Large-scale climatic phenomena drive fluctuations in macroinvertebrate assemblages in lowland tropical streams, Costa Rica: The importance of ENSO events in determining long-term (15y) patterns

Understanding how environmental variables influence the distribution and density of organisms over relatively long temporal scales is a central question in ecology given increased climatic variability (e.g., precipitation, ENSO events). The primary goal of our study was to evaluate long-term (15y time span) patterns of climate, as well as environmental parameters in two Neotropical streams in lowland Costa Rica, to assess potential effects on aquatic macroinvertebrates. We also examined the relative effects of an 8y whole-stream P-enrichment experiment on macroinvertebrate assemblages against the backdrop of this long-term study. Climate, environmental variables and macroinvertebrate samples were measured monthly for 7y and then quarterly for an additional 8y in each stream. Temporal patterns in climatic and environmental variables showed high variability over time, without clear inter-annual or intra-annual patterns. Macroinvertebrate richness and abundance decreased with increasing discharge and was positively related to the number of days since the last high discharge event. Findings show that fluctuations in stream physicochemistry and macroinvertebrate assemblage structure are ultimately the result of large-scale climatic phenomena, such as ENSO events, while the 8y P-enrichment did not appear to affect macroinvertebrates. Our study demonstrates that Neotropical lowland streams are highly dynamic and not as stable as is commonly presumed, with high intra- and inter-annual variability in environmental parameters that change the structure and composition of freshwater macroinvertebrate assemblages.This study was financed by National Science Foundation (DEB 1122389) to Catherine M. Pringle. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencias del Mar y Limnología (CIMAR

Determining Temperature-Normalized Decomposition Rates

Temperature is a key environmental factor controlling rates of litter decomposition in streams and other ecosystems. Normalizing decomposition rates for temperature not only enables assessments of the importance of temperature effects but also facilitates comparisons of potential other controlling factors. This chapter describes procedures to achieve such normalization. In streams, temperature normalization of decomposition rates has almost invariably assumed a linear relationship. Accordingly, litter mass remaining at various time points after deployment of litter in the field is regressed against thermal sums (degree days) obtained from continuous temperature records. The same approach is taken when an exponential temperature dependency is assumed, commonly by using a Q10 model, with Q10 being assumed to take a given value, such as 2, or being fitted along with the decomposition rate coefficient. Here the thermal sums of the degree-day model are replaced by an integral that captures the exponential relationship. Worked examples of the calculations are provided in a supplementary spreadsheet and as computer code. Similar approaches to temperature normalization can be adopted based on dependencies encapsulated in the Arrhenius law used in the metabolic theory of ecology.publishe