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Project Progress

During the past year we completed 4 expeditions and initated a 5th to our field site at Cuatro Cienegas, Mexico. The main focal point of the year’s work was an 8-week field experiment during summer 2002 examining the effects of snail grazers and PO⁴ enrichment on the biomass, productivity, C:P stoichiometry, and microbial community structure of oncoid stromatolites. In turn, we also examined the effects of PO⁴ enrichment on snail growth rate, ribonucleic acid: deoxyribonucleic acid (RNA:DNA) ratio, and population change. The experiment revealed that snails had no discernible impact on the stromatolites but PO⁴ had massive impacts on stromatolite productivity, communitry structure, and C:P stoichiometry. In turn, PO⁴ enrichment had major impacts on the snails, resulting in greatly reduced growth rates and increased mortality. This outcome is in contrast to our experiment in 2001 in which stromatolite P enrichment led to increased snail performance. We have now developed a hypothesis that snails at Cuatro Cienegas live on a “stoichiometric knife’s edge” in which stromatolite C:P ratios are excessively high in the field but where large P enrichments can poison them. Another important activity from the past year was the use of molecular microbial analyses to confirm that subterranean water being tapped by agricultural pumping outside of the basin is in communication with the springs at Cuatro Cienegas. This finding led to unprecedented conservation measures being imposed in the basin and surrounding region via strong actions from the highest level of the Mexican government. We also successfully completed field experiments documenting that endemic pupfishes in the basin are locally adapted to environmental conditions and thus shifting hybrid zones are driven by seasonal and longer-term environmental perturbations. Our work also continues to highlight extremely high levels of biological diversity in snail fauna and in microbes at Cuatro Cienegas.