Diatom Assemblages Research Articles

Biodiversity and seasonal distribution of benthic diatom assemblages as an indicator of water quality of small karstic river in Bosnia and Herzegovina

The aims of this paper were to describe seasonal changes in the qualitative and quantitative composition of diatom taxa and the potential application of benthic diatoms for ecological status evaluation. Diatom indices (IPS and TI) were calculated from data from three different locations along a longitudinal profile of the Bunica, a small karstic river in Bosnia and Herzegovina. A total of 147 taxa were recorded in 12 samples. The most common taxa were Meridion circulare (Greville) C.Agardh and Ulnaria ulna (Nitzsch) Compère. Physical and chemical analyses showed low concentrations of nutrients, good oxygenation, typical pH for carbonate bed/origin and generally oligotrophic conditions and high ecological status. All sites had similar physico-chemical conditions and there were only few seasonal differences. Ordination of the diatom data showed that samples showed neither longitudinal nor seasonal patterns. Median value for IPS (16.8) and for TI (7.3) can be possible ‘‘expected’’ values for ecological status assessment for small karstic rivers in the Mediterranean region. We propose the use of the phytobenthos Intercalibration Common Metric (pICM - an index that combines the IPS and TI) as a national metric for countries developing WFD diatom methods at a late stage. One situation is described, and a solution, which is potentially transferable to other locations in Bosnia and Herzegovina and also to other countries facing similar challenges.

Depositional setup and characteristics of the storm deposit by the 2007 Cyclone Sidr on Kuakata Coast, Bangladesh

This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135–277 m from the coast. Laboratory analyses of grain size, TOC, TN, δ13C, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1–17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, δ13C values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.

A First Estimate of Species Diversity for Benthic Diatom Assemblages from the Revillagigedo Archipelago, Mexico

Recent investigations at previously unexplored localities on the Mexican coast have confirmed the high taxonomic potential of benthic marine diatom assemblages (BMDA) in the region. An exploratory study of epiphytic diatoms of macroalgae in the Revillagigedo Archipelago (RA) suggested that further studies would yield many more taxa, prompting the hypothesis that diversity measurements, based on ecological indices, would be among the highest in pristine environments. Thus, the aim of this research was to enrich the record of epiphytic diatom floristics of the RA, and to estimate species diversity based on information theory (H′). Floristically, 167 identified taxa are added here to the BMDA species list of the RA, bringing the total to 397 taxa overall, including 52 taxa that are potentially new records for the Mexican Pacific coast. Among the most conspicuous genera are Mastogloia with five new taxa and it remains the most diverse genus with 55 taxa overall, followed by Cocconeis (27), Nitzschia (24), Amphora (23), Navicula (19), Diploneis (17) and Grammatophora (15). As expected for a pristine environment, the computed species diversity values for the BMDA were high, ranging from H′ = 3.92–5.2, depicting stability. Future surveys are expected to further increase the species richness of BMDA for the RA.

A 5000‐year paleoclimate record from Nettilling Lake (Baffin Island) based on diatom assemblages and oxygen isotope composition

ABSTRACTSedimentary diatoms have been used to quantitatively reconstruct climate‐related variables, such as temperature at different timescales. Even though temperature is often less of a key driver of diatom ecology than other environmental parameters (water chemistry), diatom inference models have been shown to be reliable in deducing past temperature trends. In addition, the oxygen isotope composition (δ18Odiatom) preserved in buried diatom frustules has demonstrated its potential to reflect climatic and hydrological conditions at the time of frustule formation. This study combines results from both diatom‐based climate proxies to reconstruct summer water and mean annual air temperatures, and hydrological trends in Nettilling Lake, Baffin Island, from ca. 5000 to 500 cal a bp. Diatom‐inferred temperatures revealed an overall ca. 2 °C cooling throughout the Late‐Holocene. The δ18Odiatom values showed an increasing trend up to ca. 1900 cal a bp, where they reached their highest values (+24.8‰ at 15 cm) and thereafter decreased to their lowest values (+21.4‰ at 4 cm). These trends were linked to meltwater inflows associated with Penny Ice Cap thaw rate that was in turn controlled by regional climatic conditions which went from intensified cooling during the Neoglacial period to slight warming thereafter. Our results suggest that diatom‐ and diatom‐isotope‐based temperature and hydrological reconstructions can identify trends related to the natural climate system variability. The diatom oxygen isotopes are useful for paleoenvironmental studies of terrestrial aquatic ecosystems, but not for all hydrological systems are the ideal temperature proxy. Hence, the combination of proxies helps to disentangle temperature and hydrological effects for paleoclimatic reconstructions and may support future studies of postglacial environmental change in northern lakes.

