Description: Mortality is a key factor in understanding the population dynamics of fish. For studies using biotelemetry, missing individuals pose a challenge since the ultimate fate of both the animal and the tag are unknown. In this study, we document three releases of carcasses in a simulated small-scale summer fish kill in a small urban stream using juvenile white sucker ( Catostomus commersonii ). Passive integrated transponder (PIT) and radio tags were affixed to carcasses that were tracked to determine the fate of both the dead fish and the tags. Mean daily dispersal distances ranged from 0.0 to 7.6 m day −1 and included downstream and lateral dispersal. Of the 44 radio-tagged carcasses, 26 tags (and presumably carcasses) were consumed by scavengers; the majority were consumed by snapping turtles ( Chelydra serpentina ) with fewer carcasses scavenged by great blue heron ( Ardea herodias ), raccoon ( Procyon lotor ) and muskrat ( Ondatra zibethicus ). We also contrasted the decomposition rates of in-stream carcasses with those experimentally placed on the riverbank and found that while there was no significant difference in the time to carcass evisceration, the rate of decomposition was more rapid in-stream compared to on-shore. Radio tag loss during the decomposition study was moderate (one of three lost); PIT tag loss occurred when carcasses became eviscerated, typically by invertebrates. By examining the role of scavengers, dispersal and decomposition, it is possible to understand the fate of dead fish, the fate of tags and role of mortality in tagging experiments and the connections between stream and riparian habitats and organisms. This information will help inform the interpretation of potential mortalities in fish tracking studies and improve fish kill investigations.

Description: The aquatic and terrestrial realms differ in many physical properties that not only require specific physiological adaptations but also cause differences in dispersal options. We thus expect that life-history traits related to dispersal and colonization are under selection pressure because freshwater habitats are more isolated and thus more difficult to reach. We compared traits from European databases of three taxonomic groups along the passive–active dispersal gradient: plants (Plantes), snails (Mollusca: Gastropoda: Prosobranchia et Pulmonata) and hoverflies (Diptera: Syrphidae), all of which have both terrestrial and freshwater species (plants and snails) or early life stages (hoverflies). Aquatic taxa seem to be more successful long-distance dispersers than are terrestrial taxa. Our analysis also revealed lower numbers of seeds or eggs produced in the aquatic habitats. However, aquatic taxa often allocate resources to offspring guarding (vegetative propagules in plants, egg capsules in snails) and breeding-site selection (syrphids). Colonization of the aquatic realm is reinforced by increases in life span (plants), clonal spread (plants), shorter generation times (snails), selfing ability (marginal effect in pulmonate snails) or paedogenesis (two incidences in hoverflies, needs further studies). Probably, the variety of strategies reflects the different evolutionary backgrounds that elicit different combinations of trade-offs, but all traits also might increase invasibility of species.

Description: The objective of this study was to evaluate the interactions between green tide-forming macroalgae Ulva linza and red macroalgae Gracilaria lemaneiformis in the laboratory. The results demonstrated that the presence of U. linza can restrict growth (9–31 %) and photosynthesis (25–85 %) of G. lemaneiformis . In contrast, G. lemaneiformis had little apparent effect on the growth of U. linza . Culture medium experiments confirmed that allelochemicals may be released by both the tested macroalgae. The causative mechanism for the growth and photosynthesis inhibition of G. lemaneiformis was not light limitation nor increase of pH, but a combination of allelopathic effects of U. linza and nutrient competition between the two macroalgae. Moreover, the “green tide” macroalga U. linza was a stronger competitor for nutrient than G. lemaneiformis . The results from this study provide evidence for the mechanisms of “green tide” formation by U. linza : potent allelopathic effects on G. lemaneiformis and faster nutrients uptake than its competitors.

Description: Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ 13 C and δ 15 N for a demersal fish predator, Acropoma japonicum . This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ 13 C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ 13 C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.

