Live Diatoms Research Articles

Silicon and sediment transport of the Changjiang River (Yangtze River): could the Three Gorges Reservoir be a filter?

Water samples were collected from the Changjiang River (Yangtze River) in May 2005, after the impoundment of the Three Gorges Reservoir (TGR), to examine the influence of the TGR and large lakes on material delivery to the estuary of the Changjiang River. The concentrations of suspended particle material (SPM), dissolved silica (DSi) and biogenic silica (BSi) in the main stream were analyzed. The concentrations of DSi and BSi in the main channel of the Changjiang varied between 73 and 100 and 1.1–15 μmol/l, with a distance weighted average of 81 and 8.0 μmol/l, respectively. A calculation shows that live diatom comprises only an average value of 5.2 % of the BSi in the Changjiang River, and most of BSi may come from drainage basin. The concentrations of BSi and the ratios of BSi/SPM were relatively low in the Changjiang River compared to other rivers throughout the world, but the BSi carried in suspension by the Changjiang River was an important component of the rivers silicon load (i.e. ~13 %). SPM, DSi and BSi concentrations as observed in the Changjiang River tend to decrease from the upper sections of the river to the Three Gorges Dam (TGD), reflecting sedimentation associated with BSi trapping and DSi retention in the TGR in the normal-water period. SPM and BSi retention are more strongly influenced by the TGD compared to DSi. About 98 % of SPM, 72 % of BSi and 16 % of DSi were retained within the TGR in May 2005. The fluxes variations of DSi, BSi and SPM suggested that the large lakes and dams had a coupled effect on the transportation of DSi, BSi and SPM in the normal-water period. Such a change in silicon (DSi and BSi) balances of the Changjiang River will affect the ecological environment of the Changjiang estuary and its adjacent sea to some extent.

Dissolved organic matter reduces algal accumulation of methylmercury

Dissolved organic matter (DOM) significantly decreased accumulation of methylmercury (MeHg) by the diatom Cyclotella meneghiniana in laboratory experiments. Live diatom cells accumulated two to four times more MeHg than dead cells, indicating that accumulation may be partially an energy-requiring process. Methylmercury enrichment in diatoms relative to ambient water was measured by a volume concentration factor (VCF). Without added DOM, the maximum VCF was 32 × 10(4) , and the average VCF (from 10 to 72 h) over all experiments was 12.6 × 10(4) . At very low (1.5 mg/L) added DOM, VCFs dropped by approximately half. At very high (20 mg/L) added DOM, VCFs dropped 10-fold. Presumably, MeHg was bound to a variety of reduced sulfur sites on the DOM, making it unavailable for uptake. Diatoms accumulated significantly more MeHg when exposed to transphilic DOM extracts than hydrophobic ones. However, algal lysate, a labile type of DOM created by resuspending a marine diatom in freshwater, behaved similarly to a refractory DOM isolate from San Francisco Bay. Addition of 67 µM L-cysteine resulted in the largest drop in VCFs, to 0.28 × 10(4) . Although the DOM composition influenced the availability of MeHg to some extent, total DOM concentration was the most important factor in determining algal bioaccumulation of MeHg.

The role of live diatoms in bioassessment: a large-scale study of Western US streams

Diatom-based stream bioassessment is constantly being improved to meet the increasing demands of water quality management. This study examined whether percentage of live diatoms (PLD) in periphyton communities can be used as a metric of human disturbance in streams and rivers. The analyzed dataset (587 sites) was collected over the course of 3 years (2000–2002) from 12 Western US states (US EPA’s Environmental Monitoring and Assessment Program). The mean PLD in Western streams and rivers was low (34.50%) and highly variable (range 2.08–97.02%). It did not differ significantly between the Mountains (MT, 36.38%) and the Xeric (XE, 35.49%) ecoregions, but it was significantly (P < 0.05) greater than that in the Plains ecoregion (PL, 28.27%). PLD distinguished reference from impacted sites in the MT (P < 0.05) and somewhat in the PL (P = 0.05). However, PLD exhibited opposite patterns in the two ecoregions. It increased with human disturbance in the MT and decreased in the PL due to a potential subsidy-stress gradient of available resources. The different pattern may be largely interpreted by the quality of the reference conditions in each ecoregion. In the MT ecoregion, the selected reference sites may resemble very closely the natural state in this ecoregion. In contrast, human disturbance is much more pervasive in the low-land PL ecoregion and the “reference sites” may reflect the best attainable conditions in this ecoregion. PLD as a metric has potential for monitoring human disturbance of streams, if reference sites represent natural conditions and differing responses among regions are included in expected effects.

