Effects of Atmospheric Nitrogen Deposition on Phosphorus Limitation of Freshwater Zooplankton

重要的植物养分氮(N)可以在大气中运输，并通过降雨沉积到陆地和水生栖息地。近几十年来，由于人类活动，氮沉降大幅增加，这主要是由于化肥施用的扩大和化石燃料燃烧过程中硝酸盐（NO3）的形成。本研究将试图通过改变构成湖泊食物网基础的微观植物（浮游植物）的化学成分来确定N沉积是否会影响湖泊食物网。具体来说，这些研究人员假设，向湖泊输入氮的增加导致浮游植物受到另一种关键元素磷(P)的强烈限制。当磷受到限制时，浮游植物产生的生物量磷含量非常低，因此可能在营养上不适合下一个摄食水平——浮游动物的生长。（反过来，浮游动物为鱼类提供了重要的食物基础，包括那些参与娱乐和商业渔业的鱼类。）为了验证这一点，这些研究人员将在科罗拉多州中部和挪威南部接受不同水平氮沉积的湖泊中测量浮游动物生长的磷限制。这些研究人员还将进行更大规模的实地研究，在正常条件下接受低氮沉积的湖泊中，故意操纵实验中游生态系统的N输入。这项研究将有助于基本了解湖泊食物网是如何工作的，同时也提供了潜在的重要信息，关于人类活动对看似原始的生态系统及其所支持的食物网的间接但潜在的重要影响。

The important plant nutrient, nitrogen (N), can be transported in the atmosphere and deposited into terrestrial and aquatic habitats in rainfall. During recent decades, N deposition has greatly increased as a result of human activities, largely due to expansion of fertilizer application and of formation of nitrates (NO3) during fossil fuel burning. This study will try to determine if N deposition affects lake food webs by changing the chemical composition of the microscopic plants (phytoplankton) that form the base of lake food webs. Specifically, these investigators hypothesize that increasing inputs of N to lakes cause the phytoplankton to become strongly limited by another key element, phosphorus (P). When P-limited, phytoplankton produce biomass that has a very low P content and thus potentially become nutritionally unsuitable for growth of the next feeding level, the zooplankton. (Zooplankton, in turn, provide an important food base for fish, including those that are involved in recreational and commercial fisheries.) To test this, these researchers will measure P limitation of zooplankton growth in lakes receiving different levels of N deposition in central Colorado and in southern Norway. These investigators will also perform a larger scale field study that deliberately manipulates N inputs to experimental mesocosms deployed in a lake that receives low N deposition under normal conditions. This study will help in basic understanding of how lake food webs work while also providing potentially important information about an indirect but potentially important impact of human activity on seemingly pristine ecosystems and the food webs they support.