LACUSTRINE AND SUBMARINE GROUNDWATER DISCHARGE FUEL EUTROPHICATION (Invited Presentation)

Eutrophication is a major threat to lakes and coastal waters. Typical anthropogenic nutrient sources include fertilizers, manure and sewage. Nutrient import pathways, such as streams, atmospheric deposition, and groundwater seepage (lacustrine groundwater discharge LGD), need to be quantified as a first step toward effective lake management. Nutrient loading via some pathways can be quantified quite accurately but the estimation of nutrient import to lakes by LGD is usually much more difficult. For this and several other reasons that we will discuss, LGD commonly is assumed to be small and often has been disregarded in lake nutrient budgets. Similar to LGD, submarine groundwater discharge (SGD) transports nutrients to coastal water bodies. We review the international literature, give a brief overview of measurement techniques, and present volumes, concentrations and loads reported in literature for both LGD and SGD. Most measurement techniques are based on separate determinations of seepage volume and nutrient concentration of exfiltrating groundwater, and then multiplying both values. However, concentrations depend on where along a groundwater flow pathway a water sample is collected. Therefore, we also describe the fate of nitrogen and phosphorus on their subsurface pathway from the catchment through the reactive interface into the surface water body. As a case study we present results from Lake Arendsee (Germany) where LGD is the major driver of intense eutrophication in the lake. More than 50 % of the total P load is groundwater-borne.