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Growth limitation of Lemna minordue to high plant density

Steven M. Driever, Egbert H. van Nes *, Rudi M.M. Roijackers

Department of Environmental Sciences, Aquatic Ecology and Water Quality Management Group,Wageningen University, P.O. Box 8080, NL-6700 DD Wageningen, The Netherlands

Received 4 March 2004; received in revised form 11 October 2004; accepted 6 December 2004

Abstract

The effect of high population densities on the growth rate of Lemna minor(L.) was studied under

laboratory conditions at 23 8C in a medium with sufficient nutrients. At high population densities, we

found a non-linear decreasing growth rate with increasingL. minordensity. Above aL. minorbiomass

of ca. 180 g dry weight (DW) m2

, the net growth rate became negative. At a density of 9 g DW m2

,a maximum relative growth rate of ca. 0.3 d1 was found. At very low densities (

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Lemnaceae mats often becomes too anoxic for fish and macrofauna to survive (Janse and

Van Puijenbroek, 1998). Due to competition for light, also submerged macrophytes usually

cannot coexist with Lemnaceae. The duckweed mats may be persistent and it is suggested

that Lemnaceae dominance is a self-stabilizing state (Scheffer et al., 2003). In (sub)tropicalareas, other free-floating plants such as water hyacinth (Eichhornia crassipes Solms) can

form an even larger threat to biodiversity (Mehra et al., 1999).

To control free-floating Lemnacea, insight in the growth dynamics is necessary. Several

abiotic factors have been studied intensively such as nutrients and temperature. Lemnacea

need high phosphorus and nitrogen loadings (Portielje and Roijackers, 1995). With

increasing temperature, the growth rate increases approximately linearly up to an optimum

(Landolt, 1986). Duckweed may deplete nutrients (Scheffer et al., 2003) and can change

conductivity and pH of the water (McLay, 1976). This way they change their own growth

conditions (Landolt and Kandeler, 1987). Relatively little attention has been paid to cause

of intraspecific competition within mats of Lemnacea and the effect on their growth.

We studied the effect of crowding on the growth rate ofLemna minorin the laboratory.

L. minor was grown in different densities, varying from low to high, in a medium with

sufficient nutrients. From the results of this experiment, a simple model was constructed to

explore the factors involved in growth of L. minor. To validate the model, we monitored

three Lemna-dominated ditches near Wageningen (The Netherlands) for 9 weeks.

2. Materials and methods

2.1. Crowding experiment

The growth rate ofL. minorwas determined under laboratory conditions (23 8C, a 14-h

photoperiod and an irradiance of 180 mmol m2 s1 PAR). The plants were grown on a

liquid medium based on Smart and Barko (1985), optimised for Lemnacea by Szabo et al.

(2003). A series of different densities was used (5.5, 9.5, 90, 180 and 915 g dry weight

(DW) m2). For each treatment, six replicates were applied. The initial biomass was

determined as fresh weight. The plants were placed in vertical cylinders (height 10 cm,

diameter 5.9 cm), which were placed in 2-l aquaria filled with medium (Szabo et al., 2003).

After day 4, the cylinders were placed in a basin with fresh medium. After day 7, theexperiment was stopped and the fresh weight and dry weight (24 h at 70 8C) of L. minor

were measured. The initial dry weight was calculated using the dry weight to fresh weight

ratio at the end of the experiment. The relative growth rate was calculated assuming

exponential growth.

2.2. Model

A simple model was constructed to describe the effect of crowding, temperature and

nutrients:dB

dt B r fT;B; N; P l B (1)

S.M. Driever et al. / Aquatic Botany 81 (2005) 245251246

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The variation in time of the biomass of L. minor(B in g DW m2) was modelled as the

function of the maximum growth rate (r). The gross production was modified by a

limitation function (f(T, B, N, P)), which was a function of air temperature (T), biomass (B)

and nutrients (N and P). Furthermore, the production was corrected for the loss (l), whichincluded mortality, predation and respiration.

The limitation function (f(T, B, N, P)) was defined as:

fT;B;N; P T Tmin

Topt Tmin

N

N hN

P

P hP

hB

B hB(2)

Temperature (T) limitation was assumed to be linear from the minimum temperature

(Tmin, 5 8C) up to the optimum temperature (Topt, 26 8C) (Landolt, 1986; Landolt and

Kandeler, 1987). Nutrient limitation of ammonia and nitrate (N) and ortho-phosphate (P)

were modelled as Monod-type functions, with the following half saturation values:

hN = 0.04 mg N l1 and hP = 0.05 mg P l

1 (Luond, 1980). The limiting effect of biomass

was simply assumed to be another Monod-type function dependent on biomass B and with

a half saturation hB, which was determined during this study.

2.3. Field observations

Three ditches in the surroundings of Wageningen, The Netherlands, were selected. In all

ditches L. minorwas present at the start of the monitoring period. For a period of 9 weeks

(AprilJuly 2003) biomass was measured in approximately 14-days intervals using

stratified sampling. The water surface of the ditch was divided by eye into three strata, i.e.020%, 2180% and 81100% coverage. The strata were drawn on a map and within each

stratum, 10 random sampling coordinates were drawn. The biomass in the stratum with 0

20% coverage was neglected.

Duckweed was sampled using a method after McLay (1974). A 10 cm 10 cm gauzecovered iron square was positioned horizontally under the L. minorcover. The square was

lifted up through the cover of plants. All biomass not accounting for L. minorwas removed

by hand and dry weight was determined (24 h at 70 8C).

