Hydrogeomorphic Research Articles

Using multiple taxa and wetland classification schemes for enhanced detection of biological response signatures to human impairment

Wetland indices of biological integrity (IBIs) are a common component in monitoring the wetland water resources as required by the United States’ Clean Water Act (CWA). The effectiveness of an IBI to monitor disturbance is dependent on the metrics being consistently responsive to measures of human disturbance within a described classification category. We present IBIs designed for two types of commonly used wetland classification systems – the hydrogeomorphic (HGM) and the National Wetlands Inventory (NWI). The metrics making up the IBIs were derived from anuran, avian, macroinvertebrate, and vegetation communities; each representing increasing levels of resources associated with gathering the necessary data. Knowing which communities’ data best corresponds to impairment can maximize limited wetland monitoring resources, especially if the response differs based on the wetland vegetation type NWI or HGM position on the landscape. By combining these two classification schemes together, to better define a wetland’s form and context on the landscape, more of the variability in community metrics are explained by human impairment. Moreover, when multiple taxa are used within a single wetland classification scheme, the response of the multi-taxa community IBI to the human disturbance gradient is often more sensitive than one-taxa group alone. This approach, a combination of taxa, in hybrid 2-system classification schemes, creates additional utility in measuring the effectiveness of wetland assessments and, or restoration success.

Case study: On hydrological function improvement for an endemic plant habitat in Gangcheon wetland, Korea

Wetlands are one of the most important ecosystems for the biodiversity of plants and animals. Especially, Gangcheon wetland in NamHan river, Korea is a very valuable one because Aster altaicus var. uchiyamae, which is one of endemic plants is inhabited. However, Gangcheon wetland was impaired by a river improvement project called ‘Four Major Rivers Project’. After the project, Gangcheon wetland restoration work was performed, for the rehabilitation of wetland functions but the habitat of Aster altaicus var. uchiyamae could not be restored. Actually, Aster altaicus var. uchiyamae can grow in a habitat made by mixing of gravel and sand from flood inundation of the river but there was no inundation after the project because of the rise of levee height. Therefore, this study evaluated wetland functions using Hydrogeomorphic(HGM) approach and analyzed flood inundation using HEC-GeoRAS model after the project for diagnosis of the functions and the habitat. Then we proposed a plan to improve the functions of Gangcheon wetland as a habitat for Aster altaicus var. uchiyamae. The reference wetland is Gangcheon wetland before the project and the wetland functions after the project were compared with the reference one. As a result of the function assessment, hydrological, biogeochemical, and plant habitat functions were deteriorated. The study wetland has three zones of A, B, and C for Aster altaicus var. uchiyamae habitats and particularly, hydrologic functions for zone C which is poor as the habitat should be improved. In particular, the habitat of Aster altaicus var. uchiyamae was adversely affected by the decrease in inundation depth and water level of the river. These hydrologic functions should be improved and the levee height be lowered for obtaining more proper habitat of Aster altaicus var. uchiyamae. This suggestion might be helpful for establishment of the habitat of Aster altaicus var. uchiyamae. Also, it could be expected that the overall function of Gangcheon wetland will be improved because biogeochemical and plant habitat functions are rehabilitated by the improved hydrological function.

Wetland Functional Responses to Prolonged Inundation in the Active Mississippi River Floodplain

The Mississippi River experienced historic flooding during 2019, inducing >150 days of floodplain wetland inundation. We evaluated flood effects using repeated measures of hydrogeomorphic (HGM) wetland assessment variables prior to the flood (October 2018), immediately post-flood (August 2019) and one year after initial assessment (October 2019). The flood had little/no impact on 11 of 13 assessment variables, but altered the abundance of woody debris and forest floor litter. Immediately after the flood, these changes decreased the functional capacity of wetlands to 1) detain floodwater (mean − 9.7% reduction) and 2) precipitation (−17.3%); 3) cycle nutrients (−7.5%); and export organic carbon (−23.8%). Subsequent sampling documented the detain precipitation function returning to pre-flood conditions. The export organic carbon function also improved, yet remained below pre-flood levels. Other functions will likely require additional recovery time due to the persistence of accumulated excess woody debris. Across all sample intervals, floodplain wetlands displayed high wetland function capacities and appear resilient to surface water inundation. This analysis highlights the utility of the HGM assessment to detect responses to changing environmental conditions over short time intervals. The study also emphasizes the need to incorporate metrics with appropriate impact-response characteristics when developing and implementing ecological assessments.

