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Development of a partitioned-biomass model for rooted macrophyte growth
v ELSEVIER
Aquatic Botany56 (1997) 265-276
Development of a partitioned-biomass model for rooted macrophyte growth John F. Davis a,*, Archie J. McDonnell b a Department of Civil Engineering, Widener University, Chester, PA 19013, USA b Environmental Resources Research Institute, Penn State University, University Park, PA 16802, USA
Accepted 25 September 1996
Abstract
Many surface waters support excessive growth of several rooted macrophyte species. This paper presents a macrophyte growth model to facilitate modeling the growth of several macrophyte species which exhibit distinct patterns of growth and development. The model partitions the biomass of each species according to the phenological stages of plant development including growth, maturation, senescence, and dormancy. The partitioned-biomass model was demonstrated for the growth of Elodea canadensis Michx. and Potamogeton crispus L. in Spring Creek, a shallow fourth-order stream in central Pennsylvania. The structure of the model could be adapted to a wide variety of macrophytes and aquatic ecosystems. Keywords: Rooted macrophytes;Biomass models; Photosynthesis; Respiration
1. Introduction
Various macrophyte growth models have been developed and applied to examine the effect of rooted macrophytes on aquatic ecosystems and to examine strategies for controlling macrophyte growth. Although several macrophyte species may be abundant in a given waterway, some modeling studies have not differentiated the macrophyte biomass by species (Wright and McDonnell, 1986; Collins and Wlosinski, 1989). This approach may not be valid for waters supporting several macrophyte species which exhibit distinct seasonal patterns of growth and development. A macrophyte growth model is presented herein which facilitates modeling the growth and development of individual macrophyte species. A demonstration of the model is presented for the growth
* Corresponding author. Tel.: 610-499-4063;fax: 610-499-4059. 0304-3770/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-3770(96)01 103-5
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of Elodea canadensis Michx. and Potamogeton crispus L. in a shallow stream in central Pennsylvania.
2. Model description Westlake (1969) illustrated the stages of development and metabolism for rooted macrophytes during an annual cycle. These stages included growth following germination, maturation as flowers and fruits are produced, senescence as the plants begin to die and decay, and dormancy for plants which persist through the winter. The model presented in this paper partitions the biomass for each macrophyte species into distinct stages for growth (Bg), maturation (Bm), senescence (Bs), and dormancy (Bd) as shown in Fig. 1. Functions for plant photosynthesis, respiration, and decay may be programmed for each stage. The model transfers biomass from one stage to another at assigned transfer rates to simulate the observed development of individual macrophyte species. Transfer of biomass for a given species is regulated in the model according to environmental conditions such as water temperature (Sand-Jensen, 1989). The mathematical structure of the model is discussed below. 2.1. Growth
The growth stage of plant development starts following germination or dormancy and continues until maturation, senescence, or dormancy occurs. The growth stage of the model accounts for the production of plant biomass. Eq. (1) shows the finite-difference form of a first-order growth function used in this model: (1)
Bg t = Bg o + Bgo( p -- r ) d t
where Bg t is biomass in the growth stage at the end of time interval dt (g DW m-2), Bg o is biomass in the growth stage at the start of the time interval, p is plant specific photosynthetic rate (day-i), and r is plant specific respiration rate (day-l). Relationships for photosynthetic and respiration rates can be programmed into the model as
kgd~ ~¢~ . , f k d g [Dormant Biomass
Gro,ring Biot aass
kgs Sen scent ~ k Biol lass
[Mature Biomass ms
,fd Fig. 1. Illustration of the partitioned-biomass model to describe an annual cycle of macrophytegrowth. Transfer rates are designated as kxy to representthe transferof biomass from stage x to stage y. The decay rate of senescentbiomass is representedas fa.
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267
functions of light, nutrients, and water temperature similar to models by Wright and McDonnell (1986), Collins and Wlosinski (1989), and Scheffer et al. (1993). Parameter values for these functions may be calibrated for each species. 2.2. Maturation
Many plant species produce flowers and fruits at the terminal ends of their shoots as they mature. As suggested by Westlake (1969), the model sets the respiration rates equal to the photosynthetic rates for mature plants which results in net growth rates of zero. Plant biomass may transfer into the mature stage from the growth stage, but not all species exhibit the mature stage of development. The model allows biomass to remain in the mature stage for a predetermined time before transferring into the senescent stage. 2.3. Senescence
Plants transfer into the senescent stage as they begin to die and decay. The model uses a decay function to simulate the loss of senescent biomass (Westlake, 1969). Transfer of biomass into this stage follows maturation or may occur directly from the growth stage under severe environmental conditions. 2.4. D o r m a n c y
The shoots of some species persist over winter in a dormant stage (Westlake, 1969). The model sets photosynthetic and respiration rates to zero for dormant biomass. Dormant biomass may transfer into the growth stage during the spring as environmental conditions become suitable for growth. 2.5. Advective transport
In addition to the metabolic processes discussed for each stage, an advective transport function (fl) may be added to each stage to account for the loss or gain of biomass due to the transport of biomass by water currents. Since advective transport may not be significant for some ecosystems, the inclusion of an advective transport function is left as an option to the modeler. 2.6. Transfer f u n c t i o n s
The equations used in the model to simulate the growth, accumulation, transfer, and loss of biomass each stage of plant development over a time interval are given as Bg t = Bg o + Bgo( p - r -
kg m - kg~ - kgd - f l g ) d t
(2)
B m , = B m o + ( Bgokg m - Bmokms --flm B m o ) d t
(3)
Bs, = n s o + ( n g o k g s q- Bmokms - flsnSo - f a B s o ) d t
(4)
Bd, = B d o + ( Bgokg d - f l d B d o ) d t
(5)
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where kxy is the transfer rate from stage x to stage y ( d a y - l ) , fd is the decay rate for senescence ( d a y - l ) , and fix is the biomass loss rate due to washout of biomass in stage x ( d a y - l ). Values for the transfer rates can be adjusted for each species to simulate their observed development. The transfers may be set to occur at a predetermined time during the simulation or may be triggered by environmental conditions such as water temperature.
