Intertidal meiofauna of a high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living nematode communities

Meiofauna communities of four intertidal sites, two sheltered and two more exposed, in Kongsfjorden (Svalbard) were investigated in summer 2001 at two different tidal levels (i.e. the low-water line and close below the driftline, referred to as mid-water (MW) level). A total of seven meiofaunal higher taxa were recorded with nematodes, oligochaetes and turbellarians being numerically dominant. Mean total meiofaunal densities ranged between 50 ind. 10 cm⁻² and 903 ind. 10 cm⁻². Our data showed a clear decrease in total meiofaunal densities with increasing coarseness of the sediment. Total meiofaunal biomass varied from 0.2 g dwt m⁻² to 2 g dwt m⁻² and, in general, was high even at low meiofaunal densities, i.e. larger interstitial spaces in coarser sediments supported larger meiofauna, especially turbellarians. The results on the vertical distribution of meiofauna contrasted sharply with typical meiobenthic depth profiles on other beaches, probably in response to ice-scouring and concomitant salinity fluctuations. Oligochaetes were the most abundant taxon, with a peak density of 641 ind. 10 cm⁻² at Breoyane Island. They were mainly comprised of juvenile Enchytraeidae, which prohibited identification to species/genus level. Nematode densities ranged between 4 ind. 10 cm⁻² and 327 ind. 10 cm⁻². Nematodes were identified up to genus level and assigned to trophic guilds. In total, 28 nematode genera were identified. Oncholaimus and Theristus were the most abundant genera. The composition of the nematode community and a dominance of predators and deposit feeders were in agreement with results from other arctic and temperate beaches. Nematode genus diversity was higher at the more sheltered beaches than at the more exposed ones. Low-water level stations also tended to harbour a more diverse nematode communities than stations at the MW level. Differences in nematode community structure between low- and MW stations of single beaches were more pronounced than community differences between different beaches and were mainly related to resources quality and availability.     

Life histories of the copepods Pseudocalanus minutus, P. acuspes (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

The year-round variation in abundance and stage-specific (vertical) distribution of Pseudocalanus minutus and Oithona similis was studied in the Arctic Kongsfjorden, Svalbard. Maxima of vertically integrated abundance were found in November with 111,297 ind m⁻² for P. minutus and 704,633 ind m⁻² for O. similis. Minimum abundances comprised 1,088 ind m⁻² and 4,483 ind m⁻² in June for P. minutus and O. similis, respectively. The congener P. acuspes only occurred in low numbers (15–213 ind m⁻²), and successful reproduction was debatable. Reproduction of P. minutus took place in May/June, and stage distribution revealed a 1-year life cycle with copepodids CIII, CIV, and CV as the overwintering stages. Oithona similis exhibited two main reproductive peaks in June and August/September, respectively. Moreover, it reproduced more or less continuously throughout the whole year with all stages occurring during the entire sampling period, suggesting two generations per year. Both species migrated towards greater depth in November, but O. similis preferred to stay longer in the upper 100 m as compared to Pseudocalanus. The reproduction of the two species in Kongsfjorden seemed to be linked to phytoplankton dynamics.     

Zooplankton assemblages in two reservoirs: one subjected to accentuated water level fluctuations,the other with more stable water levels

The abundance, composition and dynamics of zooplankton were followed in two reservoirs of the River Douro catchment. The Serra Serrada Reservoir is subject to marked fluctuations in water levels. The highest values of total phosphorus, soluble reactive phosphorus, nitrate, water colour and chlorophyll a were found during the minimum level phase. Rotifera was dominant except in late summer and autumn when the cladoceran Ceriodaphnia quadrangula or the copepod Tropocyclops prasinus replaced them as the dominant zooplankton. Among the rotifers the most common taxa were Keratella cochlearis, Conochilus sp. and Asplanchna priodonta. Maximum rotifer density was about 80,000 ind m⁻³ in 2000, 200,000 ind m⁻³ in 2001 and 100,000 ind m⁻³ in 2002. Among the crustacean zooplankton C. quadrangula achieved densities of up to 45,000 ind m⁻³ and T. prasinus, up to 80,000 ind m⁻³. Canonical correspondence analysis revealed a strong contribution of the variation in the stored water volume, temperature, total phosphorus, chlorophyll, nitrates, and water transparency to the observed, significant association between zooplankton assemblage and environmental variables. In the Azibo Reservoir, fluctuations in water level are smaller. Only total phosphorus, cholorophyll and conductivity varied seasonally. Cladocera and Copepoda were dominant during the whole study period. The most abundant taxa were Ceriodaphnia pulchella, Daphnia longispina, Diaphanosoma brachyurum, Bosmina longirostris and Copidodiaptomus numidicus. Cladocera achieved densities of up to 25,000 ind m⁻³ and Copepoda up to 15,000 ind m⁻³. Rotifera in general reached densities of up to 6,000 ind m⁻³. On the basis of canonical correspondence analysis only temperature and conductivity were significantly associated with zooplankton assemblage.     

