Saltcedar (Tamarix ramosissima) invasion alters organic matter dynamics in a desert stream

1. We investigated the impacts of saltcedar invasion on organic matter dynamics in a spring‐fed stream (Jackrabbit Spring) in the Mojave Desert of southern Nevada, U.S.A., by experimentally manipulating saltcedar abundance. 2. Saltcedar heavily shaded Jackrabbit Spring and shifted the dominant organic matter inputs from autochthonous production that was available throughout the year to allochthonous saltcedar leaf litter that was strongly pulsed in the autumn. Specifically, reaches dominated by saltcedar had allochthonous litter inputs of 299 g ash free dry mass (AFDM) m^?2 year^?1, macrophyte production of 15 g AFDM m^?2 year^?1 and algal production of 400 g AFDM m^?2 year^?1, while reaches dominated by native riparian vegetation or where saltcedar had been experimentally removed had allochthonous litter inputs of 7–34 g AFDM m^?2 year^?1, macrophyte production of 118–425 g AFDM m^?2 year^?1 and algal production of 640–900 g AFDM m^?2 year^?1. 3. A leaf litter breakdown study indicated that saltcedar also altered decomposition in Jackrabbit Spring, mainly through its influence on litter quality rather than by altering the environment for decomposition. Decomposition rates for saltcedar were lower than for ash (Fraxinus velutina), the dominant native allochthonous litter type, but faster than for bulrush (Scirpus americanus), the dominant macrophyte in this system.     

Role of habitat in determining macroinvertebrate production in an intermittent-stream system

1. The effect of channel drying on macroinvertebrate production was studied at the habitat and reach scale in a catchment drained by intermittent streams in Maine, U.S.A. The catchment includes two first‐order streams and their second‐order confluence. Six reaches were selected for study based on differences in channel slope and habitat cover (bedrock, riffle/run, debris dam and pool). Stream water in each reach was acidic and oligotrophic. 2. The study reaches had different degrees of channel drying. In the first‐order reaches, surface flow ceased earlier in the season and for longer periods than second‐order reaches. Regardless of reach, pool and debris dam habitats retained water longer than riffle/runs and bedrock. Unlike other habitats, debris dams retained moisture for relatively long periods following cessation of surface flow. 3. Reach‐specific macroinvertebrate production ranged from approximately 1.7 to 2.9 g AFDM m⁻² year⁻¹ which are among the lowest values ever reported. Habitat‐specific production ranged from approximately 0.5 to 5.0 g AFDM m⁻² year⁻¹ (bedrock and debris dams, respectively). 4. At the reach scale, quantities of stored benthic organic matter (range approximately 200–600 g AFDM⁻²) decreased in a downstream direction. 5. A combination of differences in the timing and duration of channel drying, habitat structure and detritus standing stocks appeared to influence levels of invertebrate production among the study reaches. 6. Our interpretation of a canonical correspondence analysis indicates that drying is more important than habitat in affecting macroinvertebrate production in this intermittent stream system.     

Effects of organic matter availability on the life history and production of a top vertebrate predator (Plethodontidae: Gyrinophilus palleucus) in two cave streams

1. Surface ecosystems provide the primary source of organic matter to many cave communities. Variation in the strength of connectivity to the surface suggests that some caves may be more resource‐limited than others. To test this, we examined diet, prey availability and production of an obligate cave salamander Gyrinophilus palleucus (Plethodontidae), a top predator, in two south‐eastern U.S.A. caves with different levels of organic matter (Tony Sinks cave, 165 g AFDM m^?2; Bluff River cave, 62 g AFDM m^?2). 2. We quantified density, biomass, growth rate, production and diet of G. palleucus monthly for 21 months. Diet composition, differences in prey communities and seasonal patterns in prey consumption were also analysed. 3. Salamander density, biomass and secondary production were significantly greater in the high organic matter cave (0.10 m^?2, 0.18 g AFDM m^?2, 0.12 g AFDM m^?2 year^?1) than in the low organic matter cave (0.03 m^?2, 0.03 g AFDM m^?2, 0.01 g AFDM m^?2 year^?1). Although growth rates were not statistically different between the two cave salamander populations, low recaptures probably influenced this result. 4. Isopoda prey were the major contributor to salamander production in the high organic matter cave (69%). In the low organic matter cave, production was provided by isopods (41%) and oligochaetes (20%). The lower number of prey taxa contributing to salamander production in the high organic matter cave suggests the ability to forage more selectively. 5. The differences in foraging strategy, density, biomass and secondary production were probably related to differences in the strength of surface connectivity, which controls organic matter supply. Links between basal resource level and top predator performance show the importance of bottom‐up limitation in the food webs of caves and other detritus‐based ecosystems.     

