Eelgrass Growth Requirements
Kenly Hiller
Eelgrass is important
• Supports variety of other organisms
• Affect sedimentation rates by modifying
currents and waves
• Rival reefs in terms of C fixation
Many factors influence eelgrass
growth (Zimmerman et al, 1991).
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Temperature
Salinity
Nutrients/light
Water motion/tidal range
Grazing
** First and foremost, eelgrass needs the correct
temperature, salinity, and depth to be able to grow. These
factors determine the large-scale range of where eelgrass is
found around the world. After that, light, nutrients, sediment
characteristics, water movement, and grazing determine
whether eelgrass can survive in these places.
Seagrasses are disappearing….why??
• Consensus is that it is mostly due to light
attenuation.
• Losses are generally attributed to
anthropogenic nutrient and sediment inputs
that reduce the amount of light that reach
eelgrass beds.
• Depends on the system.
• Let’s walk through some case studies and look
at the evidence!
Temperature
• Limits broad-scale geographic range
• Too hot means too much respiration, too cold means
metabolism halts.
• Zostera Marina, the common species of eelgrass in MA,
optimizes at 5oC, but can survive up to 35oC (Marsh et al,
1986).
• Affects P-I curve!
Salinity
• Z. marina well adapted to swings in salinity
because they live in estuaries. Can survive
alterations from 5-30 psu in a single tidal cycle
(Kamermans et al, 1999).
Light and Nutrients
• Light most often controls the depth, distribution,
and productivity of eelgrass (Zimmerman et al,
1991).
• Nutrients cause phytoplankton blooms and
increase epiphyte growth, both of which can
increase shading. Suspended solids can also
block the light directly.
• Nutrients also necessary for eelgrass growth!
• Phytoplankton, epiphytes, and suspended solids
interact differently in different systems to shade
out eelgrass.
Case Study: Epiphytes, nutrients, and
shading (Twilley et al, 1985).
• found that the total amount of epiphytes
increased with nutrient concentration :
Twilley et al continuted:
• epiphytes were found to attenuate photosyntheticlly
active radiation (PAR) with a strong correlation
Why is that bad?
The consequences of epiphytes
• Light compensation levels for one species of
eelgrass (potamogeton perfoliatus) were found to
be 50-100uE/m2 (Twilley et al, 1986). At high
nutrient concentrations, with epiphytic
colonization reaching 4g dry weight per gram dry
weight of eelgrass, only about 40uE/m2 would
reach eelgrass at a depth of only 0.5m.
• eelgrass biomass between beds with and without
epiphytes differed by up to 18g dry weight per
square meter (Montfrans et al, 1984).
Phytoplankton blooms
• Bohrer et al (1995) found that eelgrass was
only gone in places with high nitrogen loading
rates (39-45gN/m2/year).
• Gallegos and Kenworthy (1996) found that
persistent seagrass growth was only possible
in areas where chlorophyll a was less than
15mg/m3 during the growing season.
• effects of phytoplankton blooms on shading
are highly variable
Total Suspended Solids
• Moore et al, 1996, found that eelgrass loss
occurred when TSS concentrations were from
15-40mg/l.
• Gallegos and Kenworthy (1996) found that in
Chesapeake Bay, sites with persistent
submerged aquatic vegetation (SAV) beds to
depths of one meter or greater occurred only
where median concentrations of TSS were less
than 15g/m3 (15mg/l),
TSS and k from an unhealthy Chesapeake system
Take home message from light and
nutrients stuff:
• Light requirements for eelgrass are, on average,
75-150uE/m2/d. if you have light attenuation
coefficients (k) above 2/m, seagrasses are
exposed to light levels close to their
compensation point when production equals
respiration (Kemp et al, 1983). They cannot grow
under these conditions. Below this
compensation point at 75uE/m2/d, not even
adaptations like more chlorophyll per leaf area or
stem elongation to reach up to the light can help.
Oxygen
• Eutrophication>>stratification>>increased
oxygen demand in the sediment>>eelgrass
oxygen transport to roots shuts down.
• Sulfides are toxic.
