Population demographics of Palaemon peringueyi (Macpherson 1990) (Decapoda: Palaemonidae) in a small intermittently open Eastern Cape estuary

The population demographics of the caridean shrimp Palaemon peringueyi was investigated monthly at four sites over a period of one year in the small, intermittently-open Grants River Estuary situated on the south-east coastline of southern Africa. Mean total abundance and biomass of P. peringueyi ranged between 0 and 23.2 ind m^?2 and 0 and 2.3mg wwt m^?2, respectively. A peak in its abundance coincided with a breaching event that occurred in December 2004, and could be attributed to the mass recruitment of juveniles (carapace length <9mm) into the estuary. Minor peaks in abundance were also associated with marine waters overtopping the sandbar that separates the estuary from the marine environment. The highest biomass values were recorded when the estuary was separated from the sea. Maximum abundance and biomass values of P. peringueyi were generally recorded in the middle reaches of the estuary where extensive beds of submerged vegetation, mainly Potamogeton pectinatus, predominated. Pearson correlation analysis indicated that the total abundance of P. peringueyi was significantly correlated (r = 0.49; p < 0.05) to percentage cover of submerged vegetation. The spatial pattern in abundance and biomass values of the shrimp is thought to reflect the availability of refuge areas against predation. A distinct spatial pattern in the community structure of the shrimp was evident, with juveniles numerically dominating the samples in the lower reaches of the estuary, while mature females predominated in the upper reaches of the system. In the middle reaches, the community comprised both juveniles and adults.     

Secondary production,longevity and resource consumption rates of freshwater shrimps in two tropical streams with contrasting geomorphology and food web structure

1. Freshwater shrimps often dominate the biomass of tropical island streams and are known to have strong effects on stream ecosystem structure and function, but little effort has been dedicated toward quantifying basic energetic and life history attributes such as growth, production and longevity. Such information is critical for understanding both the role of shrimps in ecosystem dynamics and the gravity of threats to shrimp populations posed by human activities such as shrimp harvesting, dam construction and water withdrawal. 2. We quantified growth rates and secondary production of dominant freshwater shrimps for 3 years in two Puerto Rican headwater streams that differ in food web structure because of the presence or absence of predatory fishes that are excluded from reaches above waterfalls. Using growth data, we constructed a minimum longevity model to explore the likely minimum life spans of the two dominant taxa (Atya spp. and Xiphocaris elongata). Finally, we used a bioenergetics model to quantify annual consumption rates of major basal resources by the two taxa. 3. Daily growth rates ranged from ?0.001 to 0.011 day^?1, were inversely related to body size, and were higher for small individuals of X. elongata than Atya spp. Mean annual shrimp biomass and secondary production were an order of magnitude higher in the stream that lacked predatory fishes (biomass: 4.34 g AFDM m^?2; production: 0.89 g AFDM m^?2 year^?1) than in the stream with predatory fishes (biomass: 0.12 g AFDM m^?2; production: 0.02 g AFDM m^?2 year^?1). Production : biomass ratios ranged from 0.01 to 0.38. 4. Our longevity model predicted a minimum life span of 8 years for Atya spp. and 5 years for X. elongata in the stream lacking predatory fishes. In contrast, due to a larger average size of X. elongata in the stream with predatory fishes, our model predicted a minimum life span of 11 years. Actual life spans of these taxa are likely to be much longer based on long‐term observations of marked individuals. 5. Estimated consumption rates from the bioenergetics model indicated that Atya spp. and X. elongata are important processors of organic matter resources in streams where they occur at high densities. Atya spp. and X. elongata appeared capable of consuming a large proportion of algal and insect production and the proportion of direct leaf litter inputs consumed was also appreciable (c. 40–60%). However, the consumption of suspended fine particulate organic matter (SFPOM) by Atya spp. is probably only a minor proportion of total SFPOM flux in these streams. 6. Our study suggests that geomorphic features such as waterfalls may play an important role in controlling the distribution and production of freshwater shrimps through their effects on predatory fish movement. Spatial differences in shrimp densities result in landscape‐scale variation in the significance to ecosystem processes of these long‐lived organisms, particularly as processors of major organic matter resources.     