Prediction of Wetland Hydrogeomorphic Type Using Morphometrics and Landscape Characteristics

Accurate spatial maps of wetlands are critical for regional conservation and rehabilitation assessments, yet this often remains an elusive target. Such maps ideally provide information on wetland occurrence and extent; hydrogeomorphic (HGM) type; and condition/ level of degradation. All three elements are needed to provide ancillary layers to support mapping from remote imagery and ground-truthing. Knowledge of HGM types is particularly important, because different types show differential levels of sensitivity to degradation, and modeling accuracy for occurrence. Here, we develop and test a simple approach for predicting the most likely HGM type for mapped yet unattributed wetland polygons. We used a dataset of some 11,500 wetland polygons attributed by HGM types (floodplain, depression, seep, channeled and un-channeled valley-bottom) from the Western Cape Province in South Africa. Polygons were attributed and described in terms of nine landscape metrics, at a sub-catchment scale. Using a combination of box-and-whisker plots and PCA, we identified four variables (groundwater depth, relief ratio, slope and elevation) as being the most important variables in differentiating HGM types. We divided the data into equal parts for training and testing of a simple Bayesian network model. Model validation included field assessments. HGM types were most sensitive to elevation. Model predication was good, with error rates of only 32%. We conclude that this is a useful technique that can be widely applied using readily available data, for rapid classification of HGM types at a regional scale.

Modified hydrogeomorphic approach for estimating quantitative change of riverine wetland functions

HydroGeoMorphic (HGM) approach could examine relatively more details for wetland functions. However, HGM cannot be utilized in assessing the functional improvement of wetlands because it is limited to evaluating the similarity between the reference and the target wetlands. Therefore, this study proposes the Modified HydroGeoMorphic (MHGM) approach for considering better functions of target wetlands. The values of Index of Function (FCI) and variables in HGM are in the range of 0.0 to 1.0 and so, in MHGM, we redefine the values of variables in 0.0 to 2.0 to find a better function for the target wetland. Based on the monitored variables, FCIs for the wetland functions were estimated and the functions of dynamic surface and subsurface water storage, spatial structure of habitat, and maintaining the distribution and abundance of vertebrates in Imjin riverine wetland as a target wetland showed better FCIs than those of Hwapo which is a reference. This means that MHGM reflects an actual change in FCI, leading to a more favorable evaluation of the target compared to the reference wetland. This indicates that MHGM considers actual functional progress of wetlands by improving HGM's limitations in assessing change.

A hydrogeomorphic and condition classification for Maine, USA, lakes

Deeds J, Amirhbahman A, Norton SA, Bacon LC. 2020. A hydrogeomorphic and condition classification for Maine, USA, lakes. Lake Reserv Manage. 36:122–138. The water quality of lakes is influenced by natural landscape features, anthropogenic watershed activities, and local-scale characteristics of lake basins. Lake assessment for water quality standards is enhanced by a lake classification framework that allows for comparison of lakes of similar types and the creation of benchmark water quality values from reference lakes. Conventional lake classifications, such as those based on trophic state alone, commonly do not incorporate the natural features of the landscape or lake-specific characteristics that influence lake condition. We present a hydrogeomorphic (HGM)-based lake classification, tested with linear mixed effects modeling, to determine which natural features of lakes and their watersheds show the greatest influence on lake condition. A priori classification schemes were created for model testing using combinations of various HGM features. Model strength was evaluated based on ability to predict mean lake total phosphorus and specific conductivity values. Aggregated Level IV Ecoregions stratified by 2 categories of maximum lake depth offered the most robust lake classification. We defined condition classes of reference, intermediate, and altered lakes based on the gradient of developed watershed area that is unique to each lake class. This approach to lake classification has implications for any region or study, as HGM variables relevant to the population of lakes of interest may be tested for efficacy in a variety of schemes to suit the goals of the classification.

