Effects of short-term changes in sediment temperature on the photosynthesis of two intertidal microphytobenthos communities

Intertidal microphytobenthos (MPB) has been found to maintain high productivity rates despite the variability in various key environmental parameters, namely rapid temperature changes during emersion. The effects of short-term (30 min and 2 h) changes in temperature (15, 25, 35 and 42 °C) on the photosynthetic activity of two intertidal MPB communities (Trancão and Alcochete) of the Tagus estuary were studied using imaging pulse amplitude modulated (Imaging-PAM) fluorometry. MPB communities differed in species composition and size-class distribution: Trancão was dominated by diatoms of the size-class 100–250 μm3, particularly Navicula cf. phyllepta, whereas Alcochete had higher relative abundances for size-class 250–1000 μm3, dominated by a mixture of diatom species of the genera Navicula, Thalassiosira and Gyrosigma. The Trancão MPB community had higher photosynthetic capacity (higher ETRmax), was photoacclimated to higher irradiances (higher Ek) and had lower efficiency at limiting irradiances (lower α). The different taxonomic composition and size-class distribution could explain the observed results, as small cells are usually more active due to larger surface to volume ratios. Photosynthetic capacities of the two studied MPB communities increased with temperature until 35 °C. Photosynthetic efficiencies were not affected by temperature in the 15–35 °C range and both ETRmax and α decreased at the extreme temperature of 42 °C. MPB communities were able to increase photosynthetic capacity and productivity under transient exposure to high sediment temperatures, similar to that observed during summer midday low tides.