Long-term effects of PO4 fertilization on the distribution of bryophytes in an arctic river

1. Each year since 1983, H₃PO₄ has been added continuously during the ice-free season to a P-limited tundra river (Kuparuk River, North Slope, Alaska). Effects on epilithic metabolism, invertebrate community structure and fish production developed quickly. 2. In 1990, 7 years after fertilization began, we noted extensive coverage by bryophytes within the fertilized reach of the river, where very little had been noted before. Bryophyte biomass from a limited set of quadrats taken in 1990 and 1991 yielded 17 ± 9 (SE) g dry mass m^?2 in control reaches and 322 ± 96 g dry mass m^?2 in fertilized reaches. 3. An initial survey of macroalgal and bryophyte cover in 1991 suggested that the moss Schistidium (Grimmia) agassizii was distributed in both control and fertilized reaches of the river. No clear difference in coverage by this species was found in either reach. 4. In contrast, two species of Hygrohypnum (H. alpestre and H. ochraceum) were found almost exclusively in the fertilized reach. An extensive point transect survey done in 1992, above, within and below the fertilized reach, indicated that increased cover and biomass of Hygrohypnum spp. were confined to the fertilized reach of the river. Detrended correspondence analysis clearly separated the macrophyte and macroalgal communities in the fertilized reach from those in the control and downstream reaches. 5. A fourth bryophyte species (Fontinalis neomexicana) also occurred almost exclusively in the fertilized reach, but was much less abundant than the Hygrohypnum species. 6. Analysis of total N and P in the tissues of the Hygrohypnum spp., and estimates of average coverage (～15%) and biomass (～150g dry weight m^?2) over an 8km fertilized reach, suggest that these species alone may have removed two-thirds of the P added in the fertilizer experiment. The bryophyte community in this river is likely to be the dominant sink for P in the fertilized reach.     

Effects of temperature and salinity on the standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi

The standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi to changes in temperature (15-30 °C), salinity (0-45‰) and a combination thereof was investigated. The rate of oxygen consumption of the shrimp was determined using a YSI oxygen meter. At a constant salinity of 35‰ the respiration rate of P. peringueyi increased with an increase in temperature and ranged between 0.260 and 0.982 μl O₂ mg wwt^− 1 h^− 1. The Q₁₀ value over the temperature range 15-25 °C was estimated at 3.13. At a constant temperature of 15 °C the respiration rate of P. peringueyi also increased with an increase in salinity and ranged between 0.231 and 0.860 μl O₂ mg wwt^− 1 h^− 1. For combination experiments the absence of any significant difference in the respiration rate of P. peringueyi at the four temperatures over the salinity range 15-35‰ suggests that the shrimp is well adapted to inhabiting environments characterised by variations in salinity and temperature such as those encountered within the middle and lower reaches of permanently open estuaries with substantial freshwater inflow. On the other hand, the total mortality of the shrimp recorded at salinities < 5‰ at all four temperatures suggests that the upper distribution of the shrimp may reflect physiological constraints. Similarly, the increase in the respiration rate of the shrimp at the four temperatures at salinities > 35‰ suggests that the shrimp may experience osmotic stress in freshwater deprived permanently open and intermittently open estuaries where hypersaline conditions may develop.     

Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem‐level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole‐system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem‐level N and P excretion and uptake. Finally, we used a 3‐year record of discharge and NH₄‐N concentration in the high‐biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH₄‐N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 μmol NH₄‐N shrimp^?1 h^?1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp^?1 h^?1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp^?1 h^?1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH₄‐N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between‐stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH₄‐N m^?2 h^?1, 37.5 versus 0.26 μmol TDN m^?2 h^?1 and 1.1 versus 0.015 μmol TDP m^?2 h^?1), equivalent to 21% and 0.5% of NH₄‐N uptake and 5% and <0.1% of P uptake measured in the high‐ and low‐biomass stream, respectively. Distances required for excretion to turn over the ambient NH₄‐N pool varied more than a hundredfold over the 3‐year record in the high‐shrimp stream, driven by variability in discharge and NH₄‐N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community‐level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.     

Reach- and catchment-scale determinants of the distribution of freshwater mussels (Bivalvia: Unionidae) in south-eastern Michigan, U.S.A.

