Abundance Of Diatom Species Research Articles

Diatom immigration drives biofilm recovery after chronic copper exposure

Summary1. The impact of immigration on the recovery of diatom assemblages after chronic exposure to copper was investigated in laboratory microcosms.2. We examined the recovery trajectories of copper‐contaminated biofilms after reducing copper stress and with or without the possibility of immigration from unimpaired communities.3. The biofilms mixed with unimpaired communities returned to a ‘control’ assemblage structure within 6 weeks, with recovery patterns depending on the endpoint considered (i.e. 2 weeks for relative abundances of diatom species but 6 weeks for total diatom biomass). In contrast, no recovery was observed in assemblages placed under control conditions without external immigrants.4. Immigration has important effects on the recovery of quantitative and qualitative characteristics of biofilms.

Late Holocene diatom assemblages in a lake-sediment core from Central Kamchatka, Russia

Fossil diatom assemblages in a sediment core from a small lake in Central Kamchatka (Russia) were used to reconstruct palaeoenvironmental conditions of the late Holocene. The waterbody may be a kettle lake that formed on a moraine of the Two-Yurts Lake Valley, located on the eastern slope of the Central Kamchatka Mountain Chain. At present, it is a seepage lake with no surficial outflow. Fossil diatom assemblages show an almost constant ratio between planktonic and periphytic forms throughout the record. Downcore variations in the relative abundances of diatom species enabled division of the core into four diatom assemblage zones, mainly related to changes in abundances of Aulacoseira subarctica, Stephanodiscus minutulus, and Discostella pseudostelligera and several benthic species. Associated variations in the composition and content of organic matter are consistent with the diatom stratigraphy. The oldest recovered sediments date to about 3220 BC. They lie below a sedimentation hiatus and likely include reworked deposits from nearby Two-Yurts Lake. The initial lake stage between 870 and 400 BC was characterized by acidic shallow-water conditions. Between 400 BC and AD 1400, lacustrine conditions were established, with highest contributions from planktonic diatoms. The interval between AD 1400 and 1900 might reflect summer cooling during the Little Ice Age, indicated by diatoms that prefer strong turbulence, nutrient recycling and cooler summer conditions. The timing of palaeolimnological changes generally fits the pattern of neoglacial cooling during the late Holocene on Kamchatka and in the neighbouring Sea of Okhotsk, mainly driven by the prevailing modes of regional atmospheric circulation.

Glacial‐interglacial size variability in the diatom Fragilariopsis kerguelensis: Possible iron/dust controls?

The valve area of Fragilariopsis kerguelensis, the most abundant diatom species in the Southern Ocean, strongly changes in size in response to varying conditions in the surface ocean. We examined the link, both in two iron fertilization experiments and in sediment samples covering several glacial Terminations, between size variability in this species and environmental conditions across the Antarctic Polar Front, including sea ice extent, sea surface temperature, and the input of eolian dust. The iron fertilization experiments show valve area to be positively correlated with iron concentrations in ambient waters, which suggests the possibility of a causal relation between valve size of Fragilariopsis kerguelensis and ambient surface water iron concentration. Larger valves are usually found during glacial times and thus seem to be related to lower sea surface temperature and wider sea ice coverage. Moreover, our results indicate that there usually is a strong correlation between larger valve size and increased input of eolian dust to the Southern Ocean. However, this correlation, obvious for the fertilization experiments and for glacial Terminations I, II, III, and V, does not seem to be valid for Termination VI, where size appears to be inversely correlated to dust input.

A 2,500-year record of environmental change in Highlands Hammock State Park (Central Florida, U.S.A.) inferred from siliceous microfossils

