Density,biomass, and guild structure of arboreal arthropods as related to their inhabited tree size in aCryptomeria japonica plantation

Arboreal arthropods in a 15-year-old plantation ofCryptomeria japonica were surveyed using the smoking method under open conditions and also in the enclosed condition in which the whole above-ground part of a tree was covered by a cloth bag. Per tree, the number of individuals collected was 8200–14000, with a biomass of 340–1700 mg d.wt. Collembola and Acarina were major components in number, while Diplopoda, Collembola, and Araneae occupied the larger part of the biomass. About 60–70% of total numbers of individuals dropped within two hours after the open fumigation. Clear power-form regressions between total numbers of individuals and biomasses of all animals and their host tree size (stem diameter at clear length,D _B ) showed they were approximately proportional toD _B ². The guilds of scavengers and tourists demonstrated the most significant correlations between their numbers and biomasses and the tree size. Numbers and biomasses of Collembola, Diptera, and Araneae revealed remarkable dependence on the tree size. From these regressions, numbers of individuals and biomasses per unit ground area were estimated for all arthropods to be 3755/m² and 165.87 mg d.wt./m², respectively. Power-form regressions were observed between numbers and biomasses of prey and predators. The number-and biomass ratios of all predators to all prey tended to decrease with increasing tree size. A similar trend was observed in the corresponding ratios of parasitic Hymenoptera to lepidopteran larvae, while those of Araneae to Collembola were almost constant, irrespective of tree size.     

Brittle star fauna (Echinodermata: Ophiuroidea) of the arctic northwestern Barents sea: composition,abundance, biomass and spatial distribution

 Epibenthic brittle star assemblages were investigated on the northwestern Barents Sea shelf between 81° and 77°N in July 1991. At 9 drift stations in water depths between 80 and 360 m, series of 35–71 photographs, each depicting about 1 m² of the seabed, were taken along transects of about 150- to 300-m length to assess abundances and spatial distribution patterns of adult brittle stars (disc diameter ≥1 mm). Biomass values were derived by combining abundances with size-weight relationships and size frequencies established using specimens from trawl catches. Six brittle star species were identified on the seabed images. Ophiocten sericeum was the most abundant species on shallow shelf banks (≤100 m). Up to 2,800 individuals were counted on a single photograph; median abundances per station ranged from 32 to 524 ind.m^-2 and biomass from 0.3 to 5.0 g ash-free dry weight (AFDW) m^-2. The spatial distribution along the transects (i.e. on the 100-m scale) was, however, extremely patchy. Disc diameters of O. sericeum ranged between 1.6 mm and 15.4 mm. In deeper shelf habitats (>150 m), O. sericeum was rare or absent, and Ophiacantha bidentata dominated the brittle star fauna with median densities and biomasses of 2–49 ind.m^-2 and 0.07–1.9 g AFDW m^-2, respectively. Its disc diameters ranged from 2.9 to 14.4 mm. The other species (Ophiura sarsi, Ophiopholis aculeata, Ophioscolex glacialis, Ophiopleura borealis) occurred in distinctly lower numbers. Our findings provide further evidence that brittle stars dominate epibenthic communities on Arctic shelves and locally reach very high abundances. Dense beds of Ophiocten sericeum seem to be a general phenomenon on high-Arctic shallow shelf banks. Received: 30 March 1995/Accepted: 30 June 1995     