1. We investigated the diversity and distribution of freshwater mussels at 40 sites in an agricultural catchment, the River Raisin in south‐eastern Michigan, to relate mussel assemblages and individual taxa to reach and catchment‐scale variables. Unionids were surveyed by timed searches in 100‐m reaches, and in‐stream and riparian habitat were quantified as well as flow, water chemistry and channel morphology. Land use/cover and surficial geology were determined for site subcatchments and riparian buffers. 2. Some 21 mussel species were found overall; richness ranged from 0 to 12 living species per site. From the upper to middle to lower catchment, the number of individuals, number of species, Shannon–Weaver diversity and relative abundance of intolerant unionids all declined significantly. 3. Four groupings based on overall mussel diversity and abundance were significantly related to reach‐scale habitat variables. The richest mussel assemblages were associated with sites with higher overall habitat quality, greater flow stability, less fine substratum, and lower specific conductance. 4. Stepwise multiple regressions revealed that the distribution and abundance of the total mussel assemblage, as well as the most common species, could be predicted from a combination of reach‐ and catchment‐scale variables (R² = 0.63 for total mussels, R² = 0.51–0.86 for individual species). 5. Flow stability, substratum composition and overall reach habitat quality were the most commonly identified reach‐scale variables, and measures of surficial geology were the most effective catchment‐scale variables. The spatial pattern of geology is likely to be responsible for the diversity gradient from the upper to the lower catchment. 6. Prior studies, attempting to explain mussel distributions from local habitat features alone, have found relatively weak relationships. By employing a combination of reach‐ and catchment‐scale habitat variables, this study was able to account for a substantial amount of the spatial variability in mussel distributions.     

Are high densities of fishes and shrimp associated with top‐down control of tropical benthic communities? A test in three Hong Kong streams

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River flow,zooplankton and dominant zooplanktivorous fish dynamics in a warm‐temperate South African estuary

The possible links between river flow, zooplankton abundance and the responses of zooplanktivorous fishes to physico‐chemical and food resource changes are assessed. To this end, the seasonal abundance, distribution and diet of the estuarine round‐herring Gilchristella aestuaria and Cape silverside Atherina breviceps were studied in the Kariega Estuary. Spatio‐temporal differences were determined for selected physico‐chemical variables, zooplankton abundance and zooplanktivorous fish abundance and distribution. Results indicated that, following a river flood event in winter (>30 m³ s^?1), altered physico‐chemical conditions occurred throughout the estuary and depressed zooplankton stocks. Abundance of G. aestuaria was highest in spring, with this species dominant in the upper and middle zones of the estuary, while A. breviceps was dominant in summer and preferred the middle and lower zones. The catch per unit of effort of both zooplanktivores also declined significantly following the flooding, thus suggesting that these fishes are reliant on zooplankton as a primary food source for healthy populations. Copepods dominated the stomach contents of both fish species, indicating a potential for strong interspecific competition for food, particularly in the middle reaches. Temporal differences were evident in dietary overlap between the two zooplanktivorous fish species and were correlated with river flow, zooplankton availability and fish distribution. The findings of this study emphasize the close trophic linkages between zooplankton and zooplanktivorous fishes under changing estuarine environmental conditions, particularly river flow and provide important baseline information for similar studies elsewhere in South Africa and the rest of the world.     

Inter-annual variation in responses of water chemistry and epilithon to Pacific salmon spawners in an Alaskan stream

1. Pacific salmon (Oncorhynchus spp.) deliver salmon‐derived nutrients (SDN) to the streams in which they spawn. However, many stream parameters, such as discharge and spawner abundance, can vary from year to year, which could alter the quantity and flux of SDN. 2. Over six consecutive years, we studied responses in streamwater chemistry and epilithon (i.e. the microbial community on submerged rocks) to salmon spawners in Fish Creek, southeastern Alaska, U.S.A. The lower reach of Fish Creek receives spawners of several salmon species, while the upper reach does not receive spawners because of an intervening waterfall. 3. We estimated salmon spawner biomass, analysed water chemistry [ammonium, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC)], and measured epilithon abundance [as chlorophyll a (chl a) and ash‐free dry mass (AFDM)] in Fish Creek. Measurements were made in both the upper and lower reaches, before, during and after the major salmon runs. 4. Absolute values and relative differences indicated that the presence of salmon spawners consistently increased dissolved ammonium (by 58 μg L⁻¹ on average, 41× over background), SRP (by 6 μg L⁻¹, 14×), epilithon chl a (by 35 mg m⁻², 16×), and epilithon AFDM (by 3 g m⁻², 8×). Salmon spawners did not increase nitrate or DOC in either absolute or relative amounts. The persistence and magnitude of spawner effects varied among years and appeared to reflect weather‐driven hydrology as well as spawner biomass. 5. Salmon‐derived nutrients can stimulate the growth of primary producers by increasing streamwater nutrient concentrations, but this positive influence may be modulated by other factors, such as water temperature and discharge. To better assess the ecological influence of SDN on stream biota, future studies should explicitly consider the role of key environmental factors and their temporal and spatial dynamics in stream ecosystems.     

