The use of fertilizer to enhance growth of transplanted seagrasses Zostera marina L. and Halodule wrightii Aschers

d. Exp. Mar. Biol. Ecol., 163 (1992) 141-161 © 1992 Elsevier Science Publishers BV. All rights reserved 0022-0981,/92/$05.00

141

JEMBE 01816

The use of fertilizer to enhance growth of transplanted seagrasses Zostera marina L. and Halodule wrightii Aschers W. Judson Kenworthy and Mark S. Fonseca Beaufort Laboratory NOAA. NMFS. Beaufort, North Carolina, USA

(Received 7 June 1990; revision received 28 February 1992; accepted 19 March t992)

Abstract: TiLeeffect of two slow release fertilizers on the survival and growth of transplants of~.wo seagrasses, Zostera marina L. and Halodule wrightii Aschers was examined. The two fertilizers, an unbalanced nitrogen, phosphorus, and potassium formulation (18-0-0) and a balanced (14-14-14) formulation, were applied to bare root tronsplant units (PU) of each seagrass at three doses, 10, 90, and 170 g per PU. °urvival and growth of the tra~.splants, nutrient release from the fertilizers, and several environmental characteristics of the study sites were examined. Nitrogen enrichment enhanced vegetative reproduction, the rate of area covered and leaf growth in a fall transplant of Z. marina, but only moderately stimulated growth in the spring, confirming that nitrogen may limit the rate of development of newly established populations of Z. marina. There was no effect of nitrogen on survival of Z. marina nor on the growth and survival ofH. wrightii transplants. No phosphorus was released from the balanced fertilizers in the spring and fall experiments and no nitrogen was released from the unbalanced formulation in the spring Z. marina experiment. Therefore, it was not possible to examine the effect of nitrogen and phosphorus interactions on plant growth and vegetative reproduction. Nitrogen enrichment may be used to stimulate shoot growth of Z. marina transplants, and we suggest alternative procedures of fertilizer application to overcome problems of nutrient release. Key words: Fertilizer; Growth; Hah~dule wrightii; N; Nutrient; P; Seagrass; Transplant; Zostera marina

INTRODUCTION

Seagrasses are marine vascular plants that obtain macronutrients from both the sediments and the water column (Short, 1987). Generally, the pore water of clastic sediments have concentrations of dissolved inorganic nitrogen and phosphorus that are much greater than in the overlying water column (Bulthuis & Woelkerling, 1981; Kenworthy et al., 1982; Short, 1987). These dissolved nutrients are supplemented by large reservoirs of exchangeable elements and other nutrients regenerated from organic matter in the sediments (Patriquin, 1972; Iizumi & Hattori, 1982; Kenworthy et al., 1982; Dennison et al., 1987). Physiological studies have conclusively demonstrated that nutrients derived from root uptake are readily transported throughout the plant (McRoy et al., 1972; Short & McRoy, 1984). Consequently, seagrasses appear to derive most of their mineral nutrition from the sediments. Correspondence address: W.J. Kenworth3, Beaufort Laboratory NOAA, NMFS, 101 Pivers Island Road, Beaufort, NC 28516, USA.

142

W.J. KENWORTHY AND M.S. FONSECA

Despite large reservoirs in the sediment, under certain conditions seagrasses may be nutrient limited (Orth, 1977; Bulthuis & Woelkerling, 1981; lizumi et al., 1982; Short, 1983). Sediment pools may become nitrogen depleted d~ring periods of high plant production, especially when scagrasses are growing in substrata with very little organic matter (Short, 1983). Recent evidence on seagrasses growing in carbonate sediments indicates that growth is limited by phosphorus rather than nitrogen (Short et al., 1985; Powell et al., 1989). At least one seagrass, Zostera marina, is able to reduce its dependence on external nitrogen sources by reclaiming nitrogen from organic compounds which are translocated from senescing tissues to storage organs (Borum et al., 1989). However, the extent to which this mechanism compensates for nutrient limitation in newly established plants is not yet known. The heightened interest in restoring seagrass beds and mitigating for the loss of natural resources associated with the meadows has encouraged us to examine ways of improving transplanting methodology (Fonseca et al., 1987). A proposed method of enhancing survival and growth of transplants is to supplement their nutrient resources with fertilizers in order to minimize the potential for nutrient limitation during the crucial early stage of establishment. Transplanting constitutes a perturbation to the rhizosphere of a seagrass and may retard the normal cycling and regeneration of nutrients immediately following planting. This may be especially true for bare-root plantings where none of the original sediment, organic matter and rhizosphere organisms remain, leading us to hypothesize that fertilizer supplements would increase growth and improve the survival of barc-root transplants. Experimental fertilization of sediments in naturally occurring seagrass beds have consisted of encapsulated slow release fertilizers (Pulich, 1985; Short et al., 1990; Williams, 1990), granulated formulations either deployed directly into the sediments (Orth, 1977) or dispensed in Kleenex tissue (Bulthuis & Woelkerling, 1981), and scdimcnt diffusers (Dennison et al., 1987). Nutrients also have been dispensed in the water column using porous clay pots (Harlin & Thorne-Miller, 1981). In these experinlents seagrass response to nutrient supplements has been variable, however, the one time addition of a balanced (NPK = 14~o-14~o-14~o) slow release fertilizer to transplanted plugs of Z. marina stimulated additional growth over controts (Orth & Moore, 1982). Likewise, Roberts et al. (1984) demonstrated that addition~ of balanced and unbalanced formulations of slow release fertilizer stimulated growth of transplanted Z. marina seedlings. Along the mid-Atlantic coast of the United States two species ot' seagrass co-occur in a narrow band extending from Pamlico Sound to Cape Fear, NC (Thayer el al.~ 1984). In this region there are approximately 200000 acres of seagrass beds consisting of either Z. maripla L. and/or a common subtropical speci,:s Halodule wrightii Aschers. The seasonal abundance of these ~,wo species is closely related to their thermal tolerances. Z. marina is more abundant during the relatively cooler water temperatures between October and May while H. ~,vrightii is most abundant in the warmer periods between June and October (Kenworth.J, 1981). Both seagrasses are suitable for trans-

