Seasonal variations of diatoms and dinoflagellates in a shallow,temperate estuary,with emphasis on neritic assemblages

Seasonal changes in the diatom and dinoflagellate assemblages were examined in the neritic zone of the Urdaibai estuary (north Spain) with regard to some major physical and chemical variables during an annual cycle. A total of 81 diatoms and 38 dinoflagellates were identified and quantified during the study period. Both groups displayed a distinctive pattern of seasonal succession. The seasonal distribution of the Shannon index showed a trend of increasing values from the upper estuary to the lower neritic segment. The diatom diversity maxima were observed in February, April and September, and dinoflagellate maxima in April–May, July and October. Diatoms dominated the assemblages, reaching 1×10⁶ cells l^–1 from April to September. A shift from large diatoms and dinoflagellates to small bloom-forming taxa was observed during winter–early spring. A spring diatom bloom composed of Rhizosolenia spp. was observed in April, while small chain-forming taxa (chiefly Chaetoceros spp.) dominated from June to September. Cell maxima for both groups in late summer were produced by the diatoms Chaetoceros salsugineum and Skeletonema costatum, and by the dinoflagellates Heterocapsa pygmaea and Peridinium quinquecorne. Silicate availability by river supply and strong tidal-mixing of the water column seem to determine the year-round dominance of diatoms over dinoflagellates.     

Relative influence of size,connectivity and disturbance history on plant species richness and assemblages in fragmented grasslands

Questions: What is the relative influence of size, connectivity and disturbance history on plant species richness and assemblages of fragmented grasslands? What is the contribution of small fragments to the conservation of native species pool of the region? Location: Tandilia's Range, Southern Pampa, Argentina. Methods: Cover of plants was registered within 24 fragments of tall‐tussock grassland remnants within an agricultural landscape using modified Whittaker nested sampling. We analysed the influence of site variables related to disturbance history (canopy height, litter thickness) and fragment variables (size, connectivity) on species richness (asymptotic species richness, slope of the species–area curve) as well as on species assemblages by multiple regressions analysis and canonical correspondence analyses, respectively. Cumulative area was used for analysing whether small fragments or large fragments are more important to species diversity in the landscape. Results: Asymptotic species richness was significantly influenced by site variables, in particular by Paspalum quadrifarium's canopy height, but not by fragment variables. Species assemblages were also affected by site variables (12.2% of total variation), but no additional portion of the species assemblage variability was significantly explained by fragment size and connectivity. Sampling of several small fragments rendered more exotic and native species than sampling of few large fragments of the same total area. Conclusions: Our results agree with previous studies reporting low sensitivity of species diversity to size and isolation of grassland fragments in fragmented landscapes and high sensitivity of species diversity to local variables. The higher capture of regional native species pool by small grassland fragments than by few larger ones of equivalent accumulated area highlights the value of small fragments for conservation.     

Recent changes in coral assemblages of a South African coral reef, with recommendations for long-term monitoring

Two-mile reef, Sodwana, South Africa is an unusual coral reef, being situated on a submerged fossilized sand dune and being very southerly (27°54). It is a popular Scuba diving venue receiving about 100000 dives year^–1. The line-intercept transect method, as recommended by the global coral reef monitoring network (GCRMN), was used to determine soft coral, hard coral and other benthos percentage cover. Physical coral damage, disease and bleaching were also recorded. Results were compared with those of B. Riegl (1993 – unpublished PhD thesis) 5 to 7 years earlier. The reef appears to be ecologically and highly dynamic. In the interim, there has been an increase in living benthos cover of 22.3% but also an increase in coral bleaching from 0% in 1993 to 1% in 1998. Physical damage, despite the large number of dives on the reef was minimal (1.52%), although it appears as if coral diseases may be increasing. The 20-m transects recommended by GCRMN are too long for this highly rugose reef with its distinct ridges and gullies. It is recommended that benthos cover, coral damage, bleaching and disease should be monitored annually using 40 5-m transects on the reef ridges and 40 5-m transects on the reef slopes.     

