Diatom Assemblage Composition Research Articles

The Effects of Cattle Grazing on the Composition of Diatom Assemblages in the Peatland Pools of the Southeastern Alps (Italy, Adamello-Brenta Nature Park)

Mires are significant C-sinks and important habitats for biodiversity conservation. Particularly in the Alpine peatlands, grazing and trampling cause mires to degrade, causing changes in plant communities that increase bulk density, peat erosion, and nutrient excesses. We took sixteen samples of epipelic and epiphytic diatoms from mire pools subjected to varying degrees of grazing in the Adamello-Brenta Nature Park (Trento, Italy). We identified >100 diatom species (ca. 70% included in Red List threat categories). We used nMDS to identify groups of diatom species. Their statistical significance was checked with an ANOSIM and, to identify the species that contributed most to the difference between the two groups, a BEST-SIMPER procedure was carried out. The multivariate analyses allowed us to identify significant differences among the most and least grazed sites, and to select the species that contributed most to this distinction. We confirmed that some species were more frequent and abundant in the most grazed sites and behaved as opportunists in the presence of a greater nutrient input, to the detriment of rare and sensitive species, which were more numerous and abundant in the less grazed sites. We provided useful information on grazing effects, underlining the importance of managing and protecting habitats of unique environmental value.

Macroinvertebrate, algal and diatom assemblages respond differently to both drying and wetting transitions in non‐perennial streams

Abstract Biological assemblages in streams are influenced by hydrological dynamics, particularly in non‐perennial systems. Although there has been increasing attention on how drying impacts stream organisms, few studies have investigated how specific characteristics of drying and subsequent wetting transitions influence biotic responses via resistance and resilience traits. Here, we characterized how hydrologic metrics, including those quantifying drying and wetting transitions as well as dry and wet phases, alter diversity and composition of three aquatic assemblages in non‐perennial streams in southern California: benthic macroinvertebrates, soft‐bodied algae and diatoms. We found that flow duration prior to sampling was correlated with variation in macroinvertebrate and soft‐bodied algal assemblage composition. The composition and richness of diatom assemblages, however, were predominantly influenced by the drying start date prior to sampling. Contrary to other studies, the duration of the dry phase prior to sampling did not influence the composition or richness of any assemblage. Although our study was conducted within a region in which each assemblage experienced comparable environmental conditions, we found no single hydrologic metric that influenced all assemblages in the same way. The hot‐summer Mediterranean climate of southern California likely acts as a strong environmental filter, with taxa in this region relying on resistance and resilience adaptations to survive and recolonize non‐perennial streams following wetting. The different responses of algal and diatom assemblages to hydrologic metrics suggest greater resilience to drying and wetting events, particularly for primary producers. As drying and wetting patterns continue to change, understanding biodiversity responses to hydrologic metrics could inform management actions that enhance the ecological resilience of communities in non‐perennial streams. In particular, the creation and enhancement of flow regimes in which natural timing and duration of dry and wet phases sustain refuges that support community persistence in a changing environment.

Inferences based on diatom compositions improve estimates of nutrient concentrations in streams

Nutrient concentrations in streams vary strongly with flow conditions, and routinely gathered field measurements of nutrients reflect this variability. Diatom assemblage composition has been used in previous studies to infer nutrient concentrations, and because diatoms integrate nutrient concentrations over longer periods of time, diatom inferences may be less susceptible to fluctuations in streamflow. We tested this hypothesis by leveraging differences in the flashiness of streams across a large continental data set. More specifically, we tested whether the variabilities of direct measurements and diatom inferences of dissolved phosphorus and nitrate were greater in flashy versus non-flashy streams. We further considered whether models linking landscape predictor variables to nutrient concentrations yielded consistent results across flashy and non-flashy streams. Our analysis indicated that measured nutrient concentrations were more variable in flashy compared to non-flashy streams and that landscape models identified different important predictors of nutrient concentrations when fit using data from flashy vs. non-flashy streams. In contrast, variabilities of diatom-inferred nutrient concentrations were similar among stream types, as were the important predictor variables (e.g., manure application rates for nitrate and number of wet days for dissolved phosphorus). These analyses indicate that use of diatom-inferred nutrient concentrations can potentially improve efforts to quantify stream nutrient concentrations.