Analysis of siliceous microfossils of a 79 cm long peat sediment core from Highlands Hammock State Park, Florida, revealed distinct changes in the local hydrology during the past 2,500 years. The coring site is a seasonally inundated forest where water availability is directly influenced by precipitation. Diatoms, chrysophyte statospores, sponge remains and phytoliths were counted in 25 samples throughout the core. Based on the relative abundance of diatom species, the record was subdivided into four diatom assemblage zones, which mainly reflect the hydrological state of the study site. An age-depth relationship based on radiocarbon measurements of eight samples reveals a basal age of the core of approximately 2,500 cal. yrs. BP. Two significant changes of diatom assemblage composition were found that could be linked to both, natural and anthropogenic influences. At 700 cal. yrs. BP, the diatom record documents a shift from tychoplanktonic Aulacoseira species to epiphytic Eunotia species, indicating a shortening of the hydroperiod, i.e. the time period during which a wetland is covered by water. This transition was interpreted as being triggered by natural climate change. In the middle of the twentieth century a second major turnover took place, at that time however, as a result of human impact on the park hydrology through the construction of dams and canals close to the study site.

Diatoms from Surface Sediments of Enderby Basin of Indian Sector of Southern Ocean

Abstract Diatoms from surface sediment samples collected from Enderby Basin of Indian Sector of Southern Ocean were analyzed to determine the relative abundance and distribution of seven key indicator diatom species viz. Sea ice related species Fragilariopsis rhombica, F. separanda, F. curta, F. ritscheri, Thalassiosira tumida and Actinocyclus actinochilus and Open Ocean species F. kerguelensis on the basis of modern physico-chemical parameters. The relative abundances of different species observed viz. F. rhombica – 6.25%; F. separanda – 12.5%; F. curta – 10.53-13.33%; F. ritscheri – 4.55-12.5%; F. kerguelensis – 6.25-63.64%; T. tumida – 3.13% and A. actinochilus – 9.38-13.33%. The increasing abundance of F. kerguelensis consecutively suggests the effect of Antarctic bottom water in the study area which is further substantiated by the presence and increasing abundance of F. ritscheri. The gradual decrease in abundance or absence of sea ice related species from the sampled stations indicates the summer and winter sea ice extent concentration in the study area. The nutrient concentration correlates with the distribution and abundances of diatom species.

Sedimentology and paleontology of a tsunami deposit accompanying the great Chilean earthquake of February 2010

Abstract At Pichilemu, in the northern third of the rupture area of the moment magnitude scale (Mw) 8.8 2010 Chile earthquake, deposits of the tsunami accompanying the earthquake consist of a lower layer of medium to fine sand (mean grain size of 200 μm) containing rock clasts, overlain by a thin, silty, very fine sand (mean grain size of 125 μm) layer. Based on a sedimentological model, most (90%) of the deposit is finer than 401–408 μm suggesting tsunami flow velocities were between 7 m/ s and 13.5 m/ s. Ostracods were common in the upper layer along with a small number of broken benthic foraminifera and a single planktonic foraminifera. Diatoms were abundant throughout. Species assemblages represent a mixture of diatoms from differing environments, life forms and substrate preferences. We attribute the mixed assemblages to turbulence within the water column during tsunami inundation, with erosion of beaches and salt marshes followed by redeposition of sand and mud inland. Breakage of fragile diatom valves in the lower layer may also support transport by turbulent flow. A higher abundance of diatom species with mud substrate preferences in the upper layer implies a decrease in flow velocity from lower to upper layers.

VARIABILITY IN CELL SIZE, NUTRIENT DEPLETION, AND GROWTH RATES OF THE SOUTHERN OCEAN DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE) AFTER PROLONGED IRON LIMITATION1

Diatoms are the main primary producers in the Southern Ocean, governing the major nutrient cycles. Fragilariopsis kerguelensis (O’Meara) Hust. is the most abundant diatom species in the Southern Ocean and its paleo‐oceanographic record is frequently used to reconstruct the past position and nutrient characteristics of the Antarctic polar front. Here we report on the responses of F. kerguelensis on prolonged exposure to a range of iron concentrations, allowing a characterization of morphological and nutrient‐depletion changes in relation to iron status. Under iron limitation, F. kerguelensis grew slower, cells became smaller, chains became shorter, and the nutrient‐depletion ratios changed. Prolonged exposure to iron limitation caused F. kerguelensis to decrease its surface area and volume 2‐fold, and to increase its surface‐to‐volume ratio by 25%. With the decrease in growth rates, silicon (Si) and phosphorus (P) depletion per cell remained fairly constant, but when normalized per surface area (Si) or per cell volume (P), depletion increased. In contrast, nitrogen (N) depletion per cell decreased significantly together with the decrease in growth rates but was constant when normalized per cell volume. The different response in Si, P, and N depletion resulted in changes in the nutrient‐depletion ratios, most notably in the Si:N ratio, which significantly increased, and in the N:P ratio, which significantly decreased with decreasing growth rates. It is concluded that under iron limitation, variation in cell size and/or nutrient depletion ultimately can cause changes in oceanic biogeochemical nutrient cycles. It enables the use of cell size of F. kerguelensis as a paleo‐oceanographic proxy.

