Geology topics

L. G. Larsen

Publications and source records attributed to L. G. Larsen.

3 recordsLinked to original sources

USGS2018

Less fine particle retention in a restored versus unrestored urban stream: Balance between hyporheic exchange, resuspension, and immobilization

Stream restoration goals include reducing erosion and increasing hyporheic exchange to promote biogeochemical processing and improve water quality. Little is known, however, about fine particle dynamics in response to stream restoration. Fine particles (<100 μm) are exchanged with transient storage areas near and within streambeds and banks. Fine particle retention directly impacts carbon and nutrient cycling by supporting benthic and hyporheic primary production, but overaccumulation of fine particle deposits can impair metabolism by burying benthic biofilms and reducing streambed permeability. We analyzed the transport and retention of water and fine particles at both the reach and local scales in a restored urban stream, 9 years postrestoration. We injected conservative solute and fine particle tracers under summer baseflow conditions and monitored their distribution between surface water, porewaters, and storage areas (i.e., biofilms, hyporheic zones, and slow surface waters). Comparison of the results to a nearby unrestored stream demonstrate that the restored reach had 10–45 times greater exchange of fine particles with transient storage zones, but 5 times lower rate of net particle immobilization. Local-scale results showed that restoration increased fine particle exchange with short-term storage areas but did not increase long-term particle retention. Thus, the restored stream rapidly exchanged fine sediments with transient storage areas, but did not store fine sediments as efficiently as the unrestored stream. The decreased retention of particulate organic matter in the restored stream may reduce biogeochemical processes, such as denitrification, by not providing sufficient organic carbon or the surface area required for microbial colonization.

JGR Biogeosciences↗

USGS2017

Fine particle retention within stream storage areas at base flow and in response to a storm event

Fine particles (1–100 µm), including particulate organic carbon (POC) and fine sediment, influence stream ecological functioning because they may contain or have a high affinity to sorb nitrogen and phosphorus. These particles are immobilized within stream storage areas, especially hyporheic sediments and benthic biofilms. However, fine particles are also known to remobilize under all flow conditions. This combination of downstream transport and transient retention, influenced by stream geomorphology, controls the distribution of residence times over which fine particles influence stream ecosystems. The main objective of this study was to quantify immobilization and remobilization rates of fine particles in a third-order sand-and-gravel bed stream (Difficult Run, Virginia, USA) within different geomorphic units of the stream (i.e., pool, lateral cavity, and thalweg). During our field injection experiment, a thunderstorm-driven spate allowed us to observe fine particle dynamics during both base flow and in response to increased flow. Solute and fine particles were measured within stream surface waters, pore waters, sediment cores, and biofilms on cobbles. Measurements were taken at four different subsurface locations with varying geomorphology and at multiple depths. Approximately 68% of injected fine particles were retained during base flow until the onset of the spate. Retention was evident even after the spate, with 15.4% of the fine particles deposited during base flow still retained within benthic biofilms on cobbles and 14.9% within hyporheic sediment after the spate. Thus, through the combination of short-term remobilization and long-term retention, fine particles can serve as sources of carbon and nutrients to downstream ecosystems over a range of time scales.

Virginia↗

USGS2017

Disrupted carbon cycling in restored and unrestored urban streams: Critical timescales and controls

Carbon fixation and respiration in flowing waterways play significant roles in global and regional carbon budgets, yet how land use and watershed management interact with temporal disturbances (storms) to influence metabolism remains poorly understood. Here, we combine long-term with synoptic sampling of metabolism and its variable controls in neighboring watersheds of the Chesapeake Bay to resolve limiting factors and critical timescales associated with recovery from disturbance. We found that, relative to predictions of the river continuum concept, focal streams have “disrupted” carbon cycles, with carbon balances closer to zero, and, in some cases, tighter coupling between gross primary production (GPP) and ecosystem respiration (ER), attributable to carbon limitation. Carbon became limiting to ER where flashy storm hydrographs and simplified channel geomorphology inhibited accumulation of fine sediment. Shannon entropy analysis of timescales revealed that fine sediment served as a time-release capsule for nutrients and carbon over 4–6 months, fueling biogeochemical transformations. Loss of fines through hydraulic disturbance had up to 30-d impacts on GPP and 50-d impacts on ER in the stream with carbon limitation. In contrast, where GPP and ER were not tightly coupled, recovery occurred within 1 d. Results suggest that a complex interplay between nutrient and carbon limitation and mechanical and chemical disturbance governs patterns and consequences of disrupted carbon cycling in urban streams. Carbon limitation and tight GPP/ER coupling enhance the vulnerability of stream ecosystem functions, but best management practices that target stormflow reduction and channel geomorphic diversity can break that coupling and minimize carbon cycle disruptions.

Limnology and Oceanography↗