River-floodplain systems are ecologically and economically important and fulfill a number of ecosystem services, including primary and secondary production, as well as sediment and nutrient storage (Junk et al. 1989; Jardine et al. 2012; Jardine et al. 2015; Pettit et al. 2017; Crook et al. 2019). Floodplains consist of a network of river-adjacent terrestrial habitats, as well as seasonally disconnected lakes and channels that become inundated during river flooding. Lateral connectivity between a river and its floodplain facilitates exchange of nutrients and organic material, as well as fish and other organisms that can move into newly available habitat (Junk et al. 1989; Pettit et al. 2017; Bayley et al. 2018). Macrophytes present in floodplain lakes and channels serve as refugia for macroinvertebrates, which are valued prey items for riverine fishes. The abundance of food sources and reduced water flow also make floodplain systems ideal for fish spawning and juvenile development. Thus, floodplains typically are highly productive and can support lucrative commercial fishing operations (Opperman et al. 2010).
In floodplain systems, basal resources, like aquatic macrophytes and algae, are responsible for carbon fixation and incorporation of inorganic nutrients to upper trophic levels (Wetzel 1964; Campos-Silva et al. 2021; Cazzanelli et al. 2021). Algae, in particular, contribute substantially to production in rivers and lakes (Junk et al. 1989; Thorp and Delong 1994; Doi 2009). Historically, algal primary production was thought to be dominated mostly by phytoplankton (Reynolds 1994; Kalff 2002). However, the role of epiphytic algae has emerged as equally important (Wetzel 1983; Liboriussen and Jeppesen 2006; Adame et al. 2017). In some temperate lakes, attached algae have been observed to account for 50–90% of total lake production, depending on lake depth and surface area (Vadeboncoeur and Steinman 2002; Vesterinen et al. 2015). In river-floodplain systems, the role of epiphytic algae has not been widely studied, but the influx of inorganic nutrients (i.e., nitrate, nitrite, ammonium, phosphorus) onto the floodplain during the flood pulse (Bortolini et al. 2016) provides resources needed for growth and production of photosynthetic algae (Lewis et al. 2000; Ahearn et al. 2006). Attached algae coexist with bacteria and organic material in complex matrices, creating a thin biofilm (i.e., periphyton) layer on submerged surfaces. Periphytic biofilms are the site of carbon and nutrient absorption and cycling (Wetzel 1964; Flemming 1993; Battin et al. 2016) and are sensitive to environmental changes. Alterations to river hydrologic regimes, such as frequency and duration of flooding, can impact periphytic algal abundance, assemblage composition, and production (Agostinho et al. 2004; Agostinho et al. 2008).
Nearly all floodplains in the Northern Hemisphere have been anthropogenically altered (Lewis et al. 2000), mostly for navigation or agricultural purposes or for flood control. Modifications to river-floodplain systems can have deleterious consequences for aquatic productivity and biodiversity. When floodplains become disconnected from their river sources through dam or levee construction, biological and chemical exchange between the river and floodplain is greatly reduced, threatening ecological integrity (Fernandes et al. 2009; Algarte et al. 2016). Isolation from nearby water sources can severely limit organismal dispersal and can even lead to extirpation of sensitive species (Beisner et al. 2006; Shurin et al. 2009). The Yangtze River, for example, has been substantially altered to accommodate rising population needs, and many of its seasonally inundated lakes have been permanently severed from their river connections. These disconnected lakes show a substantial reduction in the diversity of riverine fishes, largely because of reduced access to habitat, complete loss of fluvial environments, and limited access to spawning grounds (Liu and Wang 2010). Jiang et al. (2020) recently studied fish populations in connected and disconnected lakes in the Yangtze River floodplain and found that fish populations in disconnected lakes had lower levels of taxonomic distinctiveness than populations inhabiting lakes with active river connections.
In the Paraná River, Brazil, isolated floodplain lakes had greater environmental heterogeneity and higher levels of dissimilarity in macrophyte composition relative to seasonally connected lakes (Quirino et al. 2019). In addition, the diet of the invertivorous fish Moenkhausia bonita differed among isolated lakes, but not in connected lakes, indicating river connectivity was essential to food dispersal in these aquatic systems (Quirina et al. 2019). River connectivity is important for algal communities as well. In floodplain lakes with active riverine connections, periphyton communities had a higher degree of species richness compared to isolated lakes (Agostinho et al. 2008). Similar results were found for species composition of free-floating algae (Lansac-Toha et al. 2016) as well as zooplankton, which feed on phytoplankton and have the potential to significantly influence plankton assemblage dynamics (Li et al. 2019). In Brazil, phytoplankton richness and diversity were larger in lakes with active river links due to increased exchange of riverine algal species and transfer of nutrients (Bortolini et al. 2016).
The Atchafalaya River is the fifth largest river on the North American continent and is the main distributary of the Mississippi River (Ford and Nyman 2011; Piazza 2014). The Atchafalaya River Basin (ARB) supports a tremendous diversity of terrestrial, semi-aquatic, and aquatic species, thought to be fueled by river flooding events (Rutherford et al. 2001; Colon-Gaud et al. 2004; Troutman et al. 2007). These flood pulses vary annually in degree and magnitude, but will typically inundate floodplain habitats, such as bayous, floodplain lakes, and excavated canals, for periods ranging from weeks to months. This pulse facilitates nutrient and organism exchange and drives the enormous production and biodiversity characteristic of this system, which supports numerous commercial fishing enterprises that generate approximately $17 million in fish and crayfish annually (NOAA 2018). Over the last several decades, the Atchafalaya River and its basin have undergone substantial hydrologic modification. Once over 8,000 km2, the ARB has been constricted to just half of its historic size (Sabo et al. 1999; Piazza 2014). Permanent lakes, bayous, and dredged channels on the Atchafalaya River floodplain support a diverse assemblage of native and exotic macrophytes (Walley 2007), which in turn provide substrate for highly productive periphyton assemblages, as well as the organisms that exploit this rich food source (e.g., Colon-Gaud et al. 2004; Fisher et al. 2012).
In this study, we explored how river flood pulses in this modified system impacted periphytic algal assemblages. We compared periphyton composition in ARB sites with active floodplain connections to a permanently-isolated floodplain lake, Lake Verret (LV). We hypothesized that, relative to sites receiving no annual water inputs from a flood pulse, floodplain sites will: 1) have substantially greater periphyton abundance, 2) exhibit different temporal trends in assemblage composition, and 3) exhibit spatial differences in periphyton composition related to distance from the floodwater source.