Impact of light regimen and self-shading by algal cells on primary productivity in the water column of a shallow tropical lake (Lake Naivasha, Kenya)

Light is the factor that fundamentally determines the level of primary production. Primary productivity is controlled by bottom‐up factors (nutrient levels), with secondary effects due to biological factors (zooplankton grazing). The light regimen experienced by algae is determined largely by mixing of the water column. The processes of photoinhibition and light attenuation combine with the resultant light–dark adaptation to shape the vertical productivity profile and it is important to determine their importance separately because they vary spatially and temporally. Daily variations in productivity can be as large as longer‐term changes and events causing high productivity may not be important in whole‐lake terms. At approximately 0.5 m depth, there is maximal photosynthesis causing depletion of nutrients, but there is vertical homogeneity in productivity control and horizontal similarity in productivity levels. There is zero productivity below 5 m depth due to light attenuation but, if the benthos becomes productive due to an environmental change, such as decreased turbidity, overall productivity may rise appreciably. Self‐shading was investigated by incubating serial dilutions of whole lakewater with filtered water to reduce the density of algal cells. Photosynthetic efficiency was calculated as productivity per unit of chlorophyll a. Self‐shading showed different responses for conditions of ‘low’ and ‘high’ productivity. With low productivity, there was a shading loss of 17%, while for high productivity this was 46%. Thus, self‐shading is seen to have a considerable impact on potential productivity and may affect phytoplankton–cyanobacterial community structure with implications for lake management.     

Limnology and pollution of a small, shallow tropical water-body (jagüey) in North-East Mexico

The environmental characteristics and degree of pollution of a jagüey (a small, shallow tropical water‐body) in Mexico were identified by means of an environmental impact matrix. Jagüeys are important as their main sources of water are the subsurface water‐table and rainfall, however, they have not been studied because of their small size. A flow diagram was designed to incorporate the main climatic, geological and hydrological environmental factors, as well as physico‐chemical data on sediment and water qualities, and biological, technostructural and socioeconomic aspects. The resulting index of –90 in the environmental impact matrix indicates a severe negative impact on jagüeys from the input of allochtonous nutrients because of their small depth, limited circulation and the high rate of evaporation. The jagüeys studied appear in a state of hypertrophy and senility, despite geological youth.     

Chemical and biological features of a high altitude lake in the southern Alps (Laghetto Inferiore, Switzerland)

Since 1980, surveys have been performed nearly every three years on approximately 50 small high altitude alpine lakes located in the Swiss part of the Lake Maggiore watershed. On the basis of this information, since 1996 Laghetto Inferiore has been included in the Mountain Lakes Research (MOLAR) project, an extensive European cooperative research project with 23 partners. Laghetto Inferiore is located at 2074 m a.s.l.; its watershed is mainly composed of crystalline silicic rocks; it has a maximum depth of 33 m, and a theoretical renewal time of 41 days. The pH of the water is approximately 5.6; the alkalinity of the surface water is approximately 30 μmol and that of the bottom water approximately 60 μmol. Chemical studies were performed on the inputs from atmospheric deposition and the main tributary streams, the lake concentration (on a monthly basis), and the outflow. The results show that the characteristics of summer deposition (rain) differ significantly from those of winter deposition (snow). By means of a budget model it was possible to distinguish between the contribution from atmospheric deposition and the contribution from weathering. Biological studies revealed a considerable variety of phytoplankton species and a scarcity of zooplankton species. The vertical distribution of the main algal species shows a marked vertical stratification, with biomass concentrations higher in the profundal. Along this profile, the species of the same taxonomic class tend to gather in a particular depth band, while the single classes tend to be distributed at different depths, according to their specific light requirements.