Experimental modification of light intensity

Screen shot 2013-04-07 at 11.16.03 AMJames Murray

Plankton production is a function of nutrients and light. The nutrients are abundant this year as they have been each of the past two years. This year (2013) the initial nitrate concentrations were ~25 uM. We assumed that if we just did the experiment the same as last year, the biology of the mesocosms would be dominated by a strong bloom which would obscure the impact of CO2 levels that we wanted to study. So we built mesh screens around the mesocosm bags and across the top. The light profiles (measured using a Li-cor sensor) on sample day T9 are compared in Figure 1. You can see that the light in the mesocosms starts at ~70 at the surface and decreases to very low values below about 1.5 m. Comparison of light inside and outside the mesocosms shows that we lowered the light by 70/190 or ~37%. But light inside the mesocosms below 1-2 m was very low, indeed. We suspect that biofouling of the mesh bag may be a significant factor. We have been successful in preventing a strong bloom. The nitrate time series (Figure 2) shows that NO3 has decreased from the initial values of  ~25 uM to ~21 uM on T10. Not much removed after 10 days. The chlorophyll (Figure 3) however has steadily increased from <1 to ~5 mg l-1.  We suspect that this is due to the cells becoming more chlorophyll rich rather than indicating a bloom in cells.

Screen shot 2013-04-20 at 3.07.29 PM

Figure 1

Screen shot 2013-04-20 at 3.09.33 PM

Figure 2

Screen shot 2013-04-20 at 3.10.26 PM

Figure 3

We decided that we needed to give the biology a boost by increasing the light.  We had done a test earlier to see what the impact would be on light if we removed the top mesh. With this modification the light at the surface was 54/115 or 47% of the outside light. This modification would roughly triple the light at the surface (from 20 to 55). So we made this modification to all mesocosms after sampling on Friday April 19 (T10) in order to stimulate the biology in the mesocosms. Sunny days are forecast so biological changes should be on the menu.

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Figure 4

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