Some decades ago Margalef speculated that study of the exchanges across boundaries that separate different types of ecological systems would provide significant insights about properties and processes within the units that make up ecological mosaics. Although such boundaries might be difficult to define, it seemed likely that such exchanges among units would influence the function and structure of the adjoined systems. In this paper we explore exchanges across such ecological boundaries in coastal ecosystems in Cape Cod, Massachusetts, and elsewhere. We find that, indeed, definition of such boundaries is ambiguous, but study of the exchanges is more useful. In the Cape Cod system, water transport down-gradient is the dominant mechanism exerting influence on down-gradient systems. The direction of ecological control across such boundaries is largely asymmetrical, and properties of up-gradient units exert significant influence on down-gradient units. General properties of donor and receptor parcels are hard to discern, but clearly, parcels making up an ecological mosaic are not independent units, but are coupled by transfers from upgradient tesserae. Studies of controls of ecological systems need to include inter-unit influences as well as internal mechanisms.