Annual phytoplankton primary production in the tidal channels of the western Wadden Sea cannot be estimated more precise than 150±50 g C.m-2.a-1, because of large spatial and short-term temporal variations.This implies that year-to-year variations and possibly long term trends have to be very pronounced to be measurable even with a weekly sampling program. Short-term temporal variation in primary production of microphytobenthos living on the tidal flats is less pronounced but spatial variation is large. Primary production on high tidal flats is larger than on flats lower in the tidal zone. Year-to-year variation on a tidal flat station occupied now for 12 years is large. This variation cannot be explained by year-to-year variations in nutrients, light or temperature, but probably by year-to-year variations in grazing. Macrophytobenthic primary production plays a subordinate role in the western Wadden Sea. It is difficult to give one figure for annual primary production for an "average" tidal flat for reasons mentioned. Tentatively microphytobenthic plus macrophytobenthic plus phytoplanktonic primary production on and above an average tidal flat is estimated at 150± 50 g C.m-2.a-1, i.e. the same as given for phytoplankton production in the tidal channels. The western Wadden Sea receives a considerable amount of particulate organic carbon from outside the area, estimated at 240 g C.m-2.a-1. Formerly the North Sea was thought to be the only source. New data also indicate an important input from the IJsselmeer. This import of organic carbon, particulate plus dissolved, surpasses in situ primary production. IJsselmeer DOC is probably not used to a considerable extent in the Wadden Sea. IJsselmeer POC, however, consisting for 50% on average of living phytoplankton cells, may form a suitable food source for Wadden Sea invertebrates. Cell counts of the freshwater alga Scenedesmus showed that at least part of the IJsselmeer POC is retained in the western Wadden Sea. The importance of particle size in tidal as well as biological accumulation processes of POC in the Wadden Sea is stressed. There is no indication that POC content of the northern IJsselmeer has changed during the last 30 years.
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