Hydrogeomorphic controls on soil carbon composition in two classes of subalpine wetlands

Wetlands play a vital role in terrestrial carbon (C) sequestration, but the sensitivity of their C stocks to disturbance remains uncertain, requiring enhanced understanding of the processes that govern C storage and removal. The unique conditions in wetlands from different hydrogeomorphic (HGM) classes likely regulate the cycling, storage and vulnerabilities of wetland soil C stocks. To determine how differences in hydrogeomorphic setting influence soil organic carbon (SOC) processing, we compared C content and composition between depressional and slope wetlands located in the Colorado Rocky Mountains. Isolated depressional wetlands were characterized by seasonally declining water tables, slow discharge, high clay content, and thick organic horizons. Slope wetlands received perennial groundwater inputs and had coarser soil textures and thinner organic horizons. Seasonal snowmelt inputs coupled with low hydrologic discharge and higher clay content in depressional wetlands were predicted to sustain anoxic conditions, leading to high SOC content and chemically reduced C compounds. Depressional wetland soils had higher SOC content at depth and higher porewater DOC concentrations compared to slope wetland soils. Solid-state 13C nuclear magnetic resonance spectroscopy demonstrated that aliphatic compounds were the dominant SOC component in depressional wetlands compared to aromatic C forms in the slope wetlands. The higher prevalence of aliphatic carbon in depressional wetland soils suggests that stored SOC is protected by anaerobic conditions to a greater extent than in the slope wetlands, and that this SOC may be more vulnerable to drying and oxic conditions associated with wetland drainage and climate change.

Hydrogeomorphic modeling of low-marsh habitat in coastal Georgian Bay, Lake Huron

Potential impacts of global climate change on the amount of low-marsh habitat in coastal wetlands of the Great Lakes are unknown, which may have important implications for the Great Lakes fish community that use such habitat. We developed a generalized linear model that uses only hydrogeomorphic (HGM) features and lake elevations to predict the extent of low marsh in coastal wetlands of eastern and northern Georgian Bay. The McMaster Coastal Wetland Inventory was used as a reference dataset to train the model, while best available data were assembled to create a digital elevation model that was used to derive all HGM features at a lake elevation of 176.17 m (International Great Lakes Datum 1985). The best predictive model included depth, slope, and exposure as HGM variables, yielding an area under the curve (AUC) score of 0.83. We classified the model output into low-marsh and open-water habitat using a threshold value identified by maximizing the true skill statistic. The classified model output had sensitivity and specificity scores of 0.80 and 0.75, respectively, and correctly identified 81% of the low-marsh units present in the reference dataset with an average 60% areal overlap between the model prediction and reference dataset. We applied the model to two external datasets to check model performance, and found the lowest AUC to be 0.79, with associated sensitivity and specificity scores of 0.65 and 0.77, respectively. Applying this model with future water-level scenarios should provide a cost-effective alternative for forecasting changes in the amount of low marsh-habitat in Georgian Bay.

Assessing, with limited resources, the ecological outcomes of wetland restoration: a South African case

Resources for evaluating the ecological outcomes of ecosystem restoration projects are often limited, especially within government‐funded programs. In order to rapidly assess the ecological outcomes of wetland restoration, an improved approach has been developed, which was applied in the assessment of the ecological outcomes at nine restoration sites of South Africa's Working for Wetlands program. The sites encompass a diversity of restoration problems and land use contexts. The approach begins by distinguishing hydrogeomorphic (HGM) units, for which ecological condition is assessed and reported for hydrology, geomorphology, and vegetation pre‐ and post‐restoration. These three components are closely linked but, as demonstrated at some of the sites, may respond differentially to restoration interventions. For most HGM units, overall ecological condition was improved by between 10 and 30%, with the greatest contribution of restoration generally being to the hydrology component. Having determined the integrity and costs of the interventions, cost‐effectiveness is then reported in South African Rands per hectare equivalent restored, which was found to vary by more than an order of magnitude across the HGM units assessed. Cost‐effectiveness must be interpreted in the light of the long‐term integrity of the interventions, the site's landscape context, and the contribution of restoration to ecosystem services provision. Some sites may be considerably less cost‐effective than others, but the cost may nonetheless be justified if the sites make key contributions to ecosystem services provision. The study was conducted in the context of a formative evaluation and the findings are envisaged to improve wetland restoration practice.