3. Model demonstration 3.1. Field studies The partitioned-biomass model was applied to simulate the growth of rooted macrophytes Elodea canadensis Michx. and Potamogeton crispus L. in two reaches of Spring Creek, a shallow fourth-order stream in central Pennsylvania. The growth and development of these species in Upper Spring Creek and Slab Cabin Run (tributaries of Spring Creek) were studied by Davis (1990) during 1983. The studies tracked the biomass for each species from the start of growth in spring through senescence during the fall. Nutrient concentrations of the water column and sediments, solar irradiance measured as langleys per hour (ly h - l ) , water temperature, and plant biomass were monitored throughout the study and are summarized in Table 1. A complete description of the methods used for these studies is reported by Davis (1990). As summarized in Table 1, Upper Spring Creek had relatively low phosphorus concentrations and was free of wastewater discharges, while Slab Cabin Run had higher
Table 1 Summary of stream characteristics, water temperature, solar irradiance, nutrient concentrations, and peak biomass for Upper Spring Creek and Slab Cabin Run. The stream parameters are average values for June through November 1983 (Davis, 1990) Upper Spring Creek Slab Cabin Run Discharge (m3 s-I) 0.83 0.42 Velocity (m s-~ ) 0.22 0.17 Depth (m) 0.38 0.33 Water temperature (°C) 13.9 14.8 Solar irradiance (ly day- l ) 416 416 pH 8.22 8.15 Alkalinity (mg 1- l as CaCO3) 200 203 WCNH3-N(mg 1- l N) 0.19 0.17 WCNOx-N(mg 1- i N) 3.27 3.48 WCSOP (mg 1-J P) 0.015 0.102 IWSOP (mg 1-1 p) 0.055 0.150 Peak biomass (g DW m- 2 ) Elodea canadensis 87 10 Potamogeton crispus 36 186 WCNHs-N, water column ammonia-nitrogen;WCNOx-N, water column nitrate-nitrogen and nitrite-nitrogen; WCSOP, water column soluble ortho-phosphate;IWSOP, sediment interstitial water soluble ortho-phosphate.
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phosphorus concentrations due to a discharge of treated municipal wastewater and urban stormwater runoff. Both streams had high alkalinity and nitrogen concentrations which were found to provide above growth-limiting levels of carbon and nitrogen nutrients for rooted macrophyte growth (O'Shaughnessy and McDonnell, 1973). The growth of Elodea canadensis and Potamogeton crispus started during the spring in Upper Spring Creek and Slab Cabin Run. Elodea canadensis showed growth throughout the summer and reached peak biomass in early September. These plants became senescent during the fall as water temperatures dropped rapidly. After 3 weeks of senescence, the remaining Elodea canadensis biomass persisted into the winter presumably in a dormant state. In contrast to Elodea canadensis, Potamogeton crispus began to mature during the summer as evidenced by the formation of seeds and flowers. Peak biomass of Potamogeton crispus was reached at the start of the maturation process which occurred during early July in Slab Cabin Run and early August in Upper Spring Creek. The mature plants persisted for approximately 2 weeks before becoming senescent. The younger shoots continued to grow until the fall when the plants became senescent as water temperatures dropped. Senescence persisted until all the above-ground biomass had disappeared. In addition to the field surveys on Spring Creek, Kratzer (1985) conducted in situ chamber respirometer measurements of macrophyte photosynthetic and respiration rates in Upper Spring Creek and Slab Cabin Run. The objective of these studies was to measure the mid-day peak photosynthetic and respiration rates of Elodea canadensis and Potamogeton crispus. Each experiment included two concurrent respirometer measurements: one respirometer contained a plant shoot excised above the sediment and cleaned of epiphytic communities, while the other respirometer contained a plant of the same species with an intact epiphytic community. Net photosynthetic rates (Pn) of the plant specimens were measured as the change in dissolved oxygen over time per unit dry weight of plant biomass during in-stream incubation of the chamber respirometer under full sunlight from 11:00 to 13:00 h each day. Respiration rates (R) of the plant specimens were measured by covering the respirometer with black plastic to block the sunlight from 13:00 to 15:00 h each day. The description of the respirometer and procedures used for measuring the photosynthetic and respiration rates are discussed by Kratzer (1985). Specific photosynthetic ( p ) and respiration ( r ) rates were calculated from the measured net photosynthetic and respiration rates by assuming a photosynthetic quotient of 1.0, resulting in an oxygen production to carbon assimilation mass ratio of 2.67:1, and a carbon to volatile matter mass ratio of 0.465:1 (Westlake, 1969) according to the following equations p = ( Pn + R) ( 1 g VM/0.465 g C) ( B~ VM) ( 1 g / 1 0 0 0 mg) / ( P Q )
(6)
r = R(1 g V M / O . a 6 5 g C ) ( B / V M ) ( 1 g / 1 0 0 0 m g ) / ( P Q )
(7)
where p is specific photosynthetic rate (h- 1), r is specific respiration rate (h- 1), VM is plant volatile matter (g), B is plant biomass dry weight (g), and PQ is the mass ratio of oxygen production per carbon assimilation taken as 2.67 g O 2 g-I C for this study. Tables 2 and 3 summarize the data collected during the respirometer measurements.
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Table 2 Respirometer measurements for Upper Spring Creek (Kratzer, 1985) Plant
Date
I
Temp.
IWSOP
Pn
R
p
r
Ec Ec Ec Ec Ec Ec+Ep Ec+Ep Ec+Ep Ec+Ep Ec+Ep Pc Pc Pc Pc+Ep Pc+Ep Pc+Ep
22/7/1983 27/7/1983 28/10/1983 4/6/1984 25/6/1984 22/7/1983 27/7/1983 28/10/1983 4/6/1984 25/6/1984 8/7/1983 9/8/1983 15/9/1983 8/7/1983 9/8/1983 15/9/1983
72 68 38 84 58 72 68 38 84 58 82 61 61 82 61 61
16.5 16.8 10.3 16.8 14.3 16.5 16.8 10.3 16.8 14.3 16.9 18.0 14.5 16.9 18.0 14.5
0.013 0.013 0.017 0.036 0.024 0.013 0.013 0.017 0.036 0.024 0.013 0.014 0.017 0.013 0.014 0.017
2.62 3.78 0.82 5.95 4.08 1.83 3.17 NDA 5.57 3.26 0.25 2.88 6.03 NDA 4.87 1.86
1.39 1.97 0.75 1.78 2.48 2.66 2.43 0.63 3.05 1.28 2.90 1.74 0.89 3.71 1.52 1.39
0.0049 0.0076 0.0023 0.0182 0.0090 0.0049 0.0106 NDA 0.0181 0.0110 0.0038 0.0061 0.0093 NDA 0.0118 0.0056
0.0017 0.0026 0.0011 0.0042 0.0034 0.0029 0.0046 0.0019 0.0064 0.0031 0.0035 0.0023 0.0012 0.0048 0.0028 0.0024
Ec, Elodea canadensis; Pc, Potamogeton crispus; Ep, epiphytes; NDA, no data available; L solar h-radiance (ly h - l ) ; Temp., water temperature (°C); IWSOP, interstitial water soluble ortho-phosphate (mg 1-1 p); Pn, net photosynthetic rate (mg 02 g - i DW h - l ) ; R, respiration rate (mg 02 g - l DW h - I ) ; p, specific photosynthetic rate (h- i ); r, specific respiration rate (h- I ).
3.2. Photosynthetic and respiration rate functions Functions for photosynthetic and respiration rates of the macrophytes growing in Spring Creek were calibrated by Davis (1990) using regression analyses of the data in
Table 3 Respirometer measurements for Slab Cabin Run (Kratzer, 1985) Plant
Date
I
Temp.