Meso- and bathypelagic distribution and abundance of chaetognaths in the Atlantic sector of the Southern Ocean

We conducted multinet sampling during winter and summer in the Southern Ocean (Atlantic sector) to investigate the effect of water mass, season and water depth on abundance and species composition of meso- and bathypelagic chaetognaths. Eukrohnia hamata (mean 115 ind. 1,000 m⁻³) and Sagitta marri (mean 51 ind. 1,000 m⁻³) were dominant, complemented by E. bathypelagica (mean 19 ind. 1,000 m⁻³) and E. bathyantarctica (mean 19 ind. 1,000 m⁻³) below 1,000 m. A further six species were identified, among them the rare bathypelagic species Heterokrohnia fragilis and the subtropical Eukrohnia macroneura that is new to the Antarctic. Water depth and season were the principal determinants of abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. The life cycles of E. hamata and S. marri were studied additionally. Their maturity stages were vertically segregated and prolonged reproductive periods are suggested for both species.     

Population dynamics and production of the freshwater snail Chilina gibbosa Sowerby 1841 (Chilinidae,Pulmonata) in a North-Patagonian reservoir

Chilina gibbosa is an endemic snail widely distributed in Patagonia, Argentina. Due to its importance in the benthic fauna and in the diet of some fish in the oligo-mesotrophic reservoir Ezequiel Ramos Mexía (39° 30′ S, 69° 00′ W), special attention has been given to its life cycle, growth patterns and annual production. Samples were taken monthly at five littoral stations between June 1983 and July 1984. Mean abundance and biomass of C. gibbosa were much higher in vegetated stations dominated by Potamogeton berteroanus (Station 1 : 583 ind. m⁻², 5.95 g AFDM m⁻²) or by Nitella clavata (Station 5 : 275 ind. m⁻², 4.18 g AFDM m⁻²) than bare stations with low transparency or stations with other macrophytes. The snails presented a clustered spatial pattern and their abundance was significantly correlated with macrophyte wet biomass only when this was above 250 g m⁻². Analysis of size distributions showed an annual life cycle with a reproductive period in the summer. However, differences in recruitment and growth occurred probably due to differences in water temperature and food availability. Growth was maximum in summer and almost absent during winter. Hence, shell growth data fit a sigmoid curve well, and growth was somewhat higher at Station 1. Annual production at Stations 1 and 5, estimated by the ‘growth increment summation’ method (28.8 g AFDM m⁻² and 14.18 g AFDM m⁻² respectively), was among the highest recorded for pulmonate gastropods, possibly due to a low interspecific competition. The P : B ratio values were within the range for univoltine gastropods (4.84 and 3.39). The high productivity and turnover rate of these snails grant a high availability of food for the abundant molluscivore, the silverside Patagonina hatcheri.     

