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.在气候变异性日益加剧（如降水、厄尔尼诺-南方涛动（ENSO）事件）的背景下，解析环境变量如何在较长时间尺度上影响生物的分布与种群密度，是生态学领域的核心科学问题之一。本研究的核心目标为：针对哥斯达黎加低地的两条新热带（Neotropical）溪流，解析长达15年的气候及环境参数变化模式，以评估其对水生大型无脊椎动物的潜在影响。同时，本研究亦在上述长期监测的框架下，探究了为期8年的全溪流磷富集实验对大型无脊椎动物群落的相对影响效应。两条溪流的气候、环境变量及大型无脊椎动物样本均按月采样7年，随后的8年改为按季度采样。气候与环境变量的时间序列呈现出显著波动，但未呈现明确的年际或年内固定变化模式。大型无脊椎动物的物种丰富度与个体丰度均随径流量的增加而下降，且与上次高径流量事件发生后的天数呈正相关关系。研究结果表明，溪流物理化学性质及大型无脊椎动物群落结构的波动，本质上源于ENSO事件这类大尺度气候现象；而为期8年的磷富集实验并未对大型无脊椎动物产生显著影响。本研究证实，新热带低地溪流具有极强的动态性，并不像此前普遍认为的那样稳定；其环境参数存在显著的年内与年际变异，进而改变淡水大型无脊椎动物群落的结构与组成。