Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Effects of an invasive crayfish on trophic relationships in north‐temperate lake food webs

1. The introduction of invasive species is one of the main threats to global biodiversity, ecosystem structure and ecosystem processes. In freshwaters, invasive crayfish alter macroinvertebrate community structure and destroy macrophyte beds. There is limited knowledge on how such invasive species‐driven changes affect consumers at higher trophic levels. 2. In this study, we explore how the invasive rusty crayfish Orconectes rusticus, a benthic omnivore, affects benthic macroinvertebrates, as well as the broader consequences for ecosystem‐level trophic flows in terms of fish benthivory and trophic position (TP). We expected crayfish to decrease abundance of benthic macroinvertebrates, making most fish species less reliant on benthic resources. We expected crayfish specialists (e.g. Lepomis sp. and Micropterus sp.) to increase their benthic dependence. 3. In 10 northern Wisconsin lakes, we measured rusty crayfish relative abundance (catch per unit effort, CPUE), macroinvertebrate abundance, and C and N stable isotope ratios of 11 littoral fish species. We used stable isotope data and mixing models to characterise the trophic pathways supporting each fish species, and related trophic structure to crayfish relative abundance, fish body size and abiotic predictors using hierarchical Bayesian models. 4. Benthic invertebrate abundance was negatively correlated with rusty crayfish relative abundance. Fish benthivory increased with crayfish CPUE for all 11 fish species; posterior probabilities of a positive effect were >95%. TP also increased slightly with crayfish CPUE for some species, particularly smallmouth bass, largemouth bass, rock bass and Johnny darter. Moreover, both fish body size and lake abiotic variables explained variation in TP, while their effects on benthivory were small. 5. Rusty crayfish abundance explained relatively little of the overall variation in fish benthivory and TP. Although rusty crayfish appear to have strong effects on abundances of benthic macroinvertebrates, energy flow pathways and trophic niches of lentic fishes were not strongly influenced by invasive rusty crayfish.     

‘House and garden’: larval galleries enhance resource availability for a sedentary caddisfly

1. Sedentary grazers can be numerous in fresh waters, despite the constraints on resource availability and the increased predation risk inherent in this lifestyle. The retreats of sedentary grazers have been assumed to provide protection to the resident (a ‘house’), but also may provide additional fertilised food for the grazer (i.e. a ‘garden’). If retreats function as a garden, then they should (i) contain a higher quality and/or quantity of food than the alternative food source. Furthermore, the proportion of retreat‐derived carbon and nitrogen assimilated by the resident should be (ii) related to overall resource availability (more when resources are limited) or (iii) perhaps also to larval density. Alternatively, if retreats provide a less risky food source, then (iv) assimilation of material from the retreat is likely to be greater under conditions in which the risk of emerging from the retreat is high. 2. We tested these four hypotheses for the common and widespread gallery‐building grazing caddisfly Tinodes waeneri. Resource availability, larval density and biomass, and exposure were measured for populations from six lakes of differing productivity in August, October and January. 3. Galleries always contained more algal food than the surrounding epilithon, suggesting that gardening is effective. Furthermore, gallery chlorophyll a content in August, and the disparity in food quality (assessed from the C : N ratio) between gallery and epilithon (quality higher in the former) in October were positively related to the proportion of larval biomass that was derived from the gallery. Larval density and wave exposure parameters were not related to larval assimilation of gallery material. 4. Galleries that are fertilised by the occupant provide more, and sometimes also better quality, food (in terms of the C : N ratio) than is otherwise available. Thus, the gallery plays a substantial role in larval nutrition, and this role is greater at key times of food shortage.     

Migratory benthic fishes may induce regime shifts in a tropical floodplain pond

1. Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid‐ and clear‐water conditions. Here, we investigate whether such shifts in a tropical floodplain pond may be related to the effect of the flood pulse regime on the community structures of fish and macrophytes. 2. Using a long‐term data set, we demonstrate how benthic fish migration together with colonisation by submerged plants affected the transition from a turbid to a macrophyte‐dominated state in a floodplain pond without top‐down control. 3. In our study, the turbid state occurred mostly during low water phases and was largely characterised by high values for the biomass of benthic fish, chlorophyll‐a and total phosphorous. 4. During the period of rising water levels, the migration of benthic fish out of the pond occurs simultaneously with the establishment of submerged plants, while water turbidity decreases along with phytoplankton and nutrient concentrations, inducing a clear‐water phase. However, when submerged plants are absent and fish migration is low, a transient state is generated. 5. We suggest that, in contrast to temperate ponds and shallow lakes, where the main driving mechanisms establishing alternative states are related to cascading effects via the food chain, in tropical ponds and shallow lakes it is resuspension of sediments by benthic fish that plays the most significant role in establishing alternative states. However, the effect of the flood pulse regime plays an important role in the temporal dynamics of fish community structure by controlling benthic fish migration.     

