Growth of giant salvinia (

Salvinia molesta

Mitchell) under

different pH regimes was examined at the Lewisville Aquatic

Ecosystem Research Facility (LAERF) in Lewisville, Texas.(PDF has 5 pages.

Dick, Gary O.

Honnell, David R.

Owens, Chetta S.

Smart, Michael

English

Aquatic Commons

 34
 
J. Aquat. Plant Manage.
 
 43: 2005.
 
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Effects of pH on Growth
of 
 
Salvinia molesta
 
 Mitchell
 
CHETTA S. OWENS
 
1
 
, R. MICHAEL SMART
 
2
 
, DAVID R. HONNELL
 
3
 
 AND GARY O. DICK
 
3
 
ABSTRACT
 
Growth of giant salvinia (
 
Salvinia molesta 
 
Mitchell) under
different pH regimes was examined at the Lewisville Aquatic
Ecosystem Research Facility (LAERF) in Lewisville, Texas. Gi-
ant salvinia grew to completely cover a research pond over a
15-week period when pH was less than 7.5. Growth was re-
duced in a second pond maintained at a higher pH ranging
from 8.5 to 10.00.
 
 
 
Tank studies found that significantly great-
er giant salvinia biomass was produced, over a 2-fold increase,
at the lower pH (less than 7.5 units). Additionally, water
chemistry of tanks changed, especially pH and dissolved oxy-
gen, when completely covered by the resultant mat.
 
Key words: 
 
pH, nutrient availability, giant salvinia, distribution.
 
INTRODUCTION
 
Giant salvinia is a floating aquatic fern native to southeast-
ern Brazil and occurs between latitudes 24 and 32 degrees
(Forno and Harley 1979). The plant is currently found in
subtropical and tropical regions but has been reported in
more than 20 countries, where it typically was introduced as
an aquarium or water garden species (Room et al. 1981). Gi-
ant salvinia was first discovered and subsequently eradicated
from North Carolina in the mid 1990s and was later found in
1997 in a Houston, Texas schoolyard pond (USGS 2004). In
1998, giant salvinia was reported in Toledo Bend Reservoir
and by year 2000, had been found in 3 additional reservoirs
(Conroe, Sheldon, and Texana), 5 rivers (or streams) and 20
ponds in Texas. It has also been reported in eleven other
states, including AL, AZ, CA, FL, GA, HI, LA, MS, NC, SC
and VA (USGS 2004).
Temperature is probably the greatest factor limiting giant
salvinia growth, survival and spread (Owens et al. 2004,
Whiteman and Room 1991, Harley and Mitchell 1981). Gi-
ant salvinia has a distinct northern boundary corresponding
to low (below freezing) winter temperature, and appears to
be incapable of survival in locations where ice forms for ex-
tended periods (Owens et al. 2004, Whiteman and Room
1991, Harley and Mitchell 1981).
Giant salvinia has invaded several aquatic systems in south-
ern, southwest and Gulf coastal states of the United States
where it has exhibited persistent and explosive growth
(USGS 2004). Dense mats of giant salvinia can impede trans-
portation, irrigation, hydroelectric production, flood and
mosquito control, destroy habitats, degrade water quality,
and hinder endeavors such as rice cultivation and fishing
(Mitchell 1979, Holm et al. 1977). An aggressive aquatic spe-
cies under ideal conditions, giant salvinia can completely
cover water surfaces and form mats up to 1 m thick (Thomas
and Room 1986).
Because giant salvinia is a free-floating plant, nutrients
must be obtained from the water column via the modified
third leaf, which resembles roots. Nutrients such as phospho-
rus, manganese and iron can become bound in sediments
under certain conditions, such as high pH or elevated dis-
solved oxygen concentrations, and are thus unavailable for
floating plant uptake (Wetzel 1983). These nutrients are es-
sential for healthy plant growth as they are important for
photosynthesis, chlorophyll synthesis, enzymatic activity, etc.
(Raven et al. 1981), thus availability is necessary for plant
growth and survival. When pH and dissolved oxygen concen-
trations decline, many sediment bound nutrients, such as
 
1
 
Analytical Services, Inc.—Lewisville Aquatic Ecosystem Research Facil-
ity, 201 Jones St., Lewisville, TX 75056.
 
2
 
USAE—Lewisville Aquatic Ecosystem Research Facility, 201 Jones St.,
Lewisville, TX 75056.
 
3
 
UNT-Lewisville Aquatic Ecosystem Research Facility, 201 Jones St.,
Lewisville, TX 75056. Received for publication December 12, 2004 and in
revised form February 15, 2005.
 
J. Aquat. Plant Manage.
 
 43: 34-38
 J. Aquat. Plant Manage.
 
 43: 2005. 35
iron, manganese and phosphorus may be released into the
water column (Riemer 1984, Wetzel 1983). Cary and Weerts
(1984) found that greatest dry weights were obtained from
giant salvinia plants grown in nutrient solution at a pH of 6.0
over plants grown at a pH of 7.0 or greater.
The objectives of this study were to 1) document giant sal-
vinia growth in relation to different pH regimes under natu-
ral (pond) conditions; 2) determine pH effects on giant
salvinia growth; and 3) determine impacts of giant salvinia
mats on water chemistry in tank studies. This knowledge
might be utilized to predict distributional limits of giant sal-
vinia based on water chemistry or to evaluate the probability
of a new infestation reaching problematic proportions.
 
