Seagrass meadows ecosystem engineering effects are correlated to their density (which is in turn linked to seasonal cycles) and often cannot be perceived below a given threshold level of engineer density. The density and biomass of seagrass meadows (Z. marina) together with associated macrophytes undergo substantial seasonal changes, with clear declines in winter. The present study aims to test whether the seasonal changes in the density of recovering seagrass meadows affect the benthic food webs of the southern Baltic Sea (Puck Bay). It includes meiofauna, macrofauna and fish of vegetated and unvegetated habitats in summer and winter seasons. Two levels of organization have been tested – species-specific diet preferences using stable isotopes (δ13C, δ15N) in Bayesian mixing models (MixSIAR) and the community-scale food web characteristics by means of isotopic niches (SIBER). Between-habitat differences were observed for grazers, as a greater food source diversity in species from vegetated habitats was noted in both seasons. Larger between-habitat differences in winter were documented for suspension/detritus feeders. The community-wide approach showed that the differences between the habitats were greater in winter than in summer (as indicated by the lower overlap of the respective isotope niches). Overall, the presence of seagrass meadows increased ecological stability (in terms of the range of food sources utilized by consumers) in the faunal assemblage, while invertebrates from unvegetated areas shifted their diet to cope with winter conditions. Therefore, as a more complex system, not sensitive to seasonal changes, Z. marina meadows create a stable habitat with high resilience potential.

Materials, energy, and organisms from groundwater serve as resource subsidies to lotic systems. These subsidies influence food production and post-emergent fish growth and condition through nutrient inputs and water temperature changes. To test whether post-emergent fish grew faster in gaining sites, we grew hatchery post-emergent salmon in enclosures, sampled periphyton, benthic invertebrates, and wild salmon, and modeled fish growth across a gradient of groundwater – surface water exchange. Fish grew almost twice as fast in gaining (2.7%·day⁻¹) than in losing (1.5%·day⁻¹) sites. Fish from transient sites grew as much as gaining sites, but their condition was significantly lower (18.3% vs. 20.7%). Results suggest that groundwater – surface water exchange affects fish growth and energetic condition through direct and indirect pathways. Elevated nitrogen concentrations and consistently warmer water temperature in gaining sites have a strong effect on basal production with subsequent effects on invertebrate biomass, fish growth, and condition. Findings highlight the importance of groundwater – surface water exchange as a subsidy to rearing salmon and may inform strategies for restoring fish rearing habitat.

Groundwaters are increasingly viewed as resource-limited ecosystems in which fluxes of dissolved organic carbon (DOC) from surface water are efficiently mineralized by a consortium of microorganisms which are grazed by invertebrates. We tested for the effect of groundwater recharge on resource supply and trophic interactions by measuring physico-chemistry, microbial activity and biomass, structure of bacterial communities and invertebrate density at three sites intensively recharged with surface water. Comparison of measurements made in recharge and control well clusters at each site showed that groundwater recharge significantly increased fluxes of DOC and phosphate, elevated groundwater temperature, and diminished dissolved oxygen (DO). Microbial biomass and activity were significantly higher in recharge well clusters but stimulation of autochthonous microorganisms was not associated with a major shift in bacterial community structure. Invertebrate assemblages were not significantly more abundant in recharge well clusters and did not show any relationship with microbial biomass and activity. Microbial communities were bottom-up regulated by DOC and nutrient fluxes but trophic interactions between microorganisms and invertebrates were apparently limited by environmental stresses, particularly DO depletion and groundwater warming. Hydrological connectivity is a key factor regulating the function of DOC-based groundwater food webs as it influences both resource availability for microorganisms and environmental stresses which affect energy transfer to invertebrates and top-down control on microorganisms.

Food selection by adult red foxes Vulpes vulpes was studied in a mountainous environment characterised by fluctuating populations of water voles. According to optimal foraging theory, the diets of adults and cubs should be similar during vole highs, with voles as staple prey. On the other hand, a difference should occur when water voles become scarce. Adults should then bring larger alternative prey to their cubs, and consume smaller prey at the capture site. Adult and cub scats were collected around breeding dens during a 6‐year period, from the peak to a near‐extinction phase of the water vole population, and analysed. As predicted by the optimal foraging theory, no significant difference was found between the diets of cubs and adults when voles were abundant. Furthermore, cubs consumed a higher proportion of large alternative prey, i.e. lagomorphs, than adults in the first year of low vole density. Unexpectedly, this difference did not persist the following years despite the water vole scarcity. The consumption of large alternative prey at the den decreased significantly, whereas the consumption of small alternative prey, i.e. invertebrates, increased. Consequently, the composition of prey brought to the den was similar to that consumed at the capture site. Food selection was probably affected by the unpredictability of large prey availability and accessibility. It is suggested that if small alternative prey proves less profitable to the cubs than large prey, a long‐lasting shortage of water voles could possibly become a factor regulating fox numbers in the area studied.

Bryophytes are abundant in streams and are a habitat for many invertebrates, but their contribution to the diet of fluvial zoobenthos is still debated. To estimate the amount of bryophyte-derived organic matter assimilated by benthic invertebrates, we used a combination of fatty acid and stable isotope analyses during a four-year monthly study of a littoral site in the Yenisei River (Siberia, Russia). Acetylenic acids, which are highly specific biomarkers of the water moss Fontinalis antipyretica, were found in lipids of all dominant benthic animals: gammarids, ephemeropterans, chironomids and trichopterans. The dominant zoobenthic species, Eulimnogammarus viridis, had maximum levels of the biomarkers in its biomass during winter, and minimum levels in summer. The zoobenthos in the studied site regularly consume and assimilate bryophyte-derived organic matter as a minor supplemental food. This consumption increases in winter, when the main food source of the zoobenthos, epilithic biofilms, are probably scarce.

