Wow, what an abysmal smell
There is no easier and more direct way of distinguishing the different treatments of our experiment than by looking at the material sinking out of the water column. It accumulates in the sediment traps at the bottom of the mesocosms and is collected and processed by Jana each sampling day. Just look at the different colours of the samples taken from the different silicate treatments, here aligned from mesocosm 1 to 8 from left to right. A light brown in bottles 4 and 8, sampled from the treatments with the highest silicate addition, to a solid black or dark grey in bottles 3 and 5, those retrieved from the mesocosms with the lowest silicate treatment. And if the colours don’t make a lasting impression on you, open the bottles and take a smell for an unforgettable experience. Starting with 4 and 8 you’re likely to think: oh, smells like algae rotting on the beach. When placing your nose above 3 and 5, watch out for the people behind you as you may jump back in a flash of disgust. Wow, what a ghastly, abysmal smell! Poor Jana who has to process those stinky samples each and every sampling day.
But how is this possible? The only difference in how we treat the eight mesocosms is in the amount of silicate supplied with the nutrient-rich deep water. Why would silicate determine whether the sinking material comes out either brownish with a slightly uncomfortable odour or black and horribly stinky? The answer lies in the food web that develops in response to the silicate treatment. At high silicate supply, diatoms are the dominant phytoplankton, as they need silicate to build their frustules (see Andrea’s and Megi’s blog posts for some nice insights into this). Their photosynthetic pigments give them their characteristic brownish colour, just like the material collected in the high silicate mesocosms. The moderately awful smell indicates that they sink out fairly fresh. At the other end of our silicate addition spectrum, other phytoplankton that don’t need any silicate consume the regular dose of deep-water nutrients. The dark colouring and disgusting smell of the sedimented matter indicates that the biomass produced at low silicate supply has undergone heavy degradation before accumulating in the sediment traps.
To my great surprise also the amount of material collected in the sediment traps differs strongly, with much higher sedimentation rates in the high silicate treatments. Whether this means that high silicate also drives high export of carbon to the deep ocean depends on two other key parameters: the sinking speed of the particulate matter sinking to depth and the rate at which it is remineralized. These two variables are measured by Moritz every second sampling day, so I’m really excited to see what they will teach us.
The more important message the colouring and odour of the sedimented matter sends us is that silicate is not just a subordinate driver of pelagic ecology and biogeochemistry, far back in line after nitrate, phosphate, iron, temperature, CO2 or oxygen. One that can easily be ignored in global biogeochemical cycling and marine food webs. To the contrary, it seems that silicate controls who grows, who eats, how much sinks and how it sinks. In this sense the present experiment was a real eye-opener for us. Silicate, the overlooked ecological and biogeochemical driver of the pelagic ocean.
