Giblin Anne E.

Last Name

Giblin

First Name

Anne E.

ORCID

0000-0003-3851-2178

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Benthic community metabolism in deep and shallow Arctic lakes during 13 years of whole–lake fertilization

( 2015-05) Daniels, William C. ; Kling, George W. ; Giblin, Anne E.

Benthic primary production and oxygen consumption are important components of lake biogeochemical cycling. We performed whole-lake nutrient manipulations in arctic Alaska to assess the controls of lake morphometry, nutrients, and light on benthic community metabolism. One deep, stratified lake (Lake E5) and one shallow, well-mixed lake (Lake E6) in the Alaskan Arctic were fertilized with low levels of nitrogen (56 mg N m-3 y-1) and phosphorus (8 mg P m-3 y-1) from 2001-2013. Benthic primary production was not stimulated by fertilization in either lake. In the deep lake, decreased water clarity is consistent with an increase in phytoplankton biomass during fertilization. Benthic GPP decreased by 7 - 47 mg C m-2 d-1 (not statistically significant) and benthic respiration increased from 87 ± 20 to 167 ± 9 (SE) mg C m-2 d-1. The areal hypolimnetic oxygen deficit increased by 15 mg O2 m-2 d-1 each year during the 13 years of monitoring, apparently driven by lower (more negative) benthic NEP. In the shallow lake, phytoplankton concentration did not change with fertilization. As a result, the light environment did not change and benthic GPP did not decrease. Overall the data suggest that (1) benthic algae are not nutrient limited in either the deep or shallow lake, (2) lake morphometry modulated the overall nutrient impact on benthic metabolism by controlling the response of phytoplankton, and by extension, light and organic carbon supply to the benthos, (3) year-to-year variability in light attenuation explains considerable variability in benthic GPP between lakes and years, (4) correlations between both dissolved organic carbon concentrations and light attenuation coefficients (kd) between lakes suggests a regional control on light attenuation, and (5) the dissolved oxygen concentrations in the deep experimental lake are highly sensitive to nutrient enrichment.
Understanding the effects of climate change via disturbance on pristine arctic lakes-multitrophic level response and recovery to a 12-yr, low-level fertilization experiment

(Association for the Sciences of Limnology and Oceanography, 2021-08-02) Budy, Phaedra ; Pennock, Casey A. ; Giblin, Anne E. ; Luecke, Chris ; White, Daniel L. ; Kling, George W.

Effects of climate change-driven disturbance on lake ecosystems can be subtle; indirect effects include increased nutrient loading that could impact ecosystem function. We designed a low-level fertilization experiment to mimic persistent, climate change-driven disturbances (deeper thaw, greater weathering, or thermokarst failure) delivering nutrients to arctic lakes. We measured responses of pelagic trophic levels over 12 yr in a fertilized deep lake with fish and a shallow fishless lake, compared to paired reference lakes, and monitored recovery for 6 yr. Relative to prefertilization in the deep lake, we observed a maximum pelagic response in chl a (+201%), dissolved oxygen (DO, −43%), and zooplankton biomass (+88%) during the fertilization period (2001–2012). Other responses to fertilization, such as water transparency and fish relative abundance, were delayed, but both ultimately declined. Phyto- and zooplankton biomass and community composition shifted with fertilization. The effects of fertilization were less pronounced in the paired shallow lakes, because of a natural thermokarst failure likely impacting the reference lake. In the deep lake there was (a) moderate resistance to change in ecosystem functions at all trophic levels, (b) eventual responses were often nonlinear, and (c) postfertilization recovery (return) times were most rapid at the base of the food web (2–4 yr) while higher trophic levels failed to recover after 6 yr. The timing and magnitude of responses to fertilization in these arctic lakes were similar to responses in other lakes, suggesting indirect effects of climate change that modify nutrient inputs may affect many lakes in the future.
Nitrate is an important nitrogen source for Arctic tundra plants

(National Academy of Sciences, 2018-03-27) Liu, Xue-Yan ; Koba, Keisuke ; Koyama, Lina A. ; Hobbie, Sarah E. ; Weiss, Marissa S. ; Inagaki, Yoshiyuki ; Shaver, Gaius R. ; Giblin, Anne E. ; Hobara, Satoru ; Nadelhoffer, Knute J. ; Sommerkorn, Martin ; Rastetter, Edward B. ; Kling, George W. ; Laundre, James A. ; Yano, Yuriko ; Makabe, Akiko ; Yano, Midori ; Liu, Cong-Qiang

Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3−) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3− concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3− that is typically below detection limits. Here we reexamine NO3− use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3−. Soil-derived NO3− was detected in tundra plant tissues, and tundra plants took up soil NO3− at comparable rates to plants from relatively NO3−-rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3− relative to soil NO3− accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3− availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3− availability in tundra soils is crucial for predicting C storage in tundra.
Marsh-atmosphere CO2 exchange in a New England salt marsh

(John Wiley & Sons, 2015-09-29) Forbrich, Inke ; Giblin, Anne E.

