The distribution and density of hydrilla (Hydrilla verticillata (L.f.)Royle) turions and tubers in two New Zealand lakes were assessed by sampling cores of sediment from Lakes Tutira and Waikapiro each year from 1994 to 1997.  Turion and tuber density differed with water depth, with maximum numbers of tubers and turions found in the 1-2 m and 1.5-4m water depth ranges respectively.  A high turion to tuber ratio was observed, with turions accounting for over 80% of propagules.  The relatively low numbers of turions and tubers compared with other reports, and the distribution of most tubers within the shallow water is likely to be associated with black swan grazing (Cygnus atratus Latham), with maintains a canopy of hydrilla consistently 1 m below the water surface

Champion, P.D.

Clayton, J.S.

Green, J.D.

Hofstra, D.E.

English

Aquatic Commons

 J. Aquat. Plant Manage.
 
 37: 1999. 41
 
J. Aquat. Plant Manage.
 
 37: 41-44
Distribution and Density of Vegetative Hydrilla 
Propagules in the Sediments of Two
New Zealand Lakes
 
D. E. HOFSTRA
 
1
 
, J. S. CLAYTON
 
1
 
, P. D. CHAMPION
 
1
 
 AND J. D. GREEN
 
2
 
ABSTRACT
 
The distribution and density of hydrilla (
 
Hydrilla verticil-
lata
 
 (L.f.) Royle) turions and tubers in two New Zealand
lakes were assessed by sampling cores of sediment from
Lakes Tutira and Waikapiro each year from 1994 to 1997.
Turion and tuber density differed with water depth, with
maximum numbers of tubers and turions found in the 1-2 m
and 1.5-4 m water depth ranges respectively. A high turion to
tuber ratio was observed, with turions accounting for over
80% of the propagules. The relatively low numbers of turions
and tubers compared with other reports, and the distribu-
tion of most tubers within the shallow water is likely to be
associated with black swan grazing (
 
Cygnus atratus
 
 Latham),
which maintains a canopy of hydrilla consistently 1 m below
the water surface.
 
Key words: 
 
turion, tuber, sediment cores, 
 
Hydrilla verticillata
 
.
 
INTRODUCTION
 
Dioecious male hydrilla is an introduced macrophyte in
New Zealand, which was first recorded in Lake Tutira in 1963
(Clayton et al. 1995). Currently its distribution is limited to
four lakes in the Hawke’s Bay region. The two largest lakes,
Tutira (147 ha) and Waikapiro (11 ha) have extensive hyd-
rilla distribution. Control measures have not been effective
in reducing hydrilla abundance in these two lakes. In partic-
ular, diquat, the only herbicide registered for aquatic use has
failed to reduce plant biomass, while mechanical methods
have limited suitability on account of the increased risk of
spread, and short duration of control. Containment mea-
sures, aimed at minimizing the risk of fragments escaping
and becoming more widespread have been implemented on
both lakes and include the prohibition of motorized boats
and netting for fish. Hydrilla has been more effectively man-
aged in both of the smaller lakes Opouahi (6 ha) and Eland
(4 ha). In Lake Opouahi hydrilla presently has a restricted
distribution and low abundance, with some localized control
obtained using weed mat. In Lake Eland all of the original
hydrilla stands have been removed using grass carp, which
have been present for ten years (Clayton et al. 1995).
 
1
 
NIWA (National Institute of Water and Atmospheric Research), PO Box
11-115, Hamilton, New Zealand. 
 
2
 
Department of Biological Sciences, University of Waikato, Private Bag
3105, Hamilton, New Zealand. Received for publication October 14, 1998
and in revised form February 28, 2000.
 
Hydrilla turions and tubers (subterranean turions) repre-
sent different survival and dispersal strategies (Van and Stew-
ard 1990). Turions are produced on stems in the water
column and are smaller than tubers, which are produced in
the hydrosoil on geotropic shoots (Yeo et al. 1984, Spencer
et al. 1994). Turions are more likely than tubers to be dis-
persed through normal water movement, currents and tides,
and have a greater probability of growing in an unoccupied
area (Spencer and Rejmanek 1989, Miller et al
 
.
 
