Propagule Retention Research Articles

Riverine aquatic plants trap propagules and fine sediment: Implications for ecosystem engineering and management under contrasting land uses

AbstractPlants in streams act as physical ecosystem engineers, both influencing and responding to hydrogeomorphic processes such as fine sediment retention. Instream vegetation may also influence propagule dispersal and retention, shaping plant community dynamics. These plant‐sediment interactions may result in synergistic feedback promoting hydrogeomorphic complexity and biogeomorphic succession. However, the role of aquatic plants (submerged or mostly submerged) in trapping propagules, fine sediment and organic matter in degraded lowland streams is uncertain. In this study, we sampled sediment (≤5 cm depth) from eight streams ranging in land use from rural to urban, including within patches of aquatic vegetation and unvegetated locations. We conducted a propagule bank trial to identify the abundance and diversity of propagules and analysed the particle size and organic matter composition of the samples. A total of 8,365 seedlings from 113 plant species were recorded with a range of hydrological tolerances. Aquatic plants retained 56% more propagules and 32% more species, and marginal vegetation retained 250% more propagules and 48% more species, than open channel locations (the least retentive location). Similar patterns were found for fine sediment and organic matter retention. Propagule bank communities were different across land‐use types but not sampling locations. The trapping effect of aquatic vegetation diminished as catchments became more urbanised. This study provides evidence that aquatic plants retain more propagules and species, and fine sediment and organic matter than vegetation‐free channel locations. Improving aquatic vegetation in streams may be an important early step in restoring hydrogeomorphic complexity and propagule retention, and the facilitation of biogeomorphic succession in degraded streams. Unfortunately, heavily urbanised streams with flashy flow regimes are unlikely to benefit from this function unless catchment‐scale hydrology is addressed.

Avicennia Genetic Diversity and Fine-Scaled Structure Influenced by Coastal Proximity of Mangrove Fragments

Avicennia dominated mangrove forests occur from seaward to landward sites and hence are subject to different dynamics within estuarine ecosystems. Regeneration of mangrove forests primarily depends on the extent of propagule spread and subsequent establishment in suitable habitats. The complex nature of estuarine systems induces a wide variety of local conditions for within-site propagule retention and settlement thereby allowing spontaneous regeneration of mangroves. In this study, we estimated the fine-scale spatial genetic structure (FSGS) of Avicennia populations and examined whether their position relative to the seaside or the size of mangrove patches could have influenced the extant local population genetic structure. A kinship-based FSGS was performed using microsatellite markers in 523 A. marina, 189 A. rumphiana and 60 A. alba adult trees of 24 sites in The Philippines. Transects within each estuary were taken both parallel and perpendicular to the coastline or tidal river edge. The extent of local mangrove areas and various human-induced encroachments as such did not show any trend in allele diversity, heterozygosity values or inbreeding levels. However, farther inland situated mangrove patches showed a larger FSGS extent across the neighborhood (up to 75 m) though less diversity along with inbreeding, most likely due to retention of related propagules and lowered chance of external propagule input. Estimation of connectivity along a same coastline stretch supported a unidirectional steppingstone or adjacent migration model for populations of either A. marina, A. alba or A. rumphiana. These were congruent with ocean currents across mangrove estuaries of the Tablas Strait and along Western Leyte, thereby emphasizing the relevance of coastal connectivity for long term persistence. From this study, we conclude that both proximity to open water and narrowness of mangrove patches may affect their captured diversity, inbreeding and fine-scale structure caused by propagule movement within or beyond a local mangrove fragment during recent generations. Higher levels of allele diversity for seaward sites and highest likelihood of migration for adjacent mangroves both add to the importance of coastal connectivity that is the only natural cohesive force on longer term and necessary to counteract short term effects of increasingly encroached mangrove environments.

