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Growth of Zostera marina L. Seedlings under laboratory conditions of nutrient enrichment
Aquatic Botany, 20 (1984) 321--328 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
321
GROWTH OF Z O S T E R A M A R I N A L. SEEDLINGS UNDER LABORATORY CONDITIONS OF NUTRIENT ENRICHMENT
MORRIS H. ROBERTS, JR., ROBERT J. ORTH and KENNETH A. MOORE
Virginia Institute of Marine Science and School of Marine Science, College of William and Mary, Gloucester Point, VA 23062 (U.S.A.) (Accepted for publication 29 August 1984)
ABSTRACT Roberts, M.H., Jr., Orth, R.J. and Moore, K.A., 1984. Growth of Zostera marina L. seedlings under laboratory conditions of nutrient enrichment. Aquat. Bot., 20: 321-328,. The effect of increased nutrients on growth of Zostera marina L. seedlings was tested in the laboratory by adding 2 different formulations (18:6:12 and 14:14:14 Nitrogen: Phosphorus:Potassium (N:P:K), respectively) of a slow release fertilizer, Osmocote ®. Three different application rates were used with the 2 formulations by placing appropriate amounts in p e a t pots containing 1 seedling each. The addition of fertilizer to the substrate markedly stimulated the growth of seedlings. Nutrient enrichment p r o m o t e d growth both in terms of increased leaf length and vegetative production of shoots. The nitrogen rich formulation (18:6:12) p r o d u c e d less growth than the equal balance formulation (14:14:14). F o r both formulations, the highest concentrations produced greater growth than other concentrations of the same formulation. A t equal application rates with respect to nitrogen, less growth occurred in the treatments receiving less phosphorus. Results of this experiment corroborate results from previous work suggesting that addition of nutrients to the sediment can stimulate Z. marina growth.
INTRODUCTION
Several recent studies have focused on the role of nutrient limitation in regulating the development of seagrass beds by experimentally manipulating the nutrient concentration in the sediment or the water column or by monitoring the ambient concentration of nutrients during shoot and leaf growth (Orth, 1977; Bulthuis and Woelkerling, 1981; Harlin and ThorneMiller, 1981; Orth and Moore, 1982; Short, 1983). Results from some of these studies have shown increases in growth, expressed as changes in above ground standing crop, leaf length, shoot density, or below ground standing crop, following nutrient additions (Orth, 1977; Harlin and Thorne-Miller, 1981; Orth and Moore, 1982), while in one study, no increase in growth was observed (Bulthuis and Woelkering, 1981}. These studies differed in
0304-3'770/84/$03.00
© 1984 Elsevier Science Publishers B.V.
322
types and amounts of nutrient input as well as m o d e of nutrient application. Therefore growth could have been affected either by different ambient nutrient concentrations, predominant m o d e of nutrient uptake, or competition for increased nutrients with p h y t o p l a n k t o n and epiphytic algae. The objective o f the experiment described here was to evaluate the effect of 2 fertilizer formulations at several concentrations on growth of Zostera marina L. seedlings under laboratory conditions. Laboratory culture of Z. marina is essential for some types of experimental studies of life-history, physiology, growth, and reproduction and potentially has value for production of plants to reestablish grass beds in denuded areas. In both cases, it is desirable to k n o w h o w fertilization affects growth under controlled conditions. M A T E R I A L S AND METHODS
Selected seedlings consisting of 1 s h o o t each were collected manually by divers on 11 March 1980 from a seagrass bed adjacent to Guinea Marsh, York River, Virginia. In this region, seedlings are available in different stages of development from O c t o b e r to May (Orth and Moore, 1983). Sediment for the experiments was collected from the same site as the seedlings and placed in 5 × 5 ~ m square peat pots supported in plastic greenhouse trays. The median grain size of the sediment was 2.6 Phi (sorting coefficient = 0.94). A sediment core was removed from selected pots, centrifuged in a Gelman filter centrifuge tube (0.45 gm glass fiber filter) for 10 min, and the filtrate analyzed for NH~, NO2, NO3-and PO4-3 with a Technicon Autoanalyzer II (Kopp and McKee, 1979). Seedlings were planted in the peat pots and held in flowing estuarine water for 2 weeks. Seven groups of 52 seedlings were then selected on 20 March 1980 for the experiment. The fertilizers selected for the experiment were 2 formulations of a 3-month timed-release fertilizer, Osmocote@, one with a Nitrogen:Phosphorus:Potassium (N:P:K) ratio of 18:6:12, the other 14:14:14. Nitrogen was in the form of ammonium and nitrate, phosphorus as phosphoric acid (P~Os) and potassium as soluble potash (K~O). Release rates of N, P and K remain relatively constant during the 3-month period according to the manufacturer. Each fertilizer was tamped into the sediment surface at 3 application rates (g m -s) (Table I). Application rates for each formulation of Osmocote were calculated to provide the same 3 amounts of total nitrogen; 12.5, 25 and 50 g m -2. This resulted in different levels of phosphorus in equi-nitrogen treatments. Another group of plants which received no fertilizer served as the control. Plants were grown in a holding tank receiving 10 gm filtered estuarine water. Photosynthetic active radiation (400--700 nm) at the water surface, as measured with a LI-COR 185B photometer/radiometer, was approximately 57% of the incident level, and 38% of the incident level at the surface of the sediment.
