Neill Michael
Regional Water Laboratory, Environmental Protection Agency, Kilkenny, Ireland.
Mar Pollut Bull. 2005 Sep;50(9):945-55. doi: 10.1016/j.marpolbul.2005.04.002.
A method, utilising overlaid graphs for nutrients vs salinity, was developed in order to determine which nutrient is limiting for plant growth in estuarine waters-at any salinity. Dissolved inorganic nitrogen (DIN=NO(3)(-)+NO(2)(-)+NH(4)(+)) and o-phosphate (PO(4)(-)) are the main forms of N and P that are readily bio-available for plant growth in waters and these have a Redfield atomic ratio of N:P=16:1 (i.e. aquatic plants absorb N and P in the average ratio of 16 atoms of N to 1 atom of P). Graphs are prepared for (i) DIN vs salinity and (ii) o-phosphate vs salinity with the vertical scales for DIN and o-phosphate set at a ratio of N:P=16:1; when these graphs are overlaid on each other then the lowermost trendline denotes the limiting nutrient for plant/algal growth-at any salinity. The graphs also indicate the extent by which one or other of the nutrients is limiting--at any salinity. Furthermore, if there is a transition from P to N limitation somewhere along the salinity gradient, then this occurs at the salinity where the trendlines intersect. The concept was applied to three estuaries in the southeast of Ireland and the results show that, in all of these circumstances, P is the limiting nutrient throughout--except for the higher salinities (i.e. salinities 30 per thousand), where either (i) N and P may become equally limiting at salinity approximately 35 per thousand or (ii) N may become limiting at salinity 30 per thousand. Overlaid nutrients vs salinity graphs were also used to demonstrate that, in the estuaries in southeast Ireland, carbon (as dissolved inorganic carbon, DIC=CO(2)+H(2)CO(3)+HCO(3)(-)+CO(3)(2-)) is not the limiting nutrient--at any salinity.
为了确定在河口水中的任何盐度下哪种养分限制植物生长，开发了一种利用养分与盐度叠加图的方法。溶解无机氮（DIN = NO₃⁻ + NO₂⁻ + NH₄⁺）和正磷酸盐（PO₄³⁻）是水中植物生长易于生物利用的氮和磷的主要形式，它们的雷德菲尔德原子比为N:P = 16:1（即水生植物吸收氮和磷的平均比例为16个氮原子对1个磷原子）。绘制（i）DIN与盐度的关系图以及（ii）正磷酸盐与盐度的关系图，将DIN和正磷酸盐的垂直刻度设置为N:P = 16:1的比例；当这些图相互叠加时，最下面的趋势线表示在任何盐度下对植物/藻类生长的限制养分。这些图还表明了在任何盐度下一种或另一种养分的限制程度。此外，如果在盐度梯度上的某个位置从磷限制转变为氮限制，那么这种转变发生在趋势线相交的盐度处。该概念应用于爱尔兰东南部的三个河口，结果表明，在所有这些情况下，除了较高盐度（即盐度为30‰）外，磷在整个过程中都是限制养分，在较高盐度下，要么（i）在盐度约为35‰时氮和磷可能同样成为限制因素，要么（ii）在盐度为30‰时氮可能成为限制因素。叠加的养分与盐度图还用于证明，在爱尔兰东南部的河口，碳（以溶解无机碳，DIC = CO₂ + H₂CO₃ + HCO₃⁻ + CO₃²⁻形式存在）在任何盐度下都不是限制养分。