Description: The present study was aimed to determine dietary composition and feeding guild structure of the fishes inhabiting mudflat habitat of Indian Sundarbans. In addition, partitioning of food resources by the fish species belonging to the carnivorous feeding guild was also performed to understand the survival strategies of fish in a mudflat estuarine habitat. Seventeen prey categories were isolated from the stomachs of 31 studied fish species. Overall, five feeding guilds (viz. plankti-benthivore: 12 species, herbivore: one species, detritivore: three species, omnivore: one species and carnivore: 14 species) were identified on the basis of the prey abundance within their stomachs, considering 64 % Bray–Curtis similarity. Among the carnivorous, maximum trophic richness was obtained for Uroconger lepturus followed by Ophichthus apicalis . Teleost and decapods were the main animal prey items preferred by majority of the carnivorous fishes. However, O. apicalis and Terapon jarbua showed their preference toward maximum number of prey categories among carnivores, which was also ratified by the high values for standardized niche breadth presented by them. The maximum degree of interspecific dietary overlap was found between Uropterygius marmoratus and Pseudapocryptes elongatus as both of them were recognized as cranci-piscivore. The lowest was observed between Hyporhamphus limbatus and Coilia neglecta . As food resources are not limiting in the mudflats of Indian Sundarbans, the general patterns of resource partitioning and niche differentiation in resident ecological communities will improve the understanding of the mechanisms underlying species coexistence and community structure.

Description: The appropriate level of taxonomic identification, taxonomic sufficiency, for biomonitoring purposes continues to be controversial. Taxonomic sufficiency, however, fails to address the bias created by size-dependent taxonomic identification, which can result in coarse-resolution identification for immature specimens lacking distinguishing characteristics. Our study provides a direct test for this potential systematic bias in biomonitoring data by examining two morphological traits: body size and shape of key organisms (Ephemeroptera, Plecoptera, Trichoptera and Odonata) collected from standard aquatic biomonitoring samples. Direct measurement of body size and a geometric morphometric description of body shape provide consistent, quantitative variables to describe the composition of specimens identified at different levels of taxonomic resolution (genus or family). Corroborating our expectations, we observed evidence of systematic size bias in family-level identifications. Specimens that could only reliably be identified to the family level were significantly smaller than specimens identified to the genus level. Qualitative comparisons of shape variation between specimens demonstrated a high degree of variation in specimens identified only at the family level and support the conclusion that specimens identified at the family level possess multiple constituent taxa (genera or species). Thus, size-dependent taxonomy can have negative consequences for the accurate determination of biodiversity and may invalidate common biomonitoring metrics. Improvements to biomonitoring protocols through technological advances, including DNA-based taxonomy to augment specimen identification, should effectively remove the size-bias problem in the long term. In the short-term, recognizing instances of size bias, the degree to which it may impact bioassessment and exploring methods for remediation, including traits-based assessments, can enhance data quality and inferences derived from biomonitoring studies.

Description: Variable oxygen release from the root of macrophytes growing in ammonium-rich organic substrates can stimulate the process of nitrification. To verify this hypothesis, we performed seasonal measurements of potential nitrification activity in sediments with and without the perennial submersed plant Vallisneria spiralis L. (Hydrocharitaceae). Pore water and sediment features were simultaneously considered in order to provide insights into the regulation of the process. Results demonstrated a significant effect of season and plant presence on potential nitrification activity, with higher rates in winter and lower rates in summer. Vegetated sediment displayed lower pore water ammonium, but always higher potential nitrification activity compared to the unvegetated substrate, regardless the season. Nitrification activity was strongly correlated with pore water redox status, which were affected by both season and plant presence. Along its annual cycle V. spiralis promoted more oxidized conditions in the rhizosphere likely due to elevated radial oxygen loss and the consequent maintenance of a larger nitrifying community. These outcomes confirm the results of a limited number of studies that demonstrated how sediment biogeochemistry may be controlled by plant-released oxygen also in organic-rich systems.

