Hudson River Plume Research Articles

Quantification of small-scale heterogeneity in aquatic aminopeptidase activity

Leucine aminopeptidase (LAP) is one of the enzymes involved in the hydrolysis of peptides, and is sometimes used to indicate potential nitrogen limitation in microbes. Small-scale variability has the potential to confound interpretation of underlying patterns in LAP activity in time or space. An automated flow-injection analysis instrument was used to address the small-scale variability of LAP activity within contiguous regions of the Hudson River plume (New Jersey, USA). LAP activity had a coefficient of variation (CV) of ca. 0.5 with occasional values above 1.0. The mean CVs for other biological parameters—chlorophyll fluorescence and nitrate concentration—were similar, and were much lower for salinity. LAP activity changed by an average of 35 nmol l-1 h-1 at different salinities, and variations in LAP activity were higher crossing region boundaries than within a region. Differences in LAP activity were ±100 nmol l-1 h-1 between sequential samples spaced <10 m apart. Variogram analysis indicated an inherent spatial variability of 52 nmol l-1 h-1 throughout the study area. Large changes in LAP activity were often associated with small changes in salinity and chlorophyll fluorescence, and were sensitive to the sampling frequency. This study concludes that LAP measurements in a sample could realistically be expected to range from zero to twice the average, and changes between areas or times should be at least 2-fold to have some degree of confidence that apparent patterns (or lack thereof) in activity are real.

Two‐dimensional circulation and mixing in the far field of a surface‐advected river plume

AbstractField observations of the Hudson River plume are presented to discuss circulation and mixing in the far field of this coastally trapped buoyant flow. The plume was surface advected and propagated downshelf near the internal wave speed. The plume outflow was characterized by a two‐layer bulge‐like feature but became continuously stratified and vertically sheared in the far field, where Richardson numbers are generally below 0.5. High‐frequency velocity and backscatter data from a moored ADCP revealed strong vertical and horizontal oscillatory motions at the front with a wavelength approximately 7–8 times the plume thickness, consistent with Kelvin‐Helmholtz instabilities. These motions quickly died out after 2–3 cycles. The combination of vertical shear and stratification in the plume leads to a buoyancy flux toward the nose of the plume, which competes with mixing. However, the continued salinity increase of the plume as it propagated downshelf indicates that mixing overcomes this delivery of freshwater to the plume front. A simple 2‐D model is developed, which relates the time rate‐of‐change of the plume salinity to: (1) salt entrainment due to vertical mixing, and (2) freshwater flux and salt removal due to the vertical shear of the stratified plume. Estimates of an entrainment coefficient from this model are consistent with previous estimates from the near field of a river outflow. A scaling of the plume width is obtained by assuming that vertical shears are controlled by both thermal wind and a critical Richardson number. This scaling yields plume widths that are consistent with previous laboratory studies.

The coupled Hudson River estuarine-plume response to variable wind and river forcings

Observations of the Hudson River plume were taken in the spring of 2006 in conjunction with the Lagrangian Transport and Transformation Experiment using mooring arrays, shipboard observations, and satellite data. During this time period, the plume was subjected to a variety of wind, buoyant, and shelf forcings, which yield vastly different responses in plume structure including a downstream recirculating eddy. During weak and downwelling winds, the plume formed a narrow buoyant coastal current that propagated downstream near the internal wave speed. Freshwater transport during periods when the downwelling wind was closely aligned with the coast was near the river discharge values. During periods with a cross-shore component to the wind, freshwater transport in the coastal current estimated by the mooring array is less than the river discharge due to a widening of the plume that leads to the internal Rossby radius scaling for the plume width to be invalid. The offshore detachment of plume and formation of a downstream eddy that is observed surprisingly persisted for 2 weeks under a variety of wind forcing conditions. Comparison between mooring, shipboard, and satellite data reveal the downstream eddy is steady in time. Shipboard transects yield a freshwater content equal to the previous 3 days of river discharge. The feature itself was formed due to a large discharge following a strong onshore wind. The plume was then further modified by a brief upwelling wind and currents influenced by the Hudson Shelf Valley. The duration of the detachment and downstream eddy can be explained using a Wedderburn number which is largely consistent with the wind strength index described by Whitney and Garvine (J Geophys Res 110:C03014 1997).

