Brackish Diatoms Research Articles

Synchronous changes in chironomid assemblages in two Arctic delta lake ecosystems after a major saltwater intrusion event

Low-lying Arctic coastal environments are threatened by marine storm surges, which are predicted to increase in frequency and intensity as a result of decreasing sea ice, rising sea levels and altered intensity and frequency of storm activity. The Mackenzie Delta of Canada’s Northwest Territories, a vast, low-lying wetland ecosystem, is particularly susceptible to such storm surges, because much of the outer alluvial plain is below 2-m elevation. A large storm-surge event in September 1999 flooded >13,000 ha of alluvial terrain and impacted the terrestrial and freshwater ecosystems of the region. Previous research on the limnological impacts of the storm surge recorded a shift from freshwater to brackish diatom taxa, and a change in cladoceran assemblages to more saline-tolerant species. We examined the remains of Chironomidae (Insecta, Diptera) in sediment cores from two lakes impacted by the 1999 saltwater inundation to determine whether the storm surge also affected benthic macroinvertebrate communities, which are particularly important to lake ecosystem function in Arctic regions. We observed an increase in the relative abundance of saline-tolerant taxa in the two impacted lakes, including Paratanytarsus and Cricotopus/Orthocladius, and decreases in saline-intolerant Sergentia and Corynocera oliveri-type, coincident with the 1999 storm. We observed no major assemblage changes after 1999 in a control lake located beyond the zone of inundation. The number of head capsules recovered from sediments of the impacted lakes increased after the 1999 storm, suggesting no negative impact on overall chironomid abundance as a result of the shift to brackish conditions. There has, however, been no recovery of the chironomid community to the pre-1999 composition. Earlier assemblage changes in both impacted lakes likely tracked regional climate warming in the region, known to have begun in the late nineteenth century.

Reconstructing paleoseismic deformation, 2: 1000 years of great earthquakes at Chucalén, south central Chile

In this paper we adopt a quantitative biostratigraphic approach to establish a 1000-year-long coastal record of megathrust earthquake and tsunami occurrence in south central Chile. Our investigations focus on a site in the centre of the rupture segment of the largest instrumentally recorded earthquake, the AD 1960 magnitude 9.5 Chile earthquake. At Chucalén coseismic subsidence in 1960 is recorded in the lithostratigraphy and biostratigraphy of coastal marshes, with peat overlain by minerogenic sediment and changes in the assemblages of diatoms (unicellular algae) indicating an abrupt increase in relative sea level. In addition to the 1960 earthquake, the stratigraphy at Chucalén records three earlier earthquakes, the historically documented earthquake of 1575 and two prehistoric earthquakes, radiocarbon dated to AD 1270–1450 and 1070–1220. Laterally extensive sand sheets containing marine or brackish diatom assemblages suggest tsunami deposition associated with at least two of the three pre-1960 earthquakes. The record presented here suggests a longer earthquake recurrence interval, averaging 270 years, than the historical recurrence interval, which averages 128 years. The lack of geologic evidence at Chucalén of two historically documented earthquakes, in 1737 and 1837, supports the previously suggested hypothesis of variability in historical earthquake characteristics. Our estimates of coseismic land-level change for the four earthquakes range from meter-scale subsidence to no subsidence or slight uplift, suggesting earthquakes completing each ∼270 year cycle may not share a common, characteristic slip distribution. The presence of buried soils at elevations below their modern equivalents implies net relative sea-level rise over the course of the Chucalén paleoseismic record, in contrast to relative sea-level fall over preceding millennia inferred from sites on the mainland. Sea-level rise may contribute to the preservation of evidence for multiple earthquakes during the last millennium, while net relative sea-level fall over the last 2000–5000 years may explain the lack of evidence for older earthquakes.

