Stream Mercury Export in Response to Contemporary Timber Harvesting Methods (Pacific Coastal Mountains, Oregon, USA).

Land-use activities can alter hydrological and biogeochemical processes that can affect the fate, transformation, and transport of mercury (Hg). Previous studies in boreal forests have shown that forestry operations can have profound but variable effects on Hg export and methylmercury (MeHg) formation. The Pacific Northwest is an important timber producing region that receives large atmospheric Hg loads, but the impact of forest harvesting on Hg mobilization has not been directly studied and was the focus of our investigation. Stream discharge was measured continuously, and Hg and MeHg concentrations were measured monthly for 1.5 years following logging in three paired harvested and unharvested (control) catchments. There was no significant difference in particulate-bound Hg concentrations or loads in the harvested and unharvested catchments which may have resulted from forestry practices aimed at minimizing erosion. However, the harvested catchments had significantly higher discharge (32%), filtered Hg concentrations (28%), filtered Hg loads (80%), and dissolved organic carbon (DOC) loads (40%) compared to forested catchments. MeHg concentrations were low (mostly <0.05 ng L-1) in harvested, unharvested, and downstream samples due to well-drained/unsaturated soil conditions and steep slopes with high energy eroding stream channels that were not conducive to the development of anoxic conditions that support methylation. These results have important implications for the role forestry operations have in affecting catchment retention and export of Hg pollution.

Development and validation of an aquatic Fine Sediment Biotic Index.

The Fine Sediment Biotic Index (FSBI) is a regional, stressor-specific biomonitoring index to assess fine sediment (<2 mm) impacts on macroinvertebrate communities in northwestern US streams. We examined previously collected data of benthic macroinvertebrate assemblages and substrate particle sizes for 1,139 streams spanning 16 western US Level III Ecoregions to determine macroinvertebrate sensitivity (mostly at species level) to fine sediment. We developed FSBI for four ecoregion groupings that include nine of the ecoregions. The grouping were: the Coast (Coast Range ecoregion) (136 streams), Northern Mountains (Cascades, N. Rockies, ID Batholith ecoregions) (428 streams), Rockies (Middle Rockies, Southern Rockies ecoregions) (199 streams), and Basin and Plains (Columbia Plateau, Snake River Basin, Northern Basin and Range ecoregions) (262 streams). We excluded rare taxa and taxa identified at coarse taxonomic levels, including Chironomidae. This reduced the 685 taxa from all data sets to 206. Of these 93 exhibited some sensitivity to fine sediment which we classified into four categories: extremely, very, moderately, and slightly sensitive; containing 11, 22, 30, and 30 taxa, respectively. Categories were weighted and a FSBI score calculated by summing the sensitive taxa found in a stream. There were no orders or families that were solely sensitive or resistant to fine sediment. Although, among the three orders commonly regarded as indicators of high water quality, the Plecoptera (5), Trichoptera (3), and Ephemeroptera (2) contained all but one of the species or species groups classified as extremely sensitive. Index validation with an independent data set of 255 streams found FSBI scores to accurately predict both high and low levels of measured fine sediment.

Timber harvest alters mercury bioaccumulation and food web structure in headwater streams.

Timber harvest has many effects on aquatic ecosystems, including changes in hydrological, biogeochemical, and ecological processes that can influence mercury (Hg) cycling. Although timber harvest's influence on aqueous Hg transformation and transport are well studied, the effects on Hg bioaccumulation are not. We evaluated Hg bioaccumulation, biomagnification, and food web structure in 10 paired catchments that were either clear-cut in their entirety, clear-cut except for an 8-m wide riparian buffer, or left unharvested. Average mercury concentrations in aquatic biota from clear-cut catchments were 50% higher than in reference catchments and 165% higher than in catchments with a riparian buffer. Mercury concentrations in aquatic invertebrates and salamanders were not correlated with aqueous THg or MeHg concentrations, but rather treatment effects appeared to correspond with differences in the utilization of terrestrial and aquatic basal resources in the stream food webs. Carbon and nitrogen isotope data suggest that a diminished shredder niche in the clear-cut catchments contributed to lower basal resource diversity compared with the reference of buffered treatments, and that elevated Hg concentrations in the clear-cut catchments reflect an increased reliance on aquatic resources in clear-cut catchments. In contrast, catchments with riparian buffers had higher basal resource diversity than the reference catchments, indicative of more balanced utilization of terrestrial and aquatic resources. Further, following timber harvest THg concentrations in riparian songbirds were elevated, suggesting an influence of timber harvest on Hg export to riparian food webs. These data, coupled with comparisons of individual feeding guilds, indicate that changes in organic matter sources and associated effects on stream food web structure are important mechanisms by which timber harvest modifies Hg bioaccumulation in headwater streams and riparian consumers.

