Andreas Novotny

and 2 more

IntroductionThe ability for ecosystems to maintain productivity and functionality under seasonal and long term changes in resource availability relies on the diversity of functional groups (Cadotte et al. 2011). In marine food webs, functionally diverse assemblages of heterotrophic bacteria, heterotrophic protists and zooplankton transfer the organic matter from primary producers to higher trophic levels (Sommer 1989). Zooplankton regulate the flow of energy and matter in the food web through several mechanisms including grazing, respiration, excretion, and as food to support higher trophic levels (Calbet & Landry 2004; Mitra & Davis 2010; Steinberg & Landry 2017). Variation in temporal abundance, feeding traits, size, phenotypic plasticity, growth rate and predation resistance all contribute to the total diversity of zooplankton functional groups in marine food webs (Petchey & Gaston 2006). A high diversity of functional groups contributes to a large variety of resource use that is crucial for the maintenance of ecosystem services under changing conditions (Cadotte et al. 2011). To generate accurate predictions of vulnerability and estimate the resilience of marine ecosystems, a mechanistic understanding of resource use by zooplankton is needed (Bindoff et al. 2019). However, most trophic studies are based on size or phylogeny, and the complexity of zooplankton interactions is rarely considered in trophic studies because of the lack of detailed information about feeding interactions in nature. Consequently, the functional diversity of the zooplankton community and their ability to exploit similar resources is typically not accurately considered (Mitra et al. 2014).The diversity of zooplankton allows for maintaining the biomass of fish stocks over the seasons by a shift from a phytoplankton to detritus-based food webs at times when the biomass of phytoplankton is low or inedible (D’Alelio et al. 2016). While crustacean zooplankton (e.g. copepods and cladocerans) constitute the primary link between phytoplankton and planktivorous fish (Cushing 1990), microzooplankton (i.e. heterotrophic flagellates, ciliates and rotifers) can at times dominate ocean’s carbon respiration in productive coastal ecosystems(Sherr & Sherr 2002; Calbet & Landry 2004). By utilizing matter recycled by heterotrophic bacteria in the microbial loop (Azamet al. 1983), the microzooplankton serve as an additional link between primary producers and crustacean zooplankton (Gifford 1991). The possibility to switch between alternative food web states may be particularly critical in coastal ecosystems that experience an increase in filamentous cyanobacteria due to climate warming (Paerl & Huisman 2008; Cloern et al. 2016).While most trophic studies have clustered zooplankton into broad phylogenetic groups (Mitra et al. 2014), recent studies suggest that models based on traits, particularly size, reflect the true ecosystem structure more effectively (Sommer & Stibor 2002; Boyceet al. 2015). However, none of these approaches consider the entire functional group diversity of zooplankton. As an example, the rotifer phylum contains members of different size classes (belonging to both the micro- and mesozooplankton) (Arndt 1993), as well as organisms with different feeding behaviors such as micro-filtering feeders (Pourriot 1977), selective feeders (Bogdan et al. 1980; Bogdan & Gilbert 1982; Gilbert & Jack 1993), and in some cases even carnivores (Gilbert 1980). Similarly, copepods and cladocerans can perform different feeding strategies ranging from feeding-current feeding to passive/active ambush feeding (Kiørboe 2011), utilizing a wide spectrum of resources.The difficulty in resolving plankton food webs lies within method limitations. Traditional methods to study plankton food webs, such as grazing dilution techniques (Landry & Hassett 1982), biogeochemical tracers or microscopic observations (Post 2002), may not display its full complexity with enough resolution, and has created a biased knowledge towards larger organisms in the food web that are more frequently studied (Gutiérrez-Rodríguez et al. 2014). Molecular techniques, including DNA sequencing targeting plankton communities, has highlighted the complexity and diversity of plankton interactions on a global scale (Lima-Mendez et al. 2015). Further, DNA metabarcoding of gut content or selected organisms has proven to be a useful tool for resolving trophic interactions (Pompanon et al.2012) and for zooplankton, barcoding of whole organisms can resolve both trophic, parasitic and mutualistic interactions among crustacean zooplankton (De Corte et al. 2017; Zamora-Terol et al.2020). To our knowledge no study has so far aimed to estimate the diversity of functional groups of zooplankton spanning both phylum and size, using targeted DNA metabarcoding.In this study, we aimed to investigate the functional group diversity of the most abundant zooplankton genera in the Baltic Sea, a temperate coastal sea with strong seasonal variability, and where both micro- and mesozooplankton are at times dominating with well-defined abundance peaks (Fig. 1). We hypothesize that diet composition between zooplankton consumers constitutes a more realistic proxy for functional diversity compared to size and phylogeny. By sequencing 18S rRNA and16S rRNA barcoding genes, we analyzed zooplankton-associated prey of selected individuals of different size classes including a ciliate and rotifers, and compared them with the most abundant crustacean zooplankton (copepods and cladocerans). We demonstrate a larger functional group diversity in resource use within zooplankton in the Baltic Sea than previously acknowledged. The functional group diversity goes beyond both phylogenetic diversity and size and is crucial for the understanding of key ecological processes and maintenance of ecosystem functions.