Quashnet River Research Articles

Recent advances in vertical temperature profiler instrumentation and flux estimation methods facilitate groundwater – Surface water exchange studies in environments with strong discharge zones

Groundwater fluxes to many surface water systems are spatially heterogeneous with discharge focused into discrete, high-flux zones. Quantifying fluxes in these preferential discharge zones is critical to a range of surface water habitat and water quality processes, but characterization can be difficult due to short-scale spatial and temporal variability. Passive heat-as-a-tracer methods employing vertical temperature profiler (VTP) data can provide the necessary spatial and temporal resolution, but upward fluid flow strongly attenuates the thermal signals used for estimating fluxes. In preferential discharge zones it becomes difficult to measure the signals in the subsurface and the flux parameter can become insensitive in the analysis models, leading to large uncertainties. We use data from a high-flux site of contaminant-loaded groundwater discharge to the Quashnet River on Cape Cod, Massachusetts, USA, to demonstrate how recent advances in VTP instrumentation that allow for the acquisition of high-resolution (0.001 °C) temperature data at short (1 cm) offsets near the ground surface, combined with advances in flux estimation methods that exploit the information content of the high-resolution data, facilitate heat-as-a-tracer approaches for characterizing groundwater-surface water exchanges and make it possible to obtain accurate and statistically robust results in a preferential discharge zone with a specific discharge of ∼1 m/d.

Investigation of Scale-Dependent Groundwater/Surface-water Exchange in Rivers by Gradient Self-Potential Logging: Numerical Modeling and Field Experiments

Exchanges of groundwater and surface-water are fundamental to a wide range of water-supply and water-quality management issues but challenging to map beyond the reach scale. Waterborne gradient self-potential (SP) measurements are directly sensitive to water flow through riverbed sediments and can be used to infer exchange locations, direction (gain versus loss), scale, and relative changes, but to date applications to river corridor hydrology are limited. Numerical modeling and field experiments were therefore performed herein, each emphasizing waterborne gradient SP logging for identifying and locating focused vertical groundwater discharge (surface-water gain) and recharge (surface-water loss) in a river. Two and three-dimensional numerical models were constructed to simulate the polarities, appearances, and peak amplitudes of streaming-potential and electric-field anomalies on a riverbed and in the surface-water that were attributable to steady-state vertical fluxes of groundwater through high-permeability conduits in the riverbed. Effects of varied hydraulic length-scale of exchange and surface-water depth were tested through numerical modeling. Modeling results aided in data acquisition and interpretation for three repeated field experiments performed along a 1.5–2.0 km reach of the Quashnet River in Cape Cod, Massachusetts, where focused, meter-scale groundwater discharges occur at discrete locations within otherwise ubiquitous and more diffuse groundwater upwelling conditions. Strong gradient SP anomalies were repeatedly measured in the Quashnet River at previously confirmed locations of focused groundwater discharge, showing the efficacy of waterborne gradient SP logging in identifying and characterizing groundwater/surface water exchange dynamics at multiple river network scales.

Hydrogeochemical controls on brook trout spawning habitats in a coastal stream.

Brook trout (Salvelinus fontinalis) spawn in fall and overwintering egg development can benefit from stable, relatively warm temperatures in groundwater-seepage zones. However, eggs are also sensitive to dissolved oxygen concentration, which may be reduced in discharging groundwater (i.e., seepage). We investigated a 2 km reach of the coastal Quashnet River in Cape Cod, Massachusetts, USA, to relate preferred fish spawning habitats to geology, geomorphology, and discharging groundwater geochemistry. Thermal reconnaissance methods were used to locate zones of rapid groundwater discharge, which were predominantly found along the central channel of a wider stream valley section. Pore-water chemistry and temporal vertical groundwater flux were measured at a subset of these zones during field campaigns over several seasons. Seepage zones in open-valley sub-reaches generally showed suboxic conditions and higher dissolved solutes compared to the underlying glacial outwash aquifer. These discharge zones were cross-referenced with preferred brook trout redds and evaluated during 10 years of observation, all of which were associated with discrete alcove features in steep cutbanks, where stream meander bends intersect the glacial valley walls. Seepage in these repeat spawning zones was generally stronger and more variable than in open-valley sites, with higher dissolved oxygen and reduced solute concentrations. The combined evidence indicates that regional groundwater discharge along the broader valley bottom is predominantly suboxic due to the influence of near-stream organic deposits; trout show no obvious preference for these zones when spawning. However, the meander bends that cut into sandy deposits near the valley walls generate strong oxic seepage zones that are utilized routinely for redd construction and the overwintering of trout eggs. Stable water isotopic data support the conclusion that repeat spawning zones are located directly on preferential discharges of more localized groundwater. In similar coastal systems with extensive valley peat deposits, the specific use of groundwater-discharge points by brook trout may be limited to morphologies such as cutbanks, where groundwater flow paths do not encounter substantial buried organic material and remain oxygen-rich.

