Direct measurements of light attenuation by epiphytes on eelgrass Zostera marina

Abstract

Declines in the seagrass Zostera marina L. in estuaries and lagoons have been attributed in part to reductions in irradiance reaching the seagrass blades. Epiphytes growing on Z. marina have the potential to attenuate a large fraction of the light that would otherwise reach the blades. This problem has previously been studied by measuring light penetration through homogenized epiphytic slurries or through glass slides fouled with epiphytes. However, the latter may not represent the natural succession or species composition found on live Z. marina leaves and the former does not preserve the structure of the epiphytic complex. Further, past studies have not measured attenuation across the full range of epiphytic densities found in the field. In this study, we measured light penetration across a wide range of epiphytic densities by holding scraped and unscraped Z. marina blades over a submerged light sensor. Results compared well with past studies at low epiphyte densities, with strong reductions in light penetration as density increased. However, at higher densities, penetration leveled off to a relatively constant value as the epiphytes floated out from the edges of the blade. Studies using slurries did not capture this phenomenon and thus predicted decreasing penetration down to 0%.

Morphological, growth and meadow characteristics of the seagrass Posidonia sinuosa along a depth-related gradient of light availability

“…The light-attenuating effects of this epiphyte accumulation (Burt et al 1995, Brush & Nixon 2002 may contribute to the morphological anomalies at this particular depth at CS.…”

Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

“…We know from field observations and experimental mesocosm studies that much of this decline is due to a reduction in light availability and altered biogeochemical conditions from increased algal standing stocks (as macroalgae, epiphytes and/or phytoplankton; Twilley et al 1985, Neckles et al 1994, Short et al 1995, Taylor et al 1995, Nixon et al 2001. Epiphytes attenuate up to 90% of the light at the scale of the seagrass blade (Sand-Jensen 1977, Howard & Short 1986, Brush & Nixon 2002. Bloom-forming macroalgae form dense canopies up to 1 m thick over seagrass beds in eutrophic waters and can also decrease light levels by > 90% in the upper few centimeters of the mat (Krause- , Peckol & Rivers 1996, Astill & Lavery 2001.…”

Nitrate reductase activity in macroalgae and its vertical distribution in macroalgal epiphytes of seagrasses

“…Light is an important environmental factor regulating NR activity in algae, stimulating both NO 3 -uptake and expression of NR in macroalgae (Davison & Stewart 1984b, Gao et al 1992, Lopes et al 1997. The branching habit and high epiphyte loads resulted in the relatively high attenuation of light observed in A. antarctica meadows (see also Brush & Nixon 2002). However, the lack of a steep irradiance gradient in the Posidonia sinuosa meadow, where significant vertical variation in NR activity was observed, suggests that irradiance was not a major determinant of the observed trends in NR activity in seagrass epiphytes.…”