HOLOCENE SEDIMENTATION HISTORY OF THE KNIPOVICH RIDGE AREA (THE NORWEGIAN SEA) BASED ON THE ANALYSIS OF MICROFOSSILS

Climatic events of the last millennia in the northeastern part of the Norwegian-Greenland basin were reconstructed based on micropaleontological studies of sediments from the AMK-6150 core. New results were obtained from analyses of dinoflagellate cysts, diatoms, and benthic and planktonic foraminifers. These results allow us to infer the time of sediment accumulation and the prevailing natural conditions. According to preliminary data on climatostratigraphy based on the composition of microfossils, the period of sediment formation in the AMK-6150 core does not exceed 7 thousand years. The composition of microfossil assemblages and the results of reconstructions indicate repeated changes in marine environmental conditions during this time. Sediments at the depth of 23-24 cm recorded the beginning of a significant growth of temperature and salinity of surface waters due to a possible increase of the Norwegian Current influence. According to the species and quantitative composition of dinocyst and diatom assemblages, sediments in the short depth range of 14-12 cm accumulated during a period of noticeable decrease in temperature and increased influence of Arctic water masses. The analysis of dinocysts by the method of modern analogues allows reconstruction of the quantitative values of summer paleotemperatures of surface waters and the duration of ice cover. It was found that the cooling episodes and probable formation of seasonal sea ice were possible during the accumulation of sediments at the depths of 29-24, 14-12 and 3-1 cm.

The effect of habitat on benthic diatoms in subarctic ponds – differences between rock and sediment

Subarctic ecosystems represent a vulnerable environment for their biota and studying their habitats provides a better understanding of ongoing and accelerated environmental change. Literature suggests that habitat and environmental variables such as water chemistry jointly influence benthic algal species richness and assemblage composition. However, the combined effects of habitat and water chemistry on benthic algal communities are poorly understood especially in subarctic freshwaters. This study compared benthic diatom assemblages on rocks and on sediment and examined if environmental variables affect diatoms similarly in these habitats. A total of 20 subarctic ponds were sampled for diatoms on sediment and rock in summer 2022 in northern Fennoscandia. Differences in species richness and Pielou evenness between habitats were analysed by a paired Wilcoxon signed-rank test. The most significant environmental variables influencing diatom species richness were determined by generalized linear models. Differences in diatom assemblage compositions were analysed using non-metric multidimensional scaling, analysis of similarities, and Jaccard similarity index. Finally, to study how environmental variables shape diatom compositions on rock and sediment, a redundancy analysis was performed. According to our results species richness and evenness were significantly higher on sediment than on rock, and water pH was the major determinant for diatom richness in both habitats. Diatom assemblage composition did not differ between rocks and sediment, and composition was shaped by the same environmental variables such as pond surface area and water pH in both habitats. Our results support the prevailing conception that sediment habitat contains higher diatom species richness than rocks, while assemblage composition does not differ between these habitats. This study highlights the role of water pH on benthic diatoms regardless of their habitat. Given that global warming is expected to affect water pH levels, such a change will probably have a notable influence also on subarctic diatom communities in the future decades.

Litho- and biostratigraphy of the Holocene sediments in the southern part of Lake Ladoga

We present the results of study sediment core, taken in the southern part of Lake Ladoga at the depth of 44 m. The sediments are gray silt with total thickness of 0.8 m. Diatom and loss-on-ignition and grain-size analyzes and radiocarbon dating were carried out. The retrieved sediments formed during the Holocene starting from the regression of the Ancylus Lake at the Baltic ca. 10,200 cal. BP. Changes in the composition of diatom assemblages and the dynamics of organic accumulation correspond to the typical scenarios that we recorded in the Holocene sediments in the central part of Lake Ladoga. No sedimentary hiatuses were recorded. Thus, it has been established that aquatic, relatively deep-water conditions existed in this part of the Ladoga basin starting from the Early Holocene with the accumulation of fine-grained sediments.

Distribution and morphology of the diatom genus Olifantiella Riaux-Gobin & Compère in Indonesian and Australian waters, including the description of O.gondwanensis sp. nov.

