Reconsidering the physics of the Chesapeake Bay estuarine turbidity maximum

A series of cruises was carried out in the estuarine turbidity maximum (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late January shifted the salinity structure of the upper Bay towards that of a salt wedge, but most of the massive sediment load delivered by the Susquehanna River appeared to bypass the ETM zone. In contrast, suspended sediments delivered during a flood event in late October were trapped very efficiently in the ETM. The difference in sediment trapping appeared to be due to increases in particle settling speed from January to October, suggesting that the fate of sediments delivered during large events may depend on the season in which they occur. The ETM roughly tracked the limit of salt (defined as the intersection of the 1 psu isohaline with the bottom) throughout the year, but it was often separated significantly from the limit of salt with the direction of separation unrelated to the phase of the tide. This was due to a lag of ETM sediment resuspension and transport behind rapid meteorologically induced or river flow induced motion of the salt limit. Examination of detailed time series of salt, suspended sediment, and velocity collected near the limit of salt, combined with other indications, led to the conclusion that the convergence of the estuarine circulation at the limit of salt is not the primary mechanism of particle trapping in the Chesapeake Bay ETM. This convergence and its associated salinity structure contribute to strong tidal asymmetries in sediment resuspension and transport that collect and maintain a resuspendable pool of rapidly settling particles near the salt limit. Without tidal resuspension and transport, the ETM would either not exist or be greatly weakened. In spite of this repeated resuspension, sedimentation is the ultimate fate of most terrigenous material delivered to the Chesapeake Bay ETM. Sedimentation rates in the ETM channel are at least an order of magnitude greater than on the adjacent shoals, probably due to focusing mechanisms that are poorly understood.

[1]  K. Dyer,et al.  Fine Sediment Particle Transport in Estuaries , 1988 .

[2]  Hubble,et al.  Dynamics of the turbidity maximum zone in a micro‐tidal estuary: Hawkesbury River, Australia , 1998 .

[3]  Heike Zimmermann-Timm,et al.  Seasonal dynamics of aggregates and their typical biocoenosis in the Elbe Estuary , 1998 .

[4]  D. Lynch,et al.  RECENT SEDIMENTATION RATES IN CHESAPEAKE BAY , 1984 .

[5]  J. Schubel,et al.  Turbidity Maximum of the Northern Chesapeake Bay , 1968, Science.

[6]  W. Rockwell Geyer,et al.  The importance of suppression of turbulence by stratification on the estuarine turbidity maximum , 1993 .

[7]  M. Dearnaley.  Direct measurements of settling velocities in the owen tube: A comparison with gravimetric analysis , 1996 .

[8]  Jeffery D. Musiak,et al.  Particle Trapping in Estuarine Tidal Flows , 1994 .

[9]  J. Schubel.  Tidal variation of the size distribution of suspended sediment at a station in the chesapeake bay turbidity maximum , 1971 .

[10]  E. Baker,et al.  The effect of particle size on the light attenuation coefficient of natural suspensions , 1984 .

[11]  Michael R. Roman,et al.  Temporal and spatial patterns of zooplankton in the Chesapeake Bay turbidity maximum , 2001 .

[12]  H. Burchard,et al.  The Formation of Estuarine Turbidity Maxima Due to Density Effects in the Salt Wedge. A Hydrodynamic Process Study , 1998 .

[13]  P. Hir,et al.  Modelling the Turbidity Maximum in the Seine Estuary (France): Identification of Formation Processes , 1999 .

[14]  A. E. Greenberg,et al.  Standard methods for the examination of water and wastewater : supplement to the sixteenth edition , 1988 .

[15]  Julian J. Dodson,et al.  Abundance of larval rainbow smelt (Osmerus mordax) in relation to the maximum turbidity zone and associated macroplanktonic fauna of the middle St. Lawrence estuary , 1989 .

[16]  R. Uncles,et al.  The freshwater-saltwater interface and its relationship to the turbidity maximum in the Tamar Estuary, United Kingdom , 1993 .

