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11 pages, 6 figures, 3 tables.-- Under a Creative Commons license, Zostera marina and Z. noltii are two dominant meadow-forming seagrass species in temperate regions in the northern hemisphere. Seagrass meadows provide several goods and ecological services and rank amongst the most valuable ecosystems worldwide. Phenological shifts in Zostera species occur along latitudinal or temperature gradients, leading to an expectation for distinct seasonal dynamics and annual production in Spain and Ireland. Despite their global ecological importance, seasonal seagrass traits of populations exposed to contrasting climate settings from Ireland and southern Spain are poorly described. To address this gap, we evaluated the seasonal vegetative development (morphology, population structure, and productivity) of Z. marina and Z. noltii populations exposed to cold (western Ireland) and warm (southern Spain) temperatures over a year-long period. Our results highlight contrasting dynamics of Z. marina populations across their temperature range. The Irish population exhibited maximal growth during warmer months while the Spanish population revealed signs of growth reduction under maximum annual temperatures. Our results suggest that plants of Z. marina in Ireland were temperature-limited at most times, but in southern Spain exceeded their optima temperature for growth during summer. In addition, Irish and Spanish Z. noltii populations displayed differential vegetative dynamics and population structures, likely related to the contrasting local temperature regimes. These differences were particularly evident in summer when shoot size and growth rates were reduced in warm-temperate locations. This study represents an essential comparative baseline for future assessments of ecological status, and anticipated growth stress induced by climate change across the distributional gradient, of these important Zostera species, This project was supported by a College of Science of the National University of Ireland (NUI Galway) PhD Scholarship to Pedro Beca-Carretero, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

International audience; Seagrass meadows are monitored in the frame of several environmental programs worldwide, including the Water Framework Directive (WFD), to evaluate the ecological status of European coastal and transitional water bodies. The large size, spatial complexity, and interannual variability of seagrass ecosystems significantly challenge field monitoring. In this study, a multi-mission satellite time-series was used to estimate long-term changes in seagrass status in a macrotidal system dominated by Zostera noltei, at Bourgneuf Bay (French Atlantic coast). Metrics of seagrass extent and density were obtained from Earth Observation (EO) using validated and inter-calibrated Landsat, SPOT and Sentinel2 data from 1985 to 2020. The information provided by satellite data made it possible to compute and compare several seagrass indicators currently in use in several European countries (France, Portugal and UK) within the WFD. Both the seagrass extent and meadow-averaged density displayed increasing trends since 1985. A time-series of merged observations from various satellites revealed a high degree of interannual variability in seagrass extent, with abrupt losses (up to 50% within one year) alternating with periods of slow recovery (typically 4-6 years). The seagrass meadow which was in a moderate status (sensu the WFD) in the 1980s, achieved an overall recurrent good or high status since the mid-1990s. Altogether, the methods and results presented here demonstrated that EO is a reliable source of information for mapping and assessing the status of intertidal seagrass, complementing in situ measurements by providing long-term, spatial view and standardized observation framework. We recommend the systematic use of EO time-series in complement to traditional field measurements in seagrass monitoring programs such as the WFD.

In nature, seagrasses are confronted with a compound pool of low concentrations of inorganic and organic nitrogen-containing substances of varying bioavailability. Nevertheless, the majority of research on nitrogen acquisition by seagrasses has been largely limited to studies addressing a single nitrogen substrate at a time. Using a combination of one of 15N-labeled substrates and one 14N-labeled background substrate, we investigated how the rate of nitrogen uptake by the seagrass Zostera noltii varies with nitrogen background. Leaf and root mediated uptake were studied separately for different combinations of inorganic (ammonium and nitrate) and organic substrates (urea and glycine). Ammoniumuptake rates were higher than for the other substrates. However, substrate uptake was not dependent on the background nutrient. Similar patterns and uptake rates were found for above- and belowground plant parts. The dependence of uptake rate on substrate type, combined with an independence of nutrient background is explained as difference in uptake capacity, rather than substrate preference. For the dual labeled (15N and 13C) urea and glycine, strong relationships existed between nitrogen and carbon uptake, but with deviations from expectations under complete uptake of the molecules. Overall, this study indicates that at realistically low ambient concentrations, seagrasses can simultaneously use inorganic and organic sources for their nitrogen needs, and do not distinguish between substrates. In other words, they take up whatever is available.

