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.
Highlights
Cyanobacterial blooms are globally increasing in frequency and severity due to human-induced alterations of natural aquatic ecosystems
We examined whether the relative position of the thermocline to the euphotic depth had an effect on diazotrophic cyanobacteria abundance in Apr/May, whereas we had no irradiance data for such an analysis in Sep/Oct
Our data imply that high densities of diazotrophic cyanobacteria
Summary
Cyanobacterial blooms are globally increasing in frequency and severity due to human-induced alterations of natural aquatic ecosystems. This rapid proliferation of blooms motivated numerous studies in the context of eutrophication and warming of lakes and coastal seas. Much less is known about the mechanisms triggering surface cyanobacterial blooms in large, warm oligotrophic marine and freshwater systems (White et al, 2007), such as the Northern Pacific Subtropical Gyre (NPSG), Gulf of Aqaba, Lake Malawi or Lake Tanganyika. Similar to other large, stratified oligotrophic systems (Gordon et al, 1994; Post et al, 2002; Dore et al, 2008; Gondwe et al, 2008), these blooms emerge during periods of enhanced water column stratification in Lake Tanganyika, i.e. at the onset (Hecky and Kling, 1981; Narita et al, 1986; Langenberg et al, 2002; Cocquyt and Vyverman, 2005) and throughout the rainy season (Salonen et al, 1999; Vuorio et al, 2003; Descy et al, 2005)
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