 1993),
whereas tubers provide a means of perennation in the cur-
rently occupied habitat (Van and Steward 1990). Addition-
ally, tubers remain viable for longer than turions. Tubers
from monoecious hydrilla plants have survived in undis-
turbed sediment for over four years, whilst turions germinate
readily and expire after one year (Van and Steward 1990). In
New Zealand on going studies in Lake Eland sediment have
recorded viable tubers for up to 8 years, while turions were
only viable for two years (personal observation).
Tubers and turions are the major source of regrowth after
herbicide treatments, drawdown periods and grazing by her-
bivorous fish (Sutton and Portier 1985). Because of the effec-
tiveness with which these propagules hamper hydrilla control
programmes, a number of studies have reported on the con-
ditions under which tubers and turions are produced and
germinate in different strains of hydrilla (Netherland 1997,
Miller et al. 1993, Sutton et al. 1992, Thullen 1990, Spencer
and Rejmanek 1989, Van 1989, Bowes et al. 1979, Van et al
 
.
 
1978, Haller et al. 1976, Miller
 
 
 
et al. 1976). However, com-
pared with the number of studies on the production and ger-
mination of propagules very little information is available on
the relative abundance of turions and tubers (Sutton 1996).
Given that these are the primary means for regrowth and dis-
persal, there is a need to determine the relative frequency
and distribution of turions and tubers in the lake environ-
ment. Determining the relative frequency and distribution of
these propagules in New Zealand’s Lakes Tutira and Waika-
piro should enable a better understanding of the importance
of the vegetative propagule bank (Spencer and Rejmanek
1989) and the potential for hydrilla regrowth or reinfestation
following future implementation of control programmes.
 
MATERIALS AND METHODS
 
Field Sampling
 
. Lakes Tutira and Waikapiro in the Hawke’s
Bay were visited during April and December 1994, and April
1995, 1996 and 1997. A total of five sites were sampled from
each lake over the duration of the study. The sites were T1-T5
from Lake Tutira and W1-W5 from Lake Waikapiro (Figure 1).
 42
 
J. Aquat. Plant Manage.
 
 37: 1999.
At each sample site a SCUBA diver recorded the depth distri-
bution of plant species, their relative abundance, cover and
height, within a 2 m wide area from the water’s edge down to
the maximum depth of plant growth (Clayton 1983).
The distribution of vegetative propagules was assessed fol-
lowing collection of sediment samples at water depths of 0.5,
1, 1.5, 2, 3, 4, 5, 6, 7 and 8 m. At all ten sites at least six cores
(nine in 1996 and 1997) were taken at each depth and
retained in mesh bags for sorting. Sediments were washed in
mesh bags to remove most of the hydrosoil and the retained
material was sorted on trays and hydrilla turions and tubers
were counted. In 1994 different core sizes were tested (100
mm was the smallest and 435 mm was the largest), and from
1995 to 1997 the core size was standardized to 220 mm diam-
eter by 290 mm long.
 
Data Analysis
 
. Turion and tuber numbers at each site were
rank transformed because the data were not consistent with
distributions used in standard parametric models. The t-test
of the ranked data was used to determine if there were differ-
ences between the lakes at each water depth interval. A sec-
ond comparison was made using a simple sign test, between
adjacent water depths to determine if propagule numbers
had changed significantly from one depth interval to the next.
 
RESULTS
 
Vegetation. 
 
Lakes Waikapiro and Tutira vegetation was
characterized by an assemblage of shallow water, low-growing
plants between 0.2 to 1.0 m depth, that included species of
 
Ruppia, Chara, Glossostigma
 
 and 
 
Myriophyllum
 
 (Table 1)
 
, 
 
and
by large tall-growing stands of hydrilla, which were present
from a depth of 1 to 6 m, with a few shorter plants to a depth
of ca. 8 m. Hydrilla grew to a maximum height of 6 m and 5 m
in Lakes Tutira and Waikapiro respectively. 
 
Elodea canaden-
Figure 1. Lake Tutira and Lake Waikapiro sample sites. The inset map of
New Zealand (top right corner), shows the location of these lakes.
 
T
 
ABLE
 
 1. S
 
PECIES
 
 
 
COVER
 
 
 
AND
 
 
 
DISTRIBUTION
 
 
 
ALONG
 
 
 
LAKE
 
 
 
PROFILES
 
.
Site Species
Cover
*average
& (max.)
Depth
range
(m)
Tutira
(T1)
 
Hydrilla verticillata
 
 (L.f.) Royle
 
Elodea canadensis
 
 Michx.
 
Myriophyllum triphyllum
 
 Orchard
 
Chara globularis
 
 Thuill.
 
Chara corallina
 
 Willd.
 
