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Minimum light duration needed to maintain winter and summer adapted eelgrass (Zostera marina) as determined by mesocosm experiments
BOESE, B. L., K. M. MARKO, C. FOLGER, AND S. Lynch. Minimum light duration needed to maintain winter and summer adapted eelgrass (Zostera marina) as determined by mesocosm experiments. Presented at CERF 2009, Portland, OR, November 01 - 05, 2009.
The intensity and duration of photosynthetically active radiation (PAR) is one of the primary factors determining seagrass survival.
The intensity and duration of photosynthetically active radiation (PAR) is one of the primary factors determining seagrass survival. A commonly used metric to evaluate if there is sufficient PAR is Hsat, which is the number of hours per day in which photosynthetic pigments are saturated. For eelgrass (Zostera marina) the minimum Hsat needed to maintain eelgrass has been estimated at 3-5 hours, values that are often not attained for weeks at a time during the winter in Pacific Northwest estuaries. During these intervals, eelgrasses utilize rhizome carbohydrate reserves, but few studies have addressed how long these plants can survive a negative carbon balance. We directly determined the minimum Hsat needed to maintain summer and winter light adapted Z. marina. This was done using plants collected in the summer and again in the winter which were then grown hydroponically for 60 days under varying Hsat treatments (16, 8, 4,2,1,0 hours) in indoor mesocosms. A variety of plant metrics were measured at the beginning (t0), middle (t30) and end (t60) of each experiment and used to assess treatment effects. These metrics included leaf elongation, pigment concentrations, P vs. I relationships, wet and dry biomass, rhizome carbohydrates, and mortality. In both summer and winter experiments, plants subjected to Hsat treatments of 4 or more hours during the first 30 days of exposure were able to maintain leaf metrics and overall biomass, with summer plants exhibiting statistically significant biomass increases at the higher light durations In contrast, plants exposed to lower Hsat treatments (0, 1, and 2 Hsat) generally showed statistically significant reductions in these same metrics. In contrast to leaf metrics, rhizome metrics (length and number of nodes) were insensitive to irradiance treatments as they increased significantly in all treatments. Leaf metrics at t60 exhibited reductions when compared to t0 values regardless of treatment levels; however, these reductions tended to be more severe at lower Hsat levels. In both experiments leaf elongation rates and pigments were generally greater at higher Hsat levels. Mortality data indicated that eelgrass required approximately 4 hours of saturating irradiance per day for long-term survival regardless of the season in which plants were collected. In both summer and winter, the first mortalities were observed in the lower light treatments levels following approximately 30 days of treatment. After the first mortality was observed, the number of observed mortalities rapidly increased with a general peak in mortalities observed approximately two weeks after the onset of mortalities. These results confirm that 3-5 hours of Hsat are needed in both summer and winter light adapted plants for survival, with plants able to tolerate lower Hsat levels for approximately 1 month. The relationship between rhizome carbohydrates and these metrics will be discussed.