IWSOP
Pn
R
p
r
Pc Pc Pc Pc Pc Pc Pc+Ep Pc+Ep Pc+Ep Pc+Ep Pc+Ep
13/7/1983 2/8/1983 13/8/1983 8/9/1983 5/6/1984 26/6/1984 13/7/1983 2/8/1983 13/8/1983 8/9/1983 5/6/1984
78 62 72 42 78 65 78 62 72 42 78
19.0 18.8 17.6 15.0 15.4 15.9 19.0 18.8 17.6 15.0 15.4
0.019 0.024 0.024 0.074 0.035 0.030 0.019 0.024 0.024 0.074 0.035
6.23 5.17 6.83 5.75 8.69 3.85 6.20 5.89 5.72 3.84 7.18
5.45 1.93 2.37 1.89 5.65 3.45 3.38 2.49 2.24 1.20 3.41
0.0120 0.0081 0.0093 0.0089 0.0165 0.0110 0.0119 0.0111 0.01 l0 0.0067 0.0143
0.0056 0.0022 0.0024 0.0022 0.0065 0.0052 0.0042 0.0033 0.0031 0.0016 0.0046
Ec, Elodea canadensis; Pc, Potamogeton crispus; Ep, epiphytes; NDA, no data available; /, solar irradiance (ly h - l ) ; Temp., water temperature (°C); IWSOP, interstitial water soluble ortho-phosphate (mg 1-z p); Pn, net photosynthetic rate (mg 02 g-I DW h - l ) ; R, respiration rate (mg 02 g - l DW h - l ) ; p, specific photosynthetic rate (h- l ); r, specific respiration rate (h- l ).
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271
Table 4 Summary of model parameters for macrophyte photosynthetic and respiration rate functions from Davis (1990). All parameters are significant at the 0.05 level Model Value Description pma Kc Rm
0.00035ly- ] 0.0285 mg 1- t 0.290
Rb
0.00057 h- 1
0
1.036
Maximum photosyntheticrate per unit solar irradiance Half-saturation constant for IWSOP concentration Empirical slope constant for relationship between plant respiration and photosynthetic rates Empirical intercept constant for relationship between plant respiration and photosynthetic rates Temperature adjustment factor for plant respiration rates
Tables 2 and 3. Eqs. (8) and (9) show the photosynthetic and respiration rate functions incorporated into the growth component of the partitioned-biomass for the Spring Creek demonstration. p = pma I C / ( Kc + C)
(8)
r = ( R m p + Rb)O ~r-2°)
(9)
where pma is the maximum specific photosynthetic rate per unit solar irradiance (ly- ~), I is incident solar irradiance (ly h-L), Kc is the nutrient half-saturation constant (mg 1-~), C is the limiting nutrient concentration (mg l-L), Rrn is the respiration slope constant, Rb is the respiration intercept constant ( h - l ) , and 0 is the temperature adjustment factor for respiration. Parameter values for the photosynthetic and respiration rate functions as determined by Davis (1990) are listed in Table 4. These values were used to model the growth of both Elodea canadensis and Potamogeton crispus, since there was no statistical difference ( P < 0.05) in photosynthetic or respiration rates between species. Sediment interstitial water soluble ortho-phosphate (IWSOP) was used as the limiting nutrient for the photosynthetic function. The photosynthetic and respiration rate functions are illustrated in Figs. 2 and 3, respectively. Fig. 2 shows the Monod-type relationship between the measured photosynthetic rate per unit solar irradiance and the IWSOP concentrations for the data in Tables 2 and 3. The solid line in Fig. 2 represents the calibrated photosynthetic function. Fig. 3 shows the linear relationship between respiration and photosynthetic rates for the data from Tables 2 and 3. 3.3. Transfer functions The biomass transfer functions were calibrated to match the observed phenological development of each plant species. Growth was initiated during the spring for both species. The model for Elodea canadensis transferred biomass from the growth to the senescent stage during the fall when average daily water temperatures dropped rapidly below 15°C. The period of senescence lasted for three weeks. The remaining biomass persisted into the winter in a dormant state. The model for Potamogeton crispus transferred biomass from the growth to the mature stage when average daily water temperatures exceeded 17°C. Transfer of mature
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J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
0.0003 f
0.00025 At
~.~
0.0002
"
o
o.oools p~
0.oool 0.00005 0
I -Pc 0
0.02
0.04
0.06
0.08
IWSOP ( m g l - 1 ) Fig. 2. Illustration of the Monod-type relationship between plant photosynthetic rate per unit solar irradiance and sediment interstitial water soluble ortho-phosphate concentration for the Spring Creek studies. The solid line represents the photosynthetic rate function used in the model.
biomass into senescence started 15 days after maturation. Biomass that did not mature continued to grow until senescence occurred during the fall as average daily water temperatures dropped below 14°C. Senescence continued until all above-ground biomass disappeared. Values for the transfer rates (kgm, kms, kgs, and kgd) were set at 0.06 day- 1 for both species. This value results in the transfer of over half of the standing crop within a 2-week period, which is consistent with the field observations for Spring Creek. A value of 0.06 day-1 was also used for the decay rate (fd) of senescent biomass based on studies by Jewell (1971) and Hill and Webster (1982). Davis (1990) noted that advective
0.008
tee
~,~ 0 . 0 0 6
]
,Pc
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A
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o
,~ 0.004 "~
0.o02
0
,
0
i
~
i
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~
t
i
i
,
'
. . . .
'
0.005 0.01 0.015 Photosynthetic Rate (h -1)
....
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Fig. 3. Plot of plant respiration rates versus photosynthetic rates for the Spring Creek studies. The solid line represents the respiration rate function used in the model.
J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
(a)
[-E~ Mo~e, o
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!6o
o
8o
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273
2S 20
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i!: 0
J
J
A
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200
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25
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- - Pc Model
• Pc Biomass Temperature
1so
2O
5O 5
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A
S
O
N
Month Fig. 4. Comparison of predicted versus measured biomass for: (a) Upper Spring Creek; (b) Slab Cabin Run. Elodea canadensis did not grow in Slab Cabin Run during 1983.
transport of biomass during the 1983 studies on Spring Creek was negligible, so an advective transport function was not included for this application.
3.4. Model simulations Initial biomass values for Elodea canadensis and Potamogeton crispus were input to the model at the start of the simulation for each reach. Average weekly values of solar irradiance, nutrient concentration, and water temperature were read into the model, and the biomass for each species was calculated for one day time intervals. Fig. 4 shows the predicted and measured biomass for each simulation. The biomass values shown on the graph represent the sum of biomass from the growth and mature stage since the field measurements excluded senescent biomass. Average water temperatures are shown in each graph to indicate their relationship to plant growth and development.