Patterns of fungal communities among and within decaying logs, revealed by 454 sequencing

Owing to previous methodological limitations, knowledge about the fine-scale distribution of fungal mycelia in decaying logs is limited. We investigated fungal communities in decaying Norway spruce logs at various spatial scales at two environmentally different locations in Sweden. On the basis of 454 pyrosequencing of the ITS2 region of rDNA, 1914 operational taxonomic units (OTUs) were detected in 353 samples. The communities differed significantly among logs, but the physical distance between logs was not found to have a significant effect on whether fungal communities had any resemblance to each other. Within a log, samples that were closer together generally had communities that showed more resemblance to each other than those that were further apart. OTUs characteristic for particular positions on the logs could be identified. In general, these OTUs did not overlap with the most abundant OTUs, and their ecological role was often unknown. Only a few OTUs were detected in the majority of logs, whereas numerous OTUs were rare and present in only one or a few logs. Wood-decaying Basidiomycetes were often represented by higher sequence reads in individual logs than Ascomycete OTUs, suggesting that Basidiomycete mycelia spread out more rapidly when established. OTU richness tended to increase with the decay stage of the sample; however, the known wood decayers were most abundant in less-decomposed samples. The fungi identified in the logs represented different ecological strategies. Our findings differ from previously published sporocarp studies, indicating that the highly abundant fruiting species may respond to environment in different ways than the rest of the fungal community.     

Nonsignificant isolation by distance implies limited dispersal

Understanding the scale of dispersal is an important consideration in the conservation and management of many species. However, in species in which the high‐dispersal stage is characterized by tiny gametes or offspring, it may be difficult to estimate dispersal directly. This is the case for many marine species, whose pelagic larvae are dispersed by ocean currents by several days or weeks before beginning a benthic, more sedentary, adult stage. As consequence of the high‐dispersal larval stage, many marine species have low genetic structure on large spatial scales (Waples 1998 ; Hellberg 2007 ). Despite the high capacity for dispersal, some tagging studies have found that a surprising number of larvae recruit into the population they were released from (self‐recruitment). However, estimates of self‐recruitment are not informative about mean dispersal between subpopulations. To what extent are limited dispersal estimates from tagging studies compatible with high potential for dispersal and low genetic structure? In this issue, a study on five species of coral reef fish used isolation by distance (IBD) between individuals to estimate mean dispersal distances (Puebla et al. 2012 ). They found that mean dispersal was unexpectedly small (<50 km), given relatively low IBD slopes and long pelagic durations. This study demonstrates how low genetic structure is compatible with limited dispersal in marine species. A comprehensive understanding of dispersal in marine species will involve integrating methods that estimate dispersal over different spatial and temporal scales. Genomic data may increase power to resolve these issues but must be applied carefully to this question.     

Foraging costs drive female resistance to a sensory trap

Male ornaments can evolve through the exploitation of female perceptual biases such as those involved in responding to cues from food. This type of sensory exploitation may lead to confusion between the male signals and the cues that females use to find/recognize food. Such interference would be costly to females and may be one reason why females evolve resistance to the male ornaments. Using a group of species of viviparous fish where resistance to a sensory trap has evolved, we demonstrate that females exposed to an ornament that resembles food have a diminished foraging efficiency, that this effect is apparent when foraging on a food item with which the ornament shares visual attributes, and that not all species are equally affected by such confusion. Our results lend support to the model of ornamental evolution through chase-away sexual conflict.     