323 The day following nutrient addition, the n u m b e r of leaf blades/plant, n u m b e r of plant shoots, and length of longest blade were determined and recorded. At 2-week intervals thereafter, the plants were wiped gently with fingers to remove periphyton. Number of shoots, leaf blades/shoot, and length of longest blade on oldest s h o o t were determined. The seventh and final measurement was made on 13 June 1980. TABLE I Summary of fertilizer application rates. Sediment nitrogen and phosphorus concentrations were calculated from application rate and concentration in fertilizer assuming total availability of both nutrients Treatment
Fertilizer Application rate
Nitrogen (g m -L)
Phosphorus (g m -2)
0 12.5 25 50 12.5 25 50
0 12.5 25 50 4.2 8.3 16.7
(g m -s) (g/peat pot)
1 2a 2b 2c 3a 3b 3c
None 14:14:14 14:14:14 14:14:14 18:6:12 18:6:12 18:6:12
0 89.3 178.6 357.1 69.4 138.9 277.8
0 0.23 0.46 0.91 0.18 0.35 0.71
Leaf blade lengths for each t r e a t m e n t were compared for each measurem e n t interval by one-way analysis of variance and Duncan's multiple range test. Number of leaf blades/plant and n u m b e r of shoots were analyzed by non-parametric methods. RESULTS During the acclimation period and the first sampling interval, water temperature averaged 10.3°C and 10.8°C respectively while salinity declined from 17.8 to 15.7%o. Mean temperature increased in each succeeding sampling period to 27.3°C during the final interval. Salinity declined to 14.9%o during the third sampling interval, and then increased to 17.9%o during the final period. Throughout the study, dissolved o x y g e n during the midday period usually exceeded saturation. The initial concentrations of a m m o n i u m , nitrite, nitrate and phosphorus in the sediment pore water were 204 + 46, 0.83 + 0.49, 1.97 -+ 0.59 and 41.1 -~ 27.4 gM, respectively. Initially, each seedling consisted of a single shoot. At the end of the study, the mean numbers of shoots/plant were 1.2 in the control and from 2.8 to 3.5 in the fertilized groups (Fig. 1A). No control plants developed more than 3 shoots, whereas after 12 weeks, 30--47% of the plants fertilized with 14:14:14 and 21--28% of the plants fertilized with 18:6:12 had 4 or more shoots.
324
The average length of the leaf length) was 8.6--9.2 cm throughout the study period significantly different among
longest leaf (hereafter referred to as average at the start of the experiment and increased (Table II). The average leaf lengths were not the treatments until after 4 weeks growth,
l-Z _J O. I-0 O "1" Z L~
'-
X Y / / / , ~ o ~ ~'
o/////,Z'°~ 0
I0
20
30
40
,~
50
PHOSPHORUS (g/m 2 )
B 4O
=
30 ~o
~
X y / / / / ~ o ~$~ o//,////~
,oI
/o
>~
0
I0
20
30
40
50
PHOSPHORUS ( g / m 2)
Fig. 1. Growth o f Zostera marina L. after 12 weeks as a function of phosphorus and nitrogen enrichment. (A) Expressed as mean shoots/plant; (B) expressed as average leaf length (cm).