Community dynamics of the benthic fauna in a woodland springbrook

Summary The benthic macrofauna of a woodland springbrook, Morgan's Creek, Kentucky, was studied between February 1963 and September 1964 at eight collecting sites. The basic habitat type of the stream is a rubble-riffle complex. Four stations (III, IV, VI, and VII) were located in riffle sections at different points along the stream and additional stations were established at the spring source (I), a cascade area (II), a pool (V), and a reach at the mouth of the stream (VIII).A total of 33 taxa were studied in detail, 30 of which were at the species level. Gammarus minus was the most abundant benthic macroinvertebrate in the stream. It made up 82% of the 224,400 organisms collected during the first 12 months of the study. The next most abundant species were Asellus brevicaudus, 4.4%; Phagocata gracilis, 3.0%; Baetis amplus, 1.6%; Ochrotrichia unio, 1.1%; Epeorus pleuralis, 0.9%: Diplectrona modesta, 0.9% and Baetis herodes, 0.7%.Although the largest number of animals per standard (5-minute) sample was collected at Station I, the greatest density (expressed as numbers per unit area covered by a sample) actually occurred at Station II (32,370 /m²). The remaining stations ranked as follows: I (13,160/m²), IV (11,050/m²), VI (10,820/m²), III (5890/m²), V (1260/m²), VII (490/m²), and VIII (120/m²), for the period 15 February 1963 through 1 February 1964.Information on life histories, based on differences in seasonal abundance and the duration of the life cycle, is summarized for 32 species. Of these, 24 were seasonal in occurrence and had relatively long life cycles. Two-thirds of the seasonal forms were autumn-spring growers, rather than summer growing forms.The community organization of Morgan's Creek was examined from the standpoint of species diversity and total numerical variations at each station. The situation at Stations I and II is typical of communities existing under strongly limiting conditions. The greatest diversity of invertebrates, as well as a relatively abundant number of individuals, was found at Stations III and IV, indicating a probable climax situation in this area. Both species diversity and total numbers of individuals showed marked decreases below Station IV; this was attributed to the removal of forest cover in this region. The most important effect of the removal was the alteration of temperature conditions in the stream. However, since terrestrial leaf detritus was shown to be the most important source of plant material eaten by the herbivores, removal of the riparian vegetation could, under more severe conditions, cause the depletion of primary food supplies essential to the maintenance of the woodland stream community.The environmental factors: water movement, temperature, carbon dioxide, alkalinity, hydrogen ion concentration, and dissolved oxygen were measured throughout the study. The abundance and distribution of the animals are examined in relation to these components and to that of substrate. It is concluded that temperature, substrate, and water movement are among the most important factors in Morgan's Creek.High discharge following a March spate severely devastated the fauna but had little lasting effect on the community. Species with short life cycles were able to recoup their losses quite rapidly and showed an increase in numbers soon after the flood; those with long life cycles were more seriously affected and decreased in numbers or only managed to maintain themselves following resumption of more normal discharge. The nonseasonal fauna showed the greatest decreases in numbers as a result of the spate, but their recovery was faster than for species with long seasonal cycles.A number of closely related species, with the same basic food niche, apparently coexist in Morgan's Creek. Competition between these species is avoided through the interplay of differences in microhabitat preferences and spatial and temporal distribution patterns.Contribution number 101 from the Department of Biology, University of Louisville.     

Multi-scale heterogeneity in a macrobenthic epifauna community

A quantitative study, based on monthly samples taken by divers with 0.1m² quadrats, was conducted on a benthic community settling coarse sediments in an area exposed to strong water currents.The richness of the community was high both in terms of biomass (270±107 g m^–2 in ash-free dry weight) and diversity (ten zoologicals groups for a total of 98 species). Three suspension feeding species were dominant: Ophiothrix fragilis, Urticina felina and Alcyonium digitatum accounted for more than 95% of the total biomass at all sampling periods.Spatial heterogeneity analysis confirmed the role of these three dominant species. Heterogeneity revealed at a scale of 0.1 m² by dendrograms (qualitative similarity index) or by Factor Correspondence Analysis (FCA) (on biomasses) was due to the strong dispersion of rare species.The influence of the observation scale was visualised by building Rank-Frequency Diagrams using progressively aggregated replicates. The distribution of relative biomasses and the rank of the dominant species always stabilized at scale of 0.4 or 0.5 m².The three dominant species were overdispersed at a scale of 0.1 m² compared with a Poisson distribution. Spatial competition among the three species was suggested by their dispersion in the plane of the FCA. Calculation of a multiple linear regression (at the scale of 0.8 m²) showed negative relationships among these dominant species.Temporal homogeneity was indicated (at a scale of 0.8 m²) by dendrograms of species similarities among samples and by FCA performed on total biomasses.     