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J.F. Davis,A.J. McDonnell~AquaticBotany56 (1997)265-276
4. Discussion The partitioned-biomass model was able to simulate the distinct growth and development patterns of Elodea canadensis and Potamogeton crispus as indicated in Fig. 4. The model predictions track the temporal change in measured biomass for each simulation. The median error for all simulations was within 24.5% of the predicted value, and the correlation between the predicted and measured biomass was significant ( r 2 = 0.93, P < 0.05). Several points are discussed below regarding the calibration and sensitivity of the model for the Spring Creek application and other applications in general. Calibration of the photosynthetic and respiration rate functions in the partitioned-biomass model was of primary importance for predicting peak macrophyte biomass. Since the data used for calibrating the photosynthetic and respiration rate functions for Spring Creek were independent of the biomass measurements used to demonstrate the model, the close agreement between the simulated and measured peak biomass supports the calibration of the photosynthetic and respiration rate functions for this system. To further support the calibration of the photosynthetic and respiration functions, the photosynthetic and respiration rates measured in this study were compared to rates measured by Nielsen and Sand-Jensen (1991) using laboratory grown plants under near optimal conditions for growth. Nielsen and Sand-Jensen reported net photosynthetic (Pn) rates of 5.16 mg 02 g - i DW h - l for Elodea canadensis and 17.98 mg 02 g-1 DW h-1 for Potamogeton crispus. Net photosynthetic rates measured in this study averaged 3.45 mg 0 2 g - 1 DW h - 1 for Elodea canadensis (SD = 1.64) and 5.22 mg 0 2 g-1 DW h-1 for Potamogeton crispus (SD = 2.10). Since most of the rates measured in the Spring Creek study were considered to be under phosphorus limited conditions, the average of the rates measured in this study were expected to be lower than those reported by Nielsen and Sand-Jensen. However, the highest measured rates in this study (5.95 mg 02 g-1 DW h-J for Elodea canadensis and 8.69 mg 02 g-1 DW h-1 for Potamogeton crispus) are more comparable to those measured by Nieslen and Sand-Jensen. In addition to the calibration of the growth component, the calibration and sensitivity of the transfer functions were examined for the Spring Creek application. The timing of plant development for this study was based on limited observations of the life cycle of Elodea canadensis and Potamogeton crispus in Spring Creek. The timing of maturation for Potamogeton crispus occurred during July in Slab Cabin Run but not until August in Upper Spring Creek. The difference in timing of maturation for Potamogeton crispus in each stream was correlated with average daily water temperatures above 17°C. Sastroutomo (1981) also found that the maturation of Potamogeton crispus growing in Lake Ojagaike, Japan, was regulated primarily by high water temperatures during the spring. The development pattern used to model Elodea canadensis in Spring Creek is supported by observations of Elodea canadensis in other aquatic systems (Best, 1977; Nichols and Shaw, 1986). The onset of senescence for both Potamogeton crispus and Elodea canadensis in Spring Creek was related to the rapid drop in water temperatures during the fall, and Collins and Wlosinski (1989) also used the rapid drop in water temperature during the fall to trigger plant mortality.
J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
275
There may be a water temperature above which both species may die rapidly, but these conditions were not observed during the 1983 surveys on Spring Creek. Although water temperature was proposed as the key environmental factor influencing plant development in Spring Creek, factors such as the photoperiod length may also be important (Sastroutomo, 1981). Since macrophytes may acclimate to a local environment, the range in temperatures and other factors related to the development of a given species may vary with geographic location. The partitioned-biomass model may be very sensitive to the timing of the biomass transfers. To illustrate this, the onset of maturation for Potamogeton crispus for the Slab Cabin Run simulation was delayed by 1- and 2-week time intervals to examine the effect on the peak biomass prediction. The peak biomass prediction increased from 179 to 241 g DW m -2 for a 1-week delay and to 313 g DW m -2 for a 2-week delay. However, the peak biomass predictions for Potamogeton crispus in Upper Spring Creek were not sensitive to changes in the timing of the transfers due to the low net growth rates resulting from the low phosphorus concentrations in Upper Spring Creek. Although the model may be sensitive to the timing of the transfers, the model was not sensitive to the transfer rates. All transfer rates were initially set at 0.06 d a y - l for this study based on the general field observations that the biomass seemed to transfer within a 2- to 4-week time span. Sensitivity runs were made by adjusting the transfer rates for each simulation to reduce the model error. The revised rates ranged from 0.03 to 0.15 d a y - l but did not result in any dramatic improvement in model error. Based on these results, transfer rates of 0.06 d a y - l are recommended as initial estimates for other studies.
5. Conclusions The ability of the partitioned-biomass model to simulate the distinct growth patterns of Elodea canadensis and Potamogeton crispus for a shallow stream environment was demonstrated. The structure of the model could be applied to other surface waters for the growth and development of a variety of rooted macrophytes. Application of the model as a basis for water quality management decisions regarding macrophyte growth should be contingent on calibrating the photosynthetic and respiration rate functions and the transfer functions which describe the phenological development of each macrophyte species for a given waterway.
References Best, E.P.H., 1977. Seasonal changes in mineral and organic components of Ceratophyllurn demersum and Elodea canadensis. Aquat. Bot., 3: 337-348. Collins, C.D. and Wlosinski, J.H., 1989. A macrophyte submodel for aquatic ecosystems. Aquat. Bot., 33: 191-206. Davis, J.F., 1990. A post-audit study: The response of a rooted macrophyte-dominated stream, following implementation of nutrient controls using spray-irrigation of secondary effluent. Ph.D. Dissertation, The Pennsylvania State University, University Park, 387 pp.
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Hill, B.H. and Webster, J.R., 1982. Aquatic macrophyte breakdown in an appalachian river. Hydrobiologia, 89: 53-59. Jewell, W.J., 1971. Aquatic weed decay: dissolved oxygen utilization and nitrogen and phosphorus regeneration. J. Water Pollut. Control Fed., 43: 1452-1467. Kratzer, T.W., 1985. Photosynthesis and respiration rates of macrophytes and anfwuchs as measured in situ with a submersible respirometer. Master of Environmental Pollution Control Paper, Pennsylvania State University, University Park, 93 pp. Nichols, S.A. and Shaw, B.H., 1986. Ecological life histories of the three aquatic nuisance plants, Myriophyllure spicatum, Potamogeton crispus, and Elodea canadensis. Hydrobiologia, 131: 3-21. Nielsen, S.L. and Sand-Jensen, K., 1991. Variation in growth rates of submerged rooted macrophytes. Aquat. Bot., 39: 109-120. O'Shaughnessy, J.C. and McDonnell, A.J., 1973. Criteria for estimating limiting nutrients in natural streams. Res. Publ. No. 75, Institute for Research on Land and Water Resources, The Pennsylvania State University, University Park, PA, 91 pp. Sand-Jensen, K., 1989. Environmental variables and their effects on photosynthesis of aquatic plant communities. Aquat. Bot., 34: 5-25. Sastroutomo, S.S., 1981. Turion formation, dormancy, and germination of curly pondweed, Potamogeton crispus L. Aquat. Bot., 10: 161-173. Scheffer, M., Bakema, A.H. and Wortelboer, F.G., 1993. MEGAPLANT: a simulation model of the dynamics of submerged plants. Aquat. Bot., 45: 341-356. Westlake, D., 1969. Some basic data for investigations of the productivity of aquatic macrophytes. In: C. Goldman (Editor), Primary Productivity in Aquatic Environments. University of California, Berkeley, pp. 229-248. Wright, R.M. and McDonnell, A.J., 1986. Macrophyte growth in shallow streams: biomass model. J. Environ. Eng. Div., ASCE, 112: 967-982.