Long-termed changes in ground beetle (Coleoptera: Carabidae) assemblages in a field treated by organic fertilizers

In 2001–2006, ground beetles were pitfall-trapped in a temperate lowland area of South Slovakia in an experimental field divided in five plots fertilized by four different doses of manure or biosludge (25 t stable manure ha⁻¹, 50 t biosludge ha⁻¹, 50 t stable manure ha⁻¹, 100 t biosludge ha⁻¹ and without fertilization). The field was subsequently sown by spring barley, sugar beat, maize, sunflower, sugar beat and maize. The ground beetle assemblage consisted of 31 species, but only five species predominated: Pseudoophonus rufipes representing 82.6% of individuals and five species (Poecilus cupreus, Carabus scheidleri, Calathus fuscipes, Trechus quadristriatus, Pterostichus melanarius, Anchomenus dorsalis, Dolichus halensis) representing together 14.5% of individuals. Pseudophonus rufipes represented 81.7% of dry biomass and three species (Carabus scheidleri, Poecilus cupreus Pterostichus melanarius) 15.9% of biomass. There was no significant influence of organic fertilizing on assemblage structure. During the investigation, the number of individuals and their biomass increased in all plots until 2003 and than dropped to the starting values. The culmination of 2003 was preceded by a warmer and more humid season in 2002. After a cold and dry season of 2003 abundance decreased approximately to starting values. Simultaneously, the local maxima and minima of occurrence of ground beetles in individual plots shifted independently on the doses of organic material. At the same time, number of occurring species slightly decreased. The observed changes obviously represent part of long-termed fluctuations in wider surroundings.     

Seasonal changes in the photosynthetic capacity of cones on a larch (Larix kaempferi) canopy

The seasonal changes of photosynthesis of cones of Japanese larch (Larix kaempferi Carr.) trees showed that gross photosynthetic rate of young cones (P _G) was 2–3 μmol m⁻² s⁻¹ at surface area unit and P _G/R _D (dark respiration of cones) peaked about 0.7 in the same period, indicating that 70 % of respiratory CO₂ was re-fixed. With maturation, P _G and P _G/R _D sharply decreased. Chlorophyll content in cones was 3–20 % of that in leaves, which made it a limiting factor for photosynthesis and its content was closely correlated with photosynthetic capacity. Although sunken and linearly arranged stomatal organs were found on the scale of young cones, differently from the significant regulation of leaf photosynthesis, these stomata tended to be non-functional since CO₂ is not limiting factor for cone photosynthesis. Thus photosynthesis of larch cones is an additional contribution to their development.     

Seasonal variation in the composition of fish assemblages in small Amazonian forest streams: evidence for predictable changes

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra‐firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site‐specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short‐duration lateral migrations to these ponds may play an important role in the seasonal fish‐assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.     

银杏黄酮苷和萜类内酯含量的季节变化

以银杏（Ginkgo biloba L.)2年生实生苗和大树为试材,分析根、茎和叶中银杏黄酮苷及萜类内酯含量的季节变化规律。银杏叶中萜类内酯含量从春季起逐渐增加,至夏末秋初达最高值,随后逐渐减少;根和茎中萜类内酯含量的季节变化与叶中相类似,但在冬季休眠期维持较高含量,进入春季伴随叶的萌发生长降低到全年的最低点。银杏茎中萜类内酯含量最低,相当于叶含量的1／3和根含量的1／2。叶中白果内酯含量在总萜类内酯中所占比例较高,而在根和茎中所占比例则较低。随着树龄增加,银杏叶萜类内酯含量下降,这可能与萜类内酯合成能力下降有关。银杏黄酮苷含量在春季幼叶中最高,夏季和秋季相对较低且变化不明显;长枝叶中槲皮素较多,而短枝叶中山柰黄素较多。对不同季节和不同部位的不同成分含量的相关机理进行了讨论。     

Seasonal changes in energy and the energy cost of spawning in Gulf of Alaska Pacific cod

In the Gulf of Alaska, adult Pacific cod exhibited an annual cycle of condition, gonad index and liver index in which maximum values occurred in ripe fish in March and minima in July. About 30–31 % of prespawning stored energy was expended during the spawning effort. The energy associated with spawning derived from liver (24% and 18%), somatic tissue (22% and 33%) and gonad (53% and 48%) for females and males, respectively. Liver index and gonad index at the time of sampling were directly related in females, but in males gonad index was best related to liver index 1–3 months earlier.
The Pacific cod is very similar to the Atlantic cod in terms of energy cycling, maximum gonad sizes, energy expended during spawning and gonadal contribution to energy expenditure. However, in Pacific cod, somatic tissue contributes markedly to energy expended during reproduction. The Pacific cod cod differs from the walleye pollock with respect to gonad index (13% and 20%ν. 20% and 8% for females and males, respectively), spawning weight loss (25%ν. 38%), liver energy loss during spawning (71%ν. 55%) and energy cost of spawning.     

