Abstract
Along a west-to-east axis spanning the Galicia Bank region (Iberian margin) and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m). Nematode standing stock (abundance and biomass) and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude) governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter). Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes.
Highlights
Apart from benthos dependent on chemosynthesis, deep-sea sediment communities depend for their dietary requirements on organic matter (OM) that is produced in the euphotic zone
particulate organic carbon (POC) Flux There was a significant reduction in Net primary production (NPP) along the west-to-east axis from the GB region to the eastern Mediterranean basin (Fig. 2A; Spearman rank: r = -0.89, P,0.001)
Along the longitudinal transect from the Galicia Bank region to the eastern Mediterranean, downward benthic-pelagic coupling was evident in terms of phytopigment concentrations, and in standing stock, size, genus and trophic composition of nematodes in bathyal and abyssal sediments
Summary
Apart from benthos dependent on chemosynthesis, deep-sea sediment communities depend for their dietary requirements on organic matter (OM) that is produced in the euphotic zone. The processes through which POM is transferred to the deep-sea bottom are collectively termed ‘‘the biological pump’’. During its descent through the water column the POM particles are progressively broken down, and only a limited fraction (1% on average) arrives at the deep-sea bed [1]. The fraction of exported POM that reaches the deep-sea sediments, or the efficiency of the biological pump, is determined by water depth, the sinking velocity (dependent on, amongst others, the degree of POM aggregation and the seawater mineral content) and the rate of decomposition (dependent on the pelagic food web structure and seawater temperature) of the POM [3]. Laterally advected water masses may transport sinking POM away from its point of origin [4,5,6]
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