Floating matter: a neglected component of the ecological integrity of rivers

Abstract

Floating matter (FM) is a pivotal, albeit neglected, element along river corridors contributing to their ecological integrity. FM consists of particulate matter of natural (e.g. wood, branches, leaves, seeds) and anthropogenic (e.g. plastic, human waste) origin as well as of organisms that, due to its properties, is able to float on the water surface. In this paper, we provide a comprehensive overview of the FM cycle and the fundamental environmental functions FM provides along rivers. Indeed, FM serves as an important geomorphological agent, a dispersal vector for animals and plant propagules, a habitat, a resource, and a biogeochemical component. Furthermore, we collected data on the amount of FM accumulating at dams and in reservoirs, and related it to key characteristics of the respective catchments. River fragmentation truncates the natural dynamics of FM through its extraction at damming structures, alteration in the flow regime, and low morphological complexity, which may decrease FM retention. Finally, we identify key knowledge gaps in relation to the role FM plays in supporting river integrity, and briefly discuss FM management strategies.

Appendices

Box 1. Floating matter in marine systems

Currently, thousands of tons of natural and anthropogenic material are floating at the surface of oceans and seas (e.g., Thiel et al. 2011; van Sebille et al. 2015). Rivers form key corridors for the transfer of FM from land to sea, including microplastic and natural wood (e.g., Moore et al. 2011; Sadri and Thompson 2014; Steelandt et al. 2015; Hurley et al. 2018; Kooi et al. 2018). For example, the Danube River delivers on average 1533 tons of plastic to the Black Sea per year, and the River Rhone 208 tons to the Mediterranean Sea (Kooi et al. 2018). According to recent calculations, more than 62 million macro-litter items (items of natural and anhropogenic origin > 2 cm in size) are currently floating at the surface of the Mediterranean sea (Suaria and Aliani 2014). Because of these large quantities, the role of FM as a dispersal vector, a habitat, and a resource as well as a potential environmental and socio-economic threat has already received significant attention (Thiel and Gutow 2005a, b; Suaria and Aliani 2014; Thiel et al. 2011).

Rafting on floating objects is a well-known dispersal mechanism in the marine environment (Thiel and Gutow 2005a, b and references therein). More than 1200 species are reported to have used FM for dispersal of up to 1000 km or more (Thiel and Gutow 2005a, b; Schuchert 1935). Consequently, FM facilitates the colonization of islands and larger land masses (Gathorne-Hardy et al. 2000). Censky et al. (1998), for example, described the colonization of the island of Anguilla (Caribbean Sea) by green iguana floating on logs. During transport across the open ocean, even salt-intolerant species such as amphibians are able to survive (Henderson and Hamilton 1995; Schiesari et al. 2003; Measey et al. 2007; Bell et al. 2015). For example, lizards, snakes, and small mammals were observed as far as 1600 km from the mouth of the Amazon and Orinoco Rivers (Schuchert 1935). Such survival rates of terrestrial organisms over large transport distances emphasize the importance of FM for evolutionary processes too (Thiel and Haye 2006).

FM may also support the spreading of nonnative and invasive species (Kiessling et al. 2015), bloom-forming algae (Masó et al. 2003), pathogens (Zettler et al. 2013), and pollutants (Holmes et al. 2012). For marine fish and vertebrates, FM provides a shelter and additional resource, explaining why these organisms often aggregate around floating objects and can disperse over long distances (e.g., Luiz et al. 2012).

Dispersal of marine and freshwater biota can be further facilitated by the increasing amount of anthropogenic FM such as plastic (Barnes and Milner 2005). For example, Kiessling et al. (2015) reported a total of 387 taxa (pro- and eukaryotic microorganisms, seaweeds, and invertebrates) attached to artificial FM in marine environments.

Marine FM is also important for ecosystems after deposition. Deposits of FM along coastal areas (so called “wrack deposits”) are suppliers of food and habitat and can immediately boost abundance and biodiversity of primary and secondary consumers (Spiller et al. 2010; Del Vecchio et al. 2017; Brien et al. 2017). Shore wrack is especially important in hostile areas such as the Arctic region (Lastra et al. 2014).