Nutrient enrichment in the High Arctic associated with Thule Inuit whalers: a paleolimnological investigation from Ellesmere Island (Nunavut, Canada)

Cultural eutrophication is one of several environmental stressors that represents a significant ecological threat as the human population continues to grow and encroach on aquatic ecosystems. However, until recently, the environmental impacts of ancient human populations that had low population densities were thought to be largely insignificant relative to those of post-industrial urban society. Based on paleolimnological and archeological data, we have shown that Thule Inuit whalers living in small, nomadic communities were altering High Arctic pond ecology centuries before industrialization. Changes in diatom species assemblage and spectrally inferred aquatic primary production indicate a period of increased production in a pond (E-Knud) on Knud Peninsula, Ellesmere Island, which can be linked to the effects of the Thule from about 810-340 years ago. This site is near the northernmost region of past Thule occupation. Input of nutrients from bowhead whale carcasses, used by the Thule Inuit for sustenance and the construction of winter settlements, as well as other Thule activities (e.g., sealing, human waste, etc.), resulted in an increase in d 15 N, which coincided with ecologically significant diatom abundance changes, including an increase in Amphora ovalis. Diatom and nutrient changes also coincided with increased pri- mary aquatic production, as measured by spectrally inferred chlorophyll a concentration. In addition, we recorded relative increases in diatom taxa such as Craticula halophila and Achnanthidum minutissimum in the more recent sediments, which we attribute to recent climate warming, manifested by lower water levels and associated evapo-concentration. Specific conductivities recorded during three sampling years (2004, 2006, 2009) show a trend of increasing conductivity in pond E-Knud in response to declin- ing water levels. Together these changes in diatom species abundance suggest that multiple environ- mental stressors, operating several centuries apart, have led to marked limnological changes in E-Knud pond.

Phytoplankton Composition and Environmental Conditions of the Mucilage Event in the Sea of Marmara

The composition and abundance of phytoplankton and the corresponding environmental conditions were investigated during a mucilage event (October 2007-February 2008) in the Sea of Marmara. The mucilage producers Gonyaulax fragilis, Skeletonema costatum, and Cylindrotheca closterium were identified as abundant species. Thallassiosira rotula was also identified in large numbers. The maximum number of G. fragilis was observed at 83,600 cells L^{-1} in November 2007 during the first sampling occasion on the surface layer of İzmit Bay, and T. rotula was the most abundant diatom species, with 131,040 cells L^{-1} in the same period. G. fragilis was recorded at 96,250 cells L^{-1} in the dense mucilage-containing water samples collected from Değirmendere (İzmit Bay) in January 2008, and C. closterium was the dominant (161,250 cells L^{-1}) diatom species in the same sample. Species diversity values (H'_{log2}) increased from November to February, displaying maximum values of 2.5-3.5 in February at almost all stations (depths of 0.5 and 10 m), caused by the balanced increase of diatom species and their individual numbers in the total phytoplankton abundance. The surface layer waters of İzmit Bay exhibited severely low nitrogen to phosphorus ratio (N:P) values (0.1-14.4) compared to the Redfield ratio (16), which could be supported by the fact that the limiting nutrient for the Sea of Marmara is nitrogen. Even though there is a lack of data from before the mucilage period, the data obtained during the lifespan of the event indicated that a mixture of several phytoplankton species might have played a key role in the mucilage formation. The high organic carbon content of the mucilage (24% org-C) and the surrounding waters support the possibility that the event could have been related to organically rich cellular exudates of phytoplankton.