Coastal Landform Constrains Dispersal in Mangroves

Mangrove forests are dynamic ecosystems found along low-lying coastal plains along tropical, subtropical, and some warm-temperate coasts, predominantly on tidal flats fringing deltas, estuaries, bays, and oceanic atolls. These landforms present varied hydrodynamic and geomorphological settings for mangroves to persist and could influence the extent of within-site propagule transport and subsequent local regeneration. In this study, we examined how different landform characteristics may influence local genetic diversity, kinship, and neighborhood structure of mangrove populations. To do so, we considered independent populations of Avicennia marina, one of the most abundant and widespread mangrove species, located in estuarine and coastal bay environments spread across the Western Indian Ocean region. A transect approach was considered to estimate kinship-based fine-scale spatial genetic structure using 15 polymorphic microsatellite markers in 475 adult A. marina trees from 14 populations. Elevated kinship values and significant fine-scale structure up to 30, 60, or 90 m distances were detected in sheltered systems void of river discharge, suggesting a setting suitable for very local propagule retention and establishment within a neighborhood. Slopes of a linear regression over restricted distance within 150 m were significantly declining in each sheltered transect. Contrastingly, such a spatial structure has not been detected for A. marina transects bordering rivers in the estuarine systems considered, or alongside partially sheltered creeks, suggesting that recruitment here is governed by unrelated carried-away mixed-origin propagules. South African populations showed strong inbreeding levels. In general, we have shown that A. marina populations can locally experience different modes of propagule movement, explained from their position in different coastal landforms. Thus, the resilience of mangroves through natural regeneration is achieved by different responses in coastal landforms characterized by different hydrodynamic conditions, which can be important information for their management and protection within the variety of coastal environments.

Negative indirect effects of hurricanes on recruitment of range-expanding mangroves

Disturbances often have positive, direct effects on invasions by dispersing propagules or creating environmental conditions that favor invasive species. However, disturbances that alter interactions between resident and invading species could also affect invasion success. In northeast Florida, the black mangrove Avicennia germinans is expanding into salt marshes, where it interacts with the dead litter (wrack) of the native marsh cordgrass Spartina alterniflora. From 2015-2017, we performed monthly surveys before and after 2 hurricanes in 3 marsh microhabitats (bare sediment, vegetation, wrack) to quantify mangrove propagule and seedling densities. Wrack increased propagule retention up to 10 times relative to other microhabitats. Hurricanes did not directly harm mangrove propagules or seedlings. However, storm surge relocated wrack to upland environments, which indirectly inhibited mangroves by temporarily disrupting the facilitative effects of wrack on propagule recruitment and exposing intertidal bare patches that decreased propagule retention and seedling establishment. Wrack remained absent from intertidal areas for 1-3 mo. Because hurricane season overlaps with propagule recruitment, hurricane timing and wrack return time to intertidal areas influence the degree that hurricanes disrupt wrack-mangrove interactions. We demonstrate that large-scale disturbances can negatively and indirectly affect invader recruitment by altering interactions with resident species.

Flow-divergence feedbacks control propagule retention by in-stream vegetation: the importance of spatial patterns for facilitation

Facilitation (enhancement of propagule retention in this case) is increasingly recognized as an important driver of biodiversity, but it is still unknown if facilitation during dispersal and colonization is affected by self-organized spatial pattern formation. We investigated the ability of in-stream submerged macrophyte patches to trap the vegetative propagules of three species (Berula erecta, Groenlandia densa, Elodea nuttallii in two size classes: 13–22 and 40–48 cm long), and to potentially benefit the colonization of these three species. We tested the effects of propagule traits, hydrodynamic forcing, and spatial patch configuration on propagule trapping. Propagule buoyancy was negatively correlated with trapping chance, while propagule size did not influence trapping. Species-specific differences in buoyancy were maintained for weeks after fragmentation. Propagule retention was interactive and conditional upon the interplay between incoming flow velocities and vegetation spatial patterning. In the flume experiment at low flows, a patchy configuration (one patch filling 66% of the flume width) retained more surface-drifting propagules (B. erecta, G. densa), than near-homogeneous cover (two patches close together, filling the entire flume width). In contrast, retention of sinking E. nuttallii propagules increased in the two-patch configurations. In flume and field releases where patches did not completely fill the channel width, water flowed around the patches rather than over or through them. This resulted in low-flow velocity areas within patches where canopies were upright and propagules were retained, and higher velocity flows around patches. In contrast, when vegetation filled the channel width, water could not be diverted laterally around the patches and preferentially flowed over them, causing the canopies to bend and reduce their trapping capacity. In flume experiments at high flows, retention of all species decreased, regardless of vegetation configuration, as propagules passed over the reconfigured vegetation canopies. These findings on the interplay of water movement and patch reconfiguration suggest that environmental heterogeneity generated by the self-organizing behavior of aquatic plants might enhance colonization of sessile organisms, calling for landscape-scale processes like dispersal to be better investigated.

