Thermocline depth and euphotic zone thickness regulate the abundance of diazotrophic cyanobacteria in Lake Tanganyika

Abstract

Abstract. In spite of the fact that cyanobacterial blooms are classically associated with high nutrient loadings, there is also abundant evidence revealing that nitrogen fixing cyanobacteria (diazotrophs) can prevail under oligotrophic conditions. The mechanisms favouring diazotrophs in oligotrophic water bodies remain, however, poorly resolved. Here we analyse biogeochemical and ecological factors regulating the distribution of nitrogen fixing cyanobacteria in the oligotrophic Lake Tanganyika using sensor profiles of hydrodynamic conditions, nutrient and pigment analyses, as well as phytoplankton community assessment. During periods of stable or re-establishing water column stratification, we find evidence that the location of the thermocline and the euphotic depth can create a functional niche for diazotrophic cyanobacteria: Nitrogen limitation provides an ecological advantage for an apparent mutualistic interaction between diazotrophs and diatoms when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline. Diazotrophs, comprising the filamentous genera Dolichospermum and Anabaenopsis, are key players under these conditions (up to 41.7 % of phytoplankton community), while they are rare otherwise. By contrast, a thermocline located within the euphotic zone allows rapid vertical transport of nitrate for a thriving nitrate assimilating phytoplankton community that evidently outcompetes diazotrophs. Finally, multiple observations of relatively high diazotroph densities in the upwelling region in the South of Lake Tanganyika imply that they may additionally thrive under high nutrient conditions, when nitrogen is heavily deficient with respect to phosphorous. This study highlights that, under nitrogen deficient conditions, cyanobacterial blooms may form in response to reduced nutrient fluxes to the productive surface waters.

Depth-discrete metagenomics reveals the roles of microbes in biogeochemical cycling in the tropical freshwater Lake Tanganyika

“…1 and S2). In addition, the environmental profiles are similar to those collected in other studies [39][40][41][42]. Water column temperatures ranged from 24 to 28°C, and changes in DO were greatest at depths ranging from~50 to 100 m during the time of sampling dropping to 0% saturation DO around 100 m. The thermocline depths shift vertically depending on the year (Fig.…”

“…S5). Comparatively in 2018, the peak occurred around 50 m [42]. Other profiles collected in 2018 show a nitrate peak between 50 and 100 m in LT [42].…”

Depth-discrete metagenomics reveals the roles of microbes in biogeochemical cycling in the tropical freshwater Lake Tanganyika

“…Despite not having paired physiochemical profiles in 2015, temperature and dissolved oxygen from 2010 to 2013 was highly consistent and showed minimal interannual variability, particularly during our sampling period (July and October) ( Figure 1 , Figure S2 ). Additionally, the environmental profiles are similar to those collected in other studies [40–43]. Water column temperatures ranged from 24 to 28°C, and changes in DO were greatest at depths ranging from ~50 to 100 m during the time of sampling dropping to 0% saturation DO around 100m.…”

Anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in Lake Tanganyika

“…Moreover, evidence from our molecular data and rate process measurements highlight that anammox activity is tightly coupled with organic nitrogen remineralization and nitrification. Even though anammox appeared to dominate at stations 2 and 7, we do not rule out the possibility that denitrification could play an important role in Lake Tanganyika, especially following the development of large algal blooms 35 .…”

“…2c, k), which also reflects changes in the temperature gradient. The onset of the south-east trade winds in May 34 is known to induce nutrient upwelling and the upward tilting of the thermocline, resulting in a shallow and steep temperature gradient to develop in the southern basin 35 .…”