Hopkinson Charles S.

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Hopkinson

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Charles S.

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N budgets and aquatic uptake in the Ipswich River basin, northeastern Massachusetts

(American Geophysical Union, 2004-11-05) Williams, Michael ; Hopkinson, Charles S. ; Rastetter, Edward B. ; Vallino, Joseph J.

We calculated N budgets and conducted nutrient uptake experiments to evaluate the fate of N in the aquatic environment of the Ipswich River basin, northeastern Massachusetts. A mass balance indicates that the basin retains about 50% of gross N inputs, mostly in terrestrial components of the landscape, and the loss and retention of total nitrogen (TN) in the aquatic environment was about 9% of stream loading. Uptake lengths of PO4 and NH4 were measurable in headwater streams, but NO3 uptake was below detection (minimum detection limit = 0.05 μM). Retention or loss of NO3 was observed in a main stem reach bordered by wetland habitat. Nitrate removal in urban headwater tributaries was because of water withdrawals and denitrification during hypoxic events and in ponded wetlands with long water residence times. A mass balance using an entire river network indicates that basin-wide losses due to aquatic denitrification are considerably lower than estimates from several recent studies and range from 4 to 16% of TDN in stream loading. Withdrawals for domestic use restrict the runoff of headwater catchments from reaching the main stem during low base flow periods, thereby contributing to the spatial and temporal regulation of N export from headwater tributaries.
Representing the function and sensitivity of coastal interfaces in earth system models

(Nature Research, 2020-05-18) Ward, Nicholas D. ; Megonigal, J. Patrick ; Bond-Lamberty, Benjamin ; Bailey, Vanessa L. ; Butman, David ; Canuel, Elizabeth A. ; Diefenderfer, Heida ; Ganju, Neil K. ; Goni, Miguel ; Graham, Emily B. ; Hopkinson, Charles S. ; Khangaonkar, Tarang ; Langley, J. Adam ; McDowell, Nate G. ; Myers-Pigg, Allison N. ; Neumann, Rebecca B. ; Osburn, Christopher L. ; Price, René M. ; Rowland, Joel ; Sengupta, Aditi ; Simard, Marc ; Thornton, Peter E. ; Tzortziou, Maria ; Vargas, Rodrigo ; Weisenhorn, Pamela B. ; Windham-Myers, Lisamarie

Between the land and ocean, diverse coastal ecosystems transform, store, and transport material. Across these interfaces, the dynamic exchange of energy and matter is driven by hydrological and hydrodynamic processes such as river and groundwater discharge, tides, waves, and storms. These dynamics regulate ecosystem functions and Earth’s climate, yet global models lack representation of coastal processes and related feedbacks, impeding their predictions of coastal and global responses to change. Here, we assess existing coastal monitoring networks and regional models, existing challenges in these efforts, and recommend a path towards development of global models that more robustly reflect the coastal interface.
Wetland-estuarine-shelf interactions in the Plum Island Sound and Merrimack River in the Massachusetts coast

(American Geophysical Union, 2010-10-16) Zhao, Liuzhi ; Chen, Changsheng ; Vallino, Joseph J. ; Hopkinson, Charles S. ; Beardsley, Robert C. ; Lin, Huichan ; Lerczak, James A.

Wetland-estuarine-shelf interaction processes in the Plum Island Sound and Merrimack River system in the Massachusetts coast are examined using the high-resolution unstructured grid, finite volume, primitive equations, coastal ocean model. The computational domain covers the estuarine and entire intertidal area with a horizontal resolution of 10–200 m. Driven by five tidal constituents forcing at the open boundary on the inner shelf of the eastern coast of the Gulf of Maine, the model has successfully simulated the 3-D flooding/drying process, temporal variability, and spatial distribution of salinity as well as the water exchange flux through the water passage between the Plum Island Sound and Merrimack River. The model predicts a complex recirculation loop around the Merrimack River, shelf, and Plum Island Sound. During the ebb tide, salt water in the Plum Island Sound is injected into the Merrimack River, while during flood tide, a significant amount of the freshwater in the Merrimack River is forced into Plum Island Sound. This water exchange varies with the magnitude of freshwater discharge and wind conditions, with a maximum contribution of ∼30%–40% variability in salinity over tidal cycles in the mouth of the Merrimack River. Nonlinear tidal rectification results in a complex clockwise residual recirculation loop around the Merrimack River, shelf, and Plum Island Sound. The net water flux from Plum Island Sound to the Merrimack River varies with the interaction between tide, river discharge, and wind forcing. This interaction, in turn, affects the salt transport from this system to the shelf. Since the resulting water transport into the shelf significantly varies with the variability of the wind, models that fail to resolve this complex estuarine and shelf system could either overestimate or underestimate the salt content over the shelf.
Forecasting effects of sea-level rise and windstorms on coastal and inland ecosystems