Aquatic Botany56 (1997) 265-276
Development of a partitioned-biomass model for rooted macrophyte growth John F. Davis a,*, Archie J. McDonnell b a Department of Civil Engineering, Widener University, Chester, PA 19013, USA b Environmental Resources Research Institute, Penn State University, University Park, PA 16802, USA
Accepted 25 September 1996
Abstract
Many surface waters support excessive growth of several rooted macrophyte species. This paper presents a macrophyte growth model to facilitate modeling the growth of several macrophyte species which exhibit distinct patterns of growth and development. The model partitions the biomass of each species according to the phenological stages of plant development including growth, maturation, senescence, and dormancy. The partitioned-biomass model was demonstrated for the growth of Elodea canadensis Michx. and Potamogeton crispus L. in Spring Creek, a shallow fourth-order stream in central Pennsylvania. The structure of the model could be adapted to a wide variety of macrophytes and aquatic ecosystems. Keywords: Rooted macrophytes;Biomass models; Photosynthesis; Respiration
1. Introduction
Various macrophyte growth models have been developed and applied to examine the effect of rooted macrophytes on aquatic ecosystems and to examine strategies for controlling macrophyte growth. Although several macrophyte species may be abundant in a given waterway, some modeling studies have not differentiated the macrophyte biomass by species (Wright and McDonnell, 1986; Collins and Wlosinski, 1989). This approach may not be valid for waters supporting several macrophyte species which exhibit distinct seasonal patterns of growth and development. A macrophyte growth model is presented herein which facilitates modeling the growth and development of individual macrophyte species. A demonstration of the model is presented for the growth
* Corresponding author. Tel.: 610-499-4063;fax: 610-499-4059. 0304-3770/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-3770(96)01 103-5
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of Elodea canadensis Michx. and Potamogeton crispus L. in a shallow stream in central Pennsylvania.
2. Model description Westlake (1969) illustrated the stages of development and metabolism for rooted macrophytes during an annual cycle. These stages included growth following germination, maturation as flowers and fruits are produced, senescence as the plants begin to die and decay, and dormancy for plants which persist through the winter. The model presented in this paper partitions the biomass for each macrophyte species into distinct stages for growth (Bg), maturation (Bm), senescence (Bs), and dormancy (Bd) as shown in Fig. 1. Functions for plant photosynthesis, respiration, and decay may be programmed for each stage. The model transfers biomass from one stage to another at assigned transfer rates to simulate the observed development of individual macrophyte species. Transfer of biomass for a given species is regulated in the model according to environmental conditions such as water temperature (Sand-Jensen, 1989). The mathematical structure of the model is discussed below. 2.1. Growth
The growth stage of plant development starts following germination or dormancy and continues until maturation, senescence, or dormancy occurs. The growth stage of the model accounts for the production of plant biomass. Eq. (1) shows the finite-difference form of a first-order growth function used in this model: (1)
Bg t = Bg o + Bgo( p -- r ) d t
where Bg t is biomass in the growth stage at the end of time interval dt (g DW m-2), Bg o is biomass in the growth stage at the start of the time interval, p is plant specific photosynthetic rate (day-i), and r is plant specific respiration rate (day-l). Relationships for photosynthetic and respiration rates can be programmed into the model as
kgd~ ~¢~ . , f k d g [Dormant Biomass
Gro,ring Biot aass
kgs Sen scent ~ k Biol lass
[Mature Biomass ms
,fd Fig. 1. Illustration of the partitioned-biomass model to describe an annual cycle of macrophytegrowth. Transfer rates are designated as kxy to representthe transferof biomass from stage x to stage y. The decay rate of senescentbiomass is representedas fa.
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functions of light, nutrients, and water temperature similar to models by Wright and McDonnell (1986), Collins and Wlosinski (1989), and Scheffer et al. (1993). Parameter values for these functions may be calibrated for each species. 2.2. Maturation
Many plant species produce flowers and fruits at the terminal ends of their shoots as they mature. As suggested by Westlake (1969), the model sets the respiration rates equal to the photosynthetic rates for mature plants which results in net growth rates of zero. Plant biomass may transfer into the mature stage from the growth stage, but not all species exhibit the mature stage of development. The model allows biomass to remain in the mature stage for a predetermined time before transferring into the senescent stage. 2.3. Senescence
Plants transfer into the senescent stage as they begin to die and decay. The model uses a decay function to simulate the loss of senescent biomass (Westlake, 1969). Transfer of biomass into this stage follows maturation or may occur directly from the growth stage under severe environmental conditions. 2.4. D o r m a n c y
The shoots of some species persist over winter in a dormant stage (Westlake, 1969). The model sets photosynthetic and respiration rates to zero for dormant biomass. Dormant biomass may transfer into the growth stage during the spring as environmental conditions become suitable for growth. 2.5. Advective transport
In addition to the metabolic processes discussed for each stage, an advective transport function (fl) may be added to each stage to account for the loss or gain of biomass due to the transport of biomass by water currents. Since advective transport may not be significant for some ecosystems, the inclusion of an advective transport function is left as an option to the modeler. 2.6. Transfer f u n c t i o n s
The equations used in the model to simulate the growth, accumulation, transfer, and loss of biomass each stage of plant development over a time interval are given as Bg t = Bg o + Bgo( p - r -
kg m - kg~ - kgd - f l g ) d t
(2)
B m , = B m o + ( Bgokg m - Bmokms --flm B m o ) d t
(3)
Bs, = n s o + ( n g o k g s q- Bmokms - flsnSo - f a B s o ) d t
(4)
Bd, = B d o + ( Bgokg d - f l d B d o ) d t
(5)
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where kxy is the transfer rate from stage x to stage y ( d a y - l ) , fd is the decay rate for senescence ( d a y - l ) , and fix is the biomass loss rate due to washout of biomass in stage x ( d a y - l ). Values for the transfer rates can be adjusted for each species to simulate their observed development. The transfers may be set to occur at a predetermined time during the simulation or may be triggered by environmental conditions such as water temperature.
3. Model demonstration 3.1. Field studies The partitioned-biomass model was applied to simulate the growth of rooted macrophytes Elodea canadensis Michx. and Potamogeton crispus L. in two reaches of Spring Creek, a shallow fourth-order stream in central Pennsylvania. The growth and development of these species in Upper Spring Creek and Slab Cabin Run (tributaries of Spring Creek) were studied by Davis (1990) during 1983. The studies tracked the biomass for each species from the start of growth in spring through senescence during the fall. Nutrient concentrations of the water column and sediments, solar irradiance measured as langleys per hour (ly h - l ) , water temperature, and plant biomass were monitored throughout the study and are summarized in Table 1. A complete description of the methods used for these studies is reported by Davis (1990). As summarized in Table 1, Upper Spring Creek had relatively low phosphorus concentrations and was free of wastewater discharges, while Slab Cabin Run had higher
Table 1 Summary of stream characteristics, water temperature, solar irradiance, nutrient concentrations, and peak biomass for Upper Spring Creek and Slab Cabin Run. The stream parameters are average values for June through November 1983 (Davis, 1990) Upper Spring Creek Slab Cabin Run Discharge (m3 s-I) 0.83 0.42 Velocity (m s-~ ) 0.22 0.17 Depth (m) 0.38 0.33 Water temperature (°C) 13.9 14.8 Solar irradiance (ly day- l ) 416 416 pH 8.22 8.15 Alkalinity (mg 1- l as CaCO3) 200 203 WCNH3-N(mg 1- l N) 0.19 0.17 WCNOx-N(mg 1- i N) 3.27 3.48 WCSOP (mg 1-J P) 0.015 0.102 IWSOP (mg 1-1 p) 0.055 0.150 Peak biomass (g DW m- 2 ) Elodea canadensis 87 10 Potamogeton crispus 36 186 WCNHs-N, water column ammonia-nitrogen;WCNOx-N, water column nitrate-nitrogen and nitrite-nitrogen; WCSOP, water column soluble ortho-phosphate;IWSOP, sediment interstitial water soluble ortho-phosphate.
