Effects of hydropeaking on drift, stranding and community composition of macroinvertebrates: a field experimental approach in three regulated Swiss rivers

Hydropeaking operation leads to fluctuations in wetted area between base and peak flow and increases discharge-related hydraulic forces (e.g., flow velocity). These processes promote macroinvertebrate drift and stranding, often affecting benthic abundance and biomass. Our field experimental study – conducted in three hydropeaking-regulated Swiss rivers – aimed to quantify (i) the short-term effects of the combined increase in flow amplitude and up-ramping rate based on macroinvertebrate drift and stranding, as well as (ii) long-term effects based on the established community composition. Hydropeaking led to increased macroinvertebrate drift compared to base flow and to unaffected residual flow reaches. Moreover, stranding of macroinvertebrates was positively related to drift, especially during the up-ramping phase. Flow velocity and up-ramping rate were identified as major determinants for macroinvertebrate drift, while flow ratio and down-ramping rate for stranding. Particularly high sensitivity towards HP was found for Limnephilidae, whereas Heptageniidae seemed to be resistant in respect to short and long-term hydropeaking effects. In the long-term, hydropeaking did not considerably reduce benthic density of most taxa, especially of some highly resistant and resilient taxa such as Chironomidae and Baetidae, which dominated the community composition even though they showed comparably high drift and stranding responses. Therefore, we argue that high passive drift and/or stranding, especially of individual-rich taxa, does not necessarily indicate strong hydropeaking sensitivity. Finally, our results demonstrate the necessity to consider the differences in river-specific morphological complexity and hydropeaking intensity, since these factors strongly influence the community composition and short-term drift and stranding response of macroinvertebrates to hydropower pressure.

Pili allow dominant marine cyanobacteria to avoid sinking and evade predation

How oligotrophic marine cyanobacteria position themselves in the water column is currently unknown. The current paradigm is that these organisms avoid sinking due to their reduced size and passive drift within currents. Here, we show that one in four picocyanobacteria encode a type IV pilus which allows these organisms to increase drag and remain suspended at optimal positions in the water column, as well as evade predation by grazers. The evolution of this sophisticated floatation mechanism in these purely planktonic streamlined microorganisms has important implications for our current understanding of microbial distribution in the oceans and predator–prey interactions which ultimately will need incorporating into future models of marine carbon flux dynamics.

Pili allow dominant marine cyanobacteria to avoid sinking and evade predation

ABSTRACTHow oligotrophic marine cyanobacteria position themselves in the water column is currently unknown. The current paradigm is that these organisms avoid sinking due to their reduced size and passive drift within currents. Here, we show that one in four picocyanobacteria encode a type IV pilus which allows these organisms to increase drag and remain suspended at optimal positions in the water column, as well as evade predation by grazers. The evolution of this sophisticated floatation mechanism in these purely planktonic streamlined microorganisms has profound implications for our current understanding of microbial distribution in the oceans, predator-prey interactions and, ultimately, will influence future models of carbon flux dynamics in the oceans.

High Connectivity of the White Seabream (Diplodus sargus, L. 1758) in the Aegean Sea, Eastern Mediterranean Basin

Population dynamics in the marine realm can shape species’ spatial structure and genetic variability between given geographical areas. Connectivity is an important factor of species’ population structure. In this study, we examined the genetic diversity and structure of white seabream (Diplodus sargus, L. 1758) in the eastern Mediterranean basin, using a panel of four microsatellite markers. Recorded low FST values within the study area indicate little evidence of genetic differentiation among populations. Results suggest high gene flow which may imply near-panmixia between populations, indicating the possibility of a probable movement of adult migrants, or strong passive drift at sea in early life stages of the species. To this extent, bibliographically speaking, different species within the Sparidae family favor altered population dynamics patterns with respect to local populations and genetic divergence, in the context of the molecular marker used.

Influence of the summer deep-sea circulations on passive drifts among the submarine canyons in the northwestern Mediterranean Sea

Marine biophysical models can be used to explore the displacement of individuals in and between submarine canyons. Mostly, the studies focus on the shallow hydrodynamics in or around a single canyon. In the northwestern Mediterranean Sea, the knowledge on the deep-sea circulation and its spatial variability in three contiguous submarine canyons is limited. We used a Lagrangian framework with three-dimensional velocity fields from two hydrodynamics models to study the deep bottom connectivity between submarine canyons and to compare their influences on the particle transport. The particles represented eggs and larvae spawned by the deep-sea commercial shrimp Aristeus antennatus along the continental slope in summer. The passive particles mainly followed a southwest drift along the slope and drifted less than 200 km within 31 days. Two of the sub-marine canyons were connected by more than 27 % particles if they were released at sea bottom depths above 600 m. The vertical displacement of particles was depending on the submarine canyons, the depth and the can-yon wall where particles were released and it encouraged the analyses of the particle transport by canyons in-stead of generalizing the dynamics. In the two hydrodynamic models tested in this study, passive drift simulation differed depending on topography. Despite being run on a coarser grid, the hydrodynamic model using finer bathymetric resolution data and adjusted to the topography seemed to better model the passive drift of particles. Those results promote that the physical model parameterization has to be considered for improving the transport studies of deep-sea species.