Vegetation heterogeneity and ditches create spatial variability in methane fluxes from peatlands drained for forestry

Drainage of peatlands for forestry starts a succession of ground vegetation in which mire species are gradually replaced by forest species. Some mire plant communities vanish quickly following the water-level drawdown; some may prevail longer in the moister patches of peatland. Drainage ditches, as a new kind of surface, introduce another component of spatial variation in drained peatlands. These variations were hypothesized to affect methane (CH₄) fluxes from drained peatlands. Methane fluxes from different plant communities and unvegetated surfaces, including ditches, were measured at the drained part of Lakkasuo mire, Central Finland. The fluxes were found to be related to peatland site type, plant community, water-table position and soil temperature. At nutrient-rich fen sites fluxes between plant communities differed only a little: almost all plots acted as CH₄ sinks (−0.9 to −0.4 mg CH₄ m⁻² d⁻¹), with the exception of Eriophorum angustifolium Honck. communities, which emitted 0.9 g CH₄ m⁻² d⁻¹. At nutrient-poor bog site the differences between plant communities were clearer. The highest emissions were measured from Eriophorum vaginatum L. communities (29.7 mg CH₄ m⁻² d⁻¹), with a decreasing trend to Sphagna (10.0 mg CH₄ m⁻² d⁻¹) and forest moss communities (2.6 mg CH₄ m⁻² d⁻¹). CH₄ emissions from different kinds of ditches were highly variable, and extremely high emissions (summertime averages 182–600 mg CH₄ m⁻² d⁻¹) were measured from continuously water-covered ditches at the drained fen. Variability in the emissions was caused by differences in the origin and movement of water in the ditches, as well as differences in vegetation communities in the ditches. While drainage on average greatly decreases CH₄ emissions from peatlands, a great spatial variability in fluxes is emerged. Emissions from ditches constantly covered with water, may in some cases have a great impact on the overall CH₄ emissions from drained peatlands.     

Greater nitrogen and/or phosphorus availability increase plant species’ cover and diversity at a High Arctic polar semidesert

Enhanced nitrogen (N) deposition at high latitudes is a circumpolar phenomenon. Low soil phosphorus (P), however, may limit vegetation responses to increased N inputs. From 2000 to 2002, the effects of N at 0, 0.5 (a rate occurring in Greenland and Iceland) and 5 (equivalent to deposition in areas of Europe) g N m⁻² a⁻¹ and P (0.1 g m⁻² a⁻¹) treatments on plant species’ cover and diversity were determined at a polar semidesert site (ambient deposition c 0.1 g N m⁻² a⁻¹) in Svalbard (79°N). The largest response was to combined 5 g N plus 1 g P m⁻² a⁻¹, where cover of Saxifraga oppositifolia increased c fourfold, density of Salix polaris leaves c ninefold, seedlings of several ‘new’ species (Draba oxycarpa, Saxifraga caespitosa, Sagina nivalis) were established and ‘immigration’ of Bryum arcticum and ‘extinction’ of Schistidium apocarpum were observed. There were fewer, less pronounced, effects on the plant community at 0.5 g N m⁻² a⁻¹. Low P availability did indeed appear to restrict vegetation response to N. There was a trend for plant species’ richness and diversity to increase with 1 g P m⁻² a⁻¹ at 0 and 0.5 g N m⁻² a⁻¹, but not at 5 g N m⁻². Plant species showed individualistic responses so that generalisation by functional type was not possible. Such increased colonisation by moss species of bare soil, and greater densities of previously unrecorded angiosperm seedlings, are not usually observed in more closed (subarctic) tundra as a response to N and P additions. These changes are likely to influence significantly nutrient cycles, whole system carbon budgets and surface energy and water balances. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Reproduction,population dynamics and production of <Emphasis Type="Italic">Nereis falsa</Emphasis> (Nereididae: Polychaeta) on the rocky coast of El Kala National Park,Algeria

The polychaete Nereis falsa Quatrefages, 1866 is present in the area of El Kala National Park on the East coast of Algeria. Field investigations were carried out from January to December 2007 to characterize the populations’ reproductive cycle, secondary production and dynamics. Reproduction followed the atokous type, and spawning occured from mid-June to the end of August/early September when sea temperature was highest (20–23°C). The diameter of mature oocytes was approximately 180 μm. Mean lifespan was estimated to about one year. In 2007, the mean density was 11.27 ind. m⁻² with a minimum of 7.83 ind. m⁻² in April and a maximum of 14.5 ind. m⁻² in February. The mean annual biomass was 1.36 g m⁻² (fresh weight) with a minimum of 0.86 g m⁻² in December and a maximum of 2.00 g m⁻² in June. The population consisted of two cohorts distinguishable from size frequency distributions. One cohort corresponded to the recruitment of 2006 and the other appeared during the study period in September 2007. The annual production of N. falsa was 1.45 g m⁻² year⁻¹, and the production/biomass ratio was 1.07 year⁻¹.     

