Changes In Solute Flux Research Articles

Context-Specific Bioturbation Mediates Changes to Ecosystem Functioning

Species are often grouped according to their biological or functional traits to better understand their contribution to ecosystem functioning. However, it is becoming clear that a single species can perform different roles in different habitats. Austrohelice crassa, a burrow-building mud crab shifts its primary bioturbational role to that of a vertical mixer in non-cohesive sediments as frequent burrow collapse greatly enhances sediment reworking. We conducted in situ crab density manipulations in two sediment environments (a non-cohesive sand and a cohesive muddy-sand) to examine if the context-specific functional roles were linked to changes in solute fluxes across the sediment–water interface. Across both habitats, we show that A. crassa regulated nutrient cycling, creating strong density driven effects on solute exchanges. Increasing crab density increased sediment O2 demand and the flux of NH4 + from the sediment, indicating much of the response was physiologically driven. Clear interactions between A. crassa and microphytobenthos were also detected in both habitats. Despite lowering microphyte standing stock through deposit feeding, A. crassa increased benthic primary production per unit of chlorophyll a. Our experiment also revealed important context-specific differences, most notably for NH4 + fluxes, which were higher where burrows and their associated microbial communities were most stable (muddy-sand). This study highlights the need to integrate interactions between organism behavior and habitat type into functional group studies to broaden conceptual frameworks and avoid oversimplification of highly complex organism–sediment interactions.

Sediment pore‐water dynamics of Little Rock Lake, Wisconsin: Geochemical processes and seasonal and spatial variability

The nature of sediment alkalinity generation processes and the temporal and spatial variability of the pore‐water chemistry of an experimentally acidified seepage lake (Little Rock Lake, Wisconsin) were determined. Analysis of vertical gradients of solutes near the sediment‐water interface indicates that sulfate reduction and base cation production were the major mechanisms of alkalinity generation. A comparison of surficial accumulation rates and burial rates indicates that the major source of cations to the pore water occurred by release of organically bound and exchangeable cations through decomposition. Pore‐water measurements also reveal significant seasonal changes in solute fluxes, including a sudden change in sediment metabolism following a springtime algal bloom. Spatial differences in particle deposition caused pore‐water fluxes of ammonium and alkalinity to be almost an order of magnitude higher at a hypolimnetic site than at epilimnetic sites. After 2 yr of acidification, pore‐water gradients of sulfate, calcium, and alkalinity showed only minor changes, and the pore‐water pH in the acidified basin remained within 0.5 pH units of preacidification pH.

The Combined Effects of Salinity and Root Anoxia on Growth and Net Na+ and K+-accumulation in Zea mays Grown in Solution Culture

The effects of excess salinity and oxygen deficiency on growth and solute relations in Zea mays L. cv. Pioneer 3906 were examined in greenhouse experiments. The roots of plants 14 d old growing in nutrient solution containing additions of NaCl in the range 1.0–200 mol m−3 were either exposed to a severe deficiency of O2 by bubbling with nitrogen gas (N2 treatment), or maintained with a supply of air (controls), for a period of 1–7 d. The threshold NaCl concentration resulting in appreciable inhibition of leaf extension, and shoot f. wt gain in controls was between 10 and 25 mol m−3. At 25 mol m−3 NaCl the ratio of Na+/K+ transported to shoots was about 20 times greater than in plants in 1.0 mol m−3 NaCl. The effect of addition of NaCl to the nutrient solution was to enhance Na+ movement but simultaneously depress the rate of K+ transport to shoots (per g f. wt roots). Interactions between NaCl levels and aeration treatment were shown by analyses of variance to be statistically significant for leaf extension, shoot and root f. wt gains, Na+ and K+ concentrations in shoots and roots. When roots were N2-treated, shoot and root growth were depressed, the effect of aeration treatment being greatest at NaCl concentrations of 50 mol m−3 or less. Additionally, N2-treatment greatly accelerated Na- transport to shoots while depressing K+ transport still further, so that at 10 mol m−3 NaCl the ratio Na+/K+ acquired by the shoots was 230 times greater than in controls. Over the concentration range 1.0 to 50 mol m−3 NaCl, the ratio Na+/K+ transported to shoots by anoxic roots increased by a factor of 860. Mechanisms controlling changes in solute flux to the shoot, and the significance in relation to plant tolerance of excess salts or oxygen deficiency are discussed.

Phloem transport, solute flux and the kinetics of sap exudation in Ricinus communis L.

The osmotic characteristics of phloem-sap exudation were examined in soil-grown and watercultured plants of Ricinus communis L. Prolonged exudation occurred from bark incisions in water-cultured plants. Fresh incisions caused large alterations in solute flux, but phloem-sap solute potential Ψs changed by less than ±8% over a period of 7 h. This was associated with a constancy in the levels of sucrose and K(+), the principal solutes in the sap. Studies with foliar-applied tracers and leaf-excision experiments suggested that exudation was maintained by solute loading from mature leaves. A wide range of mass transfer values through the phloem was found, these being a function of exudation rate. We consider that the exudation process possesses essentially similar characteristics to phloem transport in the intact plant. The way in which bark incisions bring about large changes in solute flux is discussed in terms of the physical properties of the sieve-tube system.