Description of four new terrestrial diatom species from Luticola and Microcostatus genera from South Africa.

The knowledge about terrestrial diatom assemblages in southern Africa is rather limited, despite a long history of diatom research in this area. Terrestrial habitats are places of characteristic diatom floras, dominated by species resistant to desiccation which are able to thrive in conditions of limited water availability. The presented work expands the knowledge about these unique habitats. During the study on terrestrial moss-inhabiting diatoms from Western Cape Province (South Africa), four taxa with a unique set of valve features have been found and described herein as new species, based on light and scanning electron microscopy. These new species are: Luticolamicrocephala M. Rybak, Peszek & Kochman-Kędziora, sp. nov., Luticolaasymmetrica M. Rybak, Kochman-Kędziora & Peszek, sp. nov., Luticolaterrestris Kochman-Kędziora, M. Rybak & Peszek, sp. nov. and Microcostatusmeridionalis Peszek, M. Rybak & Kochman-Kędziora, sp. nov. Algal community composition had low species richness (9–15 taxa per sample) and samples were dominated by: Humidophilacontenta, Nitzschiabrevissima and Eunotiaaff.pseudominor. The three new Luticola species formed numerous populations, exceeding 10% of the share, whereas Microcostatusmeridionalis reached 5.4%. Basic morphological data of associated diatom flora together with detailed micrographs are also provided.

Purification of Disc-Shaped Diatoms from the Southern Ocean Sediment by a Cell Sorter to Obtain an Accurate Oxygen Isotope Record

The oxygen isotopic composition (δ18O) of diatom frustules is potentially a quantitative paleoenvironmental proxy. However, δ18O analysis has some uncertainties because of difficulties in removing clay-mineral contamination and taxon-specific separation due to the small size of diatom frustules. The clay-mineral contamination seriously affects δ18O measurements, and the δ18O of bulk diatoms contains a mixture of information from diatoms living in various seasons and water depths. To resolve such problems, we established a new method for purifying the frustules of disc-shaped diatoms using a cell sorter and succeeded in removing the clay-mineral contamination and restricting the number of species for the δ18O analysis. We developed this method using samples from a sediment core retrieved from the Conrad Rise in the Indian Ocean sector of the Southern Ocean. First, the disc-shaped diatoms were enriched and cleaned by sieving, elutriation, and heavy-liquid separation. The samples were then further purified by a cell sorter. Regardless of the diatom assemblage or the sediment type in the core sample, this method generated samples consisting of more than 95% disc-shaped diatoms, dominated by Thalassiosira lentiginosa (Janisch) Fryxell, which occurs ubiquitously throughout the Southern Ocean. The δ18O profile for the sediment core obtained from purified disc-shaped diatoms showed the last glacial cycle since ca. 40 ka, which can be correlated with the LR04 benthic δ18O stack record and the temperature record in the EPICA Dome C ice core, indicating that this method can contribute to obtaining an accurate δ18O record of diatoms associated with glacial–interglacial cycles.

Distribution and frequent occurrence of diatom taxa (Bacillariophyta) inhabiting warmer oceans in Seogwipo coast of Jeju Island, southernmost Korea

The epiphytic diatoms attached to seaweeds and benthic diatom assemblages, have been collected along the Seogwipo coast in Jeju Island, South Korea. In the last three-year survey, 80 diatom taxa inhabiting warmer oceans have been found, including those in the tropical and subtropical regions such as the Pacific Ocean, the Caribbean Sea, the western Indian Ocean, and other warmer regions. These diatom flora shows a worldwide pattern that concentrated around the Tropic of Cancer. This survey of local areas reveals the existence of many rare or retrieved taxa that were not well known until recently. Furthermore, some of them, including Aulacodiscus affinis, Leudugeria janischii, Hyalosynedra lanceolata, Trachyneis velata var. ornata, Lyrella concilians have been reported for the first time since the first entry or haven been reported only once. Of the diatom flora described here, 48 taxa are new to South Korea and previously unrecorded in the local area.