Quantifying Functional Increases Across a Large-Scale Wetland Restoration Chronosequence

Over 300,000 ha of forested wetlands have undergone restoration within the Mississippi Alluvial Valley region. Restored forest successional stage varies, providing opportunities to document wetland functional increases across a large scale restoration chronosequence using the Hydrogeomorphic (HGM) approach. Results from >600 restored study sites spanning a 25 year chronosequence indicate that: 1) wetland functional assessment variables increased toward reference conditions; 2) restored wetlands generally follow expected recovery trajectories; and 3) wetland functions display significant improvements across the restoration chronosequence. A functional lag between restored areas and mature reference wetlands persists in most instances. However, a subset of restored sites have attained mature reference wetland conditions in areas approaching or exceeding tree diameter and canopy closure thresholds. Study results highlight the importance of site selection and the benefits of evaluating a suite of wetland functions in order to identify appropriate restoration success milestones and design monitoring programs. For example wetland functions associated with detention of precipitation (a largely physical process) rapidly increased under post restoration conditions, while improvements in wetland habitat functions (associated with forest establishment and maturation) required additional time. As the wetland science community transitions towards larger scale restoration efforts, effectively quantifying restoration functional improvements will become increasingly important.

Interplay between climate and hydrogeomorphic features and their effect on the seasonal variation of dissolved organic matter in shallow temperate lakes of the Southern Andes (Patagonia, Argentina): a field study based on optical properties

AbstractThis study analyses the effects of the interplay between climate seasonality and hydrogeomorphic (HGM) lake features on dissolved organic matter (DOM) properties in two neighbouring shallow lakes of Andean Patagonia with different connectivities. The survey was conducted over three years at the end of the wet and dry seasons, assessing the seasonal and interannual variation of dissolved organic carbon (DOC) concentration, whole‐lake DOC mass, and DOM quality, through chromophoric and fluorescent DOM (CDOM and FDOM, respectively) properties. During the wet season (fall–winter), precipitation and runoff increased water discharge, water level, and inputs of terrestrial DOM with high aromaticity, humic content, and high molecular weight in both lakes. Contrastingly, during the dry season (spring–summer), in which photodegradation promoted by high irradiance and stagnant conditions drove DOM transformation, nonhumic, low‐molecular‐weight DOM prevailed. Both lakes displayed synchronicity in their DOC mass and CDOM and FDOM properties, indicative of similar responses to climate forcing, although the overall impact was modulated by their HGM features. Conversely, DOC concentration showed asynchronous responses between lakes, due to the higher intensity of the dilution or evapoconcentration processes in the connected lake, highlighting that DOC concentration is not always sensitive to climate‐driven forces. Overall, this study emphasizes the importance of variables other than DOC concentration, like whole‐lake DOC mass, DOM quality, and HGM features, to better understand the effect of climate variability on DOM dynamics. Our results allow inferring the potential impact of an environmental scenario characterized by lower precipitation and sustained warming on DOM dynamics in Northern Andean Patagonia.

Comparing tiers of a multi-tiered wetland assessment in the Prairie Pothole Region

Wetland assessment has been shown to be an important tool in understanding the condition and function of the world’s wetlands, and use of muli-tiered assessment strategy has been recommended. In order to evaluate the performance of each tier of a multi-tiered wetland assessment strategy, we sampled 255 seasonally-ponded wetlands in the Missouri Coteau, the most wetland dense ecoregion in the Prairie Pothole Region. We assessed the condition of each study wetland using four sampling methods and models (tiers) of increasing levels of effort and complexity: (1) a level 1 assessment using the geographic information system-based Landscape Wetland Condition Analysis Model (LWCAM); (2) a level 2 assessment using the North Dakota Rapid Assessment Method (NDRAM); (3) a level 3- assessment using the vegetative-based Floristic Quality Index (FQI) and (4) a level 3 assessment consisting of a Hydrogeomorphic (HGM) Model functional assessment. We compared assessment tiers to determine how similar the different levels of assessment ranked sites either by condition or function. Both the NDRAM and FQI assessments, though very different in wetland characteristics assessed, provided similar condition rankings as the more intensive level 3 HGM assessment (89 and 90% similar, respectively). Additionally, the FQI was 86% similar to the level 2 NDRAM, indicating that these two assessment methods have utility in assessing wetlands similar to a HGM assessment. Information from this study can be used as a tool for determining need specific, financial, and time appropriate wetland sampling methods.

