Data from: Effects of land use on lake nutrients: the importance of scale, hydrologic connectivity, and region

Catchment land uses, particularly agriculture and urban uses, have long been recognized as major drivers of nutrient concentrations in surface waters. However, few simple models have been developed that relate the amount of catchment land use to downstream freshwater nutrients. Nor are existing models applicable to large numbers of freshwaters across broad spatial extents such as regions or continents. This research aims to increase model performance by exploring three factors that affect the relationship between land use and downstream nutrients in freshwater: the spatial extent for measuring land use, hydrologic connectivity, and the regional differences in both the amount of nutrients and effects of land use on them. We quantified the effects of these three factors that relate land use to lake total phosphorus (TP) and total nitrogen (TN) in 346 north temperate lakes in 7 regions in Michigan, USA. We used a linear mixed modeling framework to examine the importance of spatial extent, lake hydrologic class, and region on models with individual lake nutrients as the response variable, and individual land use types as the predictor variables. Our modeling approach was chosen to avoid problems of multi-collinearity among predictor variables and a lack of independence of lakes within regions, both of which are common problems in broad-scale analyses of freshwaters. We found that all three factors influence land use-lake nutrient relationships. The strongest evidence was for the effect of lake hydrologic connectivity, followed by region, and finally, the spatial extent of land use measurements. Incorporating these three factors into relatively simple models of land use effects on lake nutrients should help to improve predictions and understanding of land use-lake nutrient interactions at broad scales.

流域土地利用（catchment land use），尤其是农业与城镇用地，长期以来被认为是地表水体营养物浓度的主要驱动因素。然而，目前鲜有能够将流域土地利用规模与下游淡水水体营养物建立关联的简易模型，且现有模型均无法适用于区域、大洲等大范围空间尺度下的大量淡水水体。本研究旨在通过探究影响淡水生态系统中土地利用与下游营养物之间关联的三类因素，提升模型性能：土地利用测量的空间尺度、水文连通性（hydrologic connectivity），以及营养物总量与土地利用对其影响的区域差异。我们针对美国密歇根州7个区域的346个北温带湖泊，量化了这三类因素在土地利用与湖泊总磷（total phosphorus, TP）、总氮（total nitrogen, TN）关联中的作用。我们采用线性混合效应模型框架（linear mixed modeling framework），以单个湖泊的营养物浓度为响应变量（response variable）、以各类土地利用类型为预测变量（predictor variable），探究空间尺度、湖泊水文类别（lake hydrologic class）以及区域对模型的重要性。我们选择该建模方法，旨在规避预测变量间多重共线性（multi-collinearity）问题以及区域内湖泊缺乏独立性的问题——这两类问题均是淡水水体大范围分析中的常见难题。研究发现，上述三类因素均会影响土地利用与湖泊营养物之间的关联。其中，湖泊水文连通性的影响证据最为充分，其次为区域差异，最后是土地利用测量的空间尺度。将这三类因素纳入土地利用对湖泊营养物影响的简易模型中，将有助于提升大范围尺度下土地利用与湖泊营养物相互作用的预测精度与认知水平。