12165 research outputs found
Sort by
Dynamics and limitations of phytoplankton biomass along a gradient in Mwanza Gulf, southern Lake Victoria (Tanzania)
No full textSummary
1.We investigated how dynamics in phytoplankton biomass are driven through light and nutrient limitations in the Mwanza Gulf, Lake Victoria (Tanzania).
2.We measured light attenuation, chlorophyll a and water quality parameters along a phytoplankton biomass gradient in the Mwanza Gulf at six sampling stations in three different seasons from August 2009 to March 2011.
3.We found a higher rate of attenuation of photosynthetic available radiation than in other gulfs of Lake Victoria which could only partially be explained by phytoplankton biomass, measured as chlorophyll a. Suspended particulate detritus strongly affected light attenuation, particularly in shallow water.
4.In shallow water in the southern part of the Mwanza Gulf, nutrients were in excess and phytoplankton biomass was limited by light. In deeper water near the entrance of the gulf, light was not limiting when nitrogen probably was, as indicated by the N:P ratio.
5.Low abundance of N-fixing phytoplankton species suggests that N-fixation was low in shallow water. Phytoplankton biomass was lower in Mwanza Gulf than in northern gulfs of Lake Victoria, but might increase in the future as a result of nutrient enrichment caused by an increase in intensive agriculture and rapid growth of the city of Mwanza.
Nitrogen fixation and respiratory electron transport in the cyanobacterium Cyanothece under different light:dark cycles
No full textIncompatibility of nitrogen fixation and oxygen production compels unicellular diazotrophic cyanobacteria to perform photosynthesis during daytime and restrict nitrogen fixation to nighttime. The marine diazotroph Cyanothece BG 043511 was grown in continuous culture under three light/dark regimes (16L : 8D, 12L : 12D, and 8L : 16D h); we monitored nitrogen fixation and potential photosynthetic efficiency simultaneously online to reveal how their temporal separation is affected by different LD regimes. An increase in nitrogen fixation rate at night coincided with a rise in pulse-amplitude modulated fluorescence, indicating that the enhanced respiratory electron transport to fuel diazotrophy affects the oxidation state of the plastoquinone pool. This may offer an alternative approach to assess instantaneous nitrogen fixation activity. Regardless of photoperiod, the maximum rate of nitrogen fixation was conserved at about 20 h after the onset of the light. Consequently, nitrogen fixation rates peaked at different moments in the dark: relatively early in the 16L : 8D cycle, at midnight in 12L : 12D, and relatively late in 8L : 16D. Under 16L : 8D, nitrogen fixation extended into the light, demonstrating the functional plasticity of nitrogen fixation in Cyanothece. Highest daily amounts of nitrogen fixed were obtained in 12L : 12D, which is consistent with the natural LD cycle of subtropical latitudes in which Cyanothece thrives.
Clutch-size variation in Western Palaearctic secondary hole-nesting passerine birds in relation to nest box design
No full text* Secondary hole-nesting birds that do not construct nest holes themselves and hence regularly breed in nest boxes constitute important model systems for field studies in many biological disciplines with hundreds of scientists and amateurs involved. Those research groups are spread over wide geographic areas that experience considerable variation in environmental conditions, and researchers provide nest boxes of varying designs that may inadvertently introduce spatial and temporal variation in reproductive parameters. * We quantified the relationship between mean clutch size and nest box size and material after controlling for a range of environmental variables in four of the most widely used model species in the Western Palaearctic: great tit Parus major, blue tit Cyanistes caeruleus, pied flycatcher Ficedula hypoleuca and collared flycatcher F. albicollis from 365 populations and 79 610 clutches. * Nest floor area and nest box material varied non-randomly across latitudes and longitudes, showing that scientists did not adopt a random box design. Clutch size increased with nest floor area in great tits, but not in blue tits and flycatchers. Clutch size of blue tits was larger in wooden than in concrete nest boxes. * These findings demonstrate that the size of nest boxes and material used to construct nest boxes can differentially affect clutch size in different species. The findings also suggest that the nest box design may affect not only focal species, but also indirectly other species through the effects of nest box design on productivity and therefore potentially population density and hence interspecific competition. [KEYWORDS: geographic location habitat latitude longitude nest box floor area nest box material]
