VARIATION IN DIATOM COMMUNITY STRUCTURE AMONG HABITATS IN SANDY STREAMS1

Community structure of benthic diatoms in two sandy streams was studied for two summers to assess differences in abundances and species composition among habitats and between years and streams. The greatest differences in abundances (cells·cm^?2) were lower abundances on rocks than on sand, whether the sand was clean or covered with flocculent organic material. Relatively little variation in abundances occurred between years and streams. Species composition of diatom communities varied more among habitats and between streams than from year to year. Species composition was most unique in floc-covered sand when communities in clean and floc-covered sand, rock, plant, and plankton were compared. Diatom species composition in these sandy streams was most similar on sand and rocks.     

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.     

Assessing scales of variability in benthic diatom community structure

Sources of variability such as sampling method, sample preparation, and sample analysis (taxonomy) might affect our ability to measure differences in community structure between sites in environmental effects studies. It is therefore important to ensure that changes in community structure related to the physical or chemical differences between sites are not hidden by other sources of variability within a site. The goal of this study was to quantify the amount of variability in benthic diatom community structure related to sampling and laboratory procedures. Our results showed that variability in community structure was minimal among replicate microscope slides (< 1%) and among samples collected within a site (1.8%). Variability in community structure was substantially higher between sites located in one stream (16.6%), and highest across streams (59.6%). This suggests that field sampling and laboratory methods do not contribute a large amount of variation into our analyses of benthic diatom community structure across sites.     

EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of many Great Basin lakes has resulted from the diversion of tributary streams for agricultural or municipal uses. At Mono Lake, an alkaline salt lake in eastern California, salinities have risen from 50 to 100 g·L^?1 in just 50 years. Experimental mesocosms were established to simulate some of the potential ecological effects that could have accompanied this change. The influence of salinity on diatom diversity, taxonomic structure, and primary production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0.5 m deep, with open tops covered by 1 mm mesh net. Five treatments (50, 75, 100, 125, and 150 g·L^?1) with four replicates per treatment were used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g·m^?2) and diversity (from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities ≥75 g·L^?1. Photosynthetic oxygen production also was significantly lower at salinities ≥75 g·L^?1. Diatom indicator taxa for these shifts included Denticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Stephanodiscus oregonicus increasing in relative abundance in higher salinity treatments, accompanied by decreases in Achnanthes minutissima, Cymbella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting dominance at moderate salinity levels (75 to 125 g·L^?1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophyte, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occurred only in salinity treatments from 50 to 100 g·L^?1. Diversion of tributary stream flow and resulting salinity increases in this lake threaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision of the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to around 75 g·L^?1 and is expected to increase the diversity and productivity of the benthic algae of this ecosystem.     

SPATIAL AND TEMPORAL VARIATION OF EPIPSAMMIC DIATOMS IN A SPRING-FED BROOK1

Epipsammic diatom communities were sampled and quantified bimonthly from June 1983 to April 1985 at four sites in a small brook in northwest Ohio. The primary objective was to determine the extent of temporal and spatial variability in the epipsammon. The four sites, approximately 250 m apart, differed in current velocity, illumination and the amount of sand-associated detritus. Diatoms were significantly more abundant in the portion of the brook flowing through an unshaded marsh than at three heavily shaded sites located upstream. Fragilaria leptostauron (Ehr.) Hust. and Achnanthes lanceolata var. dubia Grun. dominated the epipsammic assemblage at all sites throughout the entire study period. Only Meridion circulare (Grev.) Ehr. displayed a marked seasonal distribution. Although the same species were generally found at all sites, there were sufficient differences in relative abundance that communities could be discriminated according to site. Using two canonical variate axes, all 48 samples were correctly assigned to their proper sampling site based on community composition. Data from this epipsammic assemblage support the idea developed from the river continuum concept that species comprising riverine benthic assemblages continually persist and rarely become completely absent.     