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An in-depth knowledge of solutes advection and turbulent diffusion is crucial to estimate dispersion area and retention time (t R) of pollutants within seagrass habitats. However, there is little knowledge on the influence of seagrass habitat fragmentation on such mechanisms. A set of dye tracer experiments and acoustic Doppler velocimeter measurements (ADV) were conducted. Solute transport conditions were compared in between fragmented (FM) vs homogeneous (HM) intertidal meadows, and in vertical gradients (canopy vs overlaying flow). Results showed the highest horizontal diffusion coefficient (K y, c.a. 10 -3m 2s -1) on FM and at the canopy-water column interface, whereas t R (2.6-5.6min) was not affected by fragmentation. It suggests that (1) FM are more vulnerable to pollution events in terms of dispersion area and (2) at low tide, advection rather than turbulent diffusion determines t R. Furthermore, Taylor's theorem is revealed as a powerful tool to analyze vertical gradients on K y within seagrass canopies. © 2012 Elsevier Ltd., This work was supported by the Spanish National Research Project IMACHYDRO (CTM2008-0012/MAR) and by the Andalusian Excellence Research Project FUNDIV P07-RNM-2516. First author is supported by a FPI grant of the Spanish Ministry of Science and Technology.

5 páginas, 2 tablas, 1 figura., Secondary metabolites play an important role in protecting plants from herbivores or pathogens. Despite this, few studies have investigated the presence and the concentration of those compounds over broad spatial scales in seagrasses. Here we analysed the presence and abundance of specific phenolic compounds of Zostera noltii from four different meadows located across the Atlantic and the Mediterranean coast. Three phenolic acids have been found in the samples of Z. noltii: rosmarinic, caffeic and zosteric acids. The proportion of the three compounds varied substantially with the major proportion comprising rosmarinic acid (RA, mean value 94%) followed by zosteric acid (ZA, mean value 4%) and caffeic acid (CAF, mean value 2%). Z. noltii metabolic compounds varied notably between sites. The highest concentrations in RA were found in the Cadiz Bay with values of 11,254 (55 SD) mg g 1 dry wt, while the lowest were observed in the Alfacs Bay (Ebro) with values of 823 (7 SD) mg g 1 dry wt. The highest concentrations in ZA were also observed in the Cadiz Bay with values of 727 (6 SD) mg g 1 dry wt, while the lowest were observed in the Sa Nitja Bay (Menorca Island) with values of 20 (4 SD) mg g 1 dry wt. Finally, the concentrations in CAF remain very low at each site (17e69 mg g 1 dry wt), with the lowest observed in Arcachon. Their proportions remained relatively constant, indicating that most phenolic acids responded together. Documenting the presence of those compounds in living tissues of Z. noltii and how they vary in abundance between seagrass meadows across large geographical scales is a crucial first step to understand the large-scale level response of the plant to potential pathogens, herbivore outbreaks or other ecological processes., This work was supported by an ITT-CTO2002 grant from the Generalitat de Catalunya and by a grant (CGL2007-66771-C02-01) from the Spanish Ministry of Science and technology.

Mud snails Hydrobia ulvae occupy different habitats in complex estuarine ecosystems. In order to determine if fatty acid profiles displayed by mud snails can be used to identify the habitat that they occupy within the same estuary, fatty acids of H. ulvae from one mudflat and one seagrass meadow in the Ria de Aveiro (Portugal) were analyzed and compared to those displayed by microphytobenthos (MPB), the green leaves (epiphyte-free) of Zostera noltii , as well as those exhibited by the epiphytic community colonizing this seagrass. MPB and epiphytic diatom-dominated samples displayed characteristic fatty acids, such as 16:1 n- 7 and 20:5 n- 3, while 18:2 n- 6 and 18:3 n- 3 were the dominant fatty acids in the green leaves of Z. noltii . Significant differences between the fatty acid profiles of H. ulvae specimens sampled in the mudflat and the seagrass meadow could be identified, with those from the mudflat displaying higher levels of fatty acids known to be characteristic of MPB. This result points towards the well known existence of grazing activity on MPB by mud snails. The fatty acid profiles displayed by H. ulvae inhabiting the seagrass meadows show no evidence of direct bioaccumulation of the two most abundant polyunsaturated fatty acids of Z. noltii (18:2 n- 6 and 18:3 n- 3) in the mud snails, which probably indicates that either these compounds can be metabolized to produce energy, used as precursors for the synthesis of essential fatty acids, or that the snails do not consume seagrass leaves at all. Moreover, the fatty acid profiles of mud snails inhabiting the seagrass meadows revealed the existence of substantial inputs from microalgae, suggesting that the epiphytic community colonizing the leaves of Z. noltii displays an important role on the diet of these organisms. This assumption is supported by the high levels of 20:5 n- 3 and 22:6 n- 3 recorded in mud snails sampled from seagrass meadows. In conclusion, fatty acid analyses of H. ulvae can be successfully used to identify the habitat occupied by these organisms within the same estuary (e.g. mudflats and seagrass meadows) and reveal the existence of contrasting dietary regimes.