Daphnia dominance and zooplankton community structure in fishless ponds

Predation by fish has commonly been viewed as a primary driver of spatial and seasonal variation in Daphnia dominance and the size structure of zooplankton communities. Yet, previous research suggests that large Daphnia do not always dominate in the absence of predation. As alternatives to the planktivory model, numerous mechanisms have been put forth, including the effect of resource competition and its interaction with resource quantity and quality and abiotic factors (e.g. temperature). Here results are presented of a field survey of 18 fishless, permanent ponds in southwest Michigan in which spatiotemporal variation in Daphnia pulex abundance and several potential determinants of this variation are explored. Results revealed a large amount of variation in D. pulex incidence and relative biomass, with some ponds exhibiting seasonal losses, some having few or no Daphnia, and some being dominated by D. pulex for the entire sample period. Redundancy analysis of zooplankton composition and pond environmental variables (biotic and abiotic) showed no relationship between D. pulex biomass and measures of Chaoborus abundance, algal resource production, or algal resource quality (including seston C:N:P). Instead, pH and temperature (both of which covaried) showed the strongest relationship with D. pulex biomass.