Cultivation and biomass production of the diatom Thalassiosira weissflogii as a live feed for white-leg shrimp in hatcheries and commercial farms in Vietnam

This study investigated the biomass production process from the laboratory to the pilot scale in order to use the nutrient-rich biomass of the diatom Thalassiosira weissflogii as live feed for white-leg shrimp (Litopenaeus vannamei) at larval stages (zoeal, mysis, and postlarval) and in commercial production in hatcheries in Vietnam. Our results showed that T. weissflogii was successfully cultured in 1–2 L Erlenmeyer flasks, 0.2–3.5 m³ composite tanks, and 6.5 m³ tubular photobioreactors, with the highest cell density of 1.6 × 10⁶ cells mL^?1 reached after 6 days of culture. Under optimal culture conditions, the protein, lipid, and carbohydrate contents in this algal biomass were 13.2%, 20.0%, and 10.0% of dry cell weight, respectively. The fatty acid composition contains high amount of palmitic acid (C16:0, 43.11% of total fatty acid), and polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (EPA, C20:5ω-3), approximated 16.5% of total fatty acid. In a 50 L larval rearing tank, at the optimal stocking density of 125 nauplii L^?1, the survival percentage (75.55%), the total body length (from 5.376 ± 0.007 to 10.860 ± 0.030 mm), and weight (at from PL₁ to PL₁₂ stages) (from 0.145 ± 0.002 to 1.158 ± 0.005 g) of the white-leg shrimp larvae reached the highest values but the metamorphosis time (234 h) was shortest compared with the other stocking densities. Further, adding living T. weissflogii biomass to the diet of white-leg shrimp larvae at the nauplii 6 stage led to an increase in the body length, weight, and survival percentage of white-leg shrimp larvae of 21.17%, 35.7%, and 33% higher compared with those of larvae fed the control diet (without the addition of T. weissflogii), respectively. At the same time, the metamorphosis time of larvae (from Z₁ to PL₁) decreased by 4 h compared to the control group. In intensive ponds (area of 6400 m² pond^?1), using seed stocks at the postlarvae 12 stage that had been fed T. weissflogii, the final weight, yield, and survival percentage of the shrimp were increased by 7.3%, 14.2%, and 16.3%, respectively, compared with those of the control group. There were no statistically significant differences in the protein and carbohydrate contents in the shrimp flesh among the experimental and control group (p > 0.05). The lipid, omega-3, omega-6, and omega-9 fatty acid contents of shrimp flesh in experiment formula (per 100 g shrimp) were 1.21 g, 72.9 mg, 114 mg, and 86.1 mg, 11%, 29%, 21.6%, and 17.7% higher than that those in control, respectively. The obtained results show the great potential of using T. weissflogii as live feed on white-leg shrimp farms in Vietnam.

     

Growth and production of a tropical predatory shrimp, Macrobrachium hainanense (Palaemonidae), in two Hong Kong streams

