AMT Blog

On this vast deep blue slab of ocean, we have not seen any wildlife for days and it would appear on the surface that there is nothing out here except occasional white horses looming ferociously around the ship. Similarly the satellite ocean colour imagery that we receive daily from the National Earth Observation Data Archive and Analysis Service (NEODAAS) at Plymouth Marine Laboratory indicates that there is little phytoplankton in the surface layer of the ocean.

The Conductivity-Temperature-Density (CTD) trace operated by Dave and Terry and material from Chris’s zoo-plankton net hauls, however, paint a different picture. Below us between 80 and 130 meters, there is broad peak in the fluorescence profile indicating higher phytoplankton at depth which peaks at 110 meters. The sample that Chris takes from his zooplankton nets is teaming with tiny marine animals. The zooplankton become food for fish, such as anchovies, which Stuart and Jo have been detecting in the backscatter signal. They continually graze the phytoplankton crop, but are rather ‘sloppy feeders’ and as they feast on the phytoplankton cells, they release proteins, amino acids and nutrients back into the water, which Malcolm, Carolyn and Paul are measuring. These released nutrients, in turn, become food for marine bacteria and small phytoplankton. This process is known as the microbial loop whereby large zooplankton feed on small zooplankton which in turn feed on small phytoplankton. These open ocean regions are so deep (>4.5km) that nutrient rich deep waters seldom reach the surface and the sunlit, upper ocean is fuelled by re-mineralized nutrients from grazing and breakdown of phytoplankton cells by zooplankton, bacteria and marine viruses. The system is a delicate balance between phytoplankton growth, zooplankton grazing and the release and re-mineralisation of nutrients. Any adverse affects on this tightly coupled chain, can have major consequences higher up the food chain on the fish and whales that graze either the phyto- or zoo-plankton.

The sunlight is so intense at these latitudes that if the small pico and nano-phytoplankton, such as Syneccococus and Prochloroccocus, are carried to the surface, they can explode due to the intensity of the visible and ultra violet light. They therefore reside at the deeper layers of the ocean, surviving on low light and re-mineralised nutrients. Chris characterises the light field daily at solar noon. Glen measures the abundance of these small phytoplankton groups using flow cytometry and Mike, Manuela and Ross are looking at which phytoplankton groups are using which nutrients. Vas, Mario and I are measuring the fixation of carbon through photosynthesis in these different phytoplankton groups and the respiration and O2 consumption of the communities. As the mixed layer deepens, these organisms and other organic matter gets sheared upwards into high light and can be photo-chemically transformed into different components, which are either transformed into other nutrient pools or released as gases. Some of these photo-chemical processes are what Vas, Paul and myself are measuring. These vast blue deserts occupy 70% of the world’s oceans, and these tiny phytoplankton play a important role in maintaining the delicate food web that sustains life in these huge ocean ‘deserts’.

Better turn in soon less than 6 hrs until I have to get up for the next CTD.