How can integrated morphotaxonomy- and metabarcoding-based diatom assemblage analyses best contribute to the ecological assessment of streams?

Environmental conditions, such as nutrient concentrations, salinity, elevation etc., shape diatom assemblages of periphytic biofilms. These assemblages respond rapidly to environmental changes, a fact which makes diatoms valuable bioindicators. Hence, freshwater biomonitoring programmes currently use diatom indices (e.g. EU Water Framework Directive - WFD). To date, microscopy-based assessments require high taxonomic expertise for diatom identification at the species level. High-throughput technologies now provide cost-effective identification approaches that are promising, complementary or alternative tools for bioassessment. The suitability of the metabarcoding method is evaluated for the first time in the Cyprus streams WFD monitoring network, an eastern Mediterranean country with many endemic species and results are compared to the results acquired from the morphotaxonomic analysis. Morphotaxonomic identification was conducted microscopically, using the most updated taxonomic concepts, literature and online resources. At the same time, DNA metabarcoding involved the use of the rbcL 312 bp barcode, high-throughput sequencing and bioinformatic analysis. The ecological status was calculated using the IPS Index. Results show a positive correlation between morpho-taxonomic and molecular IPS scores. Discrepancies between the two methodologies are related to the limitations of both techniques. This study confirmed that Fistulifera saprophila can have a crucial role in key differences observed, as it negatively influences IPS scores and microscopy methods frequently overlook it. Importantly, gaps in the DNA barcoding reference databases lead to a positive overestimation in IPS scores. Overall, we conclude that DNA metabarcoding offsets the morphotaxonomic methodology for the ecological quality assessment of freshwaters.

Plastic habitats: Algal biofilms on photic and aphotic plastics

Plastic pollution is abundant in aquatic environments worldwide and many of its detrimental impacts are well documented, but it also represents a novel substrate available to a diversity of organisms. Biofilms – assemblages of bacteria, algae, and fungi – colonise hard surfaces in aquatic environments. They are key agents in biogeochemical cycling and are a food source for grazing organisms, representing a keystone aquatic community, and are known to influence the fate of plastic pollution in aquatic environments. In one of the most temporally thorough assessments of biofilm development on freshwater plastics, here we report on the evolution of algal biofilm assemblages on three plastic polymers (Low Density Polyethylene, Polypropylene, and Polyethylene Terephthalate) over six weeks in the photic and aphotic zones of a freshwater reservoir in Staffordshire, UK. Significant differences were found between diatom assemblages on plastics in the photic and aphotic zones, and between diatom assemblages quantified on weeks 2, 4 and 6 of the study, but total algal photosynthetic pigment concentrations did not vary significantly between polymers in either zone. Scanning Electron Microscopy indicates that degradation of polymer surfaces occurs within six weeks in the aphotic zone, with potential implications for plastic fragmentation and microplastic generation.

Applicability of diatom metabarcoding in the ecological status assessment of Hungarian lotic and soda pan habitats

Diatoms are widely applied in the ecological status assessment of aquatic ecosystems using indices calculated from pollution sensitivity and indicator values of species. Traditional, morphology-based identification of species requires in-depth taxonomic knowledge and expertise. Identifying taxa according to their barcode sequences obtained with high-throughput sequencing (metabarcoding) would be a promising alternative. In this pilot study, we tested the applicability of metabarcoding of benthic diatom assemblages for the ecological status assessment of Hungarian water bodies, comparing its performance to that of morphology-based identification of species. The barcode region of the rbcL gene was investigated in samples from running waters with various trophic states and unique lentic habitats, namely soda pans. For running waters, the Specific Pollution Sensitivity Index (IPS) and for soda pans the H index and the Indice Biologique Diatomées (IBD) were calculated from the morphology- and sequence-based abundances. The ecological quality ratio was determined for running waters. Overall, more infrageneric taxa were found with microscopy than with metabarcoding in the lotic and lentic samples. The correspondence between taxon lists detected by the two methods was relatively low but increased considerably in the case of morphologically dominant taxa. Community composition based on microscopy and DNA sequence analysis showed a significant correlation and was determined by the same main environmental drivers. Morphology-based indices strongly correlated with sequence-based indices. Both aspects indicated the same ecological status class for more than half of the lotic samples. In other cases, a status shift from good to moderate was frequent, a phenomenon that could prove problematic because the Water Framework Directive prescribes intervention for waters with a moderate or worse status. Considering discrepancies between the results obtained with microscopy and metabarcoding, using both methods in parallel could be proposed until the reference database has been suitably updated.