Dissolution of Dead Corals by Euendolithic Microorganisms Across the Northern Great Barrier Reef (Australia)

Spatial and temporal variabilities in species composition, abundance, distribution, and bioeroding activity of euendolithic microorganisms were investigated in experimental blocks of the massive coral Porites along an inshore–offshore transect across the northern Great Barrier Reef (Australia) over a 3-year period. Inshore reefs showed turbid and eutrophic waters, whereas the offshore reefs were characterized by oligotrophic waters. The euendolithic microorganisms and their ecological characteristics were studied using techniques of microscopy, petrographic sections, and image analysis. Results showed that euendolithic communities found in blocks of coral were mature. These communities were dominated by the chlorophyte Ostreobium quekettii, the cyanobacterium Plectonema terebrans, and fungi. O. quekettii was found to be the principal agent of microbioerosion, responsible for 70–90% of carbonate removal. In the offshore reefs, this oligophotic chlorophyte showed extensive systems of filaments that penetrated deep inside coral skeletons (up to 4.1 mm) eroding as much as 1 kg CaCO₃ eroded m⁻² year⁻¹. The percentage of colonization by euendolithic filaments at the surface of blocks did not vary significantly among sites, while their depths of penetration, especially that of O. quekettii (0.6–4.1 mm), increased significantly and gradually with the distance from the shore. Rates of microbioerosion (0.1–1.4 kg m⁻² after 1 year and 0.2–1.3 kg m⁻² after 3 years of exposure) showed a pattern similar to the one found for the depth of penetration of O. quekettii filaments. Accordingly, oligotrophic reefs had the highest rates of microbioerosion of up to 1.3 kg m⁻² year⁻¹, whereas the development of euendolithic communities in inshore reefs appeared to be limited by turbidity, high sedimentation rates, and low grazing pressure (rates <0.5 kg m⁻² after 3 years). Those results suggest that boring microorganisms, including O. quekettii, have a significant impact on the overall calcium carbonate budget of coral reef ecosystems, which varies according to environmental conditions.     

Benthic communities on a sandy Ligurian beach (NW Mediterranean)

The different components of the benthic community of a sandy microtidal beach (Arenzano) in Liguria (NW Mediterranean) were investigated during late spring (May) 2002 and 2003. Sampling was carried out in two transects, chosen in order to represent the characteristics of the entire beach and their eventual spatial variations. Each transect included two stations: one placed in the swash zone (SW) and one in the surf zone (SF). Although no significant differences were found in the sediment texture over the 2 years (t-tests p > 0.1 for all the granulometric fractions), notwithstanding an increase in the mean grain size (from 0.8 to 1.1 mm) between the sampling periods, 2002 was characterised by a higher quantity of organic matter (on average 14.4 vs. 3.6 gC m⁻² for the sum of proteins, carbohydrates and lipids) and higher bacterial biomass (on average 1.9 vs. 0.9 gC m⁻²). The metazoan assemblages (meiofauna and macrofauna) were also richer (density = 2.9 × 10⁵ vs. 1.0 × 10⁵ ind. m⁻², biomass = 0.09 vs. 0.03 gC m⁻² for meiofauna; density = 1988 vs. 739 ind. m⁻², biomass = 0.14 vs. 0.03 gC m⁻² for macrofauna) in 2002. A significant quantitative reduction (t-test for proteins, carbohydrates and lipids, at least p = 0.004) in the food supply in 2003 affected the abundance of the metazoans, as confirmed by a multivariate analysis that clearly differentiated the 2 years, and seemed to inhibit their relationships within the benthic food web. The bacterial biomass was always dominant, even under the least favourable trophic conditions, due to the ability of bacteria to adapt to a very harsh environment. Our results suggest that the food supply played an important role in the benthic community structures of the beach during late spring, bacteria being the key organisms within the benthic system. The communities seemed to be bottom-up controlled, while predation seemed to be irrelevant.     

