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GCE-LTER Signature Publications

Angelini, C., Griffin, J., van de Koppel, J., Derksen-Hooijberg, M., Lamers, L., Smolders, A.J.P., van der Heide, T. and Silliman, B.R. 2016. A keystone mutualism underpins resilience of a coastal ecosystem to drought. Nature Communications. 7:12473. (DOI: 10.1038/ncomms12473)

Droughts are increasing in severity and frequency, yet the mechanisms that strengthen ecosystem resilience to this stress remain poorly understood. Here, we test whether positive interactions in the form of a mutualism between mussels and dominant cordgrass in salt marshes enhance ecosystem resistance to and recovery from drought. Surveys spanning 250 km of southeastern US coastline reveal spatially dispersed mussel mounds increased cordgrass survival during severe drought by 5- to 25-times. Surveys and mussel addition experiments indicate this positive effect of mussels on cordgrass was due to mounds enhancing water storage and reducing soil salinity stress. Observations and models then demonstrate that surviving cordgrass patches associated with mussels function as nuclei for vegetative re-growth and, despite covering only 0.1–12% of die-offs, markedly shorten marsh recovery periods. These results indicate that mutualisms, in supporting stress-resistant patches, can play a disproportionately large, keystone role in enhancing ecosystem resilience to climatic extremes.

Li, S. and Pennings, S.C. 2016. Disturbance in Georgia salt marshes: variation across space and time. Ecosphere. 7(10):e01487. (DOI: 10.1002/ecs2.1487)

We documented the frequency and effect on live biomass of five different types of disturbance over 14 years in creekbank and mid-marsh zones of eight salt marshes dominated by Spartina alterniflora in Georgia, USA. Wrack (floating debris) and creekbank slumping were the most common disturbances at the creekbank, and snails were the most common disturbance agent in the mid-marsh. Disturbance frequency varied among sites due to differences in plot elevation and landscape position. Wrack disturbance at the creekbank was positively correlated with plot elevation, and both initial slumping and terminal slumping of creekbank plots were negatively correlated with plot elevation. Wrack disturbance at the creekbank and snail disturbance in the mid-marsh were also most common at barrier island vs. interior marshes. Disturbance varied up to 14-fold among years. Wrack disturbance at the creekbank was negatively correlated with river discharge and sea level, and initial slumping of creekbank plots was also negatively correlated with sea level. The different disturbance types varied in their effects on end-of-year standing plant biomass. At the creekbank, wrack disturbance reduced biomass in affected plots by ~46%, but slumping did not affect biomass until the plot was totally lost. In the mid-marsh, slumping and wrack were not important disturbances, but snail disturbance reduced biomass in affected plots by ~70%. In addition, abiotic conditions (river discharge, maximum monthly temperature, sea level, and precipitation) strongly affected year-to-year variation in biomass. Across the entire landscape, fewer than a quarter of the plots on average were disturbed, and disturbance reduced overall standing biomass by ~18% in the creekbank zone and ~3% in the mid-marsh zone. Our results indicate that wrack has fairly strong effects on end-of-year biomass at the creekbank. Overall, however, variation in abiotic conditions among years had stronger effects on end-of-year standing biomass in both marsh zones than did disturbance.

O'Donnell, J. and Schalles, J.F. 2016. Examination of Abiotic Drivers and Their Influence on Spartina alterniflora Biomass over a Twenty-Eight Year Period Using Landsat 5 TM Satellite Imagery of the Central Georgia Coast. Special Issue: Remote Sensing in Coastal Environments. Remote Sensing. 8(6):22. (DOI: 10.3390/rs8060477)

