Matthew C. Van de Bogert, Stephen R. Carpenter, Jonathan J. Cole ...

Van de Bogert et al.Benthic and pelagic metabolismonly pelagic metabolism, but rather something between thesetwo endpoints. Determining whole-lake metabolism requiresan evaluation of the spatial heterogeneity in one’s study system:How large is the littoral zone—is it 1% or 50% of lakearea? How different are simultaneous metabolism estimatesfrom near shore and at the center of the lake? In larger lakes,are plankton patchy? Questions such as these should beaddressed over a period encompassing a representative rangeof weather patterns (e.g., wind, solar radiation, precipitation),and the answers will help investigators decide whether pursuingmeasurements at higher spatial resolution is necessary oruseful for their particular goals.Although the cost of sensors is dropping, it is still significant.Thus, more work is needed to determine the minimumnumber needed for accurate estimates of whole-lake metabolismacross systems. Our data show that on days with highwind speed, measurements are not significantly differentacross sites and therefore the only benefit of many sensors isto verify the uniformity of metabolism estimates. However,on days with low wind speeds, variation among sites may behigh or low and is not predictable with the data we collected.On low wind days, this study shows that for lakes similar toPeter Lake, the number of sensors needed is >1 and will likelyneed to include enough to separately sample the littoral zone,the pelagic zone, and perhaps a few places along the gradientbetween. Although Peter Lake is small, we suspect theseresults will be applicable for many lakes given that 97% of theworld’s lakes are smaller than or equal to the area of PeterLake and that lakes the same size or smaller account for 22%of the worldwide surface area of lakes (Downing et al. 2006).This study demonstrates the potential for in-lake heterogeneityto bias single-location estimates of whole-lake metabolismby underestimating benthic contributions. The degree towhich similar bias occurs in larger lakes is a question that stillneeds to be studied.Investigators interested in partitioning benthic and pelagicmetabolism from whole-lake values will need to employ severalsensors (4 to 6 were used in this study) along a littoralpelagicgradient. Although this study did not address the optimalnumber of sondes needed to answer this question, wenote that a minimum of 3 sensor locations is needed to be ableto simultaneously estimate the two end members of metabolismand the rate of mixing along the transect.The increased use of automated data acquired from sensorsalong with declining cost is leading to enhanced potential tomeasure ecosystem processes. Free-water oxygen measurementslike those used in this study overcome many of thelimitations of bottle and chamber methods, but there is stillmuch to learn about these methods, particularly what types ofsampling regimens are necessary to adequately capture ecosystemestimates of processes like metabolism. Without furtherevaluation of sensor-based methods, process estimates will sufferambiguities similar to traditional techniques. For example,just as traditional 14 C primary productivity methods measuresome unknown value between net and gross primary production(Peterson 1980), a single centrally located oxygen sensormay measure some unknown value between pelagic metabolismand whole-lake metabolism. These ambiguities may bereduced with improved sampling designs coupled with modelsthat incorporate spatial heterogeneity in ecosystem processes.ReferencesBade, D. L., and J. J. Cole. 2006. Impact of chemicallyenhanced diffusion on dissolved inorganic carbon stableisotopes in a fertilized lake. J. Geophys. Res. Oceans 111:C01014. [doi:10.1029/2004JC002684].Caraco, N. F., and J. J. Cole. 2002. Contrasting impacts of anative and alien macrophyte on dissolved oxygen in a largeriver. Ecol. Appl. 12:1496-1509.Carpenter, S. R., et al. 2005. Ecosystem subsidies: terrestrialsupport of aquatic food webs from C-13 addition to contrastinglakes. Ecology 86:2737-2750.Carpenter, S. R., and J. F. Kitchell. 1993. 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