Seasonal changes in the Modulus of Elasticity of living branches of three coniferous species

The seasonal changes in the Modulus of Elasticity (E) of living branches ofCryptomeria japonica, Chamaecyparis obtusa andLarix leptolepis were investigated over a period of 1 year by means of a quick and non-destructive method previously introduced by the authors. Two sample trees were used for each species and 12 branches were selected from each tree. Readings of the experiments began in summer 1992 and were successively conducted in autumn 1992, winter 1993, spring 1993 and finally in summer 1993. Our investigations revealed that meanE values increased in cold seasons and decreased in warm or hot seasons. Mean values ofE estimated in summer (1993) were relatively close to those estimated in summer 1992 indicating that any changes that occurred were the direct result of the environmental factors prevailing in the intervening seasons. Extremely high meanE values were measured when the branches were in a frozen state in winter; 69.4%, 29.9% and 24.6% higher than the previous summer for branches ofCryptomeria japonica, Chamaecyparis obtusa andLarix leptolepis, respectively. This sharp increase over the initialE values measured in summer 1992 was quite likely due to the freezing effect of rime on the branches.     

飞蝗越冬卵过冷却点的季节性变化及生态学意义

运用热电偶方法测定了飞蝗LocustamigratoriaL .越冬卵过冷却点月份间的变化。结果表明 :飞蝗卵的过冷却点值波动较大 ,主要形成 2部分 ,一部分分布在 -2 5℃左右 ,另一部分分布在 -1 0℃左右 ,同时 ,这两部分的比例随着季节性气温的变化有明显的差异 ,气温较低的月份 ,-2 5℃左右的比例增加 ,而且最小过冷却点的值也更低 ;反之 ,-1 0℃左右的比例增加 ,过冷却点最高值也升高。这种现象说明了飞蝗卵对冬季低温的一种生态适应。     

Seasonal changes in alkaline phosphatase activity of bacteria and microalgae in Lake Pavin (Massif Central,France)

Seasonal changes in alkalinephosphatase activity of bacteria and microalgae werestudied in the dimictic Lake Pavin (Massif Central,France), to test whetherthis activity is primarily algal or bacterial andwhether the APA presents seasonal variations coupledwith abiotic and biotic variables. Samples werecollected at different depths from May 1992 to May1993. The specific phosphatase activities wereanalysed spectrophotometrically with p-NitrophenylPhosphate (p-NPP) as substrate and were related to theprotein concentrations. No correlation was foundbetween alkaline phosphatase activity and solublereactive phosphorus (SRP) concentrations across anannual cycle. The specific activities of the smallclass (0.2–1.2 m) were the highest and thecontribution of this picoplanktonic size class(0.2–1.2 m) increased with depth. In addition, thelinear correlations between alkaline phosphataseactivity and protein concentration seemed to indicatethat most of these enzymes are constitutive. However,it cannot be excluded that the high phosphorusconcentrations repress APA.Finally, the measure of APA does not seem to be avalid quantitative test of the deficiency ofphosphorus for aquatic microorganisms.     

Seasonal variation in haematological parameters in male and female Tinca tinca

This study was designed to investigate any seasonal (spring, summer, autumn and winter) changes in haematological parameters in the blood of Tinca tinca measuring the number of red blood cells (RBC), haematocrit, white blood cells, and total plasma proteins.The results show significant changes in RBC and haematocrit in males comparing spring and summer with autumn and winter, whereas in females the RBC remained constant for all 4 seasons but the haematocrit decreased in autumn and winter compared to spring and summer. The white blood cells of male and female animals were significantly lower in spring and winter compared to summer and autumn. In male fish total protein contents significantly decreased in autumn and winter compared to spring and summer, whereas in females protein output significantly decreased in winter compared to the other seasons. The results indicate marked seasonal variation in the blood of male and female Tinca tinca. This variation may play a important protective role for the survival of the animals.     