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phosphorus concentrations due to a discharge of treated municipal wastewater and urban stormwater runoff. Both streams had high alkalinity and nitrogen concentrations which were found to provide above growth-limiting levels of carbon and nitrogen nutrients for rooted macrophyte growth (O'Shaughnessy and McDonnell, 1973). The growth of Elodea canadensis and Potamogeton crispus started during the spring in Upper Spring Creek and Slab Cabin Run. Elodea canadensis showed growth throughout the summer and reached peak biomass in early September. These plants became senescent during the fall as water temperatures dropped rapidly. After 3 weeks of senescence, the remaining Elodea canadensis biomass persisted into the winter presumably in a dormant state. In contrast to Elodea canadensis, Potamogeton crispus began to mature during the summer as evidenced by the formation of seeds and flowers. Peak biomass of Potamogeton crispus was reached at the start of the maturation process which occurred during early July in Slab Cabin Run and early August in Upper Spring Creek. The mature plants persisted for approximately 2 weeks before becoming senescent. The younger shoots continued to grow until the fall when the plants became senescent as water temperatures dropped. Senescence persisted until all the above-ground biomass had disappeared. In addition to the field surveys on Spring Creek, Kratzer (1985) conducted in situ chamber respirometer measurements of macrophyte photosynthetic and respiration rates in Upper Spring Creek and Slab Cabin Run. The objective of these studies was to measure the mid-day peak photosynthetic and respiration rates of Elodea canadensis and Potamogeton crispus. Each experiment included two concurrent respirometer measurements: one respirometer contained a plant shoot excised above the sediment and cleaned of epiphytic communities, while the other respirometer contained a plant of the same species with an intact epiphytic community. Net photosynthetic rates (Pn) of the plant specimens were measured as the change in dissolved oxygen over time per unit dry weight of plant biomass during in-stream incubation of the chamber respirometer under full sunlight from 11:00 to 13:00 h each day. Respiration rates (R) of the plant specimens were measured by covering the respirometer with black plastic to block the sunlight from 13:00 to 15:00 h each day. The description of the respirometer and procedures used for measuring the photosynthetic and respiration rates are discussed by Kratzer (1985). Specific photosynthetic ( p ) and respiration ( r ) rates were calculated from the measured net photosynthetic and respiration rates by assuming a photosynthetic quotient of 1.0, resulting in an oxygen production to carbon assimilation mass ratio of 2.67:1, and a carbon to volatile matter mass ratio of 0.465:1 (Westlake, 1969) according to the following equations p = ( Pn + R) ( 1 g VM/0.465 g C) ( B~ VM) ( 1 g / 1 0 0 0 mg) / ( P Q )
(6)
r = R(1 g V M / O . a 6 5 g C ) ( B / V M ) ( 1 g / 1 0 0 0 m g ) / ( P Q )
(7)
where p is specific photosynthetic rate (h- 1), r is specific respiration rate (h- 1), VM is plant volatile matter (g), B is plant biomass dry weight (g), and PQ is the mass ratio of oxygen production per carbon assimilation taken as 2.67 g O 2 g-I C for this study. Tables 2 and 3 summarize the data collected during the respirometer measurements.
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Table 2 Respirometer measurements for Upper Spring Creek (Kratzer, 1985) Plant
Date
I
Temp.
IWSOP
Pn
R
p
r
Ec Ec Ec Ec Ec Ec+Ep Ec+Ep Ec+Ep Ec+Ep Ec+Ep Pc Pc Pc Pc+Ep Pc+Ep Pc+Ep
22/7/1983 27/7/1983 28/10/1983 4/6/1984 25/6/1984 22/7/1983 27/7/1983 28/10/1983 4/6/1984 25/6/1984 8/7/1983 9/8/1983 15/9/1983 8/7/1983 9/8/1983 15/9/1983
72 68 38 84 58 72 68 38 84 58 82 61 61 82 61 61
16.5 16.8 10.3 16.8 14.3 16.5 16.8 10.3 16.8 14.3 16.9 18.0 14.5 16.9 18.0 14.5
0.013 0.013 0.017 0.036 0.024 0.013 0.013 0.017 0.036 0.024 0.013 0.014 0.017 0.013 0.014 0.017
2.62 3.78 0.82 5.95 4.08 1.83 3.17 NDA 5.57 3.26 0.25 2.88 6.03 NDA 4.87 1.86
1.39 1.97 0.75 1.78 2.48 2.66 2.43 0.63 3.05 1.28 2.90 1.74 0.89 3.71 1.52 1.39
0.0049 0.0076 0.0023 0.0182 0.0090 0.0049 0.0106 NDA 0.0181 0.0110 0.0038 0.0061 0.0093 NDA 0.0118 0.0056
0.0017 0.0026 0.0011 0.0042 0.0034 0.0029 0.0046 0.0019 0.0064 0.0031 0.0035 0.0023 0.0012 0.0048 0.0028 0.0024
Ec, Elodea canadensis; Pc, Potamogeton crispus; Ep, epiphytes; NDA, no data available; L solar h-radiance (ly h - l ) ; Temp., water temperature (°C); IWSOP, interstitial water soluble ortho-phosphate (mg 1-1 p); Pn, net photosynthetic rate (mg 02 g - i DW h - l ) ; R, respiration rate (mg 02 g - l DW h - I ) ; p, specific photosynthetic rate (h- i ); r, specific respiration rate (h- I ).
3.2. Photosynthetic and respiration rate functions Functions for photosynthetic and respiration rates of the macrophytes growing in Spring Creek were calibrated by Davis (1990) using regression analyses of the data in
Table 3 Respirometer measurements for Slab Cabin Run (Kratzer, 1985) Plant
Date
I
Temp.