We examined the influence of abiotic drivers on inter-annual and phenological patterns of aboveground biomass for Marsh Cordgrass, Spartina alterniflora, on the Central Georgia Coast. The linkages between drivers and plant response via soil edaphic factors are captured in our graphical conceptual model. We used geospatial techniques to scale up in situ measurements of aboveground S. alterniflora biomass to landscape level estimates using 294 Landsat 5 TM scenes acquired between 1984 and 2011. For each scene we extracted data from the same 63 sampling polygons, containing 1222 pixels covering about 1.1 million m2. Using univariate and multiple regression tests, we compared Landsat derived biomass estimates for three S. alterniflora size classes against a suite of abiotic drivers. River discharge, total precipitation, minimum temperature, and mean sea level had positive relationships with and best explained biomass for all dates. Additional results, using seasonally binned data, indicated biomass was responsive to changing combinations of variables across the seasons. Our 28-year analysis revealed aboveground biomass declines of 33%, 35%, and 39% for S. alterniflora tall, medium, and short size classes, respectively. This decline correlated with drought frequency and severity trends and coincided with marsh die-backs events and increased snail herbivory in the second half of the study period.

Wang, Y., Castelao, R. and Di Iorio, D. 2016. Salinity Variability and Water Exchange in Interconnected Estuaries. Estuaries and Coasts. (DOI: 10.1007/s12237-016-0195-9)

A high-resolution coastal ocean model is used to investigate salinity variability and water exchange in a complex coastal system off the southern U.S. characterized by three adjacent sounds that are interconnected by a network of channels, creeks, and intertidal areas. Model results are generally highly correlated with observations from the Georgia Coastal Ecosystem Long Term Ecological Research(GCE-LTER) program, revealing a high degree of salinity variability at the Altamaha River and Doboy Sound, decreasing sharply to ward Sapelo Sound. A Lagrangian particle tracking method is used to investigate local residence time and connectivity in the system. Local residence time is highly variable, increasing with distance from the Altamaha River and decreasing with river flow, revealing that discharge plays a dominant role on transport processes and estuary-shelf ex-change. The Altamaha River and Doboy Sound are connected to each other in all seasons, with exchange occurring both via coastal and estuarine pathways. While particles released at the Altamaha and Doboy rarely reach Sapelo Sound, particles released at Sapelo Sound and the creeks surrounding its main channel can reach the entire estuarine system.

Herbert, E., Boon, P., Burgin, A.J., Neubauer, S.C., Franklin, R.B., Ardon, M., Hopfensperger, K.N., Lamers, L. and Gell, P. 2015. A global perspective on wetland salinization: Ecological consequences of a growing threat to freshwater wetlands. Ecosphere. 6(10)(206):1-43. (DOI: 10.1890/ES14-00534.1)

Salinization, a widespread threat to the structure and ecological functioning of inland and coastal wetlands, is currently occurring at an unprecedented rate and geographic scale. The causes of salinization are diverse and include alterations of freshwater flows, land-clearance, irrigation, sea level rise, wastewater effluent, applications of de-icing salts, and storm surges. Climate change and anthropogenic modifications to the hydrologic cycle are expected to further increase the extent and severity of wetland salinization. Salinization alters the fundamental physicochemical nature of the soil-water environment, increasing ionic concentrations and altering chemical equilibria and mineral solubility. Increased concentrations of solutes, especially sulfate, alter the biogeochemical cycling of major elements including carbon, nitrogen, phosphorus, sulfur, iron, and silica. The effects of salinization on wetland biogeochemistry typically include decreased inorganic nitrogen removal (with implications for water quality and climate regulation), decreased carbon storage (with implications for climate regulation and wetland accretion), and increased generation of toxic sulfides (with implications for nutrient cycling and the health/functioning of wetland biota). Indeed, increased salt and sulfide levels induce physiological stress in wetland biota and ultimately can result in large shifts in wetland communities and their associated ecosystem functions. The productivity and composition of freshwater species assemblages will be highly altered, and there is a high potential for the disruption of existing interspecific interactions. Although there is a wealth of information on how salinization impacts individual ecosystem components, relatively few studies have addressed the complex and often non-linear feedbacks that determine ecosystem-scale responses or how wetland salinization will affect landscape-level processes. While the salinization of wetlands may be unavoidable in many cases, these systems may also prove to be a fertile testing ground for broader ecological theories including (but not limited to): alternative stable states and tipping points, trophic cascades, disturbance-recovery, and the role of historical events and landscape context in driving community response to disturbance.

