The impacts of nutrient supply and imbalance on subcontinental co-occurrence networks and metacommunity composition of stream algae

The amounts and ratios of nutrients (nitrogen and phosphorus) are important determinants of producer community biodiversity and composition and their responses to climate and dispersal. However, the nutrient effects on co-occurrence network topology, particularly in freshwaters, are understudied. Here, we investigate 1) whether nutrient supply and ratio constrain topological properties of algal co-occurrence networks in streams and 2) to what extent climate and space (a surrogate for dispersal) affect co-occurrence network topology vs. metacommunity composition across nutrient supply and ratio contexts. We used a subcontinental dataset of benthic algae from 840 stream localities in the conterminous US. We constructed co-occurrence networks representing nutrient supply contexts (oligotrophic vs. eutrophic) and nutrient ratio contexts (N-limited vs. P-limited) and statistically assessed topological variability within each pair via randomization. We then used a null model framework and direct gradient analysis to ascertain the importance of climate and space in driving, respectively, network topology and metacommunity composition. Nutrient supply was only positively related to network size (node counts), which was driven by motile species, while other topological differences were non-significant. Climatic and spatial variables had pronounced and for the most part comparable effects on network topology that further depended on nutrient context. A comparative assessment of topological vs. compositional responses to climate and space across nutrient contexts identified both similarities and differences. While climate and space contributed to both network topology and metacommunity composition, space was a stronger predictor of compositional variability than climate, regardless of nutrient context. Our findings highlight the need for developing integrative multi-level approaches (from metacommunities to co-occurrence networks) to fully understand biological responses to complex and interactive abiotic forces.

Methods
The datasets herein are subsetted from 942 qualified stream sites from the original United States Geological Survey's NAWQA dataset for benthic diatoms (1993-2015) for a total of 840 stream samples across 4 nutrient categories, 193 each of oligotrophic and eutrophic streams, and 227 each of N- and P-limited streams.

营养盐的含量与比例是调控生产者群落生物多样性、群落组成，及其对气候与扩散过程响应的关键驱动因子。然而，营养盐对共现网络（co-occurrence network）拓扑结构的影响，尤其是在淡水生态系统中，仍未得到充分研究。本研究旨在探讨两个核心科学问题：其一，营养盐供给水平与比例是否会限制溪流中藻类共现网络的拓扑属性；其二，在不同营养盐供给与比例情境下，气候与空间（作为扩散过程的替代指标）对共现网络拓扑结构与元群落（metacommunity）组成的影响程度分别如何。

我们采用了覆盖美国本土840个溪流点位的底栖藻类（benthic algae）亚大陆尺度数据集。我们构建了分别代表营养盐供给情境（贫营养vs富营养）与营养盐比例情境（氮限制vs磷限制）的共现网络，并通过随机化检验对每组网络内的拓扑变异开展统计学评估。随后我们借助零模型（null model）框架与直接梯度分析，分别明确气候与空间在驱动网络拓扑结构与元群落组成中的重要性。

研究结果显示，营养盐供给仅与网络规模（节点总数）呈正相关，且该关联由运动型物种所主导，其余拓扑差异均无统计学显著性。气候与空间变量对网络拓扑结构具有显著且整体上相近的影响，且这种影响进一步依赖于营养盐情境。对不同营养盐情境下拓扑结构与群落组成对气候、空间的响应开展比较评估后，发现了二者的共性与差异。尽管气候与空间均对网络拓扑结构与元群落组成存在贡献，但无论营养盐情境如何，空间都是比气候更强的群落组成变异预测因子。本研究结果凸显了开发整合性多尺度研究方法（从元群落到共现网络）的必要性，以全面解析生物对复杂且交互性非生物因子的响应。

## 方法
本研究数据集源自美国地质调查局（United States Geological Survey, USGS）原有底栖硅藻NAWQA数据集（1993-2015年）中筛选出的942个合格溪流点位，最终纳入840个溪流样本，涵盖4类营养盐情境：贫营养、富营养溪流各193个，氮限制、磷限制溪流各227个。