Lilaeopsis ruthiana
 
 Affolter
6 (6)
5 (6)
2 (2)
1 (2)
1 (1)
1 (1)
1.0-6.5
1.0-1.5
1.0-1.5
1.5
1.0-1.5
1.0-1.5
Tutira
(T2)
 
Hydrilla verticillata
Myriophyllum triphyllum
Elodea canadensis
Ruppia polycarpa
 
 R. Mason
6 (6)
2 (3)
5 (6)
1 (2)
1.0-7.5
0.5-0.5
0.5-1.5
0.5-0.5
Tutira
(T3)
 
Hydrilla verticillata
Elodea canadensis
Ranunculus limosella
 
 F. Muell. ex Kirk
 
Chara globularis
Potamogeton ochreatus
 
 Raoul
 
Glossostigma diandrum
 
 (L.) Kuntze
6 (6)
4 (6)
1 (2)
1 (2)
2 (3)
1 (2)
1.5-7.5
0.5-1.5
0.2-0.5
0.5-1.0
0.5-1.0
0.2-0.5
Tutira
(T4)
 
Hydrilla verticillata
Elodea canadensis
Chara globularis
Chara corallina
Lilaeopsis ruthiana
 
6 (6)
5 (6)
2 (3)
1 (1)
1 (2)
1.5-7.5
0.-1.5 
0.5-1.0
0.-1.0
0.-1.0
Tutira
(T5)
 
Hydrilla verticillata
Elodea canadensis
Potamogeton crispus
 
 L.
 
Chara globularis
Ruppia polycarpa
Lilaeopsis ruthiana
 
6 (6)
6 (6)
1 (1)
1 (1)
1 (1)
1 (1)
1.0-8.5
0.5-1.0
0.5
0.5
0.5-1.0
0.5
Waikapiro
(W1)
 
Hydrilla verticillata
Glossostigma elatinoides
 
 Benth.
 
Chara corallina
Chara globularis
Potamogeton ochreatus
Potamogeton cheesemanii
 
 A. Benn.
6 (6)
2 (5)
1 (1)
1 (1)
1 (1)
1 (1)
0.5-6.5
0-0.7
0-0.7
0-0.7
0-0.7
0-0.7
Waikapiro
(W2)
 
Hydrilla verticillata 
Chara globularis
Chara corallina
Nitella hyalina
 
 (D.C.) Ag.
 
Myriophyllum triphyllum
 
6 (6)
1 (2)
2 (4)
1 (2)
1 (1)
0.1-5.0
0.1-1.0
0.3-1.0
0.1-0.3
0.1-1.0
Waikapiro
(W3)
 
Hydrilla verticillata
Chara globularis
Chara corallina
Potamogeton crispus
Lilaeopsis ruthiana
 
6 (6)
3 (6)
2 (4)
1 (1)
1 (2)
0.3-5.0
0.1-1.0
0.3-1.0
0.3-1.0
0.1-0.3
Waikapiro
(W4)
 
Hydrilla verticillata
Chara corallina
 
6 (6)
2 (3)
1.0-8.0
0-2.0
Waikapiro
(W5)
 
Hydrilla verticillata
 
6 (6) 1.0-6.5
*Average and maximum cover values correspond to the following, 6 = 96-
100% cover, 5 = 76-95% cover, 4 = 51-75% cover, 3 = 26-50% cover, 2 = 6-25%
cover, and 1 = 1-5% cover.
 J. Aquat. Plant Manage.
 
 37: 1999. 43
 
sis
 
 was not common in Lake Waikapiro, while in Lake Tutira
it occasionally occupied the same depth range as hydrilla
and often formed a shallow band of weed between the shal-
low-water, low-growing community and the hydrilla.
 
Propagule Density and Distribution.
 
 Tubers were recorded
in eight of the ten sample sites, W1, W2, W3, W5, T1, T2, T3
and T5, and were located at water depths between 1 and 5 m.
Maximum tuber densities occurred between 1 and 2 m water
depth across all sites and the maximum tuber density (mean
 
from all sites) was 8 m
 
-2
 
 at 1.5 m (Figure 2).
Turions on average accounted for 81% (SE = 7) of all
propagules, and were more common than tubers from all
sites in both lakes Tutira and Waikapiro. Turions were
recorded from all sample sites over a 1 to 7 m depth range,
with a maximum (mean from all sites) density of 40 to 55 m
 
-2
 
at 1.5 to 4 m water depths. There was no significant differ-
ence in turion and tuber numbers between lakes, but turion
densities changed significantly between water depths. There
was a significant increase in turion numbers between 1-2 m
and a decrease between 4-5 m and 5-6 m. Tuber numbers did
not differ significantly between adjacent water depths, how-
ever tubers were not found beyond 5 m.
 
DISCUSSION
 
Miller et al
 
.
 