HGM 적용을 통한 하도습지의 기능평가 제고 방안 연구

본 연구는 유수역에 적용 시 효율성을 높일 수 있는 HGM(Hydrogeomorphic) 기능 평가 틀을 제안 하였고, 이와 더불어 유수역에 적합한 기준습지를 습지보호지역 중 유수역에 해당되는 곳에 적용함으로써 기준습지의 활용성 제고를 목적으로 하였다. 기능평가는 기존의 HGM 평가 틀을 기반으로 하였으나, 유수역의 특성을 반영하여 수문학적, 생지화학적, 식물서식처, 동물서식처 기능에 따른 총 10개 항목의 기능지수(Functional Capacity Index, FCI)를 산정하였다. 유수역에 적합한 기능평가 틀의 각 항목별 지수를 하도 습지보호지역인 2개소의 습지에 적용해 본 결과 인공호안 하도습지인 담양하천습지는 제외지내 식생 분포 면적비율, 단위면적당 식물종수, 버드나무 분포 면적비율 등의 값이 높게 분석되었고, 이들 변수 값이 반영된 양분 순환(947,668.00), 식물종풍부도 및 특징적인 식생 군집 유지(6.39), 서식처의 공간구조적 유지(11.00) 등의 기능지수 값이 상대적으로 높게 분석되었다. 자연호안 하도습지인 한반도습지는 담양하천습지에 비해 하천 규모가 크고, 생물종다양성과 관련된 변수값이 높았으며, 이와 관련된 에너지 감쇄(17,805.16), 지표하 저류(0.54), 이입된 원소와 화합물 제거(103,052.73), 잔재생체량 유지(2.31), 산포 및 연결성 유지(6.50), 저서성무척추동물의 종다양성(1.60), 척추동물의 종다양성 및 종수(2.52/ 151.50) 등에서 기능지수 값이 상대적으로 높게 분석되었다. This study aims to suggest the framework of functional assessment on lotic area based on HGM(Hydrogeomorphic) approach targeting Wetland Protected Areas which are in the type of river channel, and to set up the fundamental data as a reference wetland. A total of 10 factors in terms of hydrology, biogeochemistry, plant habitat and animal habitat was analyzed based on the original approach of HGM and each Functional Capacity Index(FCI) of those factors was calculated. As the result of the modified FCI analysis, Damyang riverine wetland which is with artificial river bank had high values in the variables of area ratio of actual vegetation in the foreland, the number of plant per area and the area ratio of Salix spp., and those values were highly reflected on the factors of Nutrient Cycling(947,668.00), Species Richness and Maintain Characteristic Plant Communites(6.39) and Maintain Spatial Structure of Habitat(11.00). The Hanbando wetland which is keeping the natural bank had higher values in the variables of structural scale and species diversity, and the those values were highly reflected on the factors of Energy Dissipation(17,805.16), Subsurface Storage of Water(0.54), Removal of Imported Elements and Compounds(103,052.73), Maintain Characteristic Detrital Biomass(2.31), Maintenance of Interspersion and Connectivity (6.50), Species Diversity of Benthic macro-invertebrates(1.60) and Species Diversity of Vertebrate & Species Number of Other Animals(2.52/ 151.50), compared to the Damyang Riverine Wetland.

HGM: a call for model validation

The hydrogeomorphic (HGM) approach to wetland functional assessment has been in place for more than 20 years, yet most models developed to date have not been validated. Although HGM models are typically calibrated to a data set, we lack true validation efforts, especially as wetland scientists and managers continue to assert that measuring structure equates to an assessment of function. This paper is a call for a renewed effort at HGM validation in order to fully assure that we know when structure is measured we are also measuring function.

