Deegan Linda A.

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Linda A.

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  • Article
    Discontinuities in soil strength contribute to destabilization of nutrient‐enriched creeks
    (Ecological Society of America, 2018-08-20) Wigand, Cathleen ; Watson, Elizabeth ; Martin, Rose ; Johnson, David S. ; Warren, R. Scott ; Hanson, Alana ; Davey, Earl ; Johnson, Roxanne ; Deegan, Linda A.
    In a whole‐ecosystem, nutrient addition experiment in the Plum Island Sound Estuary (Massachusetts), we tested the effects of nitrogen enrichment on the carbon and nitrogen contents, respiration, and strength of marsh soils. We measured soil shear strength within and across vegetation zones. We found significantly higher soil percent organic matter, carbon, and nitrogen in the long‐term enriched marshes and higher soil respiration rates with longer duration of enrichment. The soil strength was similar in magnitude across depths and vegetation zones in the reference creeks, but showed signs of significant nutrient‐mediated alteration in enriched creeks where shear strength at rooting depths of the low marsh–high marsh interface zone was significantly lower than at the sub‐rooting depths or in the creek bank vegetation zone. To more closely examine the soil strength of the rooting (10–30 cm) and sub‐rooting (40–60 cm) depths in the interface and creek bank vegetation zones, we calculated a vertical shear strength differential between these depths. We found significantly lower differentials in shear strength (rooting depth < sub‐rooting depths) in the enriched creeks and in the interface zones. The discontinuities in the vertical and horizontal shear strength across the enriched marshes may contribute to observed fracturing and slumping occurring in the marsh systems. Tide gauge data also showed a pattern of rapid sea level rise for the period of the study, and changes in plant distribution patterns were indicative of increased flooding. Longer exposure times to nutrient‐enriched waters and increased hydraulic energy associated with sea level rise may exacerbate creek bank sloughing. Additional research is needed, however, to better understand the interactions of nutrient enrichment and sea level rise on soil shear strength and stability of tidal salt marshes.
  • Preprint
    Turnover rates of nitrogen stable isotopes in the salt marsh mummichog, Fundulus heteroclitus, following a laboratory diet switch
    ( 2005-09-19) Logan, John ; Haas, Heather ; Deegan, Linda A. ; Gaines, Emily F.
    Nitrogen stable isotopes are frequently used in ecological studies to estimate trophic position and determine movement patterns. Knowledge of tissue-specific turnover and nitrogen discrimination for the study organisms is important for accurate interpretation of isotopic data. We measured δ15 N turnover in liver and muscle tissue in juvenile mummichogs, Fundulus heteroclitus, following a laboratory diet switch. Liver tissue turned over significantly faster than muscle tissue suggesting the potential for a multiple tissue stable isotope approach to study movement and trophic position over different time scales; metabolism contributed significantly to isotopic turnover for both liver and muscle. Nitrogen diet-tissue discrimination was estimated at between 0.0 and 1.2‰ for liver and –1.0 and 0.2‰ for muscle. This is the first experiment to demonstrate a significant variation in δ15 N turnover between liver and muscle tissues in a fish species.
  • Article
    Nutrient enrichment induces dormancy and decreases diversity of active bacteria in salt marsh sediments
    (Nature Publishing Group, 2016-09-26) Kearns, Patrick J. ; Angell, John H. ; Howard, Evan M. ; Deegan, Linda A. ; Stanley, Rachel H. R. ; Bowen, Jennifer L.
    Microorganisms control key biogeochemical pathways, thus changes in microbial diversity, community structure and activity can affect ecosystem response to environmental drivers. Understanding factors that control the proportion of active microbes in the environment and how they vary when perturbed is critical to anticipating ecosystem response to global change. Increasing supplies of anthropogenic nitrogen to ecosystems globally makes it imperative that we understand how nutrient supply alters active microbial communities. Here we show that nitrogen additions to salt marshes cause a shift in the active microbial community despite no change in the total community. The active community shift causes the proportion of dormant microbial taxa to double, from 45 to 90%, and induces diversity loss in the active portion of the community. Our results suggest that perturbations to salt marshes can drastically alter active microbial communities, however these communities may remain resilient by protecting total diversity through increased dormancy.
