ANT XXV/3, Weekly Report No. 5
8 February 2009 to 15 February
Last week was spent anxiously watching the movements of the two buoys we had deployed in the centre of the eddy. They are specially designed to mark the movement of surface water and their exact positions could be monitored almost in real time on the manufacturer’s web site. The buoy is a sealed metal ball of 40 cm diameter that lies low in the water to avoid being pushed by winds. Its location in the surface is determined by a drogue attached to it by a thin wire and consisting of 7 m long, thin plastic sails which “anchor” it at 30 m depth. The 2 buoys were deployed 3 km apart and the northerly one was used as the centre while fertilizing the patch. We expected them to bob around where we had placed them for the duration of the eddy but we had not reckoned with the transporting abilities of an apparent force known as inertial oscillation that revealed its presence in the looping northward motion of the buoys before our apprehensive eyes.
This force transfers wind energy into circular motion of the ocean surface with a period of 16 hours at our latitude but with counter-clockwise rotation in our hemisphere (the Southern) evident in the initial loops made by the buoys. The inertial oscillation is superimposed on the clockwise rotation of our eddy and its effect may be completely obscured when currents are strong. The buoys at first looped slowly northward until they reached the band of swift currents keeping our eddy in position. This current then deflected their movement to the east and then south (we breathed a sigh of relief when this happened, even though we were expecting it to happen) and finally to their current position where they are moving slowly to the west, back into the closed eddy core. We were worried that the outer flank of our patch might “stick” to the swift band of currents and be pulled out of the eddy centre in filaments, but at the time of writing this does not seem to have occurred. The buoys are almost stationary and close together as ever, the inertial oscillation pushing northward is counter-balanced by the current field surrounding them. This results in horizontal mixing of the surface layer and elongation in a north-south direction of the once circular patch. These results are interesting in their own right as they contribute to our understanding of small-scale mixing of water masses of the Circumpolar Current on their way around Antarctica before they are submerged to the deep ocean along its northern boundary.
During the first few days of last week we were forced to interrupt the Scanfish survey of the patch and retreat to the north to avoid a vicious band of winds trailing a strong storm passing along to the south. This was a welcome respite from the hard work we had been carrying out taking station after station during the previous days. Had we weathered the storm in the eddy, no one would have been able to work or sleep because of the jarring bumps caused by the passage of huge waves. The wind blew us to the east, so after it had subsided sufficiently, we steamed due south and deployed a buoy in the centre of the companion red (warm core) eddy adjoining ours to the east but spinning counter-clockwise (see picture in first report) to monitor the intensity of its opposite rotating motion. We then returned to our eddy and headed west-north-west toward the buoys that had now, to our great relief, started moving due westward, i.e. they were returning toward the eddy centre. About an hour before we reached them, the photosynthetic efficiency signalled by the FRRF rose sharply: we had entered our fertilized patch. Although we knew the patch had to be there, and the phytoplankton continuing to respond to iron fertilization with enhanced photosynthetic efficiency, entering the patch after some days of separation is always a moment of great elation and relief.
To make sure that the twin buoys were still located reasonably well inside the patch, we passed between them and continued until we reached the other side of the patch, signalled by sharply decreasing Fv/Fm ratios and then returned to the buoys where we took the next long, in-patch station, one week after fertilization. Chlorophyll concentrations had increased significantly indicating that the phytoplankton cells had invested the iron received in raising production of this crucial molecule. Since the weather had roughened again preventing our doing further station work, we carried out east-west transects 8 km apart while moving northward to map the outer boundaries of our patch with FRRF and SF6 measurements. After completing the large-scale grid we nested a smaller one with latitudinal transects only 3 km apart on the patch to ascertain small-scale patchiness within it and mixing along its boundaries. The weather had calmed down so we took another in-station next to the buoys after making sure they were well within the patch. They were now moving westward rapidly and the ship followed their track during the 20 hour station. However, we were dismayed at finding, during the station, that the patch was not following them. So this station was not representative of the patch and hence had been a waste of effort.
We again carried out transects to locate the patch and found that it had continued moving south instead of west. So we deployed a fourth buoy in an area of strong signals and described a perfect circle of 6 km diameter around it to ensure that it was not at the edge of the patch after which we took an in-station in its centre. This proved to be a perfect in-station with chlorophyll concentrations above 1.2 mg Chl m-3, about double the initial values. Most of the other measurements also indicated that the ecosystem was responding and we shall describe the changes and their implications in future reports. The important thing was that the patch was intact, it was not moving much and was clearly going to stay within the eddy core, at least for the time being.
Process studies such as ours are inherently risky because their success depends on whether the process under study can be followed long enough to yield a meaningful data set and whether the measurements are comprehensive enough to cover all the aspects needed to understand it quantitatively. The process we are trying to understand is a link in the chain of feedback processes that shape earth’s climate. It is driven by the biology of the organisms inhabiting the surface layer of the ocean of which the vast majority are unicellular and range in size from just under one micrometer (the size of dwarf bacteria) to about a millimetre (attained by giant cells of several groups of protozoa and algae), when they just become visible to the naked eye. Life in this size range is inaccessible to our sense organs so biological and chemical oceanographers have developed a broad range of techniques to gather indirect information on the workings of this largest planetary ecosystem with the aim of understanding how it functions and what role the different types of ecosystems play in the global climate scenario. The aim of LOHAFEX is to perturb a planktonic ecosystem in a late stage of its seasonal development by stimulating phytoplankton growth, and then following what happens to the new biomass within the planktonic food web and what effect it has on the chemistry of the surface layer, in particular the air-sea exchange of gases.