Geological evidence for an unusually large tsunami on the Pacific coast of Aomori, northern Japan

To assess long-term tsunami inundation history, we studied a wetland on the Pacific coast of Aomori, Japan, at the northern end of the Japan Trench. We found five sand sheets interbedded in freshwater mud and peat, three of which contained brackish diatoms indicating deposition by marine inundation. We identified the youngest sand sheet, deposited AD 1480–1770, as a tsunami deposit based on its lateral extent (>1 km) from the shoreline at the time of its deposition. Although this area has been struck by many tsunamis generated by earthquakes along the Japan Trench as well as along the Kuril and Peru–Chile trenches, no tsunami recorded at least in the past 120 years has inundated the coast as far inland as this tsunami deposit is distributed. We therefore infer that it was deposited by an unusually large tsunami. Historical and geological evidence for tsunamis in north-east Japan suggests that possible sources are the AD 1611 Keicho earthquake along the Japan Trench, a 17th-century earthquake along the Kuril Trench, or an unknown large earthquake.

Changes in Epipelic Diatom Diversity from the Savannah River Estuary

Littoral zones can be characterized with temporal exposure of algae to diurnal desiccation at low tides. Combinations of diverse freshwater, marine, and brackish diatoms dominate exposed mud samples. With enlargement of the delta of the Savannah River, Georgia and other anthropogenic influences, changes in the rich epipelic community will not be estimated accurately without baseline data. In the current study, mud samples were taken from the Savannah River estuary along with physicochemical characteristics every two months throughout 2011. Live algal communities were assessed in every sample and live to dead diatom proportions in the communities were calculated. Cleaned diatoms were analyzed following standard protocols. Community indices were compared between sampling events and with literature reports from similar habitats in the Southeastern USA diverse diatom community of 241 species was documented and 39 of those species should be described as new to science. Decrease in species richness and diversity was due to dominance of representatives of the genera Cymatosira and Minidiscus during the summer months.

Palaeoenvironment and agriculture of ancient Loulan and Milan on the Silk Road

Ancient Loulan, an important city on the Silk Road, disappeared about 1500 years ago. The environmental conditions associated with the vicissitude of ancient Loulan have been debated since the city was rediscovered in ad 1900. However, little paleobotanical evidence concerning vegetation and environment in this area has so far been available. In this study, phytoliths and diatoms extracted from 16 samples including two fossil camel coprolites from sites of Loulan and Milan indicate that the landscape of ancient Loulan was a typical oasis, where reeds, grasses of Paniceae and Pooideae probably grew along with some shrubs. Also, some typical brackish diatoms might live in some water bodies in the catchment of an ancient lake, Lop Nor. Our results also suggest that common millet as staple crop, and foxtail millet and possibly naked barley as non-staple crops were the main food source for ancient Loulan and Milan residents between ad 50 and 770.

Planothidium iberense sp. nov., a new brackish diatom of the Ebro Estuary, northeast Spain

This study describes the diatom species Planothidium iberense sp. nov. from a salinity gradient of the Ebro River Estuary in northeastern Spain. A detailed description is given based on light and scanning electron microscopy observations, and the diatom species is compared with morphologically similar Planothidium species. Planothidium iberense is distinguished from P. linkei mainly by its morphometric characteristics, but also by the presence of multiseriate striae, in contrast to biseriate striae in P. linkei. Planothidium iberense is also distinguished from P. delicatulum and P. septentrionale by the presence of an one-side expanded central area on the sternum valve, which is absent in the latter two Planothidium species.

Phytoplankton assemblages in two compartmentalized coastal tropical lagoons (Carretas-Pereyra and Chantuto-Panzacola, Mexico)

