EFFECTS OF LIGHT AND SUBSTRATE ON THE BENTHIC DIATOMS IN AN OLIGOTROPHIC LAKE: A COMPARISON BETWEEN NATURAL AND ARTIFICIAL SUBSTRATES1

Benthic diatoms form a particularly important community in oligotrophic lakes, but factors influencing their distribution are not well known. This study reports the depth distribution of living motile and total diatoms (living plus dead diatoms) on both natural (from sand to fine organic mud) and artificial substrates in an oligotrophic lake. On artificial substrates, motile diatom densities peaked in abundance (24–30 cells · mm^?2) between 0.6 and 1.9 m depth; on natural sediment surfaces, motile diatoms were generally more numerous and peaked in abundance (925 cells · mm^?2) at 1.3 m depth. Total diatom densities on artificial substrates were highest (1260 valves · mm^?2) at 0.6 m depth, with very low values below 3 m depth; on natural sediment surfaces, total diatom abundances were generally much higher (21600 valves · mm^?2) at 3 m depth and declined gradually with depth. Significant relationships were found between light and diatom densities on the artificial substrate. Ordination analysis indicated that substrate type significantly correlated with the variation of diatom composition on artificial and natural substrates. Our results suggest that in oligotrophic lakes, light influences benthic diatom abundance, whereas substrate type has more influence on benthic diatom composition.     

Seasonal and microhabitat influences on diatom assemblages and their representation in sediment traps and surface sediments from adjacent High Arctic lakes: Cape Bounty,Melville Island,Nunavut

The spatial (i.e. microhabitat) and temporal (i.e. seasonal) characteristics of diatom assemblages in adjacent High Arctic lakes were studied intensively June–August 2004. These baseline data are used to improve understanding of modern diatom community dynamics, as well to inform paleoenvironmental reconstructions. Diatoms were collected approximately weekly through the melt season from each principal benthic substrate (moss/macrophyte, rock scrapes, littoral sediment), plankton, and sediment traps, and were compared to the uppermost 0.5 cm of a surface core obtained from the deepest part of the lake where sediment cores are routinely collected. Water samples were collected concurrently with diatom samples to investigate species–environment relationships. The lakes share approximately half of their common taxa, the most abundant overall in both lakes being small Cyclotella species. Results of detrended correspondence analysis (DCA) indicate that the largest gradient in species turnover existed between benthic and planktonic communities in both lakes, and that sediment trap and the surface core top samples most closely resemble the planktonic assemblage, with an additional contribution from the lotic environment. Our results indicate clear micro-spatial controls on species assemblages and a degree of disconnection between the benthos and deep lake sediments that manifests as an under-representation of benthic taxa in deep lake surface sediments. These findings are particularly relevant in the context of interpreting the paleoenvironmental record and assessing ecosystem sensitivity to continued climate change.     

Influence of the colonizing substrate on diatom assemblages and implications for bioassessment: a mesocosm experiment

Although diatoms are important bioindicators of ecological quality, their ecological traits are still not well understood. A major issue is that of substrate preferences, which may result in differences in production, and assemblage structure and composition, and which should therefore be taken into account for ecological quality assessment studies. Thus, in this work, the periphyton grown on sand and ceramic tiles in indoor controlled channels were compared to understand whether substrate differences lead to differences in: periphyton production (chlorophyll-a), chlorophyll-b and c concentrations, diatom assemblages (diversity-Shannon-Wiener, cell density, taxonomic composition, trait proportions), and ecological quality assessments (IPS-‘Indice de Polluosensibilité Spécifique’). A combined inoculum of periphyton from four Portuguese streams was introduced to the running channels (six sand and six tile) and left to colonize for 35 days. Epilithic (tiles) and epipsammic (sand) assemblages were sampled at days 14 and 35. We verified that there were no differences in chlorophyll-a concentration over time and between substrates. On both sampling occasions, the epipsammic assemblages had higher concentration of chlorophyll-c and diatom density but without significant differences over time in each substrate. The taxonomic composition was different between substrates and over time. However, these differences were not reflected in ecological quality assessment. The diversity was also similar between substrates in both sampling occasions, but it was higher at day 14. Mobile and stalked species were more abundant over the entire study and differed significantly between substrates, with the epipsammic assemblages presenting higher abundances of both traits. We concluded that the colonizing substrate influences diatom assemblages but not the ecological quality assessment.     