Macrograzers strongly influence patterns of epiphytic assemblages in seagrass meadows

The influence of factors, both abiotic (light and nutrients) and biotic (meadow structure, herbivory and seagrass shoot length) in the abundance and distribution of epiphytes on the Mediterranean seagrass Posidonia oceanica was investigated by means of a correlational approach over a spatial scale of ca. 500 km and at two different depths (5 and 15 m). Variability was examined from the double perspective of integrative community measures (biomass, species richness and alpha-diversity) and species composition. We assessed the influence of biotic and abiotic factors on integrative community measures using multiple correlation analyses. The influence of these factors in the structure of the whole community was investigated using a distance-based RDA for a linear model. A total of 129 taxa, mostly epiphytic algae, were recorded across all sites and depths. A large part of the variability in species composition (51%) was explained by the variables investigated. In particular, variability caused by differences in grazing pressure was the most important (25%), followed by variables related to nutrient availability (11%), meadow structure (6%), light (5%) and seagrass shoot length (4%). Among integrative community variables investigated, species richness was also best explained by grazing and nutrients.     

Seasonal changes in the constituents of Ulva lactuca

The green marine algal species Ulva lactuca, contains appreciable amounts of lipids, but there is no correlation between the lipid content and its unsaponified fraction. The sterol content of this alga depended on the proportion of the unsaponified lipid fraction in total lipids. The presence of cholesterol-like material was demonstrated. The algal material was characterized by a high content of rhamnose (28%) in April and glucuronic acid (15·74%) in February. Variations were also found in the patterns of free amino acids and of those of the isolated algal protein. A milk-clotting factor was detected in the water extract of the algal material.     

Seasonal and long-term changes in surface cover of aquatic plants in a shallow pond,Ojaga-ike,Chiba, Japan

Seasonal changes in surface cover and long-term floral changes in aquatic plants of a shallow eutrophic pond, Ojaga-ike, as well as the phenology of each species, were studied. Out of ten floating-leaved and submerged plants, the following five species showed a change in surface cover during 1978: Potamogeton crispus and Elodea nuttallii occupied 53.5% of the total pond surface in May, Trapa natans and Nelumbo nucifera 79.1% in July, N. nucifera and Hydrilla verticillata 62.0% in September and N. nucifera 41.2% in November. In addition to dramatic seasonal changes in surface cover, long-term floral changes occurred in this pond from 1971 onwards. While Nymphoides indica, Myriophyllum spicatum and H. verticillata decreased in number and in distribution ranges between 1971 and 1978, two newly established plants, N. nucifera and E. nuttallii, have now become dominant. The alteration in dominance may be due to water level fluctuations and interspecific competition for light. Seasonal changes in water quality during the study period are also summarized.     

The influence of microclimatic conditions on potential photosynthesis ofUsnea sphacelata: A model

At a boulder on a hill near Casey Station, Wilkes Land, sensors for light, temperature and humidity were installed facing the four cardinal directions. The measurements lasted for about two months of the summer season 1985/86. The data recording was carried out at intervals of 6 minutes for all probes by automatic recording instruments. Data analysis was carried out with special regard to the biological effects of the parameters analyzed. These data of the microclimatic features taken from its original place of growth were used to a regression model of potential photosynthetic activity ofUsnea sphacelata, which is a characteristic species of this area.     

Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation

• 1 The seasonal dynamics of the benthic macroinvertebrate assemblage, and the subset of this assemblage colonising naturally formed detritus accumulations, was investigated in two streams in south‐west Ireland, one draining a conifer plantation (Streamhill West) and the other with deciduous riparian vegetation (Glenfinish). The streams differed in the quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the conifer stream being more acid at high discharge. We expected the macroinvertebrate assemblage colonising detritus to differ in the two streams, due to differences in the diversity and quantity of detrital inputs.
• 2 Benthic density and taxon richness did not differ between the two streams, but the density of shredders was greater in the conifer stream, where there was a greater mass of benthic detritus. There was a significant positive correlation between shredder density and detritus biomass in both streams over the study period.
• 3 Detritus packs in the deciduous stream were colonised by a greater number of macroinvertebrates and taxa than in the conifer stream, but packs in both streams had a similar abundance of shredders. The relative abundance of taxa colonising detritus packs was almost always significantly different to that found in the source pool of the benthos.
• 4 Correspondence analysis illustrated that there were distinct faunal differences between the two streams overall and seasonally within each stream. Differences between the streams were related to species tolerances to acid episodes in the conifer stream. Canonical correspondence analysis demonstrated a distinct seasonal pattern in the detrital composition of the packs and a corresponding seasonal pattern in the structure of the detritus pack macroinvertebrate assemblage.
• 5 Within‐stream seasonal variation both in benthic and detritus pack assemblages and in detrital inputs was of similar magnitude to the between‐stream variation. The conifer stream received less and poorer quality detritus than the deciduous stream, yet it retained more detritus and had more shredders in the benthos. This apparent contradiction may be explained by the influence of hydrochemistry (during spate events) on the shredder assemblage, by differences in riparian vegetation between the two streams, and possibly by the ability of some taxa to exhibit more generalist feeding habits and thus supplement their diets in the absence of high quality detritus.

