Interactive effects of hyposalinity and nitrate loading on growth, physiology, and nitrogen status of the seagrass, Halodule wrightii

Abstract

The study goal was to examine the interactive physiological effects of two freshwater inflow stressors, nitrate pulses coupled with salinity decrease, on the seagrass Halodule wrightii . A microcosm experiment was designed to approximate an observed freshwater inflow event. Over a 13-day period plants were subjected to three sequential salinity drops (S35→ S23→ S15→ S5) with nitrate-nitrogen added simultaneously at 0, 30 or 60 µ M . For comparisons, the Control was no salinity change and no nitrate added denoted by S35/No N. Measurements of H. wrightii shoot production, photosynthesis, respiration, quantum efficiency, %N, C:N ratios and δ 15 N values which were made after each salinity drop revealed differing effects of low versus high N levels under S35 compared to reduced salinity. Compared to the Control at the experimental endpoint, leaf net photosynthesis: respiration (P:R) ratio decreased 3-fold for hyposalinity + High N addition (S5/High N) largely due to increased respiration. Leaf %N increased and C:N ratio decreased concomitantly with both stressors, with S5/High N having the highest %N and lowest C:N ratio. While the magnitude of the effect was related to the amount of added N at S35, there were different effects of Low versus High N at low salinity (S5). The trends of P:R ratio, leaf %N and C:N ratio are consistent with increased respiration, uptake of added N, and depletion of carbon reserves. However, δ 15 N suggested that added NO3- was taken up by leaves at S35, but not at S5. The increased %N at S5 may be due to translocation of amino acid N from rhizomes-roots to leaves. Metabolic networks were hypothesized to be regulated differently at 30 versus 60 µ M NO3- under conditions of hyposalinity. These findings add to the growing evidence that simultaneous stressors typical of substantial freshwater inflow events, hyposalinity and nitrate loading, could adversely affect H. wrightii .