Modelling giant red shrimp larval dispersal in the Sardinian seas: density and connectivity scenarios

AbstractThe connectivity and three‐dimensional (3D) dispersion of the larvae of giant red shrimp, potentially released from known spawning areas along the Sardinia slope in the western Mediterranean Sea, were assessed using Lagrangian simulations forced by a 3D submesoscale permitting a regional ocean model. Biophysical simulations using the hydrodynamic conditions of the year 2012 were run to track propagules released from known spawning areas during the spawning period (May to September). Passive transport (PT) and vertical migration (VM) scenarios were tested, each with two possible pelagic larval durations (PLDs) of 21 or 42 days. Dispersion of propagules in the PT and VM scenarios differed in terms of travelled distance, export out of the domain (larger for VM), and depth distribution (shallower and bimodal for VM due to the larger variability of encountered currents). Connectivity patterns were investigated among eight release areas, and four predetermined Eco‐Regions, and results showed strong connectivity among the North‐Western (NW), Western (W), and Southern (S) regions of Sardinia, whereas the Eastern region was more segregated. Differences in connectivity patterns among scenarios were related mainly to the tendency of greater retention of propagules in the release area for the PT scenarios. This finding, together with existing hypotheses of vertical migration likely occurring during first egg‐larval phases, suggest that the VM scenarios are the most probable of the two hypotheses tested. Strong connectivity between the W and S sides of Sardinia and the relative isolation of the E side could have significant implications for the protection of this important resource.

Processes and factors that affect regeneration and establishment of the invasive aquatic plant Cabomba caroliniana

Many invasive macrophytes reproduce through fragments that are transported actively or passively to new habitats. On arrival, regeneration of fragments and colonisation of the new environment are critical steps in the invasion process. The effects of environmental and plant factors that potentially affect regeneration and colonisation success of Cabomba caroliniana A. Gray (Cabombaceae; cabomba, fanwort) were studied. C. caroliniana fragments had a very high regeneration potential that was not affected by nutrient availability. However, larger and apical fragments had higher establishment rate and regenerative growth. The coarseness of the substrate did not have any effect on establishment of C. caroliniana. Overall, even a single-node fragment had 50% establishment success when planted in substrate and this increased to 100% for fragments with two or more nodes. In contrast, free-floating propagules had only a 1–30% chance of colonisation, depending on the size of the fragment. It appears that propagule pressure is of less importance for the colonisation of C. caroliniana than the process of physical establishment in the substrate which acts as a bottleneck. Future work should address the specific processes that determine retention and establishment of propagules in the sediment.

Optimal methods for fitting probability distributions to propagule retention time in studies of zoochorous dispersal.

BackgroundPropagule retention time is a key factor in determining propagule dispersal distance and the shape of “seed shadows”. Propagules dispersed by animal vectors are either ingested and retained in the gut until defecation or attached externally to the body until detachment. Retention time is a continuous variable, but it is commonly measured at discrete time points, according to pre-established sampling time-intervals. Although parametric continuous distributions have been widely fitted to these interval-censored data, the performance of different fitting methods has not been evaluated. To investigate the performance of five different fitting methods, we fitted parametric probability distributions to typical discretized retention-time data with known distribution using as data-points either the lower, mid or upper bounds of sampling intervals, as well as the cumulative distribution of observed values (using either maximum likelihood or non-linear least squares for parameter estimation); then compared the estimated and original distributions to assess the accuracy of each method. We also assessed the robustness of these methods to variations in the sampling procedure (sample size and length of sampling time-intervals).ResultsFittings to the cumulative distribution performed better for all types of parametric distributions (lognormal, gamma and Weibull distributions) and were more robust to variations in sample size and sampling time-intervals. These estimated distributions had negligible deviations of up to 0.045 in cumulative probability of retention times (according to the Kolmogorov–Smirnov statistic) in relation to original distributions from which propagule retention time was simulated, supporting the overall accuracy of this fitting method. In contrast, fitting the sampling-interval bounds resulted in greater deviations that ranged from 0.058 to 0.273 in cumulative probability of retention times, which may introduce considerable biases in parameter estimates.ConclusionsWe recommend the use of cumulative probability to fit parametric probability distributions to propagule retention time, specifically using maximum likelihood for parameter estimation. Furthermore, the experimental design for an optimal characterization of unimodal propagule retention time should contemplate at least 500 recovered propagules and sampling time-intervals not larger than the time peak of propagule retrieval, except in the tail of the distribution where broader sampling time-intervals may also produce accurate fits.