Samples from coastal tropical waters of Central Sulawesi, Bangka Island and Bawean Island in Indonesia and from the Great Barrier Reef at Fitzroy Island in Queensland, Australia were analysed for species composition of diatom assemblages with a focus on Olifantiella. Whereas samples from Fitzroy Island littoral in Australia retrieved only one species of Olifantiella, in Poso Bay, Indonesia, we observed at least six species. All established taxa were documented with light (LM) and scanning electron microscope (SEM) and principal component analysis (PCA) analysis was used to compare the species, based on the basic valve parameters of length, width, length to width ratio and striae density. A new species of the genus Olifantiella, O.gondwanensis is described from Australia. In addition, we showed the distinct nature of O.pilosellavar.rhizophorae permitting to species status. Particular attention is placed on girdle bands in this genus.

Sediment records from Lake Nipissing (ON, Canada) register a lake-wide multi-trophic response to climate change and reveal its possible role for increased cyanobacterial blooms

Recurring cyanobacterial blooms have been reported in Lake Nipissing (Ontario, Canada) since at least 2000. However, monitoring data indicate that surface water nutrient concentrations have not increased over the past ∼45 years in this large, shallow, mesotrophic lake. Applying paleolimnological methods, we establish baseline environmental conditions and extend the Lake Nipissing water quality data to pre-industrial times. Lake-wide assessment of changes in nutrient status, algal production, and effects of climate warming are provided by examining: 1) modern and pre-industrial sedimentary diatom assemblages from nine locations; 2) temporal trends in sedimentary photosynthetic pigments over ∼200 years from five locations; and 3) detailed sedimentary profiles of diatom and chironomid assemblage composition from Callander Bay, spanning the past ∼200 years. A pronounced shift in diatom assemblage composition was registered at all sites with pre-industrial sediments dominated by Aulacoseira spp. and modern samples dominated by Discostella stelligera. The detailed diatom analysis for Callander Bay indicated that this shift occurred ca. 1990. Since ca. 1975, cyanobacterial production (spectrally inferred from sediments) exceeded the ∼200-year mean and matches the observed increase in bloom reports. Twentieth century shifts in chironomid assemblages in Callander Bay from Procladius to Chironomus spp. dominance indicate lower hypolimnetic oxygen in recent decades. Collectively, these changes suggest that previously polymictic Lake Nipissing is now experiencing periods of thermal stratification and development of transient anoxia in bottom waters. These conditions are more prevalent and widespread in modern Lake Nipissing, consistent with a response to recent warming, decreased wind speed, and a longer ice-free period.

Aquatic phases have a stronger effect on lotic benthic diatoms than human-induced microhabitat variability

AbstractHere, we studied the influence of changes of aquatic phases (standing and flowing phases) and human-induced habitat variability (natural and artificial) on the composition and diversity of benthic diatom assemblages in a small lowland stream in the Pannonian Ecoregion. Significant differences in composition were hypothesized between phases and habitats. Lower diversity was hypothesized in the flowing phase and in the artificial habitat. In addition, worser ecological status were assumed in the artificial habitat and in the standing phase than in the others. Our results only partially supported our hypotheses. While there was no significant difference in the composition of the assemblages between the natural and concreted habitats, the alteration in flow conditions resulted in a significant change. No significant differences in diversity were found between aquatic phases. In contrast, biodiversity was higher in the artificial habitat than in the natural one. While the anthropogenic impact, i.e., concreted streambed has no significant influence on diatom-based ecological status, values of diatom indices were significantly higher in the flowing phase. Our results highlight that extreme weather events play a major role in shaping diatom assemblages even during a short period, which should be taken into account in water management and nature conservation measures.

Spatial distribution of surface-sediment diatom assemblages from 45 Tibetan Plateau lakes and the development of a salinity transfer function

The Tibetan Plateau (TP) is often referred to as the “Asian Water Tower”, as it plays a crucial role in supplying water for ∼1.9 billion people. Lakes in this arid mountain region have a wide salinity gradient and are sensitive to the complex interactions between climate and the water cycle, affecting aquatic ecosystems and species distributions. Here, we analyzed relationships between measured environmental variables and the spatial distribution of surface sediment diatom assemblages from 45 lakes located throughout the TP with the intention of developing a diatom-inference model for salinity. Diatom assemblage composition and diversity differed among lakes from the western (W), southern (S), northern (N), and southeastern (E) regions of the TP. The most common taxa observed in the lake set, Pantocsekiella ocellata, Nitzschia palea, and Staurosira venter, tended to have widespread distributions across the TP. In general, diatom diversity and richness tended to be highest in lakes with salinity concentrations between 0.1 and 10 g/L and were lowest in high salinity lakes (>20 g/L) of the N region. Canonical correspondence analysis (CCA) identified lake water salinity and maximum lake depth as explaining a statistically significant portion of diatom assemblage variation. Weighted-averaging (WA) was used to develop a diatom-based salinity inference model (R2jack = 0.71, RMSEPjack = 0.28) based on diatom optima from freshwater to meso-hyposaline lakes of the TP. Diatom-salinity reconstructions and changes in diatom community composition can potentially be used to track long-term patterns in climate and environmental conditions across the highly complex mountain landscape of the TP.