Diatom survivorship in ballast water during trans-Pacific crossings

Ship ballast water is believed to be responsible for global dispersal of alien biota; mid-ocean ballast water exchange is most commonly used to mitigate this process. Diatoms are among the most abundant biotic-component in ballast water, yet their invasive biology is poorly understood. To test effectiveness of MOE we examined diatom species composition and cell density in two sets of samples. First, we examined samples collected daily during one 24 days long trans-Pacific crossing in tanks with and without ballast exchanged. Second, we used samples from 23 trans oceanic vessels arriving at Vancouver harbour where diatoms were collected on arrival. Up to 86,429 live diatom cells/l were found in the tanks, ~50% of the samples share up to eight species consistently present. Cell densities and species richness declined over time and with replacement of coastal ballast water by mid-oceanic water. In both data sets diatoms survive in the tanks for as long as 33 days despite ballast exchange.

REPRODUCTION AND GROWTH OF THE DEEP-SEA BENTHIC FORAMINIFER BULIMINA MARGINATA UNDER DIFFERENT LABORATORY CONDITIONS

In this study, laboratory experiments were performed to investigate the impact of different food types and temperatures on the reproduction and growth of the benthic foraminifer Bulimina marginata . To study reproductive success, adult specimens were kept alive at 6, 8, 10, 12, and 14° C, and fed with either freeze-dried green algae or live diatoms. The effects of temperature, food type, and food quantity on test growth rate were studied under different combinations of these parameters. Lower temperature delayed reproduction, resulted in fewer offspring, and decreased growth rate. Fresh food produced better results than freeze-dried food probably because the latter reduces the amount of digestible compounds (carbohydrates and polyunsaturated fatty acids). In order to obtain reproduction and growth of the open marine species B. marginata in a limited period of time, we recommend keeping them at a temperature ranging 8–12° C (approximating their in situ temperature), and to feed them with fresh algae. The results of this study should be useful to researchers who wish to set up experiments that require culturing deep-sea foraminifera.

Should only live diatoms be used in the bioassessment of small mountain streams?

It is unclear whether differentiating live and dead diatoms would enhance the accuracy and precision of diatom-based stream bioassessment. We collected benthic diatom samples from 25 stream sites in the Northern Oregon Coast ecoregion. We counted live diatoms (cells with visible chloroplasts) and then compared the counts with those generated using the conventional method (clean counts). Non-metric multidimensional scaling (NMDS) showed that the diatom assemblages generated from the two counts were overall similar. The relationships between the two diatom assemblages (summarized as NMDS ordination axes) and the environmental variables were also similar. Both assemblages correlated well with in-stream physical habitat conditions (e.g., channel dimensions, substrate types, and canopy cover). The conventional diatom method provides taxonomic confidence while the live diatom count offers ecological reliability. Both methods can be used in bioassessment based on specific assessment objectives.

Loss of waterborne brevetoxins from exposure to phytoplankton competitors

We tested whether interactions among phytoplankton competitors affect toxin dynamics involving the red tide dinoflagellate Karenia brevis, whose brevetoxins incapacitate and kill coastal wildlife. The addition of a live diatom, Skeletonema costatum, led to decreased concentrations of brevetoxin B (PbTx-2) associated with K. brevis cells in co-culturing experiments and with two of three natural bloom samples containing K. brevis. Similar decreases in PbTx-2 concentration, but not PbTx-3 concentration, occurred when a mixture of brevetoxins (without live K. brevis cells) was exposed to S. costatum, indicating that S. costatum metabolizes waterborne PbTx-2. Liquid chromatography–mass spectrometry (LC–MS) and ELISA analyses indicated that PbTx-2 is probably not transformed into other brevetoxins or into known brevetoxin metabolites, and instead is biotransformed by a previously unrecognized mechanism. Four different S. costatum strains from around the world caused similar loss of PbTx-2, suggesting that evolutionary experience with K. brevis is not a pre-requisite for the ability to metabolize PbTx-2. Additionally, phytoplankton-associated bacteria were found to play no role in the loss of PbTx-2, as bacteria-free S. costatum strains metabolized PbTx-2. Finally, loss of waterborne PbTx-2 caused by exposure to a dinoflagellate, a cryptophyte, and two additional diatom species indicates that this phenomenon is widespread among phytoplankton. Our results unexpectedly suggest that competing phytoplankton species present during K. brevis blooms, and possibly other red tides, could mediate bloom toxicity and therefore ecosystem-level consequences of red tides.

