The River Continuum Concept (RCC) is a milestone in stream ecology because of its comprehensive evaluation of the structure and function of lotic ecosystems. Linking stream physical and geo-morphological attributes with patterns in biodiversity, functional traits, and metabolism dynamics, this theory describes downstream gradients in community composition and ecosystem processes. The aim of this review is to evaluate how the RCC, 40 years from its publication in the Canadian Journal of Fisheries and Aquatic Sciences, has influenced basic and applied research in stream ecology, focusing on the most important contributions and recent developments. This work puts into perspective the historical significance of the RCC in the scientific process and integrates past and recent theories including metacommunity and metaecosystem theories and the river network perspective to predict taxonomic and functional diversity of benthic communities. Thus, this review provides a unifying overview of the historical context of the field for exploring basic and applied ecological questions to the next generation of stream ecologists.

Emerald ash borer (EAB), has killed millions of ash trees in the United States and Canada, yet impacts on terrestrial-aquatic linkages are largely unknown. Ash tree death along streams creates canopy gaps, increasing light to riparian plants and potentially affecting organic matter subsidies. Six EAB-related canopy gaps along streams across a gradient of timing of EAB invasion in Michigan were characterized for characterized coarse woody debris (CWM), terrestrial and aquatic leaf litter and their associated bacterial communities, and macroinvertebrates upstream, downstream, and at the center of the gap. Stream sites downstream of EAB-related canopy gaps had significantly lower dissolved oxygen and macroinvertebrate diversity than sites upstream and at the gaps. Yet there was no difference in CWM or aquatic leaf litter, likely due to downstream movement of organic matter from upstream riparian sources. Low abundance bacterial amplicon sequence variants unique to gap or forest were detected in leaves and leaf litter, suggesting EAB-related canopy gaps altered leaf associated bacterial communities. Overall, EAB invasion indirectly impacted some variables, while organic matter dynamics were resistant to change.

Salmon decomposition is traditionally viewed through the lens of energy and nutrient subsidies, but not as a potential "microbial subsidy." Microbial communities residing on and within spawning salmon are directly introduced into streams after host death. This incorporation takes the form of microbes sloughing off and integrating into substrate biofilms, or indirectly, by macroinvertebrates facilitating dispersal via consumption. The objective of this study was to determine the effects of salmon carcass-derived microbial communities on stream biofilms and macroinvertebrates during an experimental salmon carcass addition in a naïve stream (i.e., no evolutionary history of salmon). Microbial communities [epilithic biofilms and within macroinvertebrates (internal)] were sampled at treatment and control sites before (September), during (October), and after (November to following August) a salmon carcass subsidy introduction in 2 successive years (September 2014-August 2016). We found a significant interaction between carcass addition and time on microbial and macroinvertebrate communities. Heptagenia (Heptageniidae: grazer) density was five times higher in the salmon reach compared to the control. In the salmon reach during year one, Stramenopiles (i.e., eukaryotic microbes) decreased in biofilm communities after 2 weeks of decomposition. The internal microbiome of Stegopterna mutata (Simuliidae: collector-filterer) varied between years but was significantly different between reaches over time during year two of the study, with four times greater abundance of melanogenesis functional pathways (function determined in silico) in the control reach. Although unique microbial taxa, introduced to this naïve stream via salmon carrion, persisted in biofilms on benthic substrate and internal to insects during both years, those taxa represented <2% of the relative abundance in microbial communities. These results highlight the importance of allochthonous carrion resources in the microbial ecology of lotic biofilms and macroinvertebrates. Furthermore, this study contributes to previous research into the complex interkingdom interactions in stream communities in response to a novel allochthonous resource.

Rivers are among the most vulnerable ecosystems to biological invasions. After introduction, predicting the timing and magnitude of population growth and range expansion is critical to decision making on where management tactics will be most efficient. Yet, management is often hindered by the lack of knowledge on species‐specific attributes and the spatially complex structure of river networks that influences connectivity between sites. We analysed the population structure and predicted the range expansion of signal crayfish (Pacifastacus leniusculus), an invasive species in North‐western Italy which was first recorded in the Valla Stream in 2009 and then spread to adjacent rivers including Erro Stream in 2020. A total of 1,284 individual P. leniusculus crayfish were collected in the Valla Stream. No significant difference was found in density or body condition along a downstream gradient for different sex and maturity classes. These empirical data were combined with the available scientific literature to obtain key life‐history information for P. leniusculus, including carrying capacity, downstream versus upstream dispersion and proliferation rate. We used the OCNet R package to simulate the Erro Stream network and applied a meta‐population model to predict the range expansion of this species over time. Results indicate P. leniusculus may completely invade the Erro Stream network in 30–40 years, depending on the proliferation rate and without additional introductions. These findings represent one of the first attempts to use optimal channel networks simulation in R to predict the meta‐population dynamics of aquatic invasive species, a potential key tool to prevent invasive species spread.

Coastal waters of Lake Superior are generally inhospitable to the establishment of invasive Dreissena spp. mussels (both Dreissena polymorpha and Dreissena bugensis). Dreissena have inhabited the Saint Louis River estuary (SLRE; largest commercial port in the Laurentian Great Lakes) for over three decades, but only in the last few years have small colonies been found in the Apostle Islands National Lakeshore (APIS, an archipelago situated 85 km to the east of SLRE) A 2017 survey determined a low abundance Dreissena spatial distribution in APIS, with the largest colonies on the north and west islands which suggested potential veliger transport from the SLRE via longshore currents. Our objective in this study was to determine if Dreissena veligers are transported by currents at low densities along the south shore of Lake Superior from the SLRE to APIS. To do so, we used both eDNA (water and passive substrate samples) and zooplankton collection methods at eight sites evenly spaced between the SLRE and APIS with three sampling times over five weeks. Dreissena veligers were consistently detected along the south shore, although at low abundances (veligers per m3 range = 0–690, median = 8), and for every 1 km increase in distance from the SLRE, both veliger counts and water eDNA copy numbers decreased on average by 5 and 7%, respectively. D. polymorpha (suited to estuary habitats) was detected two times more than D. bugensis (better suited to deep-lake habitats). There was not a trend in the veliger size distribution along the south shore, and temperature and calcium concentrations fluctuated around the threshold for Dreissena veliger and adult development, averaging 11.0°C and 14.8 ppm, respectively. Three zooplankton taxa representative of the estuary community–Daphnia retrocurva, Diaphanosoma birgei, and Mesocyclops copepodites–decreased as the distance from the SLRE increased mirroring Dreissena veliger abundance patterns. Findings represent multiple sources of evidence of a propagule “conveyor belt” for Dreissena along the south shore of Lake Superior. We conclude that veligers are functioning as a propagule, using coastal currents to spread from the point of invasion, thereby traversing coastal habitat previously reported as inhospitable to distant habitats suitable for colonization.