Ian S. Hart

Nutrient supply is an important determinant of N fixation rates in pelagic cyanobacteria. In shallow aquatic ecosystems, benthic primary producers can affect nutrient release from the sediment to the pelagic zone, yet the relationship between benthic primary producers and pelagic N fixation is not well understood. Here, we explore the effect of benthic primary producers on benthic nutrient fluxes and pelagic N fixation rates in shallow, eutrophic Lake Mývatn, Iceland. In paired benthic nutrient flux and pelagic N fixation assays, we showed that sites with increased benthic phosphate release were associated with increased pelagic N fixation. In turn, during assays of flux rates, benthic phosphate release was negatively associated with dissolved oxygen concentrations at the sediment–water interface, which was positively associated with benthic primary producer abundances. Together, these relationships suggest that benthic primary producers influence variation in within-lake pelagic N fixation via their effects on benthic nutrient flux rates.

The community of benthic crustaceans in Lake Mývatn, Iceland, is dynamic, with different taxa showing different patterns of variation through time. Here, we analyze the dynamics of eight crustaceans collected during the summer from a central station in Mývatn over 29 years, 1995–2023. We investigated the hypothesis that fluctuations in the dominant benthic tube-forming midge, Tanytarsus gracilentus , drives fluctuations in benthic crustacean abundance. We also investigated two additional drivers: predatory three-spined stickleback ( Gasterosteus islandicus ) and water clarity as a measure of pelagic phytoplankton density. The analyses identified three groups of crustaceans whose populations are correlated with other members of the same group and which show similar responses to the drivers. The group of infaunal taxa fluctuated asynchronously with T. gracilentus , with the crustaceans reaching high abundances when T. gracilentus had low abundance. The bentho-pelagic taxa and Eurycercus lamellatus (treated as a group by itself) were not strongly influenced by T. gracilentus. These patterns derived from time-series data were supported by spatial data collected from a transect along a gradient of T. gracilentus abundance in 2024; those taxa that were present in the spatial transect showed similar responses to midge abundance. While stickleback and water clarity did have an impact on some taxa, this study largely confirms that T. gracilentus is the major driving force behind changes in the benthic crustacean community dynamics. The different responses of taxa to drivers and their asynchronous fluctuations increase the stability of the benthic crustacean community by ensuring that, in any given year, some taxa are relatively abundant.

In nitrogen-limited lakes, cyanobacterial nitrogen (N) fixation rates can be high. However, the effect of cyanobacterial N fixation on ecosystem N availability depends, in part, on the retention of fixed N. Here, we explore patterns of benthic and pelagic N fixation and fixed N retention in Lake Mývatn, Iceland. Our N budget, updated from previous assessments, showed that the annual rate of lake-scale N fixation needed to account for the deficit between external N supply and N loss varies greatly from year to year, primarily due to variation in the magnitude of N loss via outflow and midge emergences that transport and deposit N on the shore. Extrapolations from recent direct measurements of pelagic and benthic N fixation under scenarios of different benthic and pelagic N-fixer abundances showed that the capacity for N fixation is much higher in the pelagic than the benthic zone. We then explored the degree to which N fixed in the pelagic zone is retained and recycled within the lake using a dynamic N budget that leverages data on patterns of N loss via the outlet river Laxá. This revealed that approximately two-thirds of organic N in the pelagic zone must be incorporated into the benthos to explain patterns of N loss via outflow from the river Laxá. Together, these results suggest N fixation in pelagic cyanobacterial blooms may supply most of the N for benthic primary production in Mývatn.