An investigation of the fine structure, cell surface carbohydrates, and appeal of the diatom Extubocellulus sp. as prey for small flagellates

The fine structure and surface exopolymers of a coastal planktonic nanodiatom of the sparsely reported genus Extubocellulus were studied respectively by scanning electron microscopy and confocal microscopy in conjunction with fluorescent lectins. Monitoring the suitability of the species as prey food for other protists was also investigated by video microscopy coupled with digital film. Cells are rectangular in girdle view, with a pervalvar axis longer than the apical axis. Valves are almost circular with a diameter of 2.8 to 3.6 microm. The valve face bears randomly distributed areolae (ca. 50 in 10 microm), which may be either open or occluded. Two small raised ocelluli occur at the apices, with a rim devoid of perforations and about 6-7 porelli. Glucose and N-acetyl-glucosamine moieties present on the surface of the live diatom were labelled with fluorescent lectins, and a differential pattern of distribution for both carbohydrates was observed. The potential role of fluorescent lectins as cellular probes of taxonomic value in small diatoms is compared with that of nucleotide and antibody probes. We provide the first illustrative evidence of the presence of Extubocellulus sp. in the cytoplasm of the nanoflagellate Goniomonas amphinema and of the egestion of diatom frustules. Results obtained are discussed in the light of the present knowledge of the role of carbohydrate-protein interactions in phagocytosis of prey by free-living protozoa.

Settling abalone veliger larvae in a free-swimming microalgal culture

In the current abalone hatchery in China, insufficient diatoms on vertically placed corrugated pvc plates at later stage often could not support the growth of postlarvae up to the stage that they can feed on live macroalgae. As a result, stripping the spats (3–5 mm) off by anaesthetization and switching the diet from live diatoms to artificial powdered diet in combination has to be performed in most of the abalone farms. This manipulation normally leads to more than 50% mortality. Here we report the direct use of the unicellular green alga Platymonas helgolandica Kylin var. tsingtaoensis as a potential alga to be used to settle the veliger larvae of the Pacific abalone Haliotis discus hannai and to feed the postlarvae. Settlement rate of 2-day-old veliger larvae in mono culture of P. helgolandica could be as high as 92% (± 4.2%) on day 10 in small scale trials, higher than that in the selected benthic diatom strain (53.6% ± 12.7%) when settled in the water in which bacteria propagation was controlled by treatment of 2 ppm of benzylpenicillinum calcium and streptomycin sulfate. Postlarvae fed solely on P. helgolandica or the selected benthic diatom Navicula-2005-A grew at rates of 40.1 (± 1.9) and 45.8 (± 3.4) μm day − 1 , respectively, when raised at 22 °C until day 50 postfertilization. P. helgolandica was shown to have distinct diurnal settling rhythm characterized with a peak of settled cells in the middle of the night for cell division and a peak of free-swimming cells in the middle of the day. High density of attached P. helgolandica cells on the inner surface of the culture facility in the night fits the nocturnal feeding behavior of the abalone spats. Judged by the promising larvae settling rate, growth and survival rates of the postlarvae fed with this alga, the free-swimming micro-green alga P. helgolandica constitutes a potential species for settling the veliger larvae and for supporting the growth of postlarvae as well.

Single Adhesive Nanofibers from a Live Diatom Have the Signature Fingerprint of Modular Proteins

The adhesive and mechanical properties of a cell-substratum adhesive secreted by live diatom cells were examined in situ using atomic force microscopy. The resulting force curves have a regular saw-tooth pattern, the characteristic fingerprint of modular proteins, and when bridged between tip and surface can repeatedly be stretched and relaxed resulting in precisely overlaying saw-tooth curves (up to ∼600 successive cycles). The average rupture force of the peaks is 0.794 ± 0.007 (mean ± SE) nN at a loading rate of 0.8 μm/s and the average persistence length is 0.026 ± <0.001 (mean ± SE) nm (fit using the worm-like chain model). We propose that we are pulling on single adhesive nanofibers, each a cohesive unit composed of a set number of modular proteins aligned in register. Furthermore, we can observe and differentiate when up to three adhesive nanofibers are pulled based upon multimodal distributions of force and persistence length. The high force required for bond rupture, high extensibility (∼1.2 μm), and the accurate and rapid refolding upon relaxation, together provide strong and flexible properties ideally suited for the cell-substratum adhesion of this fouling diatom and allow us to understand the mechanism responsible for the strength of adhesion.

