A method to determine which nutrient is limiting for plant growth in estuarine waters-at any salinity

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=NO3-+NO2-+NH4+) and o-phosphate (PO4-) 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‰), where either (i) N and P may become equally limiting at salinity ≃ 35‰ or (ii) N may become limiting at salinity ⩾30‰. Overlaid nutrients vs salinity graphs were also used to demonstrate that, in the estuaries in southeast Ireland, carbon (as dissolved inorganic carbon, DIC=CO2+H2CO3+HCO3-+CO32-) is not the limiting nutrient-at any salinity.