The development of a classification system for inland aquatic ecosystems in South Africa

A classification system is described that was developed for inland aquatic ecosystems in South Africa, including wetlands. The six-tiered classification system is based on a top-down, hierarchical classification of aquatic ecosystems, following the functionally-oriented hydrogeomorphic (HGM) approach to classification but incorporating structural attributes at the lower levels of the hierarchy. At Level 1, a distinction is made between inland, estuarine and shallow marine systems using the degree of connectivity to the open ocean as the key discriminator. Inland systems are characterised by the complete absence of marine exchange and/or tidal influence. At Level 2, inland systems are grouped according to the most appropriate spatial framework for the particular application. At Level 3, four primary Landscape Units are distinguished (Valley floor, Slope, Plain, Bench) on the basis of the topographic position within which a particular inland aquatic ecosystem is situated, in recognition of the influence that the landscape setting has over hydrological and hydrodynamic processes acting within an aquatic ecosystem. Level 4 identifies HGM Units, defined primarily according to landform, hydrological characteristics and hydrodynamics. The following primary HGM Units (or HGM Types), which represent the main units of analysis for the classification system, are distinguished at Level 4A: (1) River; (2) Floodplain Wetland; (3) Channelled Valley-Bottom Wetland; (4) Unchannelled Valley-Bottom Wetland; (5) Depression; (6) Seep; (7) Wetland Flat. Secondary discriminators are applied at Level 5 to classify the hydrological regime of an HGM Unit, and Descriptors at Level 6 to categorise a range of biophysical attributes. The HGM Unit at Level 4 and the Hydrological Regime at Level 5 together constitute a Functional Unit, which represents the focal point of the classification system. The utility of the classification system is ultimately dependent on the level to which ecosystem units are classified, which is in turn constrained by the type and extent of information available.

Geographically Isolated Wetlands: Why We Should Keep the Term

Use of the term “isolated wetlands” in the U.S. Supreme Court’s SWANCC decision created confusion, since it could imply functional isolation. In response, the term “geographically isolated wetlands” (GIWs) – wetlands surrounded by uplands – was introduced in 2003. A recent article revisits the term, concluding that it is a misnomer that adds to the confusion. Hydrogeomorphic (HGM) type and non-adjacency to jurisdictional waters are suggested as alternatives. To address this issue, I pose two questions: is there a need to identify wetlands surrounded by uplands and what to call them? Regarding the former, there is a legal/regulatory need resulting from the Court’s Rapanos decision: to help determine whether such wetlands have a significant nexus with traditional navigable waters. There is also a scientific need to understand how the normal lack of surface water connectivity affects function. Regarding the second question, neither HGM type nor non-adjacency adequately identifies wetlands surrounded by uplands. I contend that “GIW” remains the most informative option. However, the term needs to be applied properly and researchers should emphasize that being surrounded by uplands does not imply hydrological or biological isolation. Further, conclusions from one GIW type should be extrapolated to other GIWs with great care.

Effect of eutrophication on mercury (Hg) dynamics in subtropical reservoirs from a high Hg deposition ecoregion

Eutrophication can have opposite effects on mercury (Hg) bioavailability in aquatic systems, by increasing methylmercury (MeHg) production but reducing Hg biomagnification. Globally, the effect of eutrophication on Hg dynamics remains largely untested at lower latitudes such as eastern China, a productive subtropical ecoregion with Hg emission and deposition rates that are among the highest worldwide. Here, we quantify Hg (both MeHg and total Hg, THg) concentrations, Hg bioaccumulation, and Hg biomagnification rates in reservoir food webs across a gradient of eutrophication indicated by chlorophyll a (Chl a), zooplankton density, and total phosphorus (TP). We also assess the effect of hydrogeomorphic (HGM) features on Hg concentrations in water and biota. Water THg concentrations were strongly correlated with TP and were greater in reservoirs at higher elevations with short water retention times (WRT). Zooplankton and top predator THg concentrations were negatively correlated with Chl a, suggesting algal biodilution; evidence for zooplankton density dilution was also found when subtropical reservoirs were compared at a global scale with temperate lakes. Mercury bioaccumulation and biomagnification factors, respectively, did not correlate with increasing Chl a or zooplankton density suggesting no effect of plankton density on Hg trophic transfer. In subtropical reservoirs, eutrophication is associated with lower Hg concentrations in biota but does not explain Hg biomagnification; HGM features (i.e., elevation, WRT) and land use (i.e., % crop) appear to also influence Hg concentrations and bioaccumulation in reservoir food webs.

