A variety of characteristics ranging in scale from patch-level to 10's of km were necessary to account for inter-bed variability in performance of multiple functions. Moreover, large inter-annual variability makes it more difficult to assess or predict patch performance at given locations or specific times.
In the tidal freshwater Hudson River we have mapped SAV occurrence and determined its functional significance. In much of the Hudson, distinct invertebrate communities inhabit areas vegetated with SAV (Strayer, D.L., C. Lutz, H. M. Malcom, K. Munger, and W. H. Shaw. 2003. Invertebrate communities associated with a native (Vallisneria americana) and an alien (Trapa natans) macrophyte in a large river. Freshwater Biology 48: 1938-1949.) The abundance of some fishes, such as spottail shiners, can be much higher in vegetated than bare areas.
There are two predominant SAV species, native water celery (Vallisneria americana) and exotic water chestnut (Trapa natans). Water celery covers a three-fold greater area (~ 6% of the river bottom area) compared to the floating-leafed water chestnut, but water chestnut has roughly 10X the average plant biomass/m2. In further contrast, water celery has the capacity to generate oxygen, with larger plant beds having oxygen concentrations that are super-saturated for as long as 12 hours per day (Findlay, S. E. G., W. C. Nieder, and D. T. Fischer. 2006. Multi-scale controls on water quality effects of submerged aquatic vegetation in the tidal freshwater Hudson River. Ecosystems 9:84-96.). Water chestnut releases oxygen directly to the atmosphere, with submerged plant parts contributing to dissolved oxygen reductions. (Caraco, N. F., and J. J. Cole. 2002. Contrasting impacts of a native and alien macrophyte on dissolved oxygen in a large river. Ecol. Appl. 12(5):1496-1509. Hummel, M., and S. Findlay. 2006. Effects of Water Chestnut (Trapa natans) Beds on Water Chemistry in the Tidal Freshwater Hudson River. Hydrobiologia 559:169-181.)
Water clarity in shallow areas inhabited by water celery is poorer than conditions in deeper portions of the Hudson, exacerbating the light-limitation of this submerged plant. These plant beds are subject to inadvertent disturbance by recreational boaters because at high tide the plants are not visible at the water's surface. A volunteer-based monitoring effort has collected data on approximately 100 transects distributed along the length of the Hudson. These data are useful in describing inter-annual variability in plant cover, filling an important gap between years when aerial photographs are taken.