Potential risks to freshwater aquatic organisms following a silvicultural application of herbicides in Oregon's Coast Range.

Glyphosate, aminomethylphosphonic acid (AMPA), imazapyr, sulfometuron methyl (SMM), and metsulfuron methyl (MSM) were measured in streamwater collected during and after a routine application of herbicides to a forestry site in Oregon's Coast Range. Samples were collected at 3 stations: HIGH at the fish-no-fish interface in the middle of the harvest and spray unit, MID at the bottom of the unit, and LOW downstream of the unit. All herbicides were applied by helicopter in a single tank mix. AMPA, imazapyr, SMM, and MSM were not detected (ND) in any sample at 15, 600, 500, and 1000 ng/L, respectively. A pulse of glyphosate peaking at approximately equal to 62 ng/L manifested at HIGH during the application. Glyphosate pulses peaking at 115 ng/L (MID) and 42 ng/L (HIGH) were found during the first 2 postapplication storm events 8 and 10 days after treatment (DAT), respectively: glyphosate was less than 20 ng/L (ND) at all stations during all subsequent storm events. All glyphosate pulses were short-lived (4-12 h). Glyphosate in baseflow was approximately equal to 25 ng/L at all stations 3 DAT and was still approximately equal to 25 ng/L at HIGH, but ND at the other stations, 8 DAT: subsequently, glyphosate was ND in baseflow at all stations. Aquatic organisms were subjected to multiple short-duration, low-concentration glyphosate pulses corresponding to a cumulative time-weighted average (TWA) exposure of 6634 ng/L × h. Comparisons to TWA exposures associated with a range of toxicological endpoints for sensitive aquatic organisms suggests a margin of safety exceeding 100 at the experimental site, with the only potential exception resulting from the ability of fish to detect glyphosate via olfaction. For imazapyr, SMM, and MSM the NDs were at concentrations low enough to rule out effects on all organisms other than aquatic plants, and the low concentration and (assumed) pulsed nature of any exposure should mitigate this potential. Integr Environ Assess Manag 2017;13:396-409. © 2016 SETAC.

Nitrogen uptake and turnover in riparian woody vegetation.

The nutrient balance of streams and adjacent riparian ecosystems may be modified by the elimination of anadromous fish runs and perhaps by forest fertilization. To better understand nitrogen (N) dynamics within stream and riparian ecosystems we fertilized two streams and their adjacent riparian corridors in central Idaho. On each stream two nitrogen doses were applied to a swathe approximately 35 m wide centered on the stream. The fertilizer N was enriched in 15N to 18 per thousand. This enrichment is light relative to many previous labeling studies, yet sufficient to yield a traceable signal in riparian and stream biota. This paper reports pre-treatment differences in delta15N and the first-year N response to fertilizer within the riparian woody plant community. Future papers will describe the transfer of allochthonous litter N to the stream and its subsequent processing by stream biota. Pre-treatment delta15N differed between the two creeks (P=0.0002), possibly due to residual salmon nitrogen in one of the creeks. Pre-treatment delta15N of current-year needles was enriched compared to leaf litter, which was in turn enriched compared to needles aged 4 years and older. We conclude that fractionation due to retranslocation occurs in at least two phases. The first phase, which optimizes allocation of N in younger needle age classes, is distinctly different from the second, which conserves N prior to abscission. The delta15N difference between creeks was eliminated by the fertilization (P=0.42). In the two dominant conifer species, Abies lasiocarpa and Picea engelmannii, most fertilizer N was found in the current-year foliage; little was found in older needles and none was detected in litter (P=0.53). The only N-fixing shrub species, Alnus incana, took up only a small amount of fertilizer N [mean percent N derived from fertilizer (%Ndff) 5.0+/-1.6% (SE)]. Far more fertilizer N was taken up by other deciduous shrubs (mean %Ndff=33.9+/-4.5%). Fertilizer N made up 25% (+/-4.2%) of the N in deciduous shrub litter. These results demonstrate the feasibility of light labeling with 15N and the potential influence of riparian plant species composition on stream nutrient dynamics via allochthonous leaf litter inputs.