It is known that blooms can sometimes cause problems, including mortalities, in Mytilus (see previous chapter). Long-term nutrient enrichment and increasing phytoplankton production have been reported in the southern North Sea (De Jonge, 1997; Smayda, 1990) (though interestingly production off the East Anglian Coastline has been reported by Tett at al., 1994, to be only around one quarter of that off the Wadden Sea) and the Irish Sea (Allen et al., in press). Trends in the frequency of problematic phytoplankton blooms are more difficult to establish due to lack of historical data, but blooms appear to be increasingly frequent on the west of Scotland (McKay, pers. comm.) and probably in the southern North Sea (Smayda, 1990). An associated problem is that enrichment often appears to be associated with changes in the species composition of phytoplankton, often favouring smaller groups at the expense of diatoms (Smayda, 1990) and this could have unknown consequences for all filter feeding organisms including Mytilus.

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