Production of macroaggregates from dissolved exopolymeric substances (EPS) of bacterial and diatom origin

Exopolymeric substances (EPS) isolated from a pure culture of the marine bacterium Marinobacter sp. and the marine diatom Skeletonema costatum (axenic) were partially purified, chemically characterized and used as dissolved organic matter (DOM) for the production of macroaggregates. The role of organic particles such as transparent exopolymeric particles (TEP) and Coomassie stained particles (CSP) in the production of macroaggregates was experimentally assessed. Three experimental rolling tanks containing sterile medium with: (1) EPS, (2) EPS + live diatom cells and (3) EPS + killed bacteria, and three control tanks without any added EPS were used for macroaggregate production. Changes in abundance and average size of macroaggregates were monitored using image analysis, whereas TEP and CSP were enumerated microscopically. In the presence of microbial EPS, macroaggregates of a size of 23–35 mm 2 were produced. Aggregate size and abundance considerably varied with both time and source of EPS. No correlation was observed for macroaggregate size and abundance with either TEP or CSP. One-way ANOVA demonstrated significant differences in the variance of particle abundance and size in tanks having only EPS or EPS in combination with live diatom cells. Our data suggest that production of macroaggregates was influenced by polymer chemistry and surface properties of colliding particles, whereas TEP and CSP concentrations were influenced by molecular weight of EPS and the presence of growing cells. Interestingly, macroaggregates were formed in the near absence of TEP and CSP, highlighting the role of other unknown processes in the transformation of DOM to particulate organic matter (POM) in aquatic environments.

A Live Bioprobe for Studying Diatom-Surface Interactions

Atomic force microscopy has been employed to compare the adhesion of Navicula species I diatoms to surfaces of a hydrophobic elastomer, Intersleek, and a hydrophilic mineral, mica. This was accomplished using tipless atomic force microscopy cantilevers functionalized with live diatom cells. Both surfaces were tested with the same diatom bioprobe. Force versus distance curves generated during these experiments revealed comparable cell adhesion strengths on Intersleek and mica, indicating that Navicula diatoms secrete extracellular polymeric substances with hydrophobic and hydrophilic properties. A statistical analysis of force curves was carried out and the average values of works of detachment of a diatom from Intersleek and mica surfaces were determined.

Quantifying plant pigments and live diatoms in aphotic sediments of Scandinavian coastal waters confirms a major route in the pelagic–benthic coupling

The fate of pelagic diatoms in marine coastal aphotic sediments was investigated from sediment profiles in western Scandinavian waters. We used three independent methods to estimate pigment pools in the sediment: (1) fluorometry, (2) high-performance liquid chromatography and (3) pigments estimated from germinable diatom cells, using the dilution extinction method. A strong positive relationship with an intercept close to zero was observed between fucoxanthin, a marker of diatoms, and chlorophyll a. The fucoxanthin/chl a ratio was on average 1.05, which was similar to monocultures of dominating diatom taxa, indicating that sedimentary chl a was to a large extent of diatom origin. Chl a and fucoxanthin correlated significantly and positively with, and where within the same order of magnitude as, corresponding substances predicted from live diatom cell numbers obtained with the dilution extinction method. This indicates that a major part of surficial sediment chl a was bound in live cells of pelagic diatoms. There was a consistent change in viable cells with sediment depth and with timing of dominating taxa, with the non-spore-forming Skeletonema costatum dominating in the surface sediment in March and May, while the spore forming Chaetoceros spp. dominated deep in the sediment and during periods outside of the spring bloom (February and August). This indicates that chl a is bound in several different cell pools with different degradation rates, depending on diatom taxonomy. Thus, diatoms originating predominantly from the spring bloom may provide an important direct link in the pelagic–benthic coupling in this area.