Diatom distribution in southeastern Pacific surface sediments and their relationship to modern environmental variables

The quantitative analysis of diatom assemblages preserved in 52 samples from the Bellingshausen and the Amundsen Seas provides the first comprehensive view on the distribution of diatoms in surface sediments of the eastern and central Pacific sector of the Southern Ocean. On a latitudinal transect along 120°W, diatom valve accumulation rates (AR) reach maximum values (8–10 × 10 8 valves m − 2 yr − 1 ) in a zone extending over ca. 900 km between the Antarctic Polar Front and the maximum average winter sea ice extent and exceed those ARs obtained from an eastern transect along 90°W by one order of magnitude. Lowest diatom concentrations (1–3 × 10 6 valves g − 1 ) were encountered in sediments of the Sea Ice Zone, affected by winter and summer sea ice. The accumulation rate pattern of the most abundant diatom Fragilariopsis kerguelensis (> 50% abundance in 47 samples) mirrors the pattern of the total diatom valve AR and the biogenic silica (BSi) AR, making F. kerguelensis the major contributor to the BSi preserved at the sea floor. Relative abundances of diatom species and species groups were statistically compared with a selection of environmental variables, such as the mean summer sea surface temperature and salinity, mean annual surface nutrient concentration (nitrate, phosphate, silicon), mean annual water column stratification, mixed layer depth in summer, and mean summer and winter sea ice concentrations. Polynomial canonical redundancy analysis (RDA) revealed the biogeographic distribution of diatom species had the strongest relationship with summer sea surface temperature (SSST) out of the nine tested environmental variables. This relationship accounted for 69.6% of the total variance of the diatom distribution, with 29.7% explained by the first gradient (significantly correlated to SSST with r 2 = 0.941) and 15.6% explained by the second gradient (correlated to both summer and winter sea ice and silicon concentration). Azpeitia tabularis, Hemidiscus cuneiformis and Roperia tesselata were associated with warmer water conditions (> 4 °C), whereas Fragilariopsis curta, F. separanda, F. rhombica and Thalassiosira gracilis were correlated with cold SSST (< 1.5 °C). Under the second gradient relationship, Actinocyclus actinochilus and F. curta were the most important diatoms representative of the diatom distribution in relation to the observed mean summer and winter sea ice concentrations. Confirming these environmental relationships is crucial for the development of reference data sets used in quantitative estimations of palaeoclimatic and palaeoceanographic conditions with statistical methods. This new data set represents the first modernised treatment of diatom remains from the SE Pacific Ocean and generally supports the use of a circum-polar database for the determination of summer SST, sea ice and potentially biogenic silica distribution of the Southern Ocean back into the Late Quaternary.

The last 100 000 years' palaeoenvironmental changes inferred from the diatom assemblages of core MD012404 from the Okinawa Trough, East China Sea

AbstractThe diatom assemblage records preserved in a sedimentary core MD012404 retrieved from the Okinawa Trough in the East China Sea (ECS) imply palaeoenvironmental changes of the last 100 ka. Our statistical analyses of diatom species abundance data indicate that the assemblages dominate open ocean and warm water, coastal water and freshwater masses in the ECS. These proxy species were used to infer the long‐term variation in surface hydrological conditions and palaeoclimate. Cycles of ca. 23 ka observed in the relative abundance of the tychopelagic species Paralia sulcata indicate that the increases in precipitation driven by solar insolation might have controlled the diatom assemblages in the ECS. The diatom records also indicate relatively low abundances of tropical, tychopelagic and freshwater species during millennial‐scale cold events such as the Younger Dryas and Heinrich events, which have been well expressed in the high‐latitude Northern Hemisphere climate. The floral patterns are interpreted as synchronous cooling and more saline sea surface conditions in the ECS caused by the enhanced Asian winter monsoon during the Northern Hemisphere cold events. The higher abundances of tropical species during deglaciation and the Holocene might have been linked to gradual increases in the inflow of the Kuroshio into the Okinawa Trough. The records also suggest a temporary reduction in tropical species from 5 to 3 ka, corresponding to the Pulleniatina Minimum Event during which the warm‐water planktic foraminifer Pulleniatina obliquiloculata was rare. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Efficacy of algal metrics for assessing nutrient and organic enrichment in flowing waters