Biomass and density responses in tallgrass prairie legumes to annual fire and topographic position

Annually burned tallgrass prairie is purported to be a nitrogen-limited system, especially when compared to unburned prairie. To test the hypothesis that legumes, potential nitrogen-fixers, would increase in relative abundance in annually burned sites, we assessed their density and biomass for two seasons on upland and lowland soils in annually burned and unburned watersheds. Total legume density was significantly higher in burned (8.0 ± 1.0 [SE] stems/m²) than in unburned watersheds (3.0 ± 0.3 stems/m²). Species with higher (P < 0.05) densities in burned than in unburned prairie included Amorpha canescens, Dalea candida, Dalea purpurea, Lespedeza violacea, Psoralea tenuiflora, and Schrankia nuttallii. Desmodium illinoense was the only legume that responded negatively to annual fire. Total legume biomass did not differ between burned (11.3 ± 1.3 g/m²) and unburned prairie (10.5 ± 0.9 g/m²). Biomass productions of Dalea candida and Psoralea tenuiflora were higher (P < 0.05) in burned than in unburned sites, but biomasses of other legumes were similar between burn treatments. Average individual stem masses of Amorpha canescens and Baptisia bracteata were significantly greater in unburned than in burned prairie. Legumes were affected differentially by topographic location. Total legume density was higher (P < 0.05) on lowland soils (6.6 ± 1.0 stems/m²) than on upland soils (4.3 ± 0.5 stems/m²). However, total legume biomass was not different between lowland soils (12.0 ± 1.2 g/m²) and upland soils (9.9 ± 1.0 g/m²). Densities and biomasses of Amorpha canescens, Desmodium illinoense, and Lespedeza capitata were higher on lowland sites than on upland sites, whereas densities and biomasses of Baptisia bracteata and Dalea purpurea were higher on upland than on lowland soils. Most legume species are either fire tolerant or exhibit a positive response to fire and their persistence in annually burned prairie suggests that they may play an important role in the nitrogen budget of this ecosystem.     

Distribution and population characteristics of the alien talitrid amphipod Orchestia cavimana in relation to environmental conditions in the Northeastern Baltic Sea

The talitrid amphipods were found for the first time in the Northeastern Baltic Sea in 1999. Orchestia cavimana inhabited damp wracks cast up on shore within a 200 m area of Saaremaa Island. In the following year, the species expanded its range to a few kilometres. In 2002, six additional locations of O. cavimana were found in Saaremaa Island and two locations in the Northwestern part of Estonia. Abundances and biomasses were highest in the first year of the invasion. In the following years, the values stabilized on remarkably lower levels. Population characteristics of the species varied significantly between locations. The average biomass and abundance were 9 g dw m⁻² and 1975 ind m⁻², respectively. Wrack biomass and interaction between wrack biomass and exposure were the best predictors of the abundances and biomasses of O. cavimana. Humidity explained additional variability in biomasses. The size structure of the population of O. cavimana was mostly related to humidity and wrack biomass. The Southern coast of the Baltic Sea may be regarded as the initial donor region for the Estonian populations of O. cavimana. The vector of this invasion is most likely related to the natural dispersal of the drifting algae but human activities as a transport vector can not be excluded. The incredible speed of the invasion of O. cavimana and its high biomasses indicate that the species will very likely extend its distribution along the coast of the Northeastern Baltic Sea in the following years.     

Colonisation and bioerosion of experimental substrates by benthic foraminiferans from euphotic to aphotic depths (Kosterfjord, SW Sweden)