  • Preprint
    Effects of regular salt marsh haying on marsh plants, algae, invertebrates and birds at Plum Island Sound, Massachusetts
    ( 2008-10) Buchsbaum, Robert N. ; Deegan, Linda A. ; Horowitz, Julie ; Garritt, Robert H. ; Giblin, Anne E. ; Ludlam, John P. ; Shull, David H.
    The haying of salt marshes, a traditional activity since colonial times in New England, still occurs in about 400 ha of marsh in the Plum Island Sound estuary in northeastern Massachusetts. We took advantage of this haying activity to investigate how the periodic large-scale removal of aboveground biomass affects a number of marsh processes. Hayed marshes were no different from adjacent reference marshes in plant species density (species per area) and end-of-year aboveground biomass, but did differ in vegetation composition. Spartina patens was more abundant in hayed marshes than S. alterniflora, and the reverse was true in reference marshes. The differences in relative covers of these plant species were not associated with any differences between hayed and reference marshes in the elevations of the marsh platform. Instead it suggested that S. patens was more tolerant of haying than S. alterniflora. S. patens had higher stem densities in hayed marshes than it did in reference marshes, suggesting that periodic cutting stimulated tillering of this species. Although we predicted that haying would stimulate benthic chlorophyll production by opening up the canopy, we found differences to be inconsistent, possibly due to the relatively rapid regrowth of S. patens and to grazing by invertebrates on the algae. The pulmonate snail, Melampus bidendatus was depleted in its δ13C content in the hayed marsh compared to the reference, suggesting a diet shift to benthic algae in hayed marshes. The stable isotope ratios of a number of other consumer species were not affected by haying activity. Migratory shorebirds cue in to recently hayed marshes and may contribute to short term declines in some invertebrate species, however the number of taxa per unit area of marsh surface invertebrates and their overall abundances were unaffected by haying over the long term. Haying had no impact on nutrient concentrations in creeks just downstream from hayed plots, but the sediments of hayed marshes were lower in total N and P compared to references. In sum, haying appeared to affect plant species composition but had only short-term affects on consumer organisms. This contrasts with many grassland ecosystems, where an intermediate level of disturbance, such as by grazing, increases species diversity and may stimulate productivity. From a management perspective, periodic mowing could be a way to maintain S. patens habitats and the suite of species with which they are associated.
  • Preprint
    Runoff sources and land cover change in the Amazon : an end-member mixing analysis from small watersheds
    ( 2011-03) Neill, Christopher ; Chaves, Joaquin E. ; Biggs, Trent ; Deegan, Linda A. ; Elsenbeer, Helmut ; Figueiredo, Ricardo O. ; Germer, Sonja ; Johnson, Mark S. ; Lehmann, Johannes ; Markewitz, Daniel ; Piccolo, Marisa C.
    The flowpaths by which water moves from watersheds to streams has important consequences for the runoff dynamics and biogeochemistry of surface waters in the Amazon Basin. The clearing of Amazon forest to cattle pasture has the potential to change runoff sources to streams by shifting runoff to more surficial flow pathways. We applied end member mixing analysis (EMMA) to ten small watersheds throughout the Amazon in which solute composition of streamwater and groundwater, overland flow, soil solution, throughfall and rainwater were measured, largely as part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia. We found a range in the extent to which streamwater samples fell within the mixing space determined by potential flowpath end members, suggesting that some water sources to streams were not sampled. The contribution of overland flow as a source of stream flow was greater in pasture watersheds than in forest watersheds of comparable size. Increases in overland flow contribution to pasture streams ranged in some cases from 0% in forest to 27 to 28% in pasture and were broadly consistent with results from hydrometric sampling of Amazon forest and pasture watersheds that indicate 17- to 18-fold increase in the overland flow contribution to stream flow in pastures. In forest, overland flow was an important contribution to stream flow (45 to 57%) in ephemeral streams where flows were dominated by stormflow. Overland flow contribution to stream flow decreased in importance with increasing watershed area, from 21 to 57% in forest and 60 to 89% in pasture watersheds <10 ha to 0% in forest and 27 to 28% in pastures in watersheds >100 ha. Soil solution contributions to stream flow were similar across watershed area and groundwater inputs generally increased in proportion to decreases in overland flow. Application of EMMA across multiple watersheds indicated patterns across gradients of stream size and land cover that were consistent with patterns determined by detailed hydrometric sampling.