Summary1. Algal‐community metrics were calculated for periphyton samples collected from 976 streams and rivers by the U.S. Geological Survey’s National Water‐Quality Assessment (NAWQA) Programme during 1993–2001 to evaluate national and regional relations with water chemistry and to compare whether algal‐metric values differ significantly among undeveloped and developed land‐use classifications.2. Algal metrics with significant positive correlations with nutrient concentrations included indicators of trophic condition, organic enrichment, salinity, motility and taxa richness. The relative abundance of nitrogen‐fixing algae was negatively correlated with nitrogen concentrations, and the abundance of diatom species associated with high dissolved oxygen concentrations was negatively correlated with both nitrogen and phosphorus concentrations. Median algal‐metric values and nutrient concentrations were significantly lower at undeveloped sites than those draining agricultural or urban catchments.3. Total algal biovolume did not differ significantly among major river catchments or land‐use classifications, and was only weakly correlated with nitrate (positive) and suspended‐sediment (negative) concentrations. Estimates of periphyton chlorophyll a indicated an oligotrophic–mesotrophic boundary of about 21 mg m−2 and a mesotrophic–eutrophic boundary of about 55 mg m−2 based on upper and lower quartiles of the biovolume data distribution.4. Although algal species tolerance to nutrient and organic enrichment is well documented, additional taxonomic and autecological research on sensitive, endemic algal species would further enhance water‐quality assessments.

The distribution and seasonal dynamics of diatom and dinoflagellates in the İskenderun Bay (Northeastern Mediterranean)

The distribution and abundance of diatoms and dinoflagellates which important members of marine phytoplankton were examined at three stations located from coastalmost area to offshore in the A„°skenderun Bay between 2004 and 2005. The abundance changes of diatoms and dinoflagellates with regard to sampling stations were investigated and the effects of environmental parameters were tried to determined. The diatom abundance ranged 0.25x103 and 18.9x103 cells l-1, its maximum were reached in June at the coastalmost station. The most abundant diatom species were Pseudonitzschia spp. and Proboscia alata. Dinoflagellate abundance ranged from 0.35x103 to 6.4x103 cells l-1 and their maximum were found in September at the offshore station. S. trochoidea and G.acuminatum were the most abundant dinoflagellate species. It was observed that diatoms were dominant throughout the study period. The results also indicated that diatoms were abundant at the nearcoastal area, while the dinoflagellates were more abundant at the offshore area.

Diatom assemblages in surface sediments of the northwestern Weddell Sea, Antarctic Peninsula

A study of 42 surface sediment samples from the northwestern Weddell Sea documents the relationship between the modern distribution of diatom assemblages and present oceanographic conditions in the region. Three diatom assemblages were identified using R-mode principal components analysis and cluster analysis, while Q-mode cluster analysis was used to distinguish two geographically distinct biofacies. Based on the relative abundance of diatom species, the biofacies of the former Larsen A Ice Shelf (LAIS) is more closely associated with a sea ice diatom assemblage, while the biofacies of the Prince Gustav Channel/Northern Peninsula (PGC/NP) is associated with two open coastal diatom assemblages. This division is consistent with variance observed in the current oceanographic setting, whereby the LAIS is generally characterized by a longer seasonal duration of sea ice coverage and lower primary productivity relative to the PGC/NP which is marked by a shorter seasonal duration of sea ice coverage and greater surface water productivity. This relationship is further reflected in the distribution of the paleoenvironmentally significant diatom species Fragilariopsis curta, Thalassiosira antarctica, and Chaetoceros subg. Hyalochaete spp. as well as in absolute abundance data. Specifically, F. curta and the morphological form T. antarctica T1 are representative of greater seasonal duration of sea ice coverage and lower productivity. Chaetoceros subg. Hyalochaete spp. and the morphological form T. antarctica T2 are associated with decreased seasonal duration of sea ice cover and greater surface water productivity, a setting that is further reflected in high absolute diatom abundances. These findings provide a basis for future paleoceanographic work in the region, especially as studies of surface sediment diatom assemblages are vital to establishing a link between downcore diatom assemblages and those presently found in the water column.

