Effect of storms on photosynthesis, carbohydrate content and survival of eelgrass populations from a coastal lagoon and the adjacent open ocean

Aquatic Botany 74 (2002) 149–164

Effect of storms on photosynthesis, carbohydrate content and survival of eelgrass populations from a coastal lagoon and the adjacent open ocean Alejandro Cabello-Pasini a,∗ , Celina Lara-Turrent a , Richard C. Zimmerman b a

Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California, A.P. #453, Ensenada, Baja California 22800, Mexico b Moss Landing Marine Laboratories, Moss Landing, CA 95039, USA Received 10 July 2001; received in revised form 8 May 2002; accepted 27 May 2002

Abstract Annual variations in density, morphology, pigment levels, carbohydrate levels and photosynthetic characteristics of Zostera marina L. (eelgrass) from a coastal lagoon and from an unprotected area in the open coast were compared to understand the environmental regulation of this species growing near the southern limit of its distribution in the eastern Pacific. From January to April of 1997, light attenuation coefficients at the lagoon and the open coast increased six-fold as a result of sediment resuspension caused by storms in the area. During the storms, in situ irradiance was reduced two-fold at the lagoon. Irradiance values at the open coast, however, were reduced to nearly zero for >3 weeks, promoting the disappearance of the open coast population. At the open coast, eelgrass shoots died after sugar and starch content in the leaves decreased by approximately 85% after 3 weeks of light limitation. The re-appearance of eelgrass seedlings at the open coast coincided with the end of the winter storms in the area and decreasing water column turbidity. Maximum photosynthesis values of eelgrass from the open coast were two-fold greater than those from lagoon plants, except during March–June when seeds germinated at the open coast. Similarly, survival and leaf carbohydrate content of eelgrass from both sites decreased by >90% when incubated for 3 weeks in darkness. Collectively, these results indicate that the disappearance of eelgrass at the open coast is regulated by irradiance and not by an endogenous cycle. Although eelgrass can succeed in the open coast given sufficient light during the year, the accumulation and mobilization of carbon reserves appear to play a key role in the dynamics of eelgrass survival in temporally variable environments. © 2002 Elsevier Science B.V. All rights reserved. Keywords: In situ irradiance; Photosynthesis; Seasonality; Storms; Sugar; Zostera marina

∗ Corresponding author. Tel.: +526-174-4601x107; fax: +526-174-5303. E-mail address: [email protected] (A. Cabello-Pasini).

0304-3770/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 0 2 ) 0 0 0 7 6 - 1

150

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

1. Introduction The southern limit of eelgrass (Zostera marina L.) distribution in the North Pacific extends to Baja California and the Gulf of California, Mexico. The survival of annual populations of eelgrass in the Gulf of California is largely regulated by the wide seasonal temperature fluctuations and/or endogenous rhythmicity (Meling-Lopez and Ibarra-Obando, 1999; Santamaria et al., 1999). The annual die-back of these and other submerged macrophytes occurs when water temperature increases during the summer months (Pacheco-Ruiz et al., 1992; Meling-Lopez and Ibarra-Obando, 1999; Santamaria et al., 2000). In contrast to populations from the Gulf of California, the abundance and survival of eelgrass beds along its central and northern range of distribution in the Pacific coast are controlled largely by light availability and the ability of these populations to store and allocate carbohydrates during periods of light limitation (Zimmerman et al., 1995b; Alcoberro et al., 1999). Carbohydrate reserves in the leaves, rhizomes and roots of eelgrass are mobilized to support growth and metabolism during periods of light limitation (Kraemer and Alberte, 1993; Burke et al., 1996). It has been argued, however, that this species has difficulty maintaining a positive carbon balance except under high light conditions (Burke et al., 1996). The experimental shading to reduce 80% of incident irradiance for 3 weeks, for example, has been shown to decrease the carbohydrate levels by 50% in the leaves, roots and rhizomes of eelgrass, and reduce its growth and survival rates (Burke et al., 1996). In estuaries, periods of long-term light limitation often occur as a result of anthropogenic impact or an increase of sediment load through streams or rivers (Moore et al., 1997), and as a consequence, the carbohydrate reserves in eelgrass must last longer than the period of light limitation in order for them to survive. The transparency of estuaries and coastal water columns can be highly variable in time and space, and long or recurrent brief periods of extreme light attenuation are known to produce negative carbon balance which limits seagrass growth (Dunton, 1994; Zimmerman et al., 1994). In general, coastal waters are less turbid than estuaries and coastal lagoons permitting deeper light penetration (Kirk, 1994). These clearer coastal waters often permit the survival of subtidal seagrass populations to depths below 10 m (Zimmerman et al., 1996; Procaccini et al., 2001). Unlike at shallow populations, light penetration at the deep coastal beds is strongly affected by small increments of attenuation coefficient (Koch and Beer, 1996). Such periods of light limitation can reduce the carbohydrate content in the leaves and rhizomes of eelgrass and eventually lead to a decrease in shoot density (Kraemer and Alberte, 1993; Zimmerman et al., 1995a; Lee and Dunton, 1996). Furthermore, permanent or recurring periods of light limitation resulting from anthropogenic sediment loading and resuspension have lead to the permanent disappearance of eelgrass beds at some coastal areas (Cabello-Pasini, 1984; Koch and Beer, 1996). Shear stress caused by storm-generated swells has also lead to the restructuring or disappearance of submerged aquatic vegetation (North, 1994). The effect of light limitation as a result of sediment resuspension by these storm swells, however, has not been assessed and could play a key role on the survival of seagrasses. The understanding of the physiology and growth is critical to fully understand the environmental mechanisms that control eelgrass. Consequently, the objective of this study was to characterize the annual fluctuations in density, morphology, photosynthetic char-