325 b u t t h e r e a f t e r , 3--4 groups of t r e a t m ent s were definable by Duncan's multiple range test (P<:0.05). A f t er 12 weeks, all experi m ent al t r e a t m e n t s were significantly di f f er e nt f r o m t he c o n t r o l group and assorted i n t o 2 groups: T r e a t m e n t s 2a, 2b, 2c and 3c exhibited greater average leaf length t h an T r e a t m e n t s 3a and 3b. The latter difference was m u c h smaller than the difference f r om t he c o n t r o l group. TABLE II Comparisons of average leaf length (cm) for each treatment at each time interval. Values underlined were not significantly different (P>0.05) based on Duncan's multiple range test. (See Table I for treatment designations.) Time (weeks)
Treatment 3c
2a
2b
2c
0
9.2
9.2
9.1
9.1
1 8.8
3b 8.7
3a 8.6
2
3a 11.4
2b 11.2
2c 11.0
3c 11.0
2a 10.9
1 10.8
3b 10.7
4 6 8 10 12
3a
3c
2c
2b
2a
3b
1
17.7 3c 26.4 3c 29.0 2c 33.2 2c 33.4
17.6 2b 24.3 2c 28.8 3c 32.7 3c 32.8
17.3 2c 23.9 2b 28.0 2b 31.1 2a 31.9
17.1 3a 22.9 2a 26.5 2a 30.6 2b 31.8
16.0 2a 21.7 3a 25.1 3b 28.7 3a
15.7 3b 21.3 3b 24.8 3a 28.2 3b
29.1
29.0
14.9 1 17.1 1 18.0 1 20.3 1 21.5
The addition of nitrogen p r o d u c e d the greatest growth response, b o t h in terms o f shoots/plant and average leaf length (Fig. 1A and 1B, Table II). The effect of increased p h o s p h o r u s c o n c e n t r a t i o n is n o t as large, b u t t h er e is nevertheless a small i m p r o v e m e n t in grow t h when the plants were fertilized with 1 4 : 1 4 : 1 4 O s m o c o t e over equi-nitrogen e n r i c h m e n t with 1 8 : 6 : 1 2 Os mocot e. The e f f e c t of increased phosphorus c o n c e n t r a t i o n was m o s t obvious at the low n i t r o g e n - e n r i c h m e n t levels, but disappeared at the highest nitrogen e n r i c h m e n t level, in which case t here was no significant difference in leaf length f r o m t he plants receiving t he p h o s p h o r u s - p o o r formulation. The average leaf length f o r each t r e a t m e n t was p l o t t e d against nitrogen applied at th e start o f t he e x p e r i m e n t (Fig. 1A). The t r e a t m e n t s receiving 1 4 : 1 4 : 1 4 O s m o c o t e ( T r e a t m e n t s 2a, 2b and 2c) exhi bi t ed longer leaves than did those receiving 1 8 : 6 : 1 2 O s m o c o t e ( T r e a t m e n t s 3a, 3b and 3c) exc e p t at th e highest application rate. The m ean leaf length f o r each t r e a t m e n t
326 was also plotted against the amount of phosphorus applied at the start of the experiment (Fig. 1B). Leaf length increased with increasing application rate of phosphorus up to 16.7 g m -2. Clearly, at equal application rates of nitrogen, less growth occurred in the treatments receiving less phosphorus. DISCUSSION Addition of fertilizer to the substrate markedly stimulated growth of seedlings in the laboratory, both in terms o f increased leaf length and increased production of new shoots. This agrees with observations of enhanced growth of Z. marina in natural beds fertilized with commercial fertilizers (Orth, 1977). More recently, Orth and Moore (1982) have shown that nutrient addition enhances survival and growth of transplanted Z. marina plugs. Short (1981, 1983) found that leaf length of Z. marina from the Izembek Lagoon in Alaska increased with increasing sediment NH4+ pool. However, in contrast, he found that shoot density was negatively correlated with NHJ concentration. He hypothesized that nutrient additions may initially stimulate new shoot production, but in areas where high levels of nutrients are initially present increased leaf growth and self shading by the plants themselves ultimately control shoot density. With respect to increased leaf length, the nitrogen-rich phosphoruspoor formulation (18:6:12) produced less growth than the equal balance formulation (14:14:14). For both formulations, the highest concentrations produced greater growth than the other concentrations of the same formulation. Only the 50 g m -~ application rate of 18:6:12 formulation yielded growth in leaf length equal to that observed in plants receiving the 14:14:14 formulation. The production of multiple shoots/plant was pronounced in all fertilized groups. Only 4% of the control plants exhibited 3 shoots/plant, whereas more than 60% of all fertilized plants exhibited 3 or more shoots/ plant. With the 14:14:14 formulation 30--47% of the plants had 4 or more shoots/plant, the proportion increasing with increasing application rate. For the nitrogen-rich formulation, 21--28% of the plants possessed 4 or more shoots/plant, but there was no clear relationship to application rate. Although nitrogen has been considered the key limiting nutrient for seagrass growth (Short, 1983), phosphorus may, in some cases, become limiting. In our experiments, we found that at equal application rates of nitrogen, more growth occurred at the highest levels of added phosphorus. We suggest that under conditions of extreme nutrient enrichment with nitrogen, additional phosphorus may be needed for maximum growth. Our observations of increased growth of Z. marina seedlings with increased nutrient concentration differ from 2 recent seagrass bed enrichment studies. Bulthuis and Woelkerling (1981) applied separate concentrations of ammonium and phosphate to the sediment within a bed of Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Australia.
