How the Effects of Small and Flexible Seagrass (Zostera Noltii) on Flow Modify Erosion and Deposition Processes

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Latitudinal Patterns in European Seagrass Carbon Reserves: Influence of Seasonal Fluctuations versus Short-Term Stress and Disturbance Events

Seagrass meadows form highly productive and valuable ecosystems in the marine environment. Throughout the year, seagrass meadows are exposed to abiotic and biotic variations linked to (i) seasonal fluctuations, (ii) short-term stress events such as, e.g., local nutrient enrichment, and (iii) small-scale disturbances such as, e.g., biomass removal by grazing. We hypothesized that short-term stress events and smallscale disturbances may affect seagrass chance for survival in temperate latitudes. To test this hypothesis we focused on seagrass carbon reserves in the form of starch stored seasonally in rhizomes, as these have been defined as a good indicator for winter survival. Twelve Zostera noltei meadows were monitored along a latitudinal gradient in Western Europe to firstly assess the seasonal change of their rhizomal starch content. Secondly, we tested the effects of nutrient enrichment and/or biomass removal on the corresponding starch content by using a short-term manipulative field experiment at a single latitude in the Netherlands. At the end of the growing season, we observed a weak but significant linear increase of starch content along the latitudinal gradient from south to north. This agrees with the contention that such reserves are essential for regrowth after winter, which is more severe in the north. In addition, we also observed a weak but significant positive relationship between starch content at the beginning of the growing season and past winter temperatures. This implies a lower regrowth potential after severe winters, due to diminished starch content at the beginning of the growing season. Short-term stress and disturbances may intensify these patterns, because our manipulative experiments show that when nutrient enrichment and biomass loss co-occurred at the end of the growing season, Z. noltei starch content declined. In temperate zones, the capacity of seagrasses to accumulate carbon reserves is expected to determine carbon-based regrowth after winter. Therefore, processes affecting those reserves might affect seagrass resilience. With increasing human pressure on coastal systems, short- and small-scale stress events are expected to become more frequent, threatening the resilience of seagrass ecosystems, particularly at higher latitudes, where populations tend to have an annual cycle highly dependent on their storage capacity

Effects of bioturbation on the erodibility of cohesive versus non-cohesive sediments along a current-velocity gradient: A case study on cockles

Soft-bottom bioturbators are ecosystem engineers in the sense that they can have considerable effects on sediment erodibility and resuspension. The common cockle Cerastoderma edule is a bioturbating filter feeder that is widespread along the European Atlantic coastline. Its presence and activity can decrease sediment erosion thresholds in cohesive sediments but little is known about its effect on non-cohesive sediments. Using controlled annular flume experiments, we investigated the relative effects of different cockle densities on sediment re suspension in cohesive vs. non-cohesive sediments by assessing the following: (i) the mud and sand burrowing behavior of cockles, (ii) critical erosion thresholds, (iii) the mass of eroded sediment and (iv) erosion rates. Our results show that cackles were more active in non-cohesive sediment compared with cohesive sediment. Despite their lower activity, the presence of cockles in cohesive sediment increased sediment erodibility by reducing the critical erosion threshold (U-crit) and increasing both the mass of eroded sediment and erosion rate. In contrast, cockles had no effect on erodibility in non-cohesive sediment, especially on the eroded sediment mass and erosion rate. The mass eroded was not significantly different between cohesive and non-cohesive sediments when cockles were present. Our experiments show that the increased erodibility of cohesive sediment due to the. bioturbation by cockles is density dependent: higher cockle density results in stronger effects on erodibility. Moreover, this increase in cohesive sediment erosion due to cockle bioturbation was positively correlated with current velocity. In contrast, the erosion of non-cohesive sediment only depended on the current stress and was unaffected by cockle density. Considering the high abundance of C. edule, its widespread distribution and its extensive activities, the results of this study could be widely applicable to intertidal mud flats around the world.</p

Evidence for 'critical slowing down' in seagrass:a stress gradient experiment at the southern limit of its range