Limnology of 46 lakes and ponds on Banks Island, N.W.T., Canadian Arctic Archipelago

The goal of this study was to describe and quantify the physical and chemical limnological properties of 46 lakes and ponds on Banks Island, to explore the effects of ecoclimatic differences on the water chemistry of these study sites, and to establish baseline conditions for this previously unexplored limnological region, which could then be used in subsequent long-term environmental monitoring programs. A key finding was that the study sites on Banks Island represented a large nutrient concentration gradient from ultra-oligotrophic to hypereutrophic waters. In general, the study sites were relatively nutrient rich by Arctic standards (i.e. mean total nitrogen (TN_mean)=504.2μg/l and mean total phosphorus (TPU_mean)=18.0μg/l); concentrations that are amongst the highest of any previous limnological survey from similar latitudes. Dissolved organic carbon (DOC) concentrations were also some of the highest reported to date amongst all other Canadian Arctic island limnological surveys. These values reflect the milder climate, concentrated animal life and lushness of Banks Island, as compared to other Canadian Arctic Archipelago islands. Principal components analysis (PCA) separated sites along a conductivity/ionic and elevation gradient on Axis 1 (λ₁=0.343), and a metal (Fe, Zn, Al) and alkalinity-related (DIC, pH) gradient on the second axis (λ₂=0.187). Canonical variate analysis (CVA) was used to explore the classification of the study sites into Low Arctic, Mid Arctic or High Arctic designations based on their limnological characteristics.     

Diatoms in river water-monitoring studies

Diatoms grow abundantly in rivers, colonising almost all suitable habitats. They have been used extensively in studies related to the monitoring of water quality. This review attempts to draw together data from a wide variety of approaches. The advantage of sampling discrete habitats is stressed and particular attention is paid to the diatom flora of stone surfaces (epilithon) which is the favoured source in many studies. The advantages and drawbacks of the techniques are discussed and some hopefully helpful suggestions made. The historical development of the topic is briefly surveyed, but no attempt has been made to present a comprohensive review of the literature which has increased enormously in the last few decades.     

Mediterranean karst springs: diatom biodiversity hotspots under the pressure of hydrological fluctuation and nutrient enrichment

Abstract

Mediterranean karst springs are affected by strong climatic seasonality, with long, dry and hot summers and are increasingly threatened by anthropogenic pressures. In Sardinia (western Mediterranean Sea, Italy), they are largely unexplored and their biocoenoses are mostly unknown. The diatom flora from two substrates (cobbles and macrophytes) in eight springs of different areas of the island was investigated in summer 2016 and winter 2017. A total of 162 diatom taxa (58 genera) were found of which 27 (17 genera) only on cobbles and 26 (18 genera) only on macrophytes. The most abundant species from both substrates were Achnanthidium minutissimum, Amphora indistincta, Amphora pediculus, Cocconeis euglypta, Planothidium frequentissimum and P. lanceolatum. Overall, 67 taxa (40 genera) were recorded in single sites and some of these taxa showed high affinity with specific environmental conditions. Hydrological stability (water flow permanence), discharge and nutrients were the main environmental factors influencing diatom assemblages. Our results suggest that diatoms can reflect important local factors related to the vulnerability of these spring ecosystems and underline the importance of their preservation both for biodiversity and water quality maintenance.     

Relationships between DOC concentration and epilithon stable isotopes in boreal lakes

1. Littoral biota in boreal lakes are known to assimilate epilithon. Being able to predict the stable isotopic composition of these alga will help to identify those systems in which δ¹³C and δ¹⁵N analysis can be used in foodweb investigations of allochthony and biomagnification.
2. In a survey of 15 boreal lakes, the concentration of dissolved organic carbon (DOC) explained 76% of the variation in epilithon δ¹³C, and 86% of the variation in epilithon δ¹⁵N.
3. Because both δ¹³C and δ¹⁵N values were depressed and similar to terrestrial values in humic (high DOC) lakes, it will be more difficult to successfully employ stable isotopic techniques for estimating allochthony in such systems. Lower δ¹⁵N values in humic lakes also indicate that trophic positions estimated by stable isotopes are not directly comparable to those of similar biota inhabiting clearwater lakes, unless autochthonous baseline corrections are made.     

The effects of a new reservoir on the attached diatom communities in Huntington Creek,Utah, U.S.A.

A study was made to determine whether a new reservoir being constructed on Huntington Creek, Utah would have significant effects on the attached diatom communities of the stream. Cluster analysis and other statistical methods indicated only small differences in the diatom communities before and after construction of the dam.     

