Results for the Indoor Zostera Experiments:

Zostera marina (eelgrass) seedlings were grown under controlled light conditions for 20 weeks (Jan27 - Jun6, 2003), in a range of light intensities (Table 1). All but the two greatest light treatments result in a deficit of light to saturate photosynthesis, ultimately resulting in seedling mortality.  Mortality in response to light-limitation occurred from the base up, with the meristem becoming non-viable about 1-2 weeks before the leaves were apparently dead. This observed lag-time may be critical in monitoring seagrass mortality under natural situations where light-limitation is the cause of mortality.

Table 1: Seven irradiance treatments used for indoor light experiment.  Both instantaneous and integrated (12hr) irradiance fluxes are given, as well as integrated irradiance greater than the saturation irradiance (approx. 150uE) and compensation irradiance (approx. 10uE) for Zostera. Numbers in red indicate light deficits for photosynthesis. 

Fig 1: Water temperatures in the indoor tank during the 20 week experiment.  Water temperatures were recorded using Onset StowAway Tidbit loggers (www.onsetcomp.com) at half hour time intervals.

 

Fig 2: Seedlings were grown for the duration of the growth season. No branching or spatial expansion was observed in any of the seven irradiance treatments. Five of 117 seedlings flowered (4.3%), all at irradiances of 33uE or higher.

Fig 3: The length of the longest leaf on each surviving seedling was measured as a proxy for canopy height.  Leaf growth (linear extension) occurred only in the two highest light levels (irradiance > saturation).  Early mortality (leaf len = 0) was observed in the two lowest light levels, with evidence of plant stress by weeks 3 and 5 respectively, about half the time to mortality.

Fig 4: Leaf area (single surface) was calculated from maximum length x average width. Width was measured on alternative sampling dates on a random subset of 3-5 seedlings, with linear interpolation for non-sampled dates. Area is primarily influenced by leaf length, as width remained approximately constant within a given light treatment.

Fig 5: Seedling photosynthesis was measured with an OptiSciences OS-30 PEA (Plant Efficiency Analyzer).  Photosynthetic yield, expressed as a ratio Fv/Fm ranged from 0.6-0.75 in healthy seedlings, and fell below 0.5 in seedlings that were dead or dying.  No significant difference in Fv/Fm was observed in the apparently healthy (live) seedlings across all seven light treatments, indicating acclimation of the photosynthetic apparatus to the ambient light field. We conclude that the yield ratio (Fv/Fm) is not a sensitive measure of chronic stress conditions to which the seagrasses can acclimate.  The yield ratio has, however, been successfully used to track acute stress changes (e.g., dessication) on photosynthesis in seagrass and higher plants.

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