Combined use of thermal methods and seepage meters to efficiently locate, quantify, and monitor focused groundwater discharge to a sand‐bed stream

AbstractQuantifying flow of groundwater through streambeds often is difficult due to the complexity of aquifer‐scale heterogeneity combined with local‐scale hyporheic exchange. We used fiber‐optic distributed temperature sensing (FO‐DTS), seepage meters, and vertical temperature profiling to locate, quantify, and monitor areas of focused groundwater discharge in a geomorphically simple sand‐bed stream. This combined approach allowed us to rapidly focus efforts at locations where prodigious amounts of groundwater discharged to the Quashnet River on Cape Cod, Massachusetts, northeastern USA. FO‐DTS detected numerous anomalously cold reaches one to several m long that persisted over two summers. Seepage meters positioned upstream, within, and downstream of 7 anomalously cold reaches indicated that rapid groundwater discharge occurred precisely where the bed was cold; median upward seepage was nearly 5 times faster than seepage measured in streambed areas not identified as cold. Vertical temperature profilers deployed next to 8 seepage meters provided diurnal‐signal‐based seepage estimates that compared remarkably well with seepage‐meter values. Regression slope and R2 values both were near 1 for seepage ranging from 0.05 to 3.0 m d−1. Temperature‐based seepage model accuracy was improved with thermal diffusivity determined locally from diurnal signals. Similar calculations provided values for streambed sediment scour and deposition at subdaily resolution. Seepage was strongly heterogeneous even along a sand‐bed river that flows over a relatively uniform sand and fine‐gravel aquifer. FO‐DTS was an efficient method for detecting areas of rapid groundwater discharge, even in a strongly gaining river, that can then be quantified over time with inexpensive streambed thermal methods.

Practical limitations on the use of diurnal temperature signals to quantify groundwater upwelling

Groundwater upwelling to streams creates unique habitat by influencing stream water quality and temperature; upwelling zones also serve as vectors for contamination when groundwater is degraded. Temperature time series data acquired along vertical profiles in the streambed have been applied to simple analytical models to determine rates of vertical fluid flux. These models are based on the downward propagation characteristics (amplitude attenuation and phase-lag) of the surface diurnal signal. Despite the popularity of these models, there are few published characterizations of moderate-to-strong upwelling. We attribute this limitation to the thermodynamics of upwelling, under which the downward conductive signal transport from the streambed interface occurs opposite the upward advective fluid flux. Governing equations describing the advection–diffusion of heat within the streambed predict that under upwelling conditions, signal amplitude attenuation will increase, but, counterintuitively, phase-lag will decrease. Therefore the extinction (measurable) depth of the diurnal signal is very shallow, but phase lag is also short, yielding low signal to noise ratio and poor model sensitivity. Conversely, amplitude attenuation over similar sensor spacing is strong, yielding greater potential model sensitivity. Here we present streambed thermal time series over a range of moderate to strong upwelling sites in the Quashnet River, Cape Cod, Massachusetts. The predicted inverse relationship between phase-lag and rate of upwelling was observed in the field data over a range of conditions, but the observed phase-lags were consistently shorter than predicted. Analytical solutions for fluid flux based on signal amplitude attenuation return results consistent with numerical models and physical seepage meters, but the phase-lag analytical model results are generally unreasonable. Through numerical modeling we explore reasons why phase-lag may have been over-predicted by the analytical models, and develop guiding relations of diurnal temperature signal extinction depth based on stream diurnal signal amplitude, upwelling magnitude, and streambed thermal properties that will be useful in designing future experiments.

