Deep water addition: getting right

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Preparing the plankton meal

 Leila KittuDec 6, 20182 min read

Just like all human beings require nutrients for their growth, so do the phytoplankton in the surface sunlit ocean (marine microscopic plants). These nutrients are vital for ocean productivity, so much so that a proper balance exists in the ocean for how phytoplankton consume their nutrients. For phytoplankton, this ‘balance diet’ of nutrients includes nitrogen, phosphorus and silicate in precise proportions.

In this experiment, we are collecting deep water rich in these nutrients, feeding it to the phytoplankton in our mesocosms and looking at the responses across the food web. However, as we all know, nature can be unpredictable. The nutrient levels for these 3 nutrients in our collected deep water fell below our desired target levels. But hold on guys, all is not lost, we have a plan.

The plan is to artificially supplement the deep water with the three crucial nutrients in order to bring it to target levels. I am tasked with calculating how much of each nutrient we need to add and then preparing these nutrients in the lab. I am overcome with excitement because I can finally put my high school chemistry to use. However, it is also nerve-wracking because unlike in high school, I am working on a much larger scale since we need to add these nutrients to a 40,000-litre bag of deep water. We have no spare bag of deep water and time is not on our side to collect another batch of deep-water. We therefore cannot afford to get it wrong, precision and a keen eye is key. Luckily, I am in good company, other members of the group are there to offer their help and another set of eyes on the calculations. After emails and discussions back and forth, I have the greenlight with the calculations and the fun part begins with preparing the solutions in the lab at 7:30 am. One hour and a half later, I have three 20 L bottles filled with nutrients ready to be added to the deep-water bag.

We load all the equipment onto Wasserman, and we are off to the mesocosm site where the deep-water bag is leisurely floating. We pump the nutrient solutions into the deep-water bag and ensure that it is fully mixed. However, since we estimated the volume of the water in the bag for the calculations, we have to take a sample of the deep water after the nutrient addition to verify the nutrient levels before adding the water into the mesocosms, hopefully before the end of the day. Back in the lab, I am pacing back and forth as I await the nutrient data from Natalie which will confirm that everything went well…or not. The results put a smile on my face because guess what guys…, it worked out well. But as I said, precision is key; we estimated less water in the bag than was actually there and were slightly short of the target nutrient levels. However, we can still add the deep water to the mesocosms for now. The game is still on though since we have to carry out another nutrient addition to the bag before the next deep-water addition into the mesocosms can take place. One more chance, only this time, we know the exact volume of the deep-water bag. This should at least guarantee us that in the next deep-water addition, the phytoplankton in the mesocosms receive sufficient nitrogen, phosphorus and silicate from the deep water. After all they have been through in the mesocosms, we at least owe them a healthy meal!!

Leila on the way to DW addition Leila on the way out to deliver the plankton meal

Riding the DW bag Riding the deep water bag: UW pump is lowered into the bag