Plankton in the spotlight
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.Flashing its strobes at 15 times per second, the autonomous visual plankton recorder (AVPR) is lowered from the deck of the Umitaka Maru and into the freezing waters below. Upon its return to the surface from depths of up to 1000 m, scientists will see for the first time images of living plankton, of the particles of organic matter upon which they may feed (marine snow), and who knows what else. This recently developed, state-of-the-art machine has been brought from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and installed into a stainless steel frame, along with sensors to measure conductivity (salinity), temperature, depth, dissolved oxygen, turbidity, transmissivity and fluorescence (chlorophyll concentrations), and a high-definition video camera system with a powerful light to illuminate the depths.
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.As well as the copious quantities of diatoms with their skeletons made of silica, the AVPR also recorded the presence of a potentially very important but largely unknown group of organisms – the Acantharia. These animals are planktonic, single celled organisms, which produce skeletons composed of strontium sulfate. This is a very rare material for an organism to produce in such quantities, but an important trace element needed by a variety of marine animals, from squids to corals. The ratio of strontium to calcium in the skeletons of animals such as corals is often used as a paleothermometer, to infer water temperature over geologic time. In our age of rapid climate change, any organism that can significantly affect the concentrations of strontium in seawater needs to be studied. Unfortunately the skeletons of Acantharia dissolve rapidly, making them difficult to study by traditional means.
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