During an annual cycle (2002–2003), spatio-temporal differences and/or similarities in phytoplankton communities and their relationship with physical and chemical characteristics were determined in two coastal lagoons on the Mexican South Pacific coast: Chantuto-Panzacola (CHP) and Carretas-Pereyra (CP), Chiapas. Phytoplankton assemblages, characteristic of the two main seasons (dry and wet), were identified using principal component analysis (PCA), while canonical correspondence analysis (CCA) was used to examine environmental variables that may explain the patterns of variation of the phytoplankton community. The results show that both systems have similar characteristics because they have the same physical forcing (river discharge, seawater intrusion and, in general, weather). Spatial variability is especially important because it favors, during each season, the development of two different functional zones: a marine influence zone (MIZ) and a freshwater influence zone (FIZ). Phytoplankton distribution, composition and abundance were influenced by this environmental variation. Based on PCA results, during the dry season, assemblages in these lagoons are mainly comprised of neritic diatoms such as Coscinodiscus spp. and Skeletonema costatum distributed in the MIZ, whereas dinoflagellates of the genus Protoperidinium spp., and brackish diatoms such as Cyclotella sp., Entomoneis alata and Surirella spp., make up FIZ assemblages. CCA analysis indicates that salinity, temperature, dissolved oxygen and ammonium were the most relevant environmental factors affecting the variation in the distribution of these assemblages. On the other hand, during the rainy season, assemblages were correlated with silicate and orthophosphate concentrations, while their composition was dominated by freshwater groups mainly in the FIZ, whereas in the MIZ centric diatoms were dominant. In summary, in CHP and CP coastal lagoons, phytoplankton assemblages undergo changes in species composition and community structure during each season in response to environmental variation, which allows the development of associations specifically adapted to prevailing conditions.

A multiproxy approach to inferring Holocene paleobotanical changes linked to sea-level variation, paleosalinity levels, and shallow lake alternative states in Negra Lagoon, SE Uruguay

A multiproxy analysis of diatoms, chrysophyte cysts, opal phytoliths, and palynomorphs was undertaken in Negra Lagoon, to decipher Holocene changes in paleobotanical proxies associated with sea-level changes and paleolimnological conditions. Before the Holocene transgression (7000 yr BP), a terrestrial system was inferred as no aquatic palynomorphs or biogenic silica remains were recorded. During the sea-level maximum (5200 yr BP), marine/brackish conditions were established as indicated by diatoms, Chenopodiaceae, and a high content of cysts of Peridinioideae. The catchment consisted of grasslands and wetlands as indicated by the opal phytolith data. The Holocene transgression was followed by a sea-level decrease, which led to the onset of brackish/freshwater conditions as inferred from the co-dominance of freshwater and marine/brackish diatoms. This is also supported not only by the concomitant increase in non-siliceous freshwater microalgae and emergent macrophytes, but also by the presence of Myriophyllum. As sea level continued to decrease during late Holocene, a freshwater system was observed because of the complete separation from the ocean. Chrysophyte cysts were consistently recorded and brackish diatoms exhibited reduced abundances. The observed limnological changes are consistent with the alternative states hypothesis of Scheffer’s model (1998). That is, the clear water phases were explained because of high macrophyte abundances, low phytoplankton frequencies, high cyst to diatom ratios, and increase in frequencies of benthic diatom species. Conversely, turbid phases were likely to occur when low cyst to diatom ratios together with increase in planktonic diatoms and decrease in macrophytes values were detected. This highlights the importance of this approach to detect long-term changes in shallow lake alternative states.

Marine incursions of the past 1500 years and evidence of tsunamis at Suijin-numa, a coastal lake facing the Japan Trench

Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit (B) contains, in addition, graded beds suggestive of multiple waves of long wavelength and period. The uppermost unit (C) probably dates to a time in the area's written history when the lake was separated from the sea by a beach-ridge plain at least 0.5 km wide and several metres high. Units A and B postdate AD 540—870, and unit C postdates AD 1030—1640 as judged from radiocarbon dating of leaves and seeds. Unit B pre-dates AD 915 and unit C postdates that year as judged from a tephra within the peat that separates units B and C. The age constraints permit correlation of unit B with a tsunami in AD 869 that reportedly devastated at least 100 km of coast approximately centred on Sendai. Unit C may represent a later catastrophic tsunami in 1611, or perhaps a storm surge that inundated much of Sendai. The lake lacks obvious signs of tsunamis from the region's largest twentieth-century earthquakes, which were centred to the north in 1933 (M 8.1) and directly offshore in 1936 (M 7.5), and 1978 (Mw 7.6).

