Carbon limitation may override fine-sediment induced alterations of hyporheic nitrogen and phosphorus dynamics.

The hyporheic zone underneath stream channels is considered a biogeochemical hotspot reducing nutrient loads being transported downstream due to its high surface-to-volume ratio in combination with the hyporheic exchange. However, the effect of environmental stressors such as high amounts of fine sediment (FS; grain size <0.2 mm) on nutrient cycling in the hyporheic zone are not well understood. Physical clogging caused by fine sediment (FS) decreases the hyporheic exchange, thus, diminishing its potential to reduce nutrient loads despite increasing its surface-to-volume ratio. We determined the effect of physical clogging on nutrient cycling based on net change rates of dissolved inorganic nitrogen (DIN; nitrate-N, ammonium-N), soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC) for a sand and gravel hyporheic zone. We performed three experimental runs in 12 flumes with four-week duration each following a factorial design. First, we determined nutrient cycling in sand and gravel in absence of clogging, and then tested the clogging effect for each sediment type under increasing clogging (0-480 g of FS addition increasing by 60 g per level). Without clogging, gravel acted as a source of nitrate-N; and both sand and gravel released SRP. Regardless of the clogging level and the resulting reduced hyporheic exchange, we found no changes in DOC and nitrate-N dynamics but net-release of ammonium-N and SRP for gravel. In contrast, in sand, physical clogging inhibited DOC release for flumes with the higher FS. We propose that not physical clogging but DOC availability limited the nutrient uptake, as molar ratios of DOC to DIN and SRP ranged 1.2-1.5 and 77-191, respectively, indicating severe C limitation of N-uptake and partial C limitation of P-uptake. Our results suggest an interplay between nutrient molar ratios and physical clogging, which emphasize the interactions between hydrology and the stoichiometry of organic carbon, nitrogen and phosphorus in the hyporheic zone.

Management Options to Reduce Phosphorus Leaching from Vegetated Buffer Strips.

Vegetated buffer strips (VBS) between agricultural areas and surface waters are important retention areas for eroded particulate P through which they may obtain critically high degrees of P saturation imposing high risk of soluble P leaching. We tested topsoil removal and three harvesting frequencies (once, twice, or four times per year) of natural buffer vegetation to reduce P leaching with the aim to offset erosional P accumulation and high degrees of P saturation. We used a simple numerical time-step model to estimate changes in VBS soil P levels with and without harvest. Harvesting offset erosional deposition as it resulted in an annual ammonium oxalate-extractable P reduction of 0.3 to 2.8% (25-cm topsoil content) in soils of the VBS and thus, with time, reduced potential P leaching below a baseline of 50 µg L. Topsoil removal only marginally reduced potential leaching at two sites and not anywhere near this baseline. The harvest frequency only marginally affected the annual P removal, making single annual harvests the most economical. We estimate 50 to 300 yr to reach the P leaching baseline, due to substantial amounts of P accumulated in the soils. Even in high-erosion-risk situations in our study, harvesting reduced soil P content and the P leaching risk. We suggest harvesting as a practical and efficient management to combat P leaching from agricultural VBS, not just for short-term reductions of dissolved P, but also for reductions of the total soil P pool and for possible multiple benefits for VBS.

An Assessment of the Multifunctionality of Integrated Buffer Zones in Northwestern Europe.

Integrated buffer zones (IBZs) have recently been introduced in the Northwestern Europe temperate zone to improve delivery of ecosystem services compared with the services associated with long-established vegetated buffer zones. A common feature of all the studied IBZ sites is that tile drainage, which previously discharged directly into the streams, is now intercepted within the IBZ. Specifically, the design of IBZs combines a pond, where soil particles present in drain water or surface runoff can be deposited, and a planted subsurface flow infiltration zone. Together, these two components should provide an optimum environment for microbial processes and plant uptake of nutrients. Nutrient reduction capacities, biodiversity enhancement, and biomass production functions were assessed with different emphasis across 11 IBZ sites located in Denmark, Great Britain, and Sweden. Despite the small size of the buffer zones (250-800 m) and thus the small proportion of the drained catchment (mostly <1%), these studies cumulatively suggest that IBZs are effective enhancements to traditional buffer zones, as they (i) reduce total N and P loads to small streams and rivers, (ii) act as valuable improved habitats for aquatic and amphibian species, and (iii) offer economic benefits by producing fast-growing wetland plant biomass. Based on our assessment of the pilot sites, guidance is provided on the implementation and management of IBZs within agricultural landscapes.

Structural and functional characteristics of buffer strip vegetation in an agricultural landscape - high potential for nutrient removal but low potential for plant biodiversity.

Vegetated buffer strips constitute a transition zone between terrestrial and aquatic ecosystems and provide several ecosystem services. Buffer strips are often applied as a mitigation measure against diffuse pollution in agricultural areas, primarily because they may retain nutrients and in this way help protect the aquatic environment. Additionally, they can improve biodiversity in an otherwise homogenous landscape and may therefore have a value in their own right. In the present study, we characterized the structural and functional features of the vegetation in Danish buffer strips using a nationwide dataset to explore: i) their floristic quality in terms of species diversity and conservation value and ii) based on their functional characteristics, their potential to retain nutrients. Moreover, we analyzed how the structural and functional characteristics varied along gradients in the environmental features of the catchment. We found that the floristic quality of the buffer vegetation was generally low, exhibiting an average of only 3.3% of the number of species of conservation interest. Instead, Danish buffer strips were dominated by widespread and productive species that are tolerant of anthropogenic impacts in the catchment. The abundance of highly productive plant species was positively related to high intensity land use, whereas the abundance of stress-tolerant plant species was positively related to low intensity land use. The high productivity of the buffer strips implies a large bio-storage potential, and these areas might therefore offer an opportunity to remove nutrients by harvesting the plant biomass. We discuss how Danish buffer strips could be exploited via appropriate management (e.g. harvesting) to maximize nutrient retention and at the same time improve floristic quality.

Benthic invertebrate density, biomass, and instantaneous secondary production along a fifth-order human-impacted tropical river.

The aim of this study was to assess land use effects on the density, biomass, and instantaneous secondary production (IP) of benthic invertebrates in a fifth-order tropical river. Invertebrates were sampled at 11 stations along the Rio das Mortes (upper Rio Grande, Southeast Brazil) in the dry and the rainy season 2010/2011. Invertebrates were counted, determined, and measured to estimate their density, biomass, and IP. Water chemical characteristics, sediment heterogeneity, and habitat structural integrity were assessed in parallel. Total invertebrate density, biomass, and IP were higher in the dry season than those in the rainy season, but did not differ significantly among sampling stations along the river. However, taxon-specific density, biomass, and IP differed similarly among sampling stations along the river and between seasons, suggesting that these metrics had the same bioindication potential. Variability in density, biomass, and IP was mainly explained by seasonality and the percentage of sandy sediment in the riverbed, and not directly by urban or agricultural land use. Our results suggest that the consistently high degradation status of the river, observed from its headwaters to mouth, weakened the response of the invertebrate community to specific land use impacts, so that only local habitat characteristics and seasonality exerted effects.