(Ecological Society of America, 2008-06) Hopkinson, Charles S. ; Lugo, Ariel E. ; Alber, Merryl ; Covich, Alan P. ; Van Bloem, Skip J.

We identify a continental-scale network of sites to evaluate how two aspects of climate change – sea-level rise and intensification of windstorms – will influence the structure, function, and capacity of coastal and inland forest ecosystems to deliver ecosystem services (eg carbon sequestration, storm protection, pollution control, habitat support, food). The network consists of coastal wetland and inland forest sites across the US and is representative of continental-level gradients of precipitation, temperature, vegetation, frequency of occurrence of major windstorms, value of insured properties, tidal range, watershed land use, and sediment availability. The network would provide real-time measurements of the characteristics of sea-level rise and windstorm events and would allow an assessment of the responses of wetlands, streams, and inland forests at spatial and temporal scales associated with sustainability of ecosystem services. We illustrate the potential of this approach with examples of hypotheses that could be tested across the network.
Dynamics of N removal over annual time periods in a suburban river network

(American Geophysical Union, 2008-09-23) Wollheim, Wilfred M. ; Peterson, Bruce J. ; Thomas, Suzanne M. ; Hopkinson, Charles S. ; Vorosmarty, Charles J.

River systems are dynamic, highly connected water transfer networks that integrate a wide range of physical and biological processes. We used a river network nitrogen (N) removal model with daily temporal resolution to evaluate how elevated N inputs, saturation of the denitrification and total nitrate removal processes, and hydrologic conditions interact to determine the amount, timing and distribution of N removal in the fifth-order river network of a suburban 400 km2 basin. Denitrification parameters were based on results from whole reach 15NO3 tracer additions. The model predicted that between 15 and 33% of dissolved inorganic nitrogen (DIN) inputs were denitrified annually by the river system. Removal approached 100% during low flow periods, even with the relatively low and saturating uptake velocities typical of surface water denitrification. Annual removal percentages were moderate because most N inputs occurred during high flow periods when hydraulic conditions and temperatures are less favorable for removal by channel processes. Nevertheless, the percentage of annual removal occurring during above average flow periods was similar to that during low flow periods. Predicted river network removal proportions are most sensitive to loading rates, spatial heterogeneity of inputs, and the form of the removal process equation during typical base flow conditions. However, comparison with observations indicates that removal by the river network is higher than predicted by the model at moderately high flows, suggesting additional removal processes are important at these times. Further increases in N input to the network will lead to disproportionate increases in N exports due to the limits imposed by process saturation.
Surface and hyporheic transient storage dynamics throughout a coastal stream network

(American Geophysical Union, 2010-06-22) Briggs, Martin A. ; Gooseff, Michael N. ; Peterson, Bruce J. ; Morkeski, Kate ; Wollheim, Wilfred M. ; Hopkinson, Charles S.

