Planktonic phosphorus pool sizes and cycling efficiency in coastal and interior British Columbia lakes

1. Limnologists have long acknowledged the importance of phosphorus (P) in determining the organism biomass and productivity of lake ecosystems. Despite a relatively large number of studies that have examined P cycling in lake ecosystems, there remain several substantial methodological issues that have impeded our understanding of P cycling in limnetic plankton communities. Two critical issues confronting ecologists are (1) a lack of precise measurements of the dissolved inorganic phosphorus (PO 4 ) and (2) accurate or complete measurements of dissolved P regeneration rates by plankton communities. 2. Here, we examine patterns of epilimnetic planktonic P pool sizes and turnover rates in eight lakes in British Columbia, Canada over a 2-year period. We determine the concentrations and turnover times of P in various planktonic compartments (dissolved and various planktonic size fractions), using recently developed methods for estimating phosphate concentration and planktonic regeneration rates. 3. The picoand nanoplankton size fraction (0.2–20 lm) played a central role in planktonic P cycling in lakes examined by this study. On average across lakes, picoand nanoplankton contained >60% of the planktonic P, accounted for >90% PO 4 uptake, and contributed 50% of the plankton community dissolved P regeneration rate. 4. PO 4 concentrations determined by steady state bioassays (ssPO 3 4 ) were extremely low (87–611 pmol L) and were 2–3 orders of magnitude less than simultaneously measured colorimetric soluble reactive phosphorus estimates. Lake ssPO 4 concentrations increased linearly with total phosphorus (TP), and the slope of this relationship was approximately 1, indicating that PO 4 remained a consistent proportion of the TP pool across a range of TP concentrations. 5. Turnover rates of the total planktonic P pool and the <20 lm pool became more rapid with increasing lake TP, indicating that, according to this metric, planktonic P cycling efficiency increased with TP concentrations. We also detected a significant relationship between particulate phosphorus (PP) <20 lm turnover time and seston N : P ratios, with PP <20 lm turnover times becoming slower with increasing seston N : P. These findings suggest that long-standing conceptual models of nutrient cycling that predict slower cycling rates and decreasing cycling efficiency with increasing TP concentrations require further empirical examination. We postulate that patterns in lake P turnover and cycling efficiency are a result of complex interactions between plankton biomass and composition, and the ratios of multiple nutrients (C, N, P), rather than solely a function of the TP pool.