Abstract
Seagrass meadows, and other submerged vegetated habitats, support a wide range of essential ecological services, but the true extents of these services are in many ways still not quantified. One important tool needed to assess and model many of these services is accurate estimations of the systems´ primary productivity. Such productivity estimations require an understanding of the underwater light field, especially regarding the amount of light that actually reaches the plants’ photosynthetic tissue. In this study, we tested a simple practical approach to estimate leaf light exposure, relative to incoming light at the canopy, by attaching light sensitive film at different positions on leaves of Zostera marina, eelgrass, in four seagrass meadows composed of different shoot density and at two different depths. We found that the light reaching the leaves decreased linearly down through the canopy. While the upper parts of the leaves received approximately the same level of light (photosynthetic photon flux density, PPFD) as recorded with a PAR meter at the canopy top, the average light that the seagrass leaves were exposed to varied between 40 and 60% of the light on top of the canopy, with an overall average of 48%. We recommend that actual light interception is measured when assessing or modelling light depending processes in submerged vegetation, but if this is not achievable a rough estimation for vegetation similar to Z. marina would be to use a correction factor of 0.5 to compensate for the reduced light due to leaf orientation and internal shading.
Highlights
While it is well established that seagrasses benefit human needs through a wide range of ecosystem services, there is a need to understand the strength and variability of these services [1]
We aimed to measure the actual levels of photosynthetic photon flux density (PPFD) intercepted at the seagrass leaf surface within Zostera marina meadows, and to explore if a general conversion factor for light interception by leaves relative to light measured at the top of the canopy can be calculated
This study clearly showed that light measured on top of the seagrass canopy considerably overestimates the light that is intercepted by the leaves, and that such values can be corrected to better reflect the true light conditions within the seagrass meadow
Summary
While it is well established that seagrasses benefit human needs through a wide range of ecosystem services, there is a need to understand the strength and variability of these services [1]. A practical approach to measure the actual light interception by leaves in a canopy has been used on terrestrial plants by attaching a light sensitive colour acetate film with azo dyes directly to the surface of the leaves, and thereby including any changes in shading and light interception angles in the measurements [14] This method has been used successfully to study competition for light between plant individuals of Xanthium canadense [15], assess the light regime in understory vegetation of forests [14, 16], and estimating light interception in cultured orchids [17], maize [18] and tomato [19]. Such light sensitive film has been used to estimate the light interception of a commercial moss grown in water tanks [20], but has as far as we know not previously been used to study light interception in the natural marine coastal environment
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