In the cold-temperate setting of the Swedish Kosterfjord area, experimental carbonate and PVC substrates were deployed for a 6, 12 and 24-month duration along a transect from euphotic to aphotic depths in order to study bioerosion and carbonate accretion patterns. Among the organisms that contribute to the latter by secreting calcareous skeletons, epibenthic foraminiferans represent a major component, both in terms of diversity (a dozen species) as well as in the number of individuals (exceeding 50,000 individuals per m² at certain depths). The by far dominating species were found to be Cibicides lobatulus and the agglutinating Lituotuba lituiformis, along with smaller numbers of Planorbulina mediterranensis, Tholosina vesicularis and Nubecularia lucifuga. The foraminiferal distribution exhibits a pronounced abundance maximum in shallow waters at 7 and especially 15 m and a maximum in diversity at 15-50 m water depth. Some of the foraminiferans encountered, such as Cibicides lobatulus and the rare Gypsina vesicularis, were found to contribute also to the bioerosion of the calcareous substrates by etching shallow attachment scars. These prominent traces witness the former presence of benthic foraminiferans on fossil to Recent hardgrounds, inferring a potential applicability as an in situ proxy where tests are not preserved. Estimated minimum carbonate production rates for the dominant Cibicides lobatulus reach a maximum of 0.326 g/m²/year with the highest rates occurring at 7 to 30 m water depth. Carbonate production rates are up to two magnitudes higher on the PVC (0-0.326 g/m²/year) than on the carbonate substrates (0-0.010 g/m²/year) and are considerably higher than estimates previously reported from the western Baltic.     

Selective foraging by the crayfish Orconectes virilis and its impact on macroinvertebrates

SUMMARY. 1. The impact of crayfish predation on the abundance of macroinvertebrates was examined under semi-natural conditions. Female (Experiment 1) or male (Experiment 2) crayfish (Orconectes virilis) were held for 5 weeks in twelve small pools (4.67 m² surface area) at biomasses of 0. 5, 10 or 18 g m^?2 (live weight). The pools were stocked with known densities of macroinvertebrates. 2. Crayfish significantly affected the abundance of macroinvertebrates in the pools. Differences in the effects of crayfish on macroinvertebrates were related to crayfish sex, the presence of age-0 crayfish, and the species of macroinvertebrate. 3. The abundance of snails (Stagnicola elodes and Physa gyrina) was greatly reduced, in comparison with controls, by biomass of female crayfish ≥10 g m^?2 and by biomasses of male crayfish ≥5 g m^?2. The total density of non-molluscan invertebrates was inversely correlated with the biomass of female crayfish but the total biomass of non-molluscan invertebrates did not differ between treatments. This is consistent with our observation that small invertebrates (<2 mg wet weight) were less numerous, and large amphipods (32–64 mg) were more numerous, in pools stocked with female crayfish. In contrast, male crayfish had little apparent effect on the abundance of non-molluscan invertebrates. 4. Age-0 crayfish hatched at the end of Experiment 1 and were present in each pool at the start of Experiment 2. Surprisingly, male crayfish preyed little on age-0 crayfish. At the end of Experiment 2, the densities of age-0 crayfish varied between six and 116 individuals m^?2 and there was a strong inverse correlation between the mean biomass and density of age-0 crayfish recovered from the pools. This suggests age-0 crayfish were food limited in the pools and may explain the dominance of oligochaetes (which largely escape predation by burrowing) in the invertebrate community at the end of Experiment 2. 5. These results indicate that even relatively low densities of crayfish could greatly affect the abundance of macroinvertebrates in lakes. The introduction of crayfish into lakes (most lakes in Alberta currently have no crayfish) could substantially affect abundance and species composition of the macroinvertebrate community and, ultimately, the fish populations.     

Nest and soil populations of Trinervitermes spp. with particular reference to T. geminatus (Wasmann), (Isoptera), in Southern Guinea savanna near Mokwa,Nigeria

