Evidence supporting the importance of nutrient regeneration by nano- and micrograzers for phytoplankton photosynthesis in Lake Malawi/Nyasa |
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| Authors: | Stephanie J Guildford William D Taylor |
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| Affiliation: | 1. Large Lakes Observatory and Department of Biology, University of Minnesota, Duluth 2205 East 5th Street, Duluth, MN 55812-2401, USA;2. Department of Biology, University of Waterloo, 200 University Avenue West. Waterloo, ON N2L 3G1, Canada;1. Center fort Renewable Energy Development, Route de l’Observatoire, B.P. 62, Bouzaréah-Algiers, Algeria;2. Applied Automation Laboratory, University of Boumerdès, Av. de l’Indépendance, Boumerdès, Algeria;3. National Polytechnic School, Laboratory of Environmental Sciences and Technology, 10 Avenue Hassen Badi, B.P. 182, El Harrach - Algiers, Algeria;1. Zoological Institute, University of Basel, 4051 Basel, Switzerland;1. Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, USA;2. Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712, USA;3. Department of Earth and Environmental Systems, Indiana State University, Terre Haute, IN 47809, USA |
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| Abstract: | Photosynthesis and nutrient status measurements were compared in size fractioned water samples collected in southern Lake Malawi in three different seasons. The size fractions analyzed were 0–2.0 μm, 0–20 μm, and 0–200 μm. Total chlorophyll concentrations were relatively invariant at about 1 μg L?1 in all seasons (wet stratified, deep mixing, dry stratified). Over 90% of total chlorophyll was < 20 μm and 40–50% < 2 μm. Stoichiometric ratios of carbon (C):nitrogen (N) and C:phosphorus (P) for the < 2-μm seston were similar to < 20-μm and < 200-μm seston samples and indicated that all size fractions were moderately N and P deficient in all seasons. N and P uptake experiments demonstrated that when the < 2-μm picoplankton were isolated from the larger sizes, they showed high N and P nutrient debts, apparently in response to the removal of nano- and micrograzers. This effect was strongest for N debt in the dry stratified season, but was apparent for P debt in all seasons. The chlorophyll-normalized light saturated rate of photosynthesis, Pbm, was always higher in whole water samples compared to the rates of the isolated picophytoplankton which were lowest in the dry season. We infer that nano- and micrograzers were an especially important source of regenerated nutrients when the lake was stratified although their removal as a source of nutrient to the picophytoplankton may affect Pbm in all seasons. Nitrogen regeneration was especially critical to the picophytoplankton in the dry stratified season but less important in the windy deep mixing season. |
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