The effect of UV-B radiation on coastal plant communities

1. visible light UV-B 4.00 3.50 3.00 2.50 Absorption (1/µg Chl.a) 100% UV-B 2.00 7% UV-B Greenhouse 1.50 1.00 0.50 0.00 280 285 290 295 300 305 310 315 320 Wave lenght (nm) 100 80 60 Germination (%) 40 20% UV-B 20 7% UV-B 0 0 5 10 15 Time (days) The effect of UV-B radiation on coastal plant communities PIs: Evamaria Koch, University of Maryland Center for Environmental Science (UMCES), USA; Cesar S. Costa, Universidade do Rio Grande (URG), Brazil; Roy Armstrong, University of Puerto Rico (UPR), Puerto Rico. Students: Leslie Brandt (UMCES), Lisiane Silva das Neves (URG), Rodrigo A. Real (URG), Jessica Loyola (UPR), Oswaldo Cardenas (UPR). HYPOTHESIS 1 The development and reproduction of the marsh plant Salicornia is negatively affected by UV-B METHODS Plants and seeds of the genus Salicornia were grown under full UV-B exposure as well as under UV excluding films such as Mylar, Aclar, Laxan and Saran in situ (Brazil, Fig 1) or in tanks (Puerto Rico). Morphological characteristics of the plants and germination of the seeds were measured under each treatment. RESULTS Seed germination and plant morphology were negatively affected by UV-B levels. Germination rates, plant height and branch number were highest under the lowest UV-B levels (Figs. 2 and 3). The production of UV-B absorbing pigments was also reduced in UV-B shielded plants when compared to plants exposed to full UV-B levels (Fig. 4). • INTRODUCTION • The plant communities that characterize the coastal zones (i.e. salt marshes, mangrove forests, sand dune vegetation, seagrasses) protect the shorelines from erosion and serve as a food source and habitat for a variety of commercially important organisms. Despite of its importance, the impact of UV-B radiation on coastal plant communities is poorly understood. If they are negatively impacted by UV-B, the damage may go beyond the loss of plants, it may indirectly affect fisheries resources and coastal erosion. In 2002, our CRN group will meet to better understand such links. • OBJECTIVES • The general objectives of our study are to quantify the impact of UV-B on the physiology, development and reproduction of a UV-B sensitive salt marsh plant (Salicornia) and to determine the impact of UV-B on the competitive capacity of this plant. During the past year, we have focused on: • the impact of UV-B on the physiology, development and reproduction of a UV-B sensitive salt marsh plant (Salicornia) • epipiphtes as UV-B filters on seagrass leaves HYPOTHESIS 2 Seagrass leaves are colonized by organisms among which sediment particles can be deposited. These organisms and particles on seagrass leaves form the epiphytic layer. We hypothesized that: The epiphytic layer acts a UV-B filter on seagrass leaves. METHODS Artificial seagrass leaves were constructed out of UV-B transparent plastic strips (Aclar). These were then placed in a bare area within a dense seagrass (Zosteramarina) bed and retrieved after 1, 3, 5, 7 and 12 days. The transmittance of UV-B and visible light through each artificial leaf was determined using a dual spectrophotometer. RESULTS The epiphytic layer transmitted less light in the UV-B range (detrimental to photosynthesis) than in the visible range (necessary for photosynthesis) (Fig. 5). Figure 1. Study site in Brazil where UV excluding filters were positioned above the marsh plants in situ. In Puerto Rico, plants were grown in tanks covered by UV excluding filters. Figure 3. Height and number of branches of Salicorniabigelovii exposed to ambient (100% UV) as well as reduced (Lexan 3% UV and Saran 75% UV) levels of UV radiation. Figure 5. Transmittance of UV-B and visible radiation through an epiphytic layer in a seagrass habitat. The warmer the color, the longer the artificial leaves remained in situ (= the thicker the epiphytic layer). CONCLUSIONS Present UV-B seem to be detrimental to the development and reproduction of the marsh plant Salicornia. At the same time, this plant has a mechanism to increase the absorption of UV-B as this detrimental radiation increases. This is accomplished via the synthesis of UV-B absorbing pigments, a process that may take energy from other processes such as seed formation and growth. It has been previously shown that UV-B is also detrimental to seagrasses but these plants may benefit from the organisms and particles that settle on the leaf surface as these reduce the UV-B levels that reach the seagrass leaf. Figure 2. Germination of Salicorniagaudichaudiana seeds exposed to two levels of UV-B radiation. Figure 4. Absorption of Salicorniagaudichaudiana plants exposed to ambient (100% UV) as well as reduced (7%) levels of UV radiation.