Lucia Bongiorni1*, Federica Nasi2,3, Federica Fiorentino1, Rocco Auriemma2, Federico Rappazzo4, Marie C. Nordström5, Daniela Berto4
1CNR - Istituto di Scienze Marine (ISMAR), Arsenale - Tesa 104, Castello 2737/F, 30122 Venezia, Italy
2Sezione di Oceanografia, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, I-34151 Trieste, Italy
3Dipartimento di Scienze Della Vita, Università degli Studi di Trieste, Trieste, Italy
4ISPRA- Istituto Superiore per la Protezione e la Ricerca Ambientale- Loc. Brondolo (, 30015 Chioggia, Italy
5Åbo Akademi University, Environmental and Marine Biology, FI-20500 Turku, Finland
*Corresponding author: [email protected]
Manuscript
Author contribution statement:
Design and methods: LB, FN, FF Data collection: LB,FN, FF
Data analyses: LB, FN, FF, RA,FR, DB
Manuscript preparation: LB, FN, FF,RA, FR, NM, DB Project leader: LB
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Abstract
Estuaries are highly productive ecosystems that can export organic matter to the coastal seas. However this process is seldom investigated taking into account the entire adjacent ecosystem (e.g. river distributaries and deltaic area) and hydrological setting (i.e. river discharge). Therefore we investigated the proportions of various primary producers contributing to plume of suspended particulate organic matter (SPOM) pool and the extent to which subsidies from rivers may influence macrobenthic community trophic structure and biomass. In December 2014 sediments and macrofauna samples were collected in the prodelta region (9 stations), at increasing distance from the main Po River mouth (i.e. Po di Pila). Potential estuarine food sources (end members) were collected within the Po River delta in three stations representing as much as possible the variability of the whole area (i.e. main river bank, branches and saltmarshes).
By mixing model the river SPOM and fringing vegetation were the principal riverine food sources for the macrofaunal community, highlighting the wide variation of organic load from the river distributaries of this delta. The main mouth exports organic matter from terrigenous and riverine phytoplankton, whereas the other river distributaries were important for seagrass input on the coastal trophic web. The community were dominant by surface deposit feeders in the station nearby the river mouth (the polychaete Heteromastus filiformis). The great majority of suspension feeders (above all the polychaete Owenia fusiformis) which correspond to the highest biomass were observed at the station slightly distant from the main organic loads and characterized by high idrodinamism. Subsurface deposit feeders gradually increased toward the deepest and clayey stations.
Overall, the macrofaunal community from the biomass measurement seem to be more structured due to the freshly organic matter observed at Po di Pila rather than the deepest stations, related to the high values of pheopigments/chlorophyll-a ratio. These findings highlighted the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystem.
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Keywords: Food webs, Macrofauna, Stable isotopes, Food subsidy, River delta,
Introduction
Estuaries are considered among the most productive habitats due to the high in
situ primary production, and inputs of organic matter transported from rivers
(Dias et al. 2016). Due to tide water movements these environments typically exchange materials with outside receiving ecosystems, generating among the largest cross-boundary fluxes globally (Schlünz et al. 1999). These large quantities of terrestrial organic materials fuel disproportionately high rates of transformations in coastal ecosystems (estimate around 90% of modern carbon burial), impacting global bio-geochemical cycles (McKee et al. 2004), enhancing biological production and fisheries yields (Gaston et al. 2006). Organic matter and nutrients transfer across terrestrial and marine ecosystem boundaries and their incorporation by macrofauna in receiving ecosystems can influence communities’ dynamics and feeding strategies and exert a relevant positive impact on the ecosystems functioning and services of adjacent marine coastal areas (Hocking et al. 2011; Savage et al. 2012).
The benthic habitats are important ecological components of the coastal zone. Moreover, the effect of rivers plumes in supporting macrozoobenthic feeding typologies in the adjacent coastal areas and the influence of riverine trophic subsidy on coastal macrofaunal abundance, biomass and diversity and trophic interactions remain poorly studied, limited to few estuaries or not inclusive of complex transitional areas like delta- prodelta systems. Prodelta areas are the primary interface between terrestrial and ocean environments and play a key role in linking the terrestrial and marine cycles of bioactive elements such as organic carbon. Indeed, it is thought that around 45% of the organic matter (OM) burial in marine environments occurs in deltaic regions (Aller 1998 and reference therein).
The Po River prodelta area receives a significant amount of OM and sediments from one of the largest European rivers. The Po River characterized by a mean daily discharge of 1500 m3 s-1, and periodically by floods (>5000 m3 s-1)
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represents the primary source of freshwater and nutrients entering the northern Adriatic Sea (Degobbis & Gilmartin 1990). The influence of this river can be observed in all the coastal ecosystem components especially because of the high land-derived input of allochtonous nutrients and the relevant transport of particulate inorganic suspended matter (Boldrin et al. 2005). The high nutrient concentrations discharged result in a highly impacted and eutrophic marine coastal area (Justic et al. 1995) which can lead to pathological condition of sediment hypoxia and consequently to the reduction or disappearance of sensitive organisms (Vollenweider et al. 1992). Changes of macrofaunal communities (abundances, diversity and trophic habits) in response to riverine inputs have been deeply investigated in coastal areas nearest to Po River delta (Ambrogi 1990; Simonini et al. 2004; Occhipinti-Ambrogi 2002; Occhipinti-Ambrogi et al. 2005). The macrobenthic community in this area, displays a high capacity to thrive and adapt to the peculiar environmental conditions of the area; it is characterized by a high abundance of a few dominant species (the bivalve Corbula gibba and the crustacean Ampelisca diadema), which are occasionally subjected to demographic blooms, and by a low dencity of many other species (Occhipinti-Ambrogi et al. 2005; Massamba N’Siala et al. 2008).
The condition and growth of macrofaunal organisms is dependent upon the quantity and quality of the food resources they assimilate over time. Stable isotopes (δ13C and δ15N) can provide time-integrated information about the source of food assimilated by macroinvertebrates and about trophic community structure and dynamics (Layman et al. 2012). If estuarine and marine food resources have their own distinct isotopic signatures the spatial subsidy of estuarine primary production and its contribution to secondary production can be assessed using stable isotopes in combination with mixing models (Philips et al. 2014). Moreover biomass-weighted isotope signatures can be used to assess the relative importance of different organic matter sources that contribute to animal’s diet. Further, the study of trophic web over space, taking changes in species biomass provides opportunity to investigate energy pathways based on the structure and dynamics of species “feeding relationship” (Grall et al. 2006; Quillien et al. 2016). The potential of matching stable isotope analysis and mixing models with
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quantitative studies of macrobenthic communities has been recently addressed as an effective strategy to assess and disentangle the effects of multiple environmental impacts in coastal marine ecosystems (Bongiorni et al. 2016).
In this study we attempt to estimate whether the Po River runoff, in particular after a period of major river floods, influences the food subsidies for the macrofaunal community in the adjacent prodelta area. More precisely, the aim of the present study was to answer the following questions 1) Are the macrofaunal isotopic composition and feeding habits of the prodelta area affected by riverine inputs? 2) Which is the contribution of terrestrial and marine origin food sources to the prodelta benthic consumer’s diet? 3) At which trophic level the Po River subsidy influences macrofaunal biomass?
The isotopic composition of benthic infauna was investigated in the prodelta area at increasing distance from the main river distributaries, following the principal river front. The contribution of different terrestrial vs marine primary food sources to benthic consumers was evaluated using an isotopic mixing model.