A long-term goal of marine biogeochemical studies is to understand the processes involved in biological carbon sequestration in the ocean. Predicting the future course of atmospheric carbon dioxide (CO2) increases in response to anthropogenic emissions requires a complete understanding of all factors that govern the carbon cycle. A major sink of CO2 is the ocean “biological pump” – a term used to describe the transfer of carbon to the deep ocean for long term storage. Carbon is removed from the atmosphere via photosynthesis by phytoplankton, and ultimately ends up in the deep ocean as sinking organic matter.
Some of the carbon-containing organic matter that sinks to the deep ocean is found in the fecal pellets of zooplankton, especially copepods. By most estimates, copepods are the most abundant animal on the planet by weight. They feed upon sharp and spiky phytoplankton, including glass-walled diatoms, so their fecal pellets have a strong membrane for a more comfortable exit. Compared to those of other organisms, copepod fecal pellets are fairly dense, hydrodynamic, and resistant to bacterial composition. Thus they sink to bottom of ocean faster, bringing plenty of carbon down with them.
I am investigating the impact of atmospheric dust deposition on the feeding and digestion processes of zooplankton, and its implications for the biological pump.
It has been suggested that dust can increase the efficiency of the biological pump through various physical and chemical means. Dust is known to be a source of limiting nutrients and minerals to the ocean, and has shown to affect the responses of phytoplankton. However, we ultimately haven't been able to figure out the specifics of how this all works, and how to predict just how much carbon will sink.
So for my project, I am asking: what about the zooplankton, who eat the phytoplankton? How does dust deposition affect their digestion processes, and ultimately the amount of carbon sent to the ocean floor for long-term burial (via their poop)?
Our lab did some preliminary feeding experiments which yielded some very interesting results. Zooplankton were given different feeding solutions: some with only phytoplankton, and some had local dust mixed in as well. The zooplankton who ate the "dusty" solutions were eating a whole lot faster than those without dust... and they were producing more fecal pellets that had more carbon inside them.
It seems that dust particles act as "roughage" inside zooplankton guts: Food passes through them faster without being digested as efficiently. Reduced digestion means more of the carbon they consume remains trapped inside their fecal pellets, which will eventually sink to the deep ocean. On top of that, minerals in the dust make zooplankton fecal pellets bigger and denser, so they sink faster.
We are thinking that the presence of dust can result in the production of more fecal pellets, which sink faster, and contain greater amounts of carbon.... thus increasing the efficiency of the biological pump.
Dust itself is sensitive to climate, so it's important that we understand this feedback mechanism.
In the summertime, the Gulf of Aqaba is an oligotropic (nutrient-deplete) environment with a high dust input. Dust storms occur year round, with maximum inputs in late spring. This summer I will deploy sediment traps in the Gulf of Aqaba in time series and access archived samples from periods of variable dust deposition. I will also run zooplankton feeding experiments in seawater tanks at both the Gulf of Aqaba and the Mediterranean Sea. At IUI, I will collect and analyze zooplankton fecal pellets from both trap samples as well as the experiments.
Results will be compared with similar experiments conducted back in California, as well as data from fecal pellets collected from sediment trap archives across the world. Research will provide a wide range of results relevant to many disciplines. No quantitative or mechanistic understanding for these relations is currently available, and data documenting the process is limited. Investigation of this interaction will help improve our model of the ocean carbon cycle, and our understanding of carbon sequestration via the biological pump.
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