Improving estimates of primary production in lakes: a test and a case study from a peri-alpine lake (Lake Lugano)

Although primary production (i.e., the carbon fixed by autotrophs through photosynthesis) is assessed extensively by limnologists, its measurement presents methodological challenges. The objectives of this study were to (1) address some of these challenges by proposing an improvement to current mathematical models used to calculate cumulative (e.g., seasonal or annual) production rates from instantaneous (e.g., hourly) production values, and (2) explore the response of primary production to 3 decades (1983-2014) of nutrient management in Lake Lugano (Switzerland and Italy) as a case study. Objective 1 was prompted by difficulties in estimating the photosynthetic parameters P-max(b), the maximum photosynthetic potential, and I-opt, the optimum light intensity, in days without photoinhibition. We developed a model extension to simulate these parameters from irradiance and total phosphorus (TP). To this end, we adapted equations previously used in marine studies and calibrated and tested them against values of primary production measured using the carbon-14 (C-14) light-and-dark bottle method during 1983-2014. A Nash-Sutcliffe efficiency of 0.53 and a ratio between the root mean square error and the standard deviation of the observed values (RSR) of 0.68 indicated a satisfactory performance of our model extension. Therefore, we suggest that our modelling approach presents a step toward a more precise quantitative estimate of primary production. Concerning objective 2, in Lake Lugano, annual rates of primary production (measured as C) declined by 21.26 mg m(-2) d(-1) from 1983 to 2014. The environmental drivers included TP in spring, summer, and autumn, and water temperature in winter. These results suggest that primary production in Lake Lugano was limited mainly by TP availability and therefore seemed to be responding to nutrient management.