Surface water productivity, variability in export flux and deep-sea diversity how are they linked? What is this project about?
The deep-sea food chain relies on food falling from surface waters (with the exception of ecosystems driven by chemical energy, e.g. hydrothermal vents, cold seeps etc.
Phytoplankton are microscopic single celled algae that photosynthesise at the surface of the ocean. When they die, their remains sink through the water and can reach the sea floor thus providing a food source to deep-sea animals (click on figures for larger images).
Deep sea animals
One of the main scientific issues that we are trying to investigate is whether or not the organic chemicals contained in the phytodetritus arriving at the sea floor influence the types of animals living here. Deep-sea animals require certain vitamins and if they do not have a supply of these (in the phytodetritus) then it seems likely they won’t do well.
Phytoplankton, like all plants, need nutrients such as nitrogen and phosphorous to grow. In the Southern Indian Ocean, these nutrients are relatively plentiful, but one important element is missing, or is at least very scarce! Iron (Fe, is the chemical symbol, is present at a concentration of about 0.000000005 grams per litre, or less!). For most of the year, there is little photosynthesis going on because most of the essential nutrients are "trapped" in deep waters. However, from October onwards, oceanic circulation coupled with atmospheric conditions leads to deep nutrient-rich waters being driven to the surface (this is called "upwelling") to the north of Crozet. These waters may be rich in iron, because phytoplankton “bloom”. To the south of Crozet, there is no upwelling of deep water and so, no bloom.
The Figure shows some satellite data for chlorophyll abundance (a marker of phytopklankton) at the sea surface (to a few metres depth). Deeper reds are higher values. These data are a composite for 28th November – 4th December 2005 (courtesy of Dr. Tim Smyth, Remote Sensing Group at Plymouth Marine Laboratories). There is a clear and pretty intense bloom north of Crozet.
( Click on picture for a larger version!)
The organisms living at the deep-sea floor to the north and east of the islands are likely to get a seasonal pulse of material arriving from this bloom, whereas to the south, they will have no such luck! This contrast is the key to our work!
A 42-day cruise in the Austral summer of 2006 on the research ship RRS Discovery, was the third UK cruise to have sailed to the Crozet region of the Southern Indian Ocean in this century (The pioneering expedition of the HMS Challenger visited the Crozet islands almost exactly 130 years before we did!). Two cruises were carried out during the Austral summer of 2004/2005 (Discovery cruises 285 and 286).