Growth plasticity in Cymodocea nodosa stands: the importance of nutrient supply

We examine here the growth plasticity (as the variability in leaf growth and production, and shoot mortality rate) and stand structure (rhizome, root and shoot biomasses and nutrient contents, and shoot age structure), of the Mediterranean seagrass Cymodocea nodosa (Ucria) Aschers. growing across an existing gradient of nutrient supply in shallow water (from 2 μM soluble reactive phosphorus at nutrient-rich, to 0.08 μM at nutrient-poor sites). Phosphorus concentrations in plant tissues increased strongly (three-fold in leaves, and ten-fold in rhizomes and roots) from nutrient-poor to nutrient-rich sites. Leaf phosphorus concentrations at nutrient-poor sites were only half of those required for balanced seagrass growth, indicating the importance of phosphorus in limiting C. nodosa growth at the nutrient-poor sites. Phosphorus-limited plants allocated a greater proportion of the limiting resource (32% of the P standing stock, compared with about 10% at nutrient-sufficient sites), and a greater proportion of the produced biomass (about 40% of the biomass, compared with about 15% of the biomass at nutrient-sufficient sites), to root development, thereby increasing their capacity to acquire phosphorus from the sediment at the expense of impaired shoot growth and size. Resources were relocated as nutrient shortage ameliorates towards nutrient-rich sites, yielding a reduction in the relative allocation to below-ground biomass (which represented about 80% and 50% of the biomass at nutrient-poor and nutrient-rich sites, respectively). Shoot growth at the nutrient-rich site exceeded that at sites receiving intermediate and low nutrient loadings by two-fold and three-fold, respectively, yet the biomass of plants growing under high nutrient supply was almost half of that of the plants receiving intermediate nutrient loading. Thos was due to reduced shoot density at the nutrient-rich site, resulting from higher shoot mortality at higher nutrient concentrations, which suggests a shift from phosphorus as the limiting resource towards limitation by light or oxygen supply to the roots. Depsite their lower biomass, the fast growth of plants at the nutrient-rich site allowed them to support similar leaf production to plants receiving intermediate nutrient supply, and twice the phosphorus incorporation rate of those plants. The observed differences in population dynamics across the gradient in nutrient supply demonstrate the power of comparative analyses of seagrass stands as an additional approach to examine the importance of nutrient supply in controlling seagrass growth and productivity.