Water column primary production is a major term in the organic carbon cycle, particularly in large lakes with relatively reduced shoreline and near-shore influence. Presently, there is a large imbalance in the known inputs vs. outputs of organic carbon in Lake Superior. This study examined primary production in offshore Lake Superior using in situ incubations over a range of conditions representing an annual cycle. Primary producers were dominated by small (< 20 μm) cells and included a relatively large abundance of small, spherical flagellates. During conditions with a warm surface layer, chlorophyll concentrations were two- to three-fold higher within the deep chlorophyll maximum (DCM) than at the surface. Volumetric production (mass L- 1 d- 1) was maximal at 2-10 m depth, well above the typical DCM depth. On average, 22% of 14C label appeared in the dissolved pool at the end of the incubation period with the rest appearing in GF/F-strained particles. A statistical model for volumetric production explained 93% of the variance in individual measurements for depths > 2 m, using temperature and light as predictors. This model was applied to annual fields of temperature and light, and a new estimate for whole-lake annual primary production, 9.73 Tg y- 1, was derived. This combination of new measurements and modeling results brings the organic carbon cycle of Lake Superior closer to being balanced.
Bibliographical noteFunding Information:
This work is the result of research sponsored by the Minnesota Sea Grant College Program supported by the NOAA office of Sea Grant, United States Department of Commerce , under grant No. NA07OAR4170009 . The U.S. Government is authorized to reproduce and distribute reprints for government purposes, not withstanding any copyright notation that may appear hereon. This paper is journal reprint No. JR 561 of the Minnesota Sea Grant College Program. This material was also based on work supported by the National Science Foundation, while the author was working at the Foundation.