Diatom Assemblages from the Daginsky Mud Volcano Sediments (Eastern Sakhalin) and Their Implication

Diatom assemblages in mud volcanoes are quite rare and are poorly studied. The finding of a rich diatom flora in the sediments of the Daginsky Mud Volcano (DMV), located in the tidal zone of the Nyisky Bay of the Sea of Okhotsk, is of interest to study the conditions for the formation of host sediments in the zone of influence of various extreme factors, such as outflows of gases and water–mud mass, thermal springs, and tides. For this purpose, the taxonomical composition of diatom assemblages and concentration of diatoms in DMV sediments were studied. A comparison was made with the diatoms of geothermal springs of the Russian Far East associated with volcanic activity, and the stratigraphic occurrence of the found extinct diatoms was analyzed, which is important for determining their source and the age of the mud volcano roots. Diatom assemblages from DMV sediments are mainly characterized by the predominance of Metascolioneis tumida, Paralia sulcata, Odontella aurita, Pinnunavis yarrensis, Petroneis marina, Cocconeis scutellum, and Navicula digitoradiata. They consist of diatoms of different biotopes and extinct species. The diversity and abundance of brackish water and marine species indicates the predominant influence of sea waters on the formation of DMV sediments. The diverse freshwater species were mainly introduced into sediments with river runoff, but it is likely that some of these, such as the cosmopolitan alkaliphilic species, are inhabitants of geothermal springs. The presence of extinct species from the underlying Neogene sediments from where they were carried with gas–water–mud masses is the most typical for diatom assemblages of the DMV.

Cyclical browning in a subtropical lake inferred from diatom records

Changing climate and land use activity are altering inputs of colored dissolved organic carbon (cDOC) into lakes. Increased cDOC reduces water transparency (browning) and changes lake physicochemistry, with biological consequences. Identifying the drivers and effects of changing cDOC inputs is critical for mitigating the consequences of climate change on lake ecosystems through adaptive watershed management. This study focused on determining the drivers of lake browning by evaluating shifts in diatom assemblages in subtropical, oligotrophic Lake Annie (FL, United States), which is known to experience climate-driven oscillations in transparency associated with watershed inputs of cDOC. We combined long-term limnological monitoring data and paleolimnological techniques to determine how diatoms respond to changes in cDOC and to infer past cDOC fluctuations relative to records of past climate and land use changes in the watershed. Diatom assemblage composition in a 14-year phytoplankton dataset was strongly correlated with cDOC-driven transparency fluctuations. Likewise, diatom assemblages in the upper 35 cm of the sediment core, which corresponded to a 35-year lake monitoring record, were also strongly related to past water transparency, yielding a strong transfer function (paleo model, R 2 = 0.72). When the model was applied to the diatom record from a 166-cm sediment core, diatom-inferred transparency and rates of nutrient input showed that localized ditching in the 1930s enhanced the effect of climate oscillations on water transparency, intensifying cyclical browning thereafter. Integration of long-term monitoring and paleoecological data provided valuable insights into the history of the aquatic ecosystem, enabling implementation of adaptive management strategies to contend with a changing climate.

Composition, structure, and distribution of diatom assemblages in Mediterranean thermal spring ecotones affected by natural and human disturbances

Composition, structure, and distribution of diatom assemblages in Mediterranean thermal spring ecotones affected by natural and human disturbances

Evaluation of the rbcL marker for metabarcoding of marine diatoms and inference of population structure of selected genera.