Relationships between lakewater pH and sedimentary diatoms in the Matamek Watershed,Northeastern Quebec,Canada

Twenty lakes in the Matamek and nearby watersheds were sampled in the summer of 1983 for water chemistry and surficial sediments. Thirty-two physical and chemical variables, including pH, alkalinity, cations and metal concentrations were measured on samples from the epilimnion and hypolimnion of each lake. In three lakes, two to four replicate cores were collected to estimate spatial variability of the sediment flora.All lakes were acidic (pH 4.59 to 5.80), highly colored and poorly buffered. Aluminum and magnesium concentrations reached 494 and 70 µg l^-1, respectively. The pH of the lakes appears to be declining, as indicated by a comparison of our results with those from a survey done in 1970. An empirical chemical model based on the alkalinity/sulfate ratio and the regression of pH on calcium also indicated that these lakes may be undergoing acidification.Analyses of the diatom flora of the surficial sediments showed strong dominance of 6 species that cooccurred with a large number of rare species (in all, 229 taxa were found). Variability among samples within a lake was as high as among lakes. Calculation of Nygaard's alpha index for each sample and the regression of its log-transformed value on surface pH yielded a relationship that was significantly from other published models. The regression model was applied to a down-core analysis of the diatom flora of the sediments of two lakes. Although the large confidence intervals on pH values predicted by the model obscured any evidence of pH change with sediment depth, there was a significant increase of acidophilous and a decrease of circumneutral species over time, suggesting that a change in the flora, possibly correlated with pH, is taking place.In making comparisons among lakes, the surficial-sediment flora did not provide clear evidence of a relationship with the pH of surface water. Indices computed from the surficial-sediment flora are apparently insensitive to differences in pH over a narrow range, particularly when the lakes being compared are similar chemically. Nygaard's alpha index is shown to be unduly sensitive to outliers. The currently accepted assumption of a progressive linear change in communities may be inadequate for the quantification of acidification processes. Other models, derived from catastrophe theory, may prove more fruitful.     

How Currents on Different Sides of Substrates in Streams Affect Mechanisms of Benthic Algal Accumulation

Benthic diatom accumulation rates and diversity were monitored for 32 days in five habitats where current conditions were different. After 32 days, diatom abundance in a sheltered habitat where eddies occurred over substrates was over eight times the abundance in a habitat that was exposed directly to the main flow of the stream (about 30 cm/sec). Diatom immigration rates and species richness of diatom taxocenes were negatively related to exposure; whereas, diatom reproduction rates, death rates and relative abundances of the eight dominant diatom species were not related to the small differences in current velocity that occurred. During the 32-day colonization period diatom immigration rates increased, species richness of diatom taxocenes increased and evenness of species abundances decreased. Species composition of diatom taxocenes shifted from a numerical dominance by fast immigrators to dominance by fast reproducers during the 32-day colonization.     

The seasonal response of diatom communities to variable water quality in some English urban lakes

SUMMARY. 1. Five diatom assemblages and their corresponding site groups were associated with the urban lakes. When treated as a single dataset, DC A and PCA grouped these diatom assemblages along an environmental gradient from circum natural. dilute waters to strongly alkaline, concentrated, nutrient-rich waters.
2. When treated as individual datasets, 75% of the known variance was explained by the first axis of DCA in all five diatom assemblages; this indicated the existence of a strong primary environmental gradient. The PCA results showed that the identity of this primary environmental gradient can be different for each assemblage.
3. The statistical analyses showed that the physicochemical gradients were continuous and not discrete. Nevertheless, fine-scale analysis can identify smaller, more precise species and site groups.
4. Two types of species response could be identified within most sub-assemblages in the urban lakes: the first was a quasi-seasonal response and was characterized by a gradual rise to a peak relative abundance of one dominant or several co-dominant species which could be sustained for a period of weeks before a gradual decline. A quasi-seasonal response was sometimes interrupted by a storm response. The storm response was characterized by a sudden increase in the relative abundance of one or more diatom species following a storm, after which the species declined to pre-storm abundances within 7 days.
5. The effects of urban runoff on the water quality of urban lakes can override the effects of true seasonal factors such as water temperature and day length on the relative abundance of species.     

