural
irradiance regimes, (2) how CO
2
enrichment might
affect population dynamics of shoots that ultimately
determine the density and spatial extent of eelgrass
meadows, (3) if industrial flue gas containing CO
2
derived from fossil fuel combustion promotes eelgrass
productivity if deliberately injected into the water.
Understanding the impacts of CO
2
(aq) availability on
seagrasses will provide insight into both responses of
these ecologically important macrophytes to global cli-
mate change, and techniques for seagrass restoration
in turbid coastal waters.
MATERIALS AND METHODS
Experimental site.
Four outdoor flowing seawater
aquaria were constructed at the Duke Energy-North
America Power Plant (DENAPP) at Moss Landing,
California, USA. Seawater was pumped from Moss
Landing Harbor into a 20 m
3
storage silo and gravity-
fed into 4 fiberglass open top aquaria (4 m
3
each). Out-
flow from the aquaria was fed into the power plants
seawater outfall and transported offshore, more than
1 km away from the source water in Moss Landing
Harbor. Seawater volume within the aquaria turned
over approximately 10 times per day.
Source population.
Eelgrass (512 shoots) was col-
lected by hand in September 2000 from a subtidal pop-
ulation located at Seal Bend in Elkhorn Slough, CA,
USA (36.8153° N, 121.7658° W). Care was taken to sep-
arate whole shoots from the mud, keeping as many
intact root bundles and rhizome internodes as possible.
Shoots were placed in coolers containing seawater
and transported immediately to the experimental site.
Approximately 500 kg of mud, also collected from Seal
Bend, was distributed into 128 plastic nursery pots (4 l
capacity) lined with plastic bags, and 4 eelgrass shoots
were transplanted to each pot. The pots were divided
equally among the 4 outdoor flowing seawater aquaria
2 Palacios & Zimmerman: Response of eelgrass to CO
2
enrichment
(Fig. 1). The pot-grown shoots were maintained for
5 mo without CO
2
(aq) enrichment to permit recovery
from transplant effects (if any) and to evaluate the exis-
tence of any aquarium-specific effects that might con-
found the CO
2
(aq) and light treatments. Light avail-
ability in all aquaria was reduced to 33% of incident
surface irradiance using neutral density screens to sim-
ulate the natural submarine light intensity in Elkhorn
Slough, and to prevent photoinhibition of the leaves.
New shoots created by vegetative proliferation were
carefully removed and transferred to a new pot when
shoot density exceeded 4 per pot. Shoots growing
out of the pots (a result of rhizome elongation) were
replanted as necessary to keep roots and rhizomes
buried in the sediment.
The 32 pots in each aquarium were randomly segre-
gated into light-replete (33% of surface irradiance)
and light-limited (5% of surface irradiance) treatments
of 16 pots each, 5 mo after the initial collection. Light
was reduced to 5% of surface irradiance by adding
more neutral density screening to the south half of
each aquarium. The light-limited treatment was
de