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144

W.J. KENWORTHY AND M.S. FONSECA

planting, but transplants should be initiated during the time of the year closely corresponding to each species optimum growing period. The general objectives of this study were to examine whether slow-release fertilizers enhance the survival and growth of these two seagrasses. We report the results of a set of field experiments specifically designed to examine the growth response of bare-root transplants (Fonseca et al., 1982) of Z. marina and H. wrightii exposed to two types of encapsulated slow-release fertilizers applied at three dose levels. To expand our scope of inference we selected three widely separated study sites. Additionally, we investigated nutrient release characteristics of the fertilizers to verify a direct relationship between the release of specific macro-nutrients, nitrogen and phosphorus, and the growth response of the transplants.

M A T E R I A L S AND M E T H O D S

STUDY SITES

Three study sites were selected in Back Sound, Carteret County, NC (Lat. 35 ° 41' N, Long. 75 ° 31' W) based on their proximity to existing seagrass beds and the availability of unvegetated habitat. A second criteria for these sites required that the sediment organic matter content not exceed 2 ~o. Under these conditions Kenworth~ et al. (1982) demonstrated that total, exchangeable, and pore water nitrogen concentrations were significantly lower than in vegetated sediments with organic matter exceeding 23/o. Therefore, these criteria minimized exogenous nutrient inputs and approximated conditions for sandy, nutrient poor dredged material. EXPERIMENTAL DESIGN

Three experiments were conducted between October 1984 and May 1985: (1) A fall Z. marina planting (October 24, 1984); (2)a spring Z. marina planting (March 13, 1985); and (3) a late spring H. wrightii planting (May 9, 1985). In each experiment we

used two different (balanced or unbalanced) encapsulated, controlled-release fertilizers (Osmocote, Sierra Chemical Company, Milpitas, California), each added in three dosage levels (Fig. 1). Each experiment consisted of six fertilizer treatments and a control replicated at each study site (Fig. 1). One fertilizer was balanced and contained 14°,o nitrogen as N H 4 N O 3 and 14j°~, phosphorus as P205. The second was an unbalanced form containing 18 ~/0 nitrogen as NH4SO4 only. Within each site, each treatment consisted of eine planting units (PU) planted on 1-m centers in a 2 × 2 m plot. Each PU consisted of 10-15 seagrass shoots attached to a 15-cm metal staple with a 7.6-cm twist tie (Fonseca et al., 1982) (Fig. 2). We placed pre-weighed amounts of each dose level (10, 90, or 170 g) of fertilizer in 1.1-mm mesh plastic bags, stapled the bags closed, then buried them in the sediment immediately beneath each PU at the time of planting. Controls consisted of 9 PU without added

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fertilizer. In each experiment the individual treatments were randomly assigned to locations at each study site. At each site 9 fertilizer bags containing 90 g of each type were prepared and two additional 2 x 2 m treatments were established. Each time we monitored seagrass growth during the experiments we recovered a randomly selected bag of each type at each location to measure the release of macronutrients. The fertilizer was rinsed free of sediment, freeze-dried and assayed for nitrogen and phosphorus. Total nitrogen was determined with a Carlo-Erba Model 1106 elemental analyzer standardized with acetanilide. Phosphorus was extracted by digestion with potassium persulphate and inorganic phosphorus was determined colorimetrically (Koroleff, 1983). Analytical efficiencie~ for extraction of phosphorus and determination of nitrogen were calculated fi'om analyses of National Bureau of Standards Reference Material Orchard Leaves. ENVIRONMENTAL CHARACTERISTICS

Sediments Prior to each of the experiments, we obtained surface sediment samples from all treatments at each site to characterize the particle size and organic matter content. Samples taken from the top cm were dried to a constant weight at 90 °C, then two subsamples were ashed in a muffle furnace at 500 °C to determine organic matter content. Particle size was de, ermined on the remainder of each sample by dry sieving. At each site, one corner stake per treatment was placed so that the top of the stake was 50 cm above the sediment surt'ace. To assess the erosion or accretion of sediment we measured the distance between the top of these stakes and the sediment surface on two randomly selected days each month. These distance were divided by the number of days since the last measurenaent to estimate the sediment flux rate (SFR). Sediment erosion was indicated by a positive flux and accretion by a negative flux. The natural log of the percent survival of PU was regressed on SFR to examine the potential influence of sediment stability on planting survival.