We studied marsh-atmosphere exchange of carbon dioxide in a high marsh dominated salt marsh during the months of May to October in 2012–2014. Tidal inundation at the site occurred only during biweekly spring tides, during which we observed a reduction in fluxes during day and night. We estimated net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (Reco) using a modified PLIRTLE model, which requires photosynthetically active radiation, temperature, and normalized difference vegetation index (NDVI) as control variables. NDVI decreased during inundation, when the marsh canopy was submerged. Two-time series of NDVI, including and excluding effects of tidal inundation, allowed us to quantify the flux reduction during inundation. The effect of the flux reduction was small (2–4%) at our site, but is likely higher for marshes at a lower elevation. From May to October, GPP averaged −863 g C m−2, Reco averaged 591 g C m−2, and NEE averaged −291 g C m−2. In 2012, which was an exceptionally warm year, we observed an early start of net carbon uptake but higher respiration than in 2013 and 2014 due to higher-air temperature in August. This resulted in the lowest NEE during the study period (−255.9±6.9 g C m−2). The highest seasonal net uptake (−336.5±6.3 g C m−2) was observed in 2013, which was linked to higher rainfall and temperature in July. Mean sea level was very similar during all 3 years which allowed us to isolate the importance of climatic factors.
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.
Nitrogen dynamics in arctic tundra soils of varying age : differential responses to fertilization and warming

( 2013-03) Yano, Yuriko ; Shaver, Gaius R. ; Rastetter, Edward B. ; Giblin, Anne E. ; Laundre, James A.

In the northern foothills of the Brooks Range, Alaska, a series of glacial retreats has created a landscape that varies widely in time since deglaciation (= soil age), from ~10k years to more than 2M years. Productivity of the moist tundra that covers most of this landscape is generally N-limited, but varies widely, as do plant-species composition and key soil properties such as pH. These differences might be altered in the future because of the projected increase in N availability under a warmer climate. We hypothesized that future changes in productivity and vegetation composition across soil ages might be mediated through changes in N availability. To test this hypothesis, we compared readily available-N (water-soluble ammonium, nitrate, and amino acids), moderately-available N (soluble proteins), hydrolysable-N, and total-N pools across three tussock-tundra landscapes with soil ages ranging from 11.5k to 300k years. We also compared the effects of long-term fertilization and warming on these N pools for the two younger sites, in order to assess whether the impacts of warming and increased N availability differ by soil age. Readily available N was largest at the oldest site, and amino acids (AA) accounted for 80-89 % of this N. At the youngest site, however, inorganic N constituted the majority (80-97%) of total readily-available N. This variation reflected the large differences in plant functional-group composition and soil chemical properties. Long-term (8-16 years) fertilization increased soluble inorganic N by 20-100 fold at the intermediate-age site, but only by 2-3 fold at the youngest-soil site. Warming caused small and inconsistent changes in the soil C:N ratio and soluble AA, but only in soils beneath Eriophorum vaginatum, the dominant tussock-forming sedge. These differential responses suggest that the impacts of warmer climates on these tundra ecosystems are more complex than simply elevated N mineralization, and that the response of the N cycling might differ strongly depending on the ecosystem’s soil age, initial soil properties, and plant-community composition.
Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi–plant interaction

( 2009-08) Yano, Yuriko ; Shaver, Gaius R. ; Giblin, Anne E. ; Rastetter, Edward B.

