[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE | Food-chain length (FCL) is commonly used in ecological investigations to gain insight into how ecosystems function. Several studies have investigated the mechanisms underlying FCL patterns, but none has specifically examined the effect of temperature variability. In river ecosystems, water temperature variability can modify community structure, individuals' activity, and individuals' physiological rates, among other things. As such, we expected that it would negatively influence FCL. To test this prediction, we took advantage of a dataset comprising five streams, which mainly differ according to their temperature variability. At each stream, we (i) studied the species composition of macroinvertebrates and fish, and using nitrogen and carbon stable isotopes, (ii) estimated realized FCL, and (iii) examined food web structure. For macroinvertebrates, but not for fish, species composition differed among sites displaying low and high temperature variability. FCL was negatively influenced by temperature variability. Confirming this trend, we found a highly significant linear relationship between FCL and temperature variability using data from the literature. As for food web structure, the trophic position of filter-feeders/shredders may explain the FCL differences among sites. Our study gives additional support to the "dynamic stability" hypothesis and advances a step further by suggesting that temperature variability alone may reduce FCL.

Restoration of degraded freshwater ecosystems has gained considerable attention in the USA over the past decades. However, most projects focus almost entirely on the restoration of physical habitat or specific water quality parameters, while ignoring critical ecological processes related to food web re-establishment. In this study, we investigate the impact of riparian habitat in different stages of restoration on food availability for fish in four streams in Pennsylvania, USA. The riparian buffer habitats ranged from open meadow to mature forest and included new to long-term restoration sites. We quantified abundance and community composition of aquatic macroinvertebrates and riparian arthropods with aerial and ground-dwelling life history strategies. We found that riparian habitat and water temperature exert a strong influence over potential food resources for fish, with the open meadow habitat having highest abundance of terrestrial and aquatic insects, lowest taxa richness, and possible multivoltine aquatic insect life-history. Our results provide insight into the importance of riparian buffer habitat and water temperature on the composition of food availability for fish species of concern such as brook trout. The significant differences emphasize the need to include food web dynamics into riparian habitat restoration design to guide future rehabilitation projects focusing on fish conservation.

Land-water transition areas play an important role in the functioning of both aquatic and terrestrial ecosystems. Enhancing habitat complexity and heterogeneity by restoring or adding land–water transition areas to degraded aquatic ecosystems can be effective management to stimulate productivity by lower trophic levels – and hence increase food availability for biota of conservation interest, including fish and birds. Here, we studied whether hydrological connectivity can be used as an environmental indicator (connected or disconnected) for the development trajectories of newly constructed land–water transition areas in shallow lakes. We capitalized on a large-scale restoration project called “Marker Wadden”, which created new land–water transition areas with and without hydrological connectivity in a degraded shallow lake in the Netherlands (Lake Markermeer). We compared how the new areas with and without hydrological connectivity developed with respect to abiotic parameters and biomasses of benthic, pelagic, and emergent macroinvertebrates. In sites disconnected from the open water, water depths became significantly lower than in hydrologically connected sites during summer, likely due to evaporation. In these shallower waters, daytime temperatures and organic matter content of the sediment were higher, while dissolved oxygen concentrations remained sufficient. Therefore, biomasses of benthic macroinvertebrates and emergent insects became higher in the disconnected sites. These lower trophic levels could provide higher food availability for benthivorous and insectivorous birds, while remaining inaccessible to fish. This puts forward that hydrological connectivity (connected or disconnected) can be used as an environmental indicator for aquatic food web development trajectories, and that it regulates relative food availability for fish and birds. Restoring land–water transition areas without hydrological connectivity provides higher biomasses of lower trophic levels, which are only accessible to birds. Restoring areas with hydrological connectivity results in relatively lower biomasses of invertebrates, but these provide food to birds feeding on invertebrates, and fish and fish-eating birds. Creating areas including both types of land–water transition zones, connected and disconnected to open water can benefit fish and birds of both feeding guilds.

Riparian areas of riverine plains develop extensive floodable areas named riverine wetlands, which are essential to the water cycle balance and ecosystem dynamics. In this study, we contrasted the hydrological and physicochemical variables of riverine wetlands of both peri-urban areas impacted by intensive farming and those of rural areas with the indicators of the biotic structure (taxonomic richness, Shannon diversity and total density) of benthic diatoms, phytoplankton, zooplankton, macroinvertebrates, chironomids, fishes, turtles, and birds. The study was performed on riverine waters of the Pampean plain, Argentina, with four seasonal samplings conducted in 2017–2018. Our results showed that the significant deepening of the groundwater level caused by aquifer overexploitation in peri-urban areas, as well as the declining surface water quality with higher phosphorus and total nitrogen concentrations, affected the taxonomic richness, diversity, and total density of the biotic assemblages of riverine wetlands. The taxonomic richness of birds, turtles, phytoplankton, chironomids, and fishes was the most sensitive to land use. Phytoplankton, chironomid, and fish diversity showed the greatest differences between rural and peri-urban riverine waters, while the total density of chironomids and birds showed the greatest differences according to land use. The results suggest that the socioeconomic development in those riverine wetlands that still maintain conditions close to the natural ones needs to be subject to guidelines derived from integrated basin management and sustainable urban planning.