The role of surface biofilms in seas and oceans has also been recognized with respect to their role in biogeochemical processes, air-sea gas and heat exchange, source and sink of pollutants, and a habitat for distinct assemblages (Zaitsev 1997; Dandonneau et al. 2008; Wurl et al. 2017).

Box 2. FM trapped in reservoirs in relation to catchment characteristics

Dams and reservoirs represent “observational windows” where trapped FM can be monitored with respect to a specific point or period of time. Based on information available in research papers and reports of hydropower companies, we collected data on the amount and composition of FM accumulated behind 31 dams located within catchments of 13 rivers (Table S1, Supplementary Information). Our aim was to estimate whether the amount of FM observed in reservoirs can be explained by available bulk characteristics of their catchments.

Based on the results of multiple linear regressions (for details on methods and statistical analysis see Supplementary Information), we identified that bulk characteristics of the catchment such as size of the catchment area above the reservoir (as far as the next upstream trapping structure), average annual precipitation, ratio of “woodshed” (catchment area to the next upstream dam) to catchment area, percentage of forest cover, and artificial areas within 200 m of the river channel buffer (polygons with a 200 m radius from channel network data) explained around 56.5% of the variation in trapped FM. This indicates that further environmental parameters should be taken into account, e.g., flood magnitude during the time period of wood trapping, position of the flood within the annual hydrograph (e.g., Moulin and Piégay 2004; Steeb et al. 2017), or the lag effect of events that lead to the emission of FM (suggested by Fremier et al. 2010; Seo et al. 2015). We were not able to test the effects of these factors due to the limited information that is currently available. In addition, we analyzed relatively large catchments with a mean catchment size of around 13,000 km², in contrast to Seo et al. (2008) and Rickenmann (1997) who analyzed the amount of accumulated large wood in catchments between 6.2 and 2369.5 km² and between 0.76 and 6273 km² in size. In addition, in contrast to studies that correlate the characteristics of catchments with the amount of large wood only (see recent studies by Steeb et al. 2017; Senter et al. 2017a, b), in our analysis we did such kinds of correlation for the bulk amount of FM. We also suggest that flood magnitude should be considered in relation to the hydraulic capacity of the dams that are present. If the hydraulic capacity of dams located upstream is not exceeded, FM remains trapped and cannot pass downstream (see report by URS Corporation Gomez and Sullivan Engineers 2012). Furthermore, different recruitment processes that lead to the introduction of FM into water bodies are potentially important factors that should be considered (e.g., Diehl 1997; Bradley et al. 2005; Mazzorana et al. 2009, 2011; Meyer and Rimböck 2014; Steeb et al. 2017). However, more detailed case studies are needed to take into account specific recruitment processes, also including smaller spatial scales than those analysed here. Finally, reference conditions for entrapment, particularly time since the last flood, could be incorporated to indicate the potential quantity of FM that accumulates within the floodplain and is delivered to the river.

Glossary

Coarse particulate organic matter (CPOM)

Particulate organic matter larger than 1 mm in diameter with a size range spanning from seeds to entire trees (Fisher and Likens 1973; Turowski et al. 2013)

Floating mats

Buoyant accumulations that include living plant biomass, dead organic material and mineral sediments held together by rhizomes and roots secured by attachment to soils (Azza et al. 2006)

Floating matter (FM)

particulate matter of natural and anthropogenic origin (wood, branches, leaves, seeds, waste) that, due to its properties, is able to float on the water surface

Free floating macrophytes

plants that grow unattached within or upon the water layer (Hasan and Chakrabarti 2009)

Large wood

Pieces of wood larger than 1 m in length and more than 0.1 m in diameter (Montgomery et al. 2003)

Macrolitter

Items of natural and anhropogenic origin > 2 cm in size (Suaria and Aliani 2014)

Neuston

Organisms associated with the air–water interface in aquatic habitats, including small vascular plants and inactive life stages of other organisms (e.g., seeds, spores) (Marshall and Gladyshev 2009)

Small wood

Pieces of wood with a diameter 0.05–0.1 m (Lester et al. 2009)

Surface biofilms

Complex of organic compounds and microorganisms that aggregate at the water–air interface and extend a few micrometers (µm) from the surface into the bulk water (Wotton and Preston 2005)

Wrack

Organic matter washed onto shores (Harris et al. 2014)