First satellite tracks of South Atlantic sea turtle ‘lost years’: seasonal variation in trans-equatorial movement

In the South Atlantic Ocean, few data exist regarding the dispersal of young oceanic sea turtles. We characterized the movements of laboratory-reared yearling loggerhead turtles from Brazilian rookeries using novel telemetry techniques, testing for differences in dispersal during different periods of the sea turtle hatching season that correspond to seasonal changes in ocean currents. Oceanographic drifters deployed alongside satellite-tagged turtles allowed us to explore the mechanisms of dispersal (passive drift or active swimming). Early in the hatching season turtles transited south with strong southward currents. Late in the hatching season, when currents flowed in the opposite direction, turtles uniformly moved northwards across the Equator. However, the movement of individuals differed from what was predicted by surface currents alone. Swimming velocity inferred from track data and an ocean circulation model strongly suggest that turtles' swimming plays a role in maintaining their position within frontal zones seaward of the continental shelf. The long nesting season of adults and behaviour of post-hatchlings exposes young turtles to seasonally varying ocean conditions that lead some individuals further into the South Atlantic and others into the Northern Hemisphere. Such migratory route diversity may ultimately buffer the population against environmental changes or anthropologic threats, fostering population resiliency.

Passive drift or active swimming in marine organisms?

Predictions of organismal movements in a fluid require knowing the fluid's velocity and potential contributions of the organism's behaviour (e.g. swimming or flying). While theoretical aspects of this work are reasonably well-developed, field-based validation is challenging. A much-needed study recently published by Briscoe and colleagues in Proceedings of the Royal Society B compared movements and distribution of satellite-tracked juvenile sea turtles to virtual particles released in a data-assimilating hindcast ocean circulation model. Substantial differences observed between turtles and particles were considered evidence for an important role of active swimming by turtles. However, the experimental design implicitly assumed that transport predictions were insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the model. Here, we show that the magnitude of variation in physical parameters between turtles and virtual particles can profoundly alter transport predictions, potentially sufficient to explain the reported differences without evoking swimming behaviour. We present a more robust method to derive the environmental contributions to individual movements, but caution that resolving the ocean velocities experienced by individual organisms remains a problem for assessing the role of behaviour in organismal movements and population distributions.

Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes

Invertebrate drift, the downstream transport of aquatic invertebrates, is a fundamental ecological process in streams with important management implications for drift-feeding fishes. Despite long-standing interest, many aspects of drift remain poorly understood mechanistically, thereby limiting broader food web applications (e.g., bioenergetics-based habitat models for fish). Here, we review and synthesize drift-related processes, focusing on their underlying causes, consequences for invertebrate populations and broader trophic dynamics, and recent advances in predictive modelling of drift. Improving predictive models requires further resolving the environmental contexts where drift is driven by hydraulics (passive drift) versus behaviour (active drift). We posit this can be qualitatively inferred by hydraulic conditions, diurnal periodicity, and taxa-specific traits. For invertebrate populations, while the paradox of population persistence in the context of downstream loss has been generally resolved with theory, there are still many unanswered questions surrounding the consequences of drift for population dynamics. In a food web context, there is a need to better understand drift-foraging consumer–resource dynamics and to improve modelling of drift fluxes to more realistically assess habitat capacity for drift-feeding fishes.

Evaluating existing movement hypotheses in linear systems using larval stream salamanders

Because of their linear nature, streams provide a restrictive framework to understand the movement ecology of many animals. Stream movements have been characterized under two competing hypotheses. The colonization hypothesis dictates that small individuals experience passive drift, but concurrent, upstream movement by larger individuals replaces the loss of small individuals. Alternatively, the production hypothesis suggests that downstream movements are a consequence of limited resource availability. Previous research suggests that large larvae should move upstream and vice versa for small larvae, which should therefore be found downstream more often. We conducted a mark–recapture study of larval red salamanders ( Pseudotriton ruber (Sonnini de Manoncourt and Latreille, 1801)) to assess the validity of these hypotheses. We found that no larvae exhibited downstream movement (skew = 0.361, p = 0.019; biased upstream), and large larvae were the only size cohort to exhibit directional movement upstream (skew = 0.901, p = 0.035). Contrary to predictions under the colonization hypothesis, small larvae were found upstream more frequently than large larvae (N = 871, H = 16.29, df = 2, p < 0.001). Our results suggest that larval movements are related to abiotic stream conditions, and we conclude that neither hypothesis fully explains stream movement. In the absence of drift, new movement hypotheses are necessary to describe persistent upstream movement in streams. These hypotheses should consider individual causes of movement and the direction of movements that will improve the fitness of the organism.