<Emphasis Type="Italic">Manayunkia speciosa</Emphasis> Leidy (Polychaeta: Sabellidae): introduction of this nonindigenous species in the Neotropical Region (Uruguay river,South America)

We report the migration of Manayunkia speciosa from its distribution in North America into the Neotropical Region (Argentina). We collected specimens from November 2007 to March 2009 in the lower Uruguay River-at 33° 5.01′S 58° 12′W, 33° 5.9′S 58°25.2′W from sediments reaching densities of 2,890 ind. m⁻², at a mean abundance of 350 ind. m⁻². Introductions of nonindigenous species, resulting intentionally or accidentally from anthropic activities, cause significant changes in ecosystems. In aquatic environments, polychaetes are a key invasive group that increases the geographical range of several species through human activities. M. speciosa may have reached the Río de la Plata Basin through a shipping vector and thereafter the Uruguay River by self-navigation.     

Zooplankton boom and ice amphipod bust below melting sea ice in the Amundsen Gulf, Arctic Canada

Early summer in the Arctic with extensive ice melt and break-up represents a dramatic change for sympagic–pelagic fauna below seasonal sea ice. As part of the International Polar Year-Circumpolar Flaw Lead system study (IPY-CFL), this investigation quantified zooplankton in the meltwater layer below landfast ice and remaining ice fauna below melting ice during June (2008) in Franklin Bay and Darnley Bay, Amundsen Gulf, Canada. The ice was in a state of advanced melt, with fully developed melt ponds. Intense melting resulted in a 0.3- to 0.5-m-thick meltwater layer below the ice, with a strong halocline to the Arctic water below. Zooplankton under the ice, in and below the meltwater layer, was sampled by SCUBA divers. Dense concentrations (max. 1,400 ind. m⁻³) of Calanus glacialis were associated with the meltwater layer, with dominant copepodid stages CIV and CV and high abundance of nauplii. Less abundant species included Pseudocalanus spp., Oithona similis and C. hyperboreus. The copepods were likely feeding on phytoplankton (0.5–2.3 mg Chl-a m⁻³) in the meltwater layer. Ice amphipods were present at low abundance (<10 ind. m⁻²) and wet biomass (<0.2 g m⁻²). Onisimus glacialis and Apherusa glacialis made up 64 and 51% of the total ice faunal abundance in Darnley Bay and Franklin Bay, respectively. During early summer, the autochthonous ice fauna becomes gradually replaced by allochthonous zooplankton, with an abundance boom near the meltwater layer. The ice amphipod bust occurs during late stages of melting and break-up, when their sympagic habitat is diminished then lost.     

Two lowland stream riffles – linkages between physical habitats and macroinvertebrates across multiple spatial scales

The physical structure of two riffles in a lowland Danish stream was studied and its importance for the composition and density of the macroinvertebrate communities was evaluated. The two riffles were visually assessed to be very similar, but measurements revealed that they differed in overall hydraulic conditions, stability, substratum composition and consolidation. Differences affected abundance of both burrowing and surface dwelling macroinvertebrates. The unstable unconsolidated riffle had higher total macroinvertebrate abundance (4137 m⁻² vs. 1698 m⁻²), diptera abundance (2329 m⁻² vs. 386 m⁻²) and total estimated species richness (31.7 vs. 28.8) as well as lower evenness (0.77 vs. 0.83) than the compact riffle. Among samples within the unconsolidated riffle, variations in macroinvertebrate communities were related to differences in mean substratum particle size. Here a linear log–log relationship existed between macroinvertebrate abundance, the abundance of EPT taxa and the median particle size (r ² _total = 0.46, p = 0.002; r ² _EPT = 0.73, p < 0.001). No similar relationships were evident on the consolidated riffle. Moreover, macroinvertebrate communities on the unconsolidated riffle were dominated by species with a high colonising potential. Despite being assessed to the same morphological unit, physical variation between riffles was surprisingly high as the riffles differed substantially with respect to consolidation, substratum heterogeneity and overall hydraulic structure. Macroinvertebrate community structure and composition also differed between riffles despite being drawn from the same species pool. The findings address the question if we use the correct methods and parameters when assessing the macroinvertebrate communities at the scale of the morphological unit.     

