US Long-Term Ecological Research Network

In aquatic ecosystems, phytoplankton biomass may remain about constant while concentrations of the limiting nutrient are below detection (De Pinto et al. 1986). It has been thought that mixing events that add nutrients to the mixed layer may cause episodic algal blooms in eutrophic lakes. Another explanation may be that rapid rates of biotic recycling among primary producers and heterotrophic consumers could maintain high phytoplankton biomass (Sterner 1989, Poister et al. 1994). The recycling process has been difficult to observe because release and subsequent uptake may occur over a very short time scale.

To better understand the process of recycling and to gauge its importance in the nutrient budget, we used free-water oxygen measurements and an associated metabolic model to infer rates of phosphorus uptake and biotic recycling in the epilimnion of a eutrophic lake. Free-water oxygen measurements were collected using a buoy positioned at the center of Lake Mendota. Estimates of ecosystem metabolism were converted into estimates of phosphorus uptake based on the C:P ratio of seston observed in the epilimnion. Rates of uptake and recycling were compared to “external” sources of phosphorus to the epilimnion such as loading and entrainment (Kamarainen et al. 2009).

Which source was most important in supplying phosphorus to phytoplankton during the stratified summer season?

During the period when phosphorus was below the detection limit, calculated rates of phosphorus uptake remained relatively constant. Uptake exceeded the amount of phosphorus supplied by any single source, though biotic recycling contributed approximately 57% of the documented phosphorus demand (graph at left). While the majority of phytoplankton phosphorus demand could be accounted for by biotic recycling, it was important to consider the effects of entrainment in order to account fully for phosphorus uptake. Recycling occurred as a relatively consistent source of phosphorus to the epilimnion while phosphorus was supplied through entrainment during episodic events. Phosphorus supplied through recycling and entrainment, together, could account for the total required by phytoplankton during the period of low phosphorus availability.

Did the results change with different assumptions about model parameters?

These general results were relatively insensitive to model parameterization. Changing the values of the photosynthetic and respiratory quotients used in the metabolic model did not change the general pattern of the results. The relative C:P ratio of material taken up versus mineralized (or recycled), however, was an important consideration. As long as C:Pup was smaller than C:Pmin (C:Pup/C:Pmin < 1.0) the rate of P uptake exceeded that of P supplied through recycling (graph at right). We found that if C:Pmin and C:Pup are significantly different in nature (which is possible, but hard to measure), then these differences could affect our estimates of the relative balance between uptake and recycling.

This study integrates modeling and measurement tools that monitor ecosystem processes at finer temporal resolution than has previously been possible, complementing other studies that use experimental incubation and elemental tracers. Extension of this approach could enhance models that aim to integrate biological and physical processes in assessment of water quality and prediction of phytoplankton biomass.

  • De Pinto, J. V., T. C. Young, J. S. Bonner and P. W. Rodgers. 1986. Microbial recycling of phytoplankton phosphorus. Canadian Journal of Fisheries and Aquatic Sciences 43: 336-342. 
  • Poister, D., D. E. Armstrong and J. P. Hurley. 1994. A 6-year record of nutrient element sedimentation and recycling in 3 north temperate lakes. Canadian Journal of Fisheries and Aquatic Sciences 51: 2457-2466. 
  • Kamarainen, A. M., R. M. Penczykowski, M. C. Van de Bogert, P. C. Hanson and S. R. Carpenter. 2009. Phosphorus sources and demand during summer in a eutrophic lake. Aquatic Sciences 71: 214-227. 
  • Sterner, R. W. 1989. Resource competition during seasonal succession toward dominance by cyanobacteria. Ecology 70: 229-245.

 

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