Density and size-dependent bioturbation effects of the infaunal polychaete Nephtys incisa on sediment biogeochemistry and solute exchange

The impact of bioturbation on the geochemistry of aquatic sediments is known to depend on the benthic infauna species that are present. However, burrowing and activity patterns of each species may also change during the different stages of a life cycle. In this study, we examined the effects of four size classes of the polychaete Nephtys incisa on burrow networks and sediment biogeochemistry. In our experimental aquaria, the total biovolume (~ biomass) of Nephtys was kept constant, but different age classes were introduced, so the size and abundance varied between treatments. Despite differences in the geometry of burrow networks (due to varying density and size of burrows as revealed by X-radiography), the transport of nonreactive solutes (Br–) showed little difference between treatments. In contrast, the depth distribution of reactive solutes (Fe2+, Mn2+, TPO3– 4, TCO2, O2, pH) depended on oxidized sediment volumes and on spatial micro-heterogeneity related to burrowing patterns. Net fluxes of O2, TCO2, and NO– 3 fluxes were strongly affected by age-dependent burrowing patterns. Carbonate dissolution and remineralization rates (reflected by TCO 2fluxes) were enhanced as the size of individuals increased. NO– 3fluxes showed progressive change from dominance of nitrification (release) to denitrification (uptake) as burrow densities decreased with larger individuals. We conclude that different age-size classes of a single species at identical biovolume affect biogeo- chemical cycling differently, due to changes in burrow sizes and burrow densities. Because of redox reaction coupling associated with burrow geometries (Fe2+, Mn2+ oxidation patterns), similar magnitudes of nonlocal transport may be a misleading indicator of biogenic impacts. Our observations demonstrate that biogeochemical impacts must be evaluated in the context of size (age-) specific traits and population densities rather than biomass or biovolume alone.

Conditioning preferential flow by using synthetic goethite aggregates

<p>Mass transport is significantly impacted by the nature of flow and, in particular, the occurrence of preferential flows. Most of the time, studies focus on observing preferential flow and its impact on mass transport either at the lab or the field scales. In the lab, real matrices are considered and embedded into columns, and mass transport is assessed for specific solutes and under controlled conditions (constant flow rate, saturation degree, etc…). However, very few studies use synthetic matrices and need to face matrix complexity in terms of both physics and chemistry. Such a complexity provides noise, uncertainty, and difficulty for the clear identification of mechanisms. This study made use of synthetized goethite nanoparticles as the reactant (sorption sites) combined to standardized sand to make a synthetic well-controlled porous medium. The goethite texture was changed during its fabrication to form two types of goethite-sand mixture: goethite-coated sand and goethite-aggregated sand. In the first case, goethite particles deposit at the surface of sand grains (forming a kind of coating), whereas goethite forms aggregates in the second case. The two types of columns were submitted to the injection of a tracer and two solutes: nalidixic acid (NA) and silicate. Our results show that flow remains mostly homogeneous, with the tracer following a straightforward ADE advection Dispersion Equation) process and no water fractionation into mobile and immobile water fractions. The minimal content of goethite (in the order of a few percent) does not change flow pathways. In contrast, the reactive transfer of NA and silicate is significantly impacted with less sorption, and much more solute spread in goethite-aggregated columns. NA and silicate cannot reach sites inside aggregates, reducing and slowing down their adsorption. In other words, changing the deposition mode of goethite nanoparticles on sand did not impact most of the flow and non-reactive transfer. It however greatly impacted reactive transfer. In addition, our results show that even if tracer experiments are performed for columns and attest of homogeneous flow, great care must be taken for reactive solutes. Tracers may not be the right tool to provide a clear picture of local hydraulic conditions at the vicinity of sorption sites, which are of utter importance for understanding reactive solute transfer.</p>

Strong hydroclimatic controls on vulnerability to subsurface nitrate contamination across Europe

Subsurface contamination due to excessive nutrient surpluses is a persistent and widespread problem in agricultural areas across Europe. The vulnerability of a particular location to pollution from reactive solutes, such as nitrate, is determined by the interplay between hydrologic transport and biogeochemical transformations. Current studies on the controls of subsurface vulnerability do not consider the transient behaviour of transport dynamics in the root zone. Here, using state-of-the-art hydrologic simulations driven by observed hydroclimatic forcing, we demonstrate the strong spatiotemporal heterogeneity of hydrologic transport dynamics and reveal that these dynamics are primarily controlled by the hydroclimatic gradient of the aridity index across Europe. Contrasting the space-time dynamics of transport times with reactive timescales of denitrification in soil indicate that ~75% of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year. We find that neglecting the transient nature of transport and reaction timescale results in a great underestimation of the extent of vulnerable regions by almost 50%. Therefore, future vulnerability and risk assessment studies must account for the transient behaviour of transport and biogeochemical transformation processes.