Transient storage of stream water and associated solutes is expected to vary along stream networks in response to related changes in stream hydraulic conditions and morphologic gradients. These spatial changes are relevant to a wide variety of processes (e.g., biogeochemical cycling), yet data regarding these dynamics are limited and almost exclusively confined to the general storage terms of transient storage models with a single-storage zone (1-SZ). We used a transient storage model with two-storage zones (2-SZ) to simulate field data from conservative solute injections conducted in a coastal stream network in Massachusetts to separately quantify surface transient storage (STS) and hyporheic transient storage (HTS). Solute tracer additions were performed at basin-wide, low-flow conditions, and results were compared with respect to stream size. Strong positive relationships with reach contributing area indicated that the size of the main channel and the size and residence time in surface and hyporheic storage zones all increased from small to large streams. Conversely, longitudinal dispersion and the storage zone exchange coefficients had no consistent trends downstream. The influence of storage exchange on median transport time ( $F_{MED}^{200}$) was consistently large for STS and negligible for HTS. When compared to 1-SZ model estimates, we found that the general 1-SZ model storage terms did not consistently describe either STS or HTS exchange. Overall our results indicated that many zone-specific (STS and HTS) storage dynamics were sensitive to the combination of hydraulic and morphologic gradients along the stream network and followed positive trends with stream size.
Residence time distributions in surface transient storage zones in streams : estimation via signal deconvolution

(American Geophysical Union, 2011-05-11) Gooseff, Michael N. ; Benson, David A. ; Briggs, Martin A. ; Weaver, Mitchell ; Wollheim, Wilfred M. ; Peterson, Bruce J. ; Hopkinson, Charles S.

Little is known about the impact of surface transient storage (STS) zones on reach-scale transport and the fate of dissolved nutrients in streams. Exchange with these locations may influence the rates of nutrient cycling often observed in whole-stream tracer experiments, particularly because they are sites of organic matter collection and lower flow velocities than those observed in the thalweg. We performed a conservative stream tracer experiment (slug of dissolved NaCl) in the Ipswich River in northeastern Massachusetts and collected solute tracer data both in the thalweg and adjacent STS zones at three locations in a fifth-order reach. Tracer time series observed in STS zones are an aggregate of residence time distributions (RTDs) of the upstream transport to that point (RTDTHAL) and that of the temporary storage within these zones (RTDSTS). Here we demonstrate the separation of these two RTDs to determine the RTDSTS specifically. Total residence times for these individual STS zones range from 4.5 to 7.5 h, suggesting that these zones have the potential to host important biogeochemical transformations in stream systems. All of the RTDSTS show substantial deviations from the ideal prescribed by the two-state (mobile/immobile) mass transfer equations. The deviations indicate a model mismatch and that parameter estimation based on the mass transfer equations will yield misleading values.
The changing landscape : ecosystem responses to urbanization and pollution across climatic and societal gradients

(Ecological Society of America, 2008-06) Grimm, Nancy B. ; Foster, David R. ; Groffman, Peter M. ; Grove, J. Morgan ; Hopkinson, Charles S. ; Nadelhoffer, Knute J. ; Pataki, Diane E. ; Peters, Debra P. C.

Urbanization, an important driver of climate change and pollution, alters both biotic and abiotic ecosystem properties within, surrounding, and even at great distances from urban areas. As a result, research challenges and environmental problems must be tackled at local, regional, and global scales. Ecosystem responses to land change are complex and interacting, occurring on all spatial and temporal scales as a consequence of connectivity of resources, energy, and information among social, physical, and biological systems. We propose six hypotheses about local to continental effects of urbanization and pollution, and an operational research approach to test them. This approach focuses on analysis of “megapolitan” areas that have emerged across North America, but also includes diverse wildland-to-urban gradients and spatially continuous coverage of land change. Concerted and coordinated monitoring of land change and accompanying ecosystem responses, coupled with simulation models, will permit robust forecasts of how land change and human settlement patterns will alter ecosystem services and resource utilization across the North American continent. This, in turn, can be applied globally.
Constraining marsh carbon budgets using long‐term C burial and contemporary atmospheric CO2 fluxes

(John Wiley & Sons, 2018-02-06) Forbrich, Inke ; Giblin, Anne E. ; Hopkinson, Charles S.