Summary Nest and soil populations of Trinervitermes spp. were estimated on grazed secondary savanna woodland near Mokwa cattle ranch and on primary savanna woodland, 6 km from the ranch. Nest populations were estimated by obtaining a relationship between size of nest and the number of termites in the nest and using the relationship to estimate populations in measured nests within the study area.Mound populations of T. geminatus, by far the most abundant species, were 222 m^-2 at a mound density of 232 ha^-1 at the ranch, and 225 m^-2 at a mound density of 175 ha^-1 on primary savanna woodland. The mound population at the ranch represented a fresh weight biomass of 1.089 g m^-2. Changes in abundance of the mound population of T. geminatus were correlated with breeding and foraging cycles. Maximum numbers (388 m^-2, 2.03 g m^-2) in August/September were reduced by the flight of alates and loss of foragers to predators; thereafter, the population continued to decrease (126 m^-2, 0.57 g m^-2) until the cessation of foraging in April/May and numbers of larvae and nymphs began to increase. Soil and mound sampling in primary and secondary savanna showed that although T. geminatus is a mound inhabiting species, two thirds of the mound plus soil population was outside the mounds giving a total population of 737 m^-2 (3.08 g m^-2). Alate production was estimated at 15.5 m^-2 (0.19 g m^-2) and neuter production at 367 m^-2 (1.66 g m^-2); production/biomass ratio was 1.0 T. togoensis (total population of 21 m^-2) and T. occidentalis (200 m^-2) had 90–96% of the total numbers outside the mounds, indicating that these two species were primarily subterranean.     

Phytoplankton and zooplankton (Cladocera,Copepoda) relationship in the eutrophicated River Danube (Danubialia Hungarica,CXI)

The seasonal variation in primary production, individual numbers, and biomass of phyto- and zooplankton was studied in the River Danube in 1981. The secondary production of two dominant zooplankton species (Bosmina longirostris and Acanthocyclops robustus) was also estimated. In the growing season (April–Sept.) individual numbers dry weights and chlorophyll a contents of phytoplankton ranged between 30–90 × 10⁶ individuals, l^–1, 3–12 mg l^–1, and 50–170 µg l^–1, respectively. Species of Thalassiosiraceae (Bacillariophyta) dominated in the phytoplankton with a subdominance of Chlorococcales in summer. Individual numbers and dry weights of crustacean zooplankton ranged between 1400–6500 individuals m^–3, and 1.2–12 mg m^–3, respectively. The daily mean gross primary production was 970 mg C m^–3 d^–1, and the net production was 660 mg C m^–3 d^–1. Acanthocyclops robustus populations produced 0.2 mg C m^–3 d^–1 as an average, and Bosmina longirostris populations 0.07 mg C m^–3 d^–1. The ecological efficiency between phytoplankton and crustacean zooplankton was 0.03%.     

Summer Weddell sea microplankton: Assemblage structure,distribution and abundance,with special emphasis on the Tintinnina

Summary Silicoflagellates, large thecate dinoflagellates and tintinnids were counted and measured in screened (26 m pore-size) pump-samples retrieved from a depth of ca. 9 m in February-March 1987 along a two-leg north-south transect in the Weddell Sea (approx. 62°S to 78°S). Fourteen tintinnid taxa were identified and their individual abundances and biomasses were estimated. Highest biomass, in terms of organic carbon, was recorded for the Tintinnina, which averaged 4.001 mg C m^-3, with mean cell-numbers of 161 ind. l^-1 (maximum: 859 ind. l^-1); followed by the dinoflagellates (1.018 mg C m^-3, mean: 192 ind. l^-1; max.: 1176 ind. l^-1); and the silicoflagellates (0.391 mg C m^-3, mean: 467 ind. l^-1, max.: 3123 ind. l^-1). Conspicuous abundance and settling volume peaks were recorded at some distance off the edge of the ice-pack (at approx. 69°S to 72°S), and in the ice-covered area south of 74–75°S. This pattern was paralleled by changes in the specific makeup of tintinnid assemblages: in the ice-covered southern area Cymatocylis drygalskii and Laackmanniella prolongata were dominant, while in ice-free waters north of 73°S Codonellopsis gaussi, Cd. glacialis and Cymatocylis affinis/convallaria were the main components of the fauna. Overall tintinnid abundances were higher than those reported for many (but not all) extrapolar areas, and the average size of the species present was considerably larger than elsewhere; tintinnid carbon figures were therefore very high, oscillating around 2 mg C m^-3 in the northern ice-free area, and 20 mg C m^-3 in the southern ice-covered zone.     