Impact of Urban Pollution from the Hanoi Area on Benthic Diatom Communities Collected from the Red, Nhue and Tolich Rivers (Vietnam)

The effects of urban pollution from Hanoi city on the benthic diatom communities of the Nhue–Tolich river system were studied during the 2003 dry season. Benthic diatoms were allowed to grow on glass slides suspended in the water flow for 4 weeks. To reveal the relationship between water quality and diatom communities, Canonical correspondence analysis (CCA) was used on data concerning relative abundances of diatom species and environmental variables. Two diatom indices, IPS and DAIpo, were applied to evaluate water quality in the three rivers. A total of 291 diatom taxa were found in the Red, Nhue and Tolich Rivers. These were mainly cosmopolitan taxa, with some tropical, subtropical and endemic taxa. The most abundant taxa at the Red site were Aulacoseira granulata, Achnanthidium minutissimum, Encyonema minutum, Navicula recens and other halophilous taxa such as Nitzschia kurzii, Seminavis strigosa, Entomoneis paludosa, Bacillaria paradoxa. Diatom assemblages at the Tolich site consisted mainly of Nitzschia umbonata, Nitzschia palea and Eolimna minima. Diatom density ranged from 660 to 30,000 cells/cm2. Environmental variables and diatom assemblage composition at all sites were significantly correlated. Two diatom indices gave similar results and indicate the Tolich River with the lowest values as a highly polluted site.

The influence of natural environmental factors on benthic diatom communities from the Windmill Islands, Antarctica

The species composition of benthic diatom communities responds rapidly to environmental change. At four shallow benthic sites in the Windmill Islands, Casey, East Antarctica, redundancy analysis indicated that changes in sediment grain-size, light availability and water depth account for 30% of the variation observed in the relative abundances of diatom taxa. The amount of mud (< 63 μm) present within the sediments explained 18% of the variation in the relative abundances of diatom species observed in samples from all four locations. Within two locations, the amount of mud present explained 25% or more of the total variation in relative species abundance observed. Differences between locations explained 28% of the variation observed in relative species abundance; however, the majority of this was due to changes in grain-size, light availability and depth between the locations. It is unclear what other parameters contribute to observed differences between locations.

Experimental Analysis of Grazing by the Mayfly Meridialaris chiloeensis on Different Successional Stages of Stream Periphyton

AbstractIn this study we determined grazing effects of the South Andean endemic mayfly Meridialaris chiloeensis on periphyton at different stages of successional development. Grazing effects were studied through a two‐factor experimental design (colonization stages X grazer density) in a stream‐side channel in spring and winter. Our results showed an absence of proportionality between grazer density and periphyton decline in response to grazers at low and intermediate levels of periphytic biomass; however, when periphyton biomass was high a direct inverse relationship was observed between post‐grazing biomass and grazer density. The relationship between periphytic algae (chlorophyll a concentration) and periphyton (total periphytic ash‐free dry mass) (C/OM index) was used as an estimation of the autotrophic fraction in the total periphyton matrix. Grazing did not alter the C/OM index indicating that both autotrophic and heterotrophic fractions of the periphyton components were reduced in the same proportion. Ordination of samples using the relative abundance of diatom species showed that herbivore effect was less evident at intermediate and late stage of colonization than at early one. These results support the statement that the outcome of the herbivore‐periphyton interaction may depend on the successional stage of the periphyton community. In spring Fragilaria pinnata relative abundance, on the basis of cell counts, was reduced by grazing and Nitzschia palea was enhanced. In the winter experiment, grazing decreased Achnanthes minutissima relative abundance. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of environmental gradients using physico-chemical measurements and diatom densities in Nairobi River, Kenya

