Mass balancing the seasonal turnover of POC in mud and sand on a back-barrier tidal flat (southern North Sea)

Abstract

Abstract From March 1995 to October 1996, seasonal export and import fluxes of POC in themud and sand fractions of an intertidal sand flat (area: 1400*500 m=0.7 km 2 ) were investigated in the vicinity of a series of M. edulis banks behind Spiekeroog Island (East Frisian Wadden Sea). In January/February 1996 the region experienced a severe ice winter, and together with typical autumn/winter storms, the budget assessments presented in this paper are thus representative of major physical and biological disturbances common in the Wadden Sea. The contents and concentrations of mud in the sediments of the survey area variedconsiderably, being generally higher in late summer and lower at the end of winter, although almost the same basic pattern was retained over the 19 month survey period. The average POC content of the local sands was 0.106%, whereas that of the mud fractions varied in the range 1.5–4.0 wt%, lower, values being associated with high mud contents, higher ones with low mud contents. Overall, POC concentrations in the mud fractions varied from −2 , although peak values of 300 g m −2 occurred locally. Between March and September 1995 a net import of 262 t POC was recorded. In the period September 1995 to April 1996, which included the severe ice winter, a net export of 477 t POC took place. In contrast to the previous year, the summer following the ice winter, i.e. the period from April to October 1996, was characterised by a net export of 83 t POC. In all cases, 40–44% of the total POC fluxes were associated with the sand fraction, in spite of the generally low POC contents of sand. This observation reveals an important and hitherto unrecognised role of sand in total, POC fluxes in the Wadden Sea, a feature evidently linked to the large turnover of sand which contributed >90% to the total sediment flux. Contrary to widespread belief, sand does not necessarily dilute substances attached to the mud fraction in sand-mud mixtures but can actually act as a concentration mechanism because it induces substantial changes in sediment bulk density. This feature will, amongst others, have to be taken into account in the feeding ecology of benthic marine organisms, deposit feeders in particular.