Elsevier

Aquatic Botany

Volume 85, Issue 1, July 2006, Pages 21-28
Aquatic Botany

Spatio-temporal variability in the photosynthetic characteristics of Zostera tasmanica measured by PAM

https://doi.org/10.1016/j.aquabot.2006.01.009Get rights and content

Abstract

Rapid light curves (RLCs), based on pulse amplitude modulated (PAM) fluorometry, were used to investigate the spatio-temporal variability in photosynthesis versus irradiance parameters (α, Ik and Pmax) and the Fv/Fm ratio of the seagrass Zostera tasmanica (formerly Heterozostera tasmanica). Spatial variation was examined across scales ranging from within a leaf (cms) to across the bed (ms), using a nested analysis of covariance sampling design. Overall, significant variation was identified at all scales examined, excluding the largest scale (area). Patterns of variability differed among individual parameters; however a high percentage of the variation was consistently assigned to the covariates, age (within and between leaves) for all parameters, except Pmax.

Using these results, a sampling strategy focusing on the leaf level was used to examine temporal changes to the photosynthesis parameters and the Fv/Fm ratio and to minimize spatial considerations. Both Pmax and the Fv/Fm ratio showed a significant relationship with in situ irradiance. However, a large percentage of the variation, for all parameters, was left unexplained by in situ irradiance. When the effect of in situ irradiance was removed there was no autocorrelation between measurements, taken at 3 min intervals, of the photosynthesis parameters and the Fv/Fm ratio. These results suggest that small-scale spatial and not temporal variation must be taken into account when seeking to characterise Z. tasmanica photosynthetic performance using PAM fluorometry.

Introduction

Seagrass photosynthesis is highly dynamic on a variety of spatio-temporal scales. Differences have been observed at temporal scales ranging from hours to seasons and spatial scales ranging from within a leaf to across the bed. These changes have been linked to physical factors, such as the availability of light (Mazzella and Alberte, 1986), nutrients (Bulthuis et al., 1992) and temperature (Bulthuis, 1987), and/or biological processes such as age (Durako and Kunzelman, 2002, Alcoverro et al., 1995, Alcoverro et al., 1998, Modigh et al., 1998) and various endogenous regulatory mechanisms of the plant (Pirc, 1986, Mazzella and Alberte, 1986). A number of limitations have been identified in measuring such changes with traditional techniques such as O2 exchange and 14C labelling, particularly at smaller scales. Despite these difficulties, understanding the processes that generate small-scale variability in seagrass photosynthesis and how they operate over different scales of time and space remains an important process from which other models and hypotheses can be derived (Underwood, 1981).

One of the more recent developments in photosynthesis research is the submersible pulse amplitude modulated (PAM) fluorometer. This instrument was designed to make small-scale (minute and cms) assessments of chlorophyll a fluorescence, from which the relative electron transport rates (rETRs), quantum yield and maximal effective quantum yield (Fv/Fm ratio) can be determined. The photosynthetic parameters α, Pmax and Ik (initial slope, maximum photosynthetic rate and saturation irradiance) can also be calculated by plotting rETRs against photon flux. Studies investigating the use of this instrument in photosynthesis research have found a linear or curvilinear relationship between 14C labelling or O2 evolution and PAM fluorometry in a number of aquatic organisms (Beer et al., 1998a, Geel et al., 1997, Hartig et al., 1998, Hofstraat et al., 1994, Kromkamp et al., 1998, Schreiber et al., 1997, Ting and Owens, 1992) including six species of seagrass (Beer and Björk, 2000, Beer et al., 1998b).

Several studies have demonstrated that small-scale (i.e. within and between leaves and diurnal) spatio-temporal variability in photosynthesis can be significant for some seagrass species but not others (Ralph, 1996, Ralph et al., 1998, Ralph et al., 2005, Beer and Björk, 2000, Silva and Santos, 2003, Durako and Kunzelman, 2002, Enriquez et al., 2002). Their work suggests that recent light history plays a significant role in diurnal changes (Ralph, 1996, Ralph et al., 1998) while age is more important for small-scale spatial considerations (Durako and Kunzelman, 2002, Enriquez et al., 2002, Ralph et al., 2005). Small-scale variability has the potential to confound larger scale studies, when not taken into account. However, few studies have tested how these variables (light and age) influence the photosynthesis parameters (α, Pmax and Ik) and the Fv/Fm ratio at smaller spatio-temporal scale (cms and minutes) (Durako and Kunzelman, 2002, Enriquez et al., 2002). Moreover, beyond the work of Durako and Kunzelman (2002), no attempt has been to measure seagrass photosynthesis over a range of small spatio-temporal scales. Knowledge of the scale(s) at which there is predictable variation is important for designing sampling strategies and must be determined before PAM fluorometry is to be used as a routine tool for measuring seagrass photosynthesis and calculating primary productivity.

In this study, PAM fluorometry measurements of rapid light curves (RLCs) and the Fv/Fm ratio were used to investigate spatio-temporal variability in Zostera tasmanica photosynthesis. Our objectives were (1) to examine patterns of variability in the photosynthetic parameters (Ik, α, Pmax) and the Fv/Fm ratio over a range of spatial (within a leaf to across the bed) and temporal scales (minutes to hours); (2) to identify the scale(s) at which there was predictable variation and (3) to examine the role of light and age at the scale(s) identified.

Section snippets

Methods

Shoots of Zostera tasmanica G. Martens ex Asch. (formerly known as Heterozostera tasmanica, Les et al., 2002) were collected from an intertidal area (ranging between 0.2 and 5 m deep) at Avalon Beach, Port Phillip Bay, Vic., Australia, (54°14′N, 8°23′E) in June and August 1999. This seagrass reproduces vegetatively, producing both erect and rhizome shoots. Erect shoots bear the most leaves and may be branched or unbranched with a cluster of leaves at the apex of each branch. In this study only

Results

Both the hyperbolic tangent model (Jassby and Platt, 1976, Platt et al., 1980) and the non-linear model (Eilers and Peeters, 1988) showed similar spatial and temporal trends. Therefore, only data for the photosynthetic parameters calculated using the hyperbolic tangent model are shown here (Jassby and Platt, 1976, Platt et al., 1980).

The pattern of within-leaf variation showed that α and the Fv/Fm ratio decreased with distance from the leaf base (Fig. 1). Although there was considerable

Discussion

PAM fluorescence measurements of the photosynthetic parameters and Fv/Fm ratio of Z. tasmanica varied spatially and temporally. The covariates, distance from leaf base and leaf length, were significant for all parameters examined, excluding Pmax. At larger scales, Pmax and the Fv/Fm ratio differed significantly between shoots, clusters and leaves, Ik varied between shoots and leaves and α differed between clusters and leaves. Both Pmax, and the Fv/Fm ratio were found to vary with the in situ

Acknowledgement

We are grateful to EPA Victoria for the generous loan of their Underwater PAM and logistic support.

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