Short-term variations in the fluxes and composition of seston in near-bottom traps in the southern North Sea

Abstract

In this study we attempted to measure the potential food availability for macrobenthic interface feeders using a new device (sediment recorder) which traps bed load particles at a height of 10 cm above the sea floor. The recorder is positioned flat on the seabed and alternately exposes a shallow collector mounted in a smooth surface mimicking the sediment surface. The samples were analyzed for their content of chlorophyll a (Chl) and dry weight (DM). We compared the sediment recorder data with the results obtained with a sediment trap suspended 3.2 m above the sea floor. Measurements were carried out in spring and fall at two stations in the southern North Sea with different sediment type, maximum current velocities and biomass (sand, station B vs silty sand, station F). The recorder deployments in November yielded higher Chl fluxes and a higher quality of particles (Chl:DM ratio) than the trap at both stations. Unlike the trap samples, in which Chl and DM were always positively and strongly correlated, maximum Chl and DM fluxes in the recorder samples were out of phase, particularly at station B, pointing to a highly variable quality during the tidal cycle. Unexpectedly, Chl fluxes and Chl:DM ratios in the recorder samples at station B were higher than at station F in both seasons, although the benthic biomass at the latter station was five times higher. The highest Chl fluxes and particle quality (Chl:DM) at station B were measured during slack tides when interface feeding would be least hampered by drag forces. Station B is nevertheless characterized by a low macrobenthic biomass and a scarcity of interface feeders, specifically of suspension-feeding bivalves. Possible reasons for this apparent discrepancy are discussed.

Growth trends in three bivalve species indicate climate forcing on the benthic ecosystem in the southeastern North Sea

“…Witbaard et al (2001) showed that shell growth of Arctica islandica and Chamelea striatula living in the fine sediments of the southern North Sea is slower than that of specimens from coarse sediments. Field measurements have demonstrated that regular resuspension of the fine sediment at the Frisian Front dilutes food concentrations and lowers food quality for suspension feeders (Duineveld & Boon 2002). This may explain the depressed growth rates in fine-grained sediments, as is the case in other areas with frequent resuspension (Emerson 1990, Turner & Miller 1991, Bock & Miller 1994, Urrutia et al 1996, Grant et al 1997, Cranford et al 1998, Gremare et al 1998.…”

Growth variations in the bivalve Mya truncata : a tool to trace changes in the Frisian Front macrofauna (southern North Sea)?

“…Direct measurement of benthic food supply at offshore turbulent sites requires scarce resources, i.e. ship time and special instruments (Duineveld and Boon 2002). Growth records deposited in bivalve shells like M. truncata can offer a simple alternative as the organisms integrate conditions over relevant timescales and, moreover, allow retrospective analyses (Richardson 2001).…”

Acute impacts of bottom trawl gears on benthic metabolism and nutrient cycling

“…These sediments are subject to faunal induced mixing with the presence of burrowing organisms such as Callianassa subterranea and Upogebia deltaura (Rowden et al, 1998;Dewicke et al, 2002). Previous research has shown that the area is biologically diverse and relatively high in benthic biomass (Dauwe et al, 1998;Dewicke et al, 2002;Duineveld and Boon, 2002), biological activity (Duineveld et al, 1990;Duineveld et al, 1991), and chlorophyll a (Chl a) influx to the sediments compared with adjacent North Sea habitats. Moreover, areas with these characteristics (muddy sediment/high biomass) are known to be more vulnerable to trawling activity than sandy dynamic habitats (Duplisea et al, 2001; Biogeochemical effects from pulse and beam trawl fisheries al., 2006;Queirós et al, 2006, Allen andClarke, 2007;Sciberras et al, 2016).…”