Freshwater floodplain habitats buffer native food webs from negative effects of non-native centrarchids and bullfrogs

Our study system was the Chehalis River Basin in southwestern Washington, USA. The Chehalis River floodplain contains extensive off-channel habitats, including oxbow ponds, wetlands, and seasonally flooded fields and forests. We sampled 16 sites in spring (18 April – 4 May 2017) and resampled eight of those sites in summer (20 June – 6 July 2017), with one site sampled in the spring excluded from further study as we observed none of our focal taxa. At each site, we examined a suite of habitat characteristics, collected water samples for chemical analysis, and sampled the biotic community. Specifically, we collected the following organisms for food web analysis: dominant macrophytes, seston (from water samples; see above), filamentous algae (where present), zooplankton, macroinvertebrates, amphibians, and fishes (where present). Vertebrates were weighed and measured prior to isotopic analysis. All isotope samples were analyzed for percent carbon (C), percent nitrogen (N), δ13C, and δ15N values. Details are provided in the associated manuscript (currently in review at Freshwater Science).

本研究的研究样区为美国华盛顿州西南部的奇黑利斯河流域（Chehalis River Basin）。该流域河漫滩分布有大面积河道外生境，包括牛轭塘（oxbow ponds）、湿地以及季节性淹水农田与森林。我们于2017年春季（4月18日—5月4日）对16个采样点位开展了首次调查，并于同年夏季（6月20日—7月6日）对其中8个点位进行了重采样；因春季采样的1个点位未观测到目标类群（focal taxa），故将其排除在后续研究之外。

在每个采样点位，我们系统测定了一系列生境特征参数，采集了用于化学分析的水体样本，并对区域内的生物群落进行了采样。具体而言，我们采集了以下用于食物网分析的生物类群：优势大型水生植物（macrophytes）、悬浮有机质（seston，取自前文提及的水样）、丝状藻类（若存在）、浮游动物（zooplankton）、大型底栖无脊椎动物（macroinvertebrates）、两栖动物以及鱼类（若存在）。

在进行同位素分析前，我们对所有脊椎动物样本进行了称重与形态测量。所有同位素样本均测定了碳百分含量（C）、氮百分含量（N）、碳同位素比值（δ¹³C）以及氮同位素比值（δ¹⁵N）。详细研究细节详见已投稿至《Freshwater Science》（淡水科学）的相关论文，目前该论文处于审稿阶段。