The influence of river plume dynamics on trace metal accumulation in calanoid copepods

The concentrations of Ag, Cd, Cu, Hg(II), monomethylmercury (MeHg), Pb, and Zn were measured and modeled in calanoid copepods collected from the Hudson River plume in April 2005 and May 2006. Copepods in the buoyant plume were significantly (p < 0.05) enriched in Ag, Cu, Pb, Zn, and MeHg (2‐ to 25‐fold), but depleted in Cd (by 25–40%) relative to oceanic copepods. Median burdens of Ag, Cu, and Hg(II) differed in plume copepods collected in April 2005 and May 2006 by factors of 1.5 to 3, reflecting the influence of river discharge and associated physical processes on plankton dynamics and metal bioaccumulation. Only the concentrations of Cu and Zn in plume copepods clearly showed the expected trend of highest levels near the mouth of the Hudson River and lower levels offshore. Spatial trends in concentrations of Ag and Hg(II) in plume copepods were strikingly different from those in suspended particles and the dissolved phase, apparently reflecting differences in metal bioavailability because of shifts in available food types. Modeled concentrations of Cd, Cu, Pb, and Zn in plume copepods were within less than a factor of two of measured values, but model results showed that Hg(II) was less bioavailable, and Ag and MeHg were more bioavailable in the Hudson River plume than laboratory results would predict. Zooplankton from urban river plumes may accumulate potentially toxic levels of trace metals or act as a trophic link between metal contamination in polluted estuaries and pelagic ecosystems.

High‐frequency forcing and subtidal response of the Hudson River plume

The Lagrangian Transport and Transformation Experiments (LaTTE) document the physical, biological, and chemical evolution of the Hudson River plume during the spring seasons of 2004, 2005, and 2006. While plume variability due to river discharge, subinertial frequency variability in winds, and ambient shelf circulation are important, the observations show that the plume reacts directly to higher‐frequency forcing as well. Mooring records during 2005 and 2006 show that fortnightly variability in tidal mixing is manifested as fortnightly changes in plume stratification. Diurnal variability related to forcing by the sea‐land breeze system (SLBS) is apparent in the Hudson River plume during the 2005 experiment. The SLBS, while episodic, accounts for ∼15% of the kinetic energy in surface currents in the New York Bight apex during the summer months with individual SLBS events providing up to 50% of the total kinetic energy. Simulations of the plume, using the Regional Ocean Modeling System (ROMS), demonstrate there is a subtidal response to high‐frequency forcing. Spring‐neap variability in tidal mixing modifies the estuary outflow Rossby and Froude numbers, resulting in increased transport (80% of river discharge) in the New Jersey coastal current during spring tides with lower transport (60% of river discharge) during neap tides. SLBS variability results in greater storage of river discharge water in the recirculating bulge region and increases the net transport of freshwater along the Long Island coast while significantly reducing freshwater supplied to the New Jersey coastal current to as little as 30% of the total river discharge.

Simulation of Water Age and Residence Time in New York Bight

Abstract The time scales on which river inflows disperse in the coastal ocean are relevant to a host of biogeochemical and environmental processes. These time scales are examined in a modeling study of the Hudson River plume on its entry to the New York Bight (NYB). Constituent-oriented age and residence-time theory is applied to compute two time scales: mean age, which is calculated from the ratio of two model tracers, and residence time, which is calculated using the adjoint of the tracer conservation equation. Spatial and temporal variability associated with river discharge and wind is investigated. High river discharge lowers surface water age and shortens residence time in the apex of the NYB. Easterly winds increase surface water age and extend the duration waters along the Long Island coast remain in the NYB apex. Southerly winds increase age along the New Jersey coast but drive a decrease in age of offshore surface waters and prolong the time that surface waters close to the New Jersey coast stay in the NYB apex. Residence time along the Long Island coast is high in spring and summer because of the retention of water north of the Hudson shelf valley. Patterns of modeled surface water age and an age proxy computed from the ratio of satellite-measured irradiance in two channels show qualitative agreement. A least squares fit gives a statistically significant empirical relationship between the band ratio and modeled mean age for NYB waters.

