Search

Changes in the amount, intensity, and timing of precipitation are increasing hydrologic variability in many regions, but we have little understanding of how these changes are affecting freshwater species. Stream-breeding amphibians-a diverse group in North America-may be particularly sensitive to hydrologic variability during aquatic larval and metamorphic stages. Here, we tested the prediction that hydrologic variability in streams decreases survival through metamorphosis in the salamander Gyrinophilus porphyriticus, reducing recruitment to the adult stage. Using a 20-y dataset from Merrill Brook, a stream in northern New Hampshire, we show that abundance of G. porphyriticus adults has declined by ~50% since 1999, but there has been no trend in larval abundance. We then tested whether hydrologic variability during summers influences survival through metamorphosis, using capture-mark-recapture data from Merrill Brook (1999 to 2004) and from 4 streams in the Hubbard Brook Experimental Forest (2012 to 2014), also in New Hampshire. At both sites, survival through metamorphosis declined with increasing variability of stream discharge. These results suggest that hydrologic variability reduces the demographic resilience and adaptive capacity of G. porphyriticus populations by decreasing recruitment of breeding adults. They also provide insight on how increasing hydrologic variability is affecting freshwater species, and on the broader effects of environmental variability on species with vulnerable metamorphic stages. [ABSTRACT FROM AUTHOR]

Abstract: Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t 1/2 =53days) and 210Pbex (t 1/2 =22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function. [Copyright &y& Elsevier]

Interest in the coastal dynamics of river plumes has mainly focused on large rivers, but plumes from the more numerous smaller rivers have important local consequences and may, in aggregate, be significant contributors to coastal circulation. We studied the dynamics of the plume from the Saco River in Saco Bay, Gulf of Maine, over a 3-year period. The transport and salinity in the region are governed by river discharge, tides, winds, and interaction with the Western Maine Coastal Current. The dynamics of the flow field vary with location within the plume and discharge. The far-field dynamics of the Saco River plume are dominated by inertial processes (hence qualifying it as a small-scale river plume), during times of low discharge, with low salinity water present both up and downstream of the river mouth, but are dominated by rotational processes during times of high discharge (thus qualifying it as a large-scale river plume), with buoyant water primarily advected downshelf. Near-field dynamics are governed by weak, subcritical flow during low discharge but strongly inertial, supercritical flow during high discharge. Offshore movement of the plume is not governed by Ekman dynamics but is instead a result of discharge, wind-induced vertical mixing, and the geography of the coastline and adjacent islands. [ABSTRACT FROM AUTHOR]

This article comments on T. G. Huntington's hypothesis that immobilization of inorganic nitrogen, in combination with region-wide recovery from past disturbances may explain the decrease in stream water nitrate in recent decades that were observed in 28 White Mountain, New Hampshire streams. An additional hypothesis is proposed that might explain part of the unexpectedly low nitrate concentrations during the mid-1990s at White Mountain watersheds and elsewhere across the Northeast. It is suggested that this is due to increase nitrogen immobilization and/or denitrification driven by increased availability of terrestrial dissolved organic carbon.

Chlorine is a minor constituent of most rocks and a minor (although essential) element in plants, but it cycles rapidly through the hydrosphere and atmosphere. In forest ecosystem studies, chloride ion (Cl−) is often thought to be conservative in the sense that the sources and sinks within the ecosystem are assumed negligible compared to inputs and outputs. As such, Cl− is often used as a conservative tracer to assess sources and transformations of other ions. In this paper we summarize research on chloride over the course of 36 years (1964–2000) at the Hubbard Brook Experimental Forest (HBEF) in central New Hampshire, USA. Evidence presented here suggests that in the 1960s and 1970s the dominant source of atmospheric Cl− deposition was from pollutant sources, probably coal burning. In the 1970s the Cl− inputs in bulk deposition declined, and the lower Cl− deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl− export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs − streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl− within the system, but since about 1980 the NHF has been consistently negative, indicating an unmeasured input or source within the ecosystem. Dry deposition can account for at least part of that unmeasured source, and it appears that release of Cl− from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl− in vegetation during the 1960s and 1970s offset the unmeasured source and resulted in net ecosystem retention. Accumulation of vegetative biomass has ceased since about 1982, leading to the apparent net export (negative NHF) since that time. Although we have no direct measurements of Cl− accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha−1 year−1, or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl− to the forest floor cause increases in Cl− concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl− cycle. Harvesting or devegetation of watersheds causes an increase in the Cl− concentration and flux in stream water for several years after the disturbance. This period of release is followed by a period of reaccumulation of Cl− that may last more than 15 years. In this respect, the behavior of Cl− after disturbance parallels that of NO3−, for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO42−, for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl− export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl− within the system appears to be largely under biological, rather than geochemical, control. [ABSTRACT FROM AUTHOR]

