Dennis served on the Scientific Steering Committee of the LOICZ (Land Ocean Interactions in the Coastal Zone) program of the IGBP from 2005-2011, and has been involved with developing biogeochemical budgets of coastal systems in the Baltic and elsewhere.
His more recent work focuses on nutrient accounting in large watersheds and the relationship of NANI (Net Anthropogenic Nutrient (N&P) Inputs) to riverine nutrient fluxes. Other research has included developing methods of estimating ecosystem metabolism in estuaries, modelling nutrient transport through watersheds to the coast, and modelling ecological responses of watersheds and coastal ecosystems to climate and land use changes.