Phytoplankton community composition in relation to water quality and water-body morphometry in urban lakes, reservoirs, and ponds

Distinct differences in observed summer phytoplankton communities in relation to maximum depth suggest that constraints posed by water-body morphometry may modify the trophic control of phytoplankton-community composition and structure in urban water bodies. In deep urban sites (Z(max) greater than or equal to = 5 m), phytoplankton communities tended to be predictably related to trophic status (i.e., increases in trophy were associated with increased cyanobacterial dominance, a decreased proportion of Chrysophyceae-Synurophyceae and grazable-size algae, and a decrease in community richness), although exceptions existed, owing to factors such as human intervention, age of the water body, and flushing rates. In contrast, in shallow urban water bodies (Z(max) < 5 m), trophic status was a poor predictor of phytoplankton communities. Across meso- to hyper-eutrophic conditions, shallow urban sites were rarely dominated by cyanobacteria and, when they were, the species composition differed from nutrient-rich deep urban sites. The key requirement for cyanobacterial dominance in shallow urban sites appears to be sufficiently long water residence times, viz., greater than 8-14 days. Further study should describe how the relationship between water body residence time and species-generation time may limit the development of specific nuisance algal species, aiding in the management and rehabilitation of urban water bodies.