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

151

acteristics and carbohydrate levels of eelgrass populations growing in a low energy lagoon and a more energetic site on the unprotected open coast of Baja California, México. The capacity of internal reserves to sustain metabolic processes and survivorship of eelgrass during periods of prolonged light limitation was also assessed. Seagrass performance was related to seasonal patterns of irradiance and storm-driven turbidity at these two sites.

2. Materials and methods 2.1. Study area Eelgrass populations were studied in the coastal lagoon Estero de Punta Banda and in the open coast of Bahia Todos Santos (31◦ 45 N, 116◦ 40 W), approximately 20 km south of Ensenada, Baja California, Mexico (Fig. 1). The sites are separated by a 6 km long sand bar that defines the entrance to the lagoon. Plants were collected from the lagoon at approximately 1 m depth and from the open coast at approximately 5–7 m depth between November 1997 and March 1999 using SCUBA. Currents at the open coast site are generally >0.15 m s−1 and have an average northward direction along the coast (Alvarez-Sanchez et al., 1988). Water velocity controlled by wave swell, however, can increase to >1 m s−1 during storms in the area. Currents at the lagoon are tidally-driven and are generally <0.15 m s−1 (Pritchard et al., 1978; Pritchard et al., 1979). Downwelling photosynthetically active irradiance (Ed ) in air was monitored continuously using a cosine-corrected radiometer (LI-COR) located approximately 10 km from the study site. Attenuation coefficient for scalar irradiance (Ko ) was evaluated at both sites by determining irradiance just below the surface and at 0.2 m depth with a 4␲ PAR irradiometer (Biospherical Instruments) at solar noon. Monthly variations of Ko were calculated according to Beer’s law (Kirk, 1994) and were used to estimate the in situ irradiance at both sites. In situ surface temperature was measured at both sites approximately once a month

Fig. 1. Collecting sites at Bahia Todos Santos, Baja California, M´exico. Eelgrass beds from the open coast are separated from the lagoon population by a 6 km long sand bar that runs parallel to the coastline.

152

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

with a thermometer. Weekly mean sea surface temperature was also obtained from AVHRR satellite images. 2.2. Density and morphology Shoot density was estimated monthly at both sites using 400 cm2 quadrats (n = 10) placed haphazardly approximately every 2 m throughout the seagrass meadows. Approximately 20 shoots from both sites were collected at noon, placed in coolers filled with seawater and transported to a nearby laboratory for morphological and photosynthetic characterization, and the evaluation of pigment and carbohydrate content in the leaves. Shoot length was evaluated monthly at each study site by determining the length (n > 10) of the longest leaf with a meter tape and the width (n > 10) of the blade at the middle of the shoot with a caliper. The number of leaves per shoot and first internodal length was also evaluated in a minimum of 12 plants per study site. 2.3. Pigment content Leaf pigments were extracted within 1.5 h of collection by grinding the leaves (n = 6) in ice cold acetone (90% v/v). Chlorophyll was determined spectrophotometrically using the extinction coefficients for chlorophyll a and b described by Jeffrey and Humphrey (1975) and normalized to fresh weight (FW). 2.4. Photosynthesis and growth Photosynthetic rates of eelgrass from both sites were determined monthly using polarographically-measured rates of steady-state O2 evolution. Approximately 0.1 g of tissue (n = 8) from leaf no. 2 (youngest leaf = no.1) was incubated in filtered seawater (0.45 ␮m, pH 7.8, 2.1 mM DIC) at field temperature in jacketed chambers (5 ml) connected to a water-circulating bath after a 0.5 h preincubation darkness. Prior to light incubations, oxygen concentration in the chambers was reduced to approximately 5 mg L−1 by bubbling the medium with N2 . Quartzline halogen lamps (300 W) were used as a light source and irradiance was varied using neutral-density filters. Maximum oxygenic photosynthesis (Pmax ), the initial slope of the photosynthesis versus irradiance curve (α), the threshold for irradiance-saturated photosynthesis (Ek ) and respiration were determined by a non-linear direct fitting algorithm (Sigma Plot, Jandel Scientific) of the data to the exponential function of Webb et al. (1974). Relative growth rates were evaluated every month throughout the year by marking 10 shoots just above the meristem with a hypodermic needle (Zieman and Wetzel, 1980). Shoots were collected 2 weeks later and relative growth rates were calculated from the incremental displacement of the marks on each leaf (Zimmerman et al., 1995b). 2.5. Carbohydrate determination Sugar was extracted from the leaves within 1.5 h of collection by homogenizing approximately 0.1 g of tissue (n = 6) in hot 90% (v/v) ethanol (Zimmerman et al., 1995a). The