PERIPHYTIC DIATOM ASSEMBLAGES FROM BATHURST ISLAND, NUNAVUT, CANADIAN HIGH ARCTIC: AN EXAMINATION OF COMMUNITY RELATIONSHIPS AND HABITAT PREFERENCES

Diatoms are potentially the most important biomonitors of environmental change in high arctic lakes and ponds, but to date few autecological data are available. Because of the shallow nature of many of these water bodies, a large proportion of taxa are periphytic and planktonic diatoms are absent for the most part. By determining the microhabitat and substrate preferences of these benthic diatom taxa, the potential exists to infer past changes in available habitats from fossil diatom assemblages collected from sediment cores and ultimately to reconstruct past environmental and climatic changes responsible for these shifts in habitat availability. To refine our understanding of high arctic diatom habitat preference, the common diatom taxa found on submerged moss (bryophyte), sediment, and rock substrates from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic were examined. The relationships among key limnological variables and the common taxa from each habitat were examined. Many diatom taxa exhibited varying degrees of microhabitat preference, with moss representing the more unique habitat. In addition, the following limnological variables significantly ( P ≤ 0.05) explained the species variance for each of the three substrates: Na ⁺ and total nitrogen for moss; total phosphorus (filtered) and pH for rock; and Fe^{3 +} , total phosphorus (unfiltered), total nitrogen, temperature, and pH for sediment. These data can be used to help interpret monitoring and paleolimnological studies in this environmentally sensitive region.     

Environmental factors affecting the distribution and abundance of cyst-forming Myxobolus spp. and their cyprinid hosts in 3 lakes in Algonquin Park,Ontario

In 1999, 4 species of cyprinid were surveyed for myxozoan parasites in a watershed in Algonquin Park, Canada, Kathlyn Lake. Broadwing Lake, and Lake Sasajewun were included. Eight species of Myxobolus were found that differed in their prevalence and distribution among the 3 lakes. The oligochaetes and environmental parameters, including sediment types and aquatic plants, of these 3 lakes were surveyed the following year. Oligochaetes belonging to 17 species were collected from the 3 lakes. The distribution patterns of the oligochaete fauna, with respect to the environmental variables, were examined using canonical correspondence analysis. Naidids were predominant in all 3 lakes, particularly in the pebbly and sandy sediment of Lake Sasajewun. The highest percentage of tubificids occurred in the detritus and muddy substrate of Broadwing Lake. These findings indicate that the prevalence of certain oligochaetes is congruent with the absence or presence of particular myxozoan species and that substrates and aquatic plants influence the distribution of certain oligochaete species.     

Consistent patterns in diatom assemblages and diversity measures across water‐depth gradients from eight Boreal lakes from north‐western Ontario (Canada)

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water‐depth gradient in eight small, boreal lakes in north‐western Ontario, minimally disturbed by human activities. 2. Surface‐sediment diatom assemblages were collected within each lake along a gentle slope from near‐shore to the centre deep basin of the lake, at a resolution of ～1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high‐resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east–west transect approximately 250 km long in north‐western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near‐shore assemblage of Achnanthes (sensu lato), Nitzschia, Cymbella (sensu lato) and other benthic species; (ii) a mid‐depth assemblage of small Fragilaria (sensu lato)/small Aulacoseira and various Navicula taxa; and (iii) a deep‐water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep‐water planktonic zone and the mid‐depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near‐shore benthic species.     