In the vast tract of ocean we have investigated so far, the plankton community appears to be of the same type, dominated by small phytoplankton cells and ruled by the grazing activity of mainly two copepod species of which the larger (Calanus simillimus, picture in previous report) is about the body size of a small mosquito (3 mm in length). However, the other species Oithona similis is much smaller, about 0.7 mm in length. Both species consume much the same food: phytoplankton and protozoan cells larger than about 0.005 mm. In the past, it was believed that copepods indiscriminately fed by sieving particles from the water but the improvement in camera technology has revealed that they are actually quite selective in their feeding behaviour: they grab individual particles floating or swimming past them, eating some and rejecting others. In general, copepods seem to prefer the small ciliates: tiny cone-shaped animals which dart around at high speed by means of a ring of cilia - hair-like appendages, shorter but stronger than flagellae - surrounding their “mouths”, which they beat like propellers. They are the predators within the microbial food web which we shall describe later.
Adult C. simillimus females are rare in our samples, so this species is not multiplying. Almost all the individuals are juveniles growing in size until they reach the larval stage in which they hibernate in the deep ocean. Most have already reached this stage, i.e. their body machinery is complete, and their food is now presumably being converted into fat in preparation for hibernation which is spent deep down in the water column. Fats and oils are hydrocarbons consisting mainly of carbon and hydrogen, so the other plant nutrients: nitrogen, phosphorus and presumably also iron, are recycled back to the ecosystem. In other words, these copepods are extracting and sequestering only energy from their food and returning the building blocks (nutrient elements) for recycling by the phytoplankton. In contrast, Oithona similis, which is ubiquitous from the coast to the open ocean, spends all its life in the surface layer, so its population consists of adults as well as all the larval stages. The females are producing eggs, which are carried in egg sacs, and the numerous, minute larvae flit around in water samples concentrated with fine-meshed nets. This population is clearly retaining the nutrients it is absorbing from its food and investing the proteins and DNA in population growth. Their predators will then be recycling the nutrients back into the system, depending on whether fat reserves are being accumulated or whether food is being invested in body growth and eggs. However, as we shall see in later reports, predation on the copepod populations seems, until now, relatively low.
The substantial grazing pressure exerted on the unicellular plankton, including the phytoplankton, is evidenced by the abundance of well-chewed faecal material present in the surface layer. Copepod faeces have high sinking rates in laboratory jars so it is expected that their defecation contributes significantly to the rain of organic material transporting carbon to the deep sea known as the biological carbon pump (BCP). However, opinions are divided as to the quantitative role of the different processes fuelling the pump of which sinking algal cells, faecal material from grazing zooplankton and predation of deep-sea organisms on hibernating copepods at depth, which retains their carbon there (the survivors return to the surface the next year), are the main sources. Since the BCP is one of the major processes involved in the global carbon cycle regulating atmospheric CO2 concentrations, there is an urgent need to improve our understanding of the factors driving it.
It is difficult to track particles sinking from the surface layer through the deep water column because they are quickly diluted in the vast volume they traverse on the way down. Since quantifying the different components of the BCP is a major aim of LOHAFEX, we have on board two recently developed techniques to record this flux. One of these is the funnel-shaped neutrally buoyant sediment trap called PELAGRA, developed by a group in Southampton, UK, that are programmed to stay at a given depth (a few 100 m) for a given time period (a few days) where they collect sinking particles in 4 polycarbonate jars that are closed before the trap surfaces (see picture). We have 5 traps on board of which at least two are in operation at a given time: one under and the other outside the patch to ensure full coverage. Handling them requires a great deal of skill and experience but they have functioned very well so far. The traps, deployed at 200 and 450 m depths, have come up with almost empty collection cups, apart from some zooplankton organisms that presumably swam into them. These are painstakingly removed under a microscope, no mean feat on board a rolling ship, and the remaining particles in the collection jars examined thoroughly before being preserved for further analyses in the home lab. One interesting result already achieved is the small amount of copepod faeces produced in the surface layer that actually sink to depth, indicating that the bulk is recycled. However, some of the faecal material does sink out and it is of interest to see whether the proportion increases under the patch: more food, more faeces, more sinking, or not. We are monitoring production rate and fate of the faeces.
The other technique to estimate the amount and composition of sinking particles is to observe their depth distribution with a camera system called UVP developed by a team in Villefranche, France. It is attached to the CTD and rapidly photographs 1 litre slabs of water illuminated by strobe lights at 5 frames per second while lowered at a speed of 1m a second from the surface to 3000m depth. This camera records particles down to 0.06 mm diameter and provides images of objects larger than 0.5 mm that can be identified by eye. In the upper 100 m there are about 50 objects above 0.06 mm in diameter per picture of which half a dozen are large enough to be identified. Even in the deepest layer, between 2000 and 3000m, there are still around 3 objects per picture and an identifiable one every ten images on average which is more than expected. A carefully programmed computer sorts the recorded objects in few groups: various types of zooplankton, detrital material and small particles, thus providing vertical profiles of abundance and bio-volume. The system is particularly good at capturing copepods and their faeces, thus revealing details of copepod natural history that cannot be gleaned from net catches or acoustic techniques (see picture).
We are now well into the routine phase of the experiment: taking stations inside and outside the patch at regular intervals while monitoring its movements within the eddy. Although the work is tiring, we are happy and excited by the prospects ahead.
Wajih Naqvi and Victor Smetacek