Substrate heterogeneity and number of plant species in Everglades savannas (Florida,USA)

Environmental heterogeneity, especially that related to topography, has been proposed to influence numbers of plant species in different sized areas. Despite little variation in elevation, large numbers of vascular plant species occur in some habitats. This study explored possible relationships between number of plant species and substrateheterogeneity in two species-rich habitats, subtropical pine savannas and short-hydroperiod prairies, in the Long Pine Key region of Everglades National Park (Florida, U.S.A.). We examined relationships between numbers of vascular plant species and topographic heterogeneity by measuring numbers of species and elevations in differ-ent sizes of nested plots that spanned five orders of magnitude (0.1 m² to 1000 m² ) and that were located along two transects extending from pine savannas into short-hydroperiod prairies in different areas of Long Pine Key. We also classified substrates and soil depths in 1 m 2 sized submodules within the nested plots. Pine savannas occurred at higher elevations than adjacent short-hydroperiod prairies. Although differences occurred in substrate types and distribution within 1 m 2 plots, numbers of species were not associated with these differences. Vari-ances in elevations were similar in the smallest plots, but increased with area more rapidly in pine savannas than in short-hydroperiod prairies. Plot size explained about 85% of the variation in species numbers, which increased from 2040 per 1 m 2 to 80120 per 1000 m² . An interaction between habitat and scale explained 5% of the variation; more species occurred in shorthydroperiod prairies than pine savannas at scales <10 m² , but the re-verse occurred at scales >10 m². The number of species in pine savannas at scales of 1 m²and 10 m²was positively associated with variation in elevations; no significant relationships were obtained in short-hydroperiod prairies, which lack the fine-scale topographic variation of pine savannas. Our data indicate that substrate het-erogeneity, measured as variation in elevations, is not likely to be involved in the co-occurrence of many species within small areas of these savannas, but may influence numbers of species at larger scales of observation, es-pecially in pine savannas. Why many plant species occur within very small areas in these savannas remains un-answered.     

High benthic bacteria standing stock in deep Arctic basins

During the Arctic Expedition ARK 8/3 (August to October 1991) with RV Polarstern sediment samples from 13 staions with water depths of between 258 and 4,427 m were taken along a transect from the Barents Sea slope across the deep Arctic Eurasian Basins and the Gakkel Ridge to the Lomonosov Ridge to determine bacterial biomasses and organic carbon contents. Bacterial abundance dropped along the transect from 3.03 to 0.63×10⁸ cells/cm³, and correspondingly bacterial biomass decreased from 17.35 to 3.43 g C/cm³ sediment. Positve correlations were only found between total organic carbon concentrations of surface sediment layers and biomasses of small coccoid cells and small rods. The ridges and slopes seem to be sedimentation areas for the larger coccoid cells, presumably cyanobacteria.     

Trophodynamics of selected mesozooplankton in the west-Indian sector of the Polar Frontal Zone, Southern Ocean

Mesozooplankton community structure and grazing impact were investigated at 15 stations in the west-Indian sector of the Polar Frontal Zone during the third dynamics of Eddie impacts on Marions ecosystem cruise, conducted during April 2004. An intense frontal feature, likely the convergence of the Sub-Antarctic and Antarctic Polar Fronts, was identified running in a north-eastward direction across the survey area. Total integrated chlorophyll-a (chl-a) biomass ranged from 4.15 mg m^–2 to 22.81 mg m^–2 and was dominated by picophytoplankton at all stations. Mesozooplankton abundances ranged from 163.84 ind m^–2 to 2,478.08 ind m^–2 and biomass between 6.70 mg Dwt. m^–2 and 23.40 mg Dwt. m^–2. The mesozooplankton community was dominated almost entirely by copepods, which contributed between 35% and 79% (mean=63%; SD=±12%) of the total numbers. The pteropoda, Limacina retroversa, contributed up to 30% (mean=10%; SD=± 8%) of the total numbers. Numerical analysis identified two distinct mesozooplankton communities separated by the intense frontal feature, namely the Antarctic and the Sub-Antarctic Zone Groups. Ingestion rates of the four numerically dominant copepod species (Calanus simillimus, Clausocalanus spp., Ctenocalanus spp. and Oithona similis) and the pteropod, L. retroversa, were estimated using the gut fluorescence technique. Total grazing impact ranged from 0.156 mg (pigm) m^–2 to 2.958 mg (pigm) m^–2 or between 1% and 29% of the available chl-a per day. The four copepods contributed approximately 36% of the total daily grazing impact, while the pteropod contributed to a mean of 64%, indicating that this zooplankton group may play an important role in the Southern Ocean carbon cycle. In general, the highest daily grazing impact was exhibited in the Antarctic Zone Group (mean=12% phytoplankton standing stock per day).     

