Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Abstract

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

Isotopic signatures induced by upwelling reveal regional fish populations in Lake Tanganyika

“…the residual nitrate should be isotopically heavy and phytoplankton relatively light. In line with the higher density of N-fixing cyanobacteria (Hecky and Kling 1981;Descy et al 2010;Ehrenfels et al 2021), the generally lighter δ 15 N-POM in Apr/May (∆-1.4 ‰) point at higher inputs from N fixation compared to Sep/Oct, when nutrient fluxes are higher due to upwelling/mixing. The isotopic composition of the zooplankton community did not show clear latitudinal and seasonal patterns, pointing at additional influencing factors than the signal from the base of the food web, i.e.…”

“…2 h, j). A local nitrate maximum (4.7 µM) at 40 m at station 3 in Apr/May may have been caused by an overlying cyanobacterial bloom (Ehrenfels et al 2021). The upward tilting thermocline in the south enhanced the nutrient transport to the productive surface waters during both campaigns.…”

“…The analogous pattern in δ 15 N-POM implies that upwelling and mixing may have influenced the N sources of primary producers, where lighter values are typically interpreted as inputs from N fixation (Gu et al 2006;Morana et al 2015). POM δ 15 N values increased from -1.0 ‰ at station 3 to 2.7 ‰ at station 9 in the south in Apr/May, concurrent with a decrease in filamentous, N-fixing cyanobacteria (Ehrenfels et al 2021), whereas it fluctuated with slightly higher values (-0.3-2.6 ‰) in Sep/Oct devoid of a latitudinal or phytoplankton composition related pattern. We also observed no correlation between the presence of surface nitrate and δ 15 N-POM, which may have induced fractionation effects during nitrate-uptake.…”

“…This depth interval extends from the typical location of the nitrate peak to the bottom of the euphotic zone (Cocquyt and Vyverman 2005;Descy et al 2005;Ehrenfels et al 2021). For a more detailed description of the thermal structure of the water column see Ehrenfels et al (2021).…”

“…During two lake-wide field campaigns in the final phases of the dry and the rainy seasons, we measured the C and N isotopic and elemental compositions of the major pelagic food web members (POM, zooplankton, the bivalve Pleiodon spekii, fish). These samples were collected in concert with limnological data including the physical properties of the water column, oxygen and nutrient concentrations, chlorophyll, as well as the phyto-and zooplankton community and abundance (Ehrenfels et al 2021). Using the extensive data sets from those two contrasting time points, we first tested to which extend the regional and seasonal patterns in primary productivity induce systematic differences in the isotopic signatures of plankton, and then tracked the isotopic signals and C:N ratios through the food web to the pelagic fish.…”