The invisible forest

A drift on our research station in the pack ice north of Svalbard our senses got accustomed to a white landscape formed by ice and snow. After weeks of criss-crossing this frozen landscape by foot or skidoo one might easily forget that this is an ephemeral landscape, a thin frozen crust on top of the Arctic Ocean.

Sea ice

The dark ocean underneath revealed itself through cracks and leads in the ice or holes established by us for research purposes. However, it is not as barren as one might think. Microscopic, unicellular algae (called phytoplankton) invisible to our human eye drift with the ocean currents just below our feet. Photo: Tor Ivan Karlsen / Norwegian Polar Institute

Filter stained reddish-brown (A) by the ciliate Mesodinium rubrum (B).

Filter stained reddish-brown (A) by the ciliate Mesodinium rubrum (B).

Microscopic pictures of three diatom species, Chaetoceros atlanticus (A) and two Thalassiosira species (B,C), and a colony of Phaeocystis pouchetii (D).

Microscopic pictures of three diatom species, Chaetoceros atlanticus (A) and two Thalassiosira species (B,C) and a colony of Phaeocystis pouchetii (D).

The dark ocean underneath revealed itself through cracks and leads in the ice or holes established by us for research purposes. Peeking through these windows into the abyss below, the Arctic Ocean seems a dark and barren place. And it is indeed a dark place because the ice and particularly the snow strongly limit the amount of light reaching the ocean. However, it is not as barren as one might think. Microscopic, unicellular algae (called phytoplankton) invisible to our human eye drift with the ocean currents just below our feet. They are amongst the smallest organisms on the planet running its largest ecosystem, the oceans, and provide every second breath of oxygen we breathe. Phytoplankton come in a wide diversity of shapes and forms and span a large size-range (nm to mm) in the microscopic world, which on our macroscopic scale would translate to a size-range from mosses to the tallest trees (cm to 100 m).

Throughout the first legs of N-ICE2015, covering the dark season and early spring, phytoplankton biomass was low and dominated by the tiniest algae (< 0.01 mm). You can imagine our excitement when we saw the first signs of color on the filters we use to collect particles from seawater. The color was a rather unusual dark reddish-brown that prompted further investigation under the microscope. The first impression was that of a bee hive, swarms of small hairy lumps darting through the field of view. Closer inspection revealed it was not an algae but the ciliate Mesodinium rubrum. This is a truly enigmatic organism because it robs the chloroplasts from its prey (a small algae) to perform photosynthesis (as mentioned in a previous blog). It thus falls in neither the plant nor the animal category but can be considered a chimaera that combines the best of both worlds.

Although interesting in itself the bloom by Mesodinium rubrum dwarfs against the phytoplankton spring bloom of the Arctic shelf seas. Drifting in close proximity to the Yermak plateau we recorded the first signs of the phytoplankton spring bloom in early May. This initial increase in phytoplankton biomass was mainly due to diatoms, algae living in a glass shell. Diatoms are usually the first on the scene because they grow fast and at the same are well defended, unpalatable or simply too large for most planktonic herbivores. A large fraction of the accumulated diatom biomass sinks ungrazed to the ocean interior and the seafloor and is thus an important component of the ocean's biological carbon pump. Towards the end of May and early June another type of algae started to dominate, the colonial stage of Phaeocystis pouchetii, a species belonging to the division of algae called haptophytes. This species lives as a tiny solitary flagellate (0.005 mm in diameter) during the dark season and early spring but then starts to form colonies initially attached to large, spiny diatoms that provide protection against grazers and eventually mm-sized, free-floating colonies composed of hundreds of individual flagellates. These are just preliminary results but N-ICE2015 will surely improve our understanding of the factors that drive phytoplankton blooms in the ice covered Arctic Ocean, their timing in relation to the ice algal bloom and the appearance of pelagic grazers.