Multi-scale comparisons of cliff vegetation in Colorado

We developed a nested vegetation sampling protocol to sample the plant diversity on south-facing cliffs and cliff bases in Jefferson County, Colorado. The multi-scale plots included three nested spatial scales, 1 m², 20 m², and 40 m². We compared plant species richness and species diversity among large cliffs, medium cliffs, small cliffs, and non-cliff sites using Hill's diversity numbers (N ₀, N ₁, and N ₂) for the 1-m² quadrats. Species richness (N ₀) was calculated for the 20-m² and 40-m² plots. Our results indicate that plant species diversity on the cliff faces did not increase with increasing cliff area. This pattern was consistent at all three sampling scales. A model selection was run to determine if plant species diversity values on the cliff faces were associated with cliff variables. None of the cliff variables measured were good predictors of diversity at the 1-m² scale. However, at the 20-m² scale, canyon differences and a positive relationship with increasing cliff surface roughness explained 70% of the variability in species richness. Although most plant species sampled on the cliff faces were also found in the base plots, 13 species were sampled only on the cliff faces. Additionally, dry south facing cliffs support a mix of xeric and mesic plants indicating that cliffs may provide unique microenvironments for plant establishment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Littoral Benthos of the Northern Baltic Sea. III. Macrobenthos of the Hydrolittoral Belt of Filamentous Algae on rocky shores in Tvärminne

The community composition was studied quantitatively in two localities in 1968-1970. The average ash-free dry weights of the plants in a fully exposed and a moderately exposed locality were 48.6 g. m⁻² and 93.3 g. m⁻², respectively. The average animal ash-free dry weights were 2.52 g. m⁻² and 3.45 g. m⁻². and the average numbers of animal individuals were 5360 m⁻² and 15450. m⁻². The communities are characterized by the strong dominance of a few species. viz. Cladophora glomerata. Gammarus spp. and Chironomid larvae. The species diversity is thus very low. The seasonal aspects of species composition and abundance are presented. and some factors causing the differences between the localities are discussed.     

Distribution of benthic algae and macroinvertebrates along a thermal stream gradient

The distribution and abundance of benthic algae and macroinvertebrates were examined along a natural thermal gradient formed by hot springs in Little Geysers Creek, Sonoma Co., California, USA. Maximum water temperatures ranged from 52 °C at the uppermost station to 23 °C at a station 400 m downstream. Benthic chlorophyll a decreased exponentially from 2.5 g m^–2 at 52 °C to less than 0.1 g m^–2 at 23 °C, a pattern of decline also exhibited by algal phaeophytin. Blue-green algae dominated at higher temperatures but were replaced by filamentous green algae and diatoms at lower temperatures.Macroinvertebrates were absent at temperatures 45 °C; the highest density (> 150 000 m^–2, mainly Chironomidae) occurred at 34 °C, whereas biomass was highest (4.6 g m^–2, as dry weight) at 23 °C and species richness (15 species) was highest at 27 °C. The two predominant macroinvertebrate populations (the midge Tanytarsus sp. and the caddisfly Helicopsyche borealis) occurred at sites that were several degrees below their lethal thermal threshold, suggesting that a temperature buffer is maintained.     