Potential functional redundancy and resource facilitation between tadpoles and insect grazers in tropical headwater streams

1. We quantified production and consumption of stream‐dwelling tadpoles and insect grazers in a headwater stream in the Panamanian uplands for 2 years to assess their effects on basal resources and energy fluxes. At the onset of our study, this region had healthy, diverse amphibian populations, but a catastrophic disease‐driven decline began in September 2004, which greatly reduced amphibian populations. 2. Insect grazer production was 348 mg ash‐free dry mass (AFDM) m^?2 year^?1 during the first year of the study and increased slightly to 402 mg AFDM m^?2 year^?1 during the second year. 3. Prior to amphibian declines, resource consumption by grazers (tadpoles and insects) was estimated at 2.9 g AFDM m^?2 year^?1 of algal primary production, which was nearly twice the estimated amount available. Insect grazers alone accounted for c. 81% of total primary consumption. During the initial stages of the declines, consumption remained at c. 2.9 g AFDM m^?2 year^?1, but only 35% of the available resource was being consumed and insect grazers accounted for c. 94% of total consumption. 4. Production and resource consumption of some insect grazers increased during the second year, as tadpoles declined, indicating a potential for functional redundancy in this system. However, other insect grazer taxa declined or did not respond to tadpole losses, suggesting a potential for facilitation between tadpoles and some insects; differential responses among taxa resulted in the lack of a response by insect grazers as a whole. 5. Our results suggest that before massive population declines, tadpoles exerted strong top‐down control on algal production and interacted in a variety of ways with other primary consumers. 6. As amphibian populations continue to decline around the globe, changes in the structure and function of freshwater habitats should be expected. Although our study was focused on tropical headwater streams, our results suggest that these losses of consumer diversity could influence other aquatic systems as well and may even reach to adjacent terrestrial environments.     

Energy flow and subsidies associated with the complex life cycle of ambystomatid salamanders in ponds and adjacent forest in southern Illinois

Breeding adults and metamorphosing larval amphibians transfer energy between freshwater and terrestrial ecosystems during seasonal migrations and emergences, although rarely has this been quantified. We intensively sampled ambystomatid salamander assemblages (Ambystoma opacum,A. maculatum, and A. tigrinum) in five forested ponds in southern Illinois to quantify energy flow associated with egg deposition, larval production, and emergence of metamorphosed larvae. Oviposition by female salamanders added 7.0–761.4 g ash-free dry mass (AFDM) year⁻¹ to ponds (up to 5.5 g AFDM m⁻² year⁻¹). Larval production ranged from 0.4 to 7.4 g AFDM m⁻² year⁻¹ among populations in three ponds that did not dry during larval development, with as much as 7.9 g AFDM m⁻² year⁻¹ produced by an entire assemblage. Mean larval biomass during cohort production intervals in these three ponds ranged from 0.1 to 2.3 g AFDM m⁻² and annual P/B (production/biomass) ranged from 4 to 21 for individual taxa. Emergent biomass averaged 10% (range=2–35%) of larval production; larval mortality within ponds accounted for the difference. Hydroperiod and intraguild predation limited larval production in some ponds, but emerging metamorphs exported an average of 70.0±33.9 g AFDM year⁻¹ (range=21.0–135.2 g AFDM year⁻¹) from ponds to surrounding forest. For the three ponds where larvae survived to metamorphosis, salamander assemblages provided an average net flux of 349.5±140.8 g AFDM year⁻¹ into pond habitats. Among all ponds, net flux into ponds was highest for the largest pond and decreased for smaller ponds with higher perimeter to surface area ratios (r ² =0.94, P<0.05, n=5). These results are important in understanding the multiple functional roles of salamanders and the impact of amphibian population declines on ecosystems. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Invertebrate food webs along a stream resource gradient

