Effects of Environmental Variables on Aquatic Macroinvertebrate Assemblages in Tropical Lowland Streams
Gutierrez Fonseca, Pablo E.
Ramirez, Alonso (Consejero)
Drastic changes in environmental variables are considered a dominant disturbance in freshwater ecosystems, affecting the spatial and temporal variation of ecological processes, functions, and biota. Macroinvertebrate responses to these disturbances can be measured by changes of abundance, biomass, and taxonomic richness. While macroinvertebrates are adapted to these fluctuations, global climate change is now altering the magnitude and frequency of natural disturbances, with potentially large impacts especially on highly vulnerable tropical stream ecosystems, where changes are expected to be stronger and more unpredictable. Streams at La Selva Biological Station (LSBS) offer a unique opportunity to understand the role that environmental variables play on ecosystems. Streams form a gradient of solute concentrations, due to inputs of geothermally modified groundwater that changes the chemical composition of some lowland streams. Episodic acidification events have also been observed in some streams at LSBS, with pH changes of up to 3 units between seasons, which have been associated with the amount of precipitation from the previous dry season. Streams that receive inputs of geothermally modified groundwater are buffered from these seasonal changes due to the high concentrations of bicarbonate in the water (HCO3–), while streams that lack this chemical input experience seasonal changes in pH. The overall objective of this dissertation was to improve our understanding of how environmental variables affect aquatic macroinvertebrate assemblages in tropical streams. This objective was achieved using a combination of approaches: long-term sampling (from 1997 to 2011), experimental manipulation, and an assessment of recently emerged adult insects. The results showed that climatic (i.e., Southern Oscillation Index and precipitation) and environmental (i.e., discharge) variables had the most influence on macroinvertebrate structure and composition over the study period. Also, our results revealed that these variables can influence the body length of aquatic organism, with potential effects on their fitness and food web structure. This is of great importance, especially since current global climate change models suggest an increase in the frequency and intensity of some variables, such as precipitation. This study also demonstrated the importance of long-term studies, as short-term studies limit our ability to understand ecosystem dynamics. More long-term studies such as this are needed to better understand what to expect for tropical ecosystems in the face of climate change, which can severely alter the structure and composition of natural organisms and the ecosystem processes and services they sustain.