Natural and anthropogenic aquatic environmental changes reconstructed by paleolimnological analyses in Lake Kitaura, central Japan

In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707) are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change. In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm−2 y−1) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment of the lake after the 1970s exceed rates during the period concerned in this study.

Pyritized diatoms: a good fossil marker in the Upper Paleocene-Lower Eocene sediments from the Belgian and Dieppe-Hampshire Basins

Abstract Assemblages of brackish and marine diatoms have been examined from Upper Paleocene-Lower Eocene strata of the Belgian Basin (Knokke well) and the Dieppe-Hampshire Basin (Saint-Josse borehole and Ailly sections). The diatoms observed are invariably preserved in pyrite as internal moulds and their siliceous skeletons are completely replaced by pyrite by epigenesis. Three major diatom assemblages have been observed which can be used to approximate the position of the recently defined Paleocene-Eocene boundary (defined by the Carbon Isotope Excursion). This isotope excursion occurs just below the strong increase in the abundance of Fenestrella antiqua and in the vicinity of the abundance peak of Coscinodiscus morsianus var. moelleri. They also allow correlations of the lithostratigraphic units of the Belgian Basin with the formations of the Dieppe-Hampshire and central North Sea Basins. For instance, investigations of diatoms recorded in the Knokke Clay Member of the Knokke well indicate that this unit corresponds to the lower units of the ‘Sparnacian facies’ of the Dieppe-Hampshire Basin and to the Sele Formation of the North Sea Basin.

Holocene trophic state changes in relation to sea level variation in Lake Blanca, SE Uruguay

Paleolimnological data are presented relating trophic development to sea level variation in Lake Blanca, a small (0.6 km2), coastal fresh waterbody in southern Uruguay. Using a sediment core that extended to 7300 years BP, analyses of grain size, thin sections, organic matter, carbonate, total carbon, nutrients, diatoms and palynomorphs, allowed us to infer changes in trophic state and paleosalinities, which were closely related to Holocene sea level variation. Higher trophic states were observed during regressive events, most probably due to increases in runoff and erosion as regression progressed. Four diatom association zones (DAZ) were identified in the sediment core. The basal core section pre-dated the first Holocene marine transgression, contained no diatoms, chrysophyte cysts or non-siliceous microalgae, and showed C/N ratios values higher than 20. Thus, it is likely that the system exhibited terrestrial characteristics. In the second section (∼6500–2200 years BP, following the first Holocene transgression), there was dominance of marine/brackish diatom species. The lowest trophic states of the core were observed in this section. The third section (2200–1100 years BP), represented the system as it became separate from the Atlantic Ocean, and showed a dominance of brackish/freshwater species and increases in trophic state were observed. In the last section (after 1100 years BP), the system became fully freshwater as no marine or brackish diatom species were found, but a trend to oligotrophication was observed, probably due to nutrient depletion. However, after ∼1967 AD, eutrophication intensified because of forestry and soil fertilization in the catchment. Pollen association zones (PAZ) allowed us to identify four sections. Below 250 cm (2200 years BP), the core contained no pollen grains as redox potential and pH values were not conducive for pollen preservation. After 2200 years BP (when the system started to separate from the ocean), xerophilic taxa typical of coastal dunes colonized the catchment. Only after ∼1100 years BP (after fully freshwater conditions established) pollen grains of trees were observed.