Morphological variability in propagules of a desert annual as a function of rainfall patterns at different temporal and spatial scales

Summary Organisms living in highly variable environments have to display integrated strategies to deal with both systematic and random variation occurring at different temporal and spatial scales. Two predictions were tested by analysing geographic‐scale patterns of seed size and seed retention (serotiny) in Chorizanthe rigida, a strict winter desert annual that delays seed dispersal and releases propagules after rainfall events: (i) adaptation to systematic environmental cues occurs by means of changes in morphology, and (ii) within‐individual variation in seed size allows a differential response to rainfall cues: while some seeds germinate rapidly others are retained for future rainfall events. We quantified morphological variation and performed germination experiments on C. rigida propagules (involucres + achenes) from six populations distributed throughout the Mojave and Sonoran deserts covering: (i) a systematic, west‐to‐east, winter‐to‐bi‐seasonal (summer and winter) precipitation gradient and (ii) a winter‐rain unpredictability gradient inferred from long‐term climatic data. The propagule retention structure (i.e. base area of the pedicel) of C. rigida individuals experiencing bi‐seasonal rainfall is double the size of those that have evolved under a strict winter rainfall regime, showing that populations living in bi‐seasonal environments have higher seed retention, which allows them to avoid releasing seeds to a summer rainfall cue. Within‐individual variance of propagule size varied significantly between populations and was correlated with winter rainfall variability in each site. Germination varied as a function of propagule size; smaller seeds germinated more readily than larger seeds. Increased variability in propagule size might result in a more variable germination response. Under common experimental conditions germination varied significantly among sites and was negatively correlated with mean winter effective precipitation, suggesting that propagules from populations in drier sites have lower germination moisture thresholds. Synthesis. C. rigida propagules have larger bases in deserts with biseasonal rainfall, which allows them to avoid seed release during summer rainfall cues, and display within‐individual seed variance associated to rainfall unpredictability, a trait often interpreted as a bet‐hedging strategy. Our study provides empirical evidence of an integrated strategy that allows to cope with both random and systematic rainfall variation.

Impact of landscape structure on propagule dispersal in mangrove forests

Although many riparian and semi-aquatic plant species disperse via water currents, little is known about how this process interacts with the landscape matrix. In mangroves, the dense aerial root network could act as a strong dispersal barrier for the morphologically diverse propagules found in these trees. In this study, we combined field and laboratory experiments to test the effect of root density, propagule morphology and hydrodynamic variables on retention rates and trajectories of the propagules of 4 common species. Overall, flume experiments showed that larger propagules were more frequently retained than smaller ones. For the larger propagules, retention rates increased with increasing obstacle density in the landscape matrix. In elongated propagules, intraspecific variation was linked to floating orientation. Experimental wave action and increased water flow velocity reduced retention. Dispersal in the field was constrained by major tidal currents and experiments confirmed less retention of smaller propagules, which moved farther than larger ones. Overall, our results reveal that the pronounced morphological variation in mangrove propagules interacts with the landscape matrix, contributing to strong differences in dispersal capacity among species and morphotypes. These results may help to explain observed mangrove distribution patterns, including zonation at local, regional and global scales. Additionally, given that many mangrove biotopes are currently strongly threatened by human pressure and fragmentation, this information is important as an input variable for dispersal models that aim to predict dispersal patterns at multiple scales and species responses to environmental change.