Decline in plankton diversity and carbon flux with reduced sea ice extent along the Western Antarctic Peninsula

Since the middle of the past century, the Western Antarctic Peninsula has warmed rapidly with a significant loss of sea ice but the impacts on plankton biodiversity and carbon cycling remain an open question. Here, using a 5-year dataset of eukaryotic plankton DNA metabarcoding, we assess changes in biodiversity and net community production in this region. Our results show that sea-ice extent is a dominant factor influencing eukaryotic plankton community composition, biodiversity, and net community production. Species richness and evenness decline with an increase in sea surface temperature (SST). In regions with low SST and shallow mixed layers, the community was dominated by a diverse assemblage of diatoms and dinoflagellates. Conversely, less diverse plankton assemblages were observed in waters with higher SST and/or deep mixed layers when sea ice extent was lower. A genetic programming machine-learning model explained up to 80% of the net community production variability at the Western Antarctic Peninsula. Among the biological explanatory variables, the sea-ice environment associated plankton assemblage is the best predictor of net community production. We conclude that eukaryotic plankton diversity and carbon cycling at the Western Antarctic Peninsula are strongly linked to sea-ice conditions.

Indonesian coral reef habitats reveal exceptionally high species richness and biodiversity of diatom assemblages

We present the results of the first ever research on biodiversity and biogeography of marine benthic diatoms from coral reefs of the tropical islands of Indonesia, including West Borneo, East Java, South Celebes, Komodo, Rinca and Flores. The sampled islands offer a broad range of coral reef microhabitats and host diatom assemblages from seaweeds, sea grass, dead corals, scrapes from other biological surfaces, sand, rocks and other solid surfaces. The results of this study are based on the light (LM) and scanning electron microscopy (SEM) identification of diatoms only. Our taxonomic analysis revealed the presence of 911 diatom taxa representing 188 genera. Biodiversity indices such as Shannon, Simpson and Evenness as well as species richness and diatom relative abundance were calculated for each sample examined. We find that these indices were higher for Indonesia than those published for tropical oceanic islands or regions from both hemispheres, e.g. Galapagos and Martinique, but comparable to Madagascar. The non-metric multidimensional analysis showed that the type of habitat from which a particular sample was taken was of greater significance in the taxonomic composition than the geographical aspect. In terms of biogeography, cosmopolitan taxa were dominant in terms of relative abundance, but species richness was higher within tropical zonal and local forms. Many taxa were identified only at the genus level, and may turn out to be new to science.

Environmental parameters and diversity of diatoms in the Aïn Dalia dam, Northeast of Algeria

Abstract. Heramza K, Barour C, Djabourabi A, Khati W, Bouallag C. 2021. Environmental parameters and diversity of diatoms in the Aïn Dalia dam, Northeast of Algeria. Biodiversitas 22: 3633-3644. Diatoms have long been used as biological indicators of the quality of aquatic environments due to their important capacity to respond to environmental change. As this flora has been very little studied in Algeria, the present study aimed to monitor the Spatio-temporal dynamics of diatoms at the Aïn Dalia dam (north-eastern of Algeria) and determine the main abiotic factors contributing to these variations from June 2017 to May 2018 in four selected stations. Our results showed that the waters of Aïn Dalia dam waters vary from moderately (St1 and St3) to significantly polluted (St2 and St4). They host the community of planktonic diatom flora composed of 72 species. The interspecific biodiversity was marked in summer and autumn (57 and 56 species respectively) and in the St4 station (52 species). In terms of abundance, more than 40% of the overall density was found in the St4 station and the fall period. Among the dominant genera, Cyclotella, Thalassiosira, Synedra and Navicula are found in moderately polluted waters, while Nitzschia, Melosira and Surirella in significantly polluted waters. Some environmental parameters, including nutrients (PO43-and NO2-), SM, temperature, dissolved oxygen, and pH played a key role in the functioning and evolution of this lake. Hence, these parameters demonstrated important fluctuations during our study and influenced directly the diatom distribution. These results showed that diatom assemblages are influenced by anthropogenic disturbances, and can be used as indicators of environmental quality.

