Systematic microstructure variability in double-diffusively stable coastal waters of nonuniform density gradient

Author Posting. © American Geophysical Union, 2002. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 107, C10 (2002): 3144, doi:10.1029/2001JC000844.Conductivity microstructure, water velocity, and stratification were measured during a tow-yo transect near the New England shelf/slope front in early August 1997. Velocity data were collected with an acoustic Doppler profiler on the ship. The other data were collected with a towed platform. Estimates of χ, the rate of dissipation of temperature variance, were computed from the conductivity data with vertical resolution of 0.3 m. Relationships between χ and shear, temperature gradient, buoyancy frequency (N), and gradient Richardson number (Ri) were explored, with special focus on measurements taken in waters stable to double-diffusive processes (to avoid ambiguity of interpretation) and exhibiting variable density gradient (N ranging from 5 to 40 cph). For this subset of data, χ computed from data grouped into classes of local mean temperature gradient (inline equation/dz) was proportional to inline equation/dz to the 0.7 power, which is consistent with diapycnal thermal eddy diffusivity K being proportional to (inline equation/dz)−1.3 within the framework of the Osborn-Cox model that relates χ and the mean temperature gradient to the heat flux. No correlation between K and Ri was observed, with Ri computed at 4 m vertical scale, so that systematic inhomogeneous large-scale forcing is not responsible for a false correlation of K and inline equation/dz. Water mass salinity characteristics in the area caused N2 to be proportional to (inline equation/dz)4/5, rather than to inline equation/dz as in the isohaline case, giving rise to the steep inverse relation K = 10−10N−3.3, with N in radians/s and diffusivity in m2/s. The fit K = 2 × 10−9N−2.5 results if one questionable data ensemble is disregarded. These relations are comparable to results obtained previously from the near-bottom tow data. They are not intended to be universal formulae but are meant to describe the conditions we encountered. They are not expected to hold at high and low values of N outside of our measurement range. An interpretation is that under these conditions the less strongly stratified (lower N) layers in this shelf area are more prone to instability of the larger-scale shear than the intervening interfaces, with the subsequent greater energy dissipation in the layers leading to higher buoyancy flux KN2 in the layers than in the interfaces.This work was supported by the U.S. Office of Naval Research (Grants N-00014-95-1-0633 and N00014-95-1-1064) and by a WHOI postdoctoral scholarship award to C.R.R

Resuspension of E. coli from Direct Fecal Deposits in Stream

Direct fecal deposits from cattle provide a significant source of E. coli to streams and therefore pose a threat to human health in agricultural watersheds. Experiments were conducted in a flume (9.1 m long, 0.6 m wide, and 0.6 m deep) with flow of 0.0106 m3 s-1 , an average velocity of 11.4 cm s-1 ,and water depth of 15.24 cm to measure the resuspension and deposition of E. coli from an undisturbed standard cowpat. Water samples were collected 1.22 m and 3.66 m downstream of the deposited cowpat, and at each downstream cross-section nine samples were collected to characterize the bacterial movement. E. coli in water samples were separated into the attached and unattached phases by filtration to assess the mechanism of transport. The cumulative load contribution from a single deposited cowpat after one hour was 2.49×10 9 cfu 3.66 m downstream. The composite E. coli concentrations at all sampling points and times exceeded the federal standards for primary contact in the United States of 126 cfu/100 ml. Between 77.2 and 99.5% of all E. coli downstream of the direct deposit were associated with particulates. The resuspension rate was 5.91×107 and 9.52×104 cfu m-2 s-1 0.5 min and 60 minutes after deposition, respectively, 1.22 m downstream of the deposit and 2.19×106 and 3.14×103 cfu m-2 s-1 0.5 min and 60 min after deposition, respectively, 3.66 m downstream of the deposit. Results from this study are useful to improve modeling techniques to predict in-stream E. coli concentrations from direct fecal deposits and emphasize the need to implement management practices to reduce livestock access to streams

Importance of Extracellular DNA in the Fate and Transport of Antibiotic Resistance Genes Downstream of a Wastewater Treatment Plant

A model is developed to account for antibiotic resistance genes (ARGs) as both intracellular DNA (iDNA) and extracellular DNA (eDNA) in predicting the fate and transport of ARGs in receiving waters downstream of wastewater treatment plants (WWTPs). eDNA can contribute to iDNA concentrations of ARGs through horizontal gene transfer (i.e., transformation); however, the prevalence of eDNA and its effects have not been addressed in previous field studies and predictive models. The present model tracks eDNA and iDNA in both the water column and the sediment, and it includes physical, chemical, and biological processes. It also provides a framework for systematically identifying conditions under which accounting for eDNA is important. For example, when the timescale for transformation is small compared with the timescale for advection and the water column concentration of eDNA is large compared with that of iDNA, ignoring eDNA can underpredict the total amount of ARGs significantly (e.g., by an order of magnitude or more). The model demonstrates that the eDNA fraction of ARGs in WWTP discharges is important to include in predictions for improved risk assessment of antibiotic resistance in the aquatic environment