Woodland Caribou Alpine Range Restoration: An Application for Lichen Transplants

Woodland caribou, a Schedule 1 ‘threatened’ species under the Canadian Federal Species at Risk Act, use alpine ecosystems in the mountainous South Peace region of British Columbia for core winter habitat, in part because they are able to crater for terrestrial lichen beneath the relatively thin snow pack on the windswept mountain peaks. A number of proposed and operating open pit coal mine developments will create disturbances within these alpine ecosystems. An Implementation Plan for the Ongoing Management of the South Peace Northern Caribou has been created by the province of B.C. to direct actions for South Peace region caribou population recovery, and part of the Mitigation and Monitoring Plan that must be created for disturbances in caribou core habitat is a habitat restoration plan. A component of the alpine habitat that must be restored is terrestrial lichen that woodland caribou rely on for winter forage; however, published studies about the restoration of terrestrial lichens are scarce. The majority of studies obtainable were small-scale trials or exploratory studies in reindeer range in northern European boreal forest, or in forested and alpine ecosystems in B.C. This paper reviews the literature on the feasibility of transplanting terrestrial lichen, and interprets the results for application in alpine caribou habitat restoration. Three main areas where further research is required to develop reindeer lichen transplantation techniques are suggested: methods to promote lichen propagule retention on reclaimed sites; investigating sources and collection methods for lichen propagules; and techniques for storage of viable lichen propagules.

The role of large wood in retaining fine sediment, organic matter and plant propagules in a small, single-thread forest river

This paper investigates associations among large wood accumulations, retained sediment, and organic matter and the establishment of a viable propagule bank within a forested reach of a lowland river, the Highland Water, UK.A wood survey within the 2-km study reach, illustrates that the quantity of wood retained within the channel is typical of relatively unmanaged river channels bordered by deciduous woodland and that the wood accumulations (jams) that are present are well developed, typically spanning the river channel and comprised of wood that is well decayed.Sediment samples were obtained in a stratified random design focusing on nine subreaches within which samples were aggregated from five different types of sampling location. Two of these locations were wood-associated (within and on bank faces immediately adjacent to wood jams), and the other three locations represented the broader river environment (gravel bars, bank faces, floodplain). The samples were analysed to establish their calibre, organic, and viable plant propagule content.The gravel bar sampling locations retained significantly coarser sediment containing a lower proportion of organic matter and viable propagules than the other four sampling locations. The two wood-related sampling locations retained sediment of intermediate calibre between the gravel bar and the bank–floodplain samples but they retained significantly more organic matter and viable propagules than were found in the other three sampling locations. In particular, the jam bank samples (areas of sediment accumulation against bank faces adjacent to wood jams) contained the highest number of propagules and the largest number of propagule species.These results suggest that retention of propagules, organic matter and relatively fine sediment in and around wood jams has the potential to support vegetation regeneration, further sediment retention, and as a consequence, landform development within woodland streams, although this process is arrested by grazing at the study site. These results also suggest that self-restoration using wood is a potentially cost-effective and far-reaching river restoration strategy but that its full effects develop gradually and require the establishment of a functioning wood budget coupled with grazing levels that are in balance with vegetation growth.

Recruitment ecology of pelagic‐broadcast spawning minnows: paradigms from the ocean advance science and conservation of an imperilled freshwater fauna

