I'm hoping that the inclusion of 'extreme' in the title will get you hooked...
This is back dated, it actually happened at the start of August but I have been super busy with other field work, courses and writing literature reviews.
The aim of this mesocosm experiment is to explore the response of lake ecosystems to multiple climate change and nutrient stressors. One of the effects of climate warming is a change in the water cycle, it is predicted that northern European countries will have more rainfall annually with an increase in rainfall during the winter, overall summers will be drier but with an increase in extreme rainfall events lasting 1-5 days during the later summer (July to September) whilst southern European countries will experience a reduction in annual rainfall (Christensen and Christensen 2003). Modelled changes in precipitation patterns will result in 10-30% decreases in run-off in southern European countries whereas northern countries will experience a 10-30% increase in run-off (Milly et al 2010). So as well as a 'warming' treatment we also have an 'extreme rainfall event' treatment. Water (1500 litres) is pumped into each tank to mimic the increase in in-flow that will happen as a result of a high intensity rainfall event. The tanks hold 3000 litres so this is similar to displacing half of the water in the tanks, although this is not direct displacement as just like in a lake the water will mix with the inflowing water and so the water lost from the tank will be a mix of the water from the mesocosms and the water pumped in. This event happens once every 12 weeks, 4 events in total over the duration of the experiment.
For each extreme rainfall event we need 24,000 litres of water - at the top of the site we have two large tanks which we fill up with filtered water from a nearby reservoir (this does not contain algae but does contain nutrients which we adjust for at the next nutrient addition). The tank is open to the elements so after 12 weeks of being exposed to rainfall, leaves and what not the tanks need to be cleaned out until they are spotless. This involves lots of buckets, a step ladder and a pair of waterproof overalls.
The finished product, tank spotless and ready to be filled with water. If there were ever a picture of an empty tank that I would like to sit and stare at for a long time it would be this one, oh the satisfaction!
Now the taps get switched on, it takes 24 hours to fill both up so we leave them running overnight ready for the event the next day.
The rainfall event:
We place one pump in each large tank and attach a long hose pipe that will reach to all of our smaller experimental tanks. Each pump has a different flow rate so first we time how long it takes to fill up 50 litres in marked buckets and multiply this to work out the time needed to pump 1500 litres into each tank for the different pumps. Then we place the hose end in a bucket so that the force of the water hits the bottom of the bucket instead of churning up the sediment and switch on the pumps. Easy peasy, no stress involved at all! It's like a rocket launch for each tank - five, four, three, two, one, GO - but probably less exciting. The end of the video below shows a short clip of a tank being treated with this treatment, to get an idea of the magnitude of the event the duration of the treatment is 15 minutes for one pump and 21 minutes for the other pump.
At the start of the video you will see me using the algaetorch to measure chlorophyll a and cyanobacteria biomass in the tanks. I do for all tanks but twice for the rainfall treated tanks, once before the treatment and once afterwards. We also do all the usual in-situ measurements as described here.
A tank mid treatment - now the cover photo for the blog will hopefully make sense!
The day after the rainfall event we take water samples and process them as described here . Usually we sample every four weeks but when we do a rainfall event we sample the day after, a week after and three weeks after (which coincides with the usual sampling timetable).
So now that you know what's involved, lets go back to why are we interested in the response of aquatic ecosystems to changes in flow. Increased flow through a lake results in losses of algae biomass from the direct effects of flushing (algal biomass is lost in the flow) e.g. Carvalho et al (2012). Previous research has demonstrated that phytoplankton community composition is strongly modulated by the amount, duration and seasonality of flow (different taxonomic groups are more sensitive to flow than others); cyanobacteria are a slow growing taxa and prefers slow, stable and warm conditions whilst other faster growing phytoplankton like diatoms do well in fast flowing, mixed environments (e.g. Reynolds et al (2012)). We are interested in how water bodies will respond to these periodic episodes of high flow where biomass will be flushed out of the system and the effect it will have on cyanobacteria in combination with other stressors - warming and nutrient enrichment. How long does it take for the biomass to recover to its initial state? Is the composition different before and after flooding? Is the response different in tanks that are also treated with warming, excess nutrients or both? and so on.
One last thing - for the past 3 months Nur Filiz a PhD student from METU has been helping out with the mesocosm experiment. Nur is also involved in the MARS project and is looking at the 'Impact of Climate Induced Extreme Events on Phytoplankton Community in Shallow Lakes'. During her time here she has been doing some phytoplankton counts from previous sampling occasions and has been helping me with the running of the experiment. This was Nur's last day - thanks for your help Nur!
Carvalho, L., C. Miller, et al. (2012). "Water quality of Loch Leven: responses to enrichment, restoration and climate change." Hydrobiologia 681(1): 35-47.
Christensen, J. H. and O. B. Christensen (2003). "Climate modelling: Severe summertime flooding in Europe." Nature 421(6925): 805-806.
Milly, P. C. D., K. A. Dunne, et al. (2005). "Global pattern of trends in streamflow and water availability in a changing climate." Nature 438(7066): 347-350.
Reynolds, C. S., S. C. Maberly, et al. (2012). "Forty years of monitoring water quality in Grasmere (English Lake District): separating the effects of enrichment by treated sewage and hydraulic flushing on phytoplankton ecology." Freshwater Biology 57(2): 384-399.
I have managed to turn my rather unhealthy obsession with plankton in to my day job. Things don't get much better than this! This blog documents my PhD research and the plankton delights I encounter along the way.