Lake morphology influences response of phytoplankton biomass to climate change

A collective prediction among ecologists postulates that climate change (and especially rising air temperatures) will increase phytoplankton biomass and frequency of harmful phytoplankton blooms in remote oligotrophic lakes. However, there is unique variation in the structure and functions of lakes to make this statement challengeable and, perhaps, inaccurate. To consider the variation in phytoplankton biomass, we used archived Landsat TM/ETM+ satellite products to estimated epilimnetic chlorophyll-a as a proxy for phytoplankton biomass in 281 northern temperate lakes over 28 years (1984 2011). Reflectance from the Landsat three-band algorithm ((B1-B3)/B2) showed the strongest correlation with in situ data explaining 78% of the variance in chlorophyll-a. In addition to assessing climate factors (air temperature, precipitation), we considered landscape properties as proxies for nutrient sources (i.e., proportion of wetlands in catchments, size of the littoral zone, potential for wind-driven sediment resuspension as estimated by the dynamic ratio) or nutrient sinks (lake volume) in a random forest model to explain heterogeneity in peak chlorophyll-a concentration (August October). Lakes with the higher chlorophyll-a (median Chl-a: 2.4 g L-1, n = 40) had smaller volumes (< 44 104 m3) and were more sensitive to increases in temperature. These lakes responded to climate in a more direct way (higher air temperature = higher chlorophyll-a). In contrast, lakes with lower chlorophyll-a (median Chl-a: 0.6 g L-1, n = 241) had larger volumes ( 44 104 m3), catchments with smaller proportion of wetlands (< 4.5% of catchment area, n = 70), less-developed littoral zones (< 2.9 ha, n = 137), minimal wind-driven sediment resuspension (as defined by the dynamic ratio; < 0.45, n = 232), and were more sensitive to increases in precipitation. Lakes with larger volumes were generally less responsive to climate factors; however, large-volume lakes with a significant proportion of wetlands and larger littoral zones behaved similarly to lakes with smaller volumes. Our finding that lakes with different landscape properties respond differently to climate factors will help predict lakes susceptible to phytoplankton blooms under changing climate.