Effects of Epiphyte Load on Optical Properties and Photosynthetic Potential of the Seagrasses Thalassia Testudinum Banks ex König and Zostera Marina L

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. Seagrass leaves are colonized by a diverse array of epiphytic microorganisms, macroalgae, and metazoans. The epiphyte complex consists of (i) all organisms that grow attached to or crawl over the leaf surface, (ii) the associated extracellular matrix deposited on the leaves by these organisms, and (iii) mineral and organic particles embedded in the organic matrix. This complex provides a significant fraction of the overall productivity of seagrass ecosystems (e.g., Penhale 1977; Mazzella and Alberte 1986; Klumpp et al. 1992), as well as refuge and food for an assemblage of invertebrates and fish (e.g., Orth and van Montfrans 1984; van Montfrans et al. 1984; Neckles et al. 1994; Short et al. 2001). A modest epiphyte layer may benefit seagrasses by preventing damage from ultraviolet radiation (Trocine et al. 1981) or repelling potential leaf grazers (Karez et al. 2000). It has been argued that epiphytes composed of nonchlorophyte algae and cyanobacteria can accumulate to high densities without affecting 1 Corresponding author ([email protected]). Acknowledgments We are grateful to Leslie Kampschmidt, Molly Cummings, Donald Kohrs, and Sally Wittlinger for their assistance in the field and in the laboratory, and we appreciate the efforts of the staff at the Caribbean Marine Research Center on Lee Stocking Island, Bahamas. This work was supported by the Office of Naval Research, Environmental Optics Program, Coastal Benthic Optical Properties (CoBOP) initiative, grants N00014-97-1-0018 (F.C.D. and L.A.D.), N00014-02-1-0030 (L.A.D. and F.C.D.), and N00014-97-1-0032 (R.C.Z.). seagrass photosynthesis because they preferentially absorb green light, which is an inefficient driver of seagrass photosynthesis (Mazzella and Alberte 1986). Nonetheless, epiphytes may also have negative effects on their seagrass hosts—creating physical barriers to light absorption (Losee and Wetzel 1983; Dalla Via et al. 1998; Brush and Nixon 2002), nutrient uptake, gas exchange, or a combination of these factors (Sand-Jensen 1977; van Montfrans et al. 1984; Sand-Jensen et al. 1985). Eutrophication is a common feature of estuarine environments throughout the world, creating blooms of nuisance phytoplankton and increased epiphyte biomass that may have dramatic impacts on seagrass distribution, density, and productivity (e.g., Sand-Jensen and Søndergaard 1981; Orth and Moore 1983; Cambridge and McComb 1984). Epiphyte growth on seagrass leaves can be stimulated by eutrophication (e.g., Borum 1985; Twilley et al. 1985; Tomasko and Lapointe 1991; Coleman and Burkholder 1994), removal of epiphyte grazers (e.g., Caine 1980), and a combination of both factors (e.g., Neckles et al. 1993; Williams and Ruck-