1. Macrobrachium hainanense is a predatory palaemonid shrimp (total length >7 cm) that can be abundant [density 3–5 m^?2; biomass 484–606 mg ash‐free dry mass (AFDM) m^?2] in forest streams in Hong Kong, China. This study investigated the growth and production of M. hainanense during 2001 and 2002 in pools of two forested streams (one third‐ and one fourth‐order). 2. The growth of tagged individuals was recorded in situ and compared with that of tagged and untagged shrimps in laboratory tanks. Field and laboratory estimates yielded similar growth rates of 0.7 mm carapace length (CL) per month, and instantaneous growth rate was 0.004 g AFDM g^?1 day^?1. Tagging did not affect growth in the laboratory. Cohort analysis of field populations produced similar estimates of growth to that of tagged individuals, and the growth of M. hainanense was generally slower than has been reported for other Macrobrachium species. Mass‐specific growth rate of M. hainanense in the field varied with size and was two to five times higher in small individuals (<10 mm CL). In addition, growth rate varied with season and was 40% lower in the dry season when temperature was at the annual minimum. 3. Males grew bigger than females (36 versus 25 mm CL). The minimum lifespan of M. hainanense in the field, calculated from size‐specific growth rates, ranged from 29.3 months (females) to 47.6 months (males). Male lifespan derived from cohort analysis was estimated as 48 and 46 months in the two streams. Females reached maturity in 17–18 months (at 15–17 mm CL) while males matured at 24–26 months (at 18–22 mm CL). Females bred twice (at 2 and 3 years of age) while males probably bred three times (at 2, 3 and 4 years) in both streams. 4. Macrobrachium hainanense production in the fourth‐order stream, calculated by the size‐frequency method, was 900 and 1096 mg AFDM m^?2 year^?1 (for 2001 and 2002, respectively) with a production/biomass (P/B) of 2.1–2.3 year^?1. In the third‐order stream, production was 987 and 1304 mg AFDM m^?2 year^?1 (for 2001 and 2002, respectively) with a P/B of 1.7–2.1 year^?1. Production estimates based on the instantaneous growth method were half of those obtained by the size‐frequency method. 5. Although M. hainanense production at the third‐order stream exceeded that in the fourth‐order, growth rates showed the opposite pattern and were 0.31–0.43 mm CL month^?1 and 0.56–0.65 mm CL month^?1 in the third‐ and fourth‐order streams, respectively. Greater mortality in the latter may account for low production at a site where growth rate was high. 6. Production of M. hainanense in both streams was lower during 2001 when rainfall was higher. This may reflect the influence of spates associated with monsoonal rains, which could have reduced M. hainanense production through spate‐induced mortality or by reducing the abundance of prey. This study provides the first in situ estimate of secondary production by a non‐commercial Macrobrachium species in Asia or elsewhere. It involved a whole‐pool approach to sampling that allowed the estimation of production and population parameters on a realistic scale.     

Abundance,distribution, and secondary production of the apple snail Pomacea flagellata (Say, 1829) in Bacalar Lake,a tropical karstic system in southern Mexico

Pomacea flagellata is a gastropod conspicuous in freshwater environments, and represents a fishing resource. To assess their abundance, distribution, and secondary production, monthly samplings were carried out in Bacalar Lake from June 2012 to May 2013 at 12 sampling sites. In each site, three random transects were marked parallel to the shore. All snails on transect were collected and shell length and wet weight measured. The highest density occurred in September (1.27 ind.m^?2), lowest in October (0.47 ind.m^?2). Shell lengths ranged from 2 to 56 mm, with recruitment in January–March. Growth parameters were L_∞ 59.50 mm, K 0.65.year^?1; the lifetime span was 3 years. Average biomass reached 5.57 wet g.m^?2 and secondary production was 6.025 wet g.m^?2.year^-1; annual renewal rate P/B 1.08. Highest abundance and secondary production was contributed by individuals between 31 and 41 mm in length. A potential biomass of 25.06 tons of snails was estimated in the lake. Snail densities, secondary production, and turnover were very low during the year, indicating that it is not viable to consider a commercial catch without affecting the population. A ban of 10 years is proposed, and aquaculture practices of snails are recommended to recover the resource.     

Seasonal variation in the biomass, quantity and quality of agar from Gelidiella acerosa (Forsskal) Feldmann et Hamel (Gelidiales, Rhodophyta) from the Gulf of Mannar Marine Biosphere Reserve, India

The Gulf of Mannar on the southeast coast of India, which has recently been declared as biosphere reserve, is the main harvesting place of Gelidiella acerosa for the Indian agar industry. The biomass harvested from natural stocks was estimated to be over 10 000 tonnes dry weight (DW) in the last two decades and is a serious cause of concern on long term sustainability of this valuable resource from this region. The present study reports the seasonal variation in the biomass and agar yield and quality from G. acerosa in four different habitats such as Rameswaram (rocky intertidal), Ervadi (lower intertidal), Sethukarai (subtidal) and Krusadai Island (lagoon reef area). A maximum biomass of 260 ± 26 g DW m^?2 was recorded from Ervadi, whereas Sethukarai showed the highest percentage cover (69.83 ± 4.83%) and density (208.20 ± 30.16 plants m^?2). Biomass, percentage cover and density were lowest in Rameswaram and significantly lower (P < 0.001) than the other three stations. A single peak in biomass was observed with autumn maxima in Ervadi and Rameswaram and southwest monsoon maxima in Sethukarai and Krusaidai Island. Length of G. acerosa was maximum (9.65 ± 0.25 cm) in Ervadi and the branch index value was maximum (24.70 ± 4.01 br^?1 g^?1 DW) for Rameswaram. Agar yield (37.24 ± 7.59%) and gel strength (448.66 ± 6.50 g/cm²) were higher in Sethukarai plants. The agar yields attained a peak in the northeast monsoon and decreased in summer, whereas a reverse trend was observed for gel strength. Evidently, agar yield showed a strong negative correlation with gel strength (P < 0.0001) and gelling (P < 0.0001) and melting temperatures (P < 0.01). The single annual peak in biomass observed in the present study, as compared with two peaks reported earlier, are attributed to the continuous overharvesting that would have had an effect on the overall annual biomass production. The findings of this study reveal that a single harvest during the autumn months (January to March) could yield optimum biomass with moderate agar and that would be the best for sustainable usage and conservation of this resource from this region.     