Salt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here we assess how climatic variations affect marsh‐atmosphere exchange of carbon dioxide in the short term and compare it to long‐term burial rates based on radiometric dating. The 5 years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from −104 to −233 g C m−2 a−1 with a mean of −179 ± 32 g C m−2 a−1. Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110 ± 13 g C m−2 a−1). The difference between these estimates was significant and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes and highlights the importance of investigating lateral C fluxes in future studies.
Susceptibility of salt marshes to nutrient enrichment and predator removal

( 2006-03-15) Deegan, Linda A. ; Bowen, Jennifer L. ; Drake, Deanne C. ; Fleeger, John W. ; Friedrichs, Carl T. ; Galvan, Kari A. ; Hobbie, John E. ; Hopkinson, Charles S. ; Johnson, J. Michael ; Johnson, David S. ; LeMay, Lynsey E. ; Miller, Erin ; Peterson, Bruce J. ; Picard, Christian ; Sheldon, Sallie ; Sutherland, Michael ; Vallino, Joseph J. ; Warren, R. Scott

The sustainability of coastal ecosystems in the face of widespread environmental change is an issue of pressing concern throughout the world (Emeis et al. 2001). Coastal ecosystems form a dynamic interface between terrestrial and oceanic systems and are one of the most productive ecosystems in the world. Coastal systems probably serve more human uses than any other ecosystem and they have always been valued for their rich bounty of fish and shellfish. Coastal areas are also the sites of the nation’s and the world’s most intense commercial activity and population growth; worldwide, approximately 75% of the human population now lives in coastal regions (Emeis et al. 2001). Over the past three decades nutrient enrichment of coastal and estuarine waters has become the premier issue for both scientists and managers (National Research Council 2000). Our understanding of coastal eutrophication has been developed principally through monitoring of estuaries, with a focus on pelagic or subtidal habitats (National Research Council 2000, Cloern 2001). Because estuarine systems are usually nitrogen limited, NO3- is the most common nutrient responsible for cultural nutrient enrichment (Cloern 2001). Increased nitrogen delivery to pelagic habitats of estuaries produces the classic response of ecosystems to stress (altered primary producers and nutrient cycles and loss of secondary producer species and production; Nixon 1995, Rapport and Whitford 1999, Deegan et al. 2002). Salt marsh ecosystems have been thought of as not susceptible to nitrogen over-loading because early studies found added nitrogen increased marsh grass production (primarily Spartina spp., cordgrass) and concluded that salt marshes can adsorb excess nutrients in plants and salt marsh plant-derived organic matter as peat (Verhoeven et al. 2006). Detritus from Spartina is important in food webs (Deegan et al. 2000) and in creating peat that forms the physical structure of the marsh platform (Freidrichs and Perry 2001). However, the accumulation of peat and inputs of sediments and loss of peat through decomposition and sediment through erosion may be altered under high nutrient regimes and threaten the long-term stability of marsh systems. Nitrogen addition may lead to either net gain or loss of the marsh depending on the balance between increased marsh plant production and increased decomposition. Absolute change in marsh surface elevation is determined by marsh plant species composition, production and allocation to above- and belowground biomass, microbial decomposition, sedimentation, erosion and compaction (Friedrichs and Perry 2001). Levine et al. (1998) suggested that competitive dynamics among plants might be affected by nutrient enrichment, potentially altering relative abundance patterns favoring species with less belowground storage and thus lowering rates of peat formation. When combined with the observation that nutrient additions may also stimulate microbial respiration and decomposition (Morris and Bradley 1999), the net effect on the salt marsh under conditions of chronic nitrogen loading is a critical unknown. Although most research treats nutrient enrichment as a stand-alone stress, it never occurs in isolation from other perturbations. The effect of nutrient loading on species composition (both plants and animals) and the resultant structure and function of wetlands has been largely ignored when considering their ability to adsorb nutrients (Verhoeven et al. 2006). Recent studies suggest the response of estuaries to stress may depend on animal species composition (Silliman et al. 