Water-column nitrate enrichment promotes decline of eelgrass Zostera manna: evidence from seasonal mesocosm experiments

Abstract

We imposed a replicated gradient of water-column nitrate enrichment in large-scale experimental mesocosms to test the response of eelgrass to nitrate loading during the spring and fall growing seasons. In spring, controls without nitrate additions ( < l pM ambient NO3--N) were compared to treatments with pulsed daily additions of low (LOW), moderate (MOD), and high (HIGH) enrichment (ca 3.5, 7.0, and 35.0 pM NO3--N d -', respectively). Under low water exchange simulating quiet embayments, eelgrass growth and survival significantly decreased at all enrichment levels, with most rapid decline at the highest nitrate loadings. Plant death was preceded by loss of structural integrity in the aboveground meristems. The adverse effects of nitrate enrichment in spring apparently were exacerbated by increasing/high temperatures and by macroalgal growth which reduced light comparably over time in all mesocosms. During fall w e maintained identical treatments except that the former HIGH regime was used to test the response of eelgrass to residual sediment N, accumulation (RES) wlthout additional enrichments. Eelgrass survival was significantly higher in controls and the RES treatment than under MOD enrichment. Nitrate-enriched plants maintained higher N uptakektorage In the aboveground tissue, but lower carbon storage In the rhizomes. The data indicate that water-column nltrate enrichment causes decline of eelgrass especially under increasing/high temperatures, as a d~r e c t phys~ological effect unrelated to algal l~g h t attenuation. The mechanism likely involves Internal Imbalances in nutrient supply ratios resulting from sustamed nltrate uptake through the leaf tissue.

Combined nutrient and macroalgae loads lead to response in seagrass indicator properties

“…5B). Nutrient enrichment of sediment drives plants to increase photosynthetic C fixation to meet the increased demand for C. However, if photosynthetic C fixation in leaf and rhizome tissues cannot meet the increased C demand, stored C in plant leaf tissues may be used to support metabolic processes (Burkholder et al, 1992;Irlandi et al, 2004;Lee and Dunton, 1999). This may have led to the shift in balance between aboveground and belowground biomass in our study.…”

Ecophysiological and anatomical responses ofVallisneria natansto nitrogen and phosphorus enrichment

“…Several studies have shown that no significant growth response to N enrichment has been reported (Murray et al, 1992), others have argued that increased nitrogen levels often 05p7 induce increased growth and biomass (Short, 1987;Van Lent et al, 1995). In other cases, however, decline in macrophytes was observed as a direct effect of increased nitrogen in the form of nitrate (Burkholder et al, 1992) and ammonium toxicity to submerged aquatic plants has also been given much attention in a few freshwater aquatic plants (Smolders et al, 1996). Furthermore, mesocosm experiments conducted in Denmark revealed thresholds of TN and TP concentrations of 1.2−2 and 130−200 ug·L −1 , respectively, above which there is high probability of losing submerged macrophytes in temperate shallow lakes (Gonzalez Sagrario et al, 2005;Jeppesen et al, 2007).…”

“…Eutrophication of lakes is a widespread environmental problem (OECD, 1982), and a consequential decline in macrophytes has been recorded over the past three decades (Burkholder et al, 1992;Perrow et al, 1994;Middleboe and Markager, 1997;Melzer, 1999;Tracy et al, 2003;Qin et al, 2005). Although there has been no consensus in explaining this phenomenon, some incidental stresses of eutrophication have been suggested as the causal factors, including overgrowth epiphytes and macroalgae and the associated light reduction, low sediment redox potential, excessive nutrient supply and an absence of an uptake regulation mechanism for certain species (Sand-Jensen, 1977;Phillips et al, 1978;Burkholder et al, 1994;Terrados et al, 1999;Ni, 2001).…”

Toxic effects of increased sediment nutrient and organic matter loading on the seagrass Zostera noltii

“…Previous 13 studies mainly focused on the effects of high nitrate or ammonium concentrations in the surface water layer (Burkholder et al, 1992;Christianen et al, 2011;Van der Heide et al, 2008;van Katwijk et al, 1997). In a correlative study (Short, 1983b) already suggested that porewater nitrogen concentrations of 10 -100 mmol m -2 were associated with decreasing eelgrass shoot densities.…”