Hyperbenthos in the upper reaches of the Scheldt estuary (Belgium): an indispensable trophic link re-established

Erika van den Bergh Liesbeth De Neve, Gunther Van Ryckegem, Joost Vanoverbeke, Frank Van de Meutter, Alexander Van Braeckel, Jeroen Speybroeck Hyperbenthos? • A group of typically small organisms that live in the lower layers of the water column, just above the bottom o - (opossum shrimps) - Amphipoda - Isopoda o Decapod shrimps (epibenthos) o Juvenile

• Good estuarine ecosystem quality indicators • Key role in the estuarine food web Scheldt estuary

• 1950-1980 permanent hypoxia, recurrent anoxia Vlissingen ‰ no life North Sea

• 1980-2007 POLYHALINE steady improvement MESOHALINE of water quality ‰ macrobenthos The Netherlands boom & waterbirds Antwerpen OLIGOHALINE since 2007 zooplankton, Durme hyperbenthos and fish FRESHWATER communities have Belgium recolonized Gent Melle Aims

freshwater tidal zones are poorly studied and understood in Europe , the upper reaches of the Scheldt estuary offer a quite unique study system

Understanding biotic responses to water and habitat quality restoration is essential for appropriate management

‰ Intensive monitoring campaign in the mesohaline to freshwater tidal zones of the Scheldt estuary - Document recovery of hyperbenthos community after decades of absence - How do hyperbenthos communities vary in space and time? - Which environmental variables drive spatio-temporal patterns? Sample protocol

• 6 sample sites • each month Vlissingen • 2013 - 2018 North Sea

POLYHALINE

MESOHALINE M1 • M1: Paardenschor M2 • M2: St-Anna Antwerpen • O1: Ballooi OLIGOHALINE • O2: Rupel Durme • F1: Dendermonde 100 O1 • F2: Brede schoren FRESHWATER Gent Melle Variables Evironmental • Temperature (°C) • Conductivity (µS/cm) Biotic • DO (mg/l) • SPM (mg/l) • Species composition • POM (mg/l) • Abundance(N/100m³) • River discharge • Biomass (AFDW; mg/100m³) • 5days before (m³/s) • Preceding season (m³/day) • Tidal amplitude (m) • Ebb duration (h) • Sampling moment in spring/neap cycle Species richness 62 different taxa (32 occurred in >3% of samples) Redundancy Analysis (RDA) Seasonal variation Spatial variation Focal species >20% of the samples >0.1 on at least one RDA axis Total abundance Mesopodopsis slabberi

Crangon crangon (brown shrimp) Total abundance

Palaemon longirostris Total abundance eperlanus (European )

Platchithys flesus (European flounder) Total abundance integer

Pomatoschistus microps (common goby) Total biomass Effect of river discharge

seasonal pattern most apparent in latter, dry, years negative relation to short-term river discharge ‰ flushing ‰ lower short-term residence time

positive relation to long-term river discharge ‰ prevention of eutrophication Take home message

1. Hyperbenthic communities successfully recovered in the upper reaches of the Scheldt estuary after decades of absence

2. A trophic link in the food web was re-established , adding to the estuarine ecosystem resilience

3. Oligohaline and freshwater tidal areas can represent a valuable extension of the nursing and feeding grounds for marine and estuarine fish and shrimps ,

4. Short intense peaks of high river discharge as well as long lasting periods of low discharge negatively impact hyperbenthos abundance in the oligohaline and fresh tidal zones Thank you for your attention

Thanks to Jan Soors

Team Estuaria Joram De Beukelaer Dimitri Buerms Nico De Regge Thomas Terrie Olja Bezdenjesnji