Responses to a warming trend and “El Niño” events in a tropical lake in western Mexico

Abstract

Lakes are sensitive ecosystems to climatic change but in the tropics it is frequently difficult to evaluate as there are few long-term records. In this paper hydrochemistry and phytoplankton data during the 2009–2010 “El Nino” are contrasted with non-“El Nino” conditions (pre- and post- the 2009–2010 event) and with long-term (23 years) meteorological and paleolimnological data from a lake in eastern Mexico. Meteorological data provide evidence of a recent (since year 2000) warming trend, and paleolimnological data show that the diatom assemblage changed at this time to a more diverse association, including low abundances of a small Cyclotella species (C. ocellata) and a small needle shaped species (Fragilaria nanana) to the previously dominant assemblage (Ulnaria delicatissima, Achnanthidium minutissimum). Phytoplankton associations during non-“El Nino” are consistent with the paleolimnological record (U. delicatissima–A. minutissimum, + Staurastrum sp.) but they changed at the onset of “El Nino” (May 2009), when N and P co-limitation favoured a Coelastrum reticulatum–C. ocellata assemblage. During “El Nino” winter (February 2010) colder temperatures led to a longer mixing period and a whole water column deoxygenation event that favoured a particularly low diversity association (Botryococcus sp.–Sphaerocystis sp.). The low diversity C. ocellata-bloom has no precedent for the last 23 years. The previous 1998–1999 “El Nino” had a similar low diversity assemblage but dominated by U. delicatissima. The different response of the phytoplankton to the two “El Nino” events is related with the warmer conditions since the year 2000, that lead to more stable stratification periods and more frequent or intense nutrient limitation, particularly during the warmer than average “El Nino” 2009–2010 summer. Under a global warming scenario, minor changes in ecosystem’s base levels determine that even normal climatic variability events can cause unexpected changes in the ecosystem’s diversity and species composition.