1. The flow of energy through food webs with similar species can vary with both space and time. The river continuum concept (RCC) provides a useful framework for predicting variability in the biota and food availability along streams. We estimated the flow of organic matter (g m^?2 year^?1) through food webs, arrayed along a stream, that had different resource inputs. Four sites were sampled along the Little Tennessee River, North Carolina, U.S.A.: two fifth order sites, one sixth and one seventh order site. The dominant resource is leaf detritus in the upstream reach (the upstream fifth order site), algae in the mid‐reaches (the downstream fifth and sixth order sites), and suspended material downstream (seventh order site). 2. Eleven genera, contributing from 50 to 66% of the total macroinvertebrate secondary production of each site, were studied. We estimated organic matter flow from resource to consumer by combining previously measured rates of invertebrate secondary production with gut content analyses and assimilation efficiencies. 3. Organic matter flow through food webs increased in a downstream direction, while the structure of the food webs remained constant. The total food consumed by the taxa analysed increased from 34 g m^?2 year^?1 at the upstream site to 730 g m^?2 year^?1 at the most downstream site. We estimate that the organic matter consumed by the entire macroinvertebrate community ranged from 66 to 1164 g m^?2 year^?1. These results indicate that there is variation in the magnitude of organic matter flow through the food webs along this river continuum. 4. The dominant food resource consumed also changed along the gradient. Leaf detritus consumption decreased from 58% of the total consumption upstream to 6% downstream, whereas consumption of amorphous detritus increased from 18 to 64%. The proportion of animal material consumed also increased from 3 to 27%. The total consumption of autochthonous resources (diatoms and filamentous algae) increased along the continuum (from 6.41 to 34.05 g m^?2 year^?1). We conclude that these results are related to variation in resource availability, dietary shifts and invertebrate secondary production. These results link resource availability to energy flow, a relationship originally suggested by the RCC.     

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.     

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.     

The response of Chironomidae (Diptera) to a long-term exclusion of terrestrial organic matter

We examined the effects of a seven-year detrital exclusion on chironomid assemblages in an Appalachian headwater stream. We hypothesized that litter exclusion would lead to a reduction in all chironomids at both the subfamily and generic levels because organic matter serves as both food and habitat in these headwater streams. Tanytarsini total abundance and biomass significantly declined after litter exclusion. Before litter exclusion, Tanytarsini average abundance was 4271 ± 1135 S.E. m⁻² and 625 ± 98 after litter exclusion. Biomass was 3.57 ± 0.96 mg AFDM m⁻² before litter exclusion and 1.03 ± 0.9 after exclusion. In contrast, Orthocladiinae abundance and biomass did not change because a psammanophilic chironomid, Lopescladius sp., and other Orthocladiinae genera did not decline significantly. Overall chironomid taxa richness and diversity did not change as a result of litter exclusion. However, Canonical Correspondence Analysis (CCA) of genus-level biomass did show a clear separation between the litter exclusion stream and a reference stream. Separation of taxa between the two streams was due to differences in fine (r ² = 0.39) and coarse (r ² = 0.36) organic matter standing stocks and the proportion of small inorganic substrates (r ² = 0.39) present within a sample. As organic matter declined in the litter exclusion stream, overall chironomid biomass declined and the chironomid community assemblage changed. Tanytarsini were replaced by Orthocladiinae in the litter exclusion stream because they were better able to live and feed on biofilm associated with inorganic substrates. Handling editor: K. Martens     

Trophic basis of production in tropical headwater streams,Puerto Rico: an assessment of the importance of allochthonous resources in fueling food webs