Mesozooplankton assemblages in the shallow Arctic Laptev Sea in summer 1993 and autumn 1995

Mesozooplankton distribution and composition in the very shallow part of the Siberian Laptev Sea shelf were studied during the German-Russian expeditions “Transdrift I” (August/September 1993) and “Transdrift III” (October 1995). Maximum abundances were found close to the outflow of the Lena River (7,965 ind. m⁻³) and in the Yana river mouth (38,163 ind. m⁻³). Lowest abundances occurred in the northeast and west of the Laptev Sea (64–95 ind. m⁻³). Highest biomass values (104–146 mg DM m⁻³) were determined in the northern and northeastern part of the shallow Laptev Sea, as well as close to the river outflows, with a record biomass maximum in the Yana river mouth (270 mg DM m⁻³). Biomass minima were situated north of the Lena Delta and in the western part of the shallow Laptev Sea (0.3–1.0 mg DM m⁻³). Copepods dominated in terms of abundance and biomass. Cluster analyses separated four mesozooplankton assemblages: the assemblage “Lena/Yana” in the southern part, “Eastern-central” in the centre, “Kotelnyy” in the eastern part and “Taimyr” in the western part of the shallow Laptev Sea. The small-sized neritic and brackish-water copepods Drepanopus bungei, Limnocalanus grimaldii and Pseudocalanus major occurred in enormous numbers and made up the bulk of zooplankton abundance and biomass in the very shallow part of the Laptev Sea close to the rivers Lena and Yana. In the more northern and northeastern areas, Calanus glacialis, P. minutus and P. major were dominant copepod species, whereas Oithona similis and Acartia sp. became important in the western Laptev Sea. Appendicularians, as well as hydromedusae and the chaetognath Sagitta sp., contributed significantly to abundance and biomass, respectively, but not over the entire area studied. One can identify taxon-specific distribution patterns (e.g. Sagitta predominated the biomass in a zone between the area heavily influenced by Lena/Yana and the offshore area to the north), which differ from the patterns revealed by cluster analysis. Hydrographic features, especially the enormous freshwater inflow, apparently determine the occurrence and formation of zooplankton aggregations. Extremely high numbers of small-sized neritic and brackish-water copepods occurred locally, which were probably also supported by excellent feeding conditions.     

Burrowing beetles of the genus Bledius (Staphylinidae) as agents of bioturbation in the emergent areas and shores of an athalassic inland lake (Fuente de Piedra,southern of Spain)

Bledius (Elbidus) bicornis (Germ.) and B. (Eucerotobledius) furcatus (Oliv.) (Coleoptera: Staphylinidae) are the most important burrowing species in the emergent areas and shores in the athalassic lake of Fuente de Piedra (Málaga, S. of Spain). A first estimate of the importance of these organisms in this system is presented. These insects kick out sediment during their burrowing activity, which accumulates on the surface near the burrows as tumuli which can be easily eroded. The lake perimeter (17 km) is densely colonized (usual densities from 1700 to 2500 ind m⁻²). The amount of granulated material that can be potentially kicked out was 46.22 g dry wt m⁻² day⁻¹. At the same time, the material that constitutes the tumuli shows different characteristics from the compact ground below the surface. Thus, it is relatively enriched with organic matter (6.15 g per square meter), soluble phosphate (406.5 μg m⁻²) and ammonium (4856 μg m⁻²), whereas it lacks nitrate. Results of a transect from uninhabited areas to zones of maximum population density also show a similarity between the higher ground level of ammonium and phosphate concentrations and population density.     

Photosynthesis of a temperate fallow C<Subscript>3</Subscript> herbaceous ecosystem: measurements and model simulations at the leaf and canopy levels