  • Article
    Seasonal use of a New England estuary by foraging contingents of migratory striped bass
    (American Fisheries Society, 2009-12-10) Pautzke, Sarah M. ; Mather, Martha E. ; Finn, John T. ; Deegan, Linda A. ; Muth, Robert M.
    Using acoustic telemetry on migratory striped bass Morone saxatilis in Plum Island Estuary (PIE), Massachusetts, we found that striped bass (335–634 mm total length) tagged in the spring and summer of 2005 (n = 14) and 2006 (n = 46) stayed in the estuary for an average of 66.0 d in 2005 and 72.2 d in 2006. Striped bass spent the most time in two specific reaches: middle Plum Island Sound and lower Rowley River. In both years, three different use-groups of striped bass were observed in PIE. Short-term visitors (n = 24) stayed in the estuary only briefly (range = 5–20 d). Two groups of seasonal residents stayed for more than 30 d, either in the Rowley River (n = 14) or in Plum Island Sound (n = 22). Within PIE, the two seasonal-resident use-groups may be foraging contingents that learn how to feed efficiently in specific parts of the estuary. These distinct within-estuary use patterns could have different implications for striped bass condition and prey impact.
  • Article
    Microbial associations with macrobiota in coastal ecosystems : patterns and implications for nitrogen cycling
    (John Wiley & Sons, 2016-05-02) Moulton, Orissa M. ; Altabet, Mark A. ; Beman, J. Michael ; Deegan, Linda A. ; Lloret, Javier ; Lyons, Meaghan K. ; Nelson, James A. ; Pfister, Catherine
    In addition to their important effects on nitrogen (N) cycling via excretion and assimilation (by macrofauna and macroflora, respectively), many macrobiota also host or facilitate microbial taxa responsible for N transformations. Interest in this topic is expanding, especially as it applies to coastal marine systems where N is a limiting nutrient. Our understanding of the diversity of microbes associated with coastal marine macrofauna (invertebrate and vertebrate animals) and macrophytes (seaweeds and marine plants) is improving, and recent studies indicate that the collection of microbes living in direct association with macrobiota (the microbiome) may directly contribute to N cycling. Here, we review the roles that macrobiota play in coastal N cycling, review current knowledge of macrobial–microbial associations in terms of N processing, and suggest implications for coastal ecosystem function as animals are harvested and as foundational habitat is lost or degraded. Given the biodiversity of microbial associates of macrobiota, we advocate for more research into the functional consequences of these associations for the coastal N cycle.
  • Article
    Long-term survival of adult Arctic grayling (Thymallus arcticus) in the Kuparuk River, Alaska
    (National Research Council Canada, 2004-12-23) Buzby, Karen M. ; Deegan, Linda A.
    In many long-lived species such as Arctic grayling (Thymallus arcticus), population growth rate is most sensitive to changes in adult survival probabilities. Understanding the factors that regulate adult survival in this species should provide insight into the population dynamics of this and other long-lived Arctic species. Using the program MARK, we analyzed 17 years of mark–recapture data to estimate survival rates for Arctic grayling in the Kuparuk River, Alaska, from 1985 to 2000. Mean annual survival rates overall ranged from 0.39 to 1.0 and averaged 0.71 ± 0.05 for resident and 0.75 ± 0.05 for nonresident fish. Spending the summer in the more productive fertilized zone of the experimental reach had no influence on survival despite higher productivity on all trophic levels and consistently higher growth rates in Arctic grayling. None of the environmental (stream temperature, discharge, winter severity, and incidence of drought) or population parameters (growth, condition factor, and mean fish size) that we examined explained significant amounts of variance in survival rates. The lack of responsiveness of survival to annual environmental conditions was unexpected and suggests that multiyear factors or life history tactics that maintain survival at the expense of growth and fecundity likely determine survival.
  • Article
    Discontinuities concentrate mobile predators : quantifying organism–environment interactions at a seascape scale
    (John Wiley & Sons, 2016-02-26) Kennedy, Cristina G. ; Mather, Martha E. ; Smith, Joseph M. ; Finn, John T. ; Deegan, Linda A.