Water transmissivity, temperature and saliniO, The transmission of light through the water column was recorded at each site on two randomly selected high and low tides a month using a Sea Tech 25 cm Transmissometer, Average annual attenuation coefficients ( - k] were computed for each site, tide and experiment using the equation:

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= L, e

k:

(1)

where .-= average water depth relative to mean sea level; Iz = light at depth .: in the water; I,, = incident light at a reference depth, and - k = average annual attenuation coefficient. At the same high and low tides the total water depth was measured and

USE OF FERTILIZER IN TRANSPLANTED SEAGRASSES

147

the temperature and salinity were obtained with a glass thermometer and refractometer, respectively. Monthly averages of each parameter were compiled. Water currents

Water current velocities were measured at each site during a full moon tidal cycle to estimate regular, maximum velocities. Two propeller type flowmeters and an electromagnetic current meter were read simultaneously every hour during an entire tidal cycle. The maximum velocities over the tidal cycles were used to classify the hydrodynamic conditions of the sites (Fonseca et al., 1983). Plant growth parameters

About every 5-7 wk during each experiment and beginning with the initial time of planting we obtained the following plant growth parameters: (1) survival of PU; (2) the average number of shoots per PU; and (3) the area covered per PU. Survival census included all PU per treatment at each sampling time. The total number of shoots per PU was estimated from three randomly selected PU counted in each treatme~a at ee:ch site. The area covered by each of these PU was determined by placing a 0.25-m 2 quadrat subdivided into 5 x 5 cm grids over each individual PU. The number of squares completely filled by seagrass was recorded. Partially filled squares on the edge of the PU were recorded as half squares. All full and half squares were summed to give the area covered t~y the PU. Data for the number of shoots per PU and area covered per PU were combined to estimate the density of shoots per square meter. The shoot number and coverage data were In-transformed and rcgressed on time. Analysis of covariance (ANCOVA) was used to compare the slopes of these regressions among the seven treatments within each experiment (fall Z. marina, spring Z. marina and late spring H. wrightii) to determine if fertilizer type or dosage affected the rate of increase in the number of shoots and the area covered by the PU. We estimated net leaf production for the fall Z. marina experiment using a modified leaf marking technique (Jacobs, 1979). Oo April 5, 1985, 10 shoots per treatment were tagged at each of the sites (total = 210). At the time of tagging the tip of the youngest (shortest) leaf was clipped diagonally. The length and width of the clipped leaf and the next oldest leaf were measured. Fourteen days later the tagged plants were recovered and the original weights of the clipped leaves and the next oldest leaf was estimated using a regression of leaf dry weight on surface area. The amount of plant material produced per shoot since tagging was obtained by summing the weight of the clipped leaf, the next oldest leaf and any new leaves and then subtracting the original estimated weights. The difference was divided by the number of days since tagging to get new growth per shoot per day (rag dry wt.shoot-~'day-I).

148

W.J. KENWORTHY A N D M.S. FONSECA RESULTS

ENVIRONMENTAL CHARACTERISTICS

Sediments Sediment characteristics were similar both within and among sites. Mean and median particle size for all three sites ranged from 0.17 to 0.34 mm. Organic matter ranged between 0.35 and 1.03~o of sediment dry weight and met our objective for planting in sediments with relatively low organic matter content. In the late spring H. wrightii experiment, the correlation between PU survival and sediment flux rate (SFR) was -0.69 suggesting that sediment instability may have been an important factor. Only 2-10~o of the variation in PU survival could be attributed to SFR in the fall and spring Z. marina experiments.

Water transmissivity, temperature and salinity Water depths varied among sites but were similar within sites. Average low tide depths at Shackleford Shoal, Dredge Island and Kirby-Smith Island were 39, 56, and 56 cm, respectively. Average high tide depths for the same three sites were 95, 99, and 131 cm. The light regime varied between high and low tides, seasons, and among sites (Table I). Water column transmissivity was greater at high tides than low tides and consistently increased from fall to late spring indicating a seasonal improvement in water transparency. Differences among sites were greatest at low tide; the Dredge

TABLE I

Average transmissometer attenuation coellicients (k) by site, experiment, and tide for transplants of Zostera marina and Hahn&de wrightii. Experiment

Low tide

High tide

Z. marina (Fall) Z. marital (Spring) H. wrightii (Late spring)

3.65 3.34 2.96

2.74 2.02 1.72

Dredge Island Z. marina (Fall) Z. marina (Spring) H. w~ehtii (Late spring)

