Differential Responses of Maximum Versus Median Chlorophyll‐a to Air Temperature and Nutrient Loads in an Oligotrophic Lake Over 31 Years

• Published in: Water resources research 2020-07, Vol.56 (7), p.n/a
• Main Authors: Ward, Nicole K., Steele, Bethel G., Weathers, Kathleen C., Cottingham, Kathryn L., Ewing, Holly A., Hanson, Paul C., Carey, Cayelan C.
• Format: Article
• Language: English
• Subjects: Abundance; Air temperature; Algae; Aquatic ecosystems; Blooms; Chlorophyll; Chlorophyll a; Climate and land use; Climate change; Climate effects; Computer simulation; Eutrophic lakes; Eutrophication; Hydrodynamics; Lakes; Land development; Land use; Load; Loads (forces); long term; Mineral nutrients; modeling; Nutrient concentrations; Nutrient loading; Nutrient pollution; oligotrophic lake; Oligotrophic lakes; Phosphorus; Phytoplankton; Plankton; Pollution; Simulation; Summer; Temperature; Water pollution; Water quality
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2020WR027296

Globally, phytoplankton abundance is increasing in lakes as a result of climate change and land‐use change. The relative importance of climate and land‐use drivers has been examined primarily for mesotrophic and eutrophic lakes. However, oligotrophic lakes show different sensitivity to climate and land‐use drivers than mesotrophic and eutrophic lakes, necessitating further exploration of the relative contribution of the two drivers of change to increased phytoplankton abundance. Here, we investigated how air temperature (a driver related to climate change) and nutrient load (a driver related to land‐use and climate change) interact to alter water quality in oligotrophic Lake Sunapee, New Hampshire, USA. We used long‐term data and the one‐dimensional hydrodynamic General Lake Model (GLM) coupled with Aquatic EcoDyanmics (AED) modules to simulate water quality. Over the 31‐year simulation, summer median chlorophyll‐a concentration was positively associated with summer air temperature, whereas annual maximum chlorophyll‐a concentration was positively associated with the previous 3 years of external phosphorus load. Scenario testing demonstrated a 2°C increase in air temperature significantly increased summer median chlorophyll‐a concentration, but not annual maximum chlorophyll‐a concentration. For both maximum and median chlorophyll‐a concentration, doubling external nutrient loads of total nitrogen and total phosphorus at the same time, or doubling phosphorus alone, resulted in a significant increase. This study highlights the importance of aligning lake measurements with the ecosystem metrics of interest, as maximum chlorophyll‐a concentration may be more uniquely sensitive to nutrient load and that typical summer chlorophyll‐a concentration may increase due to warming alone. Plain Language Summary Clear water lakes are experiencing more frequent water quality problems due to land development and climate change. However, it is challenging to identify how land development and climate change interact to alter water quality because their effects are complex and occurring at the same time. We used three decades of observational data combined with a lake ecosystem simulation model to explore the role of land development and climate change on water quality. Our water quality indicator of focus was phytoplankton, which are small photosynthesizing organisms in the water, often referred to as “algae.” We found that the effects of land use and climate depend on if w