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We studied the effect of ecologically relevant ammonium concentrations and light on several morphological and physiological properties, nitrogen metabolism and carbon reserves of eelgrass Zostera marina L. Eelgrass was grown under mesocosm conditions at 3 levels of ammonium enrichment (target concentrations of 0, 10 and 25 µM) and 2 levels of light (low and high light). High ammonium supply combined with low light had a negative effect on several morphological and physiological response parameters, while no such effects were found when ammonium was supplied under high light. N enrichment caused an increase in the content of total N, intracellular ammonium, free amino acids and residual N in the plants and this response was more pronounced under low-light conditions than under high light. The soluble proteins content decrea sed, in contrast with external ammonium enrichment. The accumulation of free amino acids and residual N in NH 4 + -enriched plants was followed by a substantial drop in carbohydrate reserves (sucrose and starch), which was larger in plants grown under low-light conditions. Our results indicate that N enrichment increases the demand for C skeletons and energy, and that photosynthesis cannot supply enough C and energy to cover that demand under low-light conditions. Eelgrass plants exposed to reduced light conditions, for example close to their depth limit or when covered by drift macroalgae, may thus be especially susceptible to enhanced ammonium concentrations. Our study demonstrates that ammonium toxicity may explain why eelgrass and other seagrasses deteriorate under nutrient-rich, low-light conditions.

Foliar ammonium and phosphate uptake rates and their interactions were assessed in whole seagrass Zostera noltii plants incubated in 2-compartment transparent chambers. This method allowed the calculation of nutrient uptake rates by leaves and by roots and rhizomes independently, avoiding plant breakage. Overall, a direct linear relationship between foliar uptake rates and seawater nutrient concentrations (phosphate or ammonium) was found, with uptake rates much higher in the first 5 min (nutrient adsorption). This faster adsorption was followed by slower uptake rates (nutrient absorption) in the next time intervals. When both nutrients were supplied separately, foliar ammonium uptake rates were 3-fold higher than those of phosphate in the range of the nutrient concentrations assayed for the whole incubation interval (120 min). When both nutrients were added simultaneously (10 µM phosphate and 50 µM ammonium, final concentrations), ammonium uptake rates were similar to those values recorded when ammonium was provided alone, whereas phosphate uptake rates were about 55% lower than those measured when phosphate was added alone. This study reveals for the first time the inhibitory effect of ammonium on phosphate uptake in seagrasses.

Climate change intensifies the frequency and intensity of rainfall events, which increases the discharge of freshwater and nutrients to coastal areas. This may lower salinity and increase nutrient availability and, thus, affect estuarine eelgrass populations. We studied the interactive effect of increasing NH 4 + levels and low salinity on estuarine eelgrass Zostera marina, grown in microcosm at various combinations of NH 4 + enrichment (0, 10 and 25 μM) and salinity (5, 12.5 and 20). Increasing NH 4 + had a positive effect on eelgrass performance as long as salinity was kept at ambient level (20). N enrichment was followed by an increase in pigments, photosynthesis and various growth variables and a decrease in stored carbon concentrations (sucrose and starch). Low salinity had an overall negative effect on plant fitness; pigment concentration, photosynthesis and growth were reduced while mortality increased. Exposure to low salinity was also followed by a decrease in sucrose, suggesting that it was used as an osmolyte and/or that photosynthesis could not cover energy requirements needed for osmoregulation or repairing processes. Concomitant exposure to high NH 4 + and low salinity turned the positive effect of NH 4 + into a strong, negative synergistic effect. Several growth-related variables were affected significantly and mortality increased substantially. We suggest that this simultaneous exposure intensified competition for energy and C skeletons affecting other metabolic processes (e.g. growth, repair processes) negatively. Our results suggest that climate change driven alterations in precipitation and NH 4 + loading might seriously impact estuarine eelgrass communities.

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