3.61 2.39 2.15

2.89 2.13 1.88

Kirby-Smith Island Z, mar#m (Fall) Z. marina (Spring) H. wrL~htii (Late spring)

4.85 4,59 3.86

2.65 2.65 2.48

Shacklefi~rd Shoal

USE OF FERTILIZER IN TRANSPLANTED SEAGRASSES

149

Island site the clearest, Shackleford Shoal intermediate, and Kirby-Smith Island the most turbid. We computed the relative percent of incident light energy reaching the bottom from the k values and water depth measurements. Light reaching the plants ranged from 12 to 199o in the fall Z. marina experiment, from 20 to 25% in the spring Z. marina experiment, and from 26 to 359/0 in the late spring H. wrightii experiment. Water temperatures were within normal limits for the area, ranging from 6 °C in February to 30.5 °C in July (Fig. 2). Salinity increased steadily from 30 to 35 ppt until the duration of the experiments (Fig. 2). Water currents

Water velocities were low at all sites (less than 50 c m ' s -i) (Fonseca et al., 1983). Shackleford Shoal had a peak flow of 40 cm.s -i, Kirby-Smith Island 27.8 cm.s - 1 and the Dredge Island 13.5 cm.s-l. Velocities at Shackleford Shoal were high enough to regularly move sediment. Of the three sites the Dredge Island was the most exposed to wind fetch, Shackleford Shoal was intermediate and Kirby-Smith Island was the most protected. Fertilizer release

During the fall Z. marina experiment, nitrogen was released linearly for 286 days (Fig. 3a). The balanced fertilizer released app~oximat ~ly 0.044 % of the original amount of nitrogen per day yielding 4.3, 38.5, and 72.9mg N.bag-l.day -1 for the 10, 90, and 170 g dose levels, respectively. The unbalanced fertilizer released 0.058~o of the original amount of nitrogen per day yielding 5.2, 47.3 and 89.4 mg N.bag-l.day- t for the three dose levels, respectively. During the spring Z. marina experiment, the balanced fertilizer released 0.068~o of the original nitrogen per day yielding 6.8, 81.1 and 115.5 mg N.bay-t'day -~ for the 10, 90 and 170 g dose levels, respectively (Fig. 4a). The unbalanced fertilizer did not release nitrogen in the spring Z. marina experiment. During the late spring H. wrightii experiments, the balanced fertilizer released 0.088 ~o of the original nitrogen per day yielding 8.8, 79.6 and 150.3 mg N.bag -~'day -~ for the 10, 90 and 170 g dose levels, respectively (Fig. 4a). The unbalanced fertilizer released 0.113~o of the original nitrogen per day yielding 11.3, 101.4 and 191.5rag N.bag -1"day-1 for the 10, 90 and 170 g dose levels, respectively. Contrary to expectations, none of the balanced fertilizers released phosphorus (Figs. 3b and 4b). During the fall Z. marina experiment the phosphorus content of the fertilizer began to decline after 150 days but was still higher than initial values after 286 days.

150

W.J. KENWORTHY AND M.S. FONSECA

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Fig. 3, (A) Percent qitrogen remaining in fertilizer during the f:dl Z, marital e.xl~eriment. (B)Percent phosphorus remaining in fertilizer during the fall Z. marital experiment.

PLANT PARAMETERS

Planting unit survival At day 243 survival of PU in the fall Z. marina fertilizer treatments ranged from 11 to 1000o (mean 687o). Since survival in the controls ranged from 33 to 1000,0 (mean 70 ° o), we cannot conclude that survival was a function of fertilizer treatment (Table II). Survival was generally higher in the spring Z. marina experiment however, this experiment did not last as long as the fall experiment. Again, there was no apparent relationship between survival and fertilizer treatmerts. Survival in the late spring H. w~Tghtfi planting was poor. By day 59 all PU were lost at Shackleford Shoal and the Kirby-Smith site lost 60 to 90°,~, of its PU. At day 93, PU were present in all treatments and the Dredge Island site, but there was no apparent relationship between survival and fertilizer treatment.

USE OF FERTILIZER IN TRANSPLANTED SEAGRASSES

151

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Shoot addition to planting units (population growth)

A significant difference (p < 0.05) was found between slopes of the regression lines for individual dose levels in the fall Z. marina experiment (Fig. 5a). All treatments with ba{anced fertilizer had regression with numerically higher slopes. Although the test

152

W.J. KENWORTHY AND M. S. FONSECA TABLE II

Percent planting unit survival by treatment, experiment, and site for transplants of Zostera marina and Halodule wrightii. B 10, 90, and 170 = balanced (14-14-14) nitrogen, phosphorus and potassium. U 10, 90 and 170 = unbalanced (18-0-0)fertilizer. Dose levels in g. Control---unfertilized plantings. Experiment

