Research Summary

Predicting the Effect of Changing Atmostpheric CO2 on Seagrass Distributions

Anthropogenic combustion of fossil fuel is expected to double atmospheric carbon dioxide concentrations in the next century (1). Equilibration of ocean surface waters with increased atmospheric CO2 may have significant impacts on marine biogeochemical processes such as primary production. Seagrasses are severely CO2 and light limited in nature (2). When enriched with CO2 however, seagrass photosynthetic rates increase 3-fold and light requirements are reduced (3,4). Conversely, phytoplankton and seaweed productivity is little affected by increased [CO2], because their photosynthetic pathways are already CO2 saturated in nature (5). Therefore, increased CO2 in the environment will not likely increase algal growth. The projected doubling of atmospheric CO2 may increase photosynthesis of carbon-limited seagrasses by as much as 50%, potentially increasing colonization depth limits and reducing light requirements of this critical coastal marine resource.

Objective

With my mentor, Dr. Richard C. Zimmerman, I will develop then test a bio-optical model to quantitatively predict seagrass canopy productivity at environmentally relevant combinations of [CO2] and light availability. These estimates will predict how seagrass distribution might change as [CO2] increases due to human impact.

I will test predictions of the model by growing the seagrass, Zostera marina, at varying [CO2 aq] and light availability in controlled outdoor experimental tanks plumbed with running seawater and compressed CO2. Plants will be exposed to aqueous [CO2] expected at atmospheric doubling (1-3%) and concentrations that saturate photosynthesis in this species (36%). A pH-meter and solenoid valve feedback system, measuring change in pH associated with speciation of CO2 in seawater, will be used to regulate CO2 concentrations in the experimental tanks. Light levels will be experimentally manipulated to correspond with increasing depth (light decreases as depth increases). I will measure growth, O2 production and respiration, light absorption by the leaf, and leaf area index (area of leaf m-2 of substrate).

Using light and CO2 as independent factors and growth as the dependent variable, I will perform regression analysis and use the slope to verify the productivity model and test the hypothesis that increased [CO2] decreases light requirements of seagrasses. Leaf absorption and leaf area index results also will be used to verify the productivity model. Once the model is verified, I will incorporate it into a Geographic Information System (GIS) framework and map potential seagrass aerial and depth distributions at atmospheric doubling of CO2. I will overlay these predicted ranges over measured present day seagrass ranges. This will allow visualization of potential changes in primary productivity and distribution of seagrasses in nature due to a change in light availability (a result of the cleanup of the watershed) or a change in atmospheric CO2.

Literature Cited

1. Keeling, C. D. B., R. B.; Bainbridge, A. E.; Ekdahl, C. A. JR.; Guenther, P. R.; Waterman, L. S.; Chin, J. F. S. (1976). Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii. TELLUS 28(6): 538-551.

2. Beer, S. (1989). Photosynthesis and respiration of marine angiosperms. Aquat. Bot. 34: 153-166.

3. Zimmerman, R. C., Kohrs, D. G., Steller, D. L., and Alberte, R. (1995). Sucrose partitioning in Zostera marina L. in relation to photosynthesis and the daily light-dark cycle. Plant Physiol. 108: 1665-1671.

4. Zimmerman, R., Kohrs, D., Steller, D., and Alberte, R. (1997). Impacts of CO2 -enrichment on productivity and light requirements of eelgrass. Plant Physiol. 115: 599-607.

5. Raven, J. A., Walker, D. I., Johnston, A. M., Handley, L. L., and Kubler, J. (1995). Implications of 13C natural abundance measurements for photosynthetic performance by marine macrophytes in their natural environment. Mar. Ecol. Prog. Ser. 123: 193-205.