The effects of light and ammonium levels on net production, fluorescence parameters and non-structural carbohydrates of the seagrass Zostera noltii under different phosphate conditions were studied. A fully factorial design was used with light (low/high levels), ammonium supply and phosphate preculture conditions of the plants as the experimental variables. Both ammonium supply and low light caused negative and synergistic effects on net production, while ammonium toxicity was more severe at high light levels; in this case, it was independent of the non-structural carbohydrate (sucrose and starch) content. Preculturing of plant with added phosphate alleviated the ammonium toxicity, and also attenuated the negative production balance of plants grown at low light levels. The results indicated that phosphate preculture ameliorated the plant's short-term response against the assayed stressors (low light, high ammonium) significantly. An overall consumption of non-structural carbohydrates in response to environmental stressors was recorded throughout the experiment, indicating the importance of carbon and phosphorus reserves to cope with adverse conditions. In addition, phosphate deficiency increased the vulnerability of plants, which could have negative ecological consequences for seagrass species thriving under phosphate deficiency conditions, or in developing seagrass transplantation programs.KEY WORDS: Ammonium toxicity · Carbohydrates · Eutrophication · Fluorescence · Net production · Phosphate limitation · Seagrass Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 365: [67][68][69][70][71][72][73][74][75] 2008 seagrasses has been demonstrated (Burkholder et al. 1992(Burkholder et al. , 1994van Katwijk et al. 1997 and Brun et al. 2002 for ammonium) at concentrations as low as 16 µm ammonium. Seagrass leaves take up ammonium in direct proportion to the concentration in the surrounding water, but are not capable of controlling it (Thursby & Harlin 1982, Iizumi & Hattory 1982, van Katwijk et al. 1997, Touchette & Burkholder 2000. Intracellular ammonium has to be rapidly assimilated into organic compounds to prevent its accumulation to potentially harmful levels, as was shown in an experiment with the seagrass Posidonia oceanica, which maintained low intracellular ammonium levels regardless of the nitrogen enrichment levels at which plants were grown (Invers et al. 2004).Ammonium toxicity mechanisms are complex and not fully understood (Britto & Kronzucher 2002), but the physiological mechanisms are thought to be a combination (among others) of: (1) the uncoupling of ATP production during the photosynthetic electron transport that is triggered by ammonia (Goyal et al. 1982, Marschner 1995, (2) the intracellular depletion of essential cations (such as potassium and magnesium; Kirkby 1968, van Katwijk et al. 1997) that is accompanied by the increase of intracellular inorganic anions (e.g. phosphate; Cruz et al. 1993, van Katwijk et al. 1997, (3) an increa...

Seagrasses may frequently experience a combination of velocity and light stresses, as elevated hydrodynamics often enhances turbidity and the subsequent light reduction. The objective of this study was to investigate the effects that these stressors induce on morphometric and dynamic seagrass features depending on the initial biomass partitioning. For this purpose, a factorial mesocosm experiment was conducted on plants of Zostera noltii subjected to combinations of 2 contrasting light levels (2.5 ± 0.6 and 15.6 ± 2.5 mol photons m -2 d ). No interactive effects between the 2 variables were recorded, except on plant survival and leaf length, and generally, light effects prevailed over hydrodynamic ones. Plants responded to light reduction regardless of the flow velocity treatments, showing low survival rates (which improved at high velocity), high aboveground/belowground biomass ratios (AG/BG) and a poorly developed root-rhizome system compared to plants under saturating light conditions. Plant morphometry only responded to hydrodynamic stress under saturating light: at high current velocity, plants preferentially allocated biomass into BG structures, bearing short leaves and displaying high internode and root appearance rates. Overall, light reduction promoted similar responses in plants with different AG/BG biomass ratios, but dissimilarities were recorded for current velocity. Thus, it can be concluded that, under simultaneous light and hydrodynamic stresses, light effects prevailed over hydrodynamic ones in Z. noltii, while acclimation to hydrodynamics only occurred under saturating light.

Light-dependent P uptake by root-rhizomes, acropetal translocation and subsequent foliar release by Zostera noltii Hornem. was studied under laboratory conditions in two-compartment chambers using 32p. The uptake by underground parts was unaffected by light conditions but the acropetal translocation proceeded more rapidly in light than in dark, indicating a coupling to the metabolic activity of the plants. The translocated P was mainly accumulated in the youngest leaves (30%), i.e., the most actively growing parts.Foliar release of P might be considered negligible (2-4 % of the P taken up by root-rhizomes), indicating that the role of Z. noltii as a "P pump" is of minor importance in the cycling of P between sediment and water. This was calculated for part of the Oosterschelde estuary, Zeeland, The Netherlands.