**Silvia Bianchelli 1,\*, Daniele Nizzoli 2, Marco Bartoli 2, Pierluigi Viaroli 2, Eugenio Rastelli <sup>3</sup> and Antonio Pusceddu <sup>4</sup>**


Received: 16 April 2020; Accepted: 6 May 2020; Published: 12 May 2020

**Abstract:** In benthic ecosystems, organic matter (OM), prokaryotes, and meiofauna represent a functional bottleneck in the energy transfer towards higher trophic levels and all respond to a variety of natural and anthropogenic disturbances. The relationships between OM and the different components of benthic communities are influenced by multiple environmental variables, which can vary across different habitats. However, analyses of these relationships have mostly been conducted by considering the different habitats separately, even though freshwater, transitional, and marine ecosystems, physically linked to each other, are not worlds apart. Here, we investigated the quantity and nutritional quality of sedimentary OM, along with the prokaryotic and meiofauna abundance, biomass, and biodiversity, in two sampling periods, corresponding to high vs. low freshwater inputs to the sea, along a river-to-sea transect. The highest values of sedimentary organic loads and their nutritional quality, prokaryotic and meiofaunal abundance, and biomass were consistently observed in lagoon systems. Differences in the prokaryotic Operational Taxonomic Units (OTUs) and meiofaunal taxonomic composition, rather than changes in the richness of taxa, were observed among the three habitats and, in each habitat, between sampling periods. Such differences were driven by either physical or trophic variables, though with differences between seasons. Overall, our results indicate that the apparent positive relationship between sedimentary OM, prokaryote and meiofaunal abundance, and biomass across the river-lagoon-sea transect under scrutiny is more the result of a pattern of specifically adapted prokaryotic and meiofaunal communities to different habitats, rather than an actually positive 'response' to OM enrichment. We conclude that the synoptic analysis of prokaryotes and meiofauna can provide useful information on the relative effect of organic enrichment and environmental settings across gradients of environmental continuums, including rivers, lagoons, and marine coastal ecosystems.

**Keywords:** North Adriatic Sea; trophic status; prokaryotes; meiofauna; ecosystem functioning

## **1. Introduction**

Pathways and rates of sedimentary organic matter (OM) transfer to higher trophic levels in aquatic ecosystems depend on the OM quantity and nutritional quality [1,2]. In turn, both the quantity and nutritional quality of sedimentary OM depend on its origin (i.e., autotrophic, heterotrophic, and/or detrital), biochemical composition and bioavailability (i.e., refractory vs. labile fraction), and degradation rates [3]. Therefore, the benthic trophic status of an aquatic ecosystem is not only related to the availability of inorganic nutrients, which fuels in situ primary production, but also depends upon the supply rates of OM, including allochthonous and detrital (i.e., not living) sources [3–6]. This, in turn, can influence other ecosystem functions, including nutrient cycling and oxygen availability [3,7]. For instance, the accumulation of huge amounts of detrital OM in marine coastal sediments, triggering increased benthic O2 consumption and possibly inducing hypoxic and anoxic conditions, can be associated with a preferential accumulation of semi-labile compounds (e.g., the biopolymeric fraction of organic carbon (OC) [8]), particularly enriched in nitrogenous (protein-like) compounds [9]. In such conditions, the decoupling between the production/inputs of OM loads, heterotrophic consumption, and accumulation in sediments can determine strong modifications in the structure and functioning of benthic ecosystems [9–13].

The comprehension of the biogeochemical dynamics in aquatic environments characterized by variable biodiversity levels (e.g., transitional, estuaries, and coastal environments) is strongly limited by the complex and multiple interactions among different biotic components, including microbial and meio- and macrofaunal assemblages [14,15]. In this regard, it is noticeable that changes in the benthic trophic status and OM degradation rates mediated by microbes, through the so-called microbial loop, can be mirrored in changes in the composition and structure of the benthic communities, and vice-versa [16,17].

In benthic ecosystems, OM, prokaryotes, and meiofauna, being trophic resources for higher trophic levels and, at the same time, being responsible, with different roles, for OM cycling, represent a key functional bottleneck in the energy transfer towards a higher trophic level [18]. They also detectably and rapidly respond to a variety of natural and anthropogenic disturbances. Heterotrophic prokaryotes are responsible for detrital OM degradation and transformation and rapidly respond to variations in the quantity and composition of the available OM [19,20]. Meiofauna, due to their strong sensitivity to disturbances, high abundance, lack of pelagic larval dispersion, and short life cycles, rapidly respond to environmental changes in both marine [21,22] and freshwater ecosystems [23].

The relationships between OM and the different components of benthic communities are also influenced by multiple, often interacting, environmental variables, for example, currents and the substrate composition, which, in turn, enhance the levels and variance of natural disturbance, as well as habitat-specific conditions. Analyses of these relationships have mostly been conducted by considering the different benthic components separately. Moreover, freshwater, transitional, and marine coastal ecosystems, though physically linked to each other, have most often been considered as worlds apart, and such a reductive approach especially applies to the analysis of benthic ecosystems.

Here, to provide insights on this topic, we test the null hypothesis that OM quantity, biochemical composition and degradation rates, prokaryotic and meiofaunal biodiversity, and ecosystem functioning do not vary among different ecosystems along a strong salinity gradient in different periods of the year. To test this hypothesis, we investigated the OM quantity, nutritional quality, and degradation rates, along with the prokaryotic and meiofauna biodiversity, in two sampling periods (corresponding to high vs. low freshwater inputs to the sea) along a river-to-sea gradient, comprising Po River (Italy), the North Adriatic Sea, and the associated lagoonal system. The sampling strategy included stations located in the major tributaries of the Po River, in the Po main axis, in a coastal lagoon (Sacca di Goro Lagoon) intercepting Po River outflow, and in the coastal sediments of the North Adriatic Sea facing the lagoon and the Po River delta.

#### **2. Materials and Methods**
