Nutrient mass balance of the seagrass Posidonia oceanica: the importance of nutrient retranslocation

The seasonal nutrient mass balance of the dominant seagrass of the Mediterranean, Posidonia oceanica (L.) Delde, was evaluated in NE Spain in order to test the hypothesis that the effect of seasonal nutrient imbalance can be reduced by the reutilization of internal nutrient pools. To this end we investigated the seasonal and age-dependent variability of nitrogen and phosphorus concentration of the leaves, inferring from these data values of seasonal nitrogen and phosphorus incorporation, uptake, losses and retranslocation. Incorporation of nitrogen and phosphorus in leaves peaked in June and was lowest in September, thus following the seasonal growth pattern of the plant. Retranslocation of nitrogen and phosphorus was high from May to September and close to zero during the rest of the year. Losses of nitrogen and phosphorus were highest at the end of summer, associated with the major biomass losses. Nitrogen uptake by leaves reached maximum values in winter and was lowest during August-September, while phosphorus uptake was highest in spring and lowest in August-September. On an annual basis nitrogen and phosphorus uptake accounted for 60 and 41 % of the total nutrient incorporation, respectively, while retranslocation of nutrients from old tissues accounted for the remaining 40 and 59%. Although roots and rhizomes function as sources of nutrients at the beginning of the summer, their contribution to the seasonal nutrient budget seemed to be minor.This work was supported by grant STEP-0063-C of the ECC.Peer reviewe

Vertical migration of phytoplankton in coastal waters with different UVR transparency

Background: The vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter (cDOM) concentrations. The data from the corresponding water depths (50-cm intervals down to 10 m) and times (hourly, before dawn to sunset, several days) were related to the highly resolved (2 nm) underwater ultraviolet radiation (UVR)/photosynthetic active radiation (PAR) transparency (290 to 700 nm). Results: Chlorophyll a maxima of mainly motile dinoflagellates were observed in situ at all days and at both study sites (open marine, brackish waters), independent on prevailing weather conditions or cDOM concentrations. Phytoplankton migration was triggered solely by irradiance in the 400- to 700-nm wavelength range (PAR) at the particular water depth, irrespective of PAR/UVR ratios and surface UVR (290 to 400 nm), after an illumination period of about 40 min. Interestingly, the PAR tolerance levels of the phytoplankton, which have been lower in cDOM-rich waters, matched their light acclimation values determined by parallel PAM measurements. Conclusions: The response of the phytoplankton to PAR is not a sufficient protection strategy versus increasing UVR levels, which might have wide ecological implications beyond the level of primary producers to impact important ecosystem functions such as the delicate trophic interactions

Form, function and physics: The ecology of biogenic stabilization

The effect of biological cohesion on the behaviour of sediments is gaining increasing notice. This is partly supported by ecological theory in terms of the role of organisms as “ecosystem engineers” and the associated discussion of “niche construction”, suggesting an evolutionary role for habitat modification by biological action. In addition there is a strong societal and policy drive toward the “ecosystem approach” supporting an integrated examination of the functional roles of biota in selected habitats. In this context the increasing recognition of the importance of biological activity in the mediation the erosion, transport, deposition and consolidation (ETDC) cycle of sediments is important and advances in technology will improve our ability to examine these effect under their natural settings. This will shortly be combined with vastly enhanced molecular tools that will allowthe discrimination of microbial biodiversity and examination of their metabolism contribution to ecosystem function. This may lead to a step-change in our ability to research the influence of microbiota on natural sediment dynamics and opens an exciting era for new interdisciplinary research

Erosion characteristics and floc strenght of Athabasca river cohesive sediments: towards managing sediment-related issues

Purpose: Most of Canada’s tar sands exploitations are located in the Athabasca river basin. Deposited cohesive sediments in Athabasca river and tributaries are a potential source of PAHs in the basin. Erosional behavior of cohesive sediments depends not only of fluid turbulence but on sediments structure and particularly the influence of organic content. This research tries to describe this behavior in Athabasca river sediments. Methods: An experimental study of cohesive sediments dynamics in one of the tributaries, the Muskeg river, was developed in a rotating annular flume. Variation of the shear stress allowed the determination of erosional strength for beds with different consolidation periods. Particle size measurements were made with a laser diffraction device operated in a continuous flow through mode. Optical analyses of flocs (ESEM and TEM) were performed with samples taken at the end of the experiments. Results: An inverse relationship between suspended sediment concentration (SS) and the consolidation period was found. The differences are related in this research to the increasing organic content of the sediments with consolidation period. The particle size measurements during the experiments showed differences on floc strength that are also related to changing organic content during different consolidation periods. ESEM and TEM observations confirm the structural differences for beds with different consolidation periods. The effects of SFGL on floc structure and in biostabilization of the bed are discussed. Conclusions: It is recommended in this paper that consolidation period should be taken into account for the modeling of erosion of cohesive sediments in the Athabasca river. Relating to transport models of pollutants (PAHs) it is highly recommended to consider flocs organic content, particularly algae, in the resuspension module.Environment Canada, CONACY

The pervasive role of biological cohesion in bedform development

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS

Shorebirds affect ecosystem functioning on an intertidal mudflat

Ecosystem functioning and services have provided a rationale for conservation over the past decades. Intertidal muddy sediments, and the microphytobenthic biofilms that inhabit them, perform crucial ecosystem functions including erosion protection, nutrient cycling and carbon sequestration. It has been suggested that predation on sediment macrofauna by shorebirds may impact biofilms, and shorebirds are known to consume biofilm, potentially causing significant top-down effects on mudflat ecosystem functioning. We carried out an exclusion experiment on the Colne Estuary, Essex, UK to examine whether shorebird presence significantly affects sediment erodibility measured with a Cohesive Strength Meter (CSM) and microphytobenthos biomass measured using PAM fluorescence (Fo) and chlorophyll a content. We also tested for treatment effects on sediment-water nutrient fluxes (nitrate, nitrite, ammonia, phosphate and dissolved organic carbon (DOC)) during periods of both dark and light incubation. Excluding shorebirds caused statistically significant changes in regulating and provisioning ecosystem functions, including mudflat erodibility and nutrient fluxes. The presence of shorebirds lowered the sediment critical erosion threshold, reduced nitrate fluxes into the sediment under illumination, lowered nitrate efflux, and reduced phosphate uptake, compared to sediments where birds were excluded. There were no significant differences in macrofauna community composition within the sediment between treatments after 45 days of bird exclusion, suggesting a direct link between shorebird presence or absence and the significant differences in biofilm-related variables. This study introduces previously unknown effects of shorebird presence on ecosystem functions within this system and highlights an area of shorebird science that could aid joint conservation and human provisioning action