The mesocosm experiment underway this year is actually the third we have conducted at the UW FHL. The first was in 2011 and was very much a learning exercise for how to conduct such experiments. The second in 2012 was conducted in conjunction with a half-research apprenticeship at FHL. This meant that we had to share the students with other classes and this made scheduling difficult. We did three mesocosms each with control (650 ppm) and elevated (950 ppm and 1250 ppm) CO2 levels. The experiment generally went well and a poster was prepared for the AGU meeting with all students as coauthors. The waters at the dock at FHL offer a unique situation compared to previous experiments elsewhere (Norway and South Korea) in that the CO2 (pCO2 = 650 ppm) and nutrient (NO3 = 25 micromoles/l) levels are very high and the chlorophyll is low (~1 microgram/l). The seawater around San Juan Island is what oceanographers call HNLC (high nitrate low chlorophyll). When this condition is observed in the open ocean it is thought to be due to iron limitation. That is certainly not the case at FHL. Instead the origin of this water is due to upwelling of low O2, high NO3 and CO2 water from the subsurface California Undercurrent off the Washington coast. It persists without bloom development because the mixed layer around the islands is much deeper than the euphotic zone. What we learned in 2011 and 2012 was that when we put that water into the 5m deep mesocosms the plankton could take advantage of the high light and nutrients to develop a dramatic bloom. In both previous experiments NO3 was taken to zero by about day 6-7 and chlorophyll increased to 40+. The experiment was interesting but the bloom was so dramatic that it totally obscured any effect there might be from the elevated CO2 treatments.
This year (2013) we realized we needed to try something different so we built mesh bags to put around each mesocosm. This should reduce the light by more than 50%. Our hope is that this modification to the experiment will slow down the biology and either eliminate or delay the bloom. So far so good but the experiment, which officially started on Tuesday (T0) is only to the third day (T2). We are entering the critical period when we should see what happens.
We have made one more modification regarding the CO2 levels. I almost all previous experiments the pCO2 levels have been adjusted at the start but allowed to drift during the experiment. In our first experiment in 2011 when the starting pCO2 levels were 650 and 1250, biological uptake lowered pCO2 in all mesocosm to ~75 ppm by day 7! We argue that if we are designing experiments to see the impact of elevated CO2 we need to try our best to keep the CO2 constant at the initial levels. So this year we have 3 control mesocosms (again at 650ppm) and 3 high CO2 mesocosms (at 1250 ppm) where we are working hard to monitor the daily CO2 levels and adjust them back to the target levels as the CO2 is removed by biology using exact amounts of CO2 saturated seawater.
Stay tuned for new results!