Phosphorus recycling in lakes: evidence from large limnetic enclosures for the importance of shallow sediments

• 1 The dynamics of phosphorus in large field enclosures are described from a series of experimental manipulations directed primarily to the study of phytoplankton populations. The enclosures were similar in size but differed in their depth contours. The aim of the present analysis was to detect systematic differences in the phosphorus budgets of deep and shallow systems.
• 2 The sequence of events in each of six year-long experiments is described in terms of algal production. The values are compared with water-column measurements of total phosphorus (TP), the reactive fraction (SRP) and changes in the particulate fraction (PP, assumed to be TP minus SRP). Stoichiometric ratios for larger phytoplankton crops were in the range 85–110 C/P (molar).
• 3 The annual aggregate of new carbon produced in the two uniformly deep enclosures (11.5 m) in relation to the phosphorus supplied was also remarkably close to Redfield stoichiometry, with between 87 and 97mol C yielded per mol P added. However, in the graded enclosure (< 4–13.5 m), the yield was up to twice as great (maximum: 197 mol C mol^-1 P available).
• 4 There are two possible mechanisms. A graded surface permits a greater area of sediment to be in contact with the hypolimnion and for the sediment oxygen demand to deplete a greater hypolimnetic volume; the associated fall in redox would have been adequate to explain the observed increment of dissolved phosphorus to the hypolimnion, associated with the Einsele–Mortimer model (ferric → ferrous iron reduction). There was no clear evidence that this phosphorus supported autotrophic production until late in the year.
• 5 The alternative mechanism is based on the association of phosphorus ‘injection’ to the epilimnion during wind-mixed episodes, when the enhanced shear stress on shallow sediments resuspends fine, superficial deposits and their interstitial waters into the full circulation. Circumstantial evidence suggests this second pathway predominated in the graded enclosure but was ineffective in the uniformly deep enclosures.
• 6 The role of mechanical, aerobic phosphorus cycling in the nutrient economy of lakes, especially small or shallow ones, may have been underestimated in the past but it must be considered when evaluating any proposal to reduce the external nutrient loads to lakes.