Abundance,Population Structure and Production of Scrobicularia plana and Abra tenuis (Bivalvia: Scrobicularidae) in a Mediterranean Brackish Lagoon,Lake Ichkeul,Tunisia

Abundance, growth and production of the deposit‐feeding bivalves were studied in the Ichkeul wetland, northern Tunisia, from July 1993 – April 1994. Scrobicularia plana (Da Costa , 1778) occurred at annual mean densities (biomasses) of 299 ± 65 to 400 ± 100 individuals/m² (22.54 ± 3.00 to 34.27 ± 3.96 g ash‐free dry mass (AFDM)/m²) depending on the study area. The annual mean density of Abra tenuis (Montagu , 1803) amounted to 640 ± 74 individuals/m² during the whole study period, in contrast the biomass rose from 2.87 g AFDM/m² in July to 10.29 g AFDM/m² in April. Both species were largely dominated by age class I. Although not very successful, recruitment presented a two‐period pattern: the main period at the beginning of spring, and a secondary one in late summer/autumn. S. plana rarely exceeded 40 mm and lived for only 2 years, while most individuals of A. tenuis lived for only 15–18 months growing to a length of 12 mm. The annual bivalve deposit‐feeder production for the whole lagoon system (90 km²) was 8.24 g AFDM/m² (5.26 g C/m², 0.65 g N/m²). The annual P/ ratio was about 0.4 and therefore in the same order of magnitude as estimates from other brackish coastal waters. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Species diversity,niche metrics and species associations in harvested and undisturbed bogs

Abstract. Competition is considered an important force in structuring plant communities and in governing niche relations, but communities recovering from disturbance, may be less governed by species interactions and less orderly organized. To address this issue, we studied species richness, abundance and patterns of association between plant species at three spatial scales (1 m², 1/25 m², 1/625 m²) in two ombrotrophic mires in east-central Sweden. One was at a secondary successional stage following peat extraction 50 yr ago and the other was undisturbed. Peat extraction leads to a change in hydrology which is slowly restored by the formation of new peat. Niche breadth and niche overlap along the gradient of height above the water table were calculated for the five common Sphagnum species occurring at both mires in an attempt to better understand differences in species co-occurrence at each mire. Species cover differed between the mires, and the number of species per plot was higher in the undisturbed community at all scales, suggesting that the degree of species intermingling was greater than at the harvested site. At all scales, the number of non-random associations was higher, and niche overlap lower among ecologically similar species (e.g. hollow Sphagnum species) in the undisturbed mire. These differences indicate that random events are important in colonization, and that biotic interactions between neighbours later result in a higher degree of non-randomness. In addition, we surveyed a number of abandoned peat pit sites to test the effect of disturbance for species composition at a regional scale. Ombrotrophic peat pits contained several Sphagnum species normally associated with minerotrophic mires, and species of wooded mires occurred frequently in peat pits, making them more species-rich than undisturbed bogs. There were also Sphagnum species new to, or rare in, this part of Sweden which indicates effective long-distance dispersal. Even 50 yr after peat extraction had ceased, the vegetation had not recovered to its original composition.     

Halimeda contribution to organic and inorganic production in a Tahitian reef system

Of the seven species of Halimeda inhabiting a lagoon on Moorea island, three representing 10% of the algal covering and averaging 111 g of dry weight m^-2, have been studied in the course of a year. The biomass, measured bimonthly, stresses a slight seasonal variability in the species life. The main decrease was reported for H. opuntia after a fruiting event, which happened in October. The primary production was assessed, in situ, periodically over a year, by measuring oxygen variations in enclosures. Either expressed on specific-weight basis or in area units, the highest primary productions were recorded for H. opuntia. Productions and biomasses vary simultaneously during the year. The three species produce all together about 6 g C m^-2y^-1. The growth rate of the sand-dwelling H. incrassata f. ovata was followed during the year by staining, in the field, individuals with alizarin Red-S. The average rates measured were 3.3 segments ind^-1d^-1 and 0.17 gdw d^-1m^-2. The contribution of the three species to the carbonate budget of the reef was estimated by total alkalinity measurements during 24-h cycles. H. opuntia had the highest CaCO₃ production. For the three species studied, CaCO₃ production of 1.4 kg m^-2y^-1, which could correspond to a 0.4mm/year accretion of the studied reefal system, was estimated.