Rotifers from the Canadian high arctic (Devon Island,Northwest territories)

Samples from eight water bodies in the region of Truelove and Sparbo Hardy Lowland, Devon Island (75 ° 30 N, 86 ° 00 W) yielded 70 taxa (4 Bdelloidea, 66 Monogononta) of rotifers, nineteen of which are new records for the Canadian High Arctic.     

Effects of light and nutrients on the net accumulation and elemental composition of epilithon in boreal lakes

1. Two experiments in the Experimental Lakes Area (ELA) in north-western Ontario, Canada examined the effects of light and two key elements on the net accumulation and elemental composition of epilithon. In Lake (L) 224, benthic algae were grown under different light intensity and phosphorus supply, while in L302S we provided three levels of two different carbon sources (bicarbonate and glucose) to algae colonizing nutrient-diffusing substrata. After 1 month of accumulation, we sampled biofilms for chlorophyll (chl), carbon (C), phosphorus (P) and algal C.
2. Increased C supply did not significantly affect algal growth (C or chl) or elemental composition (C/P ratios) in L302S. However, P enrichment increased chl and algal C, dramatically reduced the C/P ratio of epilithon, and did not affect total organic C in L224. Phosphorus enrichment also increased the proportion of algal material in the total particulate organic matter and altered the taxonomic composition of algae in L224 biofilms. Shading had no significant effect on the C/P ratio and total organic C in epilithon from the L224 experiment.
3. Our results demonstrate that P supply affects the elemental composition of organic matter that collects on rock substrata. It thus appears that low availability of P relative to C and light drives the formation and retention of high C/P organic matter on rock surfaces in oligotrophic boreal lakes.     

加拿大北极地区黄昏鸟化石

加拿大北极地区Ｂｙｌｏｔ岛晚白垩世陆相沉积中发现一黄昏鸟化石。这是首次在极地地区发现这类不会飞行的已绝灭的潜水鸟类,该地点也是白垩纪最晚期的黄昏鸟产地。新材料的形态构造与Ｈｅａｐｅｒｏｒｎｉｓ ｒｅｇａｌｉｓ比较接近,但它个体大,颈椎椎体后部特别向两侧扩展,椎体侧凹亦特别大而深等,具特化的性状,因而建一新属新种：Ｃａｎａｄａｇａ ａｒｃｔｉｃａ ｇｅｎ．ｅｔ．ｓｐ． ｎｏｖ．。     

Environmental variability and the ecological effects of spawning Pacific salmon on stream biofilm

1. Variation in resource subsidies can create or reinforce heterogeneity in recipient ecosystems. Related activities of organisms delivering resource subsidies, such as ecosystem engineering by Pacific salmon spawners (Oncorhynchus spp.), also alter heterogeneity. We studied whether heterogeneity in stream environmental conditions and spawner abundances were reflected in the net ecological effects of salmon (i.e. enrichment by resource subsidies and disturbance by ecosystem engineering) on benthic biofilm. 2. We sampled seven Southeast Alaska streams over 3 years, both before and during the salmon run. In each stream and year, stream environmental characteristics and their influence on responses of benthic biofilm [mean and coefficient of variation of chlorophyll a (chl a), ash‐free dry mass (AFDM) and autotrophic index (AFDM:chl a)] to spawners were assessed. 3. Streams and periods before and during the salmon run were distinct based on their environmental characteristics. The responses of most biofilm metrics to spawners were stream‐ and year‐specific, suggesting that the ecological effect of spawners ranged from net enrichment to net disturbance depending on the stream or year studied. The environmental context, especially temperature, large wood, and sediment size, explained >50% of biofilm variability during the run, but <30% over the entire study, suggesting that salmon can alter environmental constraints. 4. Precision of biofilm estimates improved by increasing either the number of streams or the number of years sampled (i.e. spatial or temporal replication). However, combining data from different North Pacific Rim ecoregions inflated the confidence interval as compared with a single ecoregion, indicating the importance of regional environmental contexts for net salmon effects. 5. Our results suggest that biofilm responses to salmon can vary greatly, even within a single ecoregion, and that environmental conditions can modify net salmon effects. Consequently, generalisations about biofilm responses across the native range of salmon may be challenging.