Diatoms are one of the most important phytoplankton groups in the world's oceans. There are responsible for up to 40% of the photosynthetic activity in the Ocean, and they play an important role in the silicon and carbon cycles by decoupling carbon from atmospheric interactions through sinking and export. These processes are strongly influenced by the taxonomic composition of diatom assemblages. Traditionally, these have been assessed using microscopy, which in some cases is not reliable or reproducible. Next-generation sequencing enabled us to study diversity in a high-throughput manner and uncover new distribution patterns and diversity. However, phylogenetic markers used for this purpose, such as various 18S rDNA regions, are often insufficient because they cannot distinguish between some taxa. In this work, we demonstrate the performance of the chloroplast-encoded rbcL marker for metabarcoding marine diatoms compared to microscopy and 18S-V9 metabarcoding using a series of monthly samples from the Gulf of Trieste (GoT), northern Adriatic Sea. We demonstrate that rbcL is able to detect more taxa compared to 18S-V9 metabarcoding or microscopy, while the overall structure of the diatom assemblage was comparable to the other two methods with some variations, that were taxon dependent. In total, 6 new genera and 22 new diatom species for the study region were identified. We were able to spot misidentification of genera obtained with microscopy such as Pseudo-nitzschia galaxiae, which was mistaken for Cylindrotheca closterium, as well as genera that were completely overlooked, such as Minidiscus and several genera from the Cymatosiraceae family. Furthermore, on the example of two well-studied genera in the region, namely Chaetoceros and particularly Pseudo-nitzschia, we show how the rbcL method can be used to infer even deeper phylogenetic and ecologically significant differences at the species population level. Despite a very thorough community analysis obtained by rbcL the incompleteness of reference databases was still evident, and we shed light on possible improvements. Our work has further implications for studies dealing with taxa distribution and population structure, as well as carbon and silica flux models and networks.

Diatom assemblages in the Lateglacial-Holocene sediments of Lake Bolshoye Shchuchye (Polar Urals)

The study of the diatom assemblages from the sediment core from Lake Bolshoye Shchuchye (Polar Urals) was performed. The composition of diatom assemblages in the Lateglacial sediments characterizes alternating periods of predominance of planktonic and benthic diatoms. In the Holocene, planktonic species predominate with Aulacoseira subarctica being the most abundant. The climate amelioration during the first half of the Holocene resulted in increased proportions of large-celled diatoms, the taxa with a wide geographical distribution, as well as species preferring higher trophicity. The cooling of the Late Holocene, in turn, caused an increase in the proportion of cold-water species characteristic of oligotrophic lakes. The morphological variability of A. subarctica observed in the Holocene sediments apparently results from its adaptation to changes in hydrodynamic conditions.

Assessment of the current state of the Lake Imandra ecosystem (Russia, Murmansk region) using statistical and bioindication methods

The diatom assemblages of surface sediments of the Lake Imandra were studied. Lake Imandra is the largest freshwater reservoir in the Murmansk region. Significant differences in the composition of diatom assemblages were revealed in various parts of the water area, differing in the type and intensity of anthropogenic load. With the help of bioindication and statistical methods, the main factors influencing the development of certain diatom assemblages were identified.

Holocene diatom record of Gryazevoe Lake evolution (Magadan region)

Diatoms from core Gz-1 (Lake Gryazevoe, Magadan region) belonging to 189 species and intraspecific taxa were studied. Three local zones (LzGz) were identified based on changes in the taxonomic composition and ecological structure of fossil diatom assemblages. They correspond to different stages of the lake ecosystem evolution during the Late Pleistocene-Holocene. A sensitive reaction of diatoms to hydrological and hydrochemical changes of the lake regimes associated with paleoclimatic fluctuations has been established.

Recommendations for the preservation of environmental samples in diatom metabarcoding studies

Implementation of DNA metabarcoding for diatoms for environmental monitoring is now moving from a research to an operational phase, requiring rigorous guidelines and standards. In particular, the first steps of the diatom metabarcoding process, which consist of sampling and storage, have been addressed in various ways in scientific and pilot studies and now need to be rationalised. The objective of this study was to compare three currently applied preservation protocols through different storage durations (ranging from one day to one year) for phytobenthos and phytoplankton samples intended for diatom DNA metabarcoding analysis. The experimental design used samples from four freshwater and two marine sites of diverse ecological characteristics. The impact of the sample preservation and storage duration was assessed through diatom metabarcoding endpoints: DNA quality and quantity, diversity and richness, diatom assemblage composition and ecological index values (for freshwater samples). The yield and quality of extracted DNA only decreased for freshwater phytobenthos samples preserved with ethanol. Diatom diversity was not affected and their taxonomic composition predominantly reflected the site origin. Only rare taxa (< 100 reads) differed among preservation methods and storage durations. For biomonitoring purposes, freshwater ecological index values were not affected by the preservation method and storage duration tested (including ethanol preservation), all treatments returning the same ecological status for a site. This study contributes to consolidating diatom metabarcoding. Thus, accompanied by operational standards, the method will be ready to be confidently deployed and prescribed in future regulatory monitoring.

