Organic Matter Composition of Biomineral Flocs and its Influence on
Suspended Particulate Matter Dynamics along a Nearshore to Offshore
Transect
Abstract
The seasonal variation in concentration of transparent exopolymer
particles (TEP), particulate organic carbon (POC) and nitrogen (PON)
were investigated together with floc size and the concentration of
suspended particulate matter (SPM) along the cross-shore gradient, from
the high turbid nearshore towards the low-turbid offshore waters in the
southern Bight of the North Sea. The analyses of TEP, POC and PON result
in a set of parameters that incorporate labile and refractory organic
matter (OM) fractions. Our data demonstrate that biophysical
flocculation cannot be explained by these heterogeneous parameters, but
requires a distinction between a more reactive labile (“fresh”) and a
less reactive refractory (“mineral-associated”) fraction. Based on all
data we separated the labile and mineral-associated POC, PON and TEP
using a semi-empirical model approach. The model’s estimates of fresh
and mineral-associated OM show that great parts of the POC, PON and TEP
are associated with suspended minerals, which are present in the water
column throughout the year, whereas the occurrence of fresh TEP, POC and
PON is restricted to spring and summer months. In spite of a constantly
high abundance of total TEP throughout the entire year, it is its fresh
fraction that promotes the formation of larger and faster sinking
biomineral flocs, thereby contributing to reduce the SPM concentration
in the water column over spring and summer. Our results show that the
different components of the SPM, such as minerals, extracellular OM and
living organisms, form an integrated dynamic system with direct
interactions and feedback controls.