Lohafex

Abajo reproducimos una parte del informe de la campaña Lohafex realizado por los Drs. Wajih Naqvi y Victor Smetacek, investigadores principales del proyecto de fertilización que se ha llevado a cabo en el buque Polarstern del "Alfred Wegener Institute" (AWI) en aguas del Océano Sur. Como pueden observar, se formó un bloom (ver foto) que fue controlado por el copépodo Calanus simillimus. Éstos a su vez fueron ingeridos por anfípodos. ¿A qué les suena todo esto? Creo que estos resultados son los que esperamos en nuestro proyecto, un aumento del zooplancton. ¿Quién se come este zooplancton? Alguna otra especie tendrá que hacerlo. El problema es que ellos no se podrán quedar en el Océano Sur para ver que ocurre con dicho zooplancton. Nosotros sí podremos estudiar lo que ocurre después. Se admiten comentarios.

Report

The results of the transects showed that our patch was indeed fairly homogeneous within its boundaries, which were sharply delineated to the south and along its sides, but was trailing a long tail of diluted water to the north. The pteropod swarm was not encountered again but the VPN images (the camera system attached to the CTD) showed that there were about twice as many copepods (Calanus simillimus) per square metre inside the patch compared to outside. Even more striking were the high densities of their main predators – the amphipod Themisto gaudichaudii – we had found at 3 previous in-stations: their numbers ranged from 30 to 138 specimens per square metre. Imagine that many 2 – 3 cm large beetles living off tens of thousands of 2 mm long aphids (the equivalent of C. simillimus densities), themselves feeding on the plants on a square metre of garden and you have an impression of the biomass of larger zooplankton maintained by our patch. At our latitude the amount of sunlight available to the plants is the same as in a cloudy August in northern Europe, so, although the plankton are distributed over a deep water column, the analogy is justified.

Tomado del informe de la campaña Lohafex (AWI).

We found several species of amphipods in the RMT tows but Themisto was by far the most abundant. They are tough, active animals, equipped with a range of grappling, sharply hooked legs in the front and paddle-like legs on their abdomen with which they scurry about in the buckets in which the net catches are emptied. It is a voracious carnivore that evidently feeds on other large zooplankton such as salps, chaetognaths (arrow worms) but also the local euphausiids (cousins of the better known krill of the south). Small groups even attacked the tiny fishes (myctophids) in the net catches, reducing them to skeletons in tens of minutes. In feeding experiments on board they also captured and ate copepods. They are visual predators with large compound lenses on top of the head (the translucent “caps” in the picture) indicating that they hunt by looking for prey above them, silhouetted against the weak light of the night sky. The black spots on the sides of the head in the picture are actually the retinas below the lenses. In the virtual absence of fishes (we caught very few mesopelagic fishes in the night net tows), amphipods are the only visual predators on plankton in this stretch of ocean, so the amazing transparency of their potential prey - from copepods to salps and chaetognaths – is witness to their predation pressure and their acute eye sight.

Calanus simillimus Photo: G. Mazzochi. AWI.

Amphipod numbers increased within the patch in the weeks following fertilization and by the middle of the experiment there were ten times more inside it than outside. They probably roam about in swarms and presumably entered our patch from the sides and stayed within it because of the higher copepod (C. simillimus) density. Previous experiments have also found higher copepod densities within iron-fertilized patches compared to outside. Since copepods, unlike amphipods and krill, are too small to swim into the patch horizontally, they are believed to congregate within it by adjusting their patterns of daily vertical migration. They feed on plankton in the surface layer only during the night and descend to a depth of around 100 m (demonstrated by the VPN images) where they spend the day, presumably invisible to visual predators. Their light sensors (they do not have lenses, so cannot see images) enable them to determine the appropriate depth, which depends on the depth of light penetration, which in turn depends on the amount of light-absorbing particles, particularly phytoplankton, in the surface layer. If surface and deeper layers move at different speeds then copepods spending the day at deeper depths, i.e. under a more transparent surface layer will be transported away from it. If they encounter a surface layer with more plankton, they will stay higher up during the day and hence add to the population already there. At this stage we cannot judge whether the VPN data are representative but will have to wait until the copepods in the net catches are counted and the relative movements of surface and deeper water layers in the patch have been analysed. So we cannot yet say what attracted the amphipods to the patch.

Amphipod Themisto gaudichaudii. Photo: Humberto González, UACh-COPAS. AWI.