Wieski, K. and Pennings, S.C. 2014. Climate Drivers of Spartina alterniflora Saltmarsh Production in Georgia, USA. Ecosystems. 17(3):473-484. (DOI: 10.1007/s10021-013-9732-6)

Tidal wetlands are threatened by global changesrelated not only to sea level rise but also to alteredweather patterns. To predict consequences ofthese changes on coastal communities, it is necessaryto understand how temporally varyingabiotic conditions drive wetland production. In2000–2011, we conducted annual surveys ofSpartina alterniflora biomass in tidal marshes atnine sites in and around the Altamaha riverestuary on the coast of Georgia, USA. End of theyear live biomass was assessed in the creekbankand midmarsh zones to estimate annual net primaryproduction (ANPP). River discharge was themost important driver of S. alterniflora ANPP,especially in creekbank vegetation. Increased riverdischarge reduces water column salinity, and thiswas most likely the proximate driver of increasedproduction. In the midmarsh zone, the patternswere less distinct, although river discharge wasagain the best predictor, but maximum temperaturehad similar predictive ability. In contrast toresults from terrestrial grasslands, we found noconsistent evidence for a sharply delimited criticalperiod for any climate driver in the tidal marsh,which indicates that plant growth was responsiveto abiotic drivers at any time during the growingseason. Results were broadly consistent acrossmultiple sites within a geographic region. Ourresults differ from previous analyses of productionin S. alterniflora marshes, which either identifiedoceanic drivers of S. alterniflora production or wereunable to identify any drivers, likely because thelow-latitude sites we studied were hotter andmore affected by river discharge than those inprevious studies.

Craft, C.B. 2012. Tidal freshwater forest accretion does not keep pace with sea level rise. Global Change Biology. 18:3615-3623. (DOI: 10.1111/gcb.12009)

Soil properties, accretion, and accumulation were measured in tidal freshwater forests (tidal forests) of the Ogeechee, Altamaha, and Satilla rivers of the South Atlantic (Georgia USA) coast to characterize carbon (C) sequestration and nutrient (nitrogen-N, phosphorus-P) accumulation in these understudied, uncommon, and ecologically sensitive wetlands. Carbon sequestration and N and P accumulation also were measured in a tidal forest (South Newport River) that experiences saltwater intrusion to evaluate the effects of sea level rise (SLR) and saltwater intrusion on C, N and P accumulation. Finally, soil accretion and accumulation of tidal forests were compared with tidal fresh, brackish and salt marsh vegetation downstream to gauge how tidal forests may respond to SLR. Soil accretion determined using 137C and 210Pb averaged 1.3 and 2.2 mm yr−1, respectively, and was substantially lower than the recent rate of SLR along the Georgia coast (3.0 mm yr−1). Healthy tidal forest soils sequestered C (49–82 g m−2 yr−1), accumulated N (3.2–5.3 g m−2 yr−1) and P (0.29–0.56 g m−2 yr−1) and trapped mineral sediment (340–650 g m−2 yr−1). There was no difference in long-term accretion, C sequestration, and nutrient accumulation between healthy tidal forests and tidal forests of the South Newport River that experience saltwater intrusion. Accelerated SLR is likely to lead to decline of tidal forests and expansion of oligohaline and brackish marshes where soil accretion exceeds the current rate of SLR. Conversion of tidal forest to marshes will lead to an increase in the delivery of some ecosystem services such as C sequestration and sediment trapping, but at the expense of other services (e.g. denitrification, migratory songbird habitat). As sea level rises in response to global warming, tidal forests and their delivery of ecosystem services face a tenuous future unless they can migrate upriver, and that is unlikely in most areas because of topographic constraints and increasing urbanization of the coastal zone.