The Shrimp Caridina nilotica in Lake Victoria (East Africa), Before and After the Nile Perch Increase

The shrimp Caridina nilotica is a major prey of the introduced Nile perch in Lake Victoria. In spite of heavy predation, the density of shrimps increased after the Nile perch boom and the concomitant disappearance of the haplochromine cichlids. In the same period, the mean size of gravid shrimps and the size at first maturity declined. This seems to indicate an increased predation pressure on adult shrimps. Before the Nile perch upsurge, specialised shrimp eaters and piscivores, among the haplochromine cichlids, only took adult shrimps, whereas we assume that most haplochromines used to include juvenile shrimps into their diet. Another important predator on adult shrimps was Bagrus docmak. The combined density of predators on adult shrimps in the pre-Nile perch era was estimated at 10 kg ha⁻¹ and the potential predators on juveniles were estimated at 170 kg ha⁻¹. After the Nile perch upsurge, only Nile perch up to 10 cm TL and Rastrineobola argentea fed on juvenile shrimps (ca. 36 kg ha⁻¹) and Nile perch from 10 to 50 cm TL (ca. 13 kg ha⁻¹) fed on adults. These rough estimates of the biomass of predators on shrimps before and after the Nile perch upsurge indicate a reduced predation pressure on juvenile shrimps. The disappearance of the haplochromines may have released competition with small Nile perch for juvenile shrimps, thus enhancing the recruitment of Nile perch.     

Population dynamics of the venerid bivalve Tawera gayi (Hupé, 1854) in the Ushuaia Bay, Beagle Channel

Growth, productivity and potential for exploitation of the clam Tawera gayi from shallow waters (3–5 m) of Ushuaia Bay, Beagle Channel were investigated. Mean abundance and biomass in the study area were 1091 ± 737 ind. m^?2 and 901.83 g SFWM m^?2 (shell‐free wet mass), respectively. Individual growth was described best by the von Bertalanffy growth model with the parameter values H_∞ = 28.03 mm, K = 0.288 year^?1, t₀ = ?0.34 (r² = 0.83). Annual production of the population was estimated to be 120.45 g SFWM m^?2 year^?1, corresponding to a production‐to‐biomass ratio (P/B) of 0.134 year^?1. The single negative exponential mortality model does not fit the population mortality pattern, but predation by gastropods (Xymenopsis muriciformis, Trophon geversianus, Natica sp.) appears to be the major cause of mortality. These highly mobile predators together with the comparatively slow growth and low turnover of T. gayi in Ushuaia Bay limit its potential for sustainable commercial exploitation.     

Distribution, abundance, and roles of freshwater clams (Bivalvia, Unionidae) in the freshwater tidal Hudson River

1. An extensive series of PONAR grabs was used to determine the distribution and abundance of unionid clams in the freshwater tidal Hudson River. 2. The five species of unionids collected were distributed very unevenly within the river. Mean river-wide density and biomass of unionids were 8.0m^?2 and 6.2 g DM m^?2 (shell-free)., respectively. 3. The environmental variables that we measured (water depth, distance from shore, sediment granulometry and organic content, presence or absence of macrophytes, and the chlorophyll a and particulate organic matter content of the water) explained little of the variation in abundance of unionids. 4. The distributions of the various species of clams did not differ significantly with respect to the environmental variables measured. 5. We estimate that unionids filter a significant amount of water (0.14m³ m^?2 day^?1, on average) in the Hudson River estuary, roughly equivalent in magnitude to downstream flushing. 6. We project that unionids will serve as a major substratum for the settlement of the zebra mussel (Dreissena polymorpha), which is now invading the estuary. We emphasize that unionids may play important non-trophic roles in large river ecosystems.     