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

153

homogenate was extracted (2X) at 80 ◦ C for 15 min and evaporated at room temperature with a stream of air or by vacuum. Starch was extracted in 1 N KOH overnight at room temperature from the ethanol insoluble residue. Sugar (sucrose + fructose + glucose) and starch levels were assayed spectrophotometrically using anthrone (9,10-dihydro-9-oxoanthracene) standardized to sucrose (Yemm and Willis, 1954) and normalized to fresh weight. 2.6. Carbohydrate levels under laboratory conditions Shoots from both populations were collected in November 2000 and transported in coolers with seawater (approximately 17 ◦ C) to the laboratory facility in Ensenada, Baja California. Individuals (n = 20) with at least six internodes were anchored to the bottom of 40 L containers with seawater at 16 ◦ C. Approximately 300 ␮mol quanta m−2 s−1 PAR with a photoperiod of 16:8 (L:D) was provided to one group of individuals while the other group was kept in darkness. Water was changed twice weekly and individuals were kept under constant aeration. Shoot survival and the levels of carbohydrates in leaf no. 2 of eelgrass from both sites (n = 4) were monitored as described previously at the beginning of the experiment and then once weekly for 1 month. 2.7. Statistical analysis Temporal and site differences in density, morphological characteristics and photosynthetic parameters were evaluated using a two-way ANOVA (site × time) after testing for normality and homoscedasticity of the data (Sokal and Rohlf, 1981). Some data were log transformed before analysis to meet these criteria. Statistical differences in carbohydrate levels in eelgrass incubated under laboratory conditions were evaluated by a one-way ANOVA. All pairwise multiple comparisons were conducted using Tukey’s test with a minimum significance level established at P < 0.05.

3. Results 3.1. Environmental parameters Daily integrated downwelling irradiance (in air) varied seasonally from a low in December (approximately 20 mol quanta m−2 per day) to a high in May–June (approximately 55 mol quanta m−2 per day, Fig. 2). Fog and clouds introduced considerable shortterm variability to the clear seasonal patter. Water temperature obtained from satellite images followed closely those obtained with a thermometer in the field, and in general, water temperature between the lagoon and the open coast differed by 1 ◦ C or less. Water temperature was coldest (approximately 16 ◦ C) in March–April and warmest (approximately 22 ◦ C) during September–October. Scalar attenuation coefficients (Ko ) were generally two- to six-fold greater in the lagoon than at the open coast (Fig. 3A). The values of the Ko were approximately eight-fold higher at both sites during the winter storm season due to sediment resuspension and influx from local streams. Thus, submarine irradiance at the top of the seagrass meadows

154

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

Fig. 2. Daily integrated surface irradiance throughout the study period (A). Surface water temperature (B) determined from satellite images (䊊) and thermometer (䊉) at the lagoon and open coast throughout the year.

was determined by the combined effects of temporal variations in surface irradiance and Ko . Submarine irradiance at the top of the eelgrass canopy was lower on the open coast than in the lagoon during winter and spring, but was two- to three-fold higher on the open coast site than in the lagoon during summer and fall (Fig. 3B). In situ irradiance in the lagoon fluctuated between 3 and 10 mol quanta m−2 per day throughout the year. In contrast to irradiance in the lagoon, the calculated in situ irradiance dropped to nearly zero during March and April on the open coast, but exceeded 20 mol quanta m−2 per day in the summer. 3.2. Density and morphology Shoot density varied significantly throughout the year at both sites (Fig. 4, Table 1). Density in the lagoon was approximately 175 shoots m−2 through most of the year, except during August and September in which it increased to approximately 250 shoots m−2 . On the open coast, all shoots disappeared in March of 1998. Seedlings re-appeared in May and reached 300 shoots m−2 during November 1998. In March of 1999, all shoots disappeared