Accumulation of algal pigments and live planktonic diatoms in aphotic sediments during the spring bloom in the transition zone of the North and Baltic Seas

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 248:41-54 (2003) - doi:10.3354/meps248041 Accumulation of algal pigments and live planktonic diatoms in aphotic sediments during the spring bloom in the transition zone of the North and Baltic Seas Jørgen L. S. Hansen*, Alf B. Josefson National Environmental Research Institute, 4000 Roskilde, Denmark *Email: joh@dmu.dk ABSTRACT: The sediment contents of algal pigments and live planktonic diatoms were measured in cores sampled at 6 stations with aphotic sediments at depths between 27 and 55 m located in the transition zone between the Baltic Sea and the Skagerrak. Five of the stations were sampled before and after the spring phytoplankton bloom in 2001. Within the area, there was a highly significant increase after the bloom in the sediment content of viable planktonic diatoms, quantified by the dilution extinction method, and in the sediment content of the pigments chlorophyll a and fucoxanthin. The composition of algal pigments as well as the number of germinable diatoms suggested that live pelagic diatoms were the source of almost the entire pool of chlorophyll a in the sediment both before and after the spring bloom. In the northern Kattegat, Great Belt, Femer Belt and in the Arkona Sea, the pools of diatoms increased 10- to 100-fold during the spring bloom. In total, the sediment received between 0.3 and 4 million germinable units of pelagic diatoms cm-2. In terms of organic nitrogen, the enrichment of live diatoms in the sediment corresponded to on average 202% of the total spring bloom production, calculated from the pool of nitrate consumed in the water column during the spring bloom. A similar calculation based on the enrichment of the pigments chlorophyll a and fucoxanthin indicated input corresponding to 24 and 64% respectively of the potential production during the spring bloom. There was considerable variation among stations; in the central Kattegat there was no detectable sedimentation of diatoms or pigments. Here, the pools of pigments and diatoms decreased during the spring bloom and the shape of the pigment profiles also indicated that these pools did not originate from a recent input. Other stations received twice as much diatom biomass than could be produced from the pool of inorganic nitrogen in the mixed layer before the bloom. It is hypothesized that the unequal distribution of the spring bloom sedimentation is a result of episodic sedimentation events in combination with advection of the water masses. The potentials of using sediment profiles of pigments to quantify spring bloom sedimentation are discussed. KEY WORDS: Diatoms · Spring bloom · Benthic-pelagic coupling · Sediment · Chlorophyll a · Fucoxanthin Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 248. Online publication date: February 20, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Characterization of the Adhesive Mucilages Secreted by Live Diatom Cells using Atomic Force Microscopy

Atomic Force Microscopy (AFM) resolved the topography and mechanical properties of two distinct adhesive mucilages secreted by the marine, fouling diatom Craspedostauros australis. Tapping mode images of live cells revealed a soft and cohesive outer mucilage layer that encased most of the diatom's siliceous wall, and force curves revealed an adhesive force of 3.58 nN. High loading force, contact mode imaging resulted in cantilever 'cleaned' cell walls, which enabled the first direct observation of the active secretion of soft mucilage via pore openings. A second adhesive mucilage consisted of strands secreted at the raphe, a distinct slit in the silica wall involved in cell-substratum attachment and motility. Force measurements revealed a raphe adhesive strand(s) resistant to breaking forces up to 60 nN, and these strands could only be detached from the AFM cantilever probe using the manual stepper motor.

GROWTH OF SOME MARINE DIATOMS IN SEA WATER POLLUTED WITH CERTAIN PETROLEUM PRODUCTS

In this investigation live diatom species were grown in sea water polluted with Kerosine and Mazot. The results indicated that the diatoms which tolerated petroleum pollution were the bentho-planktonic taxa Melosira moniliformis (O.Mull.) Ag. and Grammatophora marina (Lyngb.) Kutz. in comparing with the planktonic species Chaetoceros curvisetus Cl., Coscinodiscus granii Gough. and Ditylum brightwellii (West) Grun.. This latter species was the highly susceptible one. The effect of the two petroleum products on the growth of the investigated diatoms varied greatly. Whereas, the number of cells of the tested diatoms decreased under the influence of Kerosine, certain doses of Mazot had stimulatory effect on the growth of some studied diatoms.

