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Dissolved organic matter (DOM) helps regulate aquatic ecosystem structure and function. In small streams, DOM concentrations are controlled by transport of terrestrial materials to waterways, and are thus highly variable. As rivers become larger, the River Continuum Concept hypothesizes that internal primary production is an increasingly important DOM source, but direct evidence is limited. Recently, the Pulse-Shunt Concept postulated that terrestrial DOM concentrations in larger rivers increase with flow and temperature, which seemingly contradicts previously reported DOM chemostasis in large rivers. This study estimates daily gross primary production (GPP) in 13 streams and rivers across the Connecticut River watershed (watershed areas 0.4–25,019 km2) from 2015 through 2017. Chemostasis of DOM concentrations is maintained by a switch from autochthonous sources of DOM at low flows to terrestrial sources of DOM at high flows in a large temperate river and to a lesser degree in smaller tributaries. At low flow, autochthonous DOM linked to aquatic GPP is the dominant fraction of the DOM pool in large rivers. This autochthonous DOM maintains chemostasis in the main stem and to a lesser extent upstream. Thus, in larger rivers, low-flow autochthonous production stabilizes DOM concentrations during the summer, a critical time for riverine ecology. Consistent with the Pulse-Shunt Concept, terrigenous DOM is the dominant fraction of DOM during higher flow periods and about 70% of annual DOM fluxes to the coast are terrestrial. This pattern of DOM switching is potentially widespread in temperate watersheds with implications to both inland waters and coastal ecosystems.

Building closures related to the coronavirus disease (COVID-19) pandemic resulted in increased water stagnation in commercial building plumbing systems that heightened concerns related to the microbiological safety of drinking water post re-opening. The exact impact of extended periods of reduced water demand on water quality is currently unknown due to the unprecedented nature of widespread building closures. We analyzed 420 tap water samples over a period of six months, starting the month of phased reopening (i.e., June 2020), from sites at three commercial buildings that were subjected to reduced capacity due to COVID-19 social distancing policies and four occupied residential households. Direct and derived flow cytometric measures along with water chemistry characterization were used to evaluate changes in plumbing-associated microbial communities with extended periods of altered water demand. Our results indicate that prolonged building closures impacted microbial communities in commercial buildings as indicated by increases in microbial cell counts, encompassing greater proportion cells with high nucleic acids. While flushing reduced cell counts and increased disinfection residuals, the microbial community composition in commercial buildings were still distinct from those at residential households. Nonetheless, increased water demand post-reopening enhanced systematic recovery over a period of months, as microbial community fingerprints in commercial buildings converged with those in residential households. Overall, our findings suggest that sustained and gradual increases in water demand may play a more important role in the recovery of building plumbing-associated microbial communities as compared to short-term flushing, after extended periods of altered water demand that result in reduced flow volumes.

Dissolved organic carbon (DOC) impacts water quality, the carbon cycle, and the ecology of aquatic systems. Understanding what controls DOC is therefore critical for improving large-scale models and best management practices for aquatic ecosystems. The two main processes of DOC transformation and removal, photochemical and microbial DOC degradation, work in tandem to modify and remineralize DOC within natural waters. Here, we examined both the photo- and microbial remineralization of DOC (photolability and biolability), and the indirect phototransformation of DOC into biolabile DOC (photoprimed biolability) for samples that capture the spatiotemporal and hydrological gradients of the Connecticut River watershed. The majority of DOC exported from this temperate watershed was photolabile and the concentration of photolabile DOC correlated with UV absorbance at 254 nm (r2 = 0.86). Phototransformation of DOC also increased biolability, and the total photolabile DOC (sum of photolabile and photoprimed biolabile DOC) showed a stronger correlation with UV absorbance at 254 nm (r2 = 0.92). We estimate that as much as 49% (SD = 3.3%) and 10% (SD = 1.1%) of annual DOC export from the Connecticut River is directly photolabile and photoprimable, respectively. Thus, 2.82 Gg C year−1 (SD = 0.67 Gg C year−1) or 1.13 Mg C km−2 year−1 (SD = 0.27 km−2 year−1) of total photolabile DOC escapes photochemical degradation within the river network to be exported from the Connecticut River each year.