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

155

Fig. 3. Attenuation coefficient (A) at the lagoon and the open coast throughout the year. Arrows indicate the arrival of major storms in the area. Calculated in situ irradiance (B) throughout the year at both study sites. In situ irradiance was calculated at 1.5 m at the lagoon and at 5 m in the open coast from attenuation coefficient values and integrated surface irradiance. Symbols represent mean ± S.D. (n = 12).

on the open coast. This disappearance coincided with a 3 week period of high attenuation coefficient, low in situ irradiance and strong winter storms. Shoot length and width varied significantly (P < 0.05) between the lagoon and the open coast and also as a function of time of year (Fig. 5A, Table 1). Shoot length in the lagoon population reached approximately 80 cm during May and then decreased to 40 cm in November. The open coast population attained a maximum shoot length (55 cm) in January 1998, just before the shoots disappeared in March. After the emergence of eelgrass seedlings in May, shoot length of the open coast population fluctuated between 30 and 40 cm. In contrast to shoot length and width, the number of leafs per shoot (X¯ = 5.5 ± 0.5) did not vary significantly throughout the year at either the lagoon or the open coast (Table 1). First internode length fluctuated significantly (P < 0.05) at both study sites throughout the year (Fig. 5B, Table 1). The lagoon population had longer internodes than the open coast population during the winter period of low light availability. Internodal length decreased

156

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

Table 1 Results of two-way ANOVA testing the effects of time and study site on density, shoot length, internodal length, Pmax , α, Ek , specific growth rate, chlorophyll a + b and leaf sugar and starch levels Dependent variable

Independent variable

d.f.

MS

F

P

Significance

Density (shoots m2 )

Time Site Time × site Within

10 1 10 208

88,933 255,645 59,292 1,371

64 186 43

<0.001 <0.001 <0.001

∗∗∗

Time Site Time × site Within

8 1 8 127

587 10,074 4,185 47

12 212 88

<0.001 <0.001 <0.001

∗∗∗

Time Site Time × site Within

5 1 5 80

0.055 0.118 0.027 0.003

15 34 7

<0.001 <0.001 <0.001

∗∗∗

Leaf number

Time Site Time × site Within

6 1 6 100

1.13 0.36 0.11 0.68

0.14 0.46 0.98

n.s. n.s. n.s.

Internodal length (cm)

Time Site Time × site Within

8 1 8 113

4.8 20.8 2.6 0.3

16 69 8

<0.001 <0.001 <0.001

∗∗∗

Time Site Time × site Within

6 1 6 70

1.41 4.29 2.43 0.01

103 314 178

<0.001 <0.001 <0.001

∗∗∗

Shoot length (cm)

Shoot width (cm)

Pmax (␮mo1 O2 g−1 FW min−1 )

α (␮mo1 O2 g−1 FW min−1 ) (␮mol quanta m−2 s−1 )−1

1.65 0.53 0.16

∗∗∗ ∗∗∗

∗∗∗ ∗∗∗

∗∗∗ ∗∗∗

∗∗∗ ∗∗∗

∗∗∗ ∗∗∗

6

0.00280

97

<0.001

∗∗∗

Site Time × site Within

1 6 70

0.00210 0.00110 0.00002

74 38

<0.001 <0.001

∗∗∗

Time Site Time × site Within

6 1 6 70

52,221 72,747 28,736

39 54 21

<0.001 <0.001 <0.001

∗∗∗

Specific growth rate (% per day)

Time Site Time × site Within

6 1 6 126

0.13 0.45 0.44

0.20 0.70 0.67

0.97 0.40 0.66

n.s. n.s. n.s.

Chlorophyll a + b (mg g−1 FW)

Time Site Time × site Within

6

0.63 0.41 0.37 0.09

6.4 4.2 3.7

<0.001 0.044 0.003

∗∗∗

Ek (␮mol quanta m−2 s−1 )

Time

6 70

∗∗∗

∗∗∗ ∗∗∗

∗ ∗∗

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

157

Table 1 (Continued ) Dependent variable

Independent variable

d.f.

Sugar (␮mol g−1 FW)

Time Site Time × site Within

6 1 6 69

67,662 15,436 64,106 1,307

Time Site Time × site Within

6 1 6 70

272 7 112 7

Starch (␮mol g−1 FW)

MS

F

P

Significance

51 11 49

<0.001 0.001 <0.001

∗∗∗

37.7 0.9 15.6

<0.001 0.330 <0.001

∗∗ ∗∗∗

∗∗∗

n.s. ∗∗∗

slightly at the beginning the storm season in eelgrass from both the lagoon and the open coast and increased two-fold after the stormy period. 3.3. Photosynthesis and growth Except for May and June, Pmax values of shoots from the open coast were approximately two-fold greater (P < 0.05) than from the lagoon (Fig. 6A, Table 1). Temporal variation in Pmax did not follow a clear seasonal pattern in the lagoon population. At both sites, ␣ values were lowest during January and September and increased approximately three- to six-fold during May–June (Fig. 6B). The increase in ␣ values coincided with the increase of Ko and low integrated surface irradiance in the field. Values of Ek followed an opposite

Fig. 4. Annual fluctuation of eelgrass density at the lagoon and the open coast. Symbols represent mean ± S.D. (n = 12). Dotted symbol indicates statistical differences (P < 0.05) within sites.

