Publication Date:
2019-06-14
Description:
Benthic fluxes of dissolved nutrients and oxygen measured in the southern North Sea using ex situ incubation
chambers indicate a prominent annual cycle characterized by low level from mid-autumn (Oct) to early spring
(Mar) and enhanced values from mid-spring (Apr) to early autumn (Sep) with peak in late summer (late Aug/early
Sep). The same cycle is also shown in the budget of total organic carbon (TOC) and macrobenthic biomass in
surface sediments. The significant positive correlations between the benthic nutrient fluxes, oxygen, sedimentary
TOC and macrobenthos suggest that their variation might respond to a common source, i.e. the primary production. However, the linkages between these quantities and pelagic primary production, which exhibits a dominant
bloom in early spring (Mar/Apr) and a secondary bloom in early summer (Jun/Jul) in the study area, is not
straightforward.
We present a numerical study to unravel the complex linkages. A 3-D coupled hydrodynamic-biogeochemical
model (ECOSMO) was used to provide benthic boundary conditions for a 1-D biogeochemical model in the
sediment (TOCMAIM) that mechanistically resolves the interaction between macrobenthos and organic matter
through bioturbation. Simulation results based on a satisfactory hindcast from 1948 to 2015 reveal that although
the spring algal bloom normally starts in late winter (Feb) and peaks in early spring (Mar/Apr), deposition of labile
OC to seafloor is limited in this period due to energetic hydrodynamic conditions. Sedimentation and accumulation
of labile OC (originated from fresh planktonic detritus) in seafloor surface sediments are facilitated in summer
when wind-waves become weak enough. This drives the blooming of macrobenthos, with peak of biomass in late
summer (Aug). Bioturbation intensity, which is dependent upon macrobenthic biomass, community structure as
well as local food resource, peaks also in later summer. Enhanced bioturbation and benthic metabolism result in an
increased oxygen flux into sediments, promoting remineralization of OC and release of nutrients. The following
period (late Sep/Oct) is characterized by low level of pelagic primary production in combination with enhanced
wind-waves, which not only reduce the input of labile OC into sediments substantially but also remobilize surface
material (sediments and OC) on a major part of the shallow coastal seafloor. Depletion of labile OC in the
uppermost centimeters of sediments by a combined effect of erosion, macrobenthic uptake and downward mixing
(through bioturbation) accounts for the rapid decline of benthic nutrient fluxes in Oct, which remain low through
the stormy winter until the next spring.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Conference
,
notRev
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