1. Seasonal relationships between macrophytes and phytoplankton may alter considerably as lakes undergo eutrophication and an understanding of these changes may be key to the interpretation of changes in ecological processes that occur over much longer (decadal-centennial) timescales.\ud 2. We investigated the seasonal dynamics of macrophyte and phytoplankton populations in 39 shallow lakes (29 x U.K. and 10 x Denmark) covering broad gradients for nutrients and plant abundance. Each lake was monitored for water chemistry and chlorophyll-a on a bi-monthly basis (monthly/bi-monthly for a year in the U.K. and monthly in summer Denmark) and macrophyte surveys were undertaken in early (June) and late summer (August).\ud 3. Below a total phosphorus (TP) level of around 40-50 g l-1, chlorophyll-a was consistently low irrespective of plant abundance. Above this level of TP more scatter was evident in the TP-chlorophyll-a relationship with macrophytes abundant or sparse and with phytoplankton blooms occurring during different seasons but with no obvious pattern along the TP gradient.\ud 4. Within the plant-dominated lakes two major seasonal patterns were evident with plant density either remaining relatively stable or exhibiting a dramatic ‘crash’ between the June and August sampling dates. In the crashing lakes one or a number of the following species; Potamogeton pectinatus, Potamogeton pusillus and Zannichellia palustris were particularly characteristic, whereas Ceratophyllum demersum and Chara spp. were much more abundant in the stable lakes. The crashing lakes were characterised by a substantial increase in phytoplankton abundance after the loss of plants, while in the stable lakes the abundance of phytoplankton was less seasonally variable.\ud 5. Over recent centuries many eutrophic lakes have experienced dynamic changes in macrophyte composition and diversity with an often observed shift from a diverse community with charophytes to one dominated by a few species tolerant of eutrophication. Our data suggest that such changes may often be accompanied by a substantial reduction in the plant covered period leading to increased opportunities for phytoplankton establishment. Thus, over a long time span (10-100s years) gradual losses of macrophyte species may render a shallow lake increasingly vulnerable to invasion by a phytoplankton-dominated community. In our model of plant loss we see no need for a perturbation as required by the theory of alternative stable state
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