Uptake of nitrogen (N) via root-mycorrhizal associations accounts for a significant portion of total N supply to many vascular plants. Using stable isotope ratios (δ15N) and the mass balance among N pools of plants, fungal tissues, and soils, a number of efforts have been made in recent years to quantify the flux of N from mycorrhizal fungi to host plants. Current estimates of this flux for arctic tundra ecosystems rely on the untested assumption that the δ15N of labile organic N taken up by the fungi is approximately the same as the δ15N of bulk soil. We report here hydrolysable amino acids are more depleted in 15N relative to hydrolysable ammonium and amino sugars in arctic tundra soils near Toolik Lake, Alaska, USA. We demonstrate, using a case study, that recognizing the depletion in 15N for hydrolysable amino acids (δ15N = -5.6 ‰ on average) would alter recent estimates of N flux between mycorrhizal fungi and host plants in an arctic tundra ecosystem.
The importance of dissimilatory nitrate reduction to ammonium (DNRA) in the nitrogen cycle of coastal ecosystems

(The Oceanography Society, 2013-09) Giblin, Anne E. ; Tobias, Craig R. ; Song, Bongkeun ; Weston, Nathaniel ; Banta, Gary T. ; Rivera-Monroy, Victor H.

Until recently, it was believed that biological assimilation and gaseous nitrogen (N) loss through denitrification were the two major fates of nitrate entering or produced within most coastal ecosystems. Denitrification is often viewed as an important ecosystem service that removes reactive N from the ecosystem. However, there is a competing nitrate reduction process, dissimilatory nitrate reduction to ammonium (DNRA), that conserves N within the ecosystem. The recent application of nitrogen stable isotopes as tracers has generated growing evidence that DNRA is a major nitrogen pathway that cannot be ignored. Measurements comparing the importance of denitrification vs. DNRA in 55 coastal sites found that DNRA accounted for more than 30% of the nitrate reduction at 26 sites. DNRA was the dominant pathway at more than one-third of the sites. Understanding what controls the relative importance of denitrification and DNRA, and how the balance changes with increased nitrogen loading, is of critical importance for predicting eutrophication trajectories. Recent improvements in methods for assessing rates of DNRA have helped refine our understanding of the rates and controls of this process, but accurate measurements in vegetated sediment still remain a challenge.
Modeling denitrification in aquatic sediments

( 2008-10-10) Fennel, Katja ; Brady, Damian C. ; DiToro, Dominic ; Fulweiler, Robinson W. ; Gardner, Wayne S. ; Giblin, Anne E. ; McCarthy, Mark J. ; Rao, Alexandra ; Seitzinger, Sybil P. ; Thouvenot-Korppoo, Marie ; Tobias, Craig R.

Sediment denitrification is a major pathway of fixed nitrogen loss from aquatic systems. Due to technical difficulties in measuring this process and its spatial and temporal variability, estimates of local, regional and global denitrification have to rely on a combination of measurements and models. Here we review approaches to describing denitrification in aquatic sediments, ranging from mechanistic diagenetic models to empirical parameterizations of nitrogen fluxes across the sediment-water interface. We also present a compilation of denitrification measurements and ancillary data for different aquatic systems, ranging from freshwater to marine. Based on this data compilation we reevaluate published parameterizations of denitrification. We recommend that future models of denitrification use (1) a combination of mechanistic diagenetic models and measurements where bottom waters are temporally hypoxic or anoxic, and (2) the much simpler correlations between denitrification and sediment oxygen consumption for oxic bottom waters. For our data set, inclusion of bottom water oxygen and nitrate concentrations in a multivariate regression did not improve the statistical fit.
Biogeography of ammonia oxidizers in New England and Gulf of Mexico salt marshes and the potential importance of comammox

(Springer, 2021-03-29) Bernhard, Anne E. ; Beltz, Jack ; Giblin, Anne E. ; Roberts, Brian J.

Few studies have focused on broad scale biogeographic patterns of ammonia oxidizers in coastal systems, yet understanding the processes that govern them is paramount to understanding the mechanisms that drive biodiversity, and ultimately impact ecosystem processes. Here we present a meta-analysis of 16 years of data of ammonia oxidizer abundance, diversity, and activity in New England (NE) salt marshes and 5 years of data from marshes in the Gulf of Mexico (GoM). Potential nitrification rates were more than 80x higher in GoM compared to NE marshes. However, nitrifier abundances varied between regions, with ammonia-oxidizing archaea (AOA) and comammox bacteria significantly greater in GoM, while ammonia-oxidizing bacteria (AOB) were more than 20x higher in NE than GoM. Total bacterial 16S rRNA genes were also significantly greater in GoM marshes. Correlation analyses of rates and abundance suggest that AOA and comammox are more important in GoM marshes, whereas AOB are more important in NE marshes. Furthermore, ratios of nitrifiers to total bacteria in NE were as much as 80x higher than in the GoM, suggesting differences in the relative importance of nitrifiers between these systems. Communities of AOA and AOB were also significantly different between the two regions, based on amoA sequences and DNA fingerprints (terminal restriction fragment length polymorphism). Differences in rates and abundances may be due to differences in salinity, temperature, and N loading between the regions, and suggest significantly different N cycling dynamics in GoM and NE marshes that are likely driven by strong environmental differences between the regions.
Western Maine Coastal Current reduces primary production rates, zooplankton abundance and benthic nutrient fluxes in Massachusetts Bay