158

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

Fig. 5. Annual variations of eelgrass shoot length (A) and rhizome internodal length (B) at the lagoon and the open coast. Symbols represent mean ± S.D. (n = 12). Dotted symbol indicates statistical differences within sites.

pattern for those observed in a at the lagoon and open coast (Fig. 6C). Maximum Ek values for eelgrass from the open coast were observed in January and September–November, and minimum values in May–June, after the storm period in the field. Values of Ek for eelgrass from the lagoon were greatest (P < 0.05) during March and decreased after the storms in the field. Average growth rates were 4.8 ± 0.8% per day at both sites and did not vary significantly throughout the year (Table 1). 3.4. Pigments and carbohydrate levels Chlorophyll a + b levels from eelgrass leaves showed statistically significant differences as a function of the study site, time of year and interaction (site × time) (Fig. 7A, Table 1). While chlorophyll levels in the leaves did not fluctuate at the lagoon throughout the year, pigment levels were statistically greater (P < 0.05) prior to the storms than after shoot germination at the open coast. After the germination of eelgrass at the open coast, chlorophyll a + b levels remained constant at approximately 2 mg g−1 FW at both sites for the rest of the year.

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

159

Fig. 6. Annual fluctuations of (A) maximum photosynthesis (Pmax ), (B) the initial slope of the photosynthesis vs. irradiance curve (␣), and (C) threshold for irradiance-saturated photosynthesis (Ek ) of eelgrass (X ± S.D., n = 8) at the lagoon and the open coast. Dotted symbol indicates statistical differences within sites.

Carbohydrate levels in the leaves fluctuated significantly (P < 0.05) between the lagoon and the open coast throughout the year. At the open coast, sugar levels (sucrose +fructose + glucose) decreased sharply (P < 0.05) when in situ submarine irradiance decreased after November 1997 (Fig. 7B, Table 1). Leaf sugar levels increased to approximately 400 ␮mol sucrose g−1 FW in May when submarine irradiance was high. While significant fluctuations in sugar levels in eelgrass from the lagoon were observed, there was no strong seasonal pattern as observed for the open coast population. Although starch represented <7% of the labile carbohydrate pool at both study sites, the temporal variation of starch was similar to that of sugar (Fig. 7C, Table 1). The significant decrease (P < 0.05) in leaf starch observed in January coincided with a decrease in irradiance at both sites. Although starch levels varied as a function of time, there was no significant difference between sites (Table 1). Sugar levels in the leaves of eelgrass from the lagoon and the open coast maintained in darkness for 21 day under laboratory conditions decreased to almost undetectable levels

160

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

Fig. 7. Chlorophyll a+b in eelgrass from the lagoon and open coast (A). Soluble (B) and insoluble (C) carbohydrate in the leaves of eelgrass at both study sites. Symbols represent mean ± S.D. (n = 6). Dotted symbol indicates statistical differences within sites.

(P < 0.05, Fig. 8). Sugar levels remained low in plants kept in darkness throughout the rest of the experiment. In contrast, the levels of sugar in the leaves of eelgrass from both sites exposed to saturating irradiance levels did not decrease throughout the experimental period. Survival of eelgrass maintained in darkness decreased significantly after 2 weeks and all shoots were dead by the end of the experiment. In contrast, all plants maintained under light-saturated conditions survived throughout the experimental period.

4. Discussion Light has been identified as the factor that most often regulates the survival and depth distribution of seagrasses in coastal lagoons throughout the world (Duarte, 1991; Zimmerman et al., 1991; Koch and Beer, 1996). This study shows that prolonged periods of storm-driven

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

161

Fig. 8. Leaf sugar content (X ± S.D., n = 4) of eelgrass from the lagoon (squares) and open coast (squares) incubated in darkness (dark symbols) and under light-saturated conditions (clear symbols).