IWSOP
Pn
R
p
r
Pc Pc Pc Pc Pc Pc Pc+Ep Pc+Ep Pc+Ep Pc+Ep Pc+Ep
13/7/1983 2/8/1983 13/8/1983 8/9/1983 5/6/1984 26/6/1984 13/7/1983 2/8/1983 13/8/1983 8/9/1983 5/6/1984
78 62 72 42 78 65 78 62 72 42 78
19.0 18.8 17.6 15.0 15.4 15.9 19.0 18.8 17.6 15.0 15.4
0.019 0.024 0.024 0.074 0.035 0.030 0.019 0.024 0.024 0.074 0.035
6.23 5.17 6.83 5.75 8.69 3.85 6.20 5.89 5.72 3.84 7.18
5.45 1.93 2.37 1.89 5.65 3.45 3.38 2.49 2.24 1.20 3.41
0.0120 0.0081 0.0093 0.0089 0.0165 0.0110 0.0119 0.0111 0.01 l0 0.0067 0.0143
0.0056 0.0022 0.0024 0.0022 0.0065 0.0052 0.0042 0.0033 0.0031 0.0016 0.0046
Ec, Elodea canadensis; Pc, Potamogeton crispus; Ep, epiphytes; NDA, no data available; /, solar irradiance (ly h - l ) ; Temp., water temperature (°C); IWSOP, interstitial water soluble ortho-phosphate (mg 1-z p); Pn, net photosynthetic rate (mg 02 g-I DW h - l ) ; R, respiration rate (mg 02 g - l DW h - l ) ; p, specific photosynthetic rate (h- l ); r, specific respiration rate (h- l ).
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271
Table 4 Summary of model parameters for macrophyte photosynthetic and respiration rate functions from Davis (1990). All parameters are significant at the 0.05 level Model Value Description pma Kc Rm
0.00035ly- ] 0.0285 mg 1- t 0.290
Rb
0.00057 h- 1
0
1.036
Maximum photosyntheticrate per unit solar irradiance Half-saturation constant for IWSOP concentration Empirical slope constant for relationship between plant respiration and photosynthetic rates Empirical intercept constant for relationship between plant respiration and photosynthetic rates Temperature adjustment factor for plant respiration rates
Tables 2 and 3. Eqs. (8) and (9) show the photosynthetic and respiration rate functions incorporated into the growth component of the partitioned-biomass for the Spring Creek demonstration. p = pma I C / ( Kc + C)
(8)
r = ( R m p + Rb)O ~r-2°)
(9)
where pma is the maximum specific photosynthetic rate per unit solar irradiance (ly- ~), I is incident solar irradiance (ly h-L), Kc is the nutrient half-saturation constant (mg 1-~), C is the limiting nutrient concentration (mg l-L), Rrn is the respiration slope constant, Rb is the respiration intercept constant ( h - l ) , and 0 is the temperature adjustment factor for respiration. Parameter values for the photosynthetic and respiration rate functions as determined by Davis (1990) are listed in Table 4. These values were used to model the growth of both Elodea canadensis and Potamogeton crispus, since there was no statistical difference ( P < 0.05) in photosynthetic or respiration rates between species. Sediment interstitial water soluble ortho-phosphate (IWSOP) was used as the limiting nutrient for the photosynthetic function. The photosynthetic and respiration rate functions are illustrated in Figs. 2 and 3, respectively. Fig. 2 shows the Monod-type relationship between the measured photosynthetic rate per unit solar irradiance and the IWSOP concentrations for the data in Tables 2 and 3. The solid line in Fig. 2 represents the calibrated photosynthetic function. Fig. 3 shows the linear relationship between respiration and photosynthetic rates for the data from Tables 2 and 3. 3.3. Transfer functions The biomass transfer functions were calibrated to match the observed phenological development of each plant species. Growth was initiated during the spring for both species. The model for Elodea canadensis transferred biomass from the growth to the senescent stage during the fall when average daily water temperatures dropped rapidly below 15°C. The period of senescence lasted for three weeks. The remaining biomass persisted into the winter in a dormant state. The model for Potamogeton crispus transferred biomass from the growth to the mature stage when average daily water temperatures exceeded 17°C. Transfer of mature
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J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
0.0003 f
0.00025 At
~.~
0.0002
"
o
o.oools p~
0.oool 0.00005 0
I -Pc 0
0.02
0.04
0.06
0.08
IWSOP ( m g l - 1 ) Fig. 2. Illustration of the Monod-type relationship between plant photosynthetic rate per unit solar irradiance and sediment interstitial water soluble ortho-phosphate concentration for the Spring Creek studies. The solid line represents the photosynthetic rate function used in the model.
biomass into senescence started 15 days after maturation. Biomass that did not mature continued to grow until senescence occurred during the fall as average daily water temperatures dropped below 14°C. Senescence continued until all above-ground biomass disappeared. Values for the transfer rates (kgm, kms, kgs, and kgd) were set at 0.06 day- 1 for both species. This value results in the transfer of over half of the standing crop within a 2-week period, which is consistent with the field observations for Spring Creek. A value of 0.06 day-1 was also used for the decay rate (fd) of senescent biomass based on studies by Jewell (1971) and Hill and Webster (1982). Davis (1990) noted that advective
0.008
tee
~,~ 0 . 0 0 6
]
,Pc
I
--Model
]
-
-
A
9
5
~
o
,~ 0.004 "~
0.o02
0
,
0
i
~
i
,
~
t
i
i
,
'
. . . .
'
0.005 0.01 0.015 Photosynthetic Rate (h -1)
....
0.02
Fig. 3. Plot of plant respiration rates versus photosynthetic rates for the Spring Creek studies. The solid line represents the respiration rate function used in the model.
J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
(a)
[-E~ Mo~e, o
~
!6o
o
8o
i
4O
0
...... " o' ~ " ~
Ec Biomass Pc Model
273
2S 20
I . Pc Bi°mass
i!: 0
J
J
A
S
0
N
200
(b)
25
,
- - Pc Model
• Pc Biomass Temperature
1so
2O
5O 5
0
0
J
J
A
S
O
N
Month Fig. 4. Comparison of predicted versus measured biomass for: (a) Upper Spring Creek; (b) Slab Cabin Run. Elodea canadensis did not grow in Slab Cabin Run during 1983.
transport of biomass during the 1983 studies on Spring Creek was negligible, so an advective transport function was not included for this application.
3.4. Model simulations Initial biomass values for Elodea canadensis and Potamogeton crispus were input to the model at the start of the simulation for each reach. Average weekly values of solar irradiance, nutrient concentration, and water temperature were read into the model, and the biomass for each species was calculated for one day time intervals. Fig. 4 shows the predicted and measured biomass for each simulation. The biomass values shown on the graph represent the sum of biomass from the growth and mature stage since the field measurements excluded senescent biomass. Average water temperatures are shown in each graph to indicate their relationship to plant growth and development.