Soil macropore-matrix mass exchange tracer experiments that account for sorption at macropore walls

<p>The transfer of water and solutes between soil matrix and macropores controls preferential flow. Mass transfer depends on soil structural geometry and on properties of biopore walls and crack coatings that can differ from those of the matrix with respect to texture, organic matter, bulk density, and porosity. Agrochemicals and other solutes can react during transport along macropores, which has yet not been well-considered. The objective of this study was to study the specific effects of sorption on the reduction of mass exchange due to the effects of sorption at the macropore-matrix interface. Field and lab percolation experiments under unsaturated flow conditions were carried out with intact soil columns to simulate movement of bromide as a conservative and Brilliant Blue, iodide, and Na-Fluorescein as a reactive tracer. Sorption properties were determined separately for the biopore walls and crack coatings. The results suggest that preferential transport of reactive solutes depends even more strongly on the geometry and properties at flow paths surface than conservative solutes. If these properties can be determined, mass transfer coefficients in two-domain models can be related to soil structure and management.</p>

Solute Transport and Transformation in an Intermittent, Headwater Mountain Stream with Diurnal Discharge Fluctuations

Time-variable discharge is known to control both transport and transformation of solutes in the river corridor. Still, few studies consider the interactions of transport and transformation together. Here, we consider how diurnal discharge fluctuations in an intermittent, headwater stream control reach-scale solute transport and transformation as measured with conservative and reactive tracers during a period of no precipitation. One common conceptual model is that extended contact times with hyporheic zones during low discharge conditions allows for increased transformation of reactive solutes. Instead, we found tracer timescales within the reach were related to discharge, described by a single discharge-variable StorAge Selection function. We found that Resazurin to Resorufin (Raz-to-Rru) transformation is static in time, and apparent differences in reactive tracer were due to interactions with different ages of storage, not with time-variable reactivity. Overall we found reactivity was highest in youngest storage locations, with minimal Raz-to-Rru conversion in waters older than about 20 h of storage in our study reach. Therefore, not all storage in the study reach has the same potential biogeochemical function and increasing residence time of solute storage does not necessarily increase reaction potential of that solute, contrary to prevailing expectations.

Effect of Species Invasion on Transport of Solutes at Different Levels of Soft Sediment Macrofauna Diversity: Results from an Experimental Approach

Different irrigation or ventilation strategies by macrofauna may provide a competitive advantage to tolerant species invading impacted benthic systems and alter benthic-pelagic coupling. To comparatively analyze the effects of an exotic and a native polychaete burrower on sediment-water exchanges, two laboratory experiments were performed. In the first experiment, the invasive spionid polychaete Marenzelleria neglecta was added to defaunated sediments and fluxes of the inert tracer (bromide, Br−) were measured to quantify the effects of irrigation by the worm on the tracer transport. In the second experiment, M. neglecta or the native polychaete Hediste diversicolor were introduced to a relatively diverse Baltic soft-bottom macrofauna community. The effect of species on fluxes of reactive solutes (ammonium, NH4+, and phosphate, PO43−) and transport rates of Br− was estimated. The results indicate different invasion effects depending on the characteristics of the recipient habitat. In defaunated sediments, a single specimen of M. neglecta significantly enhanced originally low solute exchange rates. Total tracer flux was significantly enhanced over diffusive flux by a factor of 1.6 ± 0.14 (n = 3). In natural sediments, on the other hand, the addition of either M. neglecta or H. diversicolor had no statistically significant effects on benthic fluxes. Tracer flux estimates between control and treatment incubations differed by less than 10% on average, and both reactive solutes tended to increase by 10 to 40% after additions. One specimen of M. neglecta in cores with defaunated sediment generated approximately 20% of the tracer flux produced by the relatively diverse macrofauna community. Estimated net tracer fluxes in two experiments corresponded well with the number of adult polychaetes found in sediments (r2 = 0.73, p = 0.005, n = 12). The invasive M. neglecta produced a small effect on fluxes in biodiverse sediments, comparable to those of H. diversicolor, but it may deeply alter porewater chemistry in azoic sediment. As M. neglecta tolerates chemically reduced and sulphidic conditions, its bioirigation may favor sediment reoxidation and ultimately the recolonization by less tolerant, native species.