Hydrochemistry, ostracods and diatoms in a deep, tropical, crater lake in Western Mexico

Crater lakes are sensitive to natural and anthropogenic environmental changes and it is important to define and understand their current status in order to evaluate past changes related to climate variability and future ones associated with modern global change. We sampled lake waters, surface sediments and sediment trap samples (dry vs wet season) in lake Santa Maria del Oro (SMO), Western Mexico (21°22’58’’N, 104°34’48’’W, 750 m asl). Its present condition was assessed in terms of: i) thermal and oxygen stratification patterns, ii) hydrochemistry and nutrient status, iii) carbonate precipitation (dry vs wet season), and iv) spatial (depth) and seasonal (dry vs wet season) distribution of ostracod and diatom communities. Our results indicate that this 65-m deep lake is warm monomicitic, with a stable thermal stratification for most of the year (thermocline 16-24 m, metalimnetic gradient up to 7ºC). The water column is thermally homogeneous from late January to early March. Dissolved oxygen is vertically homogeneous only in January, when deoxygenation occurs throughout the water column. This is the first report of such a situation in a Mexican lake. Santa Maria del Oro has slightly alkaline, [HCO₃^–]-[Cl^–] and [Na⁺]>[Mg²⁺]>>[Ca²⁺] waters. Although it is a freshwater system, total dissolved solids, electrical conductivity (EC) and [Cl^–] indicate that the lake has undergone evaporative concentration. Aragonite  precipitation, occurring during the wet and warm part of the year, favours Ca2+ depletion and Cl^– enrichment of lake waters. This is a mesotrophic lake with relatively high soluble reactive phosphorus and silica levels. Nitrogen reaches its highest values during winter mixing (January), but becomes the limiting nutrient during stratification. Ostracod and diatom assemblages differ in specific regions of the lake. Ostracods are more diverse and abundant in littoral areas with dense vegetation, where the community is mainly formed by Potamocypris variegata, Cypridopsis vidua and Darwinula stevensoni. Diatoms show high diversity and low dominance in the littoral areas, where benthic, periphyticspecies (Amphora coffeaeformis, Planothidium delicatulum and Hippodonta lunemburgensis) are dominant. Deeper (>10 m) areas show low diversity and high dominance, and sediment trap data indicate a diatom bloom during winter mixing, dominated by Aulacoseria ganulata and Nitzschia amphibia.