    Understanding environmental drivers of spatial patterns is an enduring ecological problem that is critical for effective biological conservation. Discontinuities (ecologically meaningful habitat breaks), both naturally occurring (e.g., river confluence, forest edge, drop-off) and anthropogenic (e.g., dams, roads), can influence the distribution of highly mobile organisms that have land- or seascape scale ranges. A geomorphic discontinuity framework, expanded to include ecological patterns, provides a way to incorporate important but irregularly distributed physical features into organism–environment relationships. Here, we test if migratory striped bass (Morone saxatilis) are consistently concentrated by spatial discontinuities and why. We quantified the distribution of 50 acoustically tagged striped bass at 40 sites within Plum Island Estuary, Massachusetts during four-monthly surveys relative to four physical discontinuities (sandbar, confluence, channel network, drop-off), one continuous physical feature (depth variation), and a geographic location variable (region). Despite moving throughout the estuary, striped bass were consistently clustered in the middle geographic region at sites with high sandbar area, close to channel networks, adjacent to complex confluences, with intermediate levels of bottom unevenness, and medium sized drop-offs. In addition, the highest striped bass concentrations occurred at sites with the greatest additive physical heterogeneity (i.e., where multiple discontinuities co-occurred). The need to incorporate irregularly distributed features in organism–environment relationships will increase as high-quality telemetry and GIS data accumulate for mobile organisms. The spatially explicit approach we used to address this challenge can aid both researchers who seek to understand the impact of predators on ecosystems and resource managers who require new approaches for biological conservation.
  • Preprint
    Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers
    ( 2010-08-29) Deegan, Linda A. ; Neill, Christopher ; Haupert, Christie L. ; Ballester, M. Victoria R. ; Krusche, Alex V. ; Victoria, Reynaldo L. ; Thomas, Suzanne M. ; de Moor, Emily
    Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using 15N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream that were within several km of each other and on similar soils and landscape positions. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO3-) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH4+ uptake length, higher uptake rates into organic matter components and a shorter 15NH4+ residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added 15NH4+) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added 15N in organic matter compartments and exported 53% (15NH4+ =34%; 15NO3- = 19%). In contrast, the second-order pasture stream retained 75% of added 15N, predominantly in grasses (69%) and exported only 4% as 15NH4+. The fate of tracer 15N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (15NH4+ = 9%; 15NO3- = 6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.
  • Article
    Use of non-natal estuaries by migratory striped bass (Morone saxatilis) in summer
    (U.S. Dept. of Commerce, NOAA NMFS, 2009-07) Mather, Martha E. ; Finn, John T. ; Ferry, Kristen H. ; Deegan, Linda A. ; Nelson, Gary A.
    For most migratory fish, little is known about the location and size of foraging areas or how long individuals remain in foraging areas, even though these attributes may affect their growth, survival, and impact on local prey. We tested whether striped bass (Morone saxatilis Walbaum), found in Massachusetts in summer, were migratory, how long they stayed in non-natal estuaries, whether observed spatial patterns differed from random model predictions, whether fish returned to the same area across multiple years, and whether fishing effort could explain recapture patterns. Anchor tags were attached to striped bass that were caught and released in Massachusetts in 1999 and 2000, and recaptured between 1999 and 2007. In fall, tagged striped bass were caught south of where they were released in summer, confirming that fish were coastal migrants. In the first summer, 77% and 100% of the recaptured fish in the Great Marsh and along the Massachusetts coast, respectively, were caught in the same place where they were released. About two thirds of all fish recaptured near where they were released were caught 2–7 years after tagging. Our study shows that smaller (400–500 mm total length) striped bass migrate hundreds of kilometers along the Atlantic Ocean coast, cease their mobile lifestyle in summer when they use a relatively localized area for foraging (<20 km2), and return to these same foraging areas in subsequent years.
  • Article
    Stable isotope monitoring of benthic–planktonic coupling using salt marsh fish
    (Inter-Research, 2008-10-13) Fry, Brian ; Cieri, Matthew ; Hughes, Jeff ; Tobias, Craig R. ; Deegan, Linda A. ; Peterson, Bruce J.
