Effect of nitrogen and phosphorus additions on a benthic microbial mat from a hypersaline lake

The effect of the increase in availability of inorganic nutrients (nitrogen and phosphorus) on a benthic microbial mat from the hypersaline Lake Salada de Chiprana (NE Spain) was assayed by nutrient-addition experiments in mesocosms, using 16 d incubation periods. The upper layer of the mat was dominated by diatoms, whereas the cyanobacterium Microcoleus chthonoplastes and filamentous green bacteria containing Bacteriochlorophyll (Bchl) d dominated deeper layers. The addition of nitrogen alone favoured an increase in total abundance of diatoms with respect to cyanobacteria, although without significant changes in the relative abundance of different diatom genera. Furthermore, this treatment resulted in decreased rates of gross oxygenic photosynthesis. In contrast, phosphorus additions, either alone or supplemented with nitrogen, increased the relative abundance of cyanobacteria with respect to diatoms. No direct effects of the different treatments on green bacteria were observed. The balanced availability of high amounts of both N and P supported an increase in biomass of oxygenic phototrophic populations, especially cyanobacteria. This was attributed to a combination of stimulating active growth and decreasing senescence; the latter was reflected by more than 3-fold increase of the ratio of chl a to phaeophytin a. Dinitrogen fixation was measured indirectly as acetylene reduction activity (ARA). ARA rates were low at the start of the experiments and remained low in the N treatment, while a strong stimulation was found in the control, P and N+P treatments. Because M chthonoplastes is not capable of fixing dinitrogen, the stimulation of ARA was attributed to nitrogenase activities of heterotrophic bacteria. Our findings are generally in agreement with the resource ratio theory that predicts directional changes of community structure as a consequence of changing resource supply ratios. In addition to the effect of nutrient supply regimes on taxonomic structure of benthic communities, we also observed changes in some of their functional properties. However, application of this theory to sediment systems is not straightforward, because nutrient supply rates depend on the combined effect of mass transfer from the water column to the sediment, internal recycling and geochemical processes (precipitation/dissolution) in the sediment. Sediment processes are difficult to quantify.