## 7 Algal Primary Production  
### Microalgae and Cyanbacterial Mats  
A highly diverse mixture of chlorophytes, diatoms, phytoflagellates, cyanobacteria, and other microphotoautotrophs (e.g., microplankton) living on the surfaces of sediments, as epiphytes on leaves, other litter, decomposing wood, and on living stems and above-ground roots are additional sources of fixed carbon in mangrove forests; they all are of considerable trophic importance in mangrove food webs. It was once thought that under closed canopies with unfavourably low light conditions, the benthic algal biomass and production was low [172]. However, more recent studies at more locations and using more sensitive measurements have found that microalgal and cyanobacterial communities are well adapted to low light intensities and are thus more abundant and productive than previously believed [173, 174]. Few studies have measured microhypotbethic (MPB) production, so the sample size is small (\(n=35\)). The mean production averaged 1.5 \(\pm\) 2.5 (\(\pm\)1 SD) g C m\({}^{-2}\) d\({}^{-1}\) with a range of 0.0-10.5 g C m\({}^{-2}\) d\({}^{-1}\) and a median of 0.6 g C m\({}^{-2}\) d\({}^{-1}\)[54, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191]. The production did not correlate with the temperature, photon flux density, or salinity [172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 191]. These data include five data points of cyanobacterial mat production, which averaged 2.9 \(\pm\) (4.3 1 SD) g C m\({}^{-2}\) d\({}^{-1}\) with a range of 0.4-10.5 and a median of 1.2 [175; 176; 177; 178]. Considering the small sample sizes of both the microalgal and cyanobacterial NPP, further effort is imperative to more clearly gauge their production in relation to the forest type and age, sediment type, light intensity under the canopy, porewater salinity and pH, temperature, changes in sea level, precipitation, and tidal elevation. More urgent still, the areal extent and variability of these photoautotrophs within mangrove forests are required to accurately estimate their ecosystem-scale contribution to the mangrove forest NPP. In two recent studies, the benthic microalgal biomass in mangrove forests was almost double those measured in adjacent bare tidal flats, although the production in mangroves was greatly reduced. This finding was explained as a possible accumulation of algal biomass due to a slower tidal flow within the forest and an increased biomass per cell due to the expansion of its chloroplast to receive more light with photo- and physiological acclimation [173]. These results are supported by measurements in two contrasting estuarine and oceanic mangrove forests in Hong Kong, in which the microalgal abundance and diversity were equal if not higher in mangroves than on tidal flats [174]. The study found that the porewater salinity, irradiance level, and surface sediment temperature and pH explained 24% of the variation. Moreover, pennate diatoms dominated the mangroves as they do in many other mangroves [189], and cyanobacteria were common at the estuarine forest; the mangrove microphytobenthos exhibited photosynthetic performances characteristic of low light acclimation. However, the role of herbivores in grazing down biomass was not assessed but may be significant. The energetics of photosynthetic microphytobenthic communities play an important role in the production and exchange of CO\({}_{2}\) between the mangrove sediment and the atmosphere [185]. Under light conditions, photosynthesis by MPB results in a higher CO\({}_{2}\) uptake and higher CO\({}_{2}\) respiration in the dark. Further, these effects can be seasonally variable in the subtropics as CO\({}_{2}\) fluxes are more negative in winter light and more positive in the dark, indicating that MPB respiration and photosynthesis change the total sediment CO\({}_{2}\) flux patterns [189].