Although many climate predictions suggest that the frequency and intensity of large storm events might increase in the coming decades, few studies document the full impact of such events along their path. Here, we synthesize information on the impact of Hurricane Irene (formed August 21 2011) and Tropical Storm Lee (formed August 30, 2011) on erosion and sediment transport, lake metabolism, riparian hydrology and biogeochemistry, and stream water quality, from North Carolina to Maine. In almost all cases, these storms generated unprecedented changes in water quality (concentrations, loads), from tenfold increases in DOC and 100-fold increases in POC in Maryland, to 100-fold increases in TSS concentrations in Pennsylvania. Overbank flooding and up to 200-year streamflow events were recorded in New York and Vermont. In many cases, particulate loads (e.g. POC, PP, TSS) occurring during Irene and Lee represented more than 30% of the annual load. The dominance of particulate exports over solutes during Irene and Lee is consistent with the mobilization of normally immobile sediment pools, and massive erosion as reported at many locations across the Northeastern US. Several studies reported long lasting (> 1 year) effects of Irene and Lee on cyanobacterial blooms, erosion, or stream suspended sediment concentrations. However, this review also highlighted the lack of a consistent strategy in terms of methods, and measured water quality parameters. This strongly hinders our ability to fully assess the large-scale impact of such events on our environment, and ultimately their impact on our economy and society.

Understanding the role humans play in modifying ecosystems through changing land cover is central to addressing our current and emerging environmental challenges. In particular, the consequences of urban growth and land cover change on terrestrial carbon budgets is a growing issue for our rapidly urbanizing planet. Using the lowland Seattle Statistical Metropolitan Area (MSA) region as a case study, this paper explores the consequences of the past land cover changes on vegetative carbon stocks with a combination of direct field measurements and a time series of remote sensing data. Between 1986 and 2007, the amount of urban land cover within the lowland Seattle MSA more than doubled, from 1316 km 2 to 2798 km 2 , respectively. Virtually all of the urban expansion was at the expense of forests with the forested area declining from 4472 km 2 in 1986 to 2878 km 2 in 2007. The annual mean rate of urban land cover expansion was 1 ± 0.6% year −1 . We estimate that the impact of these regional land cover changes on aboveground carbon stocks was an average loss of 1.2 Mg C ha −1 yr −1 in vegetative carbon stocks. These carbon losses from urban expansion correspond to nearly 15% of the lowland regional fossil fuel emissions making it an important, albeit typically overlooked, term in regional carbon emissions budgets. As we plan for future urban growth and strive for more ecologically sustainable cities, it is critical that we understand the past patterns and consequences of urban development to inform future land development and conservation strategies.

Most of our global population and its CO2 emissions can be attributed to urban areas. The process of urbanization changes terrestrial carbon stocks and fluxes, which, in turn, impact ecosystem functions and atmospheric CO2 concentrations. Using the Seattle, WA, region as a case study, this paper explores the relationships between aboveground carbon stocks and land cover within an urbanizing area. The major objectives were to estimate aboveground live and dead terrestrial carbon stocks across multiple land cover classes and quantify the relationships between urban cover and vegetation across a gradient of urbanization. We established 154 sample plots in the Seattle region to assess carbon stocks as a function of distance from the urban core and land cover [urban (heavy, medium, and low), mixed forest, and conifer forest land covers]. The mean (and 95% CI) aboveground live biomass for the region was 89 � 22MgCha � 1 with an additional 11.8 � 4MgCha � 1 of coarse woody debris biomass. The average live biomass stored within forested and urban land covers was 140 � 40 and 18 � 14MgCha � 1 , respectively, with a 57% mean vegetated canopy cover regionally. Both the total carbon stocks and mean vegetated canopy cover were surprisingly high, even within the heavily urbanized areas, well exceeding observations within other urbanizing areas and the average US forested carbon stocks. As urban land covers and populations continue to rapidly increase across the globe, these results highlight the importance of considering vegetation in urbanizing areas within the terrestrial carbon cycle.

[1] We incorporate high-resolution time-series data to calculate the total amount of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) transported during Hurricane Irene in Esopus Creek in New York (August 2011). During this 200-yr event the Esopus Creek experienced a 330-fold discharge increase and a 4-fold increase in concentration, resulting in the export of roughly 43% and 31% of its average annual DOC and DON fluxes, respectively, in just 5 days. The source of this large dissolved organic matter (DOM) flux also shifted during its course and showed an increased contribution of aromatic organic matter. We conclude that more frequent large events due to climate change will increase the export of terrigenous dissolved organic matter, and potentially impact the water quality and biogeochemistry of lakes and coastal systems. In addition, we show that the use of conventional models for extreme events lead to erroneous flux calculations, unless supported by high resolution data collected during the events.