Bi-decadal changes in nutrient concentrations and ratios in marine coastal ecosystems: The case of the Arcachon bay, France

International audienceLarge amounts of nutrients have been released to the coastal ecosystems during the 20th century. Since then, management policies have been implemented and these amounts decreased in the economically developed countries. We examined the bi-decadal changes in nutrients (nitrate + nitrite, ammonium, orthophosphate and silicic acid) in the Arcachon bay, a semi-enclosed lagoon that hosts one of the largest but declining seagrass meadow in Europe. Seven sites have been sampled for nutrients and biogeochemical parameters during twenty years at low and/or high tide. In addition, continental and climatic data as well as hydro-climatic indices were used. Dynamic linear models were used to assess the bi-decadal changes in nutrient concentrations and ratios, their seasonality, and the bi-decadal changes of their potential drivers. Partial least square path modeling were used to investigate the relationships between potential abiotic drivers and nutrients. During the study period, the concentration of N and Si nutrients increased whereas the concentration of orthophosphate decreased, leading to deep changes in nutrient ratios. Clear relationships between abiotic drivers (local climate, continental inputs and the bay hydrodynamism) and N, P and Si nutrients were highlighted. However, the bi-decadal change in nutrient concentrations and ratios was mainly ascribed to the seagrass meadow decline through direct (less nutrient consumption) and indirect (increase in phytoplankton biomass) processes. Changes in temperature and wind direction may also influenced the nutrients concentrations through processes of remineralisation and flushing time, respectively. This study illustrates (1) the top-down control of seagrass on the nutrients concentrations and stoichiometry, and (2) the competition between primary producers (seagrass vs phytoplankton) for their nutrients resource

Seasonal and latitudinal variation in seagrass mechanical traits across Europe: The influence of local nutrient status and morphometric plasticity

Seagrasses are marine flowering plants distributed worldwide. They are however threatened, mostly due to the increase of human activities. Seagrasses have the capacity to adapt their morphological, physiological, and mechanical traits to their local conditions. Mechanical traits have been identified as a good tool to investigate a plant-species capacity to withstand physical forces or disturbances but are still sparsely studied in seagrasses. With this study, we aimed to assess how the mechanical traits of a broadly spread seagrass species vary along a latitudinal gradient in relation to its morphometric plasticity and nutrient status. We found that seagrasses acclimate their mechanical traits in relation to their physiological or morphological traits, both over the growing season and across a latitudinal range: leaves were weaker and thinner in northern areas, particularly at the end of the growing season. Besides the influence of the latitudinal gradient, leaf mechanical strength and stiffness were both strongly affected by their morphometric plasticity. Moreover, we showed that leaves mechanical traits change depending on their nutrient status: leaves were stronger and stiffer in oligotrophic conditions as compared to more eutrophic conditions. Thus, our results imply that, under eutrophication, leaves become weaker and thus more vulnerable to physical forces. This vulnerability is higher in the north at the end of the growing season. The latter is consistent with the more ephemeral character of northern seagrass meadows, in contrast to the more evergreen southern meadows

Pollen limitation may be a common Allee effect in marine hydrophilous plants: implications for decline and recovery in seagrasses

International audiencePollen limitation may be an important factor in accelerated decline of sparse or fragmented populations. Little is known whether hydrophilous plants (pollen transport by water) suffer from an Allee effect due to pollen limitation or not. Hydrophilous pollination is a typical trait of marine angiosperms or seagrasses. Although seagrass flowers usually have high pollen production, floral densities are highly variable. We evaluated pollen limitation for intertidal populations of the seagrass Zostera noltei in The Netherlands and found a significant positive relation between flowering spathe density and fruit-set, which wassuboptimal at <1200 flowering spathes m−2 (corresponding to <600 reproductive shoots m−2). A fragmented population had ≈35 % lower fruit-set at similar reproductive density than a continuous population. 75 % of all European populations studied over a large latitudinal gradient had flowering spathe densities below that required for optimal fruitset, particularly in Southern countries. Literature review of the reproductive output of hydrophilous pollinated plants revealed that seed- or fruit-set of marine hydrophilous plants is generally low, as compared to hydrophilous freshwater and wind-pollinated plants. We conclude that pollen limitation as found in Z. noltei may be a common Allee effect for seagrasses, potentially accelerating decline and impairing recovery even after environmental conditions have improved substantially

Mudflat Ecosystem Engineers and Services

International audienceEcosystem engineers play a fundamental role in the creation, maintenance and transformation of habitats in tidal flats. Highly diverse in terms of size, phylogeny, and effect on their environment, they can facilitate or hinder a number of organisms, but generally have a positive influence on both the abundance and the diversity of mudflat organisms. The magnitude of the engineering effect is, however, largely dependent on the biotic and abiotic environment of the engineer. In particular, stressful habitats such as mudflats host a large number of ecosystem engineers; understanding interactions between them, and how they vary with abiotic variables, is therefore of crucial importance, to evaluate how ecosystem engineers affect benthic communities and ecosystem functioning. Such understanding will also help human populations which benefit from mudflat organisms and/or functioning (i.e. which derive ecosystem services from them), to maintain and manage the sustainably of tidal flats, in a way which maintains human health and well-being