Waardgronden

The Waardgronden are used to illustrate the dynamics of benthic diatoms in an area of highlying tidal flats with a short period of submersion. The vertical distribution of benthic diatom biomass in the sediment clearly shows that both the biomass and the production is maximal in spring . The large uptake of CO2 from the pore water in the sediment for the primary production leads to an increase in pH and also to a net flux of oxygen from the sediment. From September to March the oxygen consumption by mineralization of organic material is higher than the oxygen production by benthic diatoms and in that period there is a net flux of oxygen into the sediment.

The dynamics of benthic diatoms depend on nutrients as well as on the light. In the figure below the dynamics of 2 sites are compared. The first site(Waardgronden) has a long period of emersion in each tidal cycle, in contrast to the second site (Lutjeswaard) which is submerged nearly the whole tidal period. Not only the exposure to sunlight, due to the difference in elevation(respectively 0.6 and 2.5 meter below NAP), but also the nutrient supply to both sites is different. 

Lutjeswaard-site

The Lutjeswaard-site is found near a large gully, close to the sluices of Kornwerderzand where large amounts of nutrients are discharged from Lake IJssel. This implies that the underwater light climate at the Lutjeswaard site in spring is controlled by the local (high) phytoplankton biomass and that consequently nearly no light reaches the tidal flat during submergence. Only in late summer when pelagic nutrients are depleted, the benthic diatoms win the competition for the remaining nutrients with the phytoplankton because they are closer to the last source of nutrients: the sediment. On the Waardgronden the main source of nutrients is the sediment from early spring on and there the benthic diatoms become nutrient-limited only when the nutrients in the sediment are exhausted.