Photosynthetic responses of Zostera marina L. (Eelgrass) to in situ manipulations of light intensity

Abstract

Photosynthetic responses of the temperate seagrass, Zostera marina L., were examined by manipulations of photon flux density in an eelgrass bed in Great Harbor, Woods Hole, MA during August 1981. Sun reflectors and light shading screens were placed at shallow (1.3 m) and deep (5.5 m) stations in the eelgrass bed to increase (+35% to +40%) and decrease (-55%) ambient photon flux densities. The portion of the day that light intensities exceeding the light compensation point for Z. marina (H ) and the light saturation point (H) were determined to assess the impact of the reflectors and shades. The H and H periods at the deep station shading screen were most strongly affected; H was reduced by 11% and H was reduced by 52%. Light-saturated photosynthetic rates, dark respiration rates, leaf chlorophyll content, chlorophyll a/b, PSU size, PSU density, leaf area, specific leaf area, leaf turnover times and leaf production rates were determined at the end of three sets of 1- to 2-week experiments. None of the measured parameters were affected by the photon flux density manipulations at the shallow station; however, at the deep station leaf production rates were significantly reduced under the shading screen and chlorophyll a/b ratios were higher at the reflector. These results indicate that adjustment to short-term changes in light regime in Z. marina is largely by leaf production rates. Further, the most dramatic changes in the periods of compensating or saturating photon flux densities had the greatest impact on the measured photosynthetic responses.

Effects of shading onVallisneria natans(Lour.) H. Hara growth

“…Reductions in light levels have been shown to have adverse effects on Zostera marina L. photosynthesis (Dennison and Alberte, 1982), growth (Dennison and Alberte, 1985;Dennison, 1987), community structure (Backman and Barilotti, 1976) and, ultimately, individual long-term survival (Zimmerman et al, 1995;Moore et al, 1996). Growth of more vigorous aquatic plants can create competitive interactions through shading effects affecting individual plant fitness (e.g.…”

Role of Seagrass Photosynthesis in Root Aerobic Processes

“…In the present study, we sought to examine the flood tolerance of the submerged marine angiosperm Zostera marina (eelgrass). This temperate seagrass inhabits coastal areas ofthe Northern Hemisphere which are characterized, for the most part, by the presence of reducing sediments (9) and light limitation for growth (7). Despite this apparently inhospitable environment, Z. marina is one of the most productive marine primary producers known (14).…”

“…The electrode chamber was kept in total darkness during measurement, while the shoot chamber was illuminated with photosynthetically saturating light (150 ME m 2 s-' PAR; see Ref. 7) or kept in darkness. The 02 exchange rates (uptake and release) were recorded continuously for up to 8 h during shoot illumination and darkness.…”

“…Since Z. marina leaf biomass is directly proportional to leaf area (7), a minimum leaf area (based on the 02 exchange data) of about 4 x 10-3 m2/shoot is required for 02 release. A typical mature eelgrass shoot has a leaf area of about 5 x 10-3 m2 when the canopy is at its seasonal maximum (7).…”

“…4) and leaf areas are capable of transporting 02 in excess of respiratory demand to their roots which can be released into the rhizome. It is also likely that this excess 02 is utilized by roots of small developing shoots branching off the main rhizome which probably do not carry out net photosynthesis as light levels are well below photosynthetic saturation (approximately 20-30 4E m 2 s-') at the bottom of dense eelgrass canopies (7,13).…”