Water samples for chemical analyses of N and P were taken at the last three sampling

dates. NNO3 + NNO2

, NNH4+ and PPO4

3were analysed using a Technicon Auto-

analyser II. Air temperature data were obtained from a nearby weather station.

3. Results

3.1. Crowding

Growth rates ofL. minordecreasing with increasing density (Fig. 1). The highest growth

rate of 0.30 d1 was observed at a biomass of 10 g DW m2. Remarkably, the lowest

biomass (5 g DW m2) had a significantly lower growth rate of 0.23 d1 (MannWhitney

P < 0.008). The highest densities of 180 g DW m

2 and 915 g DW m

2 showed a negativenet growth rate of0.02 and 0.08 d1, respectively. These experimental data were usedto calibrate the model, assuming no nutrient limitation. The maximum growth rate, r, was

S.M. Driever et al. / Aquatic Botany 81 (2005) 245251 247

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obtained by extrapolation and was corrected for the optimum temperature of 26 8C

(0.41 d1). The half saturation constant, hB, (26 g DW m2) was calculated from the

equilibrium biomass. The resulting model was used to describe the net relative growth for

different biomasses (Fig. 1).

3.2. Field

Fig. 2 shows the field data and model prediction for each ditch. Growth rate, r, was fitted

for each ditch separately assuming different limiting factors for each ditch. With an

increase in air temperature (day 4247; Fig. 2), the duckweed biomass in all ditches

increased, as the model predicted.

For fN (N) and fP (P), the assumption was made that nutrients were not limiting

(Table 1), supported by the fact thatL. minorwas present at the start of the sampling period.

4. Discussion

Using a combination of a laboratory-scale experiment and a simple model, we obtained

insight in the growth dynamics ofL. minorin the field. The laboratory experiment showed

the relation between crowding and growth rate. We were able to model field growth in

Dutch ditches, suggesting that we captured the main processes in the model.

Yet several processes were neglected or oversimplified in the model. For instance, the

model assumed that the plants were homogenous distributed over the ditch. In reality, this

is usually not true because of the influence of birds and wind (Duffield and Edwards, 1981).

Indeed the distribution of L. minor in the sampled ditches was very heterogeneous, also

within the three strata.

In the model the loss, l, was assumed to be constant for all ditches (0.05), which equals alifetime of about 35 days, a realistic value for individual fronds (Landolt, 1986). The loss

included respiration, grazing and mortality implicitly. It was neglected that respiration is


Fig. 1. Growth rate as a function of the initial biomass of L. minor (g DW m2

). Dots indicate the measuredgrowth rate. The solid line represents the curve described by the model at a constant temperature of 23 8C.

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strongly dependent on temperature. Differences between ditches in grazing pressure by

birds and invertebrates were not known and could not be accounted for in the mortality

rates. The influences of temperature, crowding and decomposition on mortality are

unknown. In our model, we used air temperature instead of water temperature. It is known

that temperature can vary widely between water, air and within floating mats (Dale and

Gillespie, 1976), so we assumed that the temperature of the fronds in the upper layers is

better described by the maximum air temperature than by the water temperature.

Despite these simplifications, the negative effect of crowding on growth rate wasaccurately described in the model. The density dependent reduction of growth rate was


Fig. 2. Biomass (g DW m2) ofL. minorin three ditches and maximum air temperature (8C) from day 0 (17 April

2003) to day 63 (18 June 2003). Field data are indicated as black squares, open triangles and black dots for

Opheusden, Zetten and Sinderhoeve, respectively with standard errors (N= 10). Model output is shown as a

dashed and dotted line, dashed line and solid line for Opheusden, Zetten and Sinderhoeve, respectively. Maximum

air temperature is indicated by the dotted line.

Table 1

Characteristics of the ditches

Characteristic Sinderhoeve Zetten Opheusden

Length (m) 40.0 45.5 48.0

Width (m) 3.0 1.83.0 0.7

Depth (m) 1.6 0.5 0.4

Sediment Sand Clay Clay

NNH4+ (mg l1) 0.07 (0.03) 0.08 (0.01) 0.14(0.10)

NNO3 (mg l1) 0.05 (0.05) 0.04 (0.03) 0.99(1.38)PPO4

3 (mg l1) 0.42(0.56) 0.51 (0.68) 1.62 (0.65)

The nutrient levels are mean values for the last three sampling dates.

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clearly not linear, like in logistic growth, but it decreased approximately logarithmically. It

could well be described as a Monod-type growth limitation by biomass. The mechanism

causing this density dependency is not completely clear. In a dense mat, fronds are piled up

in several layers. One could expect that in such case layers can be subdivided into twoparts: an upper part with nutrient limitation and a lower part with light limitation

(Clatworthy and Harper, 1962) or CO2 limitation. However, since we observed only

decompositon in the lower parts, it seems not very likely that nutrient limitation of the

upper part was a major factor.

The laboratory experiments suggested that there is an inverse density dependence at low

densities. In waters partly covered byL. minor(biomass below 9.5 g DW m2), the relative

growth rate increased significantly with increasing Lemna density. A likely explanation for

this effect seems to be the increase of temperature in a closed deck, due to solar radiation

(Dale and Gillespie, 1976). In this way, there is facilitation at low densities once there is full

coverage, i.e. the higher temperature within the mat will increase the relative growth rate.

For other floating plants, a similar facilitation may occur. For instance, the tropical floating

Salvenia molesta is known to increase temperature locally (Room and Kerr, 1983).

Acknowledgements

We thank Gertie Arts and Dick Belgers from Alterra (Wageningen, NL) for useful

suggestions and discussions and Frank van Herpen, who helped a great deal with the

practical work.

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