Effects of short-term changes in sediment temperature on the photosynthesis of two intertidal microphytobenthos communities Sónia Vieira a, b , Lourenço Ribeiro a , Jorge Marques da Silva b, c , Paulo Cartaxana a, * a Centro de Oceanografia, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal b Centro de Biodiversidade, Genómica Integrativa e Funcional (BioFIG), Faculdade de Ciências da Universidade de Lisboa,1749-016 Lisboa, Portugal c Departamento de Biologia Vegetal, Faculdade de Ciências da Universidade de Lisboa,1749-016 Lisboa, Portugal article info Article history: Received 5 July 2012 Accepted 2 January 2013 Available online 17 January 2013 Keywords: microphytobenthos chlorophyll fluorescence photosynthesis diatoms migration Tagus estuary abstract Intertidal microphytobenthos (MPB) has been found to maintain high productivity rates despite the variability in various key environmental parameters, namely rapid temperature changes during emer- sion. 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 100e250 mm 3 , particularly Navicula cf. phyllepta, whereas Alcochete had higher relative abundances for size-class 250e1000 mm 3 , dominated by a mixture of diatom species of the genera Navicula, Thalassiosira and Gyrosigma. The Trancão MPB community had higher photosynthetic capacity (higher ETR max ), was photoacclimated to higher irradiances (higher E k ) and had lower efficiency at limiting irradiances (lower a). 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 15e35  C range and both ETR max and a 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. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The intertidal flats of estuaries are characterized by the presence of microphytobenthos (MPB) communities dominated by diatoms, forming highly dense biofilms on the sediment surface. Intertidal MPB communities are responsible for a significant fraction of the total primary productivity of estuaries (Underwood and Kromkamp, 1999), despite the extreme variability in various key environmental parameters (e.g. irradiance, salinity or temperature). Intertidal sediment temperature fluctuations occur on long (seasonal) and short (daily and hourly) time scales, depending on factors such as meteorological conditions, time of day and tidal inundation. In summer, intertidal sediment temperature can easily change 10e15  C during an emersion period, reaching values as high as 35  C at midday (Blanchard et al., 1997; Serôdio and Catarino, 1999). Short-term (hours) temperature changes, similar to those experienced by intertidal MPB communities during a tidal cycle, have been shown to have a significant effect on the photo- synthesis of cultured benthic diatoms (Admiraal, 1984; Morris and Kromkamp, 2003; Salleh and McMinn, 2011) and suspensions of intertidal MPB (Blanchard et al., 1996, 1997). In these studies, the described relationship between maximum photosynthetic capacity (P max ) and temperature is a progressive increase of P max with increasing temperature up to an optimum value, beyond which P max declines rapidly (Blanchard et al., 1996). Although both ap- proaches (cultures and suspensions) may provide valuable infor- mation regarding the potential photosynthetic responses of benthic diatoms to short-term changes in temperature, the results thus obtained may not accurately represent the photosynthesis of these organisms while part of an MPB biofilm. The aim of this study was to characterize the effects of short- term temperature changes on the photosynthetic activity of two intertidal MPB communities of the Tagus estuary. Most studies on MPB ignore species composition and treat the assemblages as a black box (Underwood, 2005). In this study, we present a detailed * Corresponding author. E-mail address: pcartaxana@fc.ul.pt (P. Cartaxana). Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.01.001 Estuarine, Coastal and Shelf Science 119 (2013) 112e118