Benthic diatoms are an important component of the microphytobenthos that inhabit intertidal mudflats. These organisms excrete a substantial amount of photosynthetically fixed carbon as Extracellular Polymeric substances (EPS), which mainly consist of carbohydrates. In this way, diatoms are able to form biofilms at the mudflat surface, which protects them from the highly fluctuating conditions that prevail in intertidal areas. The research in this thesis focused on two questions 1) What are mechanisms of EPS excretion by benthic diatoms; 2) What is the role of EPS excretion and biofilm formation by benthic diatoms on the morphological development of intertidal mudflats. In order to assess these questions both culture and field experiments were conducted.

Culture experiments showed that the diatoms that were investigated produced two types of EPS that differed in production dynamics, composition and localization. One EPS-fraction was produced as a result of overflow metabolism due to limited intracellular storage capacity of the diatoms. This EPS represented an extracelluar storage carbohydrate that could be utilized by the diatoms themselves during darkness. Production of the second EPS-fraction was probably related to the ability of diatoms to move through the sediment. During field experiments it was observed that EPS-production mainly occurred as a result of overflow metabolism. Large quantities of this EPS were produced in the light while during darkness it was rapidly removed.

The development of diatom biofilms had a clear effect on the morphological development of intertidal mudflats. When biofilms were present, sediment stability increased and fine grained sediment particles were trapped in the biofilm matrix leading to net deposition of sediment on top of the mudflat. Furthermore, diatom biofilms effectively prevented sediment erosion during periods of high hydrodynamic forcing. In the absence of diatom biofilms, net sediment erosion occurred and physical processes governed morphodynamics of the mudflat. Although the stabilizing effect of benthic diatoms is temporal, they play an important role in the seasonal dynamics in the distribution of suspended matter between mudflats and the watercolumn in estuaries.

The electronic version of the thesis is available at: http://webdoc.ubn.kun.nl/mono/b/brouwer_j_de/
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