A GIS-BASED ASSESSMENT OF POTENTIAL AQUACULTURAL NONPOINT SOURCE LOADING IN AN ATLANTIC BAY (FRANCE)

Abstract

Spatial-explicit quantitative dissolved inorganic nitrogen (DIN) and ortho- phosphorus (OP) loadings were developed from aquatic earthen ponds to the adjacent salt marshes. These loadings were based on water renewal, the concentration of constituents available for all land use, and the spatial positions of constituent sources. Potential loading was assessed at every sub-basin gauge station by adding the individual contribution of every land use polygon. Kriging was also used to spatialize loading from control stations to salt marshes. A grid of concentration values was established specific to the bay stream network using an eight-direction-point model. Analyses showed a more important concentration near intensive activities, from - 15 p,mol/L to 30 pmol/L of DIN and from 2 pmol/L to 7 pmol/L of OP. These values decreased rapidly near the ocean, where a dilution of about 75% was noticed for DIN and 87% for OP. Constituent concentrations were quite sensitive to parameter changes, and the system reacted more intensively to point source concentration changes than to water drain. The use of regional parameters to assess DIN and OP loadings from earthen ponds to salt marshes, based on land use and topography, may be useful when working on a regional scale. Modeling nonpoint sources of DIN and OP from aquacultural land to surface water can be both complex and data intensive. Our goal was to develop a simple model that would account for spatial pattern in topography and land use using Geographic Information System (GIS) databases.