In the last post I broadly described some of the ecological changes underway in the Palmer LTER. If we consider only the biological components of any ecosystem (excluding the chemical and physical components) we can diagram their interactions as a food web, with primary producers at the base of the web and top predators at the top. Changes to the structure of the food web are generally driven by changes at the top (‘top-down’ effects) or at the bottom (‘bottom-up’ effects). Whaling during the 19th century for example, exerted a very strong top-down effect on the structure of Antarctic food webs. Rapidly changing sea ice conditions during this century are exerting a strong bottom-up effect by altering the pattern of primary production.
The classic Antarctic food web. The traditional food web is a good learning tool, but is incomplete. In reality a substantial amount of biomass cycles through the marine microbial community. Taken from http://science.jrank.org/kids/pages/63/FITTING-ALL-TOGETHER.html.
The traditional food web that we all learned in elementary school is (not surprisingly) an incomplete representation of the trophic structure of an ecosystem. If we consider the carbon flowing through such a food web we see it move only in one direction, up from the primary producers to the top consumers. A rule of thumb is that the biomass of each trophic level is one tenth of the one below it, so as carbon moves up through the food web most of it must be exiting the system. Where does it go? Much of it is lost through respiration – we and nearly all other multicellular organisms exhale carbon dioxide. The rest is lost through inefficient feeding, excretion, and death in the consumer trophic levels.
A pelagic food web with the microbial loop. The diagram represents an open-ocean environment different from the west Antarctic Peninsula, but does a good job at highlighting the function of the microbial loop (at left). DOC leaks from the food web at every trophic level. Bacteria incorporate this DOC into microbial biomass and are consumed by small zooplankton, returning the carbon to the food web. Taken from http://oceanworld.tamu.edu/resources/oceanography-book/microbialweb.htm.
If that was the end of the story carbon would be raining down to the deep ocean at a rapid pace in the form of dead phytoplankton, dead consumers, and fecal pellets (this is actually the goal of ocean fertilization), and the photic zone, the sunlight portion of the ocean that supports photosynthesis, would be awash in small particles of organic carbon (we call this dissolved organic carbon or DOC) that aren’t practical for larger organisms to eat.
What limits this in the ocean are bacteria, which consume DOC and partially degrade detrital particles. Bacteria are large enough to be prey items for some consumers, so the carbon they incorporate is recycled into the food web in a process known as the microbial loop. Bacteria take up a huge amount of the carbon that leaves the food web, but not all of it is looped back up to the higher trophic levels. Like multicellular organisms heterotrophic bacteria respire carbon dioxide. If that carbon dioxide isn’t take back up by phytoplankton (a further loop embedded in the food web) it will eventually leave the system.
Depth integrated bacterial production for the Palmer LTER during the 2014 LTER cruise, estimated from the uptake of tritiated leucine.
How much carbon is taken up by primary production, incorporated into bacterial biomass, and exported from the photic zone are huge research questions. None of these questions are particularly easy to answer, and the numbers vary widely across time and space. On the Palmer LTER cruise we used three different methods to estimate these values for the west Antarctic Peninsula. I’ll talk a little about the first two here. Bacterial production is measured by the uptake of tritiated leucine. Leucine is an amino acid that is broadly taken up by the marine microbial community. Tritium is a radioactive isotope of hydrogen, tritiated leucine is leucine with tritium in place of hydrogen atoms. It is possible to quantitatively measure very small amounts of radioactivity, which makes it possible to measure the incorporation of very small amounts of tritiated leucine into microbial biomass.
To do this we take water samples from multiple depths at each station and incubate them with tritiated leucine at the in-situ temperature. After a few hours the bacteria in the water samples are killed (“fixed”), carefully washed, and measured for radioactivity. From this a rate of leucine incorporation is calculated. This number tells us something about the rate at which bacteria are increasing their biomass. What we actually want to know however, is how much carbon they are taking up to do this. Many years of experiments have allowed the development of a conversion factor to calculate the rate of carbon incorporation from the rate of leucine incorporation. What this method doesn’t tell us however, is the amount of carbon that is consumed during microbial respiration – a much harder number to get at!
Depth integrated primary production for the Palmer LTER during the 2014 LTER cruise. Estimated from the uptake of 14C bicarbonate.
The method for measuring primary production is similar to that for measuring bacterial production, except that radioactive bicarbonate (labeled with the 14C isotope of carbon) is used in place of radioactive leucine. In seawater the bicarbonate exists as carbon dioxide and is incorporated into phytoplankton biomass during photosynthesis.
All of this fixed carbon exits the system in one of two ways. Either it is respired as carbon dioxide as it works its way up the food web, eventually ending up in the atmosphere, or it is exported out of the photic zone as sinking detrital particles. Particulate matter that is not consumed by bacteria below the photic zone will eventually end up on the seafloor. The consumption of this carbon by the microbial community is often very high right at the seafloor, but rapidly diminishes below the sediment surface as oxidized chemical species are depleted. Carbon that makes it to this depth is effectively sequestered, until a geological process (such as subduction) returns it to the surface. Measuring the flux of particulate matter to the seafloor relies on the use of sediment traps or the measurement of radioactive thorium-234, a process that is far more complex than the measure of tritiated leucine or 14C bicarbonate. To learn more about it check out the website of Café Thorium, Ken Buessler’s research group at WHOI, particularly this article.
The two figures above show our estimates of primary and bacterial production across the study area. Both sets of values are pretty high. There was a strong phytoplankton bloom this year (perhaps connected to improved sea ice coverage) and the bacterial community seems to have responded in kind to this input of fixed carbon. Note however, that bacterial production is an order of magnitude below primary production. Most of the carbon is exiting the system through respiration or particle export. A small amount is contained within the biomass of phytoplankton, krill, and the higher trophic levels. If you look carefully at the two plots you’ll also see that bacterial production is somewhat decoupled from primary production, being high where primary production is low and sometimes low when it is high. The devil is in the detail and it will take some digging to understand the dynamics of carbon transfer in this system!
The bacterial abundance, production, and export team for the 2014 Palmer LTER cruise, on the deck of the ARSV Laurence M. Gould. The odd looking contraption behind us is a French made sediment trap. Sediment traps are invaluable tools, but famously undersample particles sinking to the seafloor. The French design is supposed to minimize these effects. Fun fact – tenured WHOI faculty are impervious to the cold and to head injuries.