Large seasonal water-level fluctuations may influence isotopic signatures of primary producers and the types and amounts of these potential food sources accessible to aquatic fauna of Poyang Lake, the largest freshwater lake in China. In this study, the isotopic signatures of primary producers and consumers were determined, stable carbon and nitrogen isotope analysis and mixing models were combined to investigate the influence of water levels on the diet and isotopic composition of Poyang Lake fish and invertebrates. Five potential food sources (seston, benthic organic matter, aquatic macrophytes, attached algae, and terrestrial plants), 4 species of invertebrates, and 10 species of fish were collected from the lake area during dry and wet seasons between January 2009 and April 2010. The δ 13C values of invertebrates and most fish were within the range of δ 13C values of the potential food sources for both seasons. The δ 13C values of invertebrates and most fish were lower in the dry season than in the wet season, whereas the δ 15N values exhibited different patterns for different species. Mixing models indicated that the most important food sources for common lake fauna were seston in the dry season and aquatic macrophytes and terrestrial plants in the wet season. The fauna were more omnivorous in the wet season than in the dry season. The food web dynamics of Poyang Lake are strongly influenced by changes in the abundance and accessibility of different basal food sources that occur because of seasonal flood pulses. The trophic links within the aquatic communities of Poyang Lake are modified by water-level fluctuations.

In winter 2004/2005, 1532 Water Pipits were recorded during 37 censuses carried out along an established route on a sewage farm flooded with wastewater (Wrocław, SW Poland). Single birds were seen in nearly 39% of all 299 encounters, while the largest concentrations, between 16–28 individuals, accounted for 9%. 78% of all birds were observed on meadows flooded with communal wastewater. The remaining ones stayed around irrigation ditches (n = 172, 11.5%), sedimentation basins (n = 88, 5.9%) and reedbeds (n = 72, 4.8%). The mean size of the Water Pipit concentration was largest on the meadows (mean ± SE = 6.54 ± 0.50 individuals) and smallest at the sedimentation basins (mean = 1.44 ± 0.14). In this winter season (December-first half of March), rainfall enlarged numbers of birds to forage on the meadows, and the thickness of the snow cover was positively correlated with bird abundance at the sedimentation basins. The dominant available prey items inhabiting the warm wastewater were Diptera larvae (96%), 88% of which belonged to the genus Eristalis. The mean (± SD) invertebrate biomass was highest in the basin sediments (1.03 ± 1.14 g/dm³ of deposits), and lowest on the flooded meadows (0.20 ± 0.37 g/dm³ of deposits). The results point to the significance of the artificial environmental conditions created by warm sewage water, which enable the birds to remain largely independent of the weather and thus to overwinter in a cold region of central Europe.

Estuaries are dynamic ecosystems which vary widely in loading of the contaminant methylmercury (MeHg), and in environmental factors which control MeHg exposure to the estuarine foodweb. Inputs of organic carbon and rates of primary production are important influences on MeHg loading and bioaccumulation, and are predicted to increase with changes in climate and land use pressures. To further understand these influences on MeHg levels in estuarine biota, we used a field study approach in sites across different temperature regions, and with varying organic carbon levels. In paired comparisons of sites with high vs. low organic carbon, fish had lower MeHg bioaccumulation factors (normalized to water concentrations) in high carbon sites, particularly subsites with large coastal wetlands and large variability in dissolved organic carbon levels in the water column. Across sites, MeHg level in the water column was strongly tied to dissolved organic carbon, and was the major driver of MeHg concentrations in fish and invertebrates. Higher primary productivity (chlorophyll-a) was associated with increased MeHg partitioning to suspended particulates, but not to the biota. These findings suggest that increased inputs of MeHg and loss of wetlands associated with climate change and anthropogenic land use pressure will increase MeHg concentrations in estuarine food webs.

Previous studies conducted in the Elk River watershed showed that selenium concentrations are higher in aquatic biota in lentic compared to lotic habitats of the system having similar water selenium concentrations. Studies have also shown that water selenium concentrations have increased over time (∼10% per year) and recent annual average concentrations have ranged up to 0.044 mg/L in areas downstream from mine discharges. For the present study, trophic transfer of selenium was characterized in lotic versus lentic habitats using concentrations measured in field‐collected samples and assuming a three‐step food chain of water to the base of the food web (biofilm), to benthic invertebrates, and then to westslope cutthroat trout (WCT) ovaries. Food chain models were developed for each habitat type (lotic and lentic) by combining linear regression equations for the three transfer relationships, allowing for prediction of fish ovary concentrations from water concentrations. Greater accumulation of selenium in lentic areas was mostly attributable to greater uptake at the base of the food chain compared to lotic areas. Enrichment/trophic transfer factors for selenium at all levels of the lotic and lentic food chains decreased and then became near constant as exposure concentrations increased. The lotic model predicted little increase in WCT ovary selenium concentrations over an eightfold increase in water concentrations (∼0.005–0.040 mg/L), accounting for the lack of observed increase in within‐area fish tissue concentrations over time despite increasing trends in water concentrations. Environ. Toxicol. Chem. 2012;31:672–680. © 2011 SETAC