Thresholds of acidification impacts on macroinvertebrates adapted to seasonally acidified tropical streams: potential responses to extreme drought-driven pH declines

Background Drought-driven acidification events of increasing frequency and severity are expected as a consequence of climate change, and these events may expose macroinvertebrate taxa to increased acidification beyond their tolerance levels. Recent work in lowland Costa Rica has shown that poorly-buffered tropical streams exhibit natural seasonal variation in pH, with extremely low levels (<4.5) after extreme dry seasons). Our goal was to determine the threshold of pH effects on survival of three tropical stream macroinvertebrate taxa. Methods We conducted laboratory mesocosm experiments to determine acidification effects (using diluted HCl) on three focal macroinvertebrate taxa collected from a poorly-buffered stream at La Selva Biological Station: (1) mayfly naiads (Ephemeroptera: Leptophlebiidae: Traverella holzenthali), (2) adult shrimp (Decapoda: Palaemonidae: Macrobrachium olfersii), and (3) larval midges (Diptera: Chironomidae). We also compared the effect of pH on survival and growth rates of larval midges from a poorly-buffered (pH 4.3–6.9) vs. a naturally well-buffered (pH 5.1–6.9) stream. Results/Discussion Mayfly and shrimp survival decreased between pH 4.0 and 3.5, overlapping with the range of lowest pH levels (3.6–4.0) recorded during a previous extreme El Niño Southern Oscillation event in 1998 and suggesting that increasingly extreme acidification events induced by climate change may negatively affect their survival. In contrast, survival of larval midges was unaffected by pH regimes at/above 3.5, indicating tolerance to pH levels experienced in poorly-buffered stream during seasonal acidification, which has presumably occurred over millennia. These findings highlight the potential importance of historical pH regimes in structuring macroinvertebrate communities. These results are relevant not only to lowland Neotropical streams, but also signal the need for further research in lotic ecosystems worldwide where drought-driven pH declines have been documented or are probable in the future.

ISÓTOPOS ESTÁVEIS E OUTRAS FERRAMENTAS EM ESTUDOS TRÓFICOS DE PEIXES EM RIACHOS TROPICAIS

Stable isotope analysis (SIA) has become an essential tool in Ecology, being increasingly used in studies of fish in tropical streams. The SIA allows to reconstruct the animals' diet and its intraspecific variation, to trace the contributions of the main basal resources throughout the food web, to calculate trophic position and trophic niche, including niche overlap and its alterations, and to build food webs. The SIA is also used to understand the impact of environmental changes on fish communities and other aquatic organisms. The number of such studies is increasing in Brazil, but there are still several gaps. In this study, we present the basic concepts on stable isotopes analysis in trophic studies of stream fish - especially those of carbon, nitrogen and hydrogen - their premises, uses and limitations, presenting examples from tropical streams. We also explore new conceptual tools in this area, still not used for stream fish in Brazil, such as fatty acids, stable isotopes in specific compounds, nutritional geometry and ecological stoichiometry concepts such as the Threshold Elemental Ratio.