Complex interactions between autotrophs in shallow marine and freshwater ecosystems: implications for community responses to nutrient stress

The relative biomass of autotrophs (vascular plants, macroalgae, microphyto benthos, phytoplankton) in shallow aquatic ecosystems is thought to be cont rolled by nutrient inputs and underwater irradiance. Widely accepted concep tual models indicate that this is the case both in marine and freshwater sy stems. In this paper we examine four case studies and test whether these mo dels generally apply. We also identify other complex interactions among the autotrophs that may influence ecosystem response to cultural eutrophicatio n, The marine case studies focus on macroalgae and its interactions with se diments and vascular plants. The freshwater case studies focus on interacti ons between phytoplankton, epiphyton, and benthic microalgae. In Waquoit Ba y, MA (estuary), controlled experiments documented that blooms of macroalga e were responsible for the loss of eelgrass beds at nutrient-enriched locat ions. Macroalgae covered eelgrass and reduced irradiance to the extent that the plants could not maintain net growth. In Hog Island Bay, VA (estuary), a dense lawn of macroalgae covered the bottom sediments. There was reduced sediment-water nitrogen exchange when the algae were actively growing and high nitrogen release during algal senescence. In Lakes Brobo (West Africa) and Okechobee (FL), there were dramatic seasonal changes in the biomass an d phosphorus content of planktonic versus attached algae, and these changes were coupled with changes in water level and abiotic turbidity. Deeper wat er and/or greater turbidity favored dominance by phytoplankton. In Lake Bro bo there also was evidence that phytoplankton growth was stimulated followi ng a die-off of vascular plants. The ease studies from Waquoit Bay and Lake Okeechobee support conceptual models of succession from vascular plants to benthic algae to phytoplankton along gradients of increasing nutrients and decreasing underwater irradiance. The case studies from Hog Island Bay and Lake Brobo illustrate additional effects (modified sediment-water nutrient fluxes, allelopathy or nutrient release during plant senescence) that coul d Flay a role in ecosystem response to nutrient stress. (C) 2001 Elsevier S cience Ltd. All rights reserved.