Treatment B 10

U 10

B90

U90

B 170

U 170

Control

Shackleford Shoal Fall Z. marina Day 59 Day 243

89 78

100 100

67 67

89 78

33 33

100 100

100 100

Spring Z. marina Day 56 Day 125

67 67

I00 100

56 56

100 100

67 67

44 44

100 100

Spring H. wrightii Day 59 Day 93

0 0

0 0

0 0

0 0

0 0

0 0

0 0

Dredge Island Fall Z. marina Day 59 Day 243

100 100

100 89

78 78

100 44

100 89

78 11

100 78

Spring Z. mcrina Day 56 Day 125

I00 I00

100 100

100 100

89 8'~

100 100

89 89

78 78

Spring H. ~trightii Day 59 Day 95

I1 I1

67 67

56 56

100 ll){I

56 44

44 44

44 44

Kirby-Smith Island Fall Z. marina Day 59 Day 243

(~7 56

89 33

100 56

100 78

78 78

78 56

100 33

Spring Z. marina Day 56 Day 125

78 67

100 100

33 33

67 67

33 33

100 100

67 67

Spring H. wrightii Day 59 Day 93

0 0

44 44

0 0

44 44

11 I1

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56 56

d e t e c t e d differences in d o s e levels, there w e r e n o c o n s i s t e n t p a t t e r n s relative t o the q u a n t i t a t i v e levels with either type o f fertilizer. I n d i v i d u a l d o s e levels h a d n o signific a n t effect on s h o o t a d d i t i o n in either the spring, Z . m a r i n a o r late spring H . wrightii e x p e r i m e n t s (Figs. 6 a a n d 7a). A l t h o u g h w e h a d p l a n n e d t o p o o l the d o s e levels a n d c o m p a r e t r e a t m e n t s b y fertilizer type o n l y ( b a l a n c e d vs u n b a l a n c e d vs c o n t r o l ) , t h e s e p l a n n e d c o m p a r i s o n s w e r e c o n f o u n d e d b e c a u s e the b a l a n c e d fertilizers did n o t r e l e a s e

USE OF FERTILIZER IN TRANSPLANTED SEAGRASSES

Fall R a n t i n g ~.

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Days Fig. 5. Plot of the regression lines for the natural ]o8 transformed number of shoots per PU of Z. marina by treatment (dose level) over time in the fall experiment. In A, B = balanced fertilizer, U = unbalanced fertilizer and dose levels of 10, 90 and 170 g; Con = control of unfertilized PU. In B, treatments were pooled so that Fert = balanced + unbalanced, and Con = control or unfertilized PU. These abbreviations also apply to Figs. 5-10.

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Days Fig. 6. Plot of the regression lines for the natural log transformed number of shoots per PU of Z. marina by treatment (dose level) over time in the spring experiment. Abbreviations: see Fig. 5.

154

W.J. K E N W C R T H Y A N D M.S. FONSECA

phosphorus and the original classification of fertilizer !ypes was invalidated. Because the unbalanced fertilizer in the spring H. wrightii experiment did not release nitrogen, our original comparisons for this experiment also were invalidated. In a post facto examination of the data, we rearranged our original comparisons into a set of unplanned comparisons. Because there was no phosphorus released in the fall Z. marina transplant we pooled the unbalanced and balanced treatments and compared them against the control with ANCOVA (Fig. 5b). There was a significant difference in the slopes (p < 0.05) suggesting that the release of nitrogen enhanced the shoot addition rate. Since the unbalanced fertilizer did not release nitrogen in the spring Z. marina experiment, we pooled the unbalanced and control data and compared the balanced to the pooled data with ANCOVA (Fig. 6b). The p value for this comparison, 0.1034, indicated that nitrogen fertilization improved the shoot addition rate in the spring, but not as much as in the fall planting. For the late spring H. wrightii experiment, we pooled the unbalanced and balanced fertilizer data and compared them to the control using ANCOVA (Fig. 7b). There was no significant difference between the slopes suggesting that nitrogen enrichment failed to stimulate the rate of shoot addition. A real coverage of planting units

There was an increase in cover in every treatment, including the controls (Figs. 810). The original planned comparisons for area covered were rearranged and a,aalyzed

Spring Planting

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U S E O F F E R T I L I Z E R IN T R A N S P L A N T E D S E A G R A S S E S

155

Fall Planting Z. marina ,.:. :) 0.50, I~

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Fig. 8. Plot of the regression lines for the natural log transformed area covered per PU of Z. marina by treatment (dose level) over time in the fall experiment. Abbreviations: see Fig. 5.

Spring Planting Z. marina 0.050

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156

W.J. KENWORTHY A N D M.S. FONSECA

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in the same manner as for analysis of shoot addition. The only significant difference occurred in the fall Z. marina experiment (p<0.05) where the pooled balanced and unbalanced fertilization data were compared to the control. This difference suggests that nitrogen enrichment resulted in a significant increase in area covered by the Z. marina transplants, but only in the fall. By comparison, the fall transplants covered a much greater area than the spring plantings for the same period of time. This difference was evident even in the controls. During the first 148 days the fidl transplants covered an average of 10 cmZ.d-t while during the 125 day spring planting the average PU covered 1 cm2.d-~. The net result of this large difference in the rate of area covered was an enormous difference in the equivalent densities of shoots per square meter. After 148 days the shoot densities for the fall planting were 112.m -2 while after only 125 days in the spring shoot densities reached 1550.m -2, Net leaf production

Net leaf production for the fall Z. matqna experiment yielded results similar to the shoot addition data. Net production was higher in the balanced treatment than either the unbalanced or control (Fig. 11). Since the unbalanced fertilizer did not release phosphorus, we pooled the balanced and unbalanced treatments and compared this to the control with a t-test. There was a significant difference ( p < 0.05) between the treatments indicating that nitrogen enrichment enhanced net leaf production.