2005). Animal species composition may alter the balance between marsh gain and loss as animals may increase or decrease primary production, decomposition or N recycling (Pennings and Bertness 2001). Failure to understand interactions between nutrient loading and change in species composition may lead to underestimating the impacts of these stresses. The 'bottom up or top down' theory originated from the observation that nutrient availability (bottom up)sets the quantity of primary productivity, while other studies have shown that species composition (top down), particularly of top consumers, has a marked and cascading effect on ecosystems, including controlling species composition and nutrient cycling (Matson and Price 1992, Pace et al. 1999). Most examples of trophic cascades are in aquatic ecosystems with fairly simple, algal grazing pelagic food webs (Strong 1992). The rarity of trophic cascades in terrestrial systems has been attributed to the importance of detrital food webs (Polis 1999). Detritus-based aquatic ecosystems, such as salt marshes, bogs, and swamps, have classically been considered bottom-up or physically controlled ecosystems. Recent experiments, however, suggest that salt marshes may exhibit top-down control at several trophic levels (Silliman and Zeiman. 2001, Silliman and Bertness 2002, Quiñones-Rivera and Fleeger 2005). One abundant, ubiquitous predator, a small (<10 cm total length) killifish (Fundulus heteroclitus, mummichog) has been suggested to control benthic algal through a trophic cascade because they prey on the invertebrates that graze on the benthic algae (Kneib 1997, Sarda et al. 1998). In late summer, killifish are capable of consuming 3-10 times the creek meiofauna production and meiofauna in the absence of predators appear capable of grazing over 60% of the microalgal community per day (Carman et al. 1997). Strong top-down control by grazers is considered a moderating influence on the negative effects of elevated nutrients on algae (Worm et al. 2000). Small-scale nutrient additions and predator community exclusion experiments have demonstrated bottom-up and top-down control of macroinfauna in mudflats associated with salt marsh creeks (Posey et al. 1999, Posey et al. 2002). Together, these observations suggest mummichogs are at the top of a trophic cascade that controls benthic algae (Sarda et al. 1998). Mummichogs are also omnivorous and ingest algae, bulk detritus and the attached microbial community (D’Avanzo and Valiela 1990). As a result, marsh decomposition rates may be limited by top-down controls through trophic pathways or by release from competition with algae for nutrients. Whole-ecosystem experiments have shown that responses to stress are often not predictable from studies of the individual components (Schindler 1998). Developing the information needed to predict the interacting impacts of nutrient loading and species composition change requires experiments with realistic alterations carried out at scales of space and time that include the complexities of real ecosystems. Whole ecosystem manipulation experiments have been used effectively in other ecosystems (Bormann and Likens 1979, Carpenter et al. 1995), but they are rare in coastal research. Experiments in salt marshes have traditionally been less than a few m2. Our understanding of the response of salt marsh plants to nutrient enrichment is from small (<10 m2), plot-level additions where uniform levels of dry inorganic fertilizer (20 to > 1000 g N m-2 y-1) are sprinkled on the marsh surface at low tide. Dry fertilizer additions were usually made every two weeks or monthly and the duration of elevated nutrient levels after these additions was usually not determined. Tidal water is the primary vector for N delivery to coastal marshes, suggesting that dry fertilizer addition to the marsh surface may not be the best basis for determining if Spartina production responds to nutrient enrichment of tidal waters. Similarly, our understanding of top-down controls in salt marshes also relies on small (1 - 4 m2) exclusion experiments that use cages to isolate communities from top consumers. While the design of these cage experiments has improved, there are some remaining drawbacks. For example, it is impossible to selectively exclude single species using cages, and recruitment or size-selective movement into or out of the cages may obscure interpretations. In addition, while these small-scale experiments provide insight into controls on isolated ecosystem processes, they do not allow for interaction among different parts of the ecosystem which may buffer or alter the impacts and are not appropriate for determining the