Cai, W.-J. 2011. Estuarine and Coastal Ocean Carbon Paradox: CO2 Sinks or Sites of Terrestrial Carbon Incineration? Annual Review of Marine Science. 3:123-145. (DOI: 10.1146/annurev-marine-120709-142723)

Estuaries are a major boundary in the land-ocean interaction zone where organic carbon (OC) and nutrients are being processed, resulting in a high water-to-air carbon dioxide (CO2) flux (0.25 Pg C y−1). The continental shelves, however, take up CO2 (0.25 Pg C y−1) from the atmosphere, accounting for approximately 17% of open ocean CO2 uptake (1.5 Pg C y−1). It is demonstrated here that CO2 release in estuaries is largely supported by microbial decomposition of highly productive intertidal marsh biomass. It appears that riverine OC, however, would bypass the estuarine zone, because of short river-transit times, and contribute to carbon cycling in the ocean margins and interiors. Low-latitude ocean margins release CO2 because they receive two-thirds of the terrestrial OC. Because of recent CO2 increase in the atmosphere, CO2 releases from low latitudes have become weaker and CO2 uptake by mid- and high-latitude shelves has become stronger, thus leading to more dissolved inorganic carbon export to the ocean.

Hollibaugh, J.T., Gifford, S., Sharma, S., Bano, N. and Moran, M.A. 2011. Metatranscriptomic analysis of ammonia-oxidizing organisms in an estuarine bacterioplankton assemblage. ISME Journal. 5:866-878. (DOI: 10.1038/ismej.2010.172)

Quantitative PCR (qPCR) analysis revealed elevated relative abundance (1.8% of prokaryotes) of marine group 1 Crenarchaeota (MG1C) in two samples of southeastern US coastal bacterioplankton, collected in August 2008, compared with samples collected from the same site at different times (mean 0.026%). We analyzed the MG1C sequences in metatranscriptomes from these samples to gain an insight into the metabolism of MG1C population growing in the environment, and for comparison with ammonia-oxidizing bacteria (AOB) in the same samples. Assemblies revealed low diversity within sequences assigned to most individual MG1C open reading frames (ORFs) and high homology with ‘Candidatus Nitrosopumilus maritimus’ strain SCM1 genome sequences. Reads assigned to ORFs for ammonia uptake and oxidation accounted for 37% of all MG1C transcripts. We did not recover any reads for Nmar_1354–Nmar_1357, proposed to encode components of an alternative, nitroxyl-based ammonia oxidation pathway; however, reads from Nmar_1259 and Nmar_1667, annotated as encoding a multicopper oxidase with homology to nirK, were abundant. Reads assigned to two homologous ORFs (Nmar_1201 and Nmar_1547), annotated as hypothetical proteins were also abundant, suggesting that their unknown function is important to MG1C. Superoxide dismutase and peroxiredoxin-like transcripts were more abundant in the MG1C transcript pool than in the complete metatranscriptome, suggesting an enhanced response to oxidative stress by the MG1C population. qPCR indicated low AOB abundance (0.0010% of prokaryotes), and we found no transcripts related to ammonia oxidation and only one RuBisCO transcript among the transcripts assigned to AOB, suggesting they were not responding to the same environmental cues as the MG1C population.

Porubsky, W.P., Joye, S.B., Moore, W.S., Tuncay, K. and Meile, C. 2011. Field measurements and modeling of groundwater flow and biogeochemistry at Moses Hammock, a backbarrier island on the Georgia coast. Biogeochemistry. 104:69-90. (DOI: 10.1007/s10533-010-9484-8)

A combination of field measurements, laboratory experiments and model simulations were used to characterize the groundwater biogeochemical dynamics along a shallow monitoring well transect on a coastal hammock. A switch in the redox status of the dissolved inorganic nitrogen (DIN) pool in the well at the upland/saltmarsh interface occurred over the spring-neap tidal transition: the DIN pool was dominated by nitrate during spring tide and by ammonium during neap tide. A density-dependent reaction-transport model was used to investigate the relative importance of individual processes to the observed N redox-switch. The observed N redox-switch was evaluated with regard to the roles of nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), ammonium adsorption, and variations in inflowing water geochemistry between spring and neap tides.Transport was driven by measured pressure heads and process parameterizations were derived from field observations, targeted laboratory experiments, and the literature. Modeling results suggest that the variation in inflow water chemistry was the dominant driver of DIN dynamics and highlight the importance of spring-neap tide variations in the high marsh, which influences groundwater biogeochemistry at the marsh-upland transition.