The relative importance of allochthonous and autochthonous resources in fueling tropical headwater streams remains an open topic. We combined estimates of secondary production and assessment of its trophic basis to determine which resources were responsible for animal production. We studied benthic insect assemblages in two streams in the Luquillo Experimental Forest, Puerto Rico. Habitat-weighted production estimates were similar in both streams (528.5 and 591.5 mg m⁻² year⁻¹), but production was over twice as high in pool versus riffle habitats. The mayfly Neohagenulus (Leptophlebiidae) was a major contributor to total production (259.1 and 352.2 mg m⁻² year⁻¹). All taxa relied heavily on amorphous detritus and plant tissue. Aquatic insect production was similar to that reported for shrimp assemblages in the same study area, but low relative to temperate region estimates. The trophic basis of production appears to be allochthonous organic matter, which agrees with the small size and closed canopy cover over the study streams. This is the first study quantifying the production and trophic basis of the non-shrimp macroinvertebrate assemblage in tropical island streams. We also provide support for the importance of riparian vegetation as the main energy sources for stream tropical stream food webs.

     

Size frequency,dispersal distances and variable growth rates of young sharks in a multi-species aggregation

During a mark–recapture survey from November 2014 until April 2017, 333 neonatal and juvenile blacktip reef sharks Carcharhinus melanopterus and 302 neonatal and juvenile sicklefin lemon sharks Negaprion acutidens were tagged and measured at the uninhabited and isolated St. Joseph Atoll (Republic of Seychelles). Both species demonstrated seasonal reproductive synchronicity and relatively large sizes at birth. Despite the extended times at liberty > 2.5 years, the majority of recaptures were found in close proximity to the initial tagging location (< 500 m). Annual growth rates of C. melanopterus (n = 24) and N. acutidens (n = 62) ranged from 6.6 to 31.7 cm year⁻¹ (mean ± SE; 16.2 ± 1.2 cm year⁻¹) and 0.2 to 32.2 cm year⁻¹ (11.8 ± 1 cm year⁻¹), respectively and are to date the most variable ever recorded in wild juvenile sharks. High abundances of both species coupled with long-term and repeated recaptures are indicative of a habitat where juveniles can reside for their first years of life. However, large variability in annual growth rates in both species may suggest high intra and interspecific competition induced by a possibly resource limited, isolated habitat.     

Microbial and macroinvertebrate communities,but not leaf decomposition,change along a mining‐induced salinity gradient

相似文献   

Secondary production and diet of an invasive snail in freshwater wetlands: implications for resource utilization and competition

Invasive species can monopolize resources and thus dominate ecosystem production. In this study we estimated secondary production and diet of four populations of Pomacea canaliculata, a freshwater invasive snail, in wetlands (abandoned paddy, oxbow pond, drainage channel, and river meander) in monsoonal Hong Kong (lat. 22°N). Apple snail secondary production (ash-free dry mass [AFDM]) ranged from 165.9 to 233.3 g m⁻² year⁻¹, and varied between seasons. Production was lower during the cool dry northeast monsoon, when water temperatures might have limited growth, but fast growth and recruitment of multiple cohorts were possible throughout much (7–10 months) of the year and especially during the warm, wet southwest monsoon. The diet, as revealed by stomach-content analysis, consisted mainly of detritus and macrophytes, and was broadly consistent among habitats despite considerable variation in the composition and cover of aquatic plants. Apple snail annual production was >10 times greater than production estimates for other benthic macroinvertebrates in Hong Kong (range 0.004–15 g AFDM m⁻² year⁻¹, n = 29). Furthermore, annual production estimates for three apple snail populations (i.e. >230 g AFDM m⁻² year⁻¹) were greater than published estimates for any other freshwater snails (range 0.002–194 g AFDM m⁻² year⁻¹, n = 33), regardless of climatic regime or habitat type. High production by P. canaliculata in Hong Kong was attributable to the topical climate (annual mean ~24°C), permitting rapid growth and repeated reproduction, together with dietary flexibility including an ability to consume a range of macrophytes. If invasive P. canaliculata can monopolize food resources, its high productivity indicates potential for competition with other macroinvertebrate primary consumers. Manipulative experiments will be needed to quantify these impacts on biodiversity and ecosystem function in wetlands, combined with management strategies to prevent further range extension by P. canaliculata.     