Determination of water mass ages using radium isotopes as tracers: Implications for phytoplankton dynamics in estuaries

Despite a relatively short residence time of water in many shallow, semi-enclosed estuaries, phytoplankton blooms in nutrient enriched systems are a common phenomenon. This poses the question how is it possible to have phytoplankton populations bloom in response to local conditions of shallow estuaries, if the water residence times are similar to cell division times? To address this paradox we used the radium quartet as a tool to measure water mass age in coastal systems (Waquoit Bay, MA, USA) subject to different degrees of land-derived nitrogen load and hence differences in phytoplankton biomass. Recently, the radium quartet has been used as geochemical tracers to determine age of water masses. Based on a number of samples collected over the course of one year, the average radium-derived age (±stdev) of water in three sub-estuaries of Waquoit Bay (Childs River, Quashnet River, and Sage Lot Pond) was ~7 (±4.7), 11 (±6.2), and 17 (±7.5)days, respectively. These values are significantly longer than previous estimates based on more traditional hydrodynamic methods. Furthermore, peak chlorophyll concentrations were associated with older water masses in the heavily freshwater-influenced sub-estuaries (Childs and Quashnet). Our results suggest that water age, temperature, and nutrients all play a role in controlling phytoplankton biomass however, water age was more important at the time of the year when temperature limits phytoplankton growth (late spring, early summer). We conclude that radium-derived age models, which are similar to artificial tracer-based approaches, may be the most appropriate method for studying the role of hydrodynamics on estuarine ecology.

Coupled radon, methane and nitrate sensors for large-scale assessment of groundwater discharge and non-point source pollution to coastal waters

We constructed a survey system of radon/methane/nitrate/salinity to find sites of submarine groundwater discharge (SGD) and groundwater nitrate input. We deployed the system in Waquoit Bay and Boston Harbor, MA where we derived SGD rates using a mass balance of radon with methane serving as a fine resolution qualitative indicator of groundwater. In Waquoit Bay we identified several locations of enhanced groundwater discharge, out of which two (Childs and Quashnet Rivers) were studied in more detail. The Childs River was characterized by high nitrate input via groundwater discharge, while the Quashnet River SGD was notable but not a significant source of nitrate. Our radon survey of Boston Harbor revealed several sites with significant SGD, out of these Inner Harbor and parts of Dorchester Bay and Quincy Bay had groundwater fluxes accompanied by significant water column nitrogen concentrations. The survey system has proven effective in revealing areas of SGD and non-point source pollution.

Relative influence of grazing and nutrient supply on growth of the green macroalga Ulva lactuca in estuaries of Waquoit Bay, Massachusetts.