MATERIALS AN D METHODS
Pond Study
 
During 2001, three ponds at the LAERF were manipulated
to achieve different pH or water levels. The ponds used were
earthen, clay-lined, rectangular, and ranged in size from 0.25
to 0.32 hectares. When full, most ponds are approximately 2.0
m at the deepest end and average about 1.0 m deep overall
(Smart et al. 1995). The first two ponds were used to compare
growth rates of giant salvinia colonies under high and low pH.
Pond one (hereafter referred to as low pH, deep) received ap-
proximately 120 L of muriatic acid (HCl) to reduce alkalinity
and lower the pH to below 7.5. This was followed by the addi-
tion of 25 bales of hay to provide longer-term pH amendment
through organic decomposition, CO
 
2
 
 production and organic
acid accumulation. The second pond (hereafter referred to as
high pH, deep) was unamended and the water chemistry was
typical of LAERF ponds in which pH ranges from 8-10.5 de-
pending on time of day, season and submersed plant commu-
nity (Smart et al. 1995). The depth of these 2 ponds was
maintained at approximately 1.5 m at the deep end. A third
pond (hereafter referred to as high pH, shallow) was un-
amended, but depth was held at approximately 0.3 m. This
pond had a corresponding smaller surface area than other
treatments and was included to simulate shallow backwater ar-
eas to evaluate early growth under unamended pH conditions
with the roots in close proximity to the sediments.
Six 114-L containers of giant salvinia were added to each
pond during August 2001. Water was periodically added to each
pond to replace evaporative losses. Hydrolab DataSondes®
(Hydrolab Corp., Austin, TX) were deployed at a depth of 25
cm to record hourly pH (units), temperature (C), dissolved ox-
ygen (mg/L) and conductivity (µs/cm) for each of the deeper
ponds. After 15 weeks of growth, the ponds were surveyed us-
ing GPS (Trimble TSC1, Sunnyvale, CA) and analyzed using
GIS (ArcView 3.1, ESRI, Redlands, CA) to demonstrate the ex-
tent of giant salvinia growth under each treatment. No water
chemistry was collected on the shallow pond due to inability of
the DataSonde to record in shallow water.
 
Tank Studies
 
In the first study, water chemistry effects on giant salvinia
growth were determined over a 6-week period in 2002 using
outdoor tanks. To ensure adequate nutrients were available,
approximately 5.1 cm of LAERF pond sediment was added to
the bottom of eighteen 416-L plastic tanks (Toter Incorp.,
Statesville, NC). Tanks were then filled with LAERF pond wa-
ter. Nine tanks received unamended LAERF pond water (high
pH) and nine tanks received amended LAERF pond water.
Low pH (low pH: 7.0-6.0) was maintained by adding 19 L of
Canadian peat moss directly to the tanks at the beginning of
the study. Five 5-leaf pairs of giant salvinia were added to each
tank at a total average biomass (g dry weight) of 0.45 g. Biom-
ass was harvested from three tanks of each treatment every two
weeks, dried at 65 C in a convection-drying oven and weighed.
Surface area (cm
 
2
 
) coverage was also measured. Additionally, a
DataSonde was used to record pH, temperature and dissolved
oxygen in each tank at least once every third day.
In a second tank study, giant salvinia effects on water
chemistry were investigated. Eighteen 416-L outdoor plastic
tanks were filled with approximately 5.1 cm depth of LAERF
pond sediment followed by LAERF pond water. No amend-
ments were made to alter the water chemistry. Treatments in-
cluded six tanks with 100% giant salvinia coverage, six tanks
with 25% giant salvinia surface area coverage, and six tanks
containing no giant salvinia (control). DataSondes were dis-
tributed into one tank of each treatment to record hourly
pH, temperature and dissolved oxygen. Additionally, discrete
water chemistry data for the same parameters were collected
at least every three days from all tanks. This study was con-
ducted over a two-week period.
 
RESULTS AND DISCUSSION
 
GPS (Geographic Positioning Systems) data collected
from treatment ponds revealed substantial differences be-
tween giant salvinia populations. After 15 weeks of growth,
most of the low pH, deep pond was covered with giant salvin-
ia (Figure 1). Additionally, the dominant growth form was
the erect leaf mat-forming stage. pH was generally main-
tained below 7.5 and dissolved oxygen fell to well below 5.0
Figure 1. GIS map showing extent of surface coverage of giant salvinia after
15-weeks during summer 2001 in the 3 treatment LAERF ponds. The treat-
ments were A) high pH (natural LAERF water chemistry), shallow, B) low
pH, (amended with hay and muriatic acid), deep, and C) high pH (natural
LAERF water chemistry), deep. Ponds are actual GPS locations.
 36
 
J. Aquat. Plant Manage.
 
 43: 2005.
 