Morphology and distribution of two brackish diatoms (Bacillariophyceae): Cyclotella atomus Hustedt and Thalassiosira guillardii Hasle in the estuary of Urdaibai (northern Spain)

Morphology and distribution of two brackish diatoms (Bacillariophyceae): Cyclotella atomus Hustedt and Thalassiosira guillardii Hasle in the estuary of Urdaibai (northern Spain)

Holocene environmental change in a marine-estuarine-lacustrine sediment sequence, King George Island, South Shetland Islands

Sedimentological features and cluster analysis of diatom assemblages were used to investigate a local Holocene prograding sequence of marine-estuarine-lacustrine sediments. It consists of upward finning and thinning sediment cycles formed at the mouth of a meltwater stream during regional isostatic uplift, which followed early Holocene deglaciation and marine inundation events. The sequence begins in the lower Holocene sublittoral sand (marine diatoms and abundant molluscs) overlying, with a transgressive base, the deltic (?) clastic sediment marking probably one of the pre-Holocene interglacial periods (index diatom Actinocyclus ingens suggests an age >0.62 Ma). The lower Holocene marine sand was truncated by middle Holocene gravity flows, bearing volcanic ash. They were deposited in a high energy estuarine environment (brackish diatoms). The beach subsequently formed separated the estuary from the sea and changed it into a freshwater lake. Accumulation of moss and gyttja, containing a freshwater diatom assemblage, marks the final late Holocene stage of this coastal sedimentary sequence, which can be considered as typical for deglaciation periods in the maritime Antarctic.

ENTOMONEIS CENTROSPINOSASP. NOV., A BRACKISH DIATOM WITH RAPHE-BEARING KEEL

A new brackish diatom with a raphe-bearing keel, Entomoneis centrospinosa Osada & H. Kobayasi, sp. nov., is described from Odaito in Notsuke-wan (Notsuke Bay), Bekkai-cho, Nemuro, Hokkaido, Japan. This species is characterized mainly by the conspicuous spines, 1(0)-3 in number, present on both sides of the central nodule externally and by the structure of the wings, which have decussate rows of small puncta.

Geomorphic Development of the Yukuhashi Alluvial Plain, Northeastern Kyushu

The Yukuhashi Plain, about 13km and 10km across from east to west and north to south, respectively, is located at the northeastern part of Kyushu and faces western Seto Inland Sea. The plain is surrounded by the mountains composed of Paleozoic limestone, metamorphics, Mesozoic granites and Quaternary volcanics except in its eastern margin. The plain is divided into two parts in consideration of its geomorphic features. The northern part of the plain consists of alluvial lowlands along the Obase and the Nagao Rivers flowing from west to east. The southern part of the plain is characterized by alluvial fans formed by the Harai and the Ima Rivers flowing south to north.The sequence of Recent deposits along the shoreline is shown in Fig. 4. The thickness of the deposits exceeds 16.5m at the hurried valley. From their textural characteristics, the deposits are divided into five layers as follows:1) Lowermost Sand and Gravel layer mainly consists of rounded gravels which are 0.5-4cm in diameter. Along the Harai rever, the layer is distributed to near surface.2) Lower Sand layer containing marine shells is considered as foreset bed of the deltaic sediment. At the lower part of the layer is dated 7, 710±210 14C years B. P. Marine diatom species are prominent at the upper part of the layer.3) Middle Mud layer containing marine shells (e.g. Tegillarca granosa) and humic materials is made up of silt or sandy silt. The layer also contains a lot of marine and brackish diatom species (e. g. Cyclotella striata, Melosira sulcata etc.), therefore the layer is considered to have been formed as bottomset bed sediments. At the period of the Middle Mud deposition, Postglacial transgression was reached at its maximum stage, and the inner bay widely spread in the plain. The period is estimated around 4, 800 years B. P.4) Upper Sand layer holds shell-fragments. This layer is considered to indicate the sedimentary environment of delta front. After the deposition of Middle Mud layer, deltaic sedimentation has been continuing.5) Uppermost Mud layer consists of organic silt deposited at backmarsh or lagoonal environment.In the plain, marine and brackish diatom species are appeared up to 1.8m above sea level. It denotes the highest sea level of the Postglacial transgression in the Yukuhashi Plain, but its age is unknown. The present high tide which exceeds 2m above mean sea level in this area may suggest that the present sea level is the highest during the Postglacial transgression.