Riverine impacts on benthic biodiversity and functional traits: A comparison of two sub-Arctic fjords

Paul E. Renaud,Maeve McGovern,Eivind Oug,Amanda E. Poste,Hilde Cecilie Trannum

doi:10.1016/j.ecss.2020.106774

Abstract

Climate change is leading to increases in freshwater discharge to coastal environments with implications for benthic community structure and functioning. Freshwater inputs create strong environmental gradients, which potentially affect the community structure of benthic infauna. In turn, changes in functional trait composition have the potential to affect the processing of terrestrially-derived nutrients and organic matter along the freshwater to marine continuum. We investigated the effects of riverine inputs on benthic community structure, functional traits, taxonomic and functional diversity, and utilization of terrestrial organic matter in two contrasting northern Norwegian fjords. Results of this study revealed extensive impacts of riverine inputs on community structure and functional traits. Communities directly affected by the river were characterized by diminished taxonomic and functional diversity, with species and trait composition indicative of an environment influenced by high sedimentation rates. Large, deep-dwelling, biodiffusors and upward conveyors dominated these communities. High community biomass at the river outlet as well as indications of terrestrial organic matter utilization evidenced by stable isotope analyses, suggest that such river-influenced communities may be important for the cycling of terrestrial carbon and nutrients in the coastal zone.

Highlights

Fjords are characterized by strong spatial gradients in salinity and stratification, resulting from a complex interplay between internal structuring factors and external forcing from both terrestrial and open ocean systems (Landaeta et al, 2012)
Functional redundancy (1-(FDRao/H’)) was relatively high both in Kaldfjord compared to Målselvfjord and for shallow stations compared to deep stations
Results of this study suggest that environmental filtering of the benthic community, driven by river influence on sedi ment grain size and high rates of sediment deposition, leads to decreased taxonomic and functional diversity in the river-impacted fjord

Summary

Introduction

Fjords are characterized by strong spatial gradients in salinity and stratification, resulting from a complex interplay between internal structuring factors and external forcing from both terrestrial and open ocean systems (Landaeta et al, 2012). Positioned at the land-ocean interface, fjords are potential hotspots of organic carbon burial, and may play an important role in regulating climate change on glacial to interglacial timescales (Cui et al, 2016; Gilbert et al, 2002; Smith et al, 2015; Walinsky et al, 2009; Włodarska-Kowalczuk et al, 2019). Despite their complexity, fjords may be viewed as semi-enclosed systems, an asset that makes them well suited for studies of processes along natural environmental gradients. Benthic communities are reliant on sedimentation of organic material, including both marine material from the water column and terrestrial material from run-off, and the relative importance and fate of tOM and nutrients for benthic communities represent a substantial knowledge gap in our under standing of coastal systems

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