High ammonium availability amplifies the adverse effect of low salinity on eelgrass Zostera marina

Abstract

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.

The negative effects of short-term extreme thermal events on the seagrass Posidonia oceanica are exacerbated by ammonium additions

“…oceanica , a species considered to be persistent, thus leading to the conclusion that the future impact of warming could be much worse for plants subject to high ammonium loading than for plants living in relatively pristine environments. These findings are consistent with a large number of studies, which have also reported synergistic effects between two simultaneous stressors on seagrasses [39,52,55,107]. However, most of these studies have focused on colonizing and opportunistic seagrass species; further studies are therefore required to shed light on the response of this, and other, persistent seagrass species to simultaneous exposure to two or more stressors.…”

Additive response to multiple environmental stressors in the seagrass Zostera marina L.

“…Prolonged exposure to excess is toxic to plants although the underlying mechanisms remain partly unresolved (Britto et al ). Plants exposed to high levels accumulate intra‐cellular , obtain lower pigment concentrations, higher levels of oxidative damages, and impaired photosynthesis and growth (e.g., Brun et al ; Villazán et al ; Zhou et al ). Various processes may serve to reduce high concentrations of in the cytosol.…”

“…The response of seagrasses to multiple stressors has lately been addressed experimentally in a number of two‐factor studies where potential interactions between a variety of factors have been addressed (Hootsmans et al ; Kahn and Durako ; Koch et al ; de los Santos et al ; Collier et al ; Höffle et al ; Holmer et al ; La Nafie et al ; Jiang et al ; Villazán et al ; York et al ; Salo and Pedersen ; Castorani et al ; Villazán et al ; Hasler‐Sheetal et al ; Moreno‐Marín et al ). The conclusions in many of these articles infer a prevalence of synergistic effects in seagrasses when exposed to multiple stressors, although this has rarely been tested explicitly.…”

“…Ocean surface waters have warmed with an average of 0.18C per decade over the last century (IPCC 2007), but higher rates appear in coastal areas, not at least because heat waves become more frequent and intense (e.g., Lima and Wethey 2012). Temperatures above the optimum affect plants through limitation of enzyme capacities, increasing protein denaturation and destabilization of membranes including those related to the photosynthetic apparatus (Wahid et al 2007). Climate changes also lead to higher frequency and intensity of precipitation events in temperate climate zones (Trenberth 2011), which enhances discharge of nutrients rich and turbid freshwaters, and thus, affect salinity, light conditions, and nutrient concentrations in coastal waters (Meier et al 2012).…”

Does Warming Enhance the Effects of Eutrophication in the Seagrass Posidonia oceanica?

“…However, experiments assessing the interactions between temperature and nutrients are rare due to experimental and budgetary constraints. Some studies on seagrasses demonstrated that synergistic interactions occur when plants are exposed to a combination of stressors, such as osmotic, light, temperature and eutrophication (Touchette and Burkholder, 2000;Collier et al, 2011;Villazán et al, 2013Villazán et al, , 2015Ontoria et al, 2019b). However, the responses were highly specific for the species assessed and varied according to the life stage of the plant.…”