( 2013-08) McManus, M. Conor ; Oviatt, Candace A. ; Giblin, Anne E. ; Tucker, Jane ; Turner, Jefferson T.

Primary production was measured from 1992-2010 in Massachusetts Bay and just outside Boston Harbor for the Massachusetts Water Resources Authority’s outfall monitoring program. In 2003, annual primary production decreased by 221-278 g C m-2 year-1, with decreased rates continuing through 2010. Based on a conceptual model, oceanographic and meteorological variables were analyzed with production rates to determine if concurrent environmental changes were responsible for the reduced primary production in Massachusetts Bay. Results indicated that stronger influx of low salinity water from the western Maine Coastal Current (WMCC) in recent years might be responsible for the decreases. The WMCC appeared to have become fresher from increased river discharge in the western Gulf of Maine. Northeasterly winds in recent years promoted WMCC intrusion into Massachusetts Bay. Correlation between primary production and surface salinities suggested the impact of the WMCC on production rates. We hypothesized that increased stratification resulted in reduced vertical mixing and nutrient concentrations in surface waters for phytoplankton growth. However, no significant correlations were observed between the annual primary production and nutrient concentrations in Massachusetts Bay. Reduced production rates in Massachusetts Bay have been associated with reduced zooplankton abundances, benthic ammonium fluxes and sediment oxygen demand in summer months.
Emerging wetlands from river diversions can sustain high denitrification rates in a Coastal Delta

(American Geophysical Union, 2021-03-31) Upreti, Kiran ; Rivera-Monroy, Victor H. ; Maiti, Kanchan ; Giblin, Anne E. ; Geaghan, James P.

It is assumed that to treat excess NO3− high soil organic matter content (%OM) is required to maintain high denitrification rates in natural or restored wetlands. However, this excess also represents a risk by increasing soil decomposition rates triggering peat collapse and wetland fragmentation. Here, we evaluated the role of %OM and temperature interactions controlling denitrification rates in eroding (Barataria Bay-BLC) and emerging (Wax Lake Delta-WLD) deltaic regions in coastal Louisiana using the isotope pairing (IPT) and N2:Ar techniques. We also assessed differences between total (direct denitrification + coupled nitrification-denitrification) and net (total denitrification minus nitrogen fixation) denitrification rates in benthic and wetland habitats with contrasting %OM and bulk density (BD). Sediment (benthic) and soil (wetland) cores were collected during summer, spring, and winter (2015–2016) and incubated at close to in-situ temperatures (30°C, 20°C, and 10°C, respectively). Denitrification rates were linearly correlated with temperature; maximum mean rates ranged from 40.1–124.1 μmol m−2 h−1 in the summer with lower rates (<26.2 ± 5.3 μmol m−2 h−1) in the winter seasons. Direct denitrification was higher than coupled denitrification in all seasons. Denitrification rates were higher in WLD despite lower %OM, lower total N concentration, and higher BD in wetland soils. Therefore, in environments with low carbon availability, high denitrification rates can be sustained as long as NO3− concentrations are high (>30 μM) and water temperature is >10°C. In coastal Louisiana, substrates under these regimes are represented by emergent supra-tidal flats or land created by sediment diversions under oligohaline conditions (<1 ppt).
New England salt pond data book

(Woods Hole Oceanographic Institution, 1990-06) Giblin, Anne E.

This volume contains information on New England salt ponds and lagoons. The first part contains abstracts of a symposium on salt ponds and lagoons held in conjunction with the New England Estuarine Research Society (NEERS) on April 21, 1988. These should provide both scientists and managers with an overview of recent research on salt ponds. The second part contains maps, morphometric data, and references for individual salt ponds in Connecticut, Rhode Island, and Massachusetts. The third section is a comprehensive bibliography of papers and reports on salt ponds, including information on ponds located outside of New England. A listing of references organized according to topic areas is also provided.
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.
Effect of continuous light on leaf wax isotope ratios in Betula nana and Eriophorum vaginatum: Implications for Arctic paleoclimate reconstructions

(Elsevier B.V., 2018-08-22) Daniels, William C. ; Huang, Yongsong ; Russell, James M. ; Giblin, Anne E.