An organic debris dam is an accumulation of organic matter in a stream which obstructs water flow. Debris dams trap sediments in the pool formed upstream from them and the dam structure itself collects particulate organic matter. This study was done at the Hubbard Brook Experimental Forest, in New Hampshire, to examine the relative importance of these structures in retention of sediment and organic matter in a small stream ecosystem. An experimental approach was used in which all organic debris dams were removed from a 175-m section of second-order stream, just above a gauging weir. The material being exported from the watershed was separated into three size categories: dissolved matter (<0.5 μm), fine particulate matter (0.5 μm to 1 mm), and coarse particualte matter (>1 mm). Export of each size fraction was monitored for at least 1 yr prior to dam removal, and for 1 yr following removal. Following dam removal, export of dissolved matter increased slightly due to an increase in the concentration of dissolved organic carbon in the stream water during periods of high discharge. Fine particulate matter export increased dramatically at high discharges following dam removal; concentrations in some instances achieved values five times higher than any observed before dam removal. Coarse particulate matter export also was greatly increase. Calculating dissolved matter and particulate matter export from the watershed, with an without organic debris dams, showed that dam removal brought about a 6% increase in the export of dissolved matter and a 500% increase in the export of both fine particulate and coarse particulate matter. [ABSTRACT FROM AUTHOR]

Avulsions are rare events that cause major realignments to fluvial systems and hold potential for significant societal consequences. We present a case study of an avulsion in a minimally-aggrading landscape that was influenced by long-term human modifications to a fluvial environment that was initially setup for an avulsion during regional Pleistocene deglaciation. The 2006 Suncook River avulsion in New Hampshire, USA, is a valuable example because minimal environmental management took place for a decade following the event, allowing natural processes to dominate initial fluvial responses. Geospatial data collected during this period allow for the evaluation of the short- and long-term morphological evolution of a river that experienced an avulsion. We review data from technical reports on the event and present a new synthesis of aerial imagery and topographic data highlighting key geomorphic features associated with the avulsion. These features include headcutting and widening associated with a 0.6-km long channel created by the avulsion. Downstream of the avulsion site, extensive channel filling, deposition of meter-scale sand sheets on floodplain surfaces, and development of a cutoff channel were major manifestations of the event. The initial setup for the avulsion can be traced to Pleistocene deglaciation, when glaciofluvial ridges deposited in the valley blocked a more direct route for the river and shifted the channel towards an area of shallow bedrock. Human modifications to the channel—including mill dam construction, floodplain narrowing, road development, and gravel mining—contributed to the avulsion setup by increasing superelevation and undermining the glaciofluvial deposits that had directed the river during the Holocene. The avulsion was triggered by a ~50-yr flood event following a 70-yr interval with no major floods but marked by continued human modification to the landscape. The unusual nature of the event combined with the wealth of available geospatial and hydrologic data make the Suncook River system an attractive target for future studies that can examine avulsion processes in greater detail. • Case study of 2006 Suncook River avulsion in New Hampshire, USA • Avulsion setup through combination of regional deglaciation and human impacts • Triggered by ~50-year flood event that followed several decades of low-intensity floods • Avulsion incised floodplain and glaciogenic sediments to create new channel. • Sediment wave caused downstream channel filling and significant overbank deposition. [ABSTRACT FROM AUTHOR]

This article examines whether nitrogen sequestration can explain the unexpected decline in nitrate concentration in New Hampshire's stream. The theory of forest nitrogen biogeochemistry suggests that retention of atmospherically deposited N can occur through vegetation uptake, accumulation in soil organic matter, abiotic immobilization, and accumulation in coarse woody debris including roots and stumps. It is argued that the immobilization of inorganic N in combination with a region-wide recovery from past disturbances may explain the decrease in stream water nitrate in recent decades.

Because the taxonomy of trematodes is based on adults, the larval stages of most digeneans cannot be identified to species based on morphology alone. Molecular data provide a means of linking larval stages to known adults. We obtained sequences from the barcode region of cytochrome oxidase I (CO1) from adult and larval parasites of fish, frogs, birds, and mammals across North America. Sequences from adult Apharyngostrigea cornu, Hysteromorpha triloba, and Alaria mustelae (Diplostomoidea: Digenea) from definitive hosts matched those of meta- and mesocercariae from fish and frogs. These data provided new information on the distributions of all 3 parasite species. Metacercariae of A. cornu, which have not been previously reported in North American hosts, were found in Notemigonus crysoleucas, Pimephales notatus, and Catostomus commersonii in the St. Lawrence River. Metacercariae of H. triloba are reported in Canadian waters and in N. hudsonius for the first time. Alaria mustelae is reported for the first time in frogs from Quebec, Canada, and an additional species of Alaria was detected in California. Sequences of internal transcribed spacer rDNA from a subset of specimens support the same species boundaries indicated by CO1 divergence. There was little divergence in CO1 sequences from an unidentified diplostomid species sampled at a large spatial scale.