Estuarine consumers utilize marine, estuarine and terrestrial organic matter and provide connectivity among these food webs

Abstract

The flux of organic matter (OM) across ecosystem boundaries can influence estuarine food web dynamics and productivity. However, this process is seldom investigated taking into account all the adjacent ecosystems (e.g. ocean, river, land) and different hydrological settings (i.e. river discharge). Therefore, we aimed to quantify the contribution of autochthonous and allochthonous OM to the lower food web along the estuarine salinity gradient, under different river discharge conditions. The carbon (C) and nitrogen (N) stable isotope ratios of pelagic (zooplankton) and benthic (Corbicula fluminea) primary consumers indicated that they rely on a mixture of autochthonous and allochthonous OM, including terrestrial-derived OM. Unexpectedly, the highest contribution of terrestrial-derived OM to the estuarine food web was observed during a low river discharge period (up to 70%), that succeeded a large winter flood, showing that extreme weather events may produce prolonged effects on estuarine food webs. The contribution of marine-derived OM was higher during low river discharge periods (up to 88%) and was restricted to the seaward end of the estuary. Concomitantly, the contribution of phytoplankton to primary consumers was the highest observed (up to 91%). Further, both pelagic and benthic consumers also relied on benthic C (i.e. sediment OM and microphytobenthos). This study demonstrates that primary consumers enhance connectivity between estuarine ecosystems by utilizing subsidies of terrestrial and marine origin, and also between estuarine habitats through reliance on pelagic and benthic OM.

Benthic food webs support the production of sympatric flatfish larvae in estuarine nursery habitat

“…solea larvae relied principally on OM sources with benthic origin (MPB and SOM), indicating dependence on the estuarine benthic food web. One possible explanation may be the low phytoplankton availability in the POM pool in this estuary, especially during high river discharge conditions (~ 40%; Dias et al ., ). Previous studies indicate that flatfish larvae start to feed exogenously during the yolk‐sac stage and that their main prey includes dinoflagellates, tintinnids, copepods and other zooplankton (Last, ).…”

“…Thus, the most likely mechanisms by which they obtain their energy from the benthic food web is indirect, i.e., they feed on prey that relies on benthic OM. In a study conducted during the same period, it was found that calanoid copepods, which are the most abundant copepods in this estuary (Vieira et al ., ), relied mostly on SOM in the brackish portion of the estuary (up to 60%; Dias et al ., ). Also, it is possible that epibenthic prey, such as polychaete, harpaticoid copepods, small bivalves and gastropods, were also consumed (Lagardère et al ., ).…”

“…Platichthys flesus presents lower δ 13 C and δ 15 N values than S . solea , suggesting a greater dependence on a detritus‐based food web as a result of the assimilation of SOM and surface terrestrial‐derived POM (C : N POM > 10; Hedges et al ., , ; Dias et al ., ). These results suggest that, to some extent, food source partitioning may occur.…”

“…). This is the period when flatfish larvae begin their recruitment into northern Portuguese estuaries (Ramos et al ., ), and corresponds to the end of the high river discharge, during which phytoplankton availability in the estuary is potentially low (Dias et al ., ).…”

Characterisation of sphaeractinomyxon types (Cnidaria: Myxozoa) from marine and freshwater oligochaetes in a Portuguese estuary, with the demise of the endocapsa collective group

“…Salinity values at the fyke-nets in the upper estuary are generally below 0.5‰, increasing slightly during dry summer periods. In turn, salinity values in the lower estuary range between 15 and 40‰ throughout the year (see Dias et al 2016).…”