Spring and Summer Larval Fish Assemblages in the Surf Zone and Nearshore off Northern New Jersey, USA

Larval fish use of surf zone and nearshore habitats at northern latitudes has received little attention. Consequently, potential impacts of beach nourishment and other forms of disturbance are not well understood. This study, on a northwestern Atlantic coastline spanning May through July over 4 years, demonstrates that recently hatched larvae are common in both surf zone and nearshore habitats. Taxonomic compositions of surf zone and nearshore assemblages were similar to each other and those from an adjacent estuary. An influence of upwelling events was apparent in coincident changes in abundance and/or size of several species in the surf zone. Other changes over the late spring–summer transition, including buoyancy-driven flows from the Hudson River plume, demonstrate the dynamic nature of larval fish assemblages in the New York Bight area.

Dispersal of the Hudson River Plume in the New York Bight: Synthesis of Observational and Numerical Studies During LaTTE

Dispersal of the Hudson River Plume in the New York Bight: Synthesis of Observational and Numerical Studies During LaTTE

Dynamics of turbid buoyant plumes and the feedbacks on near‐shore biogeochemistry and physics

The near‐shore waters of the New York/New Jersey Bight in April 2005 exhibited distinct regions of turbid water with clearly differing optical properties associated with the Hudson River plume. We examined the effect of variable light attenuation on the hydrodynamics and ecological response of the Hudson River plume and its environs using field observations and a 3‐dimensional bio‐physical model. Important feedback mechanisms between the attenuation of light and the resulting impact on the mixed layer depth were revealed from the modeling results. High concentrations of chlorophyll, detritus and colored dissolved organic matter in the upper water column as a result of enhanced stratification increase the attenuation of light and modify the buoyancy driven circulation. This further impacts the growth of phytoplankton in the model and subsequently modifies the vertical profile of the attenuation coefficient, which in turn feeds back into the overall heat budget.

The Effect of Wind on the Dispersal of the Hudson River Plume

Abstract The dispersal of the Hudson River plume in response to idealized wind forcing is studied using a three-dimensional model. The model domain includes the Hudson River and its estuary, with a realistic coastline and bottom topography of the New York Bight. Steady low river discharge typical of mean conditions and a high-discharge event representative of the spring freshet are considered. Without wind forcing the plume forms a southward coastally trapped current at low river discharge and a large recirculating bulge of low-salinity water during a high-discharge event. Winds affect the freshwater export through the mouth of the estuary, which is the trajectory the plume takes upon entering the waters of the Mid-Atlantic Bight inner shelf, and the rate at which freshwater drains downstream. The dispersal trajectory is also influenced by the particular geography of the coastline in the apex of the New York Bight. Northward wind causes offshore displacement of a previously formed coastally trapped plume and drives a new plume along the Long Island coast. Southward wind induces a strong coastal jet that efficiently drains freshwater to the south. Eastward wind aids freshwater export from the estuary and favors the accumulation of freshwater in the recirculating bulge outside the mouth of Raritan Bay. Westward wind delays freshwater export from Raritan Bay. The momentum balance of the modeled plume shows that buoyancy and wind forces largely determine the pattern of horizontal freshwater dispersal, including the spreading of freshwater over ambient, more saline water and the bulge formation.

Biogeochemical impact of summertime coastal upwelling on the New Jersey Shelf

The alternative hypothesis that observed regions of recurrent hypoxia on the New Jersey inner shelf are more related to coastal upwelling than riverine inputs of nutrients was investigated through a series of multidisciplinary research programs beginning in 1993. The largest variations in ocean temperatures along the New Jersey coast, other than seasonal, are found to be caused by episodic summertime upwelling events driven by southwesterly winds associated with the atmospheric Bermuda High. Off the southern coast of New Jersey, topographic variations associated with ancient river deltas cause upwelled water to evolve into an alongshore line of recurrent upwelling centers that are colocated with historical regions of low dissolved oxygen. Recurrent upwelling centers are observed every summer in a 9‐year data set. The most significant upwelling events occur in summers following colder than usual falls and winters. Size and duration of individual events are correlated and are found to depend on the wind forcing history that effects the inner side of the Middle Atlantic Cold Pool, the precipitation history that effects the strength of the Hudson River plume, and the mixing storm frequency. Upwelling results in a significant enhancement of particulate organic carbon. The typical carbon enhancement associated with the upwelling is sufficient to deplete 75% of the oxygen in the bottom water, making it borderline hypoxic. This indicates that topographically controlled coastal upwelling, rather than riverine inputs, is the more probable mechanism for generating the historical regions of recurrent hypoxia observed along the New Jersey coast.