Drivers predicting biodiversity and community assembly of benthic diatoms in a karst spring environment

Geographical location, landscape heterogeneity, and their position at the edge of Quaternary glaciations are the major reasons why the river basins of the Western Carpathians are of the utmost importance for research on European aquatic biodiversity. Moreover, unlike streams, springs have several features determining their insularity in the river continuum. This study aimed to uncover the mechanism forming the diatom assemblages in the karst springs environment, using a two-dimensional approach, including taxonomic and trait data. Thus, (1) the congruence between observed and expected values of null models of taxonomic/functional diversity and functional dispersion and (2) Generalized mixed models and Variation partitioning were used to assess how taxonomic/functional diversity and composition were associated with the local conditions, regional affiliation and spatial springs distance. In 64 rheocrene karst springs (10 karst mountain ranges) of the Western Carpathians, 138 epilithic diatom taxa were found. Null model analyses did not uncover any clear evidence of non-random events forming the spring diatom community. Although, the observed functional diversity did not differ from the null model for most springs, most observed values were lower than the mean value of the null model, suggesting that, except for random events, environmental filtering was an important driver of community assembly. Substratum type, spring connectivity and phosphorus concentration drove the value of taxonomic richness, whereas functional diversity was controlled by silica and bicarbonate concentration as well as spring size. In both models, affiliation to the mountain range (defined as a random factor) was significant. The Variation Partitioning Analysis showed that diatom assemblage composition was mainly determined by local and regional environmental conditions, whilst the distance between springs had only a weak influence.

Have stream diatom assemblages changed a decade after the loss of a foundation riparian tree species in a headwater Appalachian watershed?

Terrestrial and stream ecosystems are interconnected, with changes in one system cascading to affect the other. Tree mortality related to nonnative pest infestation has increased, yet we do not understand the effects that riparian tree species losses will have on the structural characteristics of stream ecosystems. We compared riparian canopy cover, algal standing crop, and diatom assemblage composition in 8 southern Appalachian headwater stream reaches within the Coweeta Hydrologic Laboratory in North Carolina, USA, before and after a massive die-off of riparian eastern hemlock (Tsuga canadensis [L.] Carr.) related to the introduction of hemlock woolly adelgid (Adelges tsugae Annand), an invasive insect. We analyzed historical preserved algal samples collected prior to hemlock die-off, in both summer (September 2005) and winter (January 2006), and used multivariate analyses to compare them to samples collected in the same 8 reaches and seasons 13 y following hemlock die-off (2018–2019). We predicted that increased canopy openness following hemlock death would change light availability and lead to higher standing crop and increases in the relative biovolume of high-profile diatoms. Contrary to our expectations, we found little difference in canopy cover and lower algal standing crop 13 y post-hemlock die-off, likely due to increased basal area of the understory shrub, Rhododendron maximum L., that replaced shade cover formerly provided by hemlock. Diatom assemblages were dominated by acidophilic taxa (e.g., Eunotia spp.) across all stream reaches before and after hemlock die-off. We observed some shifts in diatom assemblage composition, but the relative biovolume of a majority of the top 20 most abundant taxa, including Eunotia incisa W. Sm. ex W. Greg., Nupela lapidosa (Krasske) Lange-Bert., and Meridion alansmithii Brant (an endemic taxon), remained relatively unchanged 13 y after hemlock mortality. Diatom assemblages differed post-hemlock die-off in summer but not winter due to a decrease in the relative biovolume of Eunotia metamondon complex and an increase in the relative biovolume of Eunotia rhomboidea complex. Our study highlights the relative stability of diatom assemblage composition in southern Appalachian streams and the importance of long-term studies following a major riparian disturbance.