AbstractPelagic‐broadcast spawning is a dominant mode of fish reproduction in the oceans and is associated with Fundamental Triad (i.e. nutrient enrichment, nutrient concentration, propagule retention) and Loophole (i.e. disruption of larval competition and predation) processes that are mediated by water currents. Pelagic‐broadcast spawning is uncommon in freshwater, but is employed by an evolutionarily convergent spawning guild in rivers on the Great Plains, North America. We reviewed ecological studies of pelagic‐broadcast spawning minnows to evaluate whether Fundamental Triad and Loophole concepts explain the adaptive significance of this behaviour. Pelagic‐broadcast spawning minnows exhibit spawning periodicity dependent on floods. Nutrient enrichment, nutrient concentration and propagule retention are enhanced during flood recession. Retention is also enhanced by high fecundity and rapid development of pelagic‐broadcast propagules. Predation and competition threats are relatively low in temporary, patchily distributed nurseries present during flood recession. Extended spawning periods and diffusive spread of juveniles and adults ensure annual recruitment despite the unpredictable and often harsh conditions in rivers on the plains. Over the last century, pelagic‐broadcast spawners have suffered declines due to dewatering, flood control and sediment deprivation that disrupt natural Fundamental Triad and Loophole processes. We propose that restoration of conditions necessary for Fundamental Triad and Loophole processes to operate will be most successful at stopping decline and extinction of pelagic‐broadcast spawning minnows of the Great Plains.

Assessing Dispersal Patterns of Fish Propagules from an Effective Mediterranean Marine Protected Area

Successfully enforced marine protected areas (MPAs) have been widely demonstrated to allow, within their boundaries, the recovery of exploited species and beyond their boundaries, the spillover of juvenile and adult fish. Little evidence is available about the so-called ‘recruitment subsidy’, the augmented production of propagules (i.e. eggs and larvae) due to the increased abundance of large-sized spawners hosted within effective MPAs. Once emitted, propagules can be locally retained and/or exported elsewhere. Patterns of propagule retention and/or export from MPAs have been little investigated, especially in the Mediterranean. This study investigated the potential for propagule production and retention/export from a Mediterranean MPA (Torre Guaceto, SW Adriatic Sea) using the white sea bream, Diplodus sargus sargus, as a model species. A multidisciplinary approach was used combining 1) spatial distribution patterns of individuals (post-settlers and adults) assessed through visual census within Torre Guaceto MPA and in northern and southern unprotected areas, 2) Lagrangian simulations of dispersal based on an oceanographic model of the region and data on early life-history traits of the species (spawning date, pelagic larval duration) and 3) a preliminary genetic study using microsatellite loci. Results show that the MPA hosts higher densities of larger-sized spawners than outside areas, potentially guaranteeing higher propagule production. Model simulations and field observation suggest that larval retention within and long-distance dispersal across MPA boundaries allow the replenishment of the MPA and of exploited populations up to 100 km down-current (southward) from the MPA. This pattern partially agrees with the high genetic homogeneity found in the entire study area (no differences in genetic composition and diversity indices), suggesting a high gene flow. By contributing to a better understanding of propagule dispersal patterns, these findings provide crucial information for the design of MPAs and MPA networks effective to replenish fish stocks and enhance fisheries in unprotected areas.

Tidal events and salt‐marsh structure influence black mangrove (Avicennia germinans) recruitment across an ecotone

Field experiments were conducted at a black mangrove-salt-marsh ecotone in southwest Florida (U.S.A.) to investigate retention of propagules of the black mangrove, Avicennia germinans, by salt-marsh plants as a mechanism of facilitation operating on recruitment success at landward boundaries. Buoyant A. germinans propagules are dispersed by tides, and stranding is required for establishment; therefore, processes that enable stranding should facilitate mangrove recruitment. We expected the physical structure of salt-marsh vegetation to define propagule retention capacity, and we predicted that salt-marsh plants with distinct growth forms would differentially retain propagules. Experimental monoculture plots (1 m2) of salt-marsh plants with different growth forms (Sporobolus virginicus [grass], Sesuvium portulacastrum [succulent forb], and Batis maritima [succulent scrub]) were created, and A. germinans propagules were emplaced into these plots and monitored over time. For comparison, propagules were also placed into natural polyculture plots (1 m2). Polyculture plots contained at least two of the salt-marsh plant taxa selected for monoculture treatments, and S. virginicus was always present within these polyculture plots. Natural polyculture plots retained 59.3% +/- 11.0% (mean +/- SE) of emplaced propagules. Monocultures varied in their propagule retention capacities with plots of S. virginicus retaining on average 65.7% +/- 11.5% of transplanted propagules compared to 7.2% +/- 1.8% by B. maritima and 5.0% +/- 1.9% by S. portulacastrum. Plots containing S. virginicus retained a significantly greater percentage of emplaced propagules relative to the two succulent salt-marsh taxa. Furthermore, propagule entrapment, across all treatments, was strongly correlated with salt-marsh structure (r2 = 0.6253, P = 0.00001), which was estimated using an indirect quantitative metric (lateral obstruction) calculated from digital images of plots. Overall, our findings imply that entrapment of propagules by salt-marsh plants may be facilitative if propagules are dispersed beyond the established tree line by spring or storm tides, and that facilitation may be sustained over time. We conclude that salt-marsh ecotone permeability may modulate landward encroachment by A. germinans, and that interactions among the early life history stages of black mangroves and neighboring plants may direct community responses to climate change.

