Open AccessNitrogen assessment points: development and application to high‐elevation lakes in the Sierra Nevada, California
Author(s) -
Heard Andrea M.,
Sickman James O.
Publication year - 2016
Publication title -
ecosphere
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.255
H-Index - 57
ISSN - 2150-8925
DOI - 10.1002/ecs2.1586
Subject(s) - phytoplankton , environmental science , eutrophication , hydrology (agriculture) , nutrient , lake ecosystem , water quality , ecosystem , growing season , dry season , watershed , national park , wet season , ecology , biology , geology , geotechnical engineering , machine learning , computer science
Increased nitrogen (N) inputs to oligotrophic high‐elevation lakes are contributing to long‐term eutrophication, changes in nutrient cycles, and shifts in phytoplankton communities. State of California water quality standards do not adequately protect mountain lake ecosystems from atmospheric deposition, including lakes in national parks and other protected areas. The development of quantitative nutrient assessment points based on measurable ecological effects specific to high‐elevation lakes is an important step in long‐term protection of these ecosystems. We conducted in situ bioassay experiments during the phytoplankton growing season and modeled the response of phytoplankton to nutrient additions using algal growth models. Phytoplankton responded to nutrient additions (N and N + phosphorus) in five of seven experiments conducted in N‐limited lakes. Results were modeled using Michaelis‐Menten, Monod, and dose–response curves and used to calculate effective doses ( ED ) for excess phytoplankton growth. The 10%, 50%, and 90% ED estimates were determined for early and late hydrologic seasons. Results for the 10% ED were 0.33 (early hydrologic season) and 0.89 (late hydrologic season), the 50% ED were 1.0 μmol/L (early hydrologic season) and 4.0 μmol/L (late hydrologic season), and the 90% ED were 3.1 μmol/L (early hydrologic season) and 18 μmol/L (late hydrologic season). We then applied these assessment points to lake chemistry data from the National Park Service's Inventory and Monitoring Program to assess the status of lakes across Yosemite, Sequoia, and Kings Canyon national parks. Mean nitrate concentration during the growing season was 4.58 μmol/L and ranged from <0.04 to 11.8 μmol/L ( n = 75). The 10% ED were exceeded by 28% (late hydrologic season) and 37% (early hydrologic season) of lakes, the 50% ED were exceeded by 18% (late hydrologic season) and 29% (early hydrologic season) of lakes, and the 90% ED were exceeded by 0.0% (late hydrologic season) and 21% (early hydrologic season) of lakes. Our results suggest that phytoplankton populations in many Sierra Nevada lakes are affected by N deposition based on exceedance of growth assessment points. The most sensitive lakes are typically found at higher elevations in watersheds with steep, north‐facing slopes and minimal vegetation.