Long-term study in an Antarctic shallow lake: evidence of the global change effect?

IZAGUIRRE, I.; PIZARRO, H.; ALLENDE, L.; RODRIGUEZ, P.; UNREIN, F.

Punta del Este, Uruguay

Conferencia; Shallow Lakes Conference; 2008

Since 1991, studies on the ecology and biodiversity of the planktonic and periphytic communities have been carried out in a shallow lake (Boeckella) from Hope Bay (Peninsula Antarctica). As other water bodies of Maritime Antarctica, this lake shows clear signs of natural eutrophication due to the inputs of organic matter of sea animals. It lies in a fluvio-glacial depression dammed by moraine sediments, which at the SW margin are covered by a layer of guano rich in calcium phosphate derived from the nearby penguin rookery. According to historical nutrient and chl a data, the lake can be classified as meso-eutrophic. First bathymetric studies carried out in the lake indicated that its maximum depth was about 4 m, it is cold polymictic, and it is usually ice-free during the austral summer.The lake was sampled during 10  summer periods, with an approximately weekly. periodicity. Different physical and chemical variables were measured in situ (temperature, pH, conductivity , dissolved oxygen, transparency, light penetration, and nutrient concentrations). Phytoplankton composition and abundance of the nano and microplankton fractions (algae > 2 µm) were studied all the summers, as well as total phytoplankton biomass (estimated by chl a). The picoplankton fraction was studied for the six last summer periods by epifluorescence and/or flow cytometry. On the other hand, the biodiversity and biomass of the benthic algal mats, and of the zooplankton community were also studied during two austral summers. Recent studies also included the quantification of other microbial planktonic components (heterotrophic nanoflagellates and ciliates), as well as molecular analyses of the nanoplankton biodiversity. As other Antarctic lakes, the food web of Boeckella Lake is relatively simple, and the copepods constitute the top of the trophic cascade.  Phytoplankton shows a relatively low diversity if compared with that of the lakes of similar trophic status at lower latitudes; this community includes a great proportion of picoplankton (Pi-Euk and Pi-Cy), and nanoplankton is dominated by flagellates (volvocaleans and mixotrophic chrysophytes). Usually, chrysophytes dominate when the lake surface is frozen. Zooplankton has also low diversity, and it is dominated by the copepod Boeckella poppei. Historical values of chl a in the lake varied from 0.04 to 9.43 µg L-1 (mean value  3.01µg L-1). At the beginning of the summer 2001, a drastic physical change took place in the lake as a result of the increase in the average temperature of recent years. There was a collapse of a portion of the lake basin, due to  the melting of the permafrost contained in the moraine material. This fact provoked an abrupt  water loss through by the effluent of the lake to the sea. The hydrometric level of the lake decreased about 3 m in only one night. An artificial damp was built in the outflow in order to partially reestablish the water level of the lake. During this summer chl a concentrations strongly increased in the lake (mean value 98.5 µg L-1). The pico-phytoplankton was the fraction that showed the most pronounced peak after this event, reaching a value of 6.98 x 106 cells ml-1.  Nutrients (DRP and DIN) and conductivity values also showed a marked increase after this episode. Contrarily, the epilithic community was negatively affected. The poor development of epilithon can be associated to: an increase in the kd and thus poor light conditions in the bottom, and to the fact that the littoral areas remained exposed and dried during a long period.