Effects of stream restoration on dispersal of plant propagules

Summary 1. Species immigration is vital for the success of restoring degraded ecosystems, but the effectiveness of enhancing dispersal following restoration is seldom evaluated. Running water is an important vector for plant dispersal. Frequency and duration of floods and channel-network complexity are important factors influencing propagule dispersal. In Sweden, these functions have been modified by channelization to facilitate timber floating, thus hampering emigration and immigration of riparian propagules. 2. During the last 10‐20 years, affected watercourses have been restored by removing barriers and replacing boulders into channels. This is hypothesized to facilitate retention of water-dispersed propagules. We studied the efficiency of propagule retention following restoration by releasing propagule mimics and by placing propagule traps in the riparian zone. 3. Retention of propagule mimics was highest in sites restored with boulders and large wood. Retention occurred at both high and low flows but was most efficient during low flows when mimics were trapped by boulders and wood. Waterborne propagules ending up at such sites are unlikely to establish unless they can reach the riparian zone later. At high flows, floating propagules are more likely to reach riparian areas suitable for establishment. According to propagule traps placed at various levels of the riparian zone, deposition of plant propagules and sediments did not increase in restored sites. 4. Synthesis and applications. Our study not only demonstrates that restoration of channel complexity through replacement of boulders and wood can enhance retention of plant propagules, but also it highlights the importance of understanding how restoration effects vary with flow. Most streams are restored to function optimally during median or average flows, whereas communities often are controlled by ecological processes acting during extreme flow events. We advocate that stream restoration should be designed for optimal function during those discharges under which the ecological processes in question are most important, which in this case is, during high flow.

Dispersal and colonisation of plants in lowland streams: success rates and bottlenecks

Plant dispersal and colonisation, including rates of dispersal, retention, colonisation and survival of dispersed propagules (shoots and seeds), were studied in a 300-m stream reach in a macrophyte-rich lowland stream during one growing season. Relationships between colonisation processes and simple flow parameters were tested. Each fortnight during a growing season, the number of dispersed plant propagules and the number of new and lost plant colonisations since the last sampling day were recorded. The retention of dispersing shoots was tested on two occasions during the growing season by releasing plant shoots and subsequently re-collecting the retained shoots in the study reach. The main bottleneck for plant colonisation in macrophyte-rich lowland streams is the primary colonisation (development of attached roots in the sediment from vegetative propagules or seedlings) of retained shoots and seeds, due in part to low retention success (1% of the dispersed shoots per 100-m reach) and to unsuccessful colonisation of retained shoots (3.4% of retained shoots colonised). The number of drifting shoots and seeds per day during the growing season were 650–6,950 and 2,970–62,780, respectively, and caused no constraint to colonisation. The survival rate of primary colonists was high with 80% surviving during the first growing season and about 50% surviving the first winter. There was no relationship between number of drifting shoots and flow, but the number of drifting seeds increased with flow. The number of colonisations between two consecutive sampling days correlated to the extent of low flow in the period. The loss rate of colonisations correlated to high flow events, but was low overall. My study strongly indicates that the number of propagules is not a constraint to colonisation in macrophyte-rich lowland streams. Rather, I conclude that primary colonisation is the main constraint to regaining vegetation in lowland streams in general and in vegetation-free rehabilitated streams in particular. Therefore, if plant colonisation is a target for stream rehabilitation, it is important to enhance retention and colonisation of propagules by creating areas of low flow and by providing physical obstacles to work as retention agents in the stream.

