Saros, J. E. 2014. Determining critical nitrogen loads to boreal lake ecosystems: The response of phytoplankton at Acadia and Isle Royale National Parks. Natural Resource Technical Report NPS/ACAD/NRTR—2014/862. National Park Service, Fort Collins, Colorado.

Saros, Jasmine

In the northeast and upper Midwest, boreal lake ecosystems are common, particularly in national parks such as Acadia (ACAD) and Isle Royale (ISRO). Atmospheric deposition is of concern in these boreal lakes, as many are situated in relatively close proximity to industrial areas and along pollution dispersal trajectories. Only slight declines in nitrogenous compounds have been observed over the last decade in these areas, and enhanced atmospheric nitrogen (N) deposition can affect aquatic ecosystems by lowering pH as well as by providing a biologically-available form of N for algal growth. Furthermore, the reduction in sulfur deposition, along with changes in climate, are affecting the delivery of dissolved organic carbon (DOC) to boreal lake ecosystems, with increasing DOC concentrations observed in surface waters in both ACAD and ISRO in recent decades. The effects of increasing DOC on phytoplankton production can vary, as DOC can add nutrients to lakes but also reduce light availability in the water column. The response of algae in lakes in ACAD and ISRO to these N and DOC changes was assessed using experiments and lake sediment cores. Experiments were used to assess which nutrients (N, phosphorus (P), or N and P) are limiting in Jordan Pond and Echo Lake in ACAD and in Sargent Lake and Richie Lake in ISRO. A second set of experiments was established in one lake in each park (Jordan Pond and Sargent Lake) for two reasons: to assess the response of algae along a N gradient, and to assess the response to DOC manipulations. To determine the type of effect that DOC has on algae (light versus nutrient effects), two types of DOC manipulations were used in this experiment: DOC shaded (no direct contact with algae), and DOC added (direct contact). The response of algae to environmental change over the past century was determined in two lakes in ACAD (Jordan Pond and Bubble Pond) and one lake in ISRO (Siskiwit Lake). This response was measured using diatom fossils and algal pigments, as well as the carbon and nitrogen content of sedimentary organic material. Algal growth in the two ACAD lakes was co-limited by N and P, whereas it was limited by N alone in the two ISRO lakes. There was thus no response to the N gradient experiment in ACAD, whereas algal biomass increased with N additions in Sargent Lake, a N-limited ISRO lake. The first positive response to N additions in Sargent Lake occurred at 10 mg L-1 NO3-- N, resulting in a 10% increase in biomass. The DOC shaded treatments had no effect in any case, whereas the DOC added treatments stimulated algal growth in both cases (in Jordan Pond and in Sargent Lake), indicating that DOC additions added nutrients. Overall, the effects of N additions were dependent on nutrient limitation status (i.e., effect observed in N-limited lakes), whereas regardless of the nutrient limitation pattern, the DOC additions always stimulated algal growth. While the nitrogen isotopes in sediment cores reflect increasing N deposition since 1950 in all three lakes, there was generally little change in diatom fossils or algal pigments over the 20th century. The exception to this was the diatom fossil record of Siskiwit Lake in ISRO, in which changes in diatom community structure in this large, deep lake suggested climate-driven changes in this lake over the past century.

Saros, J. E. 2014. Determining critical nitrogen loads to boreal lake ecosystems: The response of phytoplankton at Acadia and Isle Royale National Parks. Natural Resource Technical Report NPS/ACAD/NRTR—2014/862. National Park Service, Fort Collins, Colorado.

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