Reconstructions of climate using leaf wax D/H ratios (δDwax) require accounting for the apparent isotopic fractionation (εapp) between plant source water and waxes. There have been conflicting publications on whether plants in the Arctic growing under 24-hour continuous light, fractionate less than temperate and tropical plants. In this study, we examine the effect of diurnal light (DL) versus 24-hour continuous light (CL) on the isotopic composition of leaf n-alkanes and n-acids in greenhouse experiments using two common Arctic plants (Eriophorum vaginatum, or tussock cottongrass and Betula nana, or dwarf birch). For E. vaginatum, the δDwax values of various wax homologues were 5–11‰ more positive for CL plants relative to their DL counterparts, whereas for B. nana, CL waxes were 3–24‰ more negative, suggesting that daylight length is not a unifying control on leaf wax D/H ratios of Arctic plants. The δ13Cwax of B. nana was more negative for plants grown in continuous light compared to diurnal light, reflecting lower water-use efficiency, associated with prolonged stomatal opening in the CL treatment. We modeled the impact of increasing stomatal conductance and effective flow path lengths (mimicking variable leaf morphologies) on the isotopic composition of leaf waters (δDlw) and find that variations in leaf-water enrichment may explain the variable δDwax responses seen between E. vaginatum and B. nana. We suggest that between-species differences in the δDlw response to light, and differences in the utilization of stored carbohydrates, were important for governing δDwax. Our greenhouse results suggest that Arctic plant leaf waxes do not consistently display reduced εapp values as a result of 24-hour day light, providing additional support for field observations.
The regional and global significance of nitrogen removal in lakes and reservoirs

( 2008-06-25) Harrison, John A. ; Maranger, Roxane J. ; Alexander, Richard B. ; Giblin, Anne E. ; Jacinthe, Pierre-Andre ; Mayorga, Emilio ; Seitzinger, Sybil P. ; Sobota, Daniel J. ; Wollheim, Wilfred M.

Human activities have greatly increased the transport of biologically available N through watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and reservoirs) have the potential to act as important sinks for this reactive N as it is transported across the landscape because they offer ideal conditions for N burial in sediments or permanent loss via denitrification. However, the patterns and controls on lentic N removal have not been explored in great detail at large regional to global scales. In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N yr-1 from watersheds globally. Small lakes (< 50 km2) were critical in the analysis, retaining almost half (9.3 Tg N yr-1) of the global total. In model runs, capacity of lakes and reservoirs to remove watershed N varied substantially (0-100%) both as a function of climate and the density of lentic systems. Although reservoirs occupy just 6% of the global lentic surface area, we estimate they retain approximately 33% of the total N removed by lentic systems, due to a combination of higher drainage ratios (catchment surface area : lake or reservoir surface area), higher apparent settling velocities for N, and greater N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of NiRReLa suggests that, on-average, N removal within lentic systems will respond more strongly to changes in land use and N loading than to changes in climate at the global scale.
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.
Nutrient gradients in Panamanian estuaries : effects of watershed deforestation, rainfall, upwelling, and within-estuary transformations

(Inter-Research, 2013-05-22) Valiela, Ivan ; Giblin, Anne E. ; Barth-Jensen, Coralie ; Harris, Carolynn ; Stone, Thomas A. ; Fox, Sophia E. ; Crusius, John

To test whether deforestation of tropical forests alters coupling of watersheds, estuaries, and coastal waters, we measured nutrients in 8 watershed-estuarine systems on the Pacific coast of Panama where watershed forest cover ranged from 23 to 92%. Watersheds with greater forest cover discharged larger dissolved inorganic nitrogen concentrations and higher N/P into estuary headwaters. As freshwater mixed with seawater down-estuary, within-estuary biogeochemical processes erased the imprint of watershed deforestation, increased ammonium, lowered nitrate concentrations, and otherwise altered down-estuary water column composition. As estuarine water left mangrove estuaries, ammonium, nitrate, and phosphate, but not dissolved organic nitrogen, were exported to receiving near-shore waters. Mangrove estuaries in this region thus provide important ecological services, by uncoupling coastal waters from changes in terrestrial land covers, as well as by subsidizing adjoined receiving coastal waters by providing nutrients. The pattern of land-sea coupling and exports was disrupted during La Niña-influenced conditions. In one instance when La Niña conditions led to upwelling of deeper layers, high concentrations of marine-derived ammonium were inserted into estuaries. In another instance, La Niña-associated high rainfall diluted nutrient concentrations within estuaries and lowered salinity regionally, and the fresher upper layer impaired coastal upwelling. Regional rainfall has increased during the last decade. If La Niña rainfall continues to increase, disruptions of current land-estuary-sea couplings may become more frequent, with potentially significant changes in nutrient cycles and ecological services in these coupled ecosystems.
Methods for measuring denitrification : diverse approaches to a difficult problem

