Phosphorus, algal and zooplankton relationships across a lake productivity gradient

The focus of this research was to confirm or identify patterns in plankton biomass and composition as a function of total phosphorus (TP) concentration in Northern Irish lakes. Herbivory, lake thermal structure, and predation effects on lake trophy, plankton biomass, and community structure have been investigated in forty lakes sampled in 2005 and 2006. Mid-lake epilimnetic samples were collected and analysed using standard water chemistry analytical procedures. Surface phytoplankton samples, and zooplankton vertical haul samples were collected and preserved. Plankton were enumerated, sized, and identified. Data was transformed, and relationships were identified with LOWESS algorithms, and tested using linear regression and general linear modelling. Phytoplankton biovolume was a less useful measure of algal biomass than chlorophyll a (Chi) concentration in nutrient-algal biomass trend analysis. Total nitrogen (TN) concentration had less predictive power than TP concentration. Plankton biomass increased along the TP gradient, however there was no trend in the zoo: phytoplankton ratio along the TP gradient. The biovolume of chlorophytes and cryptophytes increased along the TP gradient. The contributions of Oaphnia sp. and cyclopoids increased along the TP gradient, whereas calanoid contribution decreased. The body length of Oaphnia sp. increased along the TP gradient. Zooplankton volume increased with phytoplankton biovolume, specifically chlorophyte biovolume. Concentrations of TP and Chi, and zooplankton volume increased with lake depth. The trophic state of shallow mixed lakes was higher than that of deep stratified lakes. Algal biomass was not significantly different between shallow mixed and deep stratified lakes. There was a greater contribution of cyanobacteria to total phytoplankton in deep stratified lakes than in shallow mixed lakes. There was no lake thermal structure or herbivory effect on the TP-Chl relationship. The presence of fish did not alter the size structure of zooplankton; however the presence of Chaoborus sp. shifted the size structure towards larger-sized species.