effects of populations of larger more motile animals on whole-ecosystems or the effects of ecosystem changes on populations. For example, interactions may be caused when a motile species alters its distribution among the habitats available to it because of an experimental treatment. Small-scale experiments generally do not allow such events to happen. Complex feedbacks among physical and biological processes can alter accumulation rates and affect marsh elevation relative to sea level rise making extrapolation of small plot level experiments to whole marsh ecosystems problematic. We are conducting an ecosystem-scale, multi-year field experiment including both nutrient and biotic manipulations to coastal salt marsh ecosystems. We are testing, for the first time at the ecosystem level, the hypothesis that nutrient enrichment and species composition change have interactive effects across multiple levels of biological organization and a range of biogeochemical processes. We altered whole salt marsh creek watersheds (~60,000 m2 of saltmarsh) by addition of nutrients (15x ambient) in flooding waters and by a 60% reduction of a key fish species, the mummichog. Small marsh creek watersheds provide an ideal experimental setting because they have the spatial complexity, species composition and processes characteristic of the larger salt marsh ecosystem, which are often hundreds of thousands of m2. Manipulating entire salt marsh creeksheds allowed us to examine effects on large motile animals and the interactive effects of motile species changes on ecosystem processes without cage artifacts. Because our manipulations were done on whole-marsh ecosystems, we are able to evaluate the integrated and interactive effects on all habitats (e.g., water column, tidal creeks and marsh) and on populations. These experiments are similar in many respects to the small watershed experiments carried out in forested catchments. Our nutrient enrichment is novel compared to past studies in two important ways. We added nutrients (N and P) directly to the flooding tidal creek waters to mimic the way in which anthropogenic nutrients reach marsh ecosystems. All previous experimental salt marsh nutrient enrichment studies used a dose-response design with spatially uniform dry fertilizer loading on small plots (<10 m2). Nutrients carried in water will interact and reach parts of the ecosystem differently than dry fertilizer. Our enrichment method also creates a spatial gradient of nutrient loading across the landscape that is proportional to the frequency and depth of inundation in the marsh. Spatial gradients in loading within an ecosystem are typical in real world situations in many terrestrial and aquatic ecosystems. Because of our enrichment method, at any location in the ecosystem, nutrient load will be a function of the nutrient concentration in the water, the frequency and depth of tidal flooding and the reduction of nutrients from the flooding waters by other parts of the ecosystem. Uniform loading misses important aspects of the spatial complexity of ecosystem exposure and response. This work is organized around two questions that are central to understanding the long-term fate of coastal marshes: 1. Does chronic nutrient enrichment via flooding water increase primary production more than it stimulates microbial decomposition? 2. Do top-down controls change the response of the salt marsh ecosystem to nutrient enrichment? Here we present findings on the first 2 years of these experiments including 1) water chemistry, 2) standing stocks and species composition of benthic microalgae, 3) microbial production, 4) species composition and ecophysiology of macrophytes, 5) invertebrates, and 6) nekton. Because even highly eutrophic waters result in nutrient loading that is an order of magnitude less than most plot level experiments, we expected little stimulation of salt marsh vascular plant growth. However, moderate levels of nutrient enrichment in the water column were expected to increase benthic algal biomass and to stimulate bacterial activity and detrital decomposition throughout the ecosystem because of direct uptake of nitrogen from the water column and availability of more high quality organic matter from increased algal production. We predicted nutrient enrichment would increase invertebrate production because of an increase of high quality microalgal and microbial production at the base of the food web. Finally, we predicted that fish reduction would reduce predation on benthic invertebrates resulting in increased abundance of benthic invertebrates that would graze down the benthic algae.
Relationship between river size and nutrient removal