A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen

We investigated the effect of climate change on Poa secunda Presl. and soils in a shrub‐steppe ecosystem in south‐eastern Washington. Intact soil cores containing P. secunda were reciprocally transplanted between two elevations. Plants and soils were examined, respectively, 4.5 and 5 years later. The lower elevation (310 m) site is warmer (28.5 °C air average monthly maximum) and drier (224 mm yr^?1) than the upper elevation (844 m) site (23.5 °C air average monthly maximum, 272 mm yr^?1). Observations were also made on undisturbed plants at both sites. There was no effect of climate change on plant density, shoot biomass, or carbon isotope discrimination in either transplanted plant population. The cooler, wetter environment significantly reduced percent cover and leaf length, while the warmer, drier environment had no effect. Warming and drying reduced percent shoot nitrogen, while the cooler, wetter environment had no effect. Culm density was zero for the lower elevation plants transplanted to the upper site and was 10.3 culms m^?2 at the lower site. There was no effect of warming and drying on the culm density of the upper elevation plants. Culm density of in situ lower elevation plants was greater than that of the in situ upper elevation plants. Warming and drying reduced total soil carbon 32% and total soil nitrogen 40%. The cooler, wetter environment had no effect on total soil C or N. Of the C and N that was lost over time, 64% of both came from the particulate organic matter fraction (POM, > 53 µ m). There was no effect of warming and drying on the upper population of P. secunda while exposing the lower population to the cooler, wetter environment reduced reproductive effort and percent cover. With the warmer and drier conditions that may develop with climate change, total C and N of semiarid soils may decrease with the active fraction of soil C also rapidly decreasing, which may alter ecosystem diversity and function.     

Shifts in reciprocal river‐riparian arthropod fluxes along an urban‐rural landscape gradient

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Distribution, biomass and primary production of an Ahnfeltia tobuchiensis (Ahnfeltiales, Rhodophyta) population in the Bay of Izmena, Kunashir Island

Regularities of distribution and primary production of an Ahnfeltia tobuchiensis (Kanno et Matsubara) Mak. population, an agar-containing red alga, were studied in the Bay ot Izmena. Experiments were conducted in a flow-through system under conditions similar to algal habitats. The population of A. tobuchiensis unattached to the ground may be from a few centimeters to as much as 1 m thick. It has been shown that only the upper part of a stratum 15–20 cm thick receives a sufficient amount ot light to realize its production potential. While 15–20% of photosynthetically active radiation (PAR) of that falling on the water surface reaches the stratum surface, only 0.1% of PAR from that falling on the water surface penetrates through stratum 15 cm thick. It has been shown for A. tobuchiensis that its photosynthetic rate curve during the daytime mainly follows the PAR intensity curve. The highest values of photosynthetic rate have been measured in the afternoon when PAR reaches its maximum. It is noted that a stratum 15–20 cm thick has peak values ot net primary production (NPP) which averages 3.2 g C m^?2 day^?1. The total area of A. tobuchiensis population was 23.4 km², and its biomass was 125 000 tons in this area. On average, the NPP of the A. tobuchiensis population made up in summer and in autumn was 46.8 and 25.0% of its biomass, respectively.     

Documentation of the presence of Gmelinoides fasciatus (Stebbing, 1899) and the native benthic fauna in the Volga River at Tver (Tver Region,Russia)

In the Volga Basin, the small Baikalian amphipod Gmelinoides fasciatus was introduced in 1965 into the Gorky reservoir in order to enhance fish production; it appeared in 1986 in the Rybinsk reservoir and we recorded it during monitoring activities in 2006 at Tver. In total, at the monitoring site Tver/Migalovo 69 benthic invertebrate taxa were identified. We compared data from three summer seasons. During summer low flow period Gmelinoides fasciatus did not exceed a share of 12.6% considering individual (ind) densities (mean abundance 165 ± 104 ind m^?2) and 14.2% considering biomass (mean biomass 0.39 ± 0.44 g m^?2). Abundances and biomass of G. fasciatus were shown to be stable over three years and no increase was observed. The monthly dataset (March–November 2008) revealed dynamics in relation to the native benthic communities and it was shown that the maximal densities of Gmelinoides did not exceed 587 ind m^–2. Understanding the effects on benthic communities caused by the invasive amphipod Gmelinoides fasciatus is crucial in order to predict further developments in European inland waters and to establish management strategies.