Dispersal of plant fragments in small streams

Summary1. Streams are subject to frequent natural and anthropogenic disturbances that cause sediment erosion and loss of submerged vegetation. This loss makes downstream transport and retention of vegetative propagules on the streambed very important for re‐establishing vegetation cover. We measured dispersal and retention of macrophyte stem fragments (15–20 cm long) along 300 m long reaches of four small to medium sized Danish lowland streams.2. The number of drifting stem fragments declined exponentially with distance below the point of release. This finding makes the retention coefficient (k, m−1) in the exponential equation a suitable measure for comparisons among different macrophyte species, and between stream reaches of different hydrology and vegetation cover.3. Buoyancy of macrophyte tissue influenced retention.Elodea canadensisstems drifted below the water surface, and were more inclined to be retained in deeper water associated with submerged plants and obstacles in the streambed.Ranunculus peltatusstems were more buoyant, drifted at the water surface, and were more inclined to be trapped in shallow water and in riparian vegetation.4. The retention coefficient of drifting stems increased with the relative contact between the flowing water and streambed, bank and vegetation. Thus, the retention coefficients were highest (0.02–0.12 m−1) in shallow reaches with a narrow, vegetation‐free flow channel. Here there were no significant differences betweenE. canadensisandR. peltatus. Retention coefficients were lowest (0.0005–0.0135 m−1) in deeper reaches with wider vegetation‐free flow channels. Retention ofE. canadensiswas up to 16 times more likely than retention ofR. peltatus.5. Overall, the longitudinal position in the stream system of source populations of species capable of producing numerous stems, the species‐specific retention coefficients of stems, and the retention capacity of stream reaches should be important for species distribution in perturbed stream systems. Retention of stems is probably constrained in headwaters by the small downstream flux of stem fragments because of the restricted source area, and constrained in downstream reaches by small retention coefficients. Macrophyte retention may, consequently, peak in medium‐sized streams.

Waterbirds as endozoochorous dispersers of aquatic organisms: a review of experimental evidence

It is commonly assumed that waterbirds act as dispersal vectors of aquatic organisms. In this paper we review experimental work focusing on the endozoochorous transport of propagules by waterbirds with the aim to determine what aspects of this mechanism have been investigated. We discuss (i) the main issues addressed in propagule feeding experiments, (ii) aspects of the gut structure that affect the retrieval, retention time and viability of propagules, and (iii) the importance of assessing propagule retention time in the gut. A total of 26 experimental studies exist, which represent a very low number when compared to studies of seed dispersal in terrestrial systems. The diversity of questions dealt with in these articles was low, with the majority focusing exclusively on whether propagules survive gut passage. More comparative studies involving batches of disperser and dispersed species should be carried out, also to establish whether they possess characteristics that favour endozoochorous transport. It is important to assess the effect of intra- and inter-specific variation in the different sections of the waterbird gut on the fate of propagules. Additionally, experimental work dealing with the mechanism of endozoochorous dispersal can be separated into a sequence of events. Each event is characterised by its own probability of occurrence. In most studies, complete data sets relating to every step of the process have not been collected. In order to quantify dispersal probabilities of propagules such data are necessary.

The influence of temperature on advective loss of Atlantic cod (Gadus morhua) eggs from the inshore environment

We use a simple model of temperature‐dependent egg development and mortality to develop several hypotheses concerning the effect of temperature on the occurrence of eggs of Atlantic cod, Gadus morhua, in Placentia Bay, Newfoundland. Predictions from this exploratory model were tested through a series of synoptic ichthyoplankton surveys throughout spawning and postspawning periods during 1997 and 1998. Although several egg mortality relationships were explored through the simulation, a constant mortality rate best represented the pattern observed in the two years of data. Peaks in late stage egg densities occurred in August of both 1997 and 1998 and were apparently decoupled from egg production peaks in April. We observed a decrease in mortality and the distance dispersed during egg development with increases in water temperature. We suggest that the effects of predation are small relative to the advective effects within this system, and that the interaction between advection and temperature‐dependent vital rates of eggs may have dramatic consequences for coastal retention of propagules produced by inshore spawning events.