Description: Human transport and active dispersal of the red swamp crayfish ( Procambarus clarkii ) contribute to its rapid spread. However, some small aquatic organisms can be transported by birds. We made two hypotheses related to waterbird-mediated passive dispersal of juvenile crayfish. The first is that, depending on water depth, recently hatched crayfish can attach to ducks, initiating passive external transport (i.e., ectozoochory). The second is that recently hatched crayfish can survive bird flight, being affected by crayfish features, flight distance, and environmental conditions. A first experiment tested the attachment of juvenile crayfish to ducks at different water depths by using a freshly dead duck and tanks with crayfish. Another set of three experiments tested crayfish survival during air transportation. To simulate bird flight, we first used a vehicle moving at bird flight speed, and we then used trained pigeons. Several flight distances, environmental conditions, and crayfish sizes were tested. Our results showed that juvenile crayfish were capable of clinging to duck feathers and were transported when ducks were removed from the water. Furthermore, some juveniles of P. clarkii were able to survive long-distance transport when suspended outside a moving vehicle or when transported by birds. The probability of success was affected by water depth, crayfish size, distance travelled, and relative humidity. Our results support the occurrence of passive transportation of this invader by means of attachment to birds. These findings indicate that waterbird-mediated passive dispersal should be taken into account to explain P. clarkii ’s rapid spread and should be considered when managing its invasions.

Description: Selection pressure induced by simultaneously occurring environmental threats is a major evolutionary driver for organisms in terrestrial, as well as in aquatic ecosystems. For example, protection against ultraviolet radiation (UVR) and predation include both morphological and behavioral components. Here we address those selective pressures on zooplankton by performing a latitudinal monitoring, combined with mechanistic experiments in the Antarctic Southern Ocean, where the UVR-threat is extremely high. We assessed vertical distributions of zooplankton along the Antarctic coast showing that animals were most abundant at 20–80 m and tended to avoid the surface at sites with clear water. UVR-threat disappeared at between 9 and 15 m at sites with low and high water transparency, respectively. Light levels were, however, sufficient for visual fish predation down to approximately 19 and 37 m, respectively. The few zooplankton that were present in surface waters had high levels of non-pigmented UVR-protective compounds (mycosporine-like amino acids) compared to deeper dwelling zooplankton. Overall they had low levels of red pigmented UVR-protective compounds (carotenoids), suggesting high predation on pigmented individuals. In a complementary laboratory study we showed that levels of UVR-protective compounds increased considerably when zooplankton were exposed to UVR in the absence of predator cues. The recently developed transparency-regulator hypothesis predicts that UVR avoidance is an important driver to diel vertical migration in transparent waters, such as in Antarctica. We, however, conclude that copepods resided well below the level where UVR had diminished to very low levels and that predator avoidance or food availability are more likely drivers of zooplankton vertical depth distribution in transparent marine systems.

Description: Benthic algae were collected from central and northern Chinese rivers to test the hypothesis that geographic location has significant contributions in shaping algal assemblages. We used Moran’s eigenvector maps (MEM) to model spatial components and variation partitioning to quantify the influences of spatial and environmental variables on regional patterns of algal richness and community composition, respectively. We found that variation in algal richness was attributed to MEM component 2, 8, and 9 and the quadratic term of N–NO 3 . Regarding abundance data, latitude, longitude, and MEM component 1, 2, and 7 were important spatial variables. Although P–PO 4 , pH, and annual mean temperature were significant environmental variables influencing algal community composition, they were all spatially structured. Among the total explained variance in both algal metrics, spatial proportions were higher than that of environmental variables. We also found that abundant species of Achnanthidium minutissimum , Cocconeis placentula , Cymbella delicatula , Cymbella affinis , Cymbella turgidula , and Synedra ulna displayed clear spatially related patterns. In conclusion, the contributions of spatial and environmental variables to regional variation of algal assemblages are scale-dependent. As for our study scale (~1,000 km), spatial control may be more important. Since spatial effects could obscure local environmental impacts on algal communities, appropriate study scale and statistical methods should be taken into account in algal bioassessment. We recommend inclusion of both algal richness and community composition in study of algal biogeography, due to their different relationships with spatial and environmental variables.