[17]  E. Houde,et al.  Retention of white perch and striped bass larvae: Biological-physical interactions in Chesapeake Bay estuarine turbidity maximum , 2001 .

[18]  R. B. Biggs.  Sources and distribution of suspended sediment in northern Chesapeake Bay , 1970 .

[19]  R. Uncles,et al.  Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries , 1997 .

[20]  Observations of Suspended Sediment Flux over a Tidal Cycle in the Region of the Turbidity Maximum of the Upper St. Lawrence River Estuary , 1988 .

[21]  Lawrence P. Sanford,et al.  Assessing the paradigm of mutually exclusive erosion and deposition of mud, with examples from upper Chesapeake Bay , 1993 .

[22]  D. W. Pritchard,et al.  Responses of upper Chesapeake Bay to variations in discharge of the Susquehanna River , 1986 .

[23]  William Panageotou,et al.  Tidal resuspension of sediments in northern Chesapeake Bay , 1991 .

[24]  Jeffery R. Cordell,et al.  Sink or swim? Copepod population maintenance in the Columbia River estuarine turbidity-maxima region , 1997 .

[25]  J. R. Schubel,et al.  Agglomeration of fine-grained suspended sediment in Northern Chesapeake Bay , 1972 .

[26]  J. Schubel,et al.  Distribution of seston in upper Chesapeake Bay , 1969 .

[27]  M. Fennessy,et al.  A comparison of in situ techniques for estuarine floc settling velocity measurements , 1996 .

[28]  T. Burt.  Field Settling Velocities of Estuary Muds , 1986 .

[29]  J. F. Festa,et al.  Turbidity maxima in partially mixed estuaries: A two-dimensional numerical model , 1978 .

[30]  John P. Downing,et al.  Laboratory apparatus for calibrating optical suspended solids sensors , 1989 .

[31]  J. Dupont,et al.  Hydrodynamics of suspended particulate matter in the tidal freshwater zone of a macrotidal estuary (the Seine Estuary, France) , 1999 .

[32]  M. W. Owen.  Determination of the settling velocities of cohesive muds , 1976 .

[33]  Laurenz Thomsen,et al.  Factors controlling aggregate formation in the benthic boundary layer of the Mecklenburg Bight (western Baltic Sea) , 1999 .

[34]  A. W. Morris,et al.  Seasonality of the Turbidity Maximum in the Humber–Ouse Estuary, UK , 1999 .

[35]  M. Nichols.  Response and recovery of an estuary following a river flood , 1977 .

[36]  C. Officer.  Discussion of the turbidity maximum in partially mixed estuaries , 1980 .

[37]  C. Olsen,et al.  Particle-borne radionuclides as tracers for sediment in the Susquehanna River and Chesapeake Bay , 1989 .

[38]  Dale A. Kiefer,et al.  Meridional variations in the concentration of chlorophyll and microparticles in the North Pacific Ocean , 1988 .

[39]  J. Baross,et al.  Particle-attached bacteria and heterotrophic plankton associated with the Columbia River estuarine turbidity maxima , 1996 .

[40]  L. Sanford.  Wave-forced resuspension of upper Chesapeake Bay muds , 1994 .

[41]  P. Hamblin.  Observations and model of sediment transport near the turbidity maximum of the upper Saint Lawrence Estuary , 1989 .

[42]  W. Boicourt,et al.  Resonant seiche motion in the Chesapeake Bay , 1989 .

[43]  W. Kimmerer,et al.  Tidally oriented vertical migration and position maintenance of zooplankton in a temperate estuary , 1998 .

[44]  J. Schubel.  SUSPENDED SEDIMENT OF THE NORTHERN CHESAPEAKE BAY , 1968 .

[45]  J. Dauvin.  Relationship between feeding incidence and vertical and longitudinal distribution of rainbow smelt larvae (Osmerus mordax) in a turbid well-mixed estuary , 1990 .