The temporal and spatial distribution, composition and abundance of Protozoa in Chaohu Lake, China: Relationship with eutrophication

Systematic investigations into the temporal and spatial distribution, composition and abundance of protozoa in two regions with different trophic levels in Chaohu Lake, a large, shallow and highly eutrophic freshwater lake in China, were conducted during 2002–2003. A total of 114 species of protozoa, including phytomastigophorans, zoomastigophorans, amoebae and ciliates, were identified from 120 polyurethane foam unit (PFU) samples exposed at four stations and from various types of natural substrates. Of the 114 taxa, 36 core species were found on PFU substrates and 23 of these were found on natural ones. Protozoan abundance and chemical–physical parameters at nine sampling stations, four in the western lake and five in the eastern part, indicate trophic gradient changes along the lake. Seasonal variations in the species composition of major groups at littoral PFU sampling stations illustrate the effect of a severe algal bloom on the protozoan community structure. Temporal and spatial distributions of individual abundance as functions of water temperature and trophic status were revealed. This study demonstrates again that the PFU artificial substrate method samples protozoan communities more effectively than routine natural substrate methods.     

SYSTEMATICS OF PRASIOLA AND PHYLOGENETIC POSITIONING OF THE PRASIOLALES (CHLOROPHYTA)

Sellman, S. M., Johansen, J. R. & Coburn, M. M. Department of Biology, John Carroll University, University Heights, OH 44118 USA The diatom composition of natural substrates at a single site were compared with that of the gut contents of three species of fish: stoneroller minnows, bluntnose minnows, and common shiners. Seven samples of each type were collected on a single day from a single pool in a headwater stream. Cluster analysis showed that diatom species composition clustered by fish species, with common shiners being a totally exclusive cluster, while natural substrates were scattered within the stoneroller and bluntnose clusters. Both richness and Shannon diversity were higher in fish gut samples than in the samples collected by humans, with stonerollers having the greatest diversity. When water quality indices calculated from the diatom floras recovered in these samples were compared, it was surprising to see that these indices consistently were better in fish gut samples than in samples collected by researchers. Common shiners particularly differed, a result possibly due to the fact that they apparently fed off of Cladophora and the highly oxygenated rock wall where the stream cascaded into the pool. In no case was the difference between stonerollers and natural substrates significantly different. Furthermore, samples from natural substrates had an internal similarity not significantly different from their similarity to samples in stoneroller and bluntnose guts. Our data thus consistently indicate that stoneroller minnows are excellent samplers that efficiently collect diverse, representative diatom samples.     

Epiphytic,littoral diatoms as bioindicators of shallow lake trophic status: Trophic Diatom Index for Lakes (TDIL) developed in Hungary

Littoral diatoms are important contributors of the primary production in shallow aquatic ecosystems and they can be used as indicators of the trophic status. The aim of the study was to develop an index to assess trophic status of Hungarian lakes as suggested by the Water Framework Directive. In spring of 2005 and 2006, epiphytic diatom samples were collected from 83 shallow lakes. Weighted average method was used to develop and test the TP model. In the developed TP model correlation between the observed and diatom inferred TP was high (r ² = 0.96, n = 67). The optimum and tolerance TP parametrics of 127 species were determined and trophic indicator and sensibility values were defined for the Trophic Diatom Index for Lakes (TDIL). The TDIL was applicable to assess the ecological status of Hungarian shallow lakes. According to the TDIL the ecological status of 4 lakes were in excellent, 25 in good, 21 in medium, 21 in tolerable and 12 in bad status. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: K. Martens     

The palaeolimnological potential of diatom assemblages in floodplain lakes of the Danube Delta, Romania: a pilot study

Within the framework of a palaeolimnological pilot study, the potential of sedimentary diatom assemblages for the reconstruction of the eutrophication history was studied in short sediment cores from five shallow lakes located in the Romanian Danube Delta. A total of 234 diatom taxa representing 57 genera could be identified but diatom assemblages are generally predominated by a few species only. Loss on ignition percentages, total diatom abundances and diatom downcore successions largely vary within the single sediment cores and between all five lakes. This suggests the presence of various lake environments within the Danube Delta, each characterized by lake-specific geomorphological, sedimentological, hydrological, and biological conditions. Highly variable depositional conditions are also confirmed by radiocesium measurements in two of the five cores suggesting distinct inter-lake differences in the sedimentation rate. Causes for the great intra-lake and inter-lake variability in diatom downcore successions are difficult to specify and may include changes in the physico-chemical conditions, aquatic vegetation cover, water depth, river influence, turbidity and inter-biotic interactions. However, based on this pilot study, it is likely that the spatial and temporal distribution of sedimentary diatom assemblages in Danube Delta lakes were not triggered by the trophic state. The delta lakes likely became meso- to eutrophic long before 1950, possibly during late-Holocene times.     