 (1976) observed an increase in tuber number
with increasing water depth between 0.3 and 1.5 m, and
described tuberisation (the production of tubers) of hydrilla
as occurring in any depth of water at which a rooted canopy
grows. Tubers in Lakes Tutira and Waikapiro were recorded
to 5 m water depth. The absence of tubers in sediment from
water deeper than 5 m was associated with a lack of hydrilla
canopy formation. A maximum shoot height of 5 and 6 m
(mean height 4 and 5 m) was recorded in Lakes Waikapiro
and Tutira respectively, with a canopy consistently 1.0 to 1.1
m below the water surface. Once water depth exceeded 5 m
the plants were less dense and no longer as close to the sur-
face of the water.
In shallow water systems where hydrilla is able to form a
canopy over most of the lake, tuber production could be
expected to be greater than in lakes with narrower littoral
margins. Reported studies that have examined sediment for
tubers appear to have sampled typically < 2.5 m (Bowes et al
 
.
 
1979, Sutton and Portier 1985, Miller et al. 1976, Sutton
1996), and make no mention of tuber density or distribution
at greater water depths. Tuber densities are generally higher
than those found in the present study (Netherland 1997).
For example, Lake Ocklawaha, which has an infestation of
female dioecious plants was reported to have 300-600 tubers
m
 
-2
 
, at water depths of 0.3 to 0.9 m (Miller et al
 
.
 
 1976). Simi-
larly, Bowes et al
 
.
 
 (1979) reported tuber densities from 20 to
510 tubers m
 
-2
 
, at a water depth of 1 m in three Florida lakes. 
The maximum turion densities observed in Lakes Tutira
and Waikapiro were 40-55 m
 
-2
 
 (mean data). Published data
on turion density would suggest that turions are rare or
absent in dioecious female hydrilla (Thullen 1990). How-
ever, a sediment survey by Sutton (1996) reported a maxi-
mum turion density of 918 turions m
 
-2 
 
in water 2 m deep in a
Florida canal in autumn. Sutton (1996) also found variation
in turion density between sampling dates and sampling sites
in the same river system, the latter being attributed to local
environmental factors, such as shade, resulting in lower
plant biomass and fewer propagules. Sutton and Portier
(1985), found no indication of seasonal fluctuation in
propagule density in five South Florida lakes. They suggested
that a steady state condition exists, in which the rate of for-
mation of new propagules equals that of germination, with
the maximum number for a body of water dependent on sed-
iment type, nutrient concentration, water quality and other
factors. In the present study turions occurred throughout a 7
m depth range, only 1 to 1.5 m less than the maximum depth
of the hydrilla plants. There was no difference in turion
number associated with sampling site, however there was sig-
nificant variation in turion distribution with water depth,
which may reflect turion production in the hydrilla stand. A
maximum number of turions between 2 to 4 m water depths,
could be attributed to high light intensity and plant biomass,
and the subsequent reduction in turion density at 5 to 7 m
could largely be due to the reduction in light intensity with
increasing water depth.
Apart from preventing motorized boat access to these
lakes, an additional factor reducing the risk of hydrilla
escape from Lakes Waikapiro and Tutira is the grazing of
weed beds by the Australian black swan. Large numbers of
these birds congregate on the littoral margins where they
crop weed bed canopies to approximately 1 m of the water
surface, also reported by Howard-Williams and Davies (1988)
in other New Zealand lakes. This has the effect of reducing
weed drift along the shoreline and may also contribute to the
lower incidence of tuber and turion densities compared to
other reported studies.
The results of this study indicate that once hydrilla has
become established, the wide depth ranges over which
tubers and turions can occur will continue to provide an
extensive resource for regrowth following either adverse
environmental conditions or control measures.
Figure 2. Mean number of propagules in lake sediments from different
water depths in Lakes Tutira and Waikapiro. Bars represent the mean num-
ber of propagules present in cores (n = 6) from all profiles. Vertical lines on
each bar indicate the standard error.
 44
 
J. Aquat. Plant Manage.
 
 37: 1999.
 
ACKNOWLEDGEMENTS
 
This research was conducted with funding assistance from
the New Zealand Foundation for Research and Scientific
Technology, and WWF New Zealand. The authors would also
like to thank the contribution of staff from the Department
of Biological Science, University of Waikato, and NIWA.
 
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Distribution and Density of Vegetative Hydrilla Propagules in the Sediments of Two New Zealand Lakes

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Distribution and Density of Vegetative Hydrilla Propagules in the Sediments of Two New Zealand Lakes

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