Enzymatic activity and organic substrates on a sandy beach of the Ligurian Sea (NW Mediterranean) influenced by anthropogenic pressure.

Enzymatic activities (leucine aminopeptidase, LA; β-glucosidase, BG; alkaline phosphatase,AP), their related substrates and the bacterial abundance and biomass were studied over 1 yr in 2 adjacent areas of a sandy beach: one occupied by the structures of a private swimming establishment and undergoing major reconstruction work (impacted area), and the other in a more natural area (reference area). Both areas were oligotrophic, characterised by low organic matter content (with a notable contribution from the autotrophic biomass) and low enzymatic activity values. Bacterial abundance did not show significant changes, either spatially or temporally, suggesting that they quickly overcame anthropogenic forcing and seasonal luctuations under the environmental and trophic conditions. The absence of positive correlations between carbohydrates and enzymatic activities suggested the adverse influence of refractory organic matter. Nevertheless, when the environmental conditions were favourable and bacteria were supported by larger N inputs from the sea, the carbohydrate became a trophic resource, thus suddenly changing the enzyme ratios. A multivariate analysis highlighted the predominance of the seasonal over the anthropogenic influence, suggesting a rapid recovery in the impacted area. Nevertheless, changes were observed for the functional relationships between the parameters, in particular those related to the trophic quality of the organic material (lower in the impacted area) and the enzymatic hydrolysis. The coupling of LA and AP was generally tight in order to overcome the P deficiency and, in the impacted area, to overcome trophic limitations related to the input of inorganic debris and refractory organic materials.