Nitrogen supply is a major control on growth of coastal macroalgae (1, 2, 3). Top-down effects in which grazing signifi cantly affects macroalgae (4, 5), and nutrient-grazer interactions (3) have also been described. In this paper we describe an experiment in which we measured net growth of a common macroalga, Ulva lactuca, in treatments that allowed different numbers of grazers to access fronds as well as incubation of fronds in estuaries with demonstrably different nutrient supplies. These treatments were intended to assess the relative infl uence of grazer and nitrogen supply on net growth rates of a coastal producer. To examine the effect of grazing on growth of U. lactuca, we constructed acrylic plastic cages with sides of 1-mm, 4-mm, or 18-mm mesh. The different mesh openings were intended to allow entry to different numbers of grazers, which we took as a proxy for grazing pressure. The cage design also allowed for light penetration and horizontal water fl ow. The 18-mm mesh permitted larger size classes and a greater number of grazers to enter the cages, while the 1-mm mesh excluded larger size classes and allowed fewer grazers. The 4-mm mesh was intended to furnish an intermediate grazer treatment. To evaluate the effect of nitrogen supply and grazing on algal growth, cages with the three mesh sides were placed in three estuaries in Waquoit Bay, Massachusetts. These three estuaries experience different nitrogen loads—Sage Lot Pond, 14 kg ha 1 y 1 ; Quashnet River, 350 kg ha 1 y 1 ; and Childs River, 601 kg ha 1 y 1 —from their watershed (6). These nitrogen loads led to different mean nitrate concentrations measured in the estuaries during July 2002, one year prior to the time of our experiments: 0.04, 6.1, and 11.75 M for Sage Lot Pond, Quashnet River, and Childs River, respectively (G. Tomasky, Boston University Marine Program, unpubl. data). To minimize effects of differences between estuaries other than our treatments, we chose sites similar in salinity, depth, and algal composition. In each estuary we placed four replicates of each of the three grazing pressure treatments, for a total of 36 cages. Three fronds of U. lactuca, each approximately 300 mg (blotted wet weight), were suspended inside each cage. To measure the effect of top-down versus bottom-up factors, we measured net growth as the dependent variable. Net growth was the growth achieved by the fronds minus the biomass consumed by grazers. To determine net growth, the U. lactuca fronds were weighed initially (blotted wet weight) and again after 10 days of fi eld incubation. First, we assess the successed of the treatments. To roughly measure the grazing pressure, we sorted and counted the potential grazers found in the cages for two replicates of each treatment at the end of the incubation. The grazers were sorted into four groups, amphipods, shrimp, crabs, and isopods. The total number of grazers in the 1-mm mesh cages was signifi cantly lower in all three estuaries than the number in the 4-mm cages (Fig. 1a; ANOVA F 31.0, P 0.0014). The number of grazers found in the 18-mm mesh cages was also signifi cantly different, although they contained lower grazer abundances than the 1-mm and 4-mm mesh cages (Fig. 1a). Predatory fi sh and large shrimp entered the cages with 18 mm mesh and likely fed on the smaller grazers, thus decreasing grazer abundances. This possible effect of predators on grazers suggests that there might be important top-down cascade effects in this system waiting to be studied. The difference in nitrogen load in the three estuaries provided quantitatively different nutrient supplies, as evident in the nitrate concentrations cited above. Bottom-up effects from these different nitrogen supplies on net growth of U. lactuca were dominant factors. Rates of net growth were higher in estuaries receiving

Impact on Marine Species of New England Recreational Fishing Policies.

Marenzelleria viridis dominated the benthos of the Quashnet River in both abundance and biomass. In the Childs River, Lumbricillus spp. and Neanthes virens were again the main contributors. Salinity appeared to be the primary factor controlling the distribution of the macroinfaunal species; but within any single salinity range, eutrophication seemed to be responsible for differences in the composition and abundance of macrofauna between estuaries. The abundance of oligochaetes and insect larvae and the occurrence of a large carnivore increased with nutrient loading, whereas the number and biomass of other typically resident species decreased. Most significantly, the ratio of polychaetes to oligochaetes was clearly lower in the Childs River (0.36) than in the Quashnet River (3.58). We contrasted the benthic assemblages within the heavily nutrient-loaded Childs River estuary to those of the more moderately loaded Quashnet River estuary by constructing plots of ranked species dominance (3). The biomass and abundance of each species were ranked from highest to lowest and plotted against the cumulative percent biomass or percent abundance (Fig. 1). Plots in which the k-dominance curve for biomass falls above the curve for abundance are indicative of communities in which the most abundant fauna are large, slower growing species. These communities are indicative of relatively unstressed conditions. Plots in which the curve for abundance falls above the curve for biomass represent communities in which the most abundant species are small and rapidly growing. These are more typical of disturbed or eutrophied communities (1). No large differences were apparent between the k-dominance curves of abundance and biomass in the upper portion of either estuary. In both the middle and lower portions of the estuaries, the evaluation suggests that Childs River, with its more urbanized watershed, is more impacted by nutrient loading than the Quashnet River. We acknowledge the MBL Associates Fellowship that supported Rafael Sarda during this work.

Effects of Nitrogen Loading and Salt Marsh Habitat on Gross Primary Production and Chlorophyll a in Estuaries of Waquoit Bay.

