Experimentally manipulated biota over a 30-40d period in two streams with distinctly different macrobiotic assemblages

Here we test the hypothesis that differences in macrobiotic assemblages can lead to differences in the quantity and quality of organic matter in benthic depositional environments among streams in montane Puerto Rico. We experimentally manipulated biota over a 30-40d period in two streams with distinctly different macrobiotic assemblages: one characterized by high densities of omnivorous shrimps (Decapoda: Atyidae and Xiphocarididae) and no predaceous fishes. To incorporate the natural hydrologic regime and to avoid confounding artifacts associated with cage enclosure/exclosure (e.g., high sedimentation), we used electricity as a mechanism for experimental exclusion, in situ. In each stream, shrimps and/or fishes were excluded from specific areas of rock substrata in four pools using electric "fences" attached to solar-powered fence chargers. In the stream lacking predaceous fishes (Sonadora), the unelectrified control treatment was almost exclusively dominated by high densities of omnivorous shrimps that constantly ingested fine particulate material from rock surfaces. Consequently, the control had significantly lower levels of inorganic sediments, organic material, carbon and nitrogen than the exclusion treatment, as well as less variability in these parameters. Tenfold more organic material (as ash-free dry mass, AFDM) and fivefold more nitrogen accrued in shrimp exclosures (10.6 g AFDM/m2, 0.2 g N/m2) than in controls (1.1 g AFDM/m2, 0.04 g N/m2). By reducing th quantity of fine particulate organic material and associated nitrogen in benthic environments, omnivorous shrimps potentially affect the the supply of this important resource to other trophic levels. The small amount of fine particulate organic matter (FPOM) that remained in control treatments (composed of sparse algal cells0 was of higher quality than that in shrimp exclosures. This is evidenced by the significantly lower carbon-to-nitrogen (C/N) ratio (an indicator of food quality, with relatively low C/N indicating higher food quality) in the control relative to the shrimp exclosure treatment. In contrast, the stream with predaceous fishes (Bisley) was characterized by very low numbers of shrimps, and macrobiota had no significant effect on benthic sediments, organic matter, C, N, and C/N. All parameters were highly variable through time, with levels and ranges in variability similar to the shrimp exclusion treatment in the Sonadora. Our experimental results are consistent with findings of an independent survey of six streams in four different drainages. Four streams that had an abundance of omnivorous shrimps but lacked predaceous fishes, had extremely low levels of fine benthic organic and inorganic material. In contrast, two streams that had low densities of shrimps and contained predaceous fishes had significantly higher levels. Results show a strong linkage between species and ecosystem characteristics: interstream differences in the quantity and quality of fine benthic organic matter resources were determined by the nature of the macrobiotic assemblage. Furthermore, patterns in the distribution of shrimp assemblages reflected landscape patterns in the benthic depositional environment among streams. Support for this work was provided by grants BSR-8811902, DEB-9411973, DEB-9705814 , DEB-0080538, DEB-0218039 , DEB-0620910 , DEB-1239764, DEB-1546686, and DEB-1831952 from the National Science Foundation to the University of Puerto Rico as part of the Luquillo Long-Term Ecological Research Program. Additional support provided by the University of Puerto Rico and the International Institute of Tropical Forestry, USDA Forest Service.

本研究旨在验证以下假设：波多黎各山区溪流中大型生物群落（macrobiotic assemblages）的差异会导致底栖沉积环境中有机质的数量与质量产生差异。我们在两条具有显著不同大型生物群落的溪流中，对生物群落进行了为期30至40天的实验操控：其中一条溪流以高密度杂食性虾类（十足目：匙指虾科与剑额虾科）为特征，且无捕食性鱼类；另一条则存在捕食性鱼类。为融入自然水文 regime 并避免网笼围隔/排除法相关的混淆干扰（如高沉积率），我们采用原位电排除法作为实验手段。在每条溪流中，通过连接太阳能围栏充电器的电“围栏”，在四个水潭的特定岩石基质区域排除虾类和/或鱼类。

在无捕食性鱼类的溪流（Sonadora）中，未通电的对照组几乎完全被高密度杂食性虾类占据，这些虾类持续摄食岩石表面的细颗粒物质。因此，对照组的无机沉积物、有机质、碳和氮含量显著低于排除组，且这些参数的变异性更小。虾类排除组的有机质（以无灰干重 AFDM 计）和氮含量分别为对照组的10倍（10.6 g AFDM/m² vs. 1.1 g AFDM/m²）和5倍（0.2 g N/m² vs. 0.04 g N/m²）。杂食性虾类通过减少底栖环境中细颗粒有机质及相关氮的数量，可能影响该重要资源向其他营养级的供应。对照组中残留的少量细颗粒有机质（FPOM，由稀疏藻类细胞组成）质量高于虾类排除组，这一点可通过对照组显著更低的碳氮比（C/N，食物质量的指标，较低的C/N值表示较高的食物质量）得到证明。

相比之下，存在捕食性鱼类的溪流（Bisley）虾类数量极少，大型生物群落对底栖沉积物、有机质、碳、氮及碳氮比无显著影响。所有参数随时间变化的变异性极高，其水平和范围与Sonadora溪流的虾类排除组相似。

我们的实验结果与对四个不同流域六条溪流的独立调查结果一致：四条富含杂食性虾类但无捕食性鱼类的溪流，其底栖细有机与无机物质含量极低；而两条虾类密度低且存在捕食性鱼类的溪流，上述物质含量显著更高。结果表明物种与生态系统特征之间存在强关联：底栖细有机质资源的数量与质量差异由大型生物群落的性质决定。此外，虾类群落的分布模式反映了溪流间底栖沉积环境的景观格局。

本研究得到美国国家科学基金会（National Science Foundation）授予波多黎各大学的资助（项目编号：BSR-8811902、DEB-9411973、DEB-9705814、DEB-0080538、DEB-0218039、DEB-0620910、DEB-1239764、DEB-1546686、DEB-1831952），作为卢基略长期生态研究计划（Luquillo Long-Term Ecological Research Program）的一部分。额外支持来自波多黎各大学及美国农业部林务局国际热带林业研究所。