(American Geophysical Union, 2006-03-30) Wollheim, Wilfred M. ; Vorosmarty, Charles J. ; Peterson, Bruce J. ; Seitzinger, Sybil P. ; Hopkinson, Charles S.

We present a conceptual approach for evaluating the biological and hydrological controls of nutrient removal in different sized rivers within an entire river network. We emphasize a per unit area biological parameter, the nutrient uptake velocity (νf), which is mathematically independent of river size in benthic dominated systems. Standardization of biological parameters from previous river network models to νf reveals the nature of river size dependant biological activity in these models. We explore how geomorphic, hydraulic, and biological factors control the distribution of nutrient removal in an idealized river network, finding that larger rivers within a basin potentially exert considerable influence over nutrient exports.
Linking ecology and economics for ecosystem management

(American Institute of Biological Sciences, 2006-02) Farber, Stephen ; Costanza, Robert ; Childers, Daniel L. ; Erickson, Jon ; Gross, Katherine ; Grove, J. Morgan ; Hopkinson, Charles S. ; Kahn, James ; Pincetl, Stephanie ; Troy, Austin ; Warren, Paige ; Wilson, Matthew

This article outlines an approach, based on ecosystem services, for assessing the trade-offs inherent in managing humans embedded in ecological systems. Evaluating these trade-offs requires an understanding of the biophysical magnitudes of the changes in ecosystem services that result from human actions, and of the impact of these changes on human welfare. We summarize the state of the art of ecosystem services–based management and the information needs for applying it. Three case studies of Long Term Ecological Research (LTER) sites—coastal, urban, and agricultural—illustrate the usefulness, information needs, quantification possibilities, and methods for this approach. One example of the application of this approach, with rigorously established service changes and valuations taken from the literature, is used to illustrate the potential for full economic valuation of several agricultural landscape management options, including managing for water quality, biodiversity, and crop productivity.
Microbial biogeography along an estuarine salinity gradient : combined influences of bacterial growth and residence time

(American Society for Microbiology, 2004-03) Crump, Byron C. ; Hopkinson, Charles S. ; Sogin, Mitchell L. ; Hobbie, John E.

Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([14C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.
Response of benthic metabolism and nutrient cycling to reductions in wastewater loading to Boston Harbor, USA

(Elsevier, 2014-10-02) Tucker, Jane ; Giblin, Anne E. ; Hopkinson, Charles S. ; Kelsey, Samuel W. ; Howes, Brian L.

We describe the long-term response of benthic metabolism in depositional sediments of Boston Harbor, MA, to large reductions in organic matter and nutrient loading. Although Boston Harbor received very high loadings of nutrients and solids it differs from many eutrophic estuaries in that severe hypoxia was prevented by strong tidal flushing. Our study was conducted for 9 years during which a series of improvements to sewage treatment were implemented, followed by 10 years after the culminating step in the clean-up, which was to divert all wastewater effluent offshore. Counter to expectations, sediment oxygen demand and nutrient effluxes initially increased at some stations, reaching some of the highest rates recorded in the literature, and were spatially and temporally quite variable. Early increases were attributed to macrofaunal effects, as sediments at some sites were rapidly colonized by tube-building amphipods, Ampelisca spp., which dominated a dense macrofaunal mat community. As reductions in loading progressed, however, mean rates in oxygen uptake and release of ammonium, nitrate, and phosphate all decreased. At the point of outfall diversion, rates and variability had already decreased substantially. By the end of the study, average oxygen uptake had decreased from 74 to 41 mmol m−2 d−1 and spatial and temporal variability had decreased. Similarly, nutrient fluxes were less than half the rates measured at the start of the project and also less variable. Other evidence of improved conditions included a decrease in the carbon content of sediments at most stations and higher Eh values at all stations, illustrating less reducing conditions. Denitrification also showed an overall decrease from the beginning to the end of the 19-year study, but was highest during the intermediate phases of the cleanup, reaching 9 mmol N m−2 d−1. At the end of the study denitrification averaged for all sites was 2.2 mmol N m−2 d−1, but when compared to current loadings, had become a more important overall sink for N within the harbor. Few long-term examinations of the responses of sediment biogeochemistry to reductions in nutrient and organic matter loading have been reported. Our findings demonstrate that benthic fluxes may respond to reductions in loading in complex ways, and sediments need not represent a long-term legacy that would impede ecosystems recovery.
Effect of historical changes in land use and climate on the water budget of an urbanizing watershed

(American Geophysical Union, 2006-03-25) Claessens, Luc ; Hopkinson, Charles S. ; Rastetter, Edward B. ; Vallino, Joseph J.

We assessed the effects of historical (1931-1998) changes in both land-use and climate on the water budget of a rapidly urbanizing watershed, Ipswich River basin (IRB), in northeastern Massachusetts. Water diversions and extremely low flow during summer are major issues in the IRB. Our study centers on a detailed analysis of diversions and a combined empirical/modeling treatment of evapotranspiration (ET) response to changes in climate and land-use. A detailed accounting of diversions showed that net diversions increased due to increases in water withdrawals (primarily ground water pumping) and export of sewage. Net diversions constitute a major component of runoff (20% of streamflow). Using a combination of empirical analysis and physically based modeling we related an increase in precipitation (2.7 mm/yr) and changes in other climate variables to an increase in ET (1.7 mm/yr). Simulations with a physically based water-balance model showed that the increase in ET could be attributed entirely to a change in climate, while the effect of land-use change was negligible. The land-use change effect was different from ET and runoff trends commonly associated with urbanization. We generalized these and other findings to predict future streamflow using climate change scenarios. Our study could serve as a framework for studying suburban watersheds, being the first study of a suburban watershed that addresses long-term effects of changes in both land-use and climate, and accounts for diversions and other unique aspects of suburban hydrology.
Separation of river network–scale nitrogen removal among the main channel and two transient storage compartments

