Does differential phosphorus processing by plankton influence the ecological state of shallow lakes?

Shallow lakes have a tendency to settle into turbid or clear-water states, the latter having lower concentrations of total phosphorus (TP). However, how P-cycling is affected by and perhaps contributes to maintaining the different states is not well understood, in part because quantifying the processes involved by traditional methods is difficult. To elucidate these processes, we conducted experiments using ³²P-PO₄ as a tracer on samples collected from the unrestored, unvegetated sections of Huizhou West Lake where turbid water prevails as well as the restored, clear-water, macrophyte-rich waters of the lake. We measured PO₄ uptake rates, ³²P-PO₄ accumulation by various plankton size-fractions (picoplankton (0.2-2 μm), nanoplankton (2-20 μm) and microplankton (>20 μm)) as well as release rates of ³²P-PO₄ by labelled plankton. Our results revealed slow PO₄ uptake in the turbid state due to low PO₄ concentration, slow recycling of the high particulate P, and high levels of particulate ³²P which may allow for continuous high growth and biomass of phytoplankton. In contrast, in the clear water state, the uptake of PO₄ was rapid due to a higher PO₄ concentration, the recycling rates of particulate ³²P were high and the levels of particulate ³²P were low, potentially constraining the phytoplankton growth. A greater proportion of particulate ³²P was in the microplankton fraction in clear waters, suggesting that grazing by microplankton may play an important role in the rapid P recycling in clear-waters. Our results provide some evidence for a reinforcement of the turbid conditions (low recycling rate) when the lake is in a turbid state and vice versa when in the clear water state. The results add new knowledge to the understanding of P cycling in shallow lakes and illustrate the utility of using P-kinetics in contrasting states in plankton communities.