Long-Term Patterns of Protozoan Colonization in Douglas Lake, Michigan

Long-term patterns of noninteractive and interactive protozoan colonization of polyurethane foam (PF) artificial substrates in Douglas Lake, Michigan, were examined for a 14-yr period. Species-time data were fitted to the Mac Arthur-Wilson equilibrium model, S =?eq(1 - e^Gt), and examined through time from 1969-1982. Comparisons were made to historical water chemistry measurements. No long-term changes in water chemistry were evident. Similarly, equilibrium species number (?eq) and colonization rate (G) oscillated about a mean through time. Protozoan colonization of PF substrates appeared stable for extended periods and showed modest variation from year to year. Examination of 7-yr-old substrates in 1982 revealed little difference from young (< 50 days) substrates. Previous reports of senescence of artificial substrate communities may have been due to habitat loss within the substrates. No evidence existed for chemical or biological degradation of the lake.     

Developing biomonitoring protocols for shallow Arctic lakes using diatoms and artificial substrate samplers

Growing concerns over effects of climate warming and other stressors on shallow Arctic lakes and ponds stimulate the need to develop and implement effective protocols to track changes in ecological integrity. This study assesses seasonal and spatial variability of periphytic diatom communities in a shallow Arctic lake in northern Yukon Territory to establish biomonitoring protocols. Artificial substrate samplers, which mimic macrophytes, allow direct measurement of biotic responses to shifting environmental conditions and control for possible confounding factors (e.g., accrual time and microhabitat type). Artificial substrate samplers were deployed at three locations and retrieved at three times (early, mid, and late) during the ice-free season. Analyses identified that diatom abundance increased exponentially and community composition changed significantly over the ice-free season, despite little variability in water chemistry, but did not differ among the three sampling locations within the lake. Patterns of seasonal succession in diatom community composition were characterized by first arrival of well-dispersed taxa, which included several planktonic taxa, followed by a transitional phase composed of planktonic and periphytic taxa, and culminated with dominance by periphytic species, mainly Achnanthes minutissima (Kützing). Results highlight the role of seasonal succession on artificial substrate colonization and the need to deploy artificial substrate samplers for the duration of the ice-free season to capture peak periphytic algal abundance. Low spatial variability of shallow Arctic lakes allows for samplers to be deployed at one single location to characterize diatom community composition.     

The reproductive cycle in the population of Saccostrea cuccullatta (Born) from the coast of Karachi,Pakistan

Periphytic diatom communities on clay tile and sterilized rock substrates were compared with those on natural substrates. Clay tiles yielded diatom communities with the least variability between replicate samples, and the greatest similarity to the natural substrate assemblage. The advantages of utilizing introduced substrate communities and problems encountered with their simulation of the natural substrate community are discussed.     

A multi-lake comparison of epilithic diatom communities on natural and artificial substrates

In a multi-season, multi-lake study of epilithic diatom communities, glass slide artificial substrate samplers provided poor representation of communities on natural substrates. Percent similarities between the two communities averaged only 37 over the course of the study. Overall, natural substrates exhibited greater species richness than artificial samplers, particularly during the summer, although this difference was slight. Patterns of selection and inhibition by artificial substrates of individual diatom species varied by both season and lake. Members of the genus Cymbella, particularly C. microcephala, appeared to be the most consistently inhibited, while Achnanthes minutissima was often selected for. In spite of the great differences between the two substrate types, replicability of artificial substrates was very high, and could prove to be a more important quality in monitoring applications than accurate representation of the natural community.     

