Arctic Tipping Points

A couple of weeks have passed and much has happened since the last blog. We have finalised our temperature experiments in the Royal Arctic Laboratory. Our simple temperature control system worked perfectly well and maintained our aquaria with kelp and eelgrass at temperatures of 10 (in situ), 12, 14, 16 and 19˚C. We grew 3 species of algae having different geographical distribution ranges, some extending to the south/mid coast of Greenland, others extending further north. During the last days in Nuuk we harvested the vegetation and measured survival, growth, photosynthesis and respiration.

The preliminary results show that some species like it hot while others do not! For example the intertidal kelp, knotted wrack (Ascophyllum nodosum) which has a geographical distribution range extending from Portugal in the south to Disko Bay on Greenlands midwest coast in the north, grew markedly faster when temperatures were raised (see figure below). Eelgrass also thrived at the warmer temperatures.

It takes 3 persons to measure 1 alg!! Photo Peter Bondo Christensen.

Birgit measuring eelgrass growth. Photo Dorte Krause-Jensen.

By contrast the subtidal kelp, longstiped tangle (Laminaria longicruris), which has a geographical distribution range extending to the north of Greenland, grew only slowly at the high temperatures (see figure) and some individuals died.

Since the temperature experiment ran well and took care of itself, the weeks in Nuuk also allowed time for field sampling. We sampled longstiped tangle and measured annual growth. Since the algae produce a new blade every year the growth can be assessed by the end of the growth season by measuring the sixe of the blade, and this was what we did. Some individuals had monster sizes (see photo).

We also sampled eelgrass and knotted wrack and measured biomass, density, growth and for knotted wrack also assessed age and population structure. Both species leaves marks that reveal their past growth. The underground rhizomes of eelgrass leave a mark for each blade that is produced and since the distance between marks is short in winter and long in summer it is possible to count the number of blades produced in a year. Knotted wrack produces a bladder every year so by counting the number of bladders we could assess the approximate age and by measuring the distance between bladders, we could assess growth. This demanded a lot of work and long days. Luckily we were 4 of us; Birgit had to go home to teach but Kitte came and assisted us with a lot of organisational skills and laboratory experience.
We now aim to compare the growth of these species in Nuuk with the growth at other latitudes in order to assess how growth varies along a climatic gradient and thereby get an indication of the response of the vegetation in the north to a future warming.

We packed down the royal laboratory and sent the equipment back home, but the work had not yet finished. The next leg of the program is that Nuria & Dorte join a 2-week cruise along Greenlands westcoast with MS FRAM, where they can sample during stops in settlements along the route. Peter joins another 2-week cruise in the Disko Bay with 3 colleagues to do vegetation and fauna surveys.