327
They found no direct effect on the growth or standing crop of this species during the experimental period. They suggested that the high ambient levels of sedimentary nutrients found there were adequate to sustain maximum growth. Harlin and Thorne-Miller (1981) showed only modest increases in growth of Z. marina and Ruppia maritima L. (sensu lato) after application of ammonium sulfate, sodium nitrate or potassium phosphate. However, the individual nutrients were released from clay pots into the water column and not the sediments. Because nutrient uptake through the root system is a major pathway for nutrient transfer in aquatic vascular plants (McRoy and Barsdate, 1970), it is possible that the nutrients were not available to the plants. Harlin and Thorne-Miller (1981) also reported the presence of large algal mats in their experiments which could have removed the nutrients before they could be assimilated by the seagrass. Results of the few nutrient enrichment studies conducted to date suggest possible interactions between sediment type, ambient nutrient concentrations, availability of sediment nutrients, single vs. combined nutrient additions and the role of trace nutrients. Future studies should concentrate on these interactions to provide meaningful insights into the growth dynamics of different seagrass species. ACKNOWLEDGMENTS This project was funded by the U.S. Environmental Protection Agency's Chesapeake Bay Program, Grant No. R805953. We thank R.E. Bendl, P. Gapsynski, and J. Hauer for their assistance in establishment and maintenance of the cultures and bi-weekly measurements of plants. Sediment nutrient analyses were performed by the VIMS Nutrient Analysis Group under the direction of Dr. B. Neflson and Ms. B. Salley. We especially appreciate the helpful review of Dr. Carl Hershner. This paper is contribution No. 1206 from the Virginia Institute of Marine Science, Gloucester Point, VA 23062, U.S.A.
REFERENCES
Bulthuis, D.A. and Woelkerling, W.J., 1981. Effects of in situ nitrogen and phosphorus enrichment of the sediments of the seagrass Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port, Victoria, Australia. J. Exp. Mar. Biol. Ecol., 53: 193--207. Harlin, M.M. and Thorne-Miller, B., 1981. Nutrient enrichment of seagrass beds in a Rhode Island coastal lagoon. Mar. Biol., 65: 221--229. Kopp, J.F. and McKee, G.D., 1979. Methods for chemical analysis of water and waste. U.S. Environmental Protection Agency. McRoy, C.P. and Barsdate, R.J., 1970. Phosphate absorption in eelgrass. Limnol. Oceanogr., 15: 6--13.
328 Orth, R.J., 1977. Effect of nutrient enrichment on growth of the eelgrass Zostera marina in the Chesapeake Bay, Virginia, U.S.A. Mar. Biol., 44: 187--194. Orth, R.J. and Moore, K.A., 1982. The effect of fertilizers on transplanted eelgrass, Zostera marina L. in the Chesapeake Bay. In: F.J. Webb (Editor), Proc. 9th Annu. Conf. Wetlands Restoration Creation. Hillsborough Community College, Tampa, Florida, pp. 104--131. Orth, R.J. and Moore, K.A., 1983. Seed germination and seedling growth of Z. marina L. (eelgrass) in the Chesapeake Bay. Aquat. Bot., 15: 117--131. Short, F.T., 1981. Nitrogen resource analysis and modeling of an eelgrass (Zostera marina L.) meadow in Izembek Lagoon, Alaska. Ph.D. Dissertation, University of Alaska, Fairbanks, Alaska, 173 pp. Short, F.T., 1983. The response of interstitial ammonium in eelgrass (Zostera marina L.) beds to environmental perturbations. J. Exp. Mar. Biol. Ecol., 68: 195--208.
