Spatial and temporal variations of nutrients and chlorophyll a in a Mediterranean coastal lagoon: Varano Lagoon, Italy

Varano lagoon, located on the northern coast of the Gargano promontory (Southern Adriatic Sea), Italy, is a shallow body of water, with a depth ranging from <1m near the coast to 5m in the central zone. It is an ecosystem with a long tradition in aquaculture practice and shows no strong anthropogenic pressure, although within its watershed, agricultural practices are evolving into intensive irrigated crops, with the result of increasing nutrients reaching the lagoon. Surface water samples during 12 cruises, from 2003 to 2005, were collected to study spatial and temporal variations of nutrients and chlorophyll a and to assess the trophic state through the use of specific indices. Statistical tests were used to investigate the relations between variables and stations, and assess the significance of the difference between means. The Spearman R index revealed a significant correlation (P<0,01) between several variables. The dissolved nutrients displayed significant correlations among themselves, except phosphate which is well correlated only with total phosphorus. A seasonal and spatial variation at the sampling stations representative of the resulting clusters show a random pattern for soluble reactive phosphorus in relatively low concentrations (0,15 ± 0,05 ?M), a well-defined seasonal pattern for both dissolved inorganic nitrogen (DIN) and N:P ratio, with highest values recorded in the wet season near the freshwater and wastewater inputs.