Bacterial production in the carbon flow of a central European stream, the Breitenbach

1. Over the last 30 years, many investigations have been performed on the dynamics of bacteria and organic matter in the Breitenbach, a first‐order stream in central Germany. The data now available allow a synthesis of the role of bacteria in the carbon budget, as an example of the general importance of bacteria in stream ecosystems. 2. Comparing measured and estimated inputs and outputs to the ecosystem, the organic matter budget of the Breitenbach is fairly balanced: 1.84 kg C m^?2 year^?1 (sum of inputs) versus 1.88 kg C m^?2 year^?1 (sum of outputs). No major missing link remains. 3. The basis of the food web in the Breitenbach is mainly allochthonous organic matter (dissolved and particulate 1.02 and 0.42 kg C m^?2 year^?1, respectively). Autochthonous gross primary production is 0.4 kg C m^?2 year^?1. Most of the organic matter leaves the stream via transport to the River Fulda (dissolved and particulate 0.74 and 0.34 kg C m^?2 year^?1, respectively), the rest by respiration (0.80 kg C m^?2 year^?1 or 43% of total outputs). 4. Bacteria constitute an important part (36%) of heterotrophic biomass (average: 0.004 kg m^?2 bacterial C of 0.011 kg m^?2 total heterotrophic C). Bacteria also account for the major fraction (71%) of heterotrophic production: 0.20 of 0.28 kg C m^?2 year^?1 total heterotrophic production. Bacterial production in the Breitenbach is similar in magnitude to the estimate of photoautotrophic net primary production: both approximately 0.20 kg C m^?2 year^?1. 5. Protozoa, the main consumers of bacteria in the Breitenbach, consume approximately one‐third of bacterial production (0.07 kg C m^?2 year^?1). Small metazoa (meiofauna, <0.5 mm) play a lesser role in the consumption of bacteria, consuming <0.01 kg bacterial C m^?2 year^?1. Larger metazoa (macrofauna, >0.5 mm) consume approximately 10% of bacterial production. Although this is a considerable amount of the carbon resources needed by the macrofauna (0.02 kg C m^?2 year^?1 of bacterial production versus 0.06 kg C m^?2 year^?1 macrofauna production plus respiration), the carbon demand of the macrofaunal community is met to a larger extent by particulate organic matter than by bacteria. 6. Bacteria are the main decomposers in the Breitenbach. They account for 78% of heterotrophic respiration (0.47 of 0.60 kg C m^?2 year^?1) and 59% of total respiration (0.47 of 0.80 kg C m^?2 year^?1).     

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.     

Seasonal and inter-stream variations in the population dynamics, growth and secondary production of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) in monsoonal Hong Kong

