Shifts in the partitioning of benthic and pelagic primary production within and across summers in Lake Mývatn, Iceland

ABSTRACT

The relative contributions of benthic and pelagic primary production affect ecosystem function, but studies documenting natural variation in the partitioning of production (i.e., autotrophic structure) are uncommon. This study examines autotrophic structure of shallow Lake Mývatn over 7 summers (2012–2018). We used routine measurements of benthic gross primary production (GPP), pelagic chlorophyll a concentrations, and pelagic production to estimate benthic and pelagic maximum productivity (P_max) across summers. With these parameters and corresponding incident light and water clarity data, we estimated in situ benthic and pelagic GPP. Our results demonstrated substantial variation in Mývatn’s autotrophic structure within and across summers. Benthic GPP often exceeded pelagic GPP. However, periods of increased phytoplankton abundance were associated with increased light attenuation and, consequently, declines in benthic GPP. These effects were strongest in 3 summers with dense cyanobacteria blooms, in which the benthic fraction of total production declined from >95% to <20%. Reduced light levels over a 2-week period were associated with low benthic P_max, implying that past shading by phytoplankton may decrease the photosynthetic potential of benthic producers. Moreover, variation in estimated benthic P_max values can affect the point at which autotrophic structure shifts toward pelagic-dominated conditions. Overall, our study demonstrates that the balance between benthic and pelagic production can vary greatly at intra- and interannual scales because of changes in the photosynthetic capacity of both pelagic and benthic primary producers. Understanding natural variation in lake autotrophic structure may inform how benthic and pelagic production respond to ongoing and future environmental changes.

KEYWORDS:

• autotrophic structure
• cyanobacteria
• epipelon
• light attenuation
• shallow lakes

Acknowledgements

We thank Árni Einarsson for his assistance in setting up the long-term monitoring, intellectual insights, ongoing logistical support at the Mývatn Research Station, and helpful comments on this manuscript. We also thank the many research interns who assisted with field and laboratory work; M. Bartrons, C. Herren, and K. Webert for overseeing data collection during 2012–2014; and C. Gratton, R. Jackson, B. Peckarsky, and J. Vander Zanden for feedback on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Science Foundation: [Grant numbers DEB-1052160 and DEB-1556208].