light limitation can reduce the survival of eelgrass populations at wave exposed sites. Carbohydrate reserves in the leaves of the open coast population appear to be critical for the survival of these populations during these periods light limitation. The variations in shoot photosynthetic performance observed here at the lagoon and the open coast are typical of seagrass populations worldwide (Dunton, 1994; Zimmerman et al., 1995b) and appear to be driven primarily by light availability. The short-term fluctuations of α and Ek in seagrasses from the open coast relative to those in the lagoon, however, appear to be driven by more transient fluctuations in light availability, especially at the lagoon. This suggests a need for more frequent or continuous in situ irradiance measurements to evaluate short period or transient fluctuations in the light environment that might influence photosynthesis and growth of eelgrass in the field. The similar temperatures observed here at the two sites is consistent with results obtained in other studies (Pritchard et al., 1978) and further suggests that metabolic differences are the result of different light regimens. Whole plant carbon balance has been implicated as a major factor determining the growth and distribution of eelgrass (Zimmerman et al., 1989; Kraemer and Alberte, 1993; Burke et al., 1996; Zimmerman and Alberte, 1996). Seasonal accumulation of carbohydrates in leaves and rhizomes during favorable growth periods provide a critical source of energy for growth and respiratory requirements during periods of light limitation (Zimmerman et al., 1995a; Burke et al., 1996). The rapid decline in leaf carbohydrates (sugar and starch) observed here during low light periods is consistent with patterns reported in eelgrass (Zimmerman et al., 1995b; Zimmerman and Alberte, 1996) and other marine macrophytes growing in environments with large seasonal amplitudes in light availability or under severe light stress (Chapman and Craigie, 1977, 1978; Dunton, 1985). The disappearance of eelgrass when in situ submarine irradiance was nearly zero during the storm periods at

162

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

the open coast paralleled the decline in leaf sugar and starch levels. This suggests that the carbohydrate storage in the leaves of eelgrass was not sufficient to sustain their metabolic processes during this period of light limitation. Such dramatic decline in carbohydrate levels was not observed in seagrasses from the lagoon where in situ irradiance levels were greater than at the open coast during storms in the area, and reinforces the hypothesis that in situ irradiance is regulating the survival of eelgrass beds here. Furthermore, light starvation experiments in the laboratory confirm that eelgrass shoots from both populations had enough carbohydrate reserves to maintain metabolic activity under light limiting conditions for only 3 weeks. While the endogenous annual life history of eelgrass from the Gulf of California appears to be driven primarily by temperature or endogenous rhythms (Meling-Lopez and Ibarra-Obando, 1999), our results indicate that shoots on the open coast of Baja California disappeared as a result of prolonged storm-driven light limitation, rather than as a consequence of endogenous rhythmicity or pure mechanical disturbance. Clearly, years with few or mild storms in the area would promote light penetration so that populations at the open coast might survive. The March disappearance of eelgrass reported here represents a common response of seagrasses to light limitation observed in other studies (Cabello-Pasini, 1984; Moore et al., 1996; Moore et al., 1997). The average minimum light requirements for marine submerged vascular plants has been shown to vary from 8 to 18% of the surface irradiance and approximately 6 h of light-saturated irradiance (Hsat ) (Duarte, 1991; Dunton, 1994; Zimmerman et al., 1995b; Olesen, 1996). In the present study, eelgrass from the lagoon were exposed to approximately 9–18% of surface irradiance while plants on the open coast experienced light levels ranging from 1 to 37% of the surface irradiance. Since surface irradiances (in air) used to calculate in situ light levels were not corrected for light reflection at the air/water interface, irradiances reaching the eelgrass canopy were actually lower than this. Clearly, eelgrass shoots from both sites maintain high sugar levels and survive if enough light to saturate photosynthesis is available. Thus, the open coast population may survive during years with mild winters that would bring few storms to the area. Carbohydrate levels in the rhizome of eelgrass are generally two- to five-fold greater than in the leaves (Kraemer and Alberte, 1993; Burke et al., 1996). These carbohydrate reserves in the rhizomes of eelgrass can be mobilized towards the leaves to support metabolism during periods of light limitation (Lee and Dunton, 1996). The smaller internodal length of eelgrass from the open coast here then suggests a lower carbohydrate storage capacity in the rhizomes compared to seagrasses from the lagoon. Such lower sugar and starch pools likely impart a critical disadvantage to the seagrasses at the open coast during prolonged periods of light limitation compared to those in the lagoon. At the open coast, leaf tissue started to disintegrate after 2 weeks of light limitation indicating that carbohydrate reserves were not enough to maintain respiratory carbon losses. At this point, mechanical stress caused by storm-driven wave swell detached the leaves from the rhizomes. The colonization of new areas by eelgrass through vegetative reproduction is very slow (Olesen and Sand-Jensen, 1994), as a consequence, the production of new beds through seed dispersal seems to be critical, especially in highly variable environments. Only dead rhizomes were found after the storms, and not a single shoot was observed to regrow from the old rhizomes. Consequently, the regrowth of eelgrass on the open coast relied completely on seeds while very few seedling recruits were observed at the lagoon (A. Cabello-Pasini,