NANOSTRUCTURE OF THE DIATOM FRUSTULE AS REVEALED BY ATOMIC FORCE AND SCANNING ELECTRON MICROSCOPY

The cell wall (frustule) of the freshwater diatom Pinnularia viridis (Nitzsch) Ehrenberg is composed of an assembly of highly silicified components and associated organic layers. We used atomic force microscopy (AFM) to investigate the nanostructure and relationship between the outermost surface organics and the siliceous frustule components of live diatoms under natural hydrated conditions. Contact mode AFM imaging revealed that the walls were coated in a thick mucilaginous material that was interrupted only in the vicinity of the raphe fissure. Analysis of this mucilage by force mode AFM demonstrated it to be a nonadhesive, soft, and compressible material. Application of greater force to the sample during repeated scanning enabled the mucilage to be swept from the hard underlying siliceous components and piled into columns on either side of the scan area by the scanning action of the tip. The mucilage columns remained intact for several hours without dissolving or settling back onto the cleaned valve surface, thereby revealing a cohesiveness that suggested a degree of cross‐linking. The hard silicified surfaces of the diatom frustule appeared to be relatively smooth when living cells were imaged by AFM or when field‐emission SEM was used to image chemically cleaned walls. AFM analysis of P. viridis frustules cleaved in cross‐section revealed the nanostructure of the valve silica to be composed of a conglomerate of packed silica spheres that were 44.8 ± 0.7 nm in diameter. The silica spheres that comprised the girdle band biosilica were 40.3 ± 0.8 nm in diameter. Analysis of another heavily silicified diatom, Hantzschia amphioxys (Ehrenberg) Grunow, showed that the valve biosilica was composed of packed silica spheres that were 37.1 ± 1.4 nm and that silica particles from the girdle bands were 38.1 ± 0.5 nm. These results showed little variation in the size range of the silica particles within a particular frustule component (valve or girdle band), but there may be differences in particle size between these components within a diatom frustule and significant differences are found between species.

The influence of particle size on seston deposition in streams

We investigated how particle size influences deposition and transport of fine particulate organic matter in streams. Field additions of very fine (VFPOM, 15–52 µm), fine (FPOM, 53–106 µm), and medium (MPOM, 107–250 µm) detritus and live diatoms (Asterionella sp.) were used to quantify the longitudinal loss rate (kP) of each material type and to derive estimates of mean transport distance (SP) and field deposition velocity (vdep). In all experiments, smaller particles deposited more slowly, and thus traveled farther, than larger size classes. Significant differences in kP were detected in four of seven paired FPOM and VFPOM particle additions. vdep estimates were neither equivalent nor closely associated with calculated quiescent water fall velocities (vfall) for all size classes. Variation in SP and vdep of FPOM and VFPOM were strongly correlated across hydrological conditions (r = 0.94 and 0.92, respectively). Variation in vdep was poorly associated with physical attributes of the stream. Transport distances were positively associated with the crossߚsectional area of the transient storage zone (AS) and the uptake length of water (SW) for both size classes. We argue that local hydrological and benthic conditions establish a minimum rate of particle deposition and that departures from this rate due to gravitational forces begin to occur at particle diameters similar to the larger size classes used in this study (50–100 µm).

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Oresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen–thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml–1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution–extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g–1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.

Shifts in habitat templates for lotic microalgae linked to interannual variation in snowmelt intensity

We investigated the importance of snowmelt as an organizing factor for epilithic microalgae in a high‐altitude montane stream during 3 yr (1995–1997) of differing melt characteristics. Changes in algal biomass and taxonomic structure in two stream reaches that differed in hydrologic characteristics were assessed relative to variation in stream‐water nutrient content, temperature, surface‐water/groundwater exchange, and algivorous macroinvertebrate assemblages. Melt‐induced increases in stream discharge were large and abrupt in 1995, nearly undetectable in 1996, and protracted and of intermediate magnitude in 1997. In 1995, algal and grazer densities were significantly reduced by melt discharge. Postmelt grazer recovery was slow, and the percentage of live cells in the diatom assemblage increased abruptly; algal biomass initially increased and subsequently varied with stream‐water N: P ratio. In 1996, snowmelt produced no evident proximate effects. Initial high grazer densities declined throughout the summer. In midsummer, an N: P ratio of 140 in the upstream reach corresponded to a more than fourfold increase in algal biovolume and a shift to dominance by large diatoms. Downstream, a lower N: P peak induced no algal response. In 1997, grazer densities declined during protracted melt runoff and increased sharply during melt recession, concurrent with decreases in live diatom percentage and algal biovolume. N: P declined in 1997 from ~16 : 1 to values indicative of N limitation and correlated with decreases in algal biovolume. Our results show that effects of snowmelt in montane streams reflect both initial melt‐induced mortality that sets initial conditions for succession and melt‐induced aquifer recharge that controls nutrient supply in the months following peak melt discharge. The influence of these two components extends beyond snowmelt recession into summer base flow, suggesting that variation in melt characteristics generates interannual differences in the functioning of these systems.