Degradation of water resources due to anthropogenic activities is a major problem all over the world. Monitoring changes in water quality is expensive but essential for management of water resources. Diatom assemblages can serve as useful indicators of changes in water quality. In order to determine changes in stream water quality in an urban environment, densities of epilithic diatom communities on both natural and artificial substrates were sampled in Nairobi River, Kenya in September 2000. Sampling of water and diatoms was carried out in almost equidistant sites along a 60 km stretch of the Nairobi River. Environmental (water physico-chemical and nutrients) data was explored by Principal Correspondence Analysis (PCA) and sites split into three groups of upper-, mid- and lower stream. Most abundant diatom taxa on both substrates were selected and their relationship with environmental variables determined. On artificial substrates, Gomphonema parvulum, Nitzschia palea and Nitzschia umbonata were the most abundant and were significantly related with total dissolved solids, nitrate, conductivity and turbidity. Temperature, altitude and pH interacted significantly with dominant diatom species on natural substrates, which included Gomphonema parvulum, Nitzschia palea and Navicula subminuscula. Non-parametric analyses showed that diatom densities in natural and artificial substrates were significantly different and were affected by different environmental variables. Our results indicate that abundant diatom species can be used to describe major types of aquatic environmental gradients and can serve as good indicators of ecological conditions in tropical streams and rivers. This study also showed that diatoms from artificial and natural substrates can be used to provide complementary information.

Gradient responses of diatom communities in the Bothnian Sea (northern Baltic Sea), with emphasis on responses to water movement

This study analyses diatom communities on submerged stones in three areas of the Bothnian Sea, a subbasin of the brackish Baltic Sea. The communities were composed of mixtures of epilithic, epiphytic, epipelic, epipsammic and occasionally some pelagic diatoms. Altogether, 120 quantitative epilithic samples, 40 per area, were collected in spring. Exposure to wave action was the major environmental factor varying among sampling sites. Relative ignition loss, used in this study as an indirect measure of macroalgal cover, was positively correlated with exposure to wave action and negatively with dry weight and diatom species diversity. This suggests that macroalgae were more abundant on stones with higher exposure to wave action. The weight of sand grains on the sampled stones was negatively correlated with exposure to wave action. Relative abundance of diatom species was recorded in two separate biovolume classes. For both classes, canonical correspondence analysis revealed that the major environmental factor stru cturing the diatom communities was exposure to wave action. Of secondary importance were differences in community composition explained by nutrient variables. Epipelic species, e.g. Navicula gregaria, N. rhynchotella and Surirella brebissonii, as well as small epipsammic species, e.g. Martyana atomus, Opephora mutabilis and Staurosira punctiformis, were typical of no or low water movement. By contrast, the relative abundance of epiphytes, e.g. Diatoma moniliformis and Tabularia fasciculata, increased with stronger water movement. This was presumably related to the higher availability of macroalgal substrata at sites exposed to wave action. Dominance of epipsammic species separated stagnant water from all other levels of water movement. Large diatom species (≥ 1000 μm3) showed a continuous gradient response to water movement; small species (< 1 000 μm3) gave a “threshold” response.

Selective feeding by the mudskipper ( Boleophthalmus pectinirostris ) on the microalgal assemblage of a tropical mudflat

Temporal variation in abundance of the microphytobenthos on an estuarine mudflat in Hong Kong was investigated over a 13-month period. Abundance, in terms of both cell counts and chlorophyll-a (chl-a) concentration, of the diatom-dominated assemblage was higher (peak at 47 mg chl-a m−2 in January) in the cooler months than in the warmer months (minimum at 11 mg chl-a m−2 in August). Variation in surface density (range: 0.25–1.9 individuals m−2) of the herbivorous mudskipper Boleophthalmus pectinirostris was also temporally variable, with a broadly inverse pattern to that of the microphytobenthos. Comparison of the relative abundance of diatom species in the gut and on the mud surface using Strauss' linear selectivity index suggests significant size-selective feeding by B. pectinirostris. Diatoms with maximum lengths >50 μm were selectively ingested whereas smaller species were rejected. This critical diatom length is close to the average distance between branchiospines of the fish's gills, suggesting their involvement in a physical sieving process that is responsible for the size-selective feeding. Significant differences in feeding pattern exist between adult (>80 mm standard length) and juvenile fish. As a result of seasonal variations in density and selective removal of the larger diatoms, B. pectinirostris exerts a significant impact on the abundance and structure of the mudflat microphytobenthos.