( 2005-07-15) Groffman, Peter M. ; Altabet, Mark A. ; Bohlke, John K. ; Butterbach-Bahl, Klaus ; David, Mark B. ; Firestone, Mary K. ; Giblin, Anne E. ; Kana, Todd M. ; Nielsen, Lars Peter ; Voytek, Mary A.

Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2), is important to primary production, water quality and the chemistry and physics of the atmosphere at ecosystem, landscape, regional and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses and future prospects for the different methods. Methodological approaches covered include; 1) acetylene-based methods, 2) 15N tracers, 3) direct N2 quantification, 4) N2/Ar ratio quantification, 5) mass balance approaches, 6) stoichiometric approaches, 7) methods based on stable isotopes, 8) in situ gradients with atmospheric environmental tracers and 9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.
Carbon turnover in Alaskan tundra soils : effects of organic matter quality, temperature, moisture and fertilizer

( 2006-02-15) Shaver, Gaius R. ; Giblin, Anne E. ; Nadelhoffer, Knute J. ; Thieler, K. K. ; Downs, M. R. ; Laundre, James A. ; Rastetter, Edward B.

Soils of tundra and boreal ecosystems contain large organic matter stocks, typically as a layer of peat that blankets the underlying mineral soil. Despite the low productivity of northern vegetation, organic matter accumulates as peat because decomposition of plant litter is limited by low soil temperatures and often wet, anaerobic conditions (Heal et al. 1981, Jonasson et al. 2001). The total C storage in this northern peat is globally significant, accounting for about one third of the global soil C stock if one includes both tundras and boreal forests (Oechel and Billings 1992, Callaghan et al. 2004a). Soils of northern ecosystems also contain large amounts of organic N that is currently unavailable to plants, but is potentially available and could support higher productivity if mineralized (Shaver et al. 1991, Nadelhoffer et al. 1992, Weintraub and Schimel 2005 a). Controls on soil C stocks and turnover, therefore, are key issues for understanding C exchanges between northern ecosystems and the atmosphere. In this paper, we determine how C losses from peaty soil organic matter are related to its chemical composition, and how that composition changes as the organic matter decomposes. To address these issues we compared four soil organic matter types from three tundra ecosystems near Toolik Lake, Alaska. The comparison included both unfertilized soils and soils that were fertilized annually for eight years before sampling. Under laboratory conditions, we determined how temperature and moisture conditions affect C losses from these organic matter types. The experiment also allowed us to determine how the chemical composition of different types of organic matter changed over four simulated “seasons” of decomposition. The chemical composition or “quality” of soil organic matter is a useful predictor of C turnover (Ågren and Bosatta 1996) although a wide range of definitions and fractionation schemes have been used (Sollins et al. 1999, Harmon and Lajtha 1999). In general, high-quality organic matter is defined as that which is more readily processed by microbes and has a higher rate of decomposition. Fresh plant litter and newly-formed organic matter are expected to be of higher quality than older, more fully decomposed organic matter in which the more labile components have been metabolized (Aerts 1997, Berg 2000). Species composition of the vegetation may also have a strong influence on litter and organic matter “quality” (Berendse 1994, Cornelissen 1996, Hobbie 1996, Hobbie and Gough 2004). In this research we characterized organic matter quality with a widely used sequential extraction procedure (Ryan et al. 1990, Harmon and Lajtha 1999) that breaks soil organic matter into 4 fractions: (1) a “non-polar extractable” (NPE) fraction extracted in methylene chloride, (2) a “water-soluble” (WS) fraction extracted in boiling water, (3) an “acid-soluble’ (AS) fraction extracted in H2SO4, and (4) an “acid-insoluble” (AIS) residue.

Giblin Anne E.

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