Effect of nutrient pulses on photosynthesis of Chaetomorpha linum from a shallow Mediterranean coastal lagoon

The influence of nitrogen and phosphorus pulses on Chaetomorpha linum (Muller) Kutzing growth and photosynthesis was studied in laboratory experiments. Photosynthesis and growth of C. linum from Tancada lagoon seems limited by both nitrogen and phosphorus, as indicated by the high rate (4.7-11.6 mg O2 g−1 dry weight h−1) of light-saturated photosynthesis (Pm) and growth rates observed under nitrogen plus phosphorus enrichment in relation to enrichment by nitrogen alone (2.9-7.6 mg O2 g−1 dry weight h−1). Significant increase in nitrogen and phosphorus content as percentage of dry weight was observed in C. linum fertilized with a single nutrient or with nitrogen plus phosphorus. In Tancada lagoon, when availability of nitrogen to primary producers is by pulses, an increase of nitrate concentration in the water column (from 6 to 100 μM) has a greater effect on growth of C. linum (growth rate: 0.13 day−1) than an increase in ammonium concentration (from 20 to 100 μM and growth rate: 0.11 day−1). For a given thallus nitrogen content (0.6-1.4% N), both Pm and the photosynthetic efficiency (α) normalized to dry weight were correlated (r2 = 0.73, p < 0.005) indicating that variations in electron transport were coupled to variations in C-fixation capacity. Optimizing both α and Pm may be a general characteristic of thin-structured opportunistic algae in more variable estuarine environments.