Data for Stream bryophytes promote cryptic productivity

This is the data and code associated with \"Stream bryophytes promote 'cryptic' productivity in highly oligotrophic headwaters. Recent observations document increased abundance of algae and bryophytes in headwater streams of Hubbard Brook Experimental Forest (HBEF). It is possible that this ‘greening up’ of HBEF streams may be due to climate change with rising temperatures, altered terrestrial phenology, and shifting hydrologic regimes induced. Alternatively, stream ‘greening’ could be due to the slow recovery of stream chemistry from decades of acid rain, which have led to rising stream water pH, declining concentrations of toxic Al3+ and extremely low solute concentrations (Likens and Buso, 2012; Rosi-Marshall, Bernhardt, et al., 2016). Three years of weekly algal measurements on contrasting substrates, more than fifteen nutrient enrichment experiments, and detailed surveys of bryophyte cover over time reveal important new insights about the interactions between these two groups of autotrophs. We predicted that light availability, hydrologic disturbance and nutrient limitation were all important determinants of algal biomass in streams. To evaluate the relative strength and hierarchy of these limiting factors, we used nutrient diffusing substrates to investigate the role of nutrients for algae and compared algal accrual rate on artificial rock vs. moss substrates in stream channels vs. constructed weir ponds to assess the role of hydrologic disturbance and scour. Our surveys and experiments spanned across seasons and local light regimes. Algal biomass was substantially higher in protected weir ponds than in stream channels, and in both habitats, algal biomass was substantially higher on artificial moss substrates than on tiles. Taken together, these results suggest that moss can provide physical protection from flood scour. Algal biomass instream on both substrate types was higher in high light seasons (pre-leaf out) and well-lit habitats indicating strong light limitation. Results from a series of fifteen nutrient diffusing substrate experiments over the course of three years provided little evidence of nutrient limitation instream (observed in 3 of 10 experiments). The most striking finding of our investigation is the previously unsuspected role of stream bryophytes in providing critical refugia for algae in these steep, heavily shaded and oligotrophic headwaters. Shifts in stream productivity over time are likely to be closely tied to changes in bryophyte cover.

本数据集与论文《溪流苔藓植物在高度贫营养源头溪流中促进“隐秘”生产力》（Stream bryophytes promote 'cryptic' productivity in highly oligotrophic headwaters）相关，附带对应研究的数据与代码。近期观测显示，哈伯布鲁克实验林（Hubbard Brook Experimental Forest, HBEF）的源头溪流中，藻类与苔藓植物的丰度有所提升。现有研究推测，HBEF溪流的此次“绿化”现象可能源于气候变化引发的气温上升、陆地物候改变以及水文格局变化。另一种可能则是，数十年酸雨导致溪流水体pH上升、有毒铝离子（Al³+）浓度下降以及溶质浓度极低，溪流化学性质正逐渐从该影响中缓慢恢复（Likens与Buso, 2012; Rosi-Marshall、Bernhardt等, 2016）。本研究通过三年来对不同基质上藻类的每周监测、超过15次营养富集实验，以及长期苔藓植物覆盖度详细调查，为这两类自养生物之间的相互作用提供了全新的重要认识。我们此前提出假设：光照可利用性、水文干扰与营养限制均是溪流中藻类生物量的关键调控因子。为评估这些限制因子的相对强度与层级关系，我们采用营养扩散基质（nutrient diffusing substrates）探究营养对藻类的作用，并在溪流渠道与人工构建堰塘中，对比人工岩石基质与苔藓基质上的藻类积累速率，以此分析水文干扰与冲刷的影响。本研究的调查与实验覆盖了不同季节与区域光照条件。受保护的堰塘中藻类生物量显著高于溪流渠道；且在两类生境中，人工苔藓基质上的藻类生物量均显著高于瓷砖基质。综合来看，这些结果表明苔藓可为藻类提供物理庇护，使其免受洪水冲刷。在光照充足的季节（展叶前）与光照条件良好的生境中，两种基质上的溪流藻类生物量均更高，这表明光照限制作用显著。三年间开展的15次营养扩散基质实验结果显示，溪流中存在营养限制的证据极少——仅在10次实验中的3次观测到了营养限制现象。本研究最显著的发现是，此前未被关注的溪流苔藓植物在这类陡峭、高遮蔽且高度贫营养的源头溪流中，为藻类提供了关键庇护生境。溪流生产力随时间的变化，很可能与苔藓植物覆盖度的变化密切相关。