1. Balitorid loaches are widespread and highly diverse in Asian streams, yet their life history and ecology have received little attention. We investigated seasonal (wet versus dry season) and spatial variation in populations of algivorous Pseudogastromyzon myersi in Hong Kong, and estimated the magnitude of secondary production by this fish in pools in four streams (two shaded and two unshaded) over a 15‐month period. 2. Mean population densities of P. myersi ranged from 6.0 to 23.2 individuals m⁻², constituting more than half (and typically >70%) of benthic fishes censused. Abundance was c. 25% greater in the wet season, when recruitment occurred. Significant density differences among streams were not related to shading conditions and were evident despite small‐scale variations in P. myersi abundance among pools. Mean biomass varied among streams from 0.85 to 3.87 g ash‐free dry weight (AFDW) m⁻². Spatial and seasonal patterns in biomass and density were similar, apart from some minor disparities attributable to differences in mean body size among populations. 3. All four P. myersi populations bred once a year in June and July, and life spans varied from 24 to 26 months. Populations consisted of three cohorts immediately after recruitment but, for most of the study period, only two cohorts were evident. Cohort‐specific growth rates did not differ significantly among streams but, in all streams, younger cohorts had higher cohort‐specific growth rates. 4. Secondary production of P. myersi estimated by the size‐frequency (SF) method was 2.7–11.5 g AFDW m⁻² year⁻¹ and almost twice that calculated by the increment‐summation (IS) method (1.2–6.6 g AFDW m⁻² year⁻¹). Annual P/B ratios were 1.17 – 2.16 year⁻¹ (IS) and 2.73 – 3.22 year⁻¹ (SF). Highest production was recorded in an unshaded stream and the lowest in a shaded stream, but site rankings by production did not otherwise match shading conditions. Wet‐season production was six times greater than dry‐season production, and daily production fell to almost zero during January and February. Cool temperatures (<17 °C) may have limited fish activity and influenced detectability during some dry‐season censuses. Estimates of abundance and annual production by P. myersi are therefore conservative. 5. Comparisons with the literature indicate that the abundance and production of P. myersi in Hong Kong was high relative to other benthic fishes in tropical Asia, or their temperate counterparts in small streams. Manipulative experiments are needed to determine the influence of P. myersi, and algivorous balitorids in general, on periphyton dynamics and energy flow in Asian streams.     

Abundance and energy requirements of eiders (Somateria spp.) suggest high predation pressure on macrobenthic fauna in a key wintering habitat in SW Greenland

The number of common eiders (Somateria mollissima borealis) in west Greenland declined dramatically during the twentieth century, supposedly because of human activities. However, their sensitivity to alternative drivers of variation, such as climate conditions, diseases or food availability, remains unstudied. In this study, we describe prey availability and assess the trophic coupling between eiders and their macrobenthic prey in a shallow inlet, Nipisat Sound; a key wintering habitat in the south-west Greenland Open Water Area. Macrobenthic species abundance and biomass were studied, and annual production was estimated by an empirical model, including environmental characteristics, fauna composition and individual biomass. In spring 2008, average macrozoobenthic abundance and biomass were 6,912 ind m⁻² and 28.4 g ash-free dry mass (AFDM) m⁻² (647 kJ m⁻²), respectively. Annual production was estimated at 13.9 g AFDM m⁻² year⁻¹ (317 kJ m⁻² year⁻¹). During the winters of 2008–2010, we monitored the number of common eiders (S. mollissima borealis) and king eiders (Somateria spectabilis) and observed a distinct peak in abundance during winter with up to 15.000 birds in Nipisat Sound. Based on physiological costs of different activities in combination with the observed behavioural pattern, we obtained an estimate of the energy required for eiders to balance their costs of living, which amounted to 58% of the estimated total annual production of macrobenthos in Nipisat Sound. This result suggests that eider predation affects macrobenthic species composition and biomass and demonstrates the potential importance of variations in prey availability for the population dynamics of eiders in Greenland.     

Effects of dissolved organic matter and ultraviolet radiation on the accrual, stoichiometry and algal taxonomy of stream periphyton

1. We investigated the effects of dissolved organic matter (DOM) and ultraviolet‐B (UVB) radiation on periphyton during a 30‐day experiment in grazer‐free, outdoor artificial streams. We established high [10–12 mg carbon (C) L⁻¹] and low (3–5 mg C L⁻¹) concentrations of DOM in artificial streams exposed to or shielded from ambient UVB radiation. Periphyton was sampled weekly for ash‐free dry mass (AFDM), chlorophyll (chl) a , algal biovolume, elemental composition [C, nitrogen (N) and phosphorus (P)], and algal taxonomic composition. 2. Regardless of the UVB environment, increased DOM concentration caused greater periphyton AFDM, chl a and total C content during the experiment. Increased DOM also significantly increased periphyton C : P and N : P (but not C : N) ratios throughout the experiment. Algal taxonomic composition was strongly affected by elevated stream DOM concentrations; some algal taxa increased and some decreased in biomass and prevalence in artificial streams receiving DOM additions. UVB removal, on the other hand, did not strongly affect periphyton biomass, elemental composition or algal taxonomic composition for most of the experiment. 3. Our results show strong effects of DOM concentration but few, if any, effects of UVB radiation on periphyton biomass, elemental composition and algal taxonomic composition. The effects of DOM may have resulted from its absorption of UVA radiation, or more likely, its provision of organic C and nutrients to microbial communities. The strong effects of DOM on periphyton biomass and elemental composition indicate that they potentially play a key role in food web dynamics and ecosystem processes in forested streams.     