Nixon (1) showed, using comparative data from different systems, that increased nitrogen load to shallow coastal estuaries increased production of phytoplankton. Furthermore, it has been well established that the growth of coastal producers is nitrogen limited (2). In Waquoit Bay, we have a complex of separate estuaries that are subject to different nitrogen loading rates (3). This variation in loading rate provides the opportunity to test, in one system, whether increased nitrogen loads result in increased production. The range of nitrogen loading to the estuaries extended from a high rate of 8.1 X 1 O3 kg N y-’ in Childs River to approximately 0.05 1 kg N y-’ in Sage Lot Pond. Because phytoplankton growth in shallow estuaries is nitrogen limited (2), increased loading rates are likely to affect activity and abundance of these primary producers. Salt marsh habitats are active sites of denitrification and nutrient uptake (2). A strip of salt marsh located between the watershed and the estuary could, therefore, intercept incoming nitrogen and significantly reduce estuarine nitrogen loading. The estuaries of Waquoit Bay are surrounded by different areas of salt marsh. We could, consequently, also evaluate the effects of salt marsh on interception of nitrogen by comparing phytoplankton abundance and activity in estuaries with different extents of fringing salt marsh. In this paper we ask, first, whether there is a relationship between nitrogen loading rate and phytoplankton abundance and productivity; and second, whether the presence of a salt marsh fringe decreases the nitrogen loading rate and, accordingly, lowers phytoplankton abundance and productivity. We measured gross primary production (GPP) and chlorophyll a concentration at two stations in each of five estuaries of Waquoit Bay (Childs River, Quashnet River, Jehu Pond, Hamblin Pond, and Sage Lot Pond). We used standard light/dark bottle technique with 5-h in situ incubation period, and the Winkler titration method to determine primary production of the estuaries. Chlorophyll a concentration was measured by the Lorenzen method (4). The nitrogen loading rate was calibrated based on total dissolved nitrogen (DIN) at shore edge, rate of water recharge, and total area of the estuary. GPP and chlorophyll a increased significantly with higher nitrogen loads (Fig. 1, top panels). For the regression of phytoplankton and loading, P < 0.003 for both GPP and Chl a. In Childs River, for example, the average chlorophyll a concentration and GPP levels were about three times as high as those in Sage Lot Pond. Both GPP rates and chlorophyll a concentration decreased in estuaries with larger areas of fringing salt marsh (Fig. 1, middle panels). The cause of this decrease is not well established. The salt marshes could be physically removing phytoplankton from the flooding estuarine water during high tide and, thus, lowering

Invertebrate Response to Nutrient-Induced Changes in Macrophyte Assemblages in Waquoit Bay.

Coastal marine systems such as Waquoit Bay, Massachusetts, have experienced a major increase in nutrient loads through groundwater in recent decades (1). Five estuaries of Waquoit Bay (Childs River, Hamblin Pond, Jehu Pond, Quashnet River, and Sage Lot Pond) are subject to different nutrient loads due to different land uses in their watersheds (1). Nutrient loading prompts replacement of eelgrass (Zosteru marina) by macroalgae-dominated communities (2). In this paper we address whether the vegetation changes caused by nutrient loading result in parallel changes in the benthic fauna of the affected areas. In each of the five estuaries, the macrophyte and invertebrate communities were sampled at 10 sites (randomly selected within representative depth strata) with an Eckman dredge (0.15 m’), June-August 1995. Samples were rinsed through a l-mm sieve. Macrophytes were sorted by species, dried, and weighed (aboveground biomass only for eelgrass). Macroinvertebrates were categorized into taxonomic groupings and counted. The estuaries varied widely in eelgrass and macroalgal biomass (Fig. I). Benthic invertebrate density did not change markedly with the decrease in eelgrass biomass from Sage Lot Pond and Jehu Pond to the other estuaries (Fig. 1, top). In contrast, invertebrate density showed a clear inverse relationship with macroalgal biomass (? = 0.5, P = 0.003) (Fig. 1, bottom). Childs River, the most nutrient-loaded site, had a consistently higher macroalgal biomass and a lower invertebrate density (Fig. 1, bottom). The other sites (shown without initials in Fig. 1) had much lower macroalgal biomass than Childs River, but still show a strong negative relationship between invertebrate density and macroalgal biomass (? = 0.3, P = 0.07) when Childs River points are excluded. The other estuaries did not, however, have consistently different macroalgal biomass from each other. Although the decrease in eelgrass biomass did not alter invertebrate density, it did change fauna1 composition. In eelgrassdominated communities (Jehu Pond, Sage Lot Pond), polychaetes composed 26% of the invertebrate abundance, whereas in macroalgae-dominated communities polychaetes increased to 4 1% of benthic invertebrates. Changes in the abundance of a single species in response to eelgrass loss have been documented for shellfish species of Waquoit Bay (1). Many factors may influence invertebrate abundance in estuaries. Low-oxygen conditions associated with accumulations of macroalgae (3) may be responsible for the decrease in invertebrate density with increasing macroalgae biomass; this oxygen

Comparison of Phytoplankton and Ecosystem Gross Production in the Quashnet River, an Estuary of Waquoit Bay, Massachusetts.

