Persistence in the longitudinal distribution of lotic insects in a changing climate: a tale of two rivers

The longitudinal distribution of many taxa in rivers is influenced by temperature. Here we took advantage of two older datasets on net-spinning caddisflies (Hydropsychidae) from contrasting European rivers to assess changes in species occurrence and relative abundance along the river by resampling the same sites, postulating that an increase in river temperature over the intervening period should have resulted in cool-adapted species retreating into the headwaters and warm adapted species expanding upstream. Distributional changes in the Welsh Usk were slight between 1968/69 and 2010, one rare species appearing at a single headwater site and one warm-adapted species disappearing from the main river. Distributional changes in the French Loire, between 1989–93 and 2005, were similarly modest, with no consistent movement of species up- or downstream. We estimate that the decadal rate of increase in the mean summer daily maximum in the Usk was only 0.1 °C at one ‘summer cool’ headwater site, while a neighbouring ‘summer warm’ tributary increased by 0.16 °C per decade, and the main river by 0.22 °C. The Loire is warmer than the Usk and the mean decadal rates of increase, over the period 1989–2005, at three sites along the lower reaches were 0.39, 0.48 and 0.77 °C. Increases in stream and river temperature, therefore, were spatially variable and were not associated with consistent upstream movement of species in either of these (very different) rivers. We conclude that either the temperature increases have hitherto been insufficient to affect species distribution or, more speculatively, that it may not be possible for river organisms (that do not respond only to temperature) to move upstream because of a developing spatial mismatch between key habitat characteristics, some of them changing with the climate but others not

Mineral grains in caddisfly pupal cases and streambed sediments: assessing resource use and its limitation across various river types

Typically, lotic caddisflies attach their mineral pupal cases to cobbles in riffles, where rapid flows facilitate respiration but also decrease case-building material availability through erosion. Effects of local grain availability on grain quantities in and architecture of (per capita grain size use) pupal cases should be more important in Resident Construction Workers (RCWs, building immediately before pupation with minerals collected near the pupation location) than in Itinerant Construction Workers (ICWs, building months before pupation with minerals collected distantly from the pupation location). I tested these hypotheses analyzing mineral grain sizes in pupal cases and streambed sediments of cobble habitats in riffles of five running water types (headwater to large river in different regions) at baseflow or exceptional droughts. When pupae were abundant, the data supported both hypotheses at the local scale of samples, as grain size use by RCWs (as a group) but not by ICWs increased across all sites with local grain availability and abundant taxa among the former responded with four types of case-architecture modifications to grain size shortage. The data also supported the idea that at larger scales such as river or habitat types, mineral grains may be a limited resource for caddisflies building pupal cases with them. These findings suggest that water currents in streams or near shores of lakes and oceans that erode finer mineral grains can create conflicts in resource requirements for invertebrates that build with locally occurring finer mineral grains and simultaneously need high oxygen renewal rates and coarse grains for attachment

Hydrodynamic abilities of riverine fish: a functional link between morphology and velocity use

To better understand the effects of perturbations (e.g. global change) or habitat restorations on fish population dynamics, it is crucial to improve the knowledge about strategies of habitat use (especially in terms of velocity use) by fish. Many recent studies accurately describe kinematics or energetic budgets of swimming activities, which are often species-specific and hardly transferable to other species. The main goal of the present study was to revive more general ecomorphological relationships between body shape and strategies of velocity use by highlighting a functional aspect of fish morphology: the hydrodynamic potential. For this purpose, potential relationships between minimum drag coefficients (Cdmin, constant at high Reynolds numbers), velocity use, fish morphology and drag in given flow conditions were investigated. To assess these relationships, dead drag values (i.e. drag values measured on dead, straight individuals) of 27 riverine species (108 individuals in total) common in France were experimentally measured under various flow conditions. These values served to estimate the Cdmin of fish. For pelagic species, Cdmin values were related to both preferred and near-maximum flow velocity used by the fish in nature. Explaining 61% of its variability, Cdmin was described using six morphological variables, which demonstrates the functional link between fish morphology and velocity use. For all studied species, a model explained 94% of drag variability using the Reynolds number of fish and three morphological variables. The link between morphology and drag force at given velocity conditions provides simple elements for modelling fish energetics in the context of physical habitat use. Moreover, the relationships between fish velocity use and their Cdmin open many applied perspectives, such as assessing the species abilities to withstand discharge modulations

Longitudinal zonation of lotic insects in the Bandama River System (Ivory Coast)

The longitudinal zonation of three lotic insect groups, viz. Hydropsychidae (Trichoptera), Philopotamidae (Trichoptera), and the Simulium damnosum complex (Diptera) in the Bandama River System (West Africa) is anomalous when compared to the patterns known from other geographical areas: In general from the highest to the lowest riffles downstream there appears to be no clear species replacement. Instead, there is a steady increase of species numbers due to the occurrence of additional species. These distributions are related to the gradient in the length of the period without flow from the north (temporary streams) to the south (permanent running waters), the consequent lack of well-defined sources and the relatively gentle slopes of the channels

Morphological adaptations of benthic invertebrates to stream flow-an old question studied by means of a new technique (laser Doppler anemometry

Summary. The generally accepted concept that dorsoventral flatness and/or small size of benthic stream invertebrates staying on the surface of the bottom substratum allows a current-sheltered life in the boundary layer (Ambtihl 1959) is checked by means of the new technique of Laser Doppler Anemometry (LDA). With LDA measurement of flow can be done nearly punctually without any mechanical disturbance. Mapping the current velocities around the body of Ecdyonurus ef. venosus (Insecta, Ephemeroptera) and Ancylus fluviatilis (Gastropoda) gives evidence that boundary layer separation occurs above the animals' bodies. Our results indicate that the velocities around the body of benthic stream invertebrates and probably the forces acting on them are much more complicate than is suggested by the currently accepted boundary layer concept

A revised description of the larva of Homilia leucophaea (Rambur 1842) (Trichoptera: Leptoceridae) and comparisons with the known western European Athripsodes larvae

International audienceCo-occurrence of mature larvae and male pharate pupae in benthos samples from the Loire River enabled descriptions of the larva of Homilia leucophaea. Using characters of the head capsule, the labrum, the meso- and metanotum, and the pro- and metathoracic legs, we compared H. leucophaea with larvae of six western European Athripsodes species, including a larval key to these species. The larva of H. leucophaea shares characters with some of the six Athripsodes species. However, without a phylogenetic analysis including larvae, pupae, and adults of Homilia species from the Afrotropical Region, it is currently impossible to decide about a potential synonymy between Athripsodes and Homilia