    Salt marshes are important coastal ecosystems whose trophic function can be monitored with stable isotopes of abundant fish biosentinel species such as the mummichog Fundulus heteroclitus and the Atlantic silverside Menidia menidia. We compared movement patterns and feeding biology of these species in the summers of 1999 and 2000 in the Rowley River salt marsh estuary north of Boston, Massachusetts, USA. A 15N tracer addition experiment showed that fish of both species were more resident than transient, with mummichogs resident at scales of 1 km or less. Natural abundance stable isotope C, N, and S distributions showed that mummichogs feed more strongly in the benthic food web while silversides feed more in the planktonic food web, with % benthic feeding respectively averaging 58 ± 5 and 32 ± 3% (mean ± 95% confidence limit, CL). For both species, isotope results indicated considerable individual specialization in foraging behavior, likely related to use of channel habitat versus use of the marsh. Power analysis showed that measuring 3 composite samples each comprising 10 to 15 individual fish should provide relatively low errors of 0.5‰ (95% CL) or less around stable isotope averages. Use of such composite samples in monitoring programs will allow detection of significant temporal and spatial changes in benthic-planktonic coupling for salt marsh ecosystems, as recorded in average fish diets. Analyzing some individual fish also is recommended to obtain more detailed information on fish food sources, feeding specializations, and end-member isotope values used in estimating importance of benthic and planktonic food sources.
  • Preprint
    Animating the carbon cycle
    ( 2013-08) Schmitz, Oswald J. ; Raymond, Peter A. ; Estes, James A. ; Kurz, Werner A. ; Holtgrieve, Gordon W. ; Ritchie, Mark E. ; Schindler, Daniel E. ; Spivak, Amanda C. ; Wilson, Rod W. ; Bradford, Mark A. ; Christensen, Villy ; Deegan, Linda A. ; Smetacek, Victor ; Vanni, Michael J. ; Wilmers, Christopher C.
    Understanding the biogeochemical processes regulating carbon cycling is central to mitigating atmospheric CO2 emissions. The role of living organisms has been accounted for, but the focus has traditionally been on contributions of plants and microbes. We develop the case that fully “animating” the carbon cycle requires broader consideration of the functional role of animals in mediating biogeochemical processes and quantification of their effects on carbon storage and exchange among terrestrial and aquatic reservoirs and the atmosphere. To encourage more hypothesis-driven experimental research that quantifies animal effects we discuss the mechanisms by which animals may affect carbon exchanges and storage within and among ecosystems and the atmosphere. We illustrate how those mechanisms lead to multiplier effects whose magnitudes may rival those of more traditional carbon storage and exchange rate estimates currently used in the carbon budget. Many animal species are already directly managed. Thus improved quantitative understanding of their influence on carbon budgets may create opportunity for management and policy to identify and implement new options for mitigating CO2 release at regional scales.
  • Article
    Plant nitrogen dynamics in fertilized and natural New England salt marshes : a paired 15N tracer study
    (Inter-Research, 2008-02-07) Drake, Deanne C. ; Peterson, Bruce J. ; Deegan, Linda A. ; Harris, Lora A. ; Miller, E. E. ; Warren, R. Scott
    We examined the effects of increased nutrient availability on nitrogen (N) dynamics in dominant New England salt marsh plants (tall and stunted Spartina alterniflora and S. patens) using paired large-scale nutrient and 15NO3– tracer additions. This study is one component of a long-term, large-scale, salt marsh nutrient and trophic manipulation study (the Trophic Cascades and Interacting Control Processes in a Detritus-based Aquatic Ecosystem [TIDE] Project). We compared physiological variables of plants in fertilized (~17× ambient N and P in incoming tidal water) and reference marsh systems to quantify NO3– uptake and uptake efficiency, allocation of N to tissues, end-of-season N resorption, leaf litter quality and other potential responses to increased nutrient availability. Reference system plants sequestered ~24.5 g NO3-N ha–1 d–1 in aboveground pools during mid-summer, while fertilized plants sequestered ~140 g NO3-N ha–1 d–1. However, NO3– uptake efficiency (% of total incoming NO3-N sequestered aboveground) was higher in the reference system (16.8%) than in the fertilized system (2.6%), suggesting that our fertilization rate (~70 µM NO3– in incoming water) approaches or exceeds the uptake saturation point for this vegetation community. Leaf litter quality was clearly affected by N availability; N resorption efficiency was lower in all plants of the fertilized system; senesced leaves from the fertilized creek contained ~43% (tall S. alterniflora), 23% (stunted S. alterniflora) and 15% (S. patens) more N per unit biomass than reference creek leaves.