The Quashnet River is a shallow (av. depth 0.5 m) estuary of Waquoit Bay, Massachusetts, that supports a large biomass of benthic macroalgae, in addition to phytoplankton producers. To evaluate the factors controlling the relative contribution of phytoplankton production to total ecosystem production in the Quashnet River, we measured both gross phytoplankton production (GPP) and gross ecosystem production (GEP), along with a suite of physical factors, for 12 consecutive days. We related change in the ratio of GPP to GEP to photosynthetically available radiation (PAR), temperature, salinity, water column depth, and wind speed and direction. Phytoplankton production was measured aily as the change of oxygen in light and dark bottles during 4to 6-h incubations; the oxygen concentrations were measured by the Winkler Method (1). Ecosystem production was estimated from die1 free water oxygen levels recorded by automated oxygen meters. ENDECO model 1184C and 6000 pulsed-sensor xygen meters were used to measure the oxygen, temperature, depth, and salinity of the surface and bottom water, respectively. Wind speed and direction were measured with an Onset Tattletale 2-B computer linked to a Digitar #7902 anemometer. The GEP and GPP fluctuated over time, going up or down in synchrony for most days (Fig. 1A). The average GPP was 1.32 g O2 me2 d-’ (SD = 0.65, SE = 0.20), and the average GEP was 3.79 g O2 mm2 d-’ (SD = 1.17, SE = 0.353). The proportion of GEP attributable to GPP has an average of 35% (SD = 13.4%, SE = 4%) (Fig. 1B). The GPP/GEP ratio ranges from 0.10 to 0.70. The ratio of GPP to GEP was higher (av. = 0.69, SD = 0.049) during the last 2 days of the deployment. This increase may have been due to progressive freshening of the surface water accompanied by increased it-radiance. Freshening of the surface waters implies increased residence time, which may allow phytoplankton populations to accumulate in the estuary. As Figure 1C shows, the Quashnet River became more stratified (expressed as delta salinity = bottom surface salinity) and fresher over time. Figure 1 D shows the GPP/GEP ratio plotted against he ratio of the bottom-to-top salinity. These points are labeled with their corresponding PAR values, the dark squares being those points with PAR values >30 E m-2 d-’ and the light squares being points with PAR values ~30 PE mm2 d-‘. The trends present in

From Watershed to Estuary: Assessment of Nutrient Loading, Retention, and Export from the Ipswich River Basin.

Childs River and Quashnet River, combined with the absence of such spikes at Sage Lot and Flat Ponds, suggests input by plumes from septic tanks. Mean NI&+ concentration in groundwater varied over a relatively small range across the three sites (Fig. 1, middle panels). NH4+ contributed less than 5% of the mean dissolved inorganic nitrogen (DIN) at Childs and Quashnet Rivers. Most of the inorganic nitrogen, therefore, enters these two estuaries in the form of N03-. Nitrogen from septic tanks and soils enters the aquifer in reduced forms, such as NH.,+ [untreated septic water contains up to 8000 PM NH4+ (J. McClelland, unpub. data)], which are oxidized to N09in the aquifer before reaching the estuary. Mean POd3+ concentrations were relatively low and similar in the three estuaries (Fig. 1, right panels). One would not expect POd3+ to vary across the sites, despite a gradient in residential area, because POd3+ is readily adsorbed by soils. Groundwater data from Waquoit Bay suggest that differences in land use can have a major effect on the concentrations of nitrogen entering a shallow estuary. To estimate the loading from groundwater to receiving estuaries and to estuarine systems, such as Waquoit Bay, concentration data such as those reported in this paper must be converted to flux by estimating actual water and nutrient transport. Calculated values of total system loading will be useful in deciding how to manage coastal estuaries threatened by eutrophication. This work was supported by Waquoit Bay Land Margin Ecosystems Research and a Research Experience for Undergraduates grant. Special thanks to Lori Saucy for operating the autoanalyzer.