Substratum
• generally grow on soft substrata such as mud
or sand (Terrados et al, 1999).
• Sediments with increased silt or clay fractions
show lower seagrass biomasses (Livingston et
al, 1998).
• Sediment Eh values in seagrass beds are
usually between -100 and 200mV in the top
ten cm (Terrados et al, 1999)
Water movement
• Rough water movement can have direct
negative effects like stretching and breaking as
well as indirect effects such as alterations in
gas exchange (Madsen et al, 2001).
• Eelgrass biomass is positively correlated with
flow velocities from 0-0.10m/s
Grazing
• Orth (1975) found that entire beds of zostera
marina can be uprooted and consumed by the
cownose ray.
• In the study done by Cambridge et al (1986)
in Cockburn Sound, Australia, grazing by sea
urchins significantly lowered biomass.
Implications for Neponset
• Results tell us what we should be monitoring in
order to get an idea of how well eelgrass would
grow in Neponset.
• Most of these parameters are already being
measured for the Neponset river. We know
salinity, TSS, light attenuation coefficients (k),
Secchi depths, chlorophyll a, and nutrient
concentrations. TSS actually looks lower than the
critical number of 15mg/l in the Neponset River
• nutrient levels look promising.
Secchi depth vs. TSS across all
available years:
What to measure?
• The problem is that we don’t have any of these data for the
estuary.
• need to know TSS concentrations, chlorophyll a, and
epiphyte growth densities to determine the level of shading
that’s occurring. Attenuation coefficients and Secchi
depths will also be critical to determining whether there is
enough light for eelgrass to grow. Sediment type and redox
potential would also be important to determine.
• TSS under 15ug/l, nutrients around 30uM, k around 1.5/m
(ideally, according to Kemp et al, 1983) would be great, but
DEPENDS ON THE SYSTEM. Need measurements!
References
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Bohrer, T. et al. 1995. Effect of epiphyte biomass on growth rate of zostera marina in estuaries subject to different
nutrient loading. Biol. Bull. 189: 260.
Cambridge, M.L. et al. 1986. The loss of seagrass in Cockburn Sound, western Austrailia. II. Possible causes of
seagrass decline. Aquatic Botany 24: 269-285.
Gallegos, C.L. and W.J. Kenworthy, 1996. Seagrass depth limits in the Indian River Lagoon (Florida, USA):
Application of an optical water quality model. Estuarine, Coastal, and Shelf Science 42: 267-288.
Kamermans, P. et al. 1999. Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass
(Zostera marina) population (Lake Grevelingen, The Netherlands). Marine Biology 133: 527-539.
Kemp, W.M. et al. 1983. The decline of submerged vascular plants in upper Chesapeake Bay: summary of results
concerning possible causes.MTS Journal: 17: 78-89.
Madsen, J.D. et al. 2001. The interaction between water movement, sediment dynamics and submersed
macrophytes. Hydrobiologia 444: 71-84.
Marsh, J.A. et al, 1986. Effects of temperature on photosynthesis and respiration in eelgrass (zoztera marina L.).
Journal of Experimental Marine Biology and Ecology 101: 257-267.
Montfrans, J. et al. 1984. Epiphyte-grazer relationships in seagrass meadows: consequences for seagrass growth
and production. Estuaries 7: 289-309.
Moore, K.A. et al. 1996. Zostera marina (eelgrass)growth and survival along a gradient of nutrients and turbidity in
the lower Chesapeake Bay. Mar. Ecol. Prog. Ser. 142: 247-259.
Terrados, J. et al. 1999. Are seagrass growth and survival constrained by the reducing conditions of the sediment?
Aquatic Botany 65: 175-197.
Twilley, R.R. et al. 1985. Nutrient enrichment of estuarine submersed vascular plant communities. 1. Algal growth
and effects on production of pants and associated communities. Mar. Ecol. Prog. Ser. 23: 179-191.
Zimmerman, R.C. et al. 1991. Assessment of environmental suitability for growth of zostera marina L. (eelgrass) in
San Francisco Bay. Aquatic Botany 39: 353-366.