The theory of critical slowing down, i.e. the increasing recovery times of complex systems close to tipping points, has been proposed as an early warning signal for collapse. Empirical evidence for the reality of such warning signals is still rare in ecology. We studied this on Zostera noltii intertidal seagrass meadows at their southern range limit, the Banc d'Arguin, Mauritania. We analyse the environmental covariates of recovery rates using structural equation modelling (SEM), based on an experiment in which we assessed whether recovery after disturbances (i.e. seagrass & infauna removal) depends on stress intensity (increasing with elevation) and disturbance patch size (1 m(2) vs. 9 m(2)). The SEM analyses revealed that higher biofilm density and sediment accretion best explained seagrass recovery rates. Experimental disturbances were followed by slow rates of recovery, regrowth occurring mainly in the coolest months of the year. Macrofauna recolonisation lagged behind seagrass recovery. Overall, the recovery rate was six times slower in the high intertidal zone than in the low zone. The large disturbances in the low zone recovered faster than the small ones in the high zone. This provides empirical evidence for critical slowing down with increasing desiccation stress in an intertidal seagrass system

Variation in flexural, morphological, and biochemical leaf properties of eelgrass (Zostera marina) along the European Atlantic climate regions

Seagrasses need to withstand hydrodynamic forces; therefore, mechanical properties such as flexibility or breaking resistance are beneficial for survival. The co-variation of leaf breaking properties with biochemical traits in seagrasses has been documented, but it is unknown if the same patterns apply to leaf flexural properties. To interpret changes in the ecological function of seagrass ecosystems based on acclimation responses to environmental changes, it is necessary to understand the factors that affect flexural leaf properties. Here, morphological and flexural leaf properties of the perennial type of Zostera marina across different environmental conditions along European Atlantic climate regions are presented together with C:N ratio and neutral detergent fibre content as descriptors of biochemical leaf composition. Eelgrass leaves from cold regions were similar to threefold more elastic and similar to tenfold more flexible, were also narrower (1.7-fold), and contained similar to 1.9-fold higher fibre content than from plants growing in warmer regions. Eelgrass also showed acclimation to local conditions such as seasonality, water depth, and hydrodynamic exposure. Leaves collected from exposed or shallower locations or during winter were more flexible, suggesting an avoidance strategy to hydrodynamic forcing, which is generally higher under those conditions. Flexural rigidity was almost equally controlled by bending modulus (35%) and leaf thickness (37%), indicating functional differences compared to leaf breaking described in the literature. Overall, the findings indicate that Zostera marina has a high flexural plasticity and high acclimation capacity to some climate change effects such as sea level rise and increase in storm frequency and intensity.German Science FoundationGerman Research Foundation (DFG) [PA 2547/1-1]Royal Swedish Academy of Sciences (KVA travel grant)FCT-Foundation for Science and TechnologyPortuguese Foundation for Science and Technology [UID/Multi/04326/2019, SFRH/BPD/119344/2016

Effects of large dams on the aquatic food web along a coastal stream with high sediment loads

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a small coastal river with high sediment loads, and make it possible to anticipate the effects of dam removal on the future river ecosystem

“Entering the age of the hypermarket cinema’: the first five years of the multiplex in the United Kingdom’

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.During the first five years of its development from the opening of The Point in Milton Keynes in 1985 the multiplex cinema radically changed the previous exhibition landscape, modernising the business of cinema exhibition, and shifting the site of film consumption to new, out-of-town shopping and leisure centres. This article considers some key developments in the first five years of the multiplex cinema’s introduction in the UK, with particular emphasis on three aspects of multiplex diffusion: the importance of regeneration and enterprise; the multiplex’s role in stimulating associated leisure and commercial developments; and out-of-town and regional shopping developments. In order to illustrate these themes, the article will consider the opening of four complexes: The Cannon in Salford Quays, and the AMC multiplexes in Telford in Shropshire, Sheffield and Dudley Merry Hill, in the West Midlands