     

Waders in winter: long-term changes of migratory bird assemblages facing climate change

Effects of climate change on species occupying distinct areas during their life cycle are still unclear. Moreover, although effects of climate change have widely been studied at the species level, less is known about community responses. Here, we test whether and how the composition of wader (Charadrii) assemblages, breeding in high latitude and wintering from Europe to Africa, is affected by climate change over 33 years. We calculated the temporal trend in the community temperature index (CTI), which measures the balance between cold and hot dwellers present in species assemblages. We found a steep increase in the CTI, which reflects a profound change in assemblage composition in response to recent climate change. This study provides, to our knowledge, the first evidence of a strong community response of migratory species to climate change in their wintering areas.     

The influence of mixing on rotifer assemblages of Michigan lakes

Seasonal changes in the rotifer assemblages of 42 lakes in northern lower Michigan was closely related to lake mixing characteristics, basin morphometry, and the presence of an oxygenated coldwater refuge. Three major classes of lakes (dimictic, discontinuous polymitic, and continuous polymictic) were evident by their capacity to maintain coldwater species as seasons progressed from winter through fall. The disappearance of coldwater assemblages from dimictic lakes coincided with oxygen depletion in the hypolimnion or with erosion of the hypolimnion through mixing. Coldwater species disappeared from large discontinuous polymictic lakes when deep epilimnetic mixing occured in late summer and fall. Species assemblages of nearly all stratified lakes converged with those of continuous polymictic lakes when the hypolimnetic refuge deteriorated in summer and fall. Local weather conditions, however, between years had a pronounced effect on the persistence of cold water species through the seasons by affecting the temperature and oxygen conditions of the hypolimnion. Large lakes of the region contain many of the coldwater species of the Laurentian Great Lakes but some taxa are conspicuosly absent. Cold stenothermal rotifers persist in the lakes of the region despite adverse environmental conditions. Their life histories and ability to form resting stages permit them to escape periods of oxygen depletion and thermal stress. In contrast, the crustacean glacial marine fauna (i.e. Mysis relicta, Limnocalanus macrurus, and Scenecella calanoides) was absent from all of the study lakes even though many of the present-day basins were once connected to the Laurentian Great Lakes. These species long life cycles, lack of diapausing stages, and limited dispersal may make them vulnerable to local extinction with the deterioration and loss of the coldwater refuge.     

The influence of water level and salinity on plant assemblages of a seasonally flooded Mediterranean wetland

We studied the key environmental variables shaping plant assemblages in Mediterranean abandoned ricefields with contrasting freshwater inputs over saline sediments. Plant species cover, water levels and soil variables were studied following a stratified random sampling design. Multivariate analysis identified water regime, particularly summer and autumn irrigation, as the most important environmental variable associated with vegetation composition. Distribution of annual and emergent macrophytes was not associated to salinity as found at the study site (0.57–4.1 mS/cm). Increased soil salinity, caused by summer irrigation near the soil surface did affect shallow-marsh assemblage distribution. These key environmental characteristics allowed us to identify six main assemblages. Annual macrophytes (such as Zannichellia palustris) were defined by high (over 10 cm) annual mean water level (MWL) and early successional conditions; emergent macrophytes (such as Typha spp., Scirpus lacustris) by annual MWL of 10 to− 25 cm and continuous shallow flooding in summer and autumn (MWL of 0–10 cm). The shallow-marsh group, correlated with annual MWL − 25 to − 100 cm, separated into two subgroups by salinity: grassland (including Paspalum distichum) with summer and autumn MWL below − 25 cm and brackish (with Juncus subulatus or Agrostis stolonifera) with summer and autumn MWL just below the soil surface (0 to − 25 cm). Water levels for the grassland subgroup may equate with a salinity ‘refuge’ for P. distichum. Time was a further determinant of variation in the full data set. Abundance of a large group of agricultural annuals (such as Sonchus tenerrimus) and damp ground annuals (including ricefield weeds such as Ammania robusta) decreased with time as bare ground disappeared. Maintenance of spatial vegetation heterogeneity in abandoned ricefields is contingent on continued water regime management.