Using Wavelet Analysis to Examine Long-Term Variability of Phytoplankton Biomass in the Tropical, Saline Lake Alchichica, Mexico

The phytoplankton biomass (chlorophyll-a, Chl-a) is directly related to the total production of lakes. Chl-a in temperate lakes oscillates on an annual scale. However, Chl-a oscillations in tropical lakes have hardly been documented, particularly over multiple years. Here, we described the periodicity of the Chl-a by performing a continuous wavelet analysis of 21 years (1998-2018), monthly Chl-a data from tropical, saline Lake Alchichica, Mexico. Parallel wavelet analyses were made on environmental time series (i.e., euphotic zone, mixed layer, temperature, dissolved oxygen concentration, dissolved inorganic nitrogen, soluble reactive phosphorus, soluble reactive silica). Throughout the time series, the wavelet transforms identified a regular and predictable annual cycle of the Chl-a associated with the warm-monomictic thermal-mixing pattern, the variability of the annual Chl-a cycle, and the presence of other cyclicities, 2-year and similar to 4-5 years, associated with external forcing agents (e.g., North Pacific Oscillation). The water quality variables display a recurrent annual cycle. At the same time, the trophic variables (nutrient concentration) showed the same cyclicity as Chl-a (1-year, 2-year, and 4-year), suggesting the external forcing agents promote Chl-a augment through nutrient increase made available from stronger, deeper, mixing periods.