321
GROWTH OF Z O S T E R A M A R I N A L. SEEDLINGS UNDER LABORATORY CONDITIONS OF NUTRIENT ENRICHMENT
MORRIS H. ROBERTS, JR., ROBERT J. ORTH and KENNETH A. MOORE
Virginia Institute of Marine Science and School of Marine Science, College of William and Mary, Gloucester Point, VA 23062 (U.S.A.) (Accepted for publication 29 August 1984)
ABSTRACT Roberts, M.H., Jr., Orth, R.J. and Moore, K.A., 1984. Growth of Zostera marina L. seedlings under laboratory conditions of nutrient enrichment. Aquat. Bot., 20: 321-328,. The effect of increased nutrients on growth of Zostera marina L. seedlings was tested in the laboratory by adding 2 different formulations (18:6:12 and 14:14:14 Nitrogen: Phosphorus:Potassium (N:P:K), respectively) of a slow release fertilizer, Osmocote ®. Three different application rates were used with the 2 formulations by placing appropriate amounts in p e a t pots containing 1 seedling each. The addition of fertilizer to the substrate markedly stimulated the growth of seedlings. Nutrient enrichment p r o m o t e d growth both in terms of increased leaf length and vegetative production of shoots. The nitrogen rich formulation (18:6:12) p r o d u c e d less growth than the equal balance formulation (14:14:14). F o r both formulations, the highest concentrations produced greater growth than other concentrations of the same formulation. A t equal application rates with respect to nitrogen, less growth occurred in the treatments receiving less phosphorus. Results of this experiment corroborate results from previous work suggesting that addition of nutrients to the sediment can stimulate Z. marina growth.
INTRODUCTION
Several recent studies have focused on the role of nutrient limitation in regulating the development of seagrass beds by experimentally manipulating the nutrient concentration in the sediment or the water column or by monitoring the ambient concentration of nutrients during shoot and leaf growth (Orth, 1977; Bulthuis and Woelkerling, 1981; Harlin and ThorneMiller, 1981; Orth and Moore, 1982; Short, 1983). Results from some of these studies have shown increases in growth, expressed as changes in above ground standing crop, leaf length, shoot density, or below ground standing crop, following nutrient additions (Orth, 1977; Harlin and Thorne-Miller, 1981; Orth and Moore, 1982), while in one study, no increase in growth was observed (Bulthuis and Woelkering, 1981}. These studies differed in
0304-3'770/84/$03.00
© 1984 Elsevier Science Publishers B.V.
322
types and amounts of nutrient input as well as m o d e of nutrient application. Therefore growth could have been affected either by different ambient nutrient concentrations, predominant m o d e of nutrient uptake, or competition for increased nutrients with p h y t o p l a n k t o n and epiphytic algae. The objective o f the experiment described here was to evaluate the effect of 2 fertilizer formulations at several concentrations on growth of Zostera marina L. seedlings under laboratory conditions. Laboratory culture of Z. marina is essential for some types of experimental studies of life-history, physiology, growth, and reproduction and potentially has value for production of plants to reestablish grass beds in denuded areas. In both cases, it is desirable to k n o w h o w fertilization affects growth under controlled conditions. M A T E R I A L S AND METHODS
Selected seedlings consisting of 1 s h o o t each were collected manually by divers on 11 March 1980 from a seagrass bed adjacent to Guinea Marsh, York River, Virginia. In this region, seedlings are available in different stages of development from O c t o b e r to May (Orth and Moore, 1983). Sediment for the experiments was collected from the same site as the seedlings and placed in 5 × 5 ~ m square peat pots supported in plastic greenhouse trays. The median grain size of the sediment was 2.6 Phi (sorting coefficient = 0.94). A sediment core was removed from selected pots, centrifuged in a Gelman filter centrifuge tube (0.45 gm glass fiber filter) for 10 min, and the filtrate analyzed for NH~, NO2, NO3-and PO4-3 with a Technicon Autoanalyzer II (Kopp and McKee, 1979). Seedlings were planted in the peat pots and held in flowing estuarine water for 2 weeks. Seven groups of 52 seedlings were then selected on 20 March 1980 for the experiment. The fertilizers selected for the experiment were 2 formulations of a 3-month timed-release fertilizer, Osmocote@, one with a Nitrogen:Phosphorus:Potassium (N:P:K) ratio of 18:6:12, the other 14:14:14. Nitrogen was in the form of ammonium and nitrate, phosphorus as phosphoric acid (P~Os) and potassium as soluble potash (K~O). Release rates of N, P and K remain relatively constant during the 3-month period according to the manufacturer. Each fertilizer was tamped into the sediment surface at 3 application rates (g m -s) (Table I). Application rates for each formulation of Osmocote were calculated to provide the same 3 amounts of total nitrogen; 12.5, 25 and 50 g m -2. This resulted in different levels of phosphorus in equi-nitrogen treatments. Another group of plants which received no fertilizer served as the control. Plants were grown in a holding tank receiving 10 gm filtered estuarine water. Photosynthetic active radiation (400--700 nm) at the water surface, as measured with a LI-COR 185B photometer/radiometer, was approximately 57% of the incident level, and 38% of the incident level at the surface of the sediment.