Ecology and production of invertebrates in a Canadian coldwater spring-springbrook system

Valley Spring is a hardwater rheocrene consisting of a single point issue and a springbrook some 60 m long. The fauna consists of > 60 taxa, with insects and mites predominating. During 1985–86, benthic densities varied both along the springbrook and at single stations over time (range 58 to 756 animals 900 cm⁻²). A clear longitudinal zonation of many of the major taxa occurred with the Diptera tending to be more abundant downstream, the Plecoptera and Acari more abundant upstream, and the Copepoda and Bivalvia most abundant in mid- and downstream sections. The Gastropoda and Trichoptera occurred throughout the system although, in the case of the latter, there were distributional differences at the species level (e.g. Diplectrona modesta downstream, Parapsyche apicalis and Lepidostoma vernale upstream). Life histories varied between major species and were grouped into: (A) those species in which there was a wide range of larval sizes for most of the year, with no easily discernible temporal succession of cohorts, e.g. L. vernale, P. apicalis, Sialis sp., Dixa sp., Metriocnemus sp., Trissopelopia sp., Micropsectra sp. and Mucronothrus nasalis (the generation time of species in this group ranged from < 1 year to > 2 years); and (B) those species in which there was a clear pattern of larval growth, with obvious temporal succession of cohorts, e.g. D. modesta, Nemoura trispinosa and Hyalella azteca (all of these species were univoltine). Differences between the “adaptive strategies” of these two life history types are discussed. Total secondary production in Valley Spring in 1985–86 was estimated to be 913.2 g wet wt yr⁻¹. Production upstream was more than twice that downstream (11.21 and 4.01 g wet wt m⁻² yr⁻¹, respectively). The three most abundant species of caddisfly together contributed > 30% of the total production, the stonefly N. trispinosa contributed > 17%, while the chironomids accounted for >16% of production at downstream stations but only 3% upstream.     

Benthic organic carbon influences denitrification in streams with high nitrate concentration

1. Anthropogenic activities have increased reactive nitrogen availability, and now many streams carry large nitrate loads to coastal ecosystems. Denitrification is potentially an important nitrogen sink, but few studies have investigated the influence of benthic organic carbon on denitrification in nitrate‐rich streams. 2. Using the acetylene‐block assay, we measured denitrification rates associated with benthic substrata having different proportions of organic matter in agricultural streams in two states in the mid‐west of the U.S.A., Illinois and Michigan. 3. In Illinois, benthic organic matter varied little between seasons (5.9–7.0% of stream sediment), but nitrate concentrations were high in summer (>10 mg N L⁻¹) and low (<0.5 mg N L⁻¹) in autumn. Across all seasons and streams, the rate of denitrification ranged from 0.01 to 4.77 μg N g⁻¹ DM h⁻¹ and was positively related to stream‐water nitrate concentration. Within each stream, denitrification was positively related to benthic organic matter only when nitrate concentration exceeded published half‐saturation constants. 4. In Michigan, streams had high nitrate concentrations and diverse benthic substrata which varied from 0.7 to 72.7% organic matter. Denitrification rate ranged from 0.12 to 11.06 μg N g⁻¹ DM h⁻¹ and was positively related to the proportion of organic matter in each substratum. 5. Taken together, these results indicate that benthic organic carbon may play an important role in stream nitrogen cycling by stimulating denitrification when nitrate concentrations are high.