Dynamics and optics of the Hudson River outflow plume

In July and August 2000 a field experiment was conducted along the New Jersey coast to explore the linkage between dynamics of the outflowing Hudson River plume and the optical character of the inshore environment. A plume of low‐salinity water periodically flows along the coast, trapped in an inshore zone of 5–15 km width and extending to over 100 km alongshore from its source. The plume is clearly distinguished in color satellite images because of its high content of particulates and colored dissolved organic matter (CDOM), which dominate the inherent optical properties (IOPs) of the inshore region. This plume water is dispersed into the interior of the shelf when winds turn to upwelling favorable. Using an array of five acoustic Doppler current profilers, we found a well‐defined first mode empirical orthogonal function that showed strong southward flow and downwelling circulation during the plume state and weak northward flow with upwelling circulation during the upwelling state. Using filtered and nonfiltered measurements, the optical absorption IOP was separated into contributions from CDOM and particulates. The contributions from particulates were further separated into phytoplankton and detritus contributions using least squares fit with model spectra. It was found that the plume could be distinguished from upwelling by larger contributions to absorption from detritus and smaller contributions from CDOM.

Turbidity as a control on phytoplankton biomass and productivity in estuaries

In many coastal plain estuaries light attenuation by suspended sediments confines the photic zone to a small fraction of the water column, such that light limitation is a major control on phytoplankon production and turnover rate. For a variety of estuarine systems (e.g. San Francisco Bay, Puget Sound, Delaware Bay, Hudson River plume), photic-zone productivity can be estimated as a function of phytoplankton biomass times mean irradiance of the photic zone. Net water column productivity also varies with light availability, and in San Francisco Bay net productivity is zero (estimated respiratory loss of phytoplankton balances photosynthesis) when the ratio of photic depth ( Z p ) to mixed depth ( Z m ) is less than about 0.2. Thus whenever Z p : Z m < 0.2, the water column is a sink for phytoplankton production. Much of the spatial and temporal variability of phytoplankton biomass or productivity in estuaries is explained by variations in the ratio of photic depth to mixed depth. For example, phytoplankton blooms often coincide with stratification events that reduce the depth of the surface mixed layer (increase Z p : Z m ). Shallow estuarine embayments (high Z p : Z m ) are often characterized by high phytoplankton biomass relative to adjacent channels (low Z p : Z m ). Many estuaries have longitudinal gradients in productivity that mirror the distribution of suspended sediments: productivity is low near the riverine source of sediments (low Z p : Z m ) and increases toward the estuary mouth where turbidity decreases. Some of these generalizations are qualitative in nature, and detailed understanding of the interaction between turbidity and estuarine phytoplankton dynamics requires improved understanding of vertical mixing rates and phytoplankton respiration.

Bacterial dynamics and distribution during a spring diatom bloom in the Hudson River plume, USA

Bacterial dynamics and distribution during a spring diatom bloom in the Hudson River plume, USA

Production and Vertical Flux of Attached Bacteria in the Hudson River Plume of the New York Bight as Studied with Floating Sediment Traps

We investigated the growth and vertical flux of attached bacteria with floating sediment traps in the Hudson River Plume of the New York Bight during the spring diatom blooms. Traps were floated at the base of the mixed layer (ca. 10 m) for 1-day periods. After recovery, we measured bacterial abundance and rates of [methyl-H]thymidine incorporation in the trap samples. The vertical flux of attached bacteria was estimated with a model formulated to distinguish between bacterial accumulation in traps due to in situ growth and that due to vertical flux. Attached bacterial flux ranged from 0.6 x 10 to 2.0 x 10 cells m day, and attached bacterial settling rates of 0.1 to 1.0 m day were observed during periods of vertical particulate organic carbon flux ranging from 254 to 1,267 mg of C m day. In situ growth of bacteria in sediment traps was unimportant as a source of bacterial increase when compared with vertical flux during our study. The vertical flux of attached bacteria removed 3 to 67% of the total daily bacterial production from the water column. Particulate organic carbon is not significantly mineralized by attached bacteria during its descent to the sea floor in the plume area during this period, when water temperature and grazing rates are at their annual minima.