The seasonal response of diatom communities to variable water quality in some English urban lakes

SUMMARY. 1. Five diatom assemblages and their corresponding site groups were associated with the urban lakes. When treated as a single dataset, DC A and PCA grouped these diatom assemblages along an environmental gradient from circum natural. dilute waters to strongly alkaline, concentrated, nutrient-rich waters.
2. When treated as individual datasets, 75% of the known variance was explained by the first axis of DCA in all five diatom assemblages; this indicated the existence of a strong primary environmental gradient. The PCA results showed that the identity of this primary environmental gradient can be different for each assemblage.
3. The statistical analyses showed that the physicochemical gradients were continuous and not discrete. Nevertheless, fine-scale analysis can identify smaller, more precise species and site groups.
4. Two types of species response could be identified within most sub-assemblages in the urban lakes: the first was a quasi-seasonal response and was characterized by a gradual rise to a peak relative abundance of one dominant or several co-dominant species which could be sustained for a period of weeks before a gradual decline. A quasi-seasonal response was sometimes interrupted by a storm response. The storm response was characterized by a sudden increase in the relative abundance of one or more diatom species following a storm, after which the species declined to pre-storm abundances within 7 days.
5. The effects of urban runoff on the water quality of urban lakes can override the effects of true seasonal factors such as water temperature and day length on the relative abundance of species.