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This study integrates diatom analysis with existing records of pollen, charcoal, elemental composition, and stable light isotopes to expand upon the 4200-year history of human activity and climate change from Laguna Los Mangos in southern Pacific Costa Rica. We counted diatoms in peroxide-treated samples and analyzed community composition using cluster analysis, revealing four distinct assemblage zones with diatom variability most closely correlated with phosphorus, titanium, and organic content. The earliest assemblage (Zone D, 4150–3430 cal yr BP) was dominated by Encyonema silesiacum and Nitzschia incognita and aligned with a period of deforestation, erosion, and abundant macrophytes. Gomphonema affine proliferated in Zone C (3430–2450 cal yr BP), reflecting increased pH and productivity likely caused by agriculture-induced nutrient loading. We attributed the preservation gap from 3290 to 2970 cal yr BP in Zone C to silica depletion and erosional deposition that induced decline in diatom abundance by diluting valve concentrations in lake sediments. Nitzschia incognita and G. affine became the dominant taxa in Zone B (2450–1400 cal yr BP), likely reflecting eutrophy, increasing conductivity, and drying climate. Dominance of Diadesmis confervacea indicated reduced lake level in Zone A (1400 cal yr BP–modern) at the onset of the Terminal Classic Drought (TCD). A hiatus in the record indicates lake desiccation from 950 to 450 cal yr BP. During the Little Ice Age (LIA), diatoms reflect conditions similar to Zone B indicating increased lake level, circumneutral pH, and eutrophy. Refilling of the lake indicates increased precipitation during the LIA despite evidence of severe regional drought reported at other sites. Variable precipitation during this period likely resulted from the combined effects of Spanish contact, agricultural collapse, forest recovery, and shifts in Atlantic and Pacific climate forcing mechanisms. Overall, the Los Mangos diatom record reflects shallow, slightly alkaline, eutrophic conditions influenced by nutrient enrichment, erosion, and deforestation associated with maize agriculture. 

Laguna Santa Elena (8.9290° N, 82.9257° W, 1055 m a.s.l.) is a small lake in the Diquís archaeological sub-region of southern Pacific Costa Rica. Previous analyses of pollen and charcoal in a sediment core from Santa Elena revealed a nearly 2,000 year history of vegetation change, maize cultivation and site occupation that is consistent with the archaeological record from the lake basin and surrounding area. Here we present the results of new loss-on-ignition, geochemical and bulk stable carbon (δ13C) and nitrogen (δ15N) isotope analyses of the Santa Elena sediments that supplement and refine the previous reconstruction. Like many lakes in Central America and the Caribbean, Laguna Santa Elena was a magnet for humans throughout its history. As a result, the lake experienced vegetation modification by humans and maize cultivation at varying intensities over a long duration. The Santa Elena sediments provide a record of palaeoenvironmental change during times of major culture change and increasing cultural complexity in the Diquís region, which occurred during intervals of broader changes driven by external forcing mechanisms, including the Terminal Classic Drought (TCD), the Little Ice Age (LIA) and the Spanish Conquest. Our high resolution lake sediment study from Santa Elena reveals details of these events at the local scale that are unobtainable by other means, including the timing of the initial intensification of maize cultivation at ca. 1,570 cal BP (AD 380) and two intervals of population decline coinciding with the TCD at ca. 1,085 cal BP (AD 865) and near the start of the LIA at ca. 683 cal BP (AD 1267).