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

163

personal observation). It has been observed that the percentage of flowering shoots is generally greater at the intertidal than at the subtidal zone, however, the germination of seeds is close to 100% at these latitudes (Phillips, 1983). The total disappearance of eelgrass from the open coast and the recruitment from seeds also implies that sexual reproduction is extremely important for the maintenance of this population. Since few flowering shoots were observed at the open coast site, it is unclear if seeds were produced locally or were transported from the lagoon. Evidence from this study strongly suggests that prolonged periods (>3 weeks) of light limitation caused by winter storms can decrease sugar levels in the leaves of eelgrass and substantially decrease survival rates. While temperature or endogenous rhythms appear regulate annual-like life histories of eelgrass in the Gulf of California, light limitation appears to control eelgrass survival in the more thermally stable Pacific coast of Baja California. It is also clear that a change in the transparency patters of the water column at the open coast site either by anthropogenic sediment loading or global weather pattern fluctuations (i.e. ENSO events) might alter the survival pattern of these populations.

Acknowledgements We thank Rodolfo Cabello Pasini and Fernando Garza for helping with the initial collection of eelgrass. This project was financed by grants from the Consejo Nacional de Ciencia y Tecnolog´ıa (I-26655-N) and Universidad Autónoma de Baja California (4023 and 4078-13). References Alcoberro, T., Zimmerman, R.C., Kohrs, D.G., Alberte, R.S., 1999. Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina. Mar. Ecol. Prog. Ser. 187, 121–131. Alvarez-Sanchez, L.G., Hernandez-Walls, R., Durazo-Arvizu, R., 1988. Patrones de trazadores lagrangeanos en la Bahia de Todos Santos. Cien. Mar. 14, 135–162. Burke, M.K., Dennison, W.C., Moore, K.A., 1996. Non-structural carbohydrate reserves of eelgrass Zostera marina. Mar. Ecol. Prog. Ser. 137, 195–201. Cabello-Pasini, A., 1984. Transplantes de Zostera marina L. en el Estero de Punta Banda, Baja California, Mexico, durante el verano de 1983 y su comportamiento a través de otoño e invierno, B.S. Thesis, Universidad Autónoma de Baja California, Baja California, Mexico, p. 40. Chapman, A.R.O., Craigie, J.S., 1977. Seasonal growth in Laminaria longicruris: relations with dissolved inorganic nutrients and internal reserves of nitrogen. Mar. Biol. 40, 197–205. Chapman, A.R.O., Craigie, J.S., 1978. Seasonal growth in Laminaria longicruris: relations with reserve carbohydrate storage and production. Mar. Biol. 46, 209–213. Duarte, C.M., 1991. Seagrass depth limits. Aquat. Bot. 40, 363–377. Dunton, K.H., 1985. Growth of dark-exposed Laminaria saccharina (L.) Lamour. and Laminaria solidungula J. Ag. (Laminariales:Phaeophyta) in the Alaskan Beaufort Sea. J. Exp. Mar. Biol. Ecol. 94, 181–189. Dunton, K.H., 1994. Seasonal growth and biomass of the subtropical seagrass Halodule wrightii in relation to continuous measurements of underwater irradiance. Mar. Biol. 120, 479–489. Jeffrey, S.W., Humphrey, G.F., 1975. New spectrophotometric equations for determining chlorophylls a, b, c1 and c2 in higher plants, algae and natural phytoplankton, algae and natural phytoplankton. Biochem. Physiol. Pflanzen 167, 191–194. Kirk, J.T., 1994. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, Cambridge, 509 pp.