Geographically Isolated Wetlands: Rethinking a Misnomer

We explore the category Bgeographically isolated wetlands^ (GIWs; i.e., wetlands completely surrounded by uplands at the local scale) as used in the wetland sciences. As currently used, the GIW category (1) hampers scientific efforts by obscuring important hydrological and ecological differences among multiple wetland functional types, (2) aggregates wetlands in a manner not reflective of regulatory and management information needs, (3) implies wetlands so described are in some way Bisolated,^ an often incorrect implication, (4) is inconsistent with more broadly used and accepted concepts of Bgeographic isolation,^ and (5) has injected unnecessary confusion into scientific investigations and discussions. Instead, we suggest other wetland classifi- cation systems offer more informative alternatives. For example, hydrogeomorphic (HGM) classes based on well- established scientific definitions account for wetland func- tional diversity thereby facilitating explorations into questions of connectivity without an a priori designation of Bisolation.^ Additionally, an HGM-type approach could be used in combination with terms reflective of current regula- tory or policymaking needs. For those rare cases in which the condition of being surrounded by uplands is the relevant distinguishing characteristic, use of terminology that does not unnecessarily imply isolation (e.g., Bupland embedded wetlands^) would help alleviate much confusion caused by the Bgeographically isolated wetlands^ misnomer.

Variation in Forest Canopy Composition of Riparian Networks from Headwaters to Large River Floodplains in the Southeast Coastal Plain, USA

Data on canopy trees (stems ≥ 15 cm DBH) in riparian wetlands, spanning from headwaters to large river floodplains, were used to test whether forest canopy composition differed among hydrogeomorphic (HGM) riverine subclasses and among physiographic sub-regions (Major Land Resource Areas; MLRA) within a given HGM subclass. Riverine stands (n = 225) were sampled in four MLRA regions of the Southeastern U.S. Atlantic and Gulf Coastal Plain Physiographic Provinces. Composition data were analyzed using Non-metric Multidimensional Scaling and Multiple-Response Permutation Procedures to evaluate differences among HGM subclasses and MLRA regions. Analyses showed that canopy composition differed among three a priori subclasses related to Strahler stream order: headwater complex (along 1st-3rd order streams), mid-gradient floodplain (4th-6th order), and low-gradient floodplain (> 6th order). Further, composition also differed by MLRA region within each subclass. Thus, not only was species composition related to riverine hydrogeomorphology across a wide physiographic area, but differences in composition within HGM subclasses were also related to sub-region. These data could be useful in defining floristic reference standards when evaluating floodplain condition in southeastern USA Coastal Plain stream networks.

Mercury dynamics in groundwater across three distinct riparian zone types of the US Midwest

Although the intense biogeochemical gradients present in riparian zones have the potential to affect mercury (Hg) cycling, Hg dynamics in riparian zones has received relatively little attention in the literature. Our study investigated groundwater filtered total mercury (THg) and methylmercury (MeHg) dynamics in three riparian zones with contrasting hydrogeomorphic (HGM) characteristics (till, alluvium, outwash) in the US Midwest. Despite high Hg deposition rates (>16 μg m(-2)) in the region, median THg (<1.05 ng L(-1)) and MeHg (<0.05 ng L(-1)) concentrations were low at the study sites. Methylmercury concentrations were significantly (p < 0.05) correlated to THg (R = 0.82), temperature (R = 0.55), and dissolved organic carbon (DOC) (R = 0.62). THg also correlated with groundwater DOC (R = 0.59). The proportion of MeHg in THg (%MeHg) was significantly correlated to temperature (R = 0.58) and MeHg (R = 0.50). Results suggest that HGM characteristics, the presence of tile drains, and the propensity for overbank flooding at a riparian site determined the extent to which stream water Hg concentrations influenced riparian groundwater Hg levels or vice versa. Differences in hydrogeomorphic characteristics between sites did not translate however in significant differences in groundwater MeHg or %MeHg. Overall, widespread Hg contamination in the most common riparian hydrogeomorphic types of the US Midwest is unlikely to be a major concern. However, for frequently flooded riparian zones located downstream from a potentially large source of Hg (e.g., concentrated urban development), Hg concentrations are likely to be higher than at other sites.