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The role of algal size as a controlling factor for nltrogen uptake kinetics is examined by comparing published values of N uptake rate and half-saturation constants in micro-and macroalgae. The uptake kinetics differ substantially among algae very different in size. Microalgae take up nitrogen much faster per unit of biomass than macroalgae at both high and low substrate concentrations, and microalgae have significantly higher affinity for nitrogen than macroalgae. These typical differences in the uptake kinetics among small and large algae are commonly attributed to size-specific differences in the relative surface area (SA:V). Regression analysis demonstrates that size-specific variations in the kinetic parameters can be attributed to changes in relative surface area over an extensive range of algal sizes, covering both micro-and macroalgae. These results agree with previously described relationships between maximum uptake rate of nutrients (nitrogen and phosphorus) and SA:V within narrow size-ranges (either phytoplankton or macroalgae), and emphasize the existence of a general coupling between physiological and morpholog~cal properties in algae. KEY WORDS: Nitrogen uptake Microalgae . Macroalgae . Allometry 0 Inter-Research 1995 Resale of full article not permitted

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1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate. 2. The influence of total dissolved inorganic carbon (DIC) and free CO2 on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1–120 μg l–1), pH (5.6–9.6) and DIC concentration (0.02–2.5 mm). Photosynthesis experiments were performed using the 14C technique in CO2‐manipulated water samples, either by changing the pH or by adding/removing CO2. 3. Lake waters were naturally CO2 supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO2 concentrations declined. Similarly, photosynthesis was limited by low CO2 concentrations during phytoplankton blooms in the hardwater alkaline lakes.

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