RELATIONSHIPS BETWEEN LITTORAL DIATOMS AND THEIR CHEMICAL ENVIRONMENT IN NORTHEASTERN GERMAN LAKES AND RIVERS1

We explored statistical relationships between the composition of littoral diatom assemblages and 21 chemical and physical environmental variables in 69 lakes and 15 river sites in the lowland of northeastern Germany. Canonical correspondence analysis with single treatment and with forward selection of environmental variables was used to detect 11 important ecological variables (dissolved inorganic carbon [DIC], Na ⁺ , total phosphorus [TP], dissolved organic carbon [DOC], total nitrogen [TN], pH, oxygen saturation, dissolved iron, SO₄^{2 ?} , NH₄ ⁺ , soluble reactive silicium) and maximum water depth or Ca^{2 +} or soluble reactive phosphorus that most independently explain major proportions of the total diatom variance among the habitats. Monte Carlo permutation tests showed that each contributed a significant additional proportion (P < 0.05) of the variance in species composition. Together, these 11 most important environmental variables explained 34% of the total variance in species composition among the sites and captured 73% of the explained variance from the full 21 parameters model. Weighted‐averaging regression and calibration of 304 indicator taxa with tolerance down‐weighting and classic deshrinking was used to develop transfer functions between littoral diatoms and DIC, pH, TP, TN, and Cl ^? . The DOC:TP ratio was introduced and a weighted‐averaging model was developed to infer allochthonous DOC effects in freshwater ecosystems. This diatom‐DOC/TP model was significant (P < 0.001) and explained 7.6% of the total diatom variance among the sites, surpassing the inferential power of the diatom‐TP‐transfer function (7.3% explained variance). The root‐mean‐square errors of prediction of the models were estimated by jack‐knifing and were comparable with published data sets from surface sediment diatom samples. The data set of littoral diatoms and environmental variables allows use of the diatom‐environmental transfer functions in biomonitoring and paleolimnological approaches across a broad array of natural water resources (such as floodplains, flushed lakes, estuaries, shallow lakes) in the central European lowland ecoregion.     

A comparison of vascular vegetation and protozoan communities in some freshwater wetlands of Northern Lower Michigan

Vascular vegetation and protozoan communities were sampled in seven wetland sites — two bogs, two fens, two marshes, and one swamp — in summer 1977. Two similarity indices were used to compare vascular vegetation and Protozoa from each site with all the other sites. Bog sites were the most distinct from other wetland types with respect to chemical and physical characteristics, dominant vascular vegetation, and protozoan species composition. The swamp site had the highest similarity to all other sites with respect to both dominant vascular vegetation and protozoan species. Protozoan communities from different wetland types were much more similar than dominant vascular species; however, the pattern of similarity between wetland sites was very similar for both groups (Pearson product-moment correlation coefficient = 0.76).Protozoan communities were also compared with those from several nearby lakes with respect to colonization rate onto polyurethane foam artificial substrates. The structure and dynamics of protozoan communities of wetlands were broadly different from those of other freshwaters, and somewhat unique to the other wetland types. Evidence for a high degree of eutrophy in certain bog lakes is presented.     

Evidence from algal bioassays of seasonal nutrient limitations in two English lakes

Comparative laboratory bioassays using Asterionella formosa and Rhodomonas lacustris as test organisms were performed from March to November 1987 on filtered water samples from two English lakes, to assess their potential fertility and to identify possible limiting nutrients. The relative growth responses (log₂ increments) per week, were measured after additions of P, Fe, Si, N, and K singly and in combinations in comparison with unenriched (control) samples. Phosphate appeared to be the major limiting element for both species throughout the year, except during the spring diatom maxima when silicon usually becomes limiting. On most occasions chelated iron increased the growth increments, particularly in combination with phosphate. In general, the bioassay results showed correspondence with the nutrient concentrations in the test waters, which showed low (< 1 µg l^–1) levels of soluble reactive phosphate during all or most of the year and depleted silicon levels in late spring. Comparison between relative (incremental) ratio and absolute (cell concentration) response was made.