Influence of phosphorus and irradiance on phytoplanktonic chlorophyll-a concentration and phosphorus contents at a diel scale in a Mediterranean reservoir

Influence of phosphorus and irradiance on phytoplanktonic chlorophyll-a concentration and phosphorus contents at a diel scale in a Mediterranean reservoirPhosphorus concentration in the water is one of the main factors regulating phytoplankton biomass and productivity in inland water bodies.However, phosphorus uptake by algae could not cause immediate growth, because other factors (light and other nutrient availability) may limit production.Nonetheless, "luxurious" phosphorus uptake (i.e.phosphorus uptake beyond the algal requirements) has been observed, and it has been interpreted as storage to use in situations of low nutrient availability.Thus, the assessment of the effect of phosphorus supply on algal growth is not straightforward, especially at very short time scales.In this study, we analyze the relationships between environmental phosphorus levels and internal phosphorus and chlorophyll-a contents in phytoplankton at a diel scale in a Mediterranean reservoir, considering the different algal (intracellular and membrane-associated) and water phosphorus pools.We also evaluated the influence of light on these relationships by sampling at two water depths with different irradiance levels.Our hypothesis is that chlorophyll-a and intracellular phosphorus contents in phytoplankton are both influenced by ambient phosphorus and irradiance levels, which are complementary resources for algae as the nutrient-light hypothesis proposes.Phosphorus concentration and relative contribution of each phosphorus fraction was similar at both sampling depths.Total phosphorus concentration was dominated by the particulate pool (70 %), and dissolved inorganic phosphate represented only one third of the dissolved pool.Total phosphorus content and the relative contribution of the different pools in algal cells were similar at both sampling depths.Intracellular phosphorus pool was on average 77 % of the total nutrient content, while phosphorus associated to membranes represented the remaining of the total pool.Mean intracellular SRP concentration was 36 % of the intracellular phosphorus content.None of the algal phosphorus pools showed significant correlations with underwater light levels at both sampling depths.Total chlorophyll-a concentration showed no significant correlations with the several water phosphorus pools at both sampling depths, because each algal group responded differently to environmental phosphorus levels, and these responses also varied with depth.We found significant relationships between the intracellular phosphorus content and the different phosphorus pools in water, but this almost 'automatic' response of algae to phosphorus levels in water was not reflected in changes in the chlorophyll-a content, at least within the 3-day time frame of this study.A possible explanation is that processes associated to nutrient uptake and biomass construction operate over different time scales.The results of this work emphasize the complexity of the links between environmental phosphorus concentration and phytoplanktonic phosphorus content and biomass, and the importance of scale in analysing such relationships.