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J.F. Davis,A.J. McDonnell~AquaticBotany56 (1997)265-276
4. Discussion The partitioned-biomass model was able to simulate the distinct growth and development patterns of Elodea canadensis and Potamogeton crispus as indicated in Fig. 4. The model predictions track the temporal change in measured biomass for each simulation. The median error for all simulations was within 24.5% of the predicted value, and the correlation between the predicted and measured biomass was significant ( r 2 = 0.93, P < 0.05). Several points are discussed below regarding the calibration and sensitivity of the model for the Spring Creek application and other applications in general. Calibration of the photosynthetic and respiration rate functions in the partitioned-biomass model was of primary importance for predicting peak macrophyte biomass. Since the data used for calibrating the photosynthetic and respiration rate functions for Spring Creek were independent of the biomass measurements used to demonstrate the model, the close agreement between the simulated and measured peak biomass supports the calibration of the photosynthetic and respiration rate functions for this system. To further support the calibration of the photosynthetic and respiration functions, the photosynthetic and respiration rates measured in this study were compared to rates measured by Nielsen and Sand-Jensen (1991) using laboratory grown plants under near optimal conditions for growth. Nielsen and Sand-Jensen reported net photosynthetic (Pn) rates of 5.16 mg 02 g - i DW h - l for Elodea canadensis and 17.98 mg 02 g-1 DW h-1 for Potamogeton crispus. Net photosynthetic rates measured in this study averaged 3.45 mg 0 2 g - 1 DW h - 1 for Elodea canadensis (SD = 1.64) and 5.22 mg 0 2 g-1 DW h-1 for Potamogeton crispus (SD = 2.10). Since most of the rates measured in the Spring Creek study were considered to be under phosphorus limited conditions, the average of the rates measured in this study were expected to be lower than those reported by Nielsen and Sand-Jensen. However, the highest measured rates in this study (5.95 mg 02 g-1 DW h-J for Elodea canadensis and 8.69 mg 02 g-1 DW h-1 for Potamogeton crispus) are more comparable to those measured by Nieslen and Sand-Jensen. In addition to the calibration of the growth component, the calibration and sensitivity of the transfer functions were examined for the Spring Creek application. The timing of plant development for this study was based on limited observations of the life cycle of Elodea canadensis and Potamogeton crispus in Spring Creek. The timing of maturation for Potamogeton crispus occurred during July in Slab Cabin Run but not until August in Upper Spring Creek. The difference in timing of maturation for Potamogeton crispus in each stream was correlated with average daily water temperatures above 17°C. Sastroutomo (1981) also found that the maturation of Potamogeton crispus growing in Lake Ojagaike, Japan, was regulated primarily by high water temperatures during the spring. The development pattern used to model Elodea canadensis in Spring Creek is supported by observations of Elodea canadensis in other aquatic systems (Best, 1977; Nichols and Shaw, 1986). The onset of senescence for both Potamogeton crispus and Elodea canadensis in Spring Creek was related to the rapid drop in water temperatures during the fall, and Collins and Wlosinski (1989) also used the rapid drop in water temperature during the fall to trigger plant mortality.
J.F. Davis, A.J. McDonnell~Aquatic Botany 56 (1997) 265-276
275
There may be a water temperature above which both species may die rapidly, but these conditions were not observed during the 1983 surveys on Spring Creek. Although water temperature was proposed as the key environmental factor influencing plant development in Spring Creek, factors such as the photoperiod length may also be important (Sastroutomo, 1981). Since macrophytes may acclimate to a local environment, the range in temperatures and other factors related to the development of a given species may vary with geographic location. The partitioned-biomass model may be very sensitive to the timing of the biomass transfers. To illustrate this, the onset of maturation for Potamogeton crispus for the Slab Cabin Run simulation was delayed by 1- and 2-week time intervals to examine the effect on the peak biomass prediction. The peak biomass prediction increased from 179 to 241 g DW m -2 for a 1-week delay and to 313 g DW m -2 for a 2-week delay. However, the peak biomass predictions for Potamogeton crispus in Upper Spring Creek were not sensitive to changes in the timing of the transfers due to the low net growth rates resulting from the low phosphorus concentrations in Upper Spring Creek. Although the model may be sensitive to the timing of the transfers, the model was not sensitive to the transfer rates. All transfer rates were initially set at 0.06 d a y - l for this study based on the general field observations that the biomass seemed to transfer within a 2- to 4-week time span. Sensitivity runs were made by adjusting the transfer rates for each simulation to reduce the model error. The revised rates ranged from 0.03 to 0.15 d a y - l but did not result in any dramatic improvement in model error. Based on these results, transfer rates of 0.06 d a y - l are recommended as initial estimates for other studies.
5. Conclusions The ability of the partitioned-biomass model to simulate the distinct growth patterns of Elodea canadensis and Potamogeton crispus for a shallow stream environment was demonstrated. The structure of the model could be applied to other surface waters for the growth and development of a variety of rooted macrophytes. Application of the model as a basis for water quality management decisions regarding macrophyte growth should be contingent on calibrating the photosynthetic and respiration rate functions and the transfer functions which describe the phenological development of each macrophyte species for a given waterway.
References Best, E.P.H., 1977. Seasonal changes in mineral and organic components of Ceratophyllurn demersum and Elodea canadensis. Aquat. Bot., 3: 337-348. Collins, C.D. and Wlosinski, J.H., 1989. A macrophyte submodel for aquatic ecosystems. Aquat. Bot., 33: 191-206. Davis, J.F., 1990. A post-audit study: The response of a rooted macrophyte-dominated stream, following implementation of nutrient controls using spray-irrigation of secondary effluent. Ph.D. Dissertation, The Pennsylvania State University, University Park, 387 pp.
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Hill, B.H. and Webster, J.R., 1982. Aquatic macrophyte breakdown in an appalachian river. Hydrobiologia, 89: 53-59. Jewell, W.J., 1971. Aquatic weed decay: dissolved oxygen utilization and nitrogen and phosphorus regeneration. J. Water Pollut. Control Fed., 43: 1452-1467. Kratzer, T.W., 1985. Photosynthesis and respiration rates of macrophytes and anfwuchs as measured in situ with a submersible respirometer. Master of Environmental Pollution Control Paper, Pennsylvania State University, University Park, 93 pp. Nichols, S.A. and Shaw, B.H., 1986. Ecological life histories of the three aquatic nuisance plants, Myriophyllure spicatum, Potamogeton crispus, and Elodea canadensis. Hydrobiologia, 131: 3-21. Nielsen, S.L. and Sand-Jensen, K., 1991. Variation in growth rates of submerged rooted macrophytes. Aquat. Bot., 39: 109-120. O'Shaughnessy, J.C. and McDonnell, A.J., 1973. Criteria for estimating limiting nutrients in natural streams. Res. Publ. No. 75, Institute for Research on Land and Water Resources, The Pennsylvania State University, University Park, PA, 91 pp. Sand-Jensen, K., 1989. Environmental variables and their effects on photosynthesis of aquatic plant communities. Aquat. Bot., 34: 5-25. Sastroutomo, S.S., 1981. Turion formation, dormancy, and germination of curly pondweed, Potamogeton crispus L. Aquat. Bot., 10: 161-173. Scheffer, M., Bakema, A.H. and Wortelboer, F.G., 1993. MEGAPLANT: a simulation model of the dynamics of submerged plants. Aquat. Bot., 45: 341-356. Westlake, D., 1969. Some basic data for investigations of the productivity of aquatic macrophytes. In: C. Goldman (Editor), Primary Productivity in Aquatic Environments. University of California, Berkeley, pp. 229-248. Wright, R.M. and McDonnell, A.J., 1986. Macrophyte growth in shallow streams: biomass model. J. Environ. Eng. Div., ASCE, 112: 967-982.