  • Article
    What happens in an estuary doesn't stay there : patterns of biotic connectivity resulting from long term ecological research
    (The Oceanography Society, 2013-09) Mather, Martha E. ; Finn, John T. ; Kennedy, Cristina G. ; Deegan, Linda A. ; Smith, Joseph M.
    The paucity of data on migratory connections and an incomplete understanding of how mobile organisms use geographically separate areas have been obstacles to understanding coastal dynamics. Research on acoustically tagged striped bass (Morone saxatilis) at the Plum Island Ecosystems (PIE) Long Term Ecological Research site, Massachusetts, documents intriguing patterns of biotic connectivity (i.e., long-distance migration between geographically distinct areas). First, the striped bass tagged at PIE migrated southward along the coast using different routes. Second, these tagged fish exhibited strong fidelity and specificity to PIE. For example, across multiple years, tagged striped bass resided in PIE waters for an average of 1.5–2.5 months per year (means: 51–72 days; range 2–122 days), left this estuary in fall, then returned in subsequent years. Third, this specificity and fidelity connected PIE to other locations. The fish exported nutrients and energy to at least three other coastal locations through biomass added as growth. These results demonstrate that what happens in an individual estuary can affect other estuaries. Striped bass that use tightly connected routes to feed in specific estuaries should have greater across-system impacts than fish that are equally likely to go anywhere. Consequently, variations in when, where, and how fish migrate can alter across-estuary impacts.
  • Preprint
    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.
  • Preprint
    Top-down and bottom-up control of infauna varies across the saltmarsh landscape
    ( 2007-12) Fleeger, John W. ; Johnson, David S. ; Galvan, Kari A. ; Deegan, Linda A.
    Responses of infaunal saltmarsh benthic invertebrates to whole-ecosystem fertilization and predator removal were quantified in Plum Island Estuary, Massachusetts, USA. Throughout a growing season, we enriched an experimental creek on each flooding tide to 70 mM NO3 - and 4 mM PO4 -3 (a 10 x increase in loading above background), and we reduced Fundulus heteroclitus density by 60% in a branch of the fertilized and a reference creek. Macroinfauna and meiofauna were sampled in creek (mudflat and creek wall), marsh edge (tall form Spartina alterniflora) and marsh platform (Spartina patens and stunted S. alterniflora) habitats before and after treatments were begun; responses were tested with BACI-design statistics. Treatment effects were most common in the mid-range of the inundation gradient. Most fertilization effects were on creek wall where ostracod abundance increased, indices of copepod reproduction increased and copepod and annelid communities were altered. These taxa may use epiphytes (that respond rapidly to fertilization) of filamentous algae as a food source. Killifish reduction effects on meiobenthic copepod abundance were detected at the marsh edge and suggest predator limitation. Fish reduction effects on annelids did not suggest top-down regulation in any habitat; however, fish reduction may have stimulated an increased predation rate on annelids by grass shrimp. Interactions between fertilization and fish reduction occurred under S. patens canopy where indirect predator reduction effects on annelids were indicated. No effects were observed in mudflat or stunted S. alterniflora habitats. Although the responses of infauna to fertilization and predator removal were largely independent and of similar mild intensity, our data suggests that the effects of ecological stressors vary across the marsh landscape.
  • Preprint
    Microbial community composition in sediments resists perturbation by nutrient enrichment
    ( 2010-11) Bowen, Jennifer L. ; Ward, Bess B. ; Morrison, Hilary G. ; Hobbie, John E. ; Valiela, Ivan ; Deegan, Linda A. ; Sogin, Mitchell L.
    Functional redundancy in bacterial communities is expected to allow microbial assemblages to survive perturbation by allowing continuity in function despite compositional changes in communities. Recent evidence suggests, however, that microbial communities change both composition and function as a result of disturbance. We present evidence for a third response: resistance. We examined microbial community response to perturbation caused by nutrient enrichment in salt marsh sediments using deep pyrosequencing of 16S rRNA and functional gene microarrays targeting the nirS gene. Composition of the microbial community, as demonstrated by both genes, was unaffected by significant variations in external nutrient supply, despite demonstrable and diverse nutrient–induced changes in many aspects of marsh ecology. The lack of response to external forcing demonstrates a remarkable uncoupling between microbial composition and ecosystem-level biogeochemical processes and suggests that sediment microbial communities are able to resist some forms of perturbation.
