SUBMERGED AQUATIC VEGETATION IN RELATION TO DIFFERENT NUTRIENT REGIMES

Submerged vegetation respond to increased nutrient loading through a shift from slow-growing seagrasses and large macroalgae to fast-growing macroalgae, and the ultimate dominance of phytoplankton at high nutrient loadings. This shift reflects a change from nutrient to light limitation along the eutrophication gradient. Slow-growing seagrasses and large macroalgae are good competitors when nutrients are limiting because they have relatively low nutrient requirements, are able of efficient internal nutrient recycling, and can access the elevated nutrient pools in the sediment. Fast-growing macroalgae and phytoplankton are superior competitors when light is limiting because they are positioned closer to the water surface, and capture and use light more efficiently. The important ecosystem consequences of altered nutrient regimes derive from the shift in dominant vegetation types. Slow-growing seagrasses and large macroalgae are longevous, decompose slowly, and experience only moderate grazing losses, whereas the production of fast growing macroalgae and phytoplankton is transferred faster to heterotrophs, through increased grazing and decomposition rates. Recovery of submerged vegetation following nutrient reduction plans is a very slow process, which involves the replacement of fast-growing for slow-growing plants. Simulation models predict recovery times to oscillate between a few years for macroalgae and fast-growing seagrasses to centuries for slow-growing seagrasses following nutrient reduction plans.