(American Geophysical Union, 2011-08-30) Stewart, Robert J. ; Wollheim, Wilfred M. ; Gooseff, Michael N. ; Briggs, Martin A. ; Jacobs, Jennifer M. ; Peterson, Bruce J. ; Hopkinson, Charles S.

Transient storage (TS) zones are important areas of dissolved inorganic nitrogen (DIN) processing in rivers. We assessed sensitivities regarding the relative impact that the main channel (MC), surface TS (STS), and hyporheic TS (HTS) have on network denitrification using a model applied to the Ipswich River in Massachusetts, United States. STS and HTS connectivity and size were parameterized using the results of in situ solute tracer studies in first- through fifth-order reaches. DIN removal was simulated in all compartments for every river grid cell using reactivity derived from Lotic Intersite Nitrogen Experiment (LINX2) studies, hydraulic characteristics, and simulated discharge. Model results suggest that although MC-to-STS connectivity is greater than MC-to-HTS connectivity at the reach scale, at basin scales, there is a high probability of water entering the HTS at some point along its flow path through the river network. Assuming our best empirical estimates of hydraulic parameters and reactivity, the MC, HTS, and STS removed approximately 38%, 21%, and 14% of total DIN inputs during a typical base flow period, respectively. There is considerable uncertainty in many of the parameters, particularly the estimates of reaction rates in the different compartments. Using sensitivity analyses, we found that the size of TS is more important for DIN removal processes than its connectivity with the MC when reactivity is low to moderate, whereas TS connectivity is more important when reaction rates are rapid. Our work suggests a network perspective is needed to understand how connectivity, residence times, and reactivity interact to influence DIN processing in hierarchical river systems.
Salt marsh ecosystem biogeochemical responses to nutrient enrichment : a paired 15N tracer study

(Ecological Society of America, 2009-09) Drake, Deanne C. ; Peterson, Bruce J. ; Galvan, Kari A. ; Deegan, Linda A. ; Hopkinson, Charles S. ; Johnson, J. Michael ; Koop-Jakobsen, Ketil ; LeMay, Lynsey E. ; Picard, Christian

We compared processing and fate of dissolved NO3− in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of 1–4 μmol NO3−/L and the other receiving experimentally fertilized flood tides containing 70–100 μmol NO3−/L. We conducted simultaneous 15NO3− (isotope) tracer additions from 23 to 28 July 2005 in the reference (8.4 ha) and fertilized (12.4 ha) systems to compare N dynamics and fate. Two full tidal cycles were intensively studied during the paired tracer additions. Resulting mass balances showed that essentially 100% (0.48–0.61 mol NO3-N·ha−1·h−1) of incoming NO3− was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO3− concentrations were 1.3–1.8 μmol/L. In contrast, only 50–60% of incoming NO3− was processed in the fertilized system when NO3− concentrations were 84–96 μmol/L; the remainder was exported in ebb tidewater. Gross NO3− processing was 40 times higher in the fertilized system at 19.34–24.67 mol NO3-N·ha−1·h−1. Dissimilatory nitrate reduction to ammonium was evident in both systems during the first 48 h of the tracer additions but <1% of incoming 15NO3− was exported as 15NH4+. Nitrification rates calculated by 15NO3− dilution were 6.05 and 4.46 mol·ha−1·h−1 in the fertilized system but could not be accurately calculated in the reference system due to rapid (<4 h) NO3− turnover. Over the five-day paired tracer addition, sediments sequestered a small fraction of incoming NO3−, although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 mol·ha−1·d−1, respectively. Macrophyte NO3− uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO3− by plants was lower in the reference system (1.75 mol NO3−·ha−1·d−1) than the fertilized system (10 mol NO3−·ha−1·d−1). Nitrogen processing efficiency decreased with NO3− load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

Hopkinson Charles S.

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