D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Thursday 6 August 2009

Organic matter processes in the Twilight Zone

Photosynthetic primary production is the basis of much of the oceanic food chain. The energy fixed by phytoplankton is transferred to grazers and to higher consumers. The efficiency of the energy transfer between producer and consumer reflects the state of the ecosystem, for example nutrient replete vs. nutrient deficient. Through sinking, organic particles from surface waters are transferred to the deep sea, where they are an important resource (i.e. food) for deep-dwelling animals, be they living in pelagic (in the water) or benthic (at the sea floor) environments. We know that in the northern Atlantic Ocean at mid- to high latitude, the deposition of organic particles is seasonally driven by the surface water spring bloom.

However, there is considerable variability in the composition, and hence the nutritional quality, of the organic particles, which is driven partly by the nature of the phytoplankton, but also by the organisms that graze on them. Many of the organic particles leaving surface waters are lost or transformed in the so-called twilight zone (i.e. 200 - 1000 m of water depth), but we know virtually nothing about these processes there.

Certain organic chemicals such as fatty acids, sterols, pigments, or amino acids can retain information on their biological origin. These chemical are often called biological markers (or biomarkers). Changes in the distributions of biomarkers and their isotopic composition can shed light on the transformations of the organic material as it sinks through the water column. In addition, biomarkers and their stable isotopic composition are often used to trace the trophic relationships of many marine communities; this will help to determine the trophic transfer efficiency (i.e. the energy transfer) of the pelagic ecosystems of the twilight zone.

Collecting particles from the water can be carried out in a variety of ways. Large volume in situ filtration systems (SAPS) and sediment traps have been used routinely for this purpose. SAPS can pump up to 2000 L of water through a filter in a short period of time (usually 1-2 hours). The particles that are collected by SAPS are rather heterogeneous and can sink at variable speeds, whereas material that is collected by sediment traps is usually heavier and sinks at higher speeds. The exchange mechanisms between these two different “pools” of particles are virtually unknown but can potentially affect the export of organic matter at depth. Comparison of the chemical composition between the different pools of particles sampled concurrently (i.e. by SAPS and traps) and at similar water depths may provide a new insight in these processes.

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