164

A. Cabello-Pasini et al. / Aquatic Botany 74 (2002) 149–164

Koch, E.W., Beer, S., 1996. Tides, light and the distribution of Zostera marina in Long Island Sound, USA. Aquat. Bot. 53, 97–107. Kraemer, G.P., Alberte, R.S., 1993. Age-related patterns of metabolism and biomass in subterranean tissues of Zostera marina (eelgrass). Mar. Ecol. Prog. Ser. 95, 193–203. Lee, K.S., Dunton, K.H., 1996. Production and carbon reserve dynamics of the seagrass Thalassia testudinum in Corpus Christi Bay, Texas, USA. Mar. Ecol. Prog. Ser. 146, 201–210. Meling-Lopez, A.E., Ibarra-Obando, S.E., 1999. Annual life cycles of two Zostera marina L. populations in the Gulf of California: contrasts in seasonality and reproductive effort. Aquat. Bot. 65, 59–69. Moore, K.A., Neckles, H.A., Orth, R.J., 1996. Zostera marina (eelgrass) growth and survival along a gradient of nutrients and turbidity in the lower Chesapeake Bay. Mar. Ecol. Prog. Ser. 142, 247–259. Moore, K.A., Wetzel, R.L., Orth, R.J., 1997. Seasonal pulses of turbidity and their relations to eelgrass (Zostera marina L.) survival in an estuary. J. Exp. Mar. Biol. Ecol. 215, 115–134. North, W.J., 1994. Review of Macrocystis biology. In: Akatsuka, I. (Ed.), Biology of Economic Algae. SPB Academic Publishing, The Netherlands, pp. 447–527. Olesen, B., 1996. Regulation of light attenuation and eelgrass Zostera marina depth distribution in a Danish embayment. Mar. Ecol. Prog. Ser. 134, 187–194. Olesen, B., Sand-Jensen, K., 1994. Patch dynamics of eelgrass Zostera marina. Mar. Ecol. Prog. Ser. 106, 147–156. Pacheco-Ruiz, I., Zertuche-Gonzalez, J.A., Cabello-Pasini, A., Brinkhuis, B.H., 1992. Growth responses and seasonal biomass variation of Gigartina pectinata Dawson (Rhodophyta) in the Gulf of California. J. Exp. Mar. Biol. Ecol. 157, 263–274. Phillips, R.C., 1983. Reproductive strategies of eelgrass (Zostera marina L.). Aquat. Bot. 16, 1–20. Pritchard, D.W., De La Paz Vela, R., Cabrera-Muro, H., Farrera-Sanz, S., Morales, E., 1978. Hidrografia fisica del Estero de Punta Banda. Parte I: Analisis de datos. Cien. Mar. 5, 1–23. Pritchard, D.W., De La Paz Vela, R., Farrera-Sanz, S., Cabrera-Muro, H., Morales, E., 1979. Hidrografia fisica del Estero de Punta Banda. Parte II. Aplicacion de un modelo para intercambio y dispersion. Cien. Mar. 6, 1–17. Procaccini, G., Orsini, L., Ruggiero, M.V., Scardi, M., 2001. Spatial patterns of genetic diversity in Posidonia oceanica, an endemic Mediterranean seagrass. Mol. Ecol. 10, 1413–1421. Santamaria, N.A., Felix-Pico, E.F., Sanchez-Lizaso, J.L., Palomares-Garcia, J.R., Mazon-Suastegui, M., 1999. Temporal coincidence of the annual eelgrass Zostera marina and juvenile scallops Agaropecten ventricosus (Sowerby II, 1842) in Bahia Concepcion, Mexico. J. Shell. Res. 18, 415–418. Santamaria, N.A., Sanchez-Lizaso, J.L., Felix-Pico, E.F., 2000. Phenology and growth cycle of annual subtidal eelgrass in a subtropical locality. Aquat. Bot. 66, 329–339. Sokal, R.R., Rohlf, F.J., 1981. Biometry. W.H. Freeman and Company, New York, 859 pp. Webb, W.L., Newton, M., Starr, D., 1974. Carbon dioxide exchange of Alnus rubra. A mathematical model. Oecologia 17, 281–291. Yemm, E.W., Willis, A.J., 1954. The estimation of carbohydrates in plant extracts by anthrone. Biochem. J. 57, 508–514. Zieman, J.C., Wetzel, R.G., 1980. Productivity in seagrasses: Methods and Rates. In: Phillips, R.C. McRoy, C.P. (Eds.), Handbook of Seagrass Biology: An Ecosystem Perspective. Garland STPM Press, New York, pp. 87–116. Zimmerman, R.C., Alberte, R.S., 1996. Effect of light/dark transition on carbon translocation in eelgrass Zostera marina seedlings. Mar. Ecol. Prog. Ser. 136, 305–309. Zimmerman, R.C., Cabello-Pasini, A., Alberte, R.S., 1994. Modeling daily production of aquatic macrophytes from irradiance measurements: a comparative analysis. Mar. Ecol. Prog. Ser. 114, 185–196. Zimmerman, R.C., Kohrs, D.G., Alberte, R.S., 1996. Top-down impact through a bottom-up mechanism: the effect of limpet grazing on growth, productivity and carbon allocation of Zostera marina L. (eelgrass). Oecologia 107, 560–567. Zimmerman, R.C., Kohrs, D.G., Steller, D.L., Alberte, R.S., 1995a. Carbon partitioning in eelgrass. Regulation by photosynthesis and the response to daily light–dark cycles. Plant Physiol. 108, 1665–1671. Zimmerman, R.C., Reguzzoni, J.L., Alberte, R.S., 1995b. Eelgrass (Zostera marina L.) transplants in San Francisco Bay: role of light availability on metabolism, growth and survival. Aquat. Bot. 51, 67–86. Zimmerman, R.C., Reguzzoni, J.L., Wyllie-Echeverria, S., Josselyn, M., Alberte, R.S., 1991. Assessment of environmental suitability for growth of Zostera marina L. (eelgrass) in San Francisco Bay. Aquat. Bot. 39, 353–366. Zimmerman, R.C., Smith, R.D., Alberte, R.S., 1989. Thermal acclimation and whole-plant carbon balance in Zostera marina L. (eelgrass). J. Exp. Mar. Biol. Ecol. 130, 93–109.