U S E O F F E R T I L I Z E R IN T R A N S P L A N T E D S E A G R A S S E S

157

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FERTILIZER TREATMENT Fig. I1. Net leaf production ( g d w . s h o o t - l . d a y - t ) of a fall transplant of Z. marina by ferti!izer treatment. B = balanced, U = unbalanced and C = control. Dose levels are 10, 90 and 170 g.

DISCUSSION

Our results suggest that nitrogen supplements increase shoot addition in fall transplants of Z. marina. The difference between nitrogen enrichment and the control remained significant even when we introduced additional variability with the lower slopes of the unbalanced treatments. Also, the area covered by the nitrogen-enriched transplants was double that of the control, indicating that nitrogen fertilization was enhancing the coverage rate as well. Increased coverage rate would also infer an enhancement of rhizome production. In the spring Z. marina experiment we pooled the results of the unbalanced fertilizer treatments with tho',e of the control. The p-value for this difference was 0.1034, indicating a moderately higher growth rate in the nitrogen-enriched treatments. Part of the difference between the highly significant response in the fall and the marginal response in the spring may have been due to the order of magnitude differences in equivaient shoot densities. Clearly the plants exhibited two different growth strategies. Rapid lateral spreading enabled fall shoot densities to remain on the lower end of the

158

W,J. KENWORTHY AND M, S. FONSECA

range of usual field densities (100-400'm-2). The spring transplants had a population growth rate similar to the fall but was accompanied by a reduced areal coverage, yielding much higher densities (1400-1600.m-2). Because of these higher densities, the spring transplants may have experienced some density dependent control mechanism that contributed to inhibiting their population growth rate response and diminished the potential effects of the added nutrients. Within the range of nitrogen added in these experiments the transplants did not show a consistent response as a function of dose level. In the fall experiment the BI70 treatment had the highest slope, but more importantly, the large difference in dose levels between the B 10 and either the U90 or B170 treatments did not translate into equivalent differences in growth suggesting that as little as 10 g ' P U - t may be adequate for stimulation of a fall planting. The difference between fall and spring results and between Z. marina and 11. wrightii may be partly explained by the seasonality of nutrient availability and the growth cycle of the plants. In relatively shallow water the optimum growing period for Z. marina in North Carolina extends from October to June. Z. marina grows all winter but achieves maximum growth between March and May (Kenworthy, 1981). Usually by mid-June growth in shallow water is slowed substantially by high water temperatures (Thayer et al., 1984), but in deeper water Z. marina is insulated from extreme temperature fluctuations and grows all summer. Z. marina can be planted in fall (October) or spring (March), but since the plants grow all winter the length of the growing season prior to the period of summer temperature stress can be extended by planting in October. This was the motivation for examining both fall and spring plantings of Z. marina. Since H. wrightii has a tropical center of distribution, this species grows optimally from late May to early October (Kenworthy, 1981; Thayer et al., 1984). No growth occurs in winter. In fact, the winter growth form of H. wrightii is barely recognizable ~ls a seagrass. The short shoots overwinter as a stubble of le~ff material and a basal meristem attached to an otherwise intact root and rhizome system. The modest enhancement for Z. marina and the lack of any response for H. wrightii in the spring may have been due to a more rapid regeneration of in situ nutrients and nitrogen fixation during the warmer temperatures, such that nitrogen was not nearly as limiting in spring as it might have been in fall and winter. When the existing pools of nitrogen were supplemented with fertilizer in the spring there may have already been sufficient nitrogen available to the plants. The extremely variable results for H. wrightii may be due to the same reason, since H. wrightii was planted later, and experienced even warmer temperatures. The lack of a response by H, wrightii is also likely to be related to the same factors that decreased survival. According to the sediment flux rate measurements, the H. wrightii planting experienced a substantial amount of sediment instzbility which may b.ave contributed to the variable results. Our results for Z. marina confirm that the population dytiamics of this seagrass and not just the individual plant productivity are affected by nitrogen availability. This higher order population level response to nutrient enrichment of the sediments suggests