The objectives of the study were to characterize photosynthesis of temperate fallow C₃herbaceous species and examine the performance of a simple photosynthesis model (based on the Farquhar’s equations) to simulate carbon fluxes at the leaf and canopy levels. The maximum rate of carboxylation at 25°C (V _m0) was estimated for sunlit leaves using in situ gas exchange data under saturating irradiance. Throughout the seasons, leaf measurements indicate that values of V _m0 were similar for the four major species of the fallow. The rate declined from March (100 μmol m⁻² s⁻¹) to July (50 μmol m⁻² s⁻¹) and remained almost constant until November. The maximum quantum yield estimated for Potentilla reptans L. (dominant species) was 0.082 mol(CO₂) mol⁻¹(photon absorbed), similar to values already published for C₃ species. Leaf area index (LAI) increased from winter (less than 0.2 m² m⁻²) to spring (up to 4 m² m⁻²). Rates of canopy photosynthesis (measured with a canopy chamber) strongly depended on LAI and temperature, in addition to irradiance. They reached a maximum of 25 μmol m⁻² s⁻¹ and were intermediate between those published for C₄ grassland or cultivated species, and on woody species. At leaf level, simulations gave realistic predictions. At canopy level, the model had the ability to reproduce the effects of environmental and seasonal conditions. However, simulations underestimated the photosynthetic activity of the fallow canopy.     

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g⁻¹ vs. 107 μg g⁻¹, respectively) and soil TP (284 μg g⁻¹ vs. 145 μg g⁻¹). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m⁻²). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly promote the alternate community. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Differentiated free-living and sediment-attached bacterial community structure inside and outside denitrification hotspots in the river–groundwater interface

This study assessed the functional significance of attached and free-living bacterial communities involved in the process of denitrification in a shallow aquifer of a riparian zone (Garonne River, SW France). Denitrification enzyme activity (DEA), bacterial density (BD) and bacterial community composition (BCC) were measured in two aquifer compartments: the groundwater and the sandy fraction of the sediment deposit. Samples were collected in wells located inside (IHD) and outside (OHD) identified hotspots of denitrification. Despite high BD values (up to 1.14 × 10¹² cells m⁻³), DEA was not detected in the water compartment (< 0.32 mg N–N₂O m⁻³ d⁻¹). The sandy fraction showed detectable DEA (up to 1,389 mg N–N₂O m⁻³ d⁻¹) and, consistent with BD pattern, higher DEA values were measured in IHD zones than in OHD zones. The BCC assessed by 16S rDNA polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) partly supported this result: attached and free-living communities were significantly different (< 30% similarity) but patterns of BCC did not cluster according to IHD and OHD zones. Targeting the denitrifying communities by means of a culture enrichment step prior to 16S rDNA PCR-DGGE showed that the free-living and sediment attached communities differed. Most sequences obtained from DGGE profiles of denitrifying communities were affiliated to Proteobacteria and showed low genetic distance with taxa that have already been detected in aquifers (e.g., Azoarcus sp., Acidovorax sp. and Pseudomonas spp.). This study confirms that in the aquifer the sediment-attached fraction exhibits different functions (DEA) from free-living communities and suggests that this functional difference is related to the communities’ structure.     

Differences in rotifer communities in two freshwater bodies of different trophic degree (Lake Ohrid and Lake Dojran,Macedonia)

We investigated monogonont rotifers in two natural Macedonian lakes that greatly differ in age, size and trophic state: Lake Ohrid and Lake Dojran. A main characteristic of Lake Ohrid is the scarcity of nutrients and consequently a low level of primary production. Lake Dojran represents a typical eutrophic lake. Results clearly indicate that species numbers are negatively correlated with trophic degree. Qualitative analyses of rotifer compositions in Lakes Ohrid and Dojran showed the presence of 70 and 55 taxa, respectively. Rotifer assemblages differed in their community structure, population densities, and the occurrence pattern of dominant species. The density of rotifers increased with increasing nutrient concentration, varying from min. 0.67 ind. L⁻¹ in June, 2006 to max. 8.2 ind. L⁻¹ in July, 2004 in Lake Ohrid, whereas min. 28.8 ind. L⁻¹ (in December, 2005) and max. 442.5 ind. L⁻¹ (in September, 2005) were recorded in Lake Dojran. Gastropus stylifer and Keratella cochlearis were the most abundant species in the pelagic zone of Lake Ohrid, averaging monthly densities of 1.2 ind. L⁻¹ and 0.6 ind. L⁻¹, respectively, thereby contributing 29% and 15% to rotifer abundance. In contrast, Lake Dojran rotifers were dominated by Brachionus spp. Brachionus diversicornis and Brachionus calyciflorus f. amphiceros were most abundant, comprising 40% and 25% of the total rotifer density. These results corroborate our idea, that the trophic state is an important factor in determining the composition of rotifer communities.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.