Figure 2. Water chemistry data for the 2 deep giant salvinia ponds showing hourly recorded data for temperature, pH, and dissolved oxygen for the 15-week
study period. Treatments were high pH (natural LAERF water chemistry) and low pH (amended with hay and muriatic acid).
 J. Aquat. Plant Manage.
 
 43: 2005. 37
mg/L toward the middle of the study period, as the giant sal-
vinia canopy began to completely cover the pond (Figure 2).
Although not measured, it is speculated that nutrients such
as iron and phosphorus became more readily available un-
der lower pH conditions, perhaps reducing growth limita-
tions on giant salvinia. As a floating plant, giant salvinia does
not have access to nutrients via the sediments and is there-
fore dependent upon nutrient availability in the water col-
umn. Neither of the high pH ponds was completely covered;
giant salvinia rimmed the edge of the high pH, deep pond
and formed a mat of erect leaves in the center of the high
pH, shallow pond (Figure 1). Giant salvinia in the low pH,
deep pond covered ca. 90% of the pond surface while both
the high pH ponds were found to have ca. 15% coverage of
the total or flooded part of the pond. It should be noted that
these growth differences based on the pH of the ponds have
been observed over several years although data was only col-
lected during this growing season (Owens, pers. obs.).
In the first tank study, significant biomass and surface area
coverage differences occurred between the plants growing
under low and high pH. By week four, giant salvinia had
grown to cover the surface of low pH tank; however, this did
not occur in the high pH tanks throughout the entire 6-week
study. In addition to differences in surface area coverage be-
tween low and high pH treatments, significant differences
were detected by ANOVA between week 2 and weeks 4 and 6
at low pH (Figure 3A), but not between any weeks at high
pH, indicating more rapid colony growth rate at low pH.
Biomass in low pH tanks exhibited significant increases
for each sampling period; however, this did not occur in high
pH tanks (Figure 3B). During the study period, pH in low
pH tanks was generally maintained at 7.0 or less while high
pH tanks were typically above 8.0 (Figure 4). Dissolved oxy-
gen was generally lower in the low pH tank and there were
no observable differences in temperature. This study indicat-
ed that pH might be a critical factor in growth of giant salvin-
ia following invasion.
Giant salvinia had a decided impact on water chemistry in
the second tank study. Tanks with 100% giant salvinia cover-
age exhibited lower dissolved oxygen readings by day 2 and
pH began to decline by day 4 (Figure 5); no differences were
observed in control or partly covered tanks. Both declines
were most likely due, in part, to reduction of algal photosyn-
thesis, caused by light blockage by the giant salvinia canopy
(Smart pers comm.). Generally, photosynthesis removes car-
bon dioxide from the water column while decomposition
consumes oxygen and drives pH down; at the same time,
photosynthesis produces oxygen as a by-product. Reduction
of photosynthesis due to low light limits oxygen production,
and dissolved oxygen concentrations will decline without al-
ternate sources of replenishment (e.g., atmospheric oxygen).
In addition to blocking light and reducing photosynthetically
produced oxygen, the giant salvinia canopy also serves as a
Figure 3. A) Measurements of giant salvinia 0, 2, 4, 6, weeks after treatment.
Bars represent means of replicates and standard error bars. Treatments
coded with same letter are not different at p = 0.05. A) Surface area (cm2/
tank), and B) Biomass (g DW/tank).
Figure 4. Temperature, pH, and dissolved oxygen in experimental tanks
during the 6-week study period. Treatments were higher pH (natural
LAERF water chemistry) and lower pH (amended with peat moss).
 38
 
J. Aquat. Plant Manage.
 
 43: 2005.
physical barrier between atmospheric and water column gas
exchange, further reducing dissolved oxygen concentrations
and potentially increasing carbon dioxide concentrations
and decreasing pH. Giant salvinia may therefore cause shifts
in pH and subsequent release of substrate nutrients (Wetzel
1983), making the environment more suitable to its growth.
No differences were observed in temperature (Figure 5).
Giant salvinia has been in the United States for several
years and is now a major detrimental factor in many lakes (ex.
Lake Wilson, HI) and wetland systems (ex. Toledo Bend, TX)
throughout states where low temperature limits are not an is-
sue (USGS 2004). This paper suggests the important effect of
pH on the explosive growth potential of giant salvinia; and
how this information could be used to identify potential areas
(low pH, high nutrients) where giant salvinia could become a
problem. Additionally, more research needs to be conducted
to study the relationships between pH and nutrient dynamics
relative to giant salvinia and to investigate the effects of other
water chemistry parameters such as conductivity and salinity.
 
ACKNOWLEDGMENTS
 
This research was conducted under the U.S. Army Corps
of Engineers Aquatic Plant Control Research Program, U.S.
Army Engineers Research and Development Center. Permis-
sion to publish this information was granted by the Chief of
Engineers. We would like to thank LeeAnn Glomski and Dr.
Michael Grodowitz for review of this paper.
 
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Effects of pH on Growth of Salvinia molesta Mitchell

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Effects of pH on Growth of Salvinia molesta Mitchell

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