323 The day following nutrient addition, the n u m b e r of leaf blades/plant, n u m b e r of plant shoots, and length of longest blade were determined and recorded. At 2-week intervals thereafter, the plants were wiped gently with fingers to remove periphyton. Number of shoots, leaf blades/shoot, and length of longest blade on oldest s h o o t were determined. The seventh and final measurement was made on 13 June 1980. TABLE I Summary of fertilizer application rates. Sediment nitrogen and phosphorus concentrations were calculated from application rate and concentration in fertilizer assuming total availability of both nutrients Treatment
Fertilizer Application rate
Nitrogen (g m -L)
Phosphorus (g m -2)
0 12.5 25 50 12.5 25 50
0 12.5 25 50 4.2 8.3 16.7
(g m -s) (g/peat pot)
1 2a 2b 2c 3a 3b 3c
None 14:14:14 14:14:14 14:14:14 18:6:12 18:6:12 18:6:12
0 89.3 178.6 357.1 69.4 138.9 277.8
0 0.23 0.46 0.91 0.18 0.35 0.71
Leaf blade lengths for each t r e a t m e n t were compared for each measurem e n t interval by one-way analysis of variance and Duncan's multiple range test. Number of leaf blades/plant and n u m b e r of shoots were analyzed by non-parametric methods. RESULTS During the acclimation period and the first sampling interval, water temperature averaged 10.3°C and 10.8°C respectively while salinity declined from 17.8 to 15.7%o. Mean temperature increased in each succeeding sampling period to 27.3°C during the final interval. Salinity declined to 14.9%o during the third sampling interval, and then increased to 17.9%o during the final period. Throughout the study, dissolved o x y g e n during the midday period usually exceeded saturation. The initial concentrations of a m m o n i u m , nitrite, nitrate and phosphorus in the sediment pore water were 204 + 46, 0.83 + 0.49, 1.97 -+ 0.59 and 41.1 -~ 27.4 gM, respectively. Initially, each seedling consisted of a single shoot. At the end of the study, the mean numbers of shoots/plant were 1.2 in the control and from 2.8 to 3.5 in the fertilized groups (Fig. 1A). No control plants developed more than 3 shoots, whereas after 12 weeks, 30--47% of the plants fertilized with 14:14:14 and 21--28% of the plants fertilized with 18:6:12 had 4 or more shoots.
324
The average length of the leaf length) was 8.6--9.2 cm throughout the study period significantly different among
longest leaf (hereafter referred to as average at the start of the experiment and increased (Table II). The average leaf lengths were not the treatments until after 4 weeks growth,
l-Z _J O. I-0 O "1" Z L~
'-
X Y / / / , ~ o ~ ~'
o/////,Z'°~ 0
I0
20
30
40
,~
50
PHOSPHORUS (g/m 2 )
B 4O
=
30 ~o
~
X y / / / / ~ o ~$~ o//,////~
,oI
/o
>~
0
I0
20
30
40
50
PHOSPHORUS ( g / m 2)
Fig. 1. Growth o f Zostera marina L. after 12 weeks as a function of phosphorus and nitrogen enrichment. (A) Expressed as mean shoots/plant; (B) expressed as average leaf length (cm).
