Objective 1: Measure nitrification rates seasonally at selected stations along a near-shore/ off-shore transect that includes a significant agricultural tributary (Sandusky River) to the Sandusky sub-basin and the Central Basin.

Objective 2: Measure denitrification rate potential in sediments and the water column seasonally along the same transect.

Objective 3: Develop an ecosystem nitrogen budget model for Lake Erie to provide additional information for constructing future management strategies.

We suggest that these zones of hypoxia may be caused, at least in part by N-loadings, and that a major ecosystem consequence of hypoxia may be the effects on N-dynamics in this lake and other Laurentian Great Lakes. The loadings of non-point source nitrate and dissolved organic N (DON) have steadily increased over the past decade. Decomposition of DON results in the release of ammonia and ammonium that can be assimilated or, more likely, oxidized to nitrite (e.g. by Nitrosomonas); subsequently nitrite is oxidized to nitrate (e.g. by Nitrobacter) in an oxygen-consumptive process. In hypoxic sediments and hypolimnetic water column, nitrate is often reduced by many species of denitrifying bacteria and released from the system as N2.

Management strategies for Lake Erie have focused solely on control of P-loading; loading of N, has been virtually neglected. The consequences of large amounts of N-loading have been largely uninvestigated, even though they may have significant effects on the health and function of the lake ecosystem. This proposal will investigate a reasonable hypothesis that unabated loading of N can contribute significantly to formation of large zones of hypoxia (through nitrification), which in turn leads to significant losses of N (through dissimilatory denitrification) from the lake ecosystem. Our findings will be integrated into a model useful for development of rational plans for management of nitrogenous resources in the Lake Erie watershed.

Method 1: Nitrate accumulation mesocosms under ambient conditions.

Method 2: Acetylene block mesocosms under ambient conditions.

Method 3: Nitrogen budget model will consider watershed and atmospheric nitrogen loadings, internal processing, and losses via hydrologic export, sedimentation, and gaseous exchange.