  • Article
    Surprisingly modest water quality impacts from expansion and intensification of large-scale commercial agriculture in the Brazilian Amazon-Cerrado region
    (Sage, 2017-08-30) Neill, Christopher ; Jankowski, KathiJo ; Brando, Paulo ; Coe, Michael T. ; Deegan, Linda A. ; Macedo, Marcia N. ; Riskin, Shelby H. ; Porder, Stephen ; Elsenbeer, Helmut ; Krusche, Alex V.
    Large-scale commercial cropping of soybeans expanded in the tropical Amazon and Cerrado biomes of Brazil after 1990. More recently, cropping intensified from single-cropping of soybeans to double-cropping of soybeans with corn or cotton. Cropland expansion and intensification, and the accompanying use of mineral fertilizers, raise concerns about whether nutrient runoff and impacts to surface waters will be similar to those experienced in commercial cropland regions at temperate latitudes. We quantified water infiltration through soils, water yield, and streamwater chemistry in watersheds draining native tropical forest and single- and double-cropped areas on the level, deep, highly weathered soils where cropland expansion and intensification typically occurs. Although water yield increased four-fold from croplands, streamwater chemistry remained largely unchanged. Soil characteristics exerted important control over the movement of nitrogen (N) and phosphorus (P) into streams. High soil infiltration rates prevented surface erosion and movement of particulate P, while P fixation in surface soils restricted P movement to deeper soil layers. Nitrogen retention in deep soils, likely by anion exchange, also appeared to limit N leaching and export in streamwater from both single- and double-cropped watersheds that received nitrogen fertilizer. These mechanisms led to lower streamwater P and N concentrations and lower watershed N and P export than would be expected, based on studies from temperate croplands with similar cropping and fertilizer application practices.
  • Article
    Supporting Spartina: Interdisciplinary perspective shows spartina as a distinct solid genus
    (Ecological Society of America, 2019-09-19) Bortolus, Alejandro ; Adam, Paul ; Adams, Janine B. ; Ainouche, Malika L. ; Ayres, Debra ; Bertness, Mark D. ; Bouma, Tjeerd J. ; Bruno, John F. ; Caçador, Isabel ; Carlton, James T. ; Castillo, Jesus M. ; Costa, Cesar S.B. ; Davy, Anthony J. ; Deegan, Linda A. ; Duarte, Bernardo ; Figueroa, Enrique ; Gerwein, Joel ; Gray, Alan J. ; Grosholz, Edwin D. ; Hacker, Sally D. ; Hughes, A. Randall ; Mateos‐Naranjo, Enrique ; Mendelssohn, Irving A. ; Morris, James T. ; Muñoz‐Rodríguez, Adolfo F. ; Nieva, Francisco J.J. ; Levin, Lisa A. ; Li, Bo ; Liu, Wenwen ; Pennings, Steven C. ; Pickart, Andrea ; Redondo‐Gómez, Susana ; Richardson, David M. ; Salmon, Armel ; Schwindt, Evangelina ; Silliman, Brian ; Sotka, Erik E. ; Stace, Clive ; Sytsma, Mark ; Temmerman, Stijn ; Turner, R. Eugene ; Valiela, Ivan ; Weinstein, Michael P. ; Weis, Judith S.
    In 2014, a DNA‐based phylogenetic study confirming the paraphyly of the grass subtribe Sporobolinae proposed the creation of a large monophyletic genus Sporobolus, including (among others) species previously included in the genera Spartina, Calamovilfa, and Sporobolus. Spartina species have contributed substantially (and continue contributing) to our knowledge in multiple disciplines, including ecology, evolutionary biology, molecular biology, biogeography, experimental ecology, biological invasions, environmental management, restoration ecology, history, economics, and sociology. There is no rationale so compelling to subsume the name Spartina as a subgenus that could rival the striking, global iconic history and use of the name Spartina for over 200 yr. We do not agree with the subjective arguments underlying the proposal to change Spartina to Sporobolus. We understand the importance of both the objective phylogenetic insights and of the subjective formalized nomenclature and hope that by opening this debate we will encourage positive feedback that will strengthen taxonomic decisions with an interdisciplinary perspective. We consider that the strongly distinct, monophyletic clade Spartina should simply and efficiently be treated as the genus Spartina.