325 b u t t h e r e a f t e r , 3--4 groups of t r e a t m ent s were definable by Duncan's multiple range test (P<:0.05). A f t er 12 weeks, all experi m ent al t r e a t m e n t s were significantly di f f er e nt f r o m t he c o n t r o l group and assorted i n t o 2 groups: T r e a t m e n t s 2a, 2b, 2c and 3c exhibited greater average leaf length t h an T r e a t m e n t s 3a and 3b. The latter difference was m u c h smaller than the difference f r om t he c o n t r o l group. TABLE II Comparisons of average leaf length (cm) for each treatment at each time interval. Values underlined were not significantly different (P>0.05) based on Duncan's multiple range test. (See Table I for treatment designations.) Time (weeks)
Treatment 3c
2a
2b
2c
0
9.2
9.2
9.1
9.1
1 8.8
3b 8.7
3a 8.6
2
3a 11.4
2b 11.2
2c 11.0
3c 11.0
2a 10.9
1 10.8
3b 10.7
4 6 8 10 12
3a
3c
2c
2b
2a
3b
1
17.7 3c 26.4 3c 29.0 2c 33.2 2c 33.4
17.6 2b 24.3 2c 28.8 3c 32.7 3c 32.8
17.3 2c 23.9 2b 28.0 2b 31.1 2a 31.9
17.1 3a 22.9 2a 26.5 2a 30.6 2b 31.8
16.0 2a 21.7 3a 25.1 3b 28.7 3a
15.7 3b 21.3 3b 24.8 3a 28.2 3b
29.1
29.0
14.9 1 17.1 1 18.0 1 20.3 1 21.5
The addition of nitrogen p r o d u c e d the greatest growth response, b o t h in terms o f shoots/plant and average leaf length (Fig. 1A and 1B, Table II). The effect of increased p h o s p h o r u s c o n c e n t r a t i o n is n o t as large, b u t t h er e is nevertheless a small i m p r o v e m e n t in grow t h when the plants were fertilized with 1 4 : 1 4 : 1 4 O s m o c o t e over equi-nitrogen e n r i c h m e n t with 1 8 : 6 : 1 2 Os mocot e. The e f f e c t of increased phosphorus c o n c e n t r a t i o n was m o s t obvious at the low n i t r o g e n - e n r i c h m e n t levels, but disappeared at the highest nitrogen e n r i c h m e n t level, in which case t here was no significant difference in leaf length f r o m t he plants receiving t he p h o s p h o r u s - p o o r formulation. The average leaf length f o r each t r e a t m e n t was p l o t t e d against nitrogen applied at th e start o f t he e x p e r i m e n t (Fig. 1A). The t r e a t m e n t s receiving 1 4 : 1 4 : 1 4 O s m o c o t e ( T r e a t m e n t s 2a, 2b and 2c) exhi bi t ed longer leaves than did those receiving 1 8 : 6 : 1 2 O s m o c o t e ( T r e a t m e n t s 3a, 3b and 3c) exc e p t at th e highest application rate. The m ean leaf length f o r each t r e a t m e n t
326 was also plotted against the amount of phosphorus applied at the start of the experiment (Fig. 1B). Leaf length increased with increasing application rate of phosphorus up to 16.7 g m -2. Clearly, at equal application rates of nitrogen, less growth occurred in the treatments receiving less phosphorus. DISCUSSION Addition of fertilizer to the substrate markedly stimulated growth of seedlings in the laboratory, both in terms o f increased leaf length and increased production of new shoots. This agrees with observations of enhanced growth of Z. marina in natural beds fertilized with commercial fertilizers (Orth, 1977). More recently, Orth and Moore (1982) have shown that nutrient addition enhances survival and growth of transplanted Z. marina plugs. Short (1981, 1983) found that leaf length of Z. marina from the Izembek Lagoon in Alaska increased with increasing sediment NH4+ pool. However, in contrast, he found that shoot density was negatively correlated with NHJ concentration. He hypothesized that nutrient additions may initially stimulate new shoot production, but in areas where high levels of nutrients are initially present increased leaf growth and self shading by the plants themselves ultimately control shoot density. With respect to increased leaf length, the nitrogen-rich phosphoruspoor formulation (18:6:12) produced less growth than the equal balance formulation (14:14:14). For both formulations, the highest concentrations produced greater growth than the other concentrations of the same formulation. Only the 50 g m -~ application rate of 18:6:12 formulation yielded growth in leaf length equal to that observed in plants receiving the 14:14:14 formulation. The production of multiple shoots/plant was pronounced in all fertilized groups. Only 4% of the control plants exhibited 3 shoots/plant, whereas more than 60% of all fertilized plants exhibited 3 or more shoots/ plant. With the 14:14:14 formulation 30--47% of the plants had 4 or more shoots/plant, the proportion increasing with increasing application rate. For the nitrogen-rich formulation, 21--28% of the plants possessed 4 or more shoots/plant, but there was no clear relationship to application rate. Although nitrogen has been considered the key limiting nutrient for seagrass growth (Short, 1983), phosphorus may, in some cases, become limiting. In our experiments, we found that at equal application rates of nitrogen, more growth occurred at the highest levels of added phosphorus. We suggest that under conditions of extreme nutrient enrichment with nitrogen, additional phosphorus may be needed for maximum growth. Our observations of increased growth of Z. marina seedlings with increased nutrient concentration differ from 2 recent seagrass bed enrichment studies. Bulthuis and Woelkerling (1981) applied separate concentrations of ammonium and phosphate to the sediment within a bed of Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Australia.
