Effects of epiphyte load on optical properties and photosynthetic potential of the seagrasses Thalassia testudinum Banks ex König and Zostera marina L.

Abstract

The biomass and optical properties of seagrass leaf epiphytes were measured to evaluate their potential impact on the photosynthetic performance of the seagrasses Thalassia testudinum Banks ex König (turtlegrass) and Zostera marina L. (eelgrass). Turtlegrass was obtained from oligotrophic waters near Lee Stocking Island, Bahamas; eelgrass was collected from a eutrophic environment in Monterey Bay, California. Leaf-epiphyte loads were characterized visually and quantified using measurements of their phospholipid biomass. Light absorption and reflectance of the intact epiphyte layer were determined spectrophotometrically. Turtlegrass epiphytes from the oligotrophic site absorbed a maximum of 36% of incident light in peak chlorophyll absorption bands, whereas higher epiphyte loads on eelgrass from the more eutrophic Monterey Bay absorbed 60% of incident light in peak chlorophyll absorption bands. The combination of intact epiphyte-leaf complexes and spectral measurements enabled us to construct a quantitative relationship between epiphyte biomass and light attenuation, and, by extension, between epiphyte biomass and seagrass photosynthesis. The model yielded a robust, positive relationship between epiphyte biomass and the absorption of photons in photosynthetically important wavelengths, and it generated a strong negative relationship between epiphyte biomass and spectral photosynthesis of their seagrass hosts. Furthermore, the calculations of photosynthesis highlighted the significant differences between PAR and spectral models of photosynthesis, illustrating that the spectral quality of the incident flux must be considered when evaluating the effects of epiphyte load on seagrass leaf photosynthesis. Verification of the model-using direct measurements of photosynthesis and a variety of epiphyte and macrophyte combinations from different locations-is warranted.

Epiphyte-cover on seagrass (Zostera marina L.) leaves impedes plant performance and radial O2 loss from the below-ground tissue

“…Light availability on the surface of the leaves of Z. marina covered by epiphytes was dramatically decreased compared to leaves without epiphytes in agreement with previous studies (Drake et al, 2003;Pedersen et al, 2014). Effectively, this means that a higher downwelling photon irradiance is needed to meet the compensation irradiance for the epiphyte covered leaf (Figure 6).…”

Optical properties of canopies of the tropical seagrass Thalassia testudinum estimated by a three‐dimensional radiative transfer model

“…The temporal effect of canopy movement on PAR absorptance and bidirectional reflectance could then be quantified and may be significant, since the distribution of leaf orientations has previously been demonstrated as a potentially important factor (Zimmerman 2003). The effect of leaf epiphytes has also been neglected here (Cebriá n et al 1999;Drake et al 2003), since epiphyte load at the six sites was low, but this could be incorporated in future work. Temporal light fluctuations due to wave focusing are an important feature of shallow water environments.…”

Remote Sensing of Shallow Coastal Benthic Substrates: In situ Spectra and Mapping of Eelgrass (Zostera marina) in the Gulf Islands National Park Reserve of Canada

“…Further, the success of requirement (1) can also be affected by epiphytic loading, which can remarkably change the reflectance of eelgrass. Studies have shown that heavy epiphyte loads can absorb up to 63% of incident light in the peak chlorophyll absorption bands, leading to greater reflectance slopes at 440 nm and 680 nm than at 550 nm [25]. Reflectance in the 575-630 nm region is markedly increased [26] while green light is physically blocked by non-photosynthetic material accumulation [25] and absorbed by non-chlorophyte epiphytes [27].…”

“…Studies have shown that heavy epiphyte loads can absorb up to 63% of incident light in the peak chlorophyll absorption bands, leading to greater reflectance slopes at 440 nm and 680 nm than at 550 nm [25]. Reflectance in the 575-630 nm region is markedly increased [26] while green light is physically blocked by non-photosynthetic material accumulation [25] and absorbed by non-chlorophyte epiphytes [27]. These effects lead to a flattening of the eelgrass spectral signature and increased variability in green reflectance values [26,28].…”