Changes in nutrient retention capacity of boreal aquatic ecosystems under climate warming: a simulation study

Loading, retention and release of nitrogen and phosphorus were studied for three growing seasons in experimental flow-through pond ecosystems, where impacts of rising temperature on the boreal zone were studied; one pond was under ambient temperature conditions and another was enclosed in a greenhouse with air temperature 2-3 degreesC higher than ambient. The ponds received a high load of nitrogen (14-18 g N m(-2) in June-October) and a moderate load of phosphorus (0.2 g P m(-2)) from the nearby mesohumic Lake Paajarvi. During the growing seasons, the ponds retained inorganic nitrogen (NO3-N), and the retention improved in the course of the experiment from 29% to 76% in the greenhouse pond and from 33% to 76% in the reference pond. For total nitrogen (totN), the retention was, however, lower (15-22% in the greenhouse pond and 16-33% in the reference pond). The ponds acted more as sources than sinks for P; with the exception of the greenhouse pond during the first growing season more P was released from than retained in the ponds. During the second and third growing seasons, the release of P exceeded the load of P in the inflow by 65% and 72% in the greenhouse pond and to a lesser extent (by 49% and 32%, respectively) in the reference pond. This was likely due to uptake of P by macrophytes from the sediment, and P released from decaying plants was later utilized by littoral algal communities. In a warmer climate, the uptake of P by macrophytes from sediment may be an important accelerator of eutrophication of lakes in the boreal zone.