USE OF FERTILIZER IN TRANSPLANTED SEAGRASSES

159

that the establishment and development of Z. marina meadows is closely coupled to the reservoir of sediment nitrogen and the processes which make nitrogen available to the plants. This is consistent with the theory discussed in earlier studies which showed that, once established, seagrasses are capable of modifying the sedimentary physicochemical environment leading to an enrichment of the organic matter and a maturation of the nutrient cycling processes within well developed meadows (Kenworthy et al., 1982; Short, 1983). As the meadows mature and shoot density increases, they thrive on the endogenous recycling of nutrients. However, the extent to which the plants will establish and the rate at which they achieve maximum density and maturity must initially depend on an existing or exogenous supply of nutrients, perhaps residing in the sediments. On coarse textured sandy sediments low in organic matter and nutrients, the early stages of seagrass meadow development are likely to be nitrogen limited. We originally planned to compare fertilizers with and without phosphorus in order to examine the question of phosphorus limitation as well as its overall effect on transplant success and growth. But, unexpectedly, phosphorus was not released nor was it released from this same fertilizer in two other separate trials (Kenworthy, unpubl, obs.). We cannot rule out the possibility that the effect of the nitrogen supplements was diminished because it was not accompanied by phosphorus enrichment. Subsequent discussions with representatives of the fertilizer manufacturer (Dick Benson, Sierra Chemical Company, Milpitas, CA) revealed that the phosphate (P205) and nitrogen ( N H 4 N O 3 ) molecules used in the fertilizers are different sizes and compete for release through pores in the coating. The coating is a copolymer of vegetable oil and resin with dicyclopentadiene that makes it pliable. The individuals pellets imbibe and the nutrients escape through very small pores. Because the nitrogen molecules are smaller than the phosphorus molecules, nitrogen is released relatively faster than phosphorus in the balanced fertilizer. The situation of no nitrogen release from the unbalanced fertilizer was described by the manufacturer as a problem with overcoating the fertilizer. Evidently the N H 4 S O 4 molecule used in this formulation interacted with the polymer such that the resin could not be evenly distributed over the encapsulated fertilizer. The manufacturer experimented with different degrees of coating and in some cases were overeoating the fertilizer to a point where the fertilizer could not escape, which is probably what occurred with the batch of unbalanced fertilizer used in the spring Z. marina experiment. The manufacturer has since discontinued production of this type of unbalanced fertilizer. Osmocote now recommends using a mixture of fertilizers for this particular type of application where nitrogen and phosphorus are coated separately. Pores in the phosphorus coating are large enough to allow the release of phosphorus without the problem of excessive loss of nitrogen. These types of mixtures have been applied to seagrass beds in San Salvador, Bahamas and in mesocosms with modest success (Short et al., 1990) and may be reasonable alternatives to the fertilizers we used. The unbalanced nitrogen fertilizer used by Short et al. (1990) consisted of urea and doesn't have the variable coating problem that we experienced with the NH.~S04 formulation.

160

W.J. KENWORTHY AND M.S. FONSECA

Despite the fact that earlier fertilizer studies suggested phosphorus limitation for Z. marina seedlings (Roberts et al., 1984) and plug transplants of Z. marina (Orth &

Moore, 1982), we believe that the issue of phosphorus limitation and interactions between nitrogen and phosphorus remains unresolved for transplants. These two earlier studies also used Osmocote fertilizers with combined nitrogen and phosphorus formulations. However, the authors neglected to examine the dynamics of nutrient release from the fertilizers. Assuming that nitrogen was released from their balanced formulations, these studies together with ours suggest that nitrogen ferti'.ization with controlled release fertilizers may be used to enhance tl-,,~growth rates of Z. marina transplants. Future studies should examine formulations of nitrogen and phosphorus to determine if the nitrogen response can be improved by adding an available form of phosphorus. ACKNOWLEDGEMENTS

This work was supported by the Environmental Impact Research Program, US Army Corps of Engineers. The authors gratefully acknowledge the assistance of K. Rittmaster, C. Currin, D. Field, H. Gordy, G.W. Thayer and T. Fredette.

REFERENCES Borunl, J., L. Murray & M. Kemp, 1989. Aspects of nitrogen acquisition and conservation in eelgrass plants. Aquat. Bot., Vol. 35, pp. 289-300. Bulthuis, D.A. & W.J. Woelkerling, 1981. Effects of in situ niteogen and phosphorus enrichment of the sediments on the seagrass Heterozostera tasmallica (Marten~ e× Aschers.) den Hartog in Western Port, Victoria, Australia. J. E.xT~.Mar. Bhd. Ecol., Vol. 53, pp. 193-207. Dennison, W.C., R.C. Aller & R. S. Alberto, 1987. Sediment amnlonium availabi¢ily and eelgrass (Zostera marOta) growth, Mar. Bhd., Vol, 94, pp. 469-477. Fonseca, M. S., W.J. Kenworthy & G. W. Thayer, 1982. A low cost planting technique for eelgrass (Zostera marina L.), Coastal Eng:,eering Technical Aid 82-6, U.S. Army Corps of Engineers, Coastal Engineering Research Center, Ft. Bol 'oir, VA, 13 pp. Fonseca, M. S., G.W. Thaycr & W.J. Kenworthy, 1987. The use of ecological data in the implementation and management of seagrass restorations. Florida Mar. Res. Pabl., Vol. 42, pp. 175-187. Fonseca, M.S., J.C. Zieman, G W. Thayer & J.S. Fisher, 1983. The role of current velocity in structuring eelgrass (Zostera marina k.)meadows. Esmarine Coastal She(f Sci., Vol. 12, pp. 367-380. Harlin, M.M. & B. Thorne-Miller, 1981. Nutrient enrichment of seagrass beds in a Rhode Island coastal lagoon. Mar. Biol., Vol. 65, pp. 221-229. Iizumi, H. & A. Hattori, 1982. Growth and organic production of eelgrass (Zostera masqna L.)in temperate waters of the Pacific coast of Japan. III. Kinetics of nitrogen uptake. Aquat. Bot., Vol. 12, pp. 245256. lizumi, H., A. Hattori & C.P. McRoy, 1982. Ammonium regeneration and assimilation in eelgrass (Zostera marina) beds. Mar. Biol., Vol. 66, pp. 59-65. Jacobs, R.P.W.M., 1979. Dist~'ibution and aspects of the production and biomass of eelgrass, Zostera matqna, at Roscoff, France. Aquat. Bot., Vol. 7, pp. 151-172.