McKay, P. and Di Iorio, D. 2010. The Cycle of Vertical and Horizontal Mixing in a Tidal Creek. Journal of Geophysical Research. 115:C01004. (DOI: 10.1029/2008JC005204)

An experimental study of vertical mixing and along-channel dispersion parameterizedin terms of horizontal mixing near the mouth of the Duplin River (a tidal creek borderedby extensive intertidal salt marshes on Sapelo Island, Georgia) was carried out overseveral spring/neap cycles in the fall of 2005. Vertical mixing is modulated on both M4and fortnightly frequencies with maximum turbulent stresses being generated near thebed on periods of maximum flood and ebb and propagating into the water columnshowing a linear dependence with depth. Values are significantly greater on spring tidethan on neap. Horizontal mixing evaluated by salt fluxes is driven and dominated bytidal dispersion, which is also modulated by the fortnightly spring/neap cycle. Net export of salt from the lower Duplin is shown to be due to residual advection modified by upstream tidal pumping. The tidal dispersion coefficient exhibits a pulsating character with greater values on spring tide followed by smaller values on neap tide.

Schaefer, S.C. and Alber, M. 2007. Temperature controls a latitudinal gradient in the proportion of watershed nitrogen exported to coastal ecosystems. Biogeochemistry. 85(3):333-346. (DOI: 10.1007/s10533-007-9144-9)

Increased export of biologically available nitrogen (N) to the coastal zone is strongly linked to eutrophication, which is a major problem in coastal marine ecosystems (NRC (2000) Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. National Academy Press, Washington, DC; Bricker et al. (1999) National Estuarine Eutrophication Assessment. Effects of nutrient enrichment in the nation’s estuaries. NOAA-NOS Special Projects Office, Silver Spring, MD). However, not all of the nitrogen input to a watershed is exported to the coast (Howarth et al. (1996) Biogeochemistry 35:75–139; Jordan and Weller (1996) Bioscience 46:655–664). Global estimates of nitrogen export to coasts have been taken to be 25% of watershed input, based largely on northeastern U.S. observations (Galloway et al. (2004) Biogeochemistry 70:153–226; Boyer et al. (2006) Global Biogeochem Cycle 20:Art. No. GB1S91). We applied the N budgeting methodology developed for the International SCOPE Nitrogen project (Howarth et al. (1996) Biogeochemistry 35:75–139; Boyer et al. (2002) Biogeochemistry 57:137–169) to 12 watersheds in the southeastern U.S., and compared them with estimates of N export for 16 watersheds in the northeastern U.S. (Boyer et al. (2002) Biogeochemistry 57:137–169). In southeastern watersheds, average N export was only 9% of input, suggesting the need for downward revision of global estimates. The difference between northern and southern watersheds is not a function of the absolute value of N inputs, which spanned a comparable range and were positively related to export in both cases. Rather, the proportion of N exported was significantly related to average watershed temperature (% N export = 58.41 e-0.11 * temperature; R 2 = 0.76), with lower proportionate nitrogen export in warmer watersheds. In addition, we identified a threshold in proportionate N export at 38°N latitude that corresponds to a reported breakpoint in the rate of denitrification at 10–12°C. We hypothesize that temperature, by regulating denitrification, results in increased proportionate N export at higher latitudes. Regardless of the mechanism, these observations suggest that temperature increases associated with future climate change may well reduce the amount of nitrogen that reaches estuaries, which will have implications for coastal eutrophication.


This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959 and OCE-1237140. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.