327
They found no direct effect on the growth or standing crop of this species during the experimental period. They suggested that the high ambient levels of sedimentary nutrients found there were adequate to sustain maximum growth. Harlin and Thorne-Miller (1981) showed only modest increases in growth of Z. marina and Ruppia maritima L. (sensu lato) after application of ammonium sulfate, sodium nitrate or potassium phosphate. However, the individual nutrients were released from clay pots into the water column and not the sediments. Because nutrient uptake through the root system is a major pathway for nutrient transfer in aquatic vascular plants (McRoy and Barsdate, 1970), it is possible that the nutrients were not available to the plants. Harlin and Thorne-Miller (1981) also reported the presence of large algal mats in their experiments which could have removed the nutrients before they could be assimilated by the seagrass. Results of the few nutrient enrichment studies conducted to date suggest possible interactions between sediment type, ambient nutrient concentrations, availability of sediment nutrients, single vs. combined nutrient additions and the role of trace nutrients. Future studies should concentrate on these interactions to provide meaningful insights into the growth dynamics of different seagrass species. ACKNOWLEDGMENTS This project was funded by the U.S. Environmental Protection Agency's Chesapeake Bay Program, Grant No. R805953. We thank R.E. Bendl, P. Gapsynski, and J. Hauer for their assistance in establishment and maintenance of the cultures and bi-weekly measurements of plants. Sediment nutrient analyses were performed by the VIMS Nutrient Analysis Group under the direction of Dr. B. Neflson and Ms. B. Salley. We especially appreciate the helpful review of Dr. Carl Hershner. This paper is contribution No. 1206 from the Virginia Institute of Marine Science, Gloucester Point, VA 23062, U.S.A.
REFERENCES
Bulthuis, D.A. and Woelkerling, W.J., 1981. Effects of in situ nitrogen and phosphorus enrichment of the sediments of the seagrass Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port, Victoria, Australia. J. Exp. Mar. Biol. Ecol., 53: 193--207. Harlin, M.M. and Thorne-Miller, B., 1981. Nutrient enrichment of seagrass beds in a Rhode Island coastal lagoon. Mar. Biol., 65: 221--229. Kopp, J.F. and McKee, G.D., 1979. Methods for chemical analysis of water and waste. U.S. Environmental Protection Agency. McRoy, C.P. and Barsdate, R.J., 1970. Phosphate absorption in eelgrass. Limnol. Oceanogr., 15: 6--13.
328 Orth, R.J., 1977. Effect of nutrient enrichment on growth of the eelgrass Zostera marina in the Chesapeake Bay, Virginia, U.S.A. Mar. Biol., 44: 187--194. Orth, R.J. and Moore, K.A., 1982. The effect of fertilizers on transplanted eelgrass, Zostera marina L. in the Chesapeake Bay. In: F.J. Webb (Editor), Proc. 9th Annu. Conf. Wetlands Restoration Creation. Hillsborough Community College, Tampa, Florida, pp. 104--131. Orth, R.J. and Moore, K.A., 1983. Seed germination and seedling growth of Z. marina L. (eelgrass) in the Chesapeake Bay. Aquat. Bot., 15: 117--131. Short, F.T., 1981. Nitrogen resource analysis and modeling of an eelgrass (Zostera marina L.) meadow in Izembek Lagoon, Alaska. Ph.D. Dissertation, University of Alaska, Fairbanks, Alaska, 173 pp. Short, F.T., 1983. The response of interstitial ammonium in eelgrass (Zostera marina L.) beds to environmental perturbations. J. Exp. Mar. Biol. Ecol., 68: 195--208.