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Kenworthy, W.J., 1981. The interrelationships between seagrass, Zostera marina m~d Halodule wrightii, and the physical and chemical properties of sediments in a coastal plain esn~arj' near Beaufort, North Carolina. M.Sc. Thesis, Univ. of Virginia, Charlottesville, VA. Kenworthy, W.J., J. C. Zieman & G. W. Thayer, 1982. Evidence for the influence of seagrasses on the benthic nitrogen cycle in a coastal plain estuary near Beaufort, N.C. (U.S.A). Oecologia (Berlin), Vol. 54, pp. 152158. Koroleff, F., 1983. Determination of Phosphorus. In, Methods of Seawater Analysis, edited by K. Grasshoff, M. Ehrhardt and K. Kremling, Verlag Chemie, Weinheim, Germany, pp. 125-139. McRoy, C.P., R.J. Barsdate & M. Nebert, 1972. Phosphorus cycling in an eelgrass (Zostera marina) ecosystem. Limnol. Oceanogr., Vol. 17, pp. 58-67. 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., Vol. 44, pp. 187-194. Orth, R.J. & K.A. Moore, 1982. The effects of fertilizers on transplanted eelgrass, Zostera marina L., in the Chesapeake Bay. In, Proceedings of the Ninth Annual Co~!ference on Wetlands Restoration and Creation, edited by F.J. Webb, Hillsborough Community College, Tampa, FL, pp. 101-131. Patriquin, D.G., 1972. The origin of nitrogen and phosphorus for growth of the marine angiosperm Thalassia testudinum. Mar. Biol., Vol. 15, pp. 35-46. Powell, G. V. N., W.J. Kenworthy & J.W. Fourqurean, 1989. Experimental evidence of nutrient limitation of seagrass growth in a tropical estuary with restricted circulation. Bull. Mar. Sci., Vol. 44, pp. 324-340. Pulich, W. M. Jr., 1985. Seasonal growth dynamics of Ruppia marithna L. s.I. and Hah~dule wright.;i A schers, in southern Texas and evaluation of sediment fertility status. Aquat. Bot., Vol. 23, pp. 53-66. Roberts, M.H., R.J. Orth & K.A. Moore, 1984. Growth of Zostera marina L. seedlings under laboratory conditions of nutrient enrichment. Aquat. Bot., Vol. 20, pp. 321-328. Short, F.T., 1983. The response of interstitial ammonium in eelgrass (Zostera marina L.) beds to environmental perturbations. J. Exp. Mar. Biol. Ecol., Vol. 68, pp. 195-208. Short, F.T., 1987. Effects of sediment nutrients on seagrasses, literature review and mesocosm experiment. Aquat. Bot., Vol. 27, pp. 41-57. Short, F.T., M.W. Davis, R.A. Gibson & C. F. Zimmerman, 1985. Evidence for phosphorus limitation in carbonate sediments of the seagrass Syringodium[il(forme. Estuarine Coastal Shel.fSt'i., Vol. 20, pp. 419430. Short, F.T., W.C. Dennison & D.G. Capone, 1990. Phosphorus limited growth of the tropical seagrass S vri,~godimnlililhnne in carbonate sediments. Mar. EcoL Prog. Ser., Vol. 62, pp. 169-174. Short, F.T. & C. P. McRoy, 1984. Nitrogen uplake by leaves and roots of the seagrass Zostera marina L. Bot. Mar., Vol. 17, pp. 547-555. Thayer, G.W., W.J. Kcnworthy & M.S. Fonseca, 1984. The ecology of eelgra~s meadows of the Atlantic Coast: a community profile. U.S. Fish aml Wildl([b Service, FWS/OBS-84/0. Williams, S.L., 1990. Experimental studies of Caribbean seagrass bed development. Etoi. Monogr., Vol, 60, pp. 449-469.