Plankton in the spotlight
Marine Census Feature...
A system of oceanographic profiling instruments and high definition video is allowing scientists to study fragile, planktonic organisms that are often destroyed by traditional sampling methods.
Among the images that this system brought back to the surface were schools of the Antarctic silverfish (Pleurogramma antarcticum), often at depths of around 400 m over the continental shelf – an awfully long way for a penguin to dive after them. This video data will be compared with sonar data from the ship and the results from trawls done at the same stations to provide a clearer picture of the behaviour and distribution of this important prey fish.
The AVPR system also photographed many different planktonic organisms, including juveniles of the comb jelly Callianira antarctica that were so fragile they were not sampled successfully in any of the seven different types of net used to survey the water column. Other fragile denizens of the Southern Ocean are the siphonophores – colonial jellyfish that form chains that are often ripped apart into their individual constituents in a plankton net and need to be pieced back together under the microscope, like a jigsaw puzzle, to determine their species. These chains are the asexual generation of the life cycle, while the sexual generation is a much smaller colony called a 'eudoxid'. The AVPR photographed a variety of these tiny eudoxids and gathered the first data on their exact distributional depths and their inferred habitat preferences.
As well as plankton photographs, the AVPR system took colour images of the other particles in the water column. To no-one's surprise the highest concentrations of particles were found in the upper hundred metres and were clearly linked to the chlorophyll concentrations in the water. This suggested that these particles were phytoplankton in origin – the plants of the open ocean.
The AVPR photographed Acantharians throughout the water column, all the way to its maximum deployment depth of 1000 m. Analyses of these results should give us a greater understanding of the role Acantharians play in the biogeochemical cycles of the Southern Ocean – one of the most important oceans affecting the climate of the Earth.
Deep Sea Ecosphere Research Team, Japan Agency for Marine-Earth Science and Technology
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