An ecological and hydrochemical study of three springs in NE Iran with the emphasis on diatom diversity

Springs are complex and taxa rich ecosystems. Diatom assemblages have received very little attention in spring ecosystems in Iran; hence, the diatom assemblage in three selected springs in northeast Iran, were investigated using multivariate analysis together with hydro chemical measures. For this purpose, water and diatom samples collected during four seasons of 2019. Hydrochemistry results revealed that water-rock interaction is the most important factor in changing the water chemistry of studied springs and their waters produced from carbonate reservoir rocks (mainly calcite). As a result, Ca-HCO3 is predominate water type in all three springs. In this study, 75 diatom taxa identified, from which 55 were included in the analysis. Cluster analysis based on diatoms relative abundance, clustered samples in two major groups and third small group. Results of the indicator species analysis for groups and DCA analysis were in the absolute conformity. Results revealed that the diatom assemblage dominated by combination of early colonizer taxa, spring indicator taxa and cosmopolitan taxa; most of them also reported from different spring types. The results also showed that the spring’s diatom composition influence by size and morphology of the springs.

Reply to formal comment on Griffiths et al. (2017) submitted by Gajewski (2020).

Gajewski offers a formal comment on Griffiths et al. (2017), a paper that explored how microclimates and their varying ice cover regimes on lakes and ponds in Arctic regions modified the diatom assemblage responses to recent warming. One of Gajewski’s main criticisms is that the microclimate classification scheme used in Griffiths et al. (2017) is merely anecdotal; a claim which ignores the value of observational evidence and misunderstands the frequency that each site was visited or surveyed. We clarify that the study sites were visited multiple times via recurrent aerial surveys and ground observations dating back to the 1970s, which supports our microclimate classification scheme. Many of Gajewski’s claims regarding climate, catchment characteristics, and ice melting properties from field locations he has not visited were refuted by veteran Arctic scientists with long-term field experience in these regions. In addition, Gajewski makes several criticisms concerning radioisotopic dating, core chronology, sediment mixing, diagenesis, and preservation of bioindicators that relate more to general paleolimnological assumptions than to conclusions reached by Griffiths et al. (2017). Research from the 1980s and 1990s, when scientific consensus on these issues was first reached, readily show that the methodologies and data interpretation of Griffiths et al. (2017) are sound. We appreciate the opportunity to expound on the finer details of the Griffiths et al. (2017) paper, work based on field research by the study’s co-authors spanning almost three decades, with additional observations from colleagues dating back to the 1970s. We address Gajewski’s criticisms with relevant literature, expert statements, and a few clarifying comments.

Spatiotemporal variation of extracellular polymeric substances (EPS) associated with the microphytobenthos of tidal flats in the Yellow Sea

The physical functions of extracellular polymeric substances (EPS), viz., by-product of microphytobenthos (MPB), in tidal flat system are well documented, but some ecological aspects remain unknown. We investigated MPB biomass (Chl-a), EPS, diatom assemblage, and erodibility in two contrasting tidal flat environments (megatidal vs. macrotidal flat) in the Yellow Sea. Thick biofilms were observed when MPB bloomed, with high Chl-a and increased EPS concentrations. Among diatom genera, Navicula was the most dominant taxa found over the year (mean 41%) in both areas. Compared with non-bloom periods, the erodibility decreased by 54–73% as biofilm thickened during the blooms. It was attributed to the elevated abundance of large-sized (>40 μm) Navicula, which was expected to secrete large amounts of EPS. Overall, we successfully demonstrated spatiotemporal differences of sediment stabilization that significantly related to ecological variations of MPB, and identified the key diatom genus as a “sediment stabilizer” in the typical tidal flats of the Yellow Sea.