A combination of hydrodynamical and statistical modelling reveals non-stationary climate effects on fish larvae distributions

Biological processes and physical oceanography are often integrated in numerical modelling of marine fish larvae, but rarely in statistical analyses of spatio-temporal observation data. Here, we examine the relative contribution of inter-annual variability in spawner distribution, advection by ocean currents, hydrography and climate in modifying observed distribution patterns of cod larvae in the Lofoten-Barents Sea. By integrating predictions from a particle-tracking model into a spatially explicit statistical analysis, the effects of advection and the timing and locations of spawning are accounted for. The analysis also includes other environmental factors: temperature, salinity, a convergence index and a climate threshold determined by the North Atlantic Oscillation (NAO). We found that the spatial pattern of larvae changed over the two climate periods, being more upstream in low NAO years. We also demonstrate that spawning distribution and ocean circulation are the main factors shaping this distribution, while temperature effects are different between climate periods, probably due to a different spatial overlap of the fish larvae and their prey, and the consequent effect on the spatial pattern of larval survival. Our new methodological approach combines numerical and statistical modelling to draw robust inferences from observed distributions and will be of general interest for studies of many marine fish species.     

Male cleaner wrasses adjust punishment of female partners according to the stakes

Punishment is an important deterrent against cheating in cooperative interactions. In humans, the severity of cheating affects the strength of punishment which, in turn, affects the punished individual's future behaviour. Here, we show such flexible adjustments for the first time in a non-human species, the cleaner wrasse (Labroides dimidiatus), where males are known to punish female partners. We exposed pairs of cleaners to a model client offering two types of food, preferred 'prawn' items and less-preferred 'flake' items. Analogous to interactions with real clients, eating a preferred prawn item ('cheating') led to model client removal. We varied the extent to which female cheating caused pay-off reduction to the male and measured the corresponding severity of male punishment. Males punished females more severely when females cheated during interactions with high value, rather than low value, model clients; and when females were similar in size to the male. This pattern may arise because, in this protogynous hermaphrodite, cheating by similar-sized females may reduce size differences to the extent that females change sex and become reproductive competitors. In response to more severe punishment from males, females behaved more cooperatively. Our results show that punishment can be adjusted to circumstances and that such subtleties can have an important bearing on the outcome of cooperative interactions.     

Bacterial communities in sediments of the shallow Lake Dongping in China

Aims: The purpose of this study was to discuss how the environmental inputs and anthropogenic activities impact bacterial communities in the sediments of a shallow, eutrophic and temperate freshwater lake. Methods and Results: Sediment cores were collected from Lake Dongping, located in Taian, Shandong, China. All samples were processed within 4 h of collection. Total nitrogen, total phosphorus (TP), total organic carbon, ammonium nitrogen and nitrate nitrogen content of samples were measured by Kjeldahl determination, sulphuric acid–perchloric acid digestion and molybdenum blue colorimetry, potassium dichromate titration, Nessler’s reagent colorimetric and the phenol disulphonic acid colorimetric method, respectively. Seasonal and temporal diversity of sediment bacterial communities at six stations in Lake Dongping were investigated using molecular approaches (terminal restriction fragment length polymorphism and 16S rDNA clone libraries). Noticeable seasonal and temporal variations were observed in bacterial diversity and composition at all six stations. Sediment bacterial communities in Lake Dongping belonged to 16 phyla: Proteobacteria (including α‐Proteobacteria, β‐Proteobacteria, δ‐Proteobacteria, ε‐Proteobacteria, γ‐Proteobacteria), Acidobacteria, Planctomycetes, Bacteroidetes, Firmicutes, Verrucomicrobia, Nitrospira, Chloroflexi, Gemmatimonadetes, Chlorobi, Cyanobacteria, Deferribacteres, Actinobacteria, OP8, Spirochaetes and OP11. Members of β‐, δ‐ and γ‐Proteobacterial sequences were predominant in 11 of 12 clone libraries derived from sediment samples. Sediment samples collected at stations 1 and 4 in July had the greatest bacterial diversity while those collected at station 2 in October had the least diversity. TP concentration was significantly correlated with the distribution of bacterial communities. Conclusions: Our results suggested that different environmental nutrient inputs contribute to seasonal and temporal variations of chemical features and bacterial communities in sediments of Lake Dongping. TP concentration was significantly correlated with the distribution of bacterial communities. Significance and Impact of the Study: This study has an important implication for the optimization of integrated ecosystem assessment of shallow temperate freshwater lake and provides interesting information for the subsequent of the ecosystem.