Thalassia seedlings in Florida Bay: Can they survive in extreme salinity
environments?
Amanda E. Kahn and Michael J. Durako
University of North Carolina at Wilmington, Center for Marine Science,
Wilmington, NC
Although much research in Florida Bay has focused on how biotic and abiotic
factors effect mature seagrasses, there is relatively little information on the effects
of these factors on seedling dispersal, recruitment and survival. Seedlings may be
critical to the re-establishment and sustainability of seagrass beds following stress
exposure or die-off. To address the effects of stress on seagrass seedling
establishment, we have initiated investigations on the effects of several
environmental variables on seedling growth and development. Since the primary
initial modification to the Florida Bay system in the Comprehensive Everglades
Restoration Program (CERP) will be hydrological, salinity was the central focus
of manipulation in this initial research. In this investigation we sought to quantify
the effects of increased and reduced salinity (0-70 PSU) on the survival and
development of Thalassia testudinum seedlings.
Thalassia seedlings were collected on August 13th, 2002 from the mangrove
fringe on Little Rabbit Key in south-central Florida Bay and transported up to the
Center for Marine Science, Wilmington, North Carolina. The seedlings were
acclimated to 30°C, 29 PSU medium salt water for 24 h. Following acclimation,
the seedlings were transferred to Magenta GA-7 flasks containing 200 ml of
autoclaved Instant Ocean-based media at 0, 10, 20, 30, 40, 50, 60 and 70 PSU.
Ten seedlings were chosen at random for each of the salinity treatments. The
flasks were incubated at 24°C in a growth chamber under 12:12 light:dark
photoperiod for three months. Weekly measurements were made of root number
and lengths as well as blade number, lengths and widths from which total leaf
area was calculated. Every four weeks, effective quantum yield and dark-adapted
maximum quantum yield were measured using the Waltz Mini-PAM fluorometer.
At the end of the experiment period, leaf osmolality was measured on the
surviving seedlings using a Wescor VAPRO vapor pressure osmometer.
The extreme low- and high-salinity treatments had a negative impact on seedling
survival after 3 weeks. All seedlings in 0 PSU media were dead after 25 days in
the treatment. A dramatic decrease in survival by day 30 was also observed in the
seedlings grown at 50, 60, and 70 PSU. The seedlings grown at 20, 30 and 40
PSU had a 100% survival over the experimental period. Morphometric
characteristics (Fig. 1) showed a general trend in increasing total root length over
time, with the greatest rate of increase in the 10 and 20 PSU treatments. Blade
length and total leaf area decreased most drastically in the 0, 50, 60 and 70 PSU
treatments. The general trend of decreased values over the course of the
experiment was possibly the result of an experimental artifact associated with the
free-floating conditions in which the seedlings were kept. Previous observations
suggest that Thalassia seedlings exhibit better growth when rooted rather than
free-floating as was the case in this experiment.
Total Root Length (cm)
Fig.1. Average morphometric charactersitics of Thalassia seedlings grown at
8 salinity treatments measured over 3 month experimental period
3
2
1
Average Blade Width (cm)
Total Blade Length (cm)
0
10
8
6
4
2
0
0.4
0.3
0 PSU
10 PSU
20 PSU
30 PSU
0.2
0.1
40 PSU
50 PSU
60 PSU
70 PSU
2
Total Leaf Area (cm )
0.0
4
3
2
1
0
0
20
40
60
Time in Treatment (days)
80
100
The results from the PAM fluorometer measurements were inconclusive and no
assumptions could be drawn from the data. The osmolality measurements on the
other hand, showed significant trend in increased tissue osmolality for each of the
more saline treatments. Upon comparison of tissue osmolality to saltwater
osmolality, the tissue was distinctly hyperosmotic and maintained approximately
the same value of hyperosmolality between each treatment (Fig. 2).
Fig. 2. Average Osmolality of Treatment Leaf Tissue and
Plain Saltwater (mean del of 646 mmol/kg ±108)
Osmolality (mmol/kg)
2500
n=1
Tissue
2000
n=9
n=10
1500
Saltwater
Media
(n=5)
n=10
1000
n=9
500
0
10
20
30
40
Salinity Treatment
50
These preliminary experimental data suggest that Thalassia seedling survival and
development is negatively impacted in extreme salinity environments. It is
possible that the cost of maintaining a hyperosmotic environment within the tissue
is detrimental to plant tissue development and growth. Further studies will
experimentally manipulate the treatment environment to examine the effects of
incremental increases/decreases in salinity on seedling development. Oxygen
electrodes will also be used to further investigate the impacts of extreme salinity
on seedling physiology, particularly rates of respiration, as well more in depth use
of the PAM flurorometer. Currently, data are being collected in an aquarium-scale
experiment of a similar experimental design involving Thalassia seedlings
growing in an aragonite shell-hash substrate. The data from this experiment, in
conjunction with the flask-scale experiment, should more clearly define the
potential contribution of seedlings to seagrass recovery in extreme salinity
environments.
Amanda E. Kahn- The University of North Carolina at Wilmington, Center for
Marine Science 5600 Marvin Moss Lane, Wilmington, NC 28409
Phone: 910-962-2374, Email: aek8122@uncwil.edu