An unintended ecological benefit from human intervention: The enhancement of carbon storage in seagrass meadows

Abstract Human interventions have had unintended consequences for the diverse functions of various ecosystems. Hydrological interventions have the potential to alter vegetated habitats in coastal nearshore ecosystems, but little is known about the impacts on organic carbon (Corg) sequestration, which plays an important role in climate change mitigation. We examined the effects of past human interventions through artificial inlet opening in a lagoonal seagrass meadow. The sediment profiles of geological and biogeochemical characteristics were measured to obtain the historical changes in the lagoon environment and Corg accumulation rates. Isotopic and elemental signatures and diatom assemblages showed that the interventions increased the duration of seawater exchanges and the extent of the seagrass meadow. Corg accumulation rates increased more than 1.9‐fold after the intervention, resulting in additional Corg storage in the seagrass meadow during last 64 years. Synthesis and applications. Artificial opening of tidal inlet can potentially provide unintended ecological benefits, such as increased carbon sequestration in seagrass meadows. Hydrological interventions could be undertaken intentionally as a potential active intervention to sequester carbon, as well as control flood risk and provide other benefits to people in estuarine systems. Our results illustrate that physical factors such as improving salinity conditions and mineral particle supply have an impact on the feasibility of hydrological interventions for enhancing carbon sequestration. We propose that biogeochemical–physical coupled approach will help stakeholders to evaluate the ecological and social risks and synergies of implementing hydrological interventions.

Diatoms of the intertidal environments of Willapa Bay, Washington, USA as a sea-level indicator

An understanding of the modern relationship between diatom species and elevation is a prerequisite for using fossil diatoms to reconstruct relative sea level (RSL). We described modern diatom distributions from seven transects covering unvegetated subtidal environments to forested uplands from four tidal wetland sites (Smith Creek, Bone River, Niawiakum River, and Naselle River) of Willapa Bay, Washington, USA. We compared our diatom dataset (320 species from 104 samples) to a series of environmental variables (elevation, grain-size, total organic carbon (TOCSOM), and porewater salinity) using hierarchical clustering and ordination. While no single variable consistently explains variations in diatom assemblages at every site, elevation, salinity, and substrate (mud fraction and TOCSOM) are variables affecting diatom distribution along our transects. Elevation was the major environmental control of diatom variability (explained 27–39% variance) at four transects (Bone River Transect 1, Niawiakum River Transect 2, Naselle River Transect 1 and 2). Salinity and substrate were the major environmental controls (explained 12–32% variance) of diatom variability at three transects (Niawiakum River Transect 1: salinity; Smith Creek Transect and Bone River Transect 2: TOCSOM). Analyses of a combined regional dataset of all transects suggest that elevation is the driver of regional diatom variability in Willapa Bay, with salinity and substrate co-varying along an elevation gradient. We identify species with narrow elevation tolerances that can serve as indicator species of specific environments. Despite the site-specific variability of our modern diatom distribution, the regional dataset provides an important dataset that can be used to reconstruct RSL in Willapa Bay.

Multiple drivers of ecological change in Arctic lakes and ponds.

Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to “…explicitly examine the role of ice cover as the dominant driver of diatom assemblage change…”. I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality (“warm”, “cool”, “cold”, and “oasis”), is confounded with other morphological and chemical variables that better explain the differences between the groups. The “cold” sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The “oasis” sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that “warm” sites have responded more rapidly to recent climate change than “cold” or “cool” sites, but their chronologies do not allow for such a conclusion. There is no clear difference between “cool” and “warm” sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ30SidSi) and biogenic silica (δ30SibSi) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ30SibSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ30SibSi data (together with δ30SidSi) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ30SidSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ30SibSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ30Si) ranging from −0.4‰ to −1.0‰. In contrast, δ30SidSi and δ30SibSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ30SidSi. A strong relationship between δ30SibSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ30SibSi. A comparison of δ30SibSi samples from the water column and from underlying core-top sediments (δ30